Recent progress in selected bio-nanomaterials and their engineering applications: An overview

Chia sẻ: Bịnh Bệnh | Ngày: | Loại File: PDF | Số trang:26

Thêm vào BST

Báo xấu

7
lượt xem 2
download

Download Vui lòng tải xuống để xem tài liệu đầy đủ

This review article is an effort to combine the recent developments, concerns and prospective applications of environmentally friendly nano- with micro-structured polymeric materials such as chitin, starch, polycaprolactone and nanocellulose.

Chủ đề:

Bình luận(0) Đăng nhập để gửi bình luận!

Lưu

Nội dung Text: Recent progress in selected bio-nanomaterials and their engineering applications: An overview

Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Review Article Recent progress in selected bio-nanomaterials and their engineering applications: An overview Raghvendra Kumar Mishra a, Sung Kyu Ha b, **, Kartikey Verma c, Santosh K. Tiwari b, d, * a International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India b Department of Mechanical Engineering, Hanyang University, South Korea c Department of Chemical Engineering, Indian Institute of Technology, Kanpur, Uttar Pradesh, India d Department of Applied Chemistry, Indian Institute of Technology (ISM), Dhanbad, Jharkhand, India a r t i c l e i n f o a b s t r a c t Article history: Nowadays, the rapid climate change, water pollution and harmful gas emissions are largely caused by the Received 3 March 2018 extensive use of petrochemicals and the burning of plastic materials. The government authorities across Received in revised form the globe and experts mentioned that the dumping of plastic waste and non-biodegradable materials is a 21 May 2018 principal problem of the environmental pollution. In their numerous chemical forms, cellulose and Accepted 28 May 2018 Available online 2 June 2018 various other biodegradable materials can be possible alternatives to resolve these challenging issues. This review article is an effort to combine the recent developments, concerns and prospective applica- tions of environmentally friendly nano- with micro-structured polymeric materials such as chitin, starch, Keywords: Green materials polycaprolactone and nanocellulose. Nanocellulose has been considered as one of the most important Biopolymers biopolymers having signiﬁcant advancements in research and their application in the various ﬁelds. Cellulose Herein, cellulose-based materials for engineering and interdisciplinary applications, comprising ap- Nanocellulose proaches for the transformation of cellulose to nanocellulose, and the fabrication method for their blends Biodegradable materials and composites have been reviewed. Moreover, the structural-functional relationship, the thermo- mechanical properties of starch, poly (Lactic) acid, polycaprolactone, lignin and some of their compos- ite and potential applications of these materials in various ﬁelds of engineering have been elaborated. © 2018 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). 1. Introduction and reused, huge ranges of these types of materials completely turn out in the form of landﬁll [2,3]. Consequently, progressive The prevailing development in polymeric composites applica- biosphere issues parallelled the growth and development of ver- tions is gaining momentum around the world [1e3]. The satile barrier bio-based product packaging materials as a reason- extraordinary features of innovative polymeric composites and able alternative when dealing with these materials [2]. Together, their establishment with existing environmentally friendly the lack of fossil fuel energy sources and the raised costs of crude analytical techniques adduce considerable triumph to improve the oil have heightened the worldwide interest in bio-based materials era of environmental research [2,3]. The food packaging materials [1e3]. It turned out by analysers that the petroleum sources obtained from petroleum-derived polymers are widely employed would appear to be inadequate in successive 60 years [2e4]. in several different functions due to their lesser density, more Controlling the forthcoming concerns because of the plastic affordable cost, and extraordinary mechanical as well as barrier wastes as well as petroleum resources triggers the production of properties [1e3]. Even though, various forms of petroleum-based leading-edge and more environmentally friendly materials in the product packaging polymer materials are generally recoverable modern era. Severe efforts are undertaken for growth and development of bio-based composites composed of renewable sources to replace petroleum-based polymers by obtaining eco- * Corresponding author. Department of Applied Chemistry, Indian Institute of friendly materials [1e4]. Edible coatings and ﬁlms involved with Technology (ISM), Dhanbad, Jharkhand, India. an appreciable unique class of packaging materials that offer an ** Corresponding author. E-mail addresses: sungha@hanyang.ac.kr (S.K. Ha), ismgraphene@gmail.com additional strategy over the traditional packaging materials likely (S.K. Tiwari). due to their outstanding biodegradable, biocompatible as well as Peer review under responsibility of Vietnam National University, Hanoi. https://doi.org/10.1016/j.jsamd.2018.05.003 2468-2179/© 2018 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
264 R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 edibility characteristics [2e5]. Although a sizeable number of bio classes are further deﬁned directly into a bunch of subgroups in extracted materials are analysed for their precise practical appli- agreement with their source [3,4]. Biopolymers are grouped, cations, scientists and experts have anticipated a fair number of dependent upon the type of the repeating unit which is composed persistent issues that conﬁne their wider industrial applications of three classes: (i) polysaccharides which are produced of sugars, [3]. For example, some bio-degradable polymer materials in (ii) proteins that come from different amino acids and (iii) nucleic general exhibit poor mechanical properties in comparison with a acids that are composed of nucleotides. Relevant to the applica- lot of petroleum-based polymers. This is exactly due to the tion, biopolymers are known for their role of bioplastics, bio- intrinsic low stiffness as well as strength characteristics of surfactant, bio detergent, bio-adhesive, bioﬂocculant, etc [6,7]. biodegradable polymer materials. Enhancing the properties of these kinds of biopolymers can often lead to innovative materials. 2.2. Biopolymer source and preparation Low production levels, competition with food crops and high costs are the important aspects that can further reduce the broader Biopolymers are usually a variety of plastics produced from technical applications of biopolymer packaging materials. Thus, environmentally friendly biomass sources, for example, corn scientists are attempting to enhance the mechanical and barrier starch, pea starch, vegetable oil, and so on [6,7]. Combined with characteristics of bio-based ﬁlms. There are several options bio-inspired polymeric materials (also many biopolymers) which available to enhance the barrier and mechanical properties of are artiﬁcially extracted from particular polymers for preferred packaging [4]. Nowadays, studies regarding bio-polymer nano- applications [6]. The existence of unique natural polymers in composites have witnessed a considerable improvement in the crops, vegetation and plants grant a bio-renewable opportunity targeted properties in both industrial and academic laboratories. for their preparation. It is noticeable that, almost all regular man- According to the recent studies, the worldwide foods and made polymers, are created in bulk after which they are moulded beverage trade are going to display progression rates of near to 8% for the purpose of scientiﬁc research works. Various types of CAGR over the expected duration [4,5]. The total food and microbes performed a key part in creating a variety of bio- beverage business enterprise in 2005 were approximately around polymers, for example, polyesters, polysaccharides, as well as USD 8 trillion and achieved around USD 15 trillion in 2015; a polyamides within the range of viscous solutions to plastics development related to, among other activities, an increasing (Table 1) [7,8]. Their physical characteristics are inﬂuenced by the middle class of the community with an ampliﬁed customer constitution, design of repeated units as well as the molecular investing potential in the Asia Paciﬁc and Latin America. The huge weight of the polymer [8]. The physical, as well as chemical research and development on this material around the globe characteristics of a variety of biopolymers synthesized by the help (Scopus data 2017, Fig. 1 a and b), have produced extensive items of microbes, may be tailored to the consistent treatment of mi- for the various industries, that would eventually raise the total croorganisms which makes it suitable for healthcare applications, biopolymer sell capacity by 20,246, as shown in Fig. 1. Therefore, for example, drug delivery and tissue mechanism [9]. Biopolymers this review article is a boon for the research outputs regarding the which are generated by making use of microorganisms bio-polymer inspired micro and nanomaterials. require speciﬁc nutrition as well as maintained surrounding environment. These are commercially developed by means of 2. Green materials and their derivatives direct fermentation or even by chemical polymerization by using repeating units that are consequently prepared by means of 2.1. Bio-polymer fermentation procedure. Generally, most of the biopolymers are biocompatible as well as biodegradable without negative impact Biopolymers are usually polymeric biomolecules consisting on biological systems [9]. The functional mechanism of monomeric units, which are generally covalently joined to fabri- manufacturing of biopolymers from the microorganism origin is cate larger sized molecules [1,2]. The term ‘bio’ implies that these widely seen either as a result of their particular defence mecha- are in fact naturally degradable materials derived from formal nism or as the storage material [9]. It is well recognized that these living microorganisms [1]. A group of materials generally manu- types of materials tend to get degraded by natural processes as factured from organic natural resources just to illustrate microbes, that of microorganisms and enzymes to ensure that it may lastly crops, or even plants are deﬁned by means of the expression be reabsorbed in the environment [9,10]. By focussing our con- “biopolymer”. Materials based synthetic routes derived from the centration, a bit more into the biopolymers, the modiﬁcation of biological resources for instance vegetable oils; sucrose, fats, fossil materials as well reducing of CO2 pollutants are achieved, resins, proteins, amino acids, etc are also referred to as biopolymer therefore promoting environmental friendly development [10]. (because of the natural compositions) [2e4]. In comparison with Among the variety of microorganisms, algae work as an artiﬁcial polymers that contain a less complicated and additional outstanding feedstock for the plastic generation because of their random conﬁguration, biopolymers are complicated molecular high output along with its potential to cultivate in a variety of assemblies that employ accurate, described 3D patterns and conditions [9,10]. The application of algae reveals the chance for architectural structures [5]. This is certainly one vital character making use of carbon as well as neutralizing greenhouse gas rendering biopolymers active molecules in-vivo. Their speciﬁed emissions from all sorts of industrial facilities. Algae-based plas- shapes together with structure are key elements of their perfor- tics have been a newly released inclination in the period of bio- mance. As an illustration, haemoglobin is unable to have oxygen in plastics in comparison with conventional strategies of employing the blood when it was not folded inside a quaternary architecture. feedstocks of corn and potatoes as plastics [1,10]. Although algae- Biopolymers are categorized in various ways according to distinct based plastics are in their infancy, once they are into commer- scales [5,6]. According to their degradability, biopolymers are split cialization they likely ﬁnd applications in a wide range of in- into two wide categories, speciﬁcally biodegradable in addition to dustries. At present, microbial plastics are viewed in the form of a non-biodegradable, and alternatively, into bio-based as well as crucial root of polymeric material which has an incredible op- non-bio-based biopolymers [6]. On the structure of their polymer portunity for commercialization in the next generation. They can main chain, biopolymers are undoubtedly grouped generally into tailor the movement abilities of ﬂuids, be encapsulating materials, the following categories: polyesters, polysaccharides, poly- ﬂocculate particles, create emulsions as well as stabilize suspen- carbonates, polyamides, as well as vinyl polymers. These types of sions [10].
R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 265 Fig. 1. (a) Scopus database (09/12/2017) for the research output in form of research articles from the top ten countries and (b) applications of bio-polymer inspired micro and nanomaterials (Scopus database 09/12/2017). Table 1 Different sources and methods of preparation for bio-polymer and related materials [1]. Polymer Source or Method Basic materials used for synthesis Hybrid plastics Introducing denatured algae biomass to petroleum-based Filamentous green algae, Cladophores plastics exactly like polyurethane and polyethene in the role of ﬁllers. Cellulose-based plastics Biopolymer of glucose 25e30% of the biomass created after extraction of algal oil is known to comprise cellulose Poly-lactic acid (PLA) Polymerization of lactic acid lactic acid Microorganism fermentation of algal biomass Bio-polyethylene Ethylene manufactured from ethanol, by a chemical Microorganism fermentation of algal biomass reaction called cracking. Ethanol derived from natural gas/petroleum sources Bio-Polyesters Biomass Microorganism like Akaligeneseurrophus, Escherichia coli, etc. Polyhydroxybutyrate (PHA) Obtained as a carbon and energy storage polymer Microorganism such as Alcaligenes eutrophus under nutrient limiting growth environments Polycaprolactone (PCL) Ring-opening polymerization using dibutyl ε-caprolactone zinc-triisobutylaluminum as a catalyst Chitosan Alkali NaOH treatment Treating shrimp and other crustacean shells such as crabs and krills Gelatin Partial hydrolysis of collagen Collagen from white connective tissue, animal bones and skin Alginic acid Treatment Brown algae in aqueous alkali solutions Brown algae, including Laminaria hyperborea, Laminaria digitata, Laminaria japonica, Ascophyllum nodosum, and Macrocystis pyrifera
266 R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 3. Structure- property relationship of green polymeric Green composites include a group of materials, which are materials gathered from renewable resources and undergo complete degra- dation through microorganisms [1e3]. These materials act as a Bio-polymers are adaptable in structure as well as properties and potential substitute for conventional petroleum-based polymeric hence regarded as the most preferred biomaterials. The demand for materials for which recycling seems to be unpractical or uneco- polymeric biomaterials in healthcare enterprise is about 60% of the nomical. The biodegradability aspect of green polymers associated global market, which has been expanding exponentially in recent with their biocompatibility makes them an efﬁcient advanced years [9]. This is due to the structure and surface properties of biomaterial [10,11]. However, lack of adequate mechanical strength, polymers, which can be tuned according to the speciﬁc desires of as well as bioactivity characteristics, insufﬁcient control with the biomaterials for various applications [9,10]. The polymer-based respect to degradation rate and poor biomimetic structural or materials derived from renewable resources support the develop- compositional features, limit their practical applications, Table 2 ment of sustainable composites via economically feasible and shows a list of frequently used polymers and their applications environmentally friendly technology [1,9]. If a polymer or a poly- [10,11]. meric blend or a polymer composite material with biodegradation property is obtained solely from a sustainable source, then it is 4. Cellulose called as a green polymeric product. Nature presents a wide range of polymers with biodegradability property and exhibits the potential Cellulose is an abundantly available natural biopolymer and can aspects to replace conventional fossil fuel-based polymers [1e3]. be readily obtained from sustainable sources [2,10]. The examples The common examples for naturally derived polymers include of ﬁbrous form of cellulose include cotton, wool and hemp. As sugar proteins, starch, chitin and cellulose [1e3]. In addition to this, nat- constitutes the monomeric units of cellulose, it falls under the ural rubber latex (NR), polylactic acid (PLA) derived from corn and category of the polysaccharide [11]. The molecular formula for an polyhydroxyalkanoates (PHA) produced from bacteria serve as ex- organic cellulose is (C6H10O5)n. This denotes polysaccharide, which amples for other green polymers. Polymers such as poly(a-hydroxy consists of hundreds or thousands of 1e4 linked D-glucose units acid)s, poly(ε-caprolactone), poly(glycolic acid), poly(methyl that are linked together in a linear fashion [10e13]. It is found that methacrylate), poly(dimethylsiloxane), PU, cellulose, silk etc. are certain bacterial species also secrete cellulose to promote the for- utilized as biomaterials for various applications like contact lenses, mation of bioﬁlms. In general, plants consist of 33% of cellulose bone cement, wound dressings, artiﬁcial organs, tissue scaffolds, content on an average [12,13]. The plant sources rich in cellulose cardio-vascular apparatus, breast implants, catheters, drug delivery, include cotton and wood. The cotton consists of ~90% cellulose, sutures and so forth [1e3]. whereas the cellulose content present in wood corresponds to ~50% Table 2 Most frequently used polymers for different applications with remarks on their pros and cons [10]. Polymer Applications Remarks Poly(methyl Contact lenses, bone cements, dentures etc. Comparable elastic modulus to bone, bio-stable or bio-inert, brittle, low methacrylate) tolerance to the organic solvents, inability to modify with biomolecules etc. Polyurethane(PU) Wound dressings, artiﬁcial organs, tissue scaffolds, Tuneable properties, blood-compatibility, biodegradable with no signiﬁcant cardio-vascular devices, etc. pH change, Toxic degradation products, lack of bio-stability for permanent implants etc. Poly(dimethylsiloxane) Contact lenses, breast implants etc. Skin protectant, bio-durable, immunogenic activation of anti-silicon antigens etc. Polyethylene Orthopaedic joint implants, components of catheters etc. Good toughness, resistance to fats and oil, cannot withstand sterilization temperature etc. Poly(ethylene glycol) Wound dressings, ﬁllers etc. Hydrophilicity, biocompatibility, low immunogenic, insufﬁcient strength, high degradation rate etc. Polycaprolactone Drug delivery, sutures and scaffolds etc. Good ductility, biocompatible, low tensile strength, slow degradation rate etc. Poly(lactide- Resorbable suture scaffolds, bone grafts, stents, Excellent biodegradability, good processability, acidic degradation product co-glycolide) drug delivery etc. etc. Gelatin Tissue engineering, wound dressing, gene transfection, Haemostatic, non-immunogenic, pro-angiogenic, biodegradable as well as drug delivery, weight loss as well as for treating biocompatible, cross-linked to form hydrogels etc. osteoarthritis, rheumatoid arthritis, for foods, cosmetics, and medicines etc. Starch Paper, textiles as well as adhesives, pharmaceutical tablets, Cheap as well as degradable, sensitivity to moisture and poor mechanical pesticides, cosmetics, detergents, oil-drilling ﬂuids etc. properties etc. Cellulose Biomedical ﬁeld, Textile applications etc. Natural biological polymer, environment-friendly, biodegradable, remarkable strength etc. Chitin Water treatment, biomedical applications such as Fibroblast It is insoluble in most of the solvents, as applicability, oxygen permeability, migration and proliferation wound dressing, raw materials water sorptivity, blood coagulating property etc. for chitosan etc. Chitosan Drug carrier, coating agent, gel former etc. The excellent ﬁlm forming property etc. Polylactic acid Medical implants, wound management, drugs delivery etc. Biodegradable thermoplastic, aliphatic polyester, soluble in many organic solvents, higher transparency compare with other biodegradable polymers, superior in weather resistance etc. Poly-vinyl-Alcohol Thickener in glues, paper-making, a sizing agent in textiles, Colourless, white, and odourless, water-soluble synthetic polymer etc. water-soluble ﬁlms useful for packing, coatings, optical, pharmaceutical, medical applications Polyvinyl acetate Biomedical, synthesis of metal nanoparticles, Polyvinyl ester family, Thermoplastic resin etc. sensing activity etc. Collagen Medical such as healing and repairing of the body's Damage the production of collagen includes sunlight, smoking, and high tissues and skin-deep application. sugar consumption.
R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 267 or more [12,13]. Apart from cellulose, several natural components chains are stacked parallel to each other by Van Der Waal's inter- namely lignin, hemicellulose and pectin are also present in plant action [17,18]. Cellulose Ib has a monoclinic unit with two hydrogen ﬁbres. As cellulose is an abundantly available raw material in nature bonding chains per unit cell. These two forms of cellulose I mutu- and has attractive features, it is considered to meet the growing ally co-exist and their percentage varies with varying sources of demand for eco-friendly products with ensured biocompatibility cellulose [18]. Cellulose obtained from primitive organisms (bac- [12,13]. It is well known that cellulose is insoluble in many solvents, teria, algae) is rich in cellulose Ia while cellulose obtained from which brings a limitation to its reactivity and processability for developed higher woody plants are rich in cellulose Ib. Cellulose Ia utilization [11e13]. In thermoplastic-based polymer matrices, cel- can be transformed into more stable Ib by annealing at around lulose ﬁbres are recognized as an efﬁcient reinforcement in the 2600 C to 2800 C in some special solvents [18]. recent years. This is mainly due to the signiﬁcant features of cel- lulose, which include low density, reduced wear problems while 4.2.2. Cellulose II processing and readily active surface for functionalization, cost- Cellulose II is much more thermodynamically stable than Cel- effective nature and easy availability [13]. The cellulose reinforced lulose I. It was in 1844 that John Mercer ﬁrst invented the technique polymer composites can be easily combusted during a recycling of mercerization for converting cellulose I to Cellulose II [17e19]. In process as compared to polymer matrices ﬁlled with inorganic the process of mercerization cellulose is treated with alkali with a ﬁllers [13,14]. Despite all these advantages, the utilization of cel- concentration of about 17%e20% w/v [17e19]. This leads to the lulose ﬁbres in the industrial scale is limited. The reason behind this swelling of cellulose ﬁbres when Naþ ions penetrate the spaces problem is the inability in attaining satisfactory dispersion levels in between the cellulose molecules without causing any dissolution. polymer matrices [14]. During the process, the parallel chain arrangement of the cellulose molecules gets reversed into antiparallel chains. In 2015, B. J. C. 4.1. The basic structure of cellulose Duchemin was able to convert cellulose I into cellulose II by using only 1% w/v NaOH solution at the temperature below 0 C without It is well known that cellulose is a polysaccharide with glucose changing the crystallinity of the cellulose microstructure. This was as its monomeric units. Structural isomers of glucose include indeed a great advancement in the mercerization technique. fructose and galactose [14,15]. Throughout the cellulose chain, Another process of dissolution and regeneration can also convert glucose molecules are linked via glycosidic bridges which are cellulose I into cellulose II [18,19]. As the name suggests, this pro- formed by loss of hydrogen atom from one monomer and hydroxyl cess involves the complete dissolution of cellulose followed by groups to another monomeric unit [15]. This leads to the formation regeneration of cellulose ﬁbres. Regeneration of cellulose can be of microﬁbrils [14]. These microﬁbrils are arranged together by the done by various processes like copper ammonium and N-methyl intermolecular hydrogen bonding to form bigger ﬁbrils. It can be morpholine N-oxide (NMMO) processes [19]. The cellulose I to II observed that a cellulose homopolymer consists of D-anhy- conversion was reported by regeneration using phosphoric acid. droglucopyranose units which are then linked via (1e4) glycosidic The alteration of cellulose I to cellulose II is an irreversible process bonds. As glucose exists as a six-membered ring in the cellulose indicating cellulose II is much more thermodynamically stable [19]. structure, it is named as pyranose [15]. The oxygen linkages pro- vide a connection between the pyranose monomeric units. Native 4.2.3. Cellulose III cellulose refers to the cellulose produced by plants, which exists in Interestingly, Cellulose III exists in two forms: Cellulose IIII and two types of crystalline structure namely cellulose I and cellulose II. Cellulose IIIII. Cellulose IIII is obtained by the ammonia merceriza- Type II cellulose is found to exist in marine algae [15,16]. Cellulose tion of Cellulose I and Cellulose IIIII is obtained by the ammonia II is usually crystalline in nature and can be produced when cel- mercerization of Cellulose II. In 1986, Yatsu did the stable trans- lulose I is subjected to undergo treatment with aqueous phase formation of cellulose I to cellulose III by immersing cellulose in sodium hydroxide. Apart from type I and type II cellulose, other ammonia solution followed by degassing [20]. The conversion to forms such as cellulose III and cellulose IV do exist. Cellulose I is Cellulose III is reversible in nature where chain orientation is not recognized to be less stable as compared to other polymorphs, changed. However, fragmentation of crystal takes place during the whereas cellulose II is known to present a highly durable structure transformation of Cellulose I to cellulose III. During the reverse among all the types [16]. transformation, distortion of the morphological structure is not restored [20]. 4.2. Polymorphisms of cellulose 4.2.4. Cellulose IV Polymorphism is the property by virtue, a compound can Cellulose IV is obtained from Cellulose III by treating it with appear in more than one form. Cellulose consists of many hydroxyl glycerol at a higher temperature. In 1946, Hermans and Weidinger groups contributing to more intramolecular and intermolecular were able to convert mercenaries' ramie cellulose ﬁbres into cel- hydrogen bonds which lead to separate ordered arrangements [17]. lulose IV by treating it at high temperature in the presence of In one cellulose bonding unit, there prevails six hydroxyl groups glycerol [21]. and three oxygen atoms. Hence, there are many possibilities of crystal packaging, different cellulose units, and change in chain 4.3. Nanocellulose polarity. The polymorphs of cellulose are classiﬁed generally into four heads [17]: Nanocellulose is one of the dominant biodegradable and sus- tainable nanomaterials found in nature. In simpler terms nano- 4.2.1. Cellulose I cellulose corresponds to cellulose in the nanometer scale [20e22]. Cellulose I is a native cellulose, and it is abundantly found in the Till 1970, humans have been making cellulose from plant materials environment [2,3]. In 1984, Atalla and Vander Hart discovered that like wood, plants, waste materials and algae [22,23]. In 1970, Tur- the native cellulose is present in two forms, namely Ia and Ib. They bak, Snyder and Sandberg could successfully synthesize micro- used 13C CP/MAS NMR spectra to characterize native cellulose into ﬁbrillated cellulose by homogenization at high temperature and two distinct allomorphs [17,18]. Cellulose Ia has a triclinic unit with high pressure accompanied by an impact ejection at a hard surface one hydrogen bonding chain per unit cell, where the cellulose [22,23]. Generally, cellulose is slightly crystalline and amorphous in
268 R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 nature [23]. It is extremely difﬁcult to break the crystalline part of [26]. However, M. denticulate microﬁbrils possess rectangular cellulose because of its remarkably strong hydrogen bonds [23]. cross-sectional area (nm2) because they are mainly of the CIb Hence, cellulose has to go through a sequence of chemical and crystalline form. Bacterial cellulose (BC) is a result of the major mechanical treatments in order to extract crystalline nanocellulose metabolic operations of a particular kind of bacteria [26,27]. The and nanocellulose ﬁbres [22]. Cellulose nanoﬁbrils (CNF) include most well-known BC-producing bacterial breeds are Gluconaceto- extremely thin (nearly 5e20 nm) and in length (up to several mm) bacter xylinus [23,26,27]. Under speciﬁc culturing environment, ﬁbrils with signiﬁcantly high surface area (aspect ratio). Every these types of bacteria create a dense gel which is made up of microﬁbril is usually deﬁned as a chain of cellulose crystals con- cellulose microﬁbrils together with 97e99% water [23,26,27]. nected along the microﬁbril axis disordered amorphous domains Bacteria cellulose crystallites are predominantly of the CIa crys- [22,23]. In small quantities, it makes a translucent gel-like material talline form along with the degree of polymerization (DP) of bac- that can be used for developing biodegradable combined with eco- terial cellulose, which is commonly between two thousand and six friendly safe, uniform, as well as dense ﬁlms for several purposes, thousand. The beneﬁt of bacterial cellulose is the fact that it is easy speciﬁcally in the biomedical area [23]. Extraction of CNF is being to adapt the culturing environment to modify the microﬁbril discussed from a variety of resources such as coir, banana, sugar conﬁguration as well as crystallization [23]. The supplementary beet, hemp, softwood, in addition to wood pulps [23]. After vital ability of bacterial cellulose is its superior chemical purity, employing a range of plasticizers, range of thermal, mechanical, which distinguishes it from the kinds of plant cellulose that are barrier, and also physical features of the cellulose are enhanced in generally related to hemicelluloses as well as lignin. In spite of this, order that it has been employed in a variety of market applications both celluloses synthesized by bacteria or cellulose obtained from a [23]. Plasticizing enhances the features such as oil and grease number of plants contain identical molecular arrangements protection, and signiﬁcant barrier against oxygen transfer particu- [15,23,26]. larly at a dry environment [23,24]. Almost all cellulose-synthesizing The different kinds of nanocellulose are generally categorized microorganisms consisting microbes, algae, tunicates, as well as into various subcategories according to their shape, dimension, considerably higher plants possess cellulose syntheses proteins, functionality, as well as generation strategy, which predominantly which catalyse the polymerization reaction of glucan chains [24]. rely on the cellulosic resource together with processing environ- At present, cellulose is generally obtained from a vast variety of ment [23,26]. Various terminologies can be employed for the crops, vegetation, plants, animals, as well as bacteria [17,23]. The different types of nanocellulose [26]. A serious problem that origin is crucial as it inﬂuences the dimensions as well as charac- needs to be eliminated for effective commercialization of cellu- teristics of the obtained cellulose. An array of the crop, vegetable lose nano-ﬁbrils is the excessive energy consumption needed for and plant materials are being analysed in relation to the extraction the mechanical disintegration of the preliminary cellulose of cellulose as well as nanocellulose, which includes timber, rice microﬁbers into nano-ﬁbres, may possibly consist of many passes husk, sisal, hemp, ﬂax, kenaf, and in the coconut husk. Cotton ﬁbres through the disintegration machine. To deal with, it looks like the have likewise been applied in form of an efﬁcient source material, mode of cellulose feedstock performs a noteworthy role in the beneﬁting from their low non-cellulosic constituent content than energy consumption; despite this, it seems to have merely any wood. Wood is an elegant initiating material for cellulose as well as effect on the resultant cellulose nano-ﬁbrils features. It should be nanocellulose isolation, due to its terriﬁc quantity [23]. It is a nat- mentioned that cellulose nano-ﬁbrils experience some speciﬁc ural composite material with a hierarchical structure comprised of unfavourable characteristics, which reduce their application in cellulose, hemicelluloses, as well as lignin [23,24]. Wood includes a several sectors, for illustration, in papermaking due to sluggish porous anisotropic conﬁguration, which displays an extraordinary dewatering or even as polymer composites because of inadequate combination of excellent strength, rigidity, resilience, as well as compatibility of hydrophilic reinforces with hydrophobic poly- lesser density [25]. The preparation of nanocellulose from wood mers. A possible method for ﬁxing this issue is the chemical involves a multistage operation concerning vigorous chemical and/ modiﬁcation of cellulose nanoﬁbrils to decrease the quantity of or mechanical treatments. Tunicates are aquatic invertebrate ani- hydrophilic hydroxy active groups [28]. Cellulose nanocrystals mals, particularly, members of the subphylum Tunicata. Most of the (CNCs) show an elongated rod-like appearance and provide very study in this ﬁeld includes a highlighted category of tunicates limited ﬂexibility in comparison to cellulose nanoﬁbrils, due to its which are typically referred to as sea squirts (Ascidiacea), that is a considerably higher crystallinity [28,29]. Cellulose nanocrystals breed of aquatic invertebrate ﬁlter feeders. Experts are working are usually called as nanocrystalline cellulose, nanowhiskers, over 2300 varieties of Ascidiacea and cellulose microﬁbrils to un- nanorods, or rod-like cellulose crystals [29]. The nanocrystalline derstand above mentioned phenomenon. For the same reason, particles structure are produced through the splitting of amor- scientists are also working on many dissimilar species like Hal- phous domains, along with the splitting of localized crystalline ocynthia roretzi, Halocynthia papillosa and Metandroxarpa uedai contacts between nanoﬁbrils, by means of hydrolysis with [23,26,27]. The tunicates create cellulose in the external tissue, concentrated acids (6e8 M). This chemical type method is referred to as tunic, from which a reﬁned cellulose portion referred accompanied by high-power mechanical or sometimes ultrasonic to as tunicin is obtained. Tunicate cellulose consists of nearly fresh treatment methods [29]. A signiﬁcant feature of cellulose nano- cellulose of CIb allomorph form with signiﬁcant crystallinity [26]. crystals made from sulfuric acid is the negative particle charge, The nano/microﬁbrils of tunicate cellulose contain a huge aspect because of the generation of sulphate ester active groups, which ratio as well as the excellent speciﬁc surface area. Algae of a variety improves the phase durability of the nanocrystalline particles in of breeds, green, red as well as brown, are also deﬁned as cellulose an aqueous environment [28,29]. The geometrical shapes and as well as nanocellulose sources. For example, Valonia, Micrasterias sizes of cellulose nanocrystals may vary extensively, with a denticulate, Micrasterias rotate, Cladophora, Boergesenia, as well as diameter ranging from 5 to 50 nm as well as a length within the other kinds of algae are being employed [26,27]. Cellulose micro- range of 100e500 nm. The dimensions, as well as the crystallinity ﬁbrils with a huge aspect ratio (>40) are generally obtained from an of a cellulose nanocrystal, are controlled by the cellulose resource algae cell wall by way of acid hydrolysis or mechanical reﬁning [27]. and consequently extraction circumstances. Researchers have The architectural structures of CMFs separated from various kinds described that nanocrystalline particles obtained from tunicates of algae change. To illustrate, Valonia microﬁbrils hold square and bacteria cellulose are likely to be bigger in comparison to cross-sectional area (nm2) as they are mainly of Ia crystalline form cellulose nanocrystals received from wood or cotton. This is
R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 269 because tunicates or bacteria cellulose are extremely crystalline bioactive ingredients, thickening agents in a variety of aqueous and possess extended nanocrystallites. Cellulose nanocrystals systems, etc [30e34]. Cellulose Nanoyarn is another form of derived from pure cellulose materials display greater crystallinity. nanocellulose, it is produced by electrospinning a solution Nanocrystalline cellulose particles manifest outstanding me- composed of cellulose or cellulose derivatives. However, it has not chanical characteristics [29,30]. The theoretical Young's modulus been widely studied till date [34e37]. A transmission electron of a cellulose nanocrystal along the cellulose chain axis is ex- microscope (TEM) image of nanocellulose forms, produced from pected to be 167.5 GPa, which is certainly just like the modulus of different sources is shown as an example in Fig. 2. Kevlar or maybe greater than the modulus of steel [30]. The experimental Young's modulus of cotton cellulose nanocrystals is 4.4. Preparation of nanocellulose 105 GPa, in addition the modulus of tunicate cellulose nano- crystals is 143 GPa. Amorphous Nanocellulose is generally When plant cell wall is exposed to powerful mechanical disin- extracted by using acid hydrolysis of regenerated cellulose with tegration, the initial structure of cellulose ﬁbre is transformed, succeeding ultrasound disintegration. Amorphous nanocellulose therefore the ﬁbres transform into nanoﬁbrils (CNF) or even their particles commonly contain an elliptical shape with typical di- microﬁbrils bundles (CMF) with diameters which range from 10 to ameters of 50e200 nm [30,31]. Due to its amorphous conﬁg- 100 nm based on the disintegration power [33e37]. Many me- uration and arrangement, amorphous nanocellulose has chanical methods are often used to obtain cellulose nanoﬁbrils or extraordinary capabilities, including a much higher functional cellulose microﬁbrils from a variety of feedstocks, which include group content, a signiﬁcant availability, an improved sorption, as homogenization, microﬂuidization, grinding, cryocrushing, as well well as an enriched thickening potential [3,17,22]. Even though, as ultrasonication, as shown in Fig. 3 [36]. The preparation of amorphous nanocellulose particles possess inadequate mechani- nanocellulose consists of many steps which includes mechanical cal characteristics because they are inappropriate to be used in the and chemical treatments. Such treatments are assigned towards form of reinforcing nanoﬁllers [3,17]. Thereby, the main applica- restructuring the speciﬁc coherent organization of microﬁbrils tions of amorphous nanocellulose are in the role of carriers for present in a natural cellulose [23,36]. The nature and properties of Fig. 2. TEM micrographs of (a) Microcrystalline Cellulose from fodder grass [32], (b) Cellulose microﬁbril from sugar beet [33], (c) Cellulose nanoﬁbril from banana peel [34], (d) Cellulose nanocrystal from ramie ﬁber [35], (e) Amorphous Nanocellulose from MCC [36].
270 R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 the ultimate nanocellulose product that we receive depend upon of cellulose are: (i) ﬁbril aggregation, whenever slurry is pumped the individual steps involved as well as the individual length of by means of the disintegration equipment in addition to (ii) step. The aspect ratio of the ﬁnal nanocellulose depends upon the excessive energy intake involved with ﬁber delamination, may length of de-structuring that the raw material undergoes [38]. possibly engage multiple feeds into the disintegration equipment until effective delamination of cell walls is attained [36e39]. The 4.4.1. Methods of nanocellulose preparation excessive energy contribution is essential with the intention to There are several different extraction techniques to collect produce the nanoﬁbers. In order to reduce the interﬁbrillar nanoﬁbrils (Fig. 3). They are often carried out by mechanical stra- hydrogen bonding, based on earlier scientiﬁc studies, an effective tegies, for example, grinding, cryocrushing in the presence of liquid pre-treatment can reduce the energy intake. The selection of pre- nitrogen, high-pressure homogenization, and so forth. Similarly, treatment methodology is based on the cellulose source [39]. It is various chemical alkali, as well as enzymatic hydrolyses, are worthwhile stating that a suitable pre-treatment of cellulose ﬁbres applied before mechanical techniques to be able to increase the supports reliability, improves the inner surface, adjusts crystal- gain access of hydroxyl active groups, which improve the inner linity, decreases the energy demand and promotes the process of surface, modify the crystallinity, split cellulose hydrogen bonds, nanocellulose generation [40]. As an example, the pre-treatment of thereby enhancing the reactivity of the ﬁbres [37,38]. vegetable, crop, fruit, plant materials enhances the total or even limited elimination of noncellulose constituents hemicellulose, 4.4.2. Pre-treatment lignin as well as the isolation of speciﬁc ﬁbres. Pre-treatment of Two main issues frequently take place during the ﬁbrillation tunicate entails the elimination of the proteins matrix, isolation of stage, and most importantly throughout the mechanical ﬁbrillation the mantel, along with the isolation of speciﬁc cellulose ﬁbrils Fig. 3. Schematic diagram for the recent process to achieve nanocellulose [39].
R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 271 [36e40]. Pre-treatment of algae usually helps in the elimination of ﬁbre ﬁbrillation operation is performed by transferring the cellu- the matrix material of algae cell walls, while pre-treatment of lose slurry between static as well as rotating grindstones rotating at bacterial NC is targeted on the elimination of microbes as well as about 1500 rpm, which exert a shearing stress to the ﬁbres [46]. The other contaminants from the slurry [39]. Pre-treatment is an ﬁbrillation procedure in the grinder makes use of shear forces to extremely crucial stage since it can adjust the structural foundation, decompose the cell wall conﬁguration and arrangement as well as crystallinity, as well as polymorphism of the cellulose, in addition separate the nanoscale ﬁbrils [45,46]. The level of ﬁbrillation relies to numerous characteristics of the pre-treated feedstock. The pre- upon the distance between the disks, the actual morphology of the treatment method is employed to assist the cell wall delamina- disk tunnels, along with the number of feeds into the grinder. tion together which produces nano-sized ﬁbrils. The well-known Regarding a homogenizer, numerous passes have been instructed pretreatment approach includes pulping techniques, bleaching, to produce the ﬁbrillated cellulose. The requirement for disk stone alkaline-acid-alkaline treatment, enzymatic treatment, ionic liq- routine maintenance as well as a replacement may be a downside uids, oxidation, and steam explosion [1,38e40]. of this method since wood pulp ﬁbres are able to deteriorate the grooves as well as grit [46,47]. But, an important beneﬁt of grinder 4.4.3. High-pressure homogenization operation is the fact that extra mechanical pre-treatments are High-pressure homogenization refers to a technique for the usually not required [47]. large-scale fabrication of CNF, along with laboratory-scale prepa- ration of nanoﬁbrils [41,42]. This approach includes pushing the 4.4.6. Cryocrushing solution by using an extremely trim channel or alternatively an Cryocrushing is a mechanical ﬁbrillation way to cellulose in a oriﬁce taking advantage of a piston, under an elevated pressure of refrigerated condition. This process generates ﬁbrils with reason- 50e2000 MPa [41,42]. The width of the homogenization gap is ably big diameters, varying within 0.1 to 1 mm. Within this method, dependent upon the viscosity of the solution along with the exerted water-swollen cellulose ﬁbres are usually refrigerated in liquid ni- pressure [42]. The consequential high solution streaming velocity trogen and after that progressively crushed [46,48]. The use of leads to an intensiﬁcation of the dynamic pressure as well as a substantial collision forces to the frozen cellulosic ﬁbres contributes lowering the static pressure underneath the vapour pressure of the to breaking of the cell walls because of the pressure implemented aqueous phase. This contributes to the production of gas bubbles, by the help of the ice crystals. This draws out the nanoﬁbers. The which breakdown instantly if the liquid departs from the homog- cryo-crushed ﬁbres can subsequently be distributed as well as enization gap, getting once again under a standard air pressure dispersed uniformly in water with the help of a routine disinte- [41,42]. The gas bubble production, as well as implosion incident, grator [49]. This technique is relevant to numerous cellulose ma- can cause the creation of shockwaves and cavitations, which trigger terials which enable it to be considered as a ﬁbre pre-treatment disturbances of the ﬁbrillar conﬁguration of the cellulose. Cellulose procedure before homogenization. Authors developed nanoﬁbers ﬁbre size drop is accomplished by means of a signiﬁcant pressure from soybean stock by means of cryocrushing together with suc- drop, excessive shear forces, turbulent ﬂow, as well as interparticle ceeding high-pressure ﬁbrillation. TEM conﬁrmed that the nano- collisions. The level of the cellulose ﬁbrillation is determined by the ﬁber diameters have been found in the 50e100 nm range [48]. The range of homogenization cycles as well as on the exerted pressure nanoﬁbers produced manifested outstanding dispersion capability [42]. in the acrylic emulsion in comparison to water. In spite of this, the cryocrushing technique offers a low efﬁciency and is not cost 4.4.4. Microﬂuidization worthy, due to its high energy expenses [49]. A microﬂuidizer is an additional technique which can be employed for cellulose nanoﬁbrils or even cellulose microﬁbrils 4.4.7. High-intensity ultrasonication production. In contrast to the homogenizer, which works at the High-intensity ultrasonication is a very common laboratory steady pressure, the microﬂuidizer performs at a consistent shear mechanical treatment method employed for cell disturbances in rate. The ﬂuid slurry is pumped via a z-shaped chamber, which aqueous conditions [50,51]. This technique produces effective attains an elevated shear force [42,43]. The pressure can achieve cavitation which includes the growth, extension, as well as ranges up to 40,000 psi, which is about 276 MPa [42]. Purposely collapsing of microscopic gas bubbles once the water molecules designed predetermined-geometry microchannels are placed in- intercept ultrasonic energy. The effect of the hydrodynamic forces side the chamber, by which the slurry speeds up to higher veloc- of the ultrasound on the pulp contributes to the deﬁbrillation of the ities. The preferred shear, as well as impact forces, are built cellulose ﬁbres [50]. Number of researchers have investigated the whenever the slurry stream strikes on wear-resistant surfaces. use of high-intensity ultrasonication (HIUS) to the separation of Several check valves enable recirculation of the slurry. Upon leaving nanoﬁbers from a variety of cellulosic resources, for example, plain the interaction section, the product is sent in a heat exchanger, cellulose, microcrystalline cellulose, pulp, culinary banana peel, recirculated in the system for additional operation, or perhaps sent rice waste, as well as microﬁbrillated cellulose [3,17,50,51]. The test from the outside to the subsequent step in the operation. It is results conﬁrm that a mixture of microscale as well as nanoscale required to perform repeatedly the process many times to adapt to ﬁbrils can be accomplished directly after ultrasonication of the different sized chambers to be able to enhance the level of ﬁbril- cellulose samples; the diameters of the extracted ﬁbrils are lation. Authors inspected the inﬂuence of the number of passes of extensively found in the range from 20 nm to several microns, microcrystalline cellulose MCC slurry in a microﬂuidizer on the suggesting that a few nanoﬁbrils are extracted from the ﬁbres, morphology of the extracted cellulose nanoﬁbrils [44]. They iden- although a few stay on the ﬁbre surface [50]. As a result, this pro- tiﬁed that the aspect ratio of the nanoﬁbrillar bundles enhanced cess provides aggregated ﬁbrils with a vast width distribution. It is after 10e15 transferring cycles, while extra ﬂows resulted in additionally noticed that the crystalline structure of certain cellu- agglomeration of the CNFs because of the expanded surface area as lose ﬁbres is modiﬁed by means of ultrasonic treatment method well as a greater level of the surface hydroxyl group [45]. [50]. These types of alterations change for individual cellulose re- sources, for instance, the crystallinity after remedy enhanced for 4.4.5. Grinding 100% pure cellulose, diminished for microcrystalline cellulose, An additional method for isolating cellulose ﬁbres into nano- although it continued to be consistent for pulp ﬁbre. Authors sized ﬁbrils is grinding process [46]. Throughout grinding process, a examined the consequences of temperature, concentration, power,
272 R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 dimensions, duration, as well as distance from the probe tip on the The main beneﬁts of hydrolysis in the presence of the solid acid are level of ﬁbrillation of plenty of cellulose ﬁbres with the help of HIUS easy restore of the solid acid, relative safe, lesser corrosion rate of treatment [51]. They claimed that outstanding ﬁbrillation was due the equipment [52e54]. Hydrolysis with gaseous acids method to a higher power as well as temperature, even though lengthier could provide numerous environmentally harmful as well as time- ﬁbres were much less deﬁbrillated. Greater pulp fraction and larger consuming steps which are used for traditional acid hydrolysis is sized distance from the probe to beaker were not beneﬁcial for the ruled out. Without any doubt, less quantity of water is required, the ﬁbrillation [51]. These researchers noticed that a conjunction of acid reusing is much easier, and in fact the dialysis stage is left out. HIUS together with HPH enhances the ﬁbrillation together with The cellulose nanocrystals output is considerably more, due to uniformity of the nanoﬁbers, in comparison to high-intensity lesser cellulose feedstock damage throughout the gaseous hydro- ultrasonication (HIUS) alone. The nanoﬁbrils cellulose output is lysis operation [53]. Hydrolysis with metal salt catalyst is per- additionally improved if TEMPO-oxidized pulp is employed for formed using a transition metal-dependent catalyst. A transition HIUS treatment method [50,51]. metal-dependent catalyst offers a satisfactory, preferential, as well as feasible hydrolysis operation with minor acidity. The 4.4.8. Acid hydrolysis valence condition of the metal ion is the paramount aspect to To extract cellulose nanocrystals, acid hydrolysis of puriﬁed induce the hydrolysis performance, in which an acidic solution (Hþ) cellulosic material can be carried out by intense mineral acids produces during the period of polarization between metal ions as (6e8 M) under-governed environment, time, agitation, and acid/ well as water molecules [55,56]. A greater valence state produces cellulose ratio conditions [52]. Various mineral acids are employed much more Hþ ions, which behave efﬁciently in the co-catalysed for this speciﬁc purpose, for example, sulfuric acid, hydrochloric, acid hydrolysis reaction in the existence of metal ions [1e3]. phosphoric, maleic, hydrobromic, nitric, as well as formic acids [3,17,52]. A combination made up of hydrochloric together with 4.5. Chitin organic acids (acetic or butyric) has been discussed in the previous sections. Sulfuric is regarded as the most widely applied acid for Chitin is a polysaccharide which is highly basic in nature. Chitin cellulose nanocrystals production. Throughout hydrolysis, disor- falls under the category of natural polymer usually found in shells dered amorphous regions, as well as interﬁbrillar contacts of cel- of crabs. After cellulose, chitin is recognised as the most abundantly lulose, are selectively hydrolysed; on the other hand, consistent available polymer in nature. Both chitin and cellulose belong to crystallites stay unchanged which enable it to be separated as rod- polysaccharide category. Chitin differs from cellulose by the pres- like nanocrystalline materials [52]. The cellulose nanocrystals ence of acetamide group rather than hydroxyl group. It is estimated dispersion in an intense acid is antiquated by using water and that the crab and shrimp shell waste generated from ﬁshing in- cleaned via consecutive centrifugations. Neutralization or even dustry contains 8e33% of chitin polymer. Chitin consists of b-1, 4- dialysis by using distilled water is conducted to take away N-acetyl-D-glucosamine monomer units arranged in a linear remaining acid from the dispersion. Supplementary steps for fashion, as shown in Fig. 4 [57,58]. example ﬁltration, centrifugation, or perhaps ultracentrifugation, The isolation of chitin starts with the choice of shells. Preferably, in addition to mechanical or possibly ultrasound disintegration, shells of the identical size, as well as kinds, are selected. For shrimp have likewise been discussed [53,54]. In case cellulose nanocrystals shells, the relatively slim walls render restoration of chitin more are produced taking advantage of cellulose hydrolysis together convenient [57,58]. The washing, as well as drying of the shells with hydrochloric acid, the uncharged nanocrystalline particles are pursued by extensive crushing, is the subsequent step in the likely to ﬂocculate in the aqueous medium [53,54]. The other hy- method. The small shell fragments are dealt with dilute hydro- drolysis techniques are hydrolysis with solid acids, hydrolysis with chloric acid to take away calcium carbonate. Proteins along with gaseous acids, hydrolysis with metal Salt catalyst (Novo et al., 2015). other organic contaminants are eliminated by an alkali treatment Fig. 4. Chemical structure of chitin and deacetylated chitin (chitosan) [58].
R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 273 method (20% sodium hydroxide). The pigments, predominantly The XRD patterns for a-chitin derived from different sources carotenoids are taken away by extraction with ethanol or even namely Lobster and Sagitta exhibited difference [61,62]. In the case acetone after the demineralization method [58]. Chitin exhibits of a-chitin derived from lobster, the diffraction peak at 001 crystal biodegradability and anti-microbial properties. Chitosan is not lattices is found whereas the similar peak position is found to be water soluble and it exhibits resistance towards alkalis, acids and absent for a-chitin derived from Sagitta [60]. The a-chitin obtained other solvents [55e58]. Chitin is being widely used in several ap- from Sagitta is found to be highly crystalline as compared to a- plications such as biosensors and medical ﬁeld. In the case of a chitin derived from the lobster [60]. Hence, much attention needs medical ﬁeld, chitin ﬁnds utilization in the form of wound dressing to be given to investigating the crystal structure of a-chitin in order material and drug delivery vehicles. Chitin exhibits biological ac- solve the ambiguities [60]. However, well-deﬁned crystal structure tivity apart from being biocompatible and biodegradable in nature. for b-chitin is well documented other than certain issues related to Chitin exhibits optical properties in addition to the ability to form unit cell parameters. The crystallinity indices of commercial chitin, polyoxy salt, ﬁlms and chelation of metal ions [56,57]. and chitin extracted from cocktail protease treatment method, Chitin does not represent block or random orientation. The were 97.9 and 81.0%, respectively (the baseline at 2q ¼ 16 ). On the properties of chitin and cellulose are entirely different. Like chi- whole, it had been noticed that the use of protease cocktail tosan, chitin is also not a water-soluble polymer and it readily decreased the crystallinity of chitin from 97.9% in commercial chitin dissolves in organic solvents. The solvents in which chitin is sol- to 88.0% in the enzymatic treatment method (Table 4) [62]. uble includes hexaﬂuoroacetone, hexaﬂuoro isopropanol and several other chloro alcohols in combination with mineral acids 4.6. Lignin [57e59]. Chitin is also known to exhibit reactivity towards certain solvents. Chitin begins to degrade even prior subjecting to melting Lignin, the 2nd most abundant biopolymer next to cellulose on due to the existence of hydrogen bonding. Chitin contains a pro- earth planet, has been considered as an auspicious alternative for tein matrix in which microﬁbrillar structure is embedded. The existing fossil fuel resources, exhibits some advanced properties diameter of microﬁbrils is found to be in the range of 2.5e2.8 nm. like antimicrobial activity, antioxidant properties, low density, good In crab shell, chitin prevails in the thixotropic state as well as stiffness, high carbon content, and ultraviolet (UV) radiation pro- liquid crystalline form. The crystalline regime of chitin is isolated tection [60e63]. With such properties, wide research leads to from squid pens and crab shells by various authors [58,59]. For explore the possibility of converting lignin into value-added in- isolation of crystalline regions of chitin, acid hydrolysis is per- dustrial products. Lignocellulosic materials chieﬂy composed of formed using hydrochloric acid. It is documented that the aspect cellulose, lignin and hemicellulose along with small fractions of ratio of chitin determines its reinforcing ability in polymer waxes, and several water-soluble mixtures. Lignocellulosic material matrices. The crystal structure of chitin and chitosan is investi- like natural ﬁbres, strength to the materials is provided by cellulose, gated in several published articles [59,60]. Three kinds of crystal- moisture absorption thermal, degradation and biodegradation by line forms are found in the chitin, these are a-, b-, and g-chitins, hemicellulose. Besides possessing high thermal stability, Lignin is these crystalline forms are dependent upon the conﬁguration and responsible for the ultraviolet (UV) biodegradation of the materials arrangement of the polymeric chains [60]. For example, authors in [63]. Being hydrophobic in nature, it makes the cell wall imper- their investigation of the crystal structure found that a-chitin and meable to water and ensures a well-organized water and nutrition b-chitin derived from shrimp shell and squid pen displayed similar transport in the cells. Lignin is known as a cross-linked macro- diffractograms when subjected to X-ray diffractometer (XRD) molecular material based on a phenylpropanoid monomer struc- analysis [61]. Additional evidence on the crystalline structure of a- ture [62,63]. Molecular masses of lignin have been reported to be in and b-chitin is acquired by analysis of XRD patterns of highly the range of 1000e20,000 g/mole, isolated from different re- crystalline specimens [61]. However, the crystallographic param- sources. Lignin consists of several types of substructures, which eters differed for both the forms of chitin. Each unit cell of a-chitin repeat in an apparently haphazard manner, and it is found invari- is found to contain two anti-parallel molecules [61]. In contrast, b- ably fragmented during extraction, so measurement of the degree chitin is reported to contain parallel construction per unit cell. In of polymerization is very difﬁcult i.e. varying functionality and high spite of exhibiting such difference in terms of crystallographic cross-linking. The process of extraction and the source from which parameters, both the forms of chitin demonstrate independent the lignin is extracted like hardwood/softwood/grass determines crystallographic unit for N-acetyl glycosyl functional group [61]. the overall physicochemical properties of the lignin [63]. Different For the examination of physicochemical properties of chitin and properties for some selected lignins are summarized in Table 5. chitosan, a variety of methods can be found from the previous Watkins et al. represented that the processing methods of the publications for the main purpose. The comprehensive informa- lignin have a strong impact on the adhesive properties of the tion is explained in Table 3. synthesized phenol-formaldehyde adhesive [63,64]. In their observation, it is found that the properties of kraft lignin-derived Table 3 phenol-formaldehyde resin were more superior to the steam- Physicochemical characteristics of chitin and chitosan and the determination exploded lignin-based phenol-formaldehyde resin [63]. This methods [61]. Physical characteristics Determination methods Table 4 Degree of deacetylation Differential scanning calorimetry, Conductometric The degree of acetylation and crystallinity index of commercial chitin, chitin ob- titration, Nuclear magnetic resonance spectroscopy tained from crude protease, commercial protease, alkali and acid treatment [62]. (1HNMR) and (13CNMR), Infrared spectroscopy, Samples The degree Crystallinity First derivative UV-spectrophotometry, of acetylation (%) index (%) Potentiometric titration Protein Bradford method Chitin obtain from acid treatment 55.25 ± 0.06 e Ash contents Gravimetric analysis Chitin obtain from commercial protease 80.34 ± 0.11 e Average Mw and/or Light scattering, Viscosimetry, Gel Permeation Chitin obtain from cocktail 82.25 ± 0.07 88.0 Mw distribution Chromatography protease treatment Moisture contents Gravimetric analysis Chitin obtain from alkaline treatment 84.46 ± 0.14 e Crystallinity X-ray Diffraction Commercial chitin 88.10 ± 0.11 97.9
274 R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 Table 5 Table 6 Constituents of different lignins [64]. Thermal stability of different kinds of lignin. Lignin type Mn (g mol1) COOH OH phenolic Methoxy Types of lignin Tg ( C) (%) (%) (%) Milled wood lignin steam explosion lignin 113e139 Soda (bagasse) 2160 13.6 5.1 10.0 Hardwood 110e130 Soda (wheat straw) 1700 7.2 2.6 16.0 Softwood 138e160 Kraft (softwood) 3000 4.2 2.6 14.0 Organosolv lignin 91e97 Organosolv (hardwood) 800 3.6 3.7 19.0 Kraft lignin 124e174 Organosolv (bagasse) 2000 7.7 3.4 15.1 Because of cleavage of the bonds, ionic strength, temperature, and superiority of kraft lignin is due to the high-density network like pH of the solution also inﬂuence the solvency of kraft lignin in an hydrogen bonding between the constituents of kraft lignin and aqueous medium [66]. Commonly, diluted NaOH is used as the therefore Glass transition temperature (Tg) is another imperative cooking chemical to produce wood pulp in soda pulping process. property that affects the properties of the ﬁnal product [63]. The Soda lignin usually exhibits similar properties such as high phenolic hydrogen bonding between stilbene and amylose of lignin is shown hydroxyl content, relatively low glass transition temperature (Tg) in Fig. 5. Tg of different Lignin Processed by Using Different tech- and low molecular weight. Gupta et al. used spray-dried lignin- niques, as displayed in Table 6. coated cellulose a bio-based ﬁller in PLA host matrix, to modify the Leskinen et al. have reported in their widespread research that rheological and thermo-mechanical properties of poly(lactic acid) Tg depends upon the amount of polysaccharides and water in the (PLA) composites. The lignin coating on CNCs improves the lignin, along with the molecular weight and functionalities present dispersion of CNCs as well as improved their interfacial interaction in the lignins [64,65]. Different chemical procedures are being used with the PLA matrix, resulting in a substantial enhancement in to process Lignin from different resources and each process has thermo-mechanical and rheological properties, which make them a some advantage and disadvantage and most of the procedures suitable candidate for the end user applications [67]. The com- follow either an acid or base-catalysed mechanism [65]. Thus posites show signiﬁcantly higher storage modulus (G0 ) than the Lignin is broken down into low molecular weight fractions while neat PLA at all loading of L-CNCs in both glassy and rubbery region handling through these processes, and its properties got affected in the Dynamic Mechanical Analysis [67]. In the presence of L-CNCs, greatly [65]. In the same line, many researchers have developed a crystallization behaviour of the PLA matrix was also found to new technique for the processing of lignin, in which sulﬁte, kraft, improve signiﬁcantly [67]. and soda, are three major processes along with many others Kai et al. synthesized a series of the composite with different [64e66]. The sulﬁte process is an acid-catalysed process, conven- loading of lignin in PLA via the ring-open polymerization of lactide tionally used in pulping technology, involves the cleavage of the a- onto selectively alkylated lignin [68]. First, this copolymer was ether linkages and b-ether linkages of lignin [65,66]. In this blended with poly(L-Lactide) (PLLA) and then the electrospun method, a chemical reaction between free sulphurous acid and process is taken place to fabricate uniform nanoﬁbres with lignin leads to the formation of lignosulfonic acid, soluble ligno- controlled ﬁbre diameters. To examine the biocompatibility of PLA- sulfonates formation with cations and lignosulfonates fragmenta- lignin composites, three different cell types ePC12, human tion along with the production of carbohydrates take place [65,66]. mesenchymal stem cells (MSCs) and human dermal ﬁbroblasts Kraft lignin is a product of kraft pulping process, which exhibit a (HDFs) were cultured. In mechanical properties study, it was found dark colour and is insoluble in solvents including water [66]. The that elongation at break and toughness of PLA/lignin composites are kraft lignin comprises the highest quantity of the phenolic eOH in ﬁve times higher than neat pure PLA. Antioxidant activities were contrast to other of lignins [65,66]. With a decrease in the molec- evaluated by DPPH assay for PLA-lignin copolymers and lignin- ular mass of lignin, an increment in the number of eOH is reported. based nanoﬁbres. It is observed that Neat PLLA nanoﬁbers show low antioxidant activities [68]. Even after 72 h, it attains only 15.5 ± 6.2% free radical inhibitions, which is much lower than lignin- containing nanoﬁbers. Such type of lignin-based nanoﬁbers used as biomaterials may reduce oxidative stress-related tissue damages or functional disorders. The biocompatibility of PLLA/PLA-lignin was studied. Due to the oxidative stress induced by the polyester itself, all three cell types show low metabolic activities on neat PLLA nanoﬁbers [67,68]. Higher cell proliferation values were found for all lignin-containing nanoﬁbers which indicate that the antioxidant activities may enhance the viability of the cells. In locally attenu- ating cellular oxidative stress, such materials could be used as tissue engineering scaffolds. In the same line, Kai D. et al. [68], again evaluated the antioxidant activities of lignin-PCLLA copolymers and their electrospun nanoﬁbres by DPPH assay. They proved that higher lignin loaded copolymer shows the higher antioxidant property. Even all PLLA/lignin-PCLLA nanoﬁbers exhibit antioxidant activity more than 70% which enables their application for bio- materials and food packaging to address issues of oxidative stress [67,68]. 4.7. Unvulcanised natural rubber particles Natural rubber (NR) is the main biopolymer and it is commonly Fig. 5. Hydrogen bonding between stilbene and amylose of lignin. utilized in many ﬁelds such as medical, tyre and glove due to its
R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 275 good physical properties [69]. Natural rubber (NR) is a bio-based weight between 106 and 107 g mol1 [72,73]. The amylopectin is a polymer which can be derived from renewable resources namely branched polymer with a short length of (1e4)-a-D-glucan units Hevea brasiliensis. The overall production of natural rubber in 2003 linked through a-(1e6) bonds. In most of the starch varieties, the was 8 million tonnes with the major producers being Thailand (2.8 amylose content varies in the range of 72e82%, whereas the million tonnes), Indonesia (1.8 million tonnes), Malaysia (0.9 amylopectin content varies from 18 to 28% [73,74]. Morphologi- million tonnes) and India (0.6 million tonnes) [69]. In India, 90% of cally, the branched amylopectin component consists of crystalline the rubber production is accounted from Kerala. NR has excellent areas and the linear amylose is mostly composed of amorphous or extrudability and launderability and has a high rate of cure. Apart semi-crystalline. Amylose is therefore soluble in hot water whereas from its application in the tyre industry, NR is employed in the amylopectin is insoluble. In Industries, the steps used for extraction production of thin-walled soft products with high strength. This is of starch from plant sources include wet grinding, washing, serving because; NR can crystallize easily upon stretching [69]. The physical and ﬁnally drying. The white powder which resembles like ﬂour properties of NR are enhanced by the addition of ﬁller, chemical after its extraction from plant sources is termed as “native starch”. If alteration and blending with other polymers such as polyethene the white powder obtained is subjected to undergo chemical (PE), propylene (PP) and nitrile rubber (NBR). The green ﬁbre and treatment to meet signiﬁcant characteristics, then it is termed as ‘natural ﬁbre’ covers a broad range of vegetable ﬁbres such as wood “modiﬁed starch” [71,73]. Native starch is classiﬁed into three ﬁbre and plant-based bast, leaf, seed, stem ﬁbre and animal ﬁbres different classes namely class A, class B and Class C. The XRD such as collagen, keratin and ﬁbroin [69,70]. Recently, they are used analysis suggests a long-range order for the starch granules irre- as reinforcements in polymer composites to improve the me- spective of three different classes. The chain length of amylopectin chanical properties. The natural ﬁbres were also blended with NR to was known to inﬂuence the crystallinity of starch biopolymer [73]. enhance its modulus and biodegradability properties [70]. In the To ﬁgure out the difference between class A and class B type of past work, the raw materials were picked up from many natural starch, authors proposed a model which represents double-helix sources such as sugar beet pulp as well as eucalyptus kraft pulp packing. It is found that the transition occurs from class A to class because H2SO4, HCl, HClO4, NaOH are capable to react with cellu- B and vice-versa via rearrangement phenomenon of double-helix lose ﬁbre [69,70]. The problem with NR composite is poor adhesion structure. It is found that the A-type adopts a closely packed between NR and cellulose ﬁbre. The factors affecting the properties arrangement in which the water molecules exist in between every of the NR-based polymer composite are ﬁbre type, ﬁbre amount, double-helix structure [73,74]. However, in the case of class B, the and chemical treatment, the shape of the ﬁbre, adhesion and packing is more open such that the water molecules exist in the arrangement of ﬁbres in the polymer matrix [69,70]. central cavity created by six double helices. Class c is reported to exhibit the combination of both the class A and class B types, which 4.8. Properties of starch, separation and applications: an overview is conﬁrmed from the diffractogram obtained from XRD analysis [74]. The class C starch is found to be present in bean starch. It has Starch is a sustainable and cheap biodegradable polymer [71]. been reported by authors that the class B and C type starch granules After cellulose, starch is recognised as the widely available polymer are larger in diameter than that of class A [74]. The diameter of class [3,71]. The main sources from which starch is obtained include rice, B and C types are found to be in the range of 400e500 nm whereas wheat, potato as well as corn. However, the certain features of the class A type demonstrated only 25e100 nm. Several reports are plants namely shape as well as size; morphology along with the available on the investigation of structural characteristics of class C composition of different plant sources used for starch production starch derived from pea seeds [74]. The reports suggest that the varies from each other [71]. The United States is reported to be as class C starch exhibit polymorphism of both the class A and class B. the World's top producers of starch. The European countries It has been found that the class B is present at the central part of the contribute next to the United States in starch production in the starch granule whereas class A exists in the surrounding or pe- World. Both of these nations play signiﬁcant contribution towards ripheral region. It is found that the starch crystals belonging to class half of the World's starch production [71e73]. Native starch is A and C show much resistance towards acid hydrolysis as compared naturally composed of nano-sized blocklets which have semi- to class B [73e75]. It has also been reported that amylose and crystalline arrangements of starch chains. It is now well established branched amylopectin contribute to the amorphous regime and that the crystalline regime is made up of thin lamellar domains by amylopectin with the short form of branched chains correspond to the intertwining of amylopectin side chains to result in the for- the crystalline regime for the starch [74,75]. However, it is not mation of double helices [71e73]. Such double helices are tightly established whether the presence of side chains of amylopectin packed such that they tend to lead to the basis of crystalline do- with clustered form leads to the partial crystallinity of the starch mains. In the amorphous regime, the molecules of starch are ar- [75]. The ratio of amylose to amylopectin is dependent on the type ranged in a single-chain conformation whereas the crystalline of plant source used for extraction. In fact, the ratio also depends on regime is so ordered such that the molecules of starch exist in the the steps involved in the extraction process. In recent decades, even double-helix state. Both the amorphous as well as crystalline re- though considerable efforts are made on the investigation of the gions get arranged resulting in the formation of the ring structure molecular structure of starch, signiﬁcant information at the mo- which in turn encompasses the initiation point for the granule lecular level remains unclear. Starch demonstrates highly complex [71e73]. The presence of ring structure which initiates the forma- structure that could be better understood when classiﬁed into tion of the granule is visible via optical and electron microscopy. several levels of the organization as shown in Fig. 6 [74e76]. Starch The morphological view of starch represents granules of spherical granules can be gelatinized in water at lower temperatures and in shape. The diameter of the spherical starch granules ranges from 2 alkaline solutions. The starch in its paste form can be used for to 100 mm, which depends on the botanical source from where the glueing or stiffening agent. The application of starch in industrial starch is produced. Irrespective of the source used for the produc- level is limited due to its functional difﬁculties. tion of starch, the density is found to be consistent and reported to These natural limitations can be substantially solved using a be 1.5 kg/m3 [72,73]. variety of modiﬁcations, including physical, chemical and enzy- Those components include amylose, an essentially linear or matic techniques [74e76]. The physical methods include heat slightly branched (1e4)-a-D-glucan, which exhibits molecular treatment to remove moisture, an annealing process, pre- mass as high as 106 g mol1, and amylopectin, with the molecular gelatinization, treatment under high pressure, radiation process
276 R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 Fig. 6. The starch structure at different levels of organization [76]. and sonication. The chemical methods include modiﬁcation using A series of terms are used to deﬁne several forms of starch in the substitution or cross-linking reactions, oxidation process and hy- literature, which includes starch crystallites, starch nanocrystals, drolysis under acidic conditions [71,73]. A good number of pub- microcrystalline starch and hydrolysed starch [77,78]. All of them lished reports have described the development of starch-based are crystalline in nature and obtained via hydrolysis. However, the polymers for decreasing the environmental effects and to increase crystallinity of different forms of starch differs from each other [78]. the range of applications for biopolymers. The two main properties The onion structure of starch granules is due to the co-existence of which make starch as unique and one of the promising biopolymers the amorphous and crystalline regime [79,80]. Through clustering are its (i) natural biodegradability in water and soil into sugar and organization procedure, the amylopectin chains become spiral. The organic acids and (ii) undeniable merits to the environment. In nanometric subunits stack together to form crystalline regions [80]. addition to this, unique chemical features of starch which are As the starch granule is inherently made up of nanoscale crystalline responsive to a large variety of chemical and enzymatic alterations blocklets, it results in the formation of starch nanocrystals [80]. By allow the incorporation of new functionalities in starch [76,77]. the addition of natural starch into acid hydrolysis, low lateral order Since starch is hydrophilic in nature, it is vulnerable to moisture areas and amorphous region present in starch start to dissolve [79]. attack which in turn leads to variations in terms of dimensional However, the highly crystalline region with thick lamellar remains stability and poor mechanical characteristics [75,76]. Moreover, the undissolved in water. Various studies conducted an experiment to crystallization, as well as retrogradation of movable starch chains, check whether starch from different sources could be used to causes adverse changes in thermo-mechanical properties [75]. synthesize starch nanocrystals and they also checked whether Such type of limitation makes the starch to be unsatisfactory for amylose content and/or botanical origin of the starch inﬂuenced packaging application. Even though certain problems can be their ﬁnal characteristics. It was revealed that diverse sources like reduced by the addition of plasticizers, it is not possible to meet all maize, wheat and potato with similar amylose content displayed no the necessities required for packaging application [77]. difference in crystal sizes. However, various crystal sizes are more In a recent study, corn-derived starch ﬁlms incorporated with noticeable when sourced from same botanical origin but with two different types of essential oils from Zataria multiﬂora Boiss and different amylose content, signifying the great inﬂuence of Mentha pulegium plasticizers are fabricated. Improvement in terms composition and molecular structure on resulting crystallite di- of ﬁlm characteristics was achieved via emulsiﬁcation process mensions [79]. Commonly starch nanocrystals are stated to be [77,78]. The water vapour barrier property for the ﬁlms is also derived from starch granule crystallites. It is obtained via disrupting found to be improved. Even though some modiﬁcations in physical the semi-crystalline arrangement of native starch at temperatures and mechanical properties are achieved, starch-based ﬁlms remain below the gelatinization temperature [80,81]. Under such condi- to be unsuitable for general packaging application [77]. A compa- tions, the amorphous regime in starch gets hydrolysed and leads to rable barrier and mechanical characteristics in comparison with the separation of nanoscale crystalline residues [80]. This is due to fossil fuel derived plastics is important if starch-based ﬁlms should the higher resistance to hydrolysis of crystalline lamellae than ﬁnd application in packaging industries [77]. Therefore, additional amorphous lamellae by either chemicals or enzymes [80]. It is enhancement in mechanical as well as barrier properties is because of the difference in acid susceptibility and crystalline required [77,78]. The most widely used way to enhance the me- dextran in starch granules can be formed by the hydrolysis of mild chanical, as well as barrier properties, is to fabricate starch-based acids in the amorphous region [81]. There seems to be same or no nanocomposites. The enhancement in the properties can rely on variation in hydrolysis temperature used, which are usually be- the shape of the particles being incorporated. The reinforced ﬁller tween 35 and 45 C. The important cause for maintaining this low- can exist in different shapes namely (i) particulates, (ii) elongated temperature range might be to avoid gelatinization of starch and materials and (iii) layered structures. The particulates can be any type of destruction crystalline structure of starch. Starch nanoparticles with the well-deﬁned morphology [78]. nanocrystals of various size and shape can be gained depending on
R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 277 the origin of starch and isolation procedure. Different sources of particles into nanocrystals which include the following: extraction starch namely barley, tapioca, potato, mung bean and chicken pea by acid hydrolysis, enzymatic hydrolysis or co-crystallization dur- are used to synthesize starch nanocrystals via acid hydrolysis [80]. ing the regeneration process [82,83]. The irreplaceable properties Even though starch granules from diverse sources differed in size of starch nanocrystals like biodegradability, outstanding mechani- and shape, no understandable variation in terms of shape between cal characteristics, lower density level and reduced permeability several types of starch nanocrystals produced. Researchers have allow it to be a perfect candidate to synthesize starch nanocrystal also found that the structure and morphology of starch nano- incorporated natural polymer composites. Since starch nano- crystals mainly relied on several factors like botanical origin, crystals are polar in nature and exhibit hydrophilicity, their crystallinity, the ratio of amylose to amylopectin and morphology dispersion level in non-polar solvents is limited. This results in the of starch granules. It was found that the acid hydrolysis process of lack of compatibility among starch nanocrystals and polymers of starch granules showed inﬂuence on the morphological features of hydrophobic nature. Auspiciously, starch nanocrystals exhibit starch nanocrystals [82,83]. It was reported that platelet-like reactive surfaces appropriate for chemical derivatization and morphology of starch nanocrystals was obtained which is grafting reactions such that their surface hydrophobicity can be conﬁrmed by TEM examinations as reported by various researchers manipulated and enable dispersion in non-polar solvents [81]. [81e85]. The plate-like starch nanocrystals exhibited a thickness of Some researchers have found that the reduction of surface energy 5e7 nm and diameters varying from 15 to 40 nm. However, in the associated with starch nanocrystals to enable its dispersion in case of other sources of starch, nanocrystals of different sizes and polymers is challenging. The surface energy of starch nanocrystals shapes were gained [81]. To cite an example, Chen et al., reported increase their dispersion level in a polymer matrix [81,82]. It is now that round and grape-like nanocrystals were obtained from potato accepted that the nature of the hydroxyl groups in starch nano- starch granules with size varying from 40 to 100 nm [84,85]. Re- crystals offer the chance of alteration through chemical reaction searchers carried out an experiment to decrease the size of starch approach [81]. The shape of nanoparticles namely rod shape in the granules from the micro level to nanoscale by using high-pressure case of cellulose, as well as chitin and plate-like morphology for homogenization technique [85]. As a result, it was experienced that starch nanocrystals, mainly depends on the source of poly- the particle size of starch can be reduced signiﬁcantly from 3 to saccharides [83,84]. Integrating the nanoﬁllers obtained from these 6 mm to as low as 10e20 nm [85,86]. This was possible only after polysaccharides in crystalline forms with uniform structures can subjecting the starch granules to 20 passes of high-pressure ho- thus be a perfect choice for making bio-nanocomposites with high mogenization [82]. Starch nanocrystals can be compounded as re- rigidity. The shelf life of unpreserved packaged products can be inforcements in diverse types polymer matrices for preparing pointedly extended by increasing the barrier properties of pack- nanocomposite because of its unique properties like nanoscale aging materials. It is known that polymeric materials with an platelet-like morphology, greater crystallinity, reduced perme- outstanding barrier to water vapour usually lacks oxygen perme- ability level and rigidity [84,85]. Researchers reported a method to ability and vice versa [83,84]. Therefore, protection of packaged obtain natural rubber-based nanocomposites reinforced with products from deterioration due to oxidation, high temperature, starch nanocrystals derived from maize. Some reports were avail- moisture and microorganisms can effectively be attained through able on natural rubber reinforced with starch nanocrystals [84]. the use of multilayer structures consisting of various polymers, Enhancing effects in terms of properties for the composites have each contributing to certain speciﬁc functions [83]. The permeation been noticed if the content of starch nanocrystals was maintained rate of most of the vapour and gas through the polymeric materials to be below 20% and vice-versa [84,85]. Sorbitol-plasticized pul- depend on its chemical nature and physicochemical properties of lulan ﬁlms containing waxy maize starch nanocrystals exhibit permeating molecules [85]. Improving the resistance to water improved mechanical and water barrier properties [78,80]. The vapour permeation and oxygen diffusion is an essential require- enhancement is due to the strong interaction that developed ment in composites/nanocomposites for the packaging of several among starch nanocrystals and the polymer matrix. Exciting results foods and drug products [84,85]. The usage of starch nanocrystals were also reported for starch nanocrystal-reinforced/plasticized with high crystallinity and their morphology along with intrinsic starch ﬁlms using glycerol and sorbitol plasticizers. Other re- tortuosity that they impart can considerably limit the migration of searchers found that if the content of starch nanocrystals was water vapour and oxygen [86]. Undeniably, platelet-shaped starch maintained to be below 2 wt% in soy protein matrix, Young's nanocrystals can potentially change the diffusion path of penetra- modulus for the composite ﬁlms increased. However, the presence tive molecules more than rod-like cellulose nanocrystals and thus of starch nanocrystals in soy protein matrix decreased the increase the barrier properties of polymer composites [85,86]. elongation-at-break (%) values [78e80]. Various groups of re- Swelling is one of the useful techniques to regulate the presence of searchers described the extraction and characterization of starch speciﬁc interaction between the ﬁllers and the polymeric matrix by nanocrystals from potato source. The same group fabricated natural forming additional crosslinking action [84,85]. From a technolog- rubber nanocomposites reinforced with starch nanocrystals using ical perspective, the interaction of polymeric materials with several sulfuric acid at a temperature condition of 40 C. The structure and solvents is very important due to the changes in the material di- performance of the nanocomposites were examined by SEM and mensions and physical characteristics imparted by the penetrating Atomic Force Microscopy (AFM). The analysis conﬁrmed the uni- solvent molecules into the polymer [84,85]. It has been well formly dispersed starch nanocrystals in the rubber latex. a established that crosslinking alteration prevents starch from moderately uniform dispersion of starch nanocrystals when it swelling. The rate of inhibition towards swelling depends on the incorporated in the rubber latex. Acid hydrolysis is the most degree of cross-linking. Several researchers have examined the commonly used approach to yield starch nanocrystals. The treat- effects of variables like nature of polymeric matrix (polar or ment with acid dissolves the area of low lateral order for revealing nonpolar), the content of starch nanocrystal, chemical alteration of the concentric lamellar portion present in starch. By using acid the nanocrystals, diverse swelling liquids (water, toluene) and re- hydrolysis method, crystalline residues insoluble in water gets action time on the swelling behaviour of the various nano- transformed into stable suspensions via the strong action of composite [82e85]. The results indicated that the rate of water shearing. For modiﬁcation of starch and its characteristics, acid uptake for most of the compositions containing starch nanocrystals hydrolysis process has been practised for several years [79,80]. improved rapidly at the earlier period of immersion. After reaching There are different approaches that can be used to convert starch a maximum point, the water uptake was found to be reduced until
278 R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 attaining the equilibrium. Consequently, the absorption kinetics in grain size of native starch before and after hydrolysis. The respec- the initial stages is faster, which is then followed by an absorption tive SEM images are presented in Fig. 7. plateau [82,83]. The detected reduction in the water uptake The nanocrystals obtained from corn-starch are polyhedral through this process can be ascribed to leaching or partial release of granules whereas those from other starches are elliptical, spherical starch nanocrystals in water, although starch exhibits insolubility in and/or irregular in shape [85e87]. The largest nanocrystal with an water starch is insoluble at low-temperature conditions [82]. average granular size of 41.3 mm was gathered from potato starch, The discharge of starch nanocrystals into aqueous phase while accompanied by legume starch. Interestingly, all the starch nano- swelling is one of the reasons for the increase in the water uptake crystals produced via acid hydrolysis exhibited a spherical shape rate in the early phases. It has been recommended that the inter- irrespective of their origin [85e87]. face between starch nanocrystals and a polymeric matrix namely natural rubber tends to decline by the swelling of the starch do- 5. Some other nanomaterials compatible to biosystem mains with respect to exposure time and hence the swelling rate seems to be improved due to “overshooting effect”. The growth of 5.1. Titanium dioxide biodegradable packaging materials with better thermal properties is the main reason to improve the processability of polymeric The arrival of green, facile and environmentally friendly modes composites. The thermal behaviour including determination of has made the development of nanomaterials simple [88]. Signiﬁ- glass transition temperature (Tg), is normally determined by dif- cant features required for such green methods include low- ferential scanning calorimetry (DSC) and dynamic mechanical temperature, fast reaction rate and reduced toxic agents [88,89]. analysis (DMA). The DMA method is more broadly used for The applications of photoactive materials are diverse which include studying polymer chain movement via a-relaxation at the molec- photocatalysts, water splitting reaction, chemical and biosensors, ular level and Tg is usually determined at the point of modulation of electrochromic displays, photoelectric conversion, and solar cells the curve of loss tangent (tan d) as a function of temperature. The [88,89]. Titanium dioxide (TiO2) is one of the most extensively thermal stability these polymeric particles can also be checked employed photo-active nanomaterials [88]. There are three distinct using thermogravimetric analysis (TGA) [84]. forms of TiO2 exist each of which can either be crystalline or It has been conﬁrmed that the nanocomposites reinforced with amorphous. They comprise anatase, rutile, brookite, or a combi- nanocrystals generally demonstrate improved thermal character- nation of the three. It is remarkable to indicate that pure crystalline istics when nanoﬁllers are well dispersed into the matrix [86,87]. anatase is the most photo-active phase of TiO2. The high stability of Excitingly, the thermal properties of polysaccharide nanocrystals photoelectric chemistry and high photoelectric conversion efﬁ- rely on the origin, synthesis process and type of surface modiﬁca- ciency provides TiO2 with peculiar properties [88]. The photo- tion [86]. For example, sulfuric acid hydrolysis leads to the deco- catalytic properties of TiO2 nanoparticles grant the potential to ration of nanocrystal's surface with sulphate ester groups of acidic immediately deteriorate pollutants in wastewater and split water nature. This, in turn, results in the reduction of degradation tem- into hydrogen and oxygen. Organic pollutants can be degraded in perature signiﬁcantly. In the recent years, nanocrystals obtained the presence of TiO2 along with an energetic light source and an from starch have shown better thermal properties and exceptional oxidizing agent [87,88]. temperature resistance [86,87]. Established data pertaining to the Many synthetic approaches are available to synthesize TiO2 crystallinity of starch granules are obviously vital for the improved photo-active nanoparticles including solegel processing, reverse large-scale manufacture of starch nanocrystals [86,87]. The main microemulsion, dialysis hydrolysis, microwave-assisted emulsion property of crystalline regime in starch granules is the poly- polymerization, alcohol-thermal method, hydrothermal, combus- morphism of a-glucans. Since native starch granules have crystal- tion, and gas-phase methods [89,90]. Amongst all the methods, the line areas, their presence can be noticed using XRD. The appearance hydrothermal approach is regarded as an extremely effective pro- of X-ray diffractograms is dependent on the extent of water content cedure to prepare TiO2. In the hydrothermal method, the reaction present in starch granules during analysis. If more is the degree of can be performed at lower temperature condition (100 C) as starch hydration, thinner will be the diffraction pattern [85e87]. compared to other methods. The particle size, morphology and Therefore, hydration remains the major and essential variables for composition are tuneable by manipulating the reaction parameters the organization of crystalline areas in starch. The change from such as temperature and heating time. Remarkably small TiO2 crystalline to amorphous occurs primarily between 60 and 70 C in nanoparticles ranging from 6 nm to 10 nm can be prepared which water by a method known as gelatinization. Commonly, starch enhances the photo-activity owing to the enormous surface area comprises of 15e45% crystalline content and XRD patterns have [88]. Some of the other green and simpliﬁed methods are also often conﬁrmed different types of starch based on crystalline arrange- used to prepare the photo-active TiO2 nanomaterials. In ment, named as A, B and C [86]. It can be considered as an appro- microwave-assisted hydrothermal process (MW-HT), TiO2 is pre- priate polymer for making biodegradable nanoparticles due to its pared from TiCl4 whereas the hydrothermal process utilizes an richness in nature and low cost. Some methods for preparing starch oxidant and a reductant in aqueous solution. Previous author's nanocrystals have been advanced and improved over recent years studies made a comparative investigation on the synthesis of TiO2 [86]. About improvement in methods, yield and resulting proper- using conventional hydrothermal process and microwave-assisted ties were found to be fruitful. Novel and improved solutions have hydrothermal technique [90]. In the microwave-assisted hydro- been discussed that aim to improve the preparation of starch thermal technique, TiO2 nanoparticles were synthesized from nanocrystals [86]. However, despite the recent growth in metal alkoxides and hydrogen peroxide with a molar ratio of manufacturing nanocrystal, there is still a good number of serious 0.05:0.12 (Ti:H2O2). The polymer-gel procedure is a straightforward issues that need to be considered. In addition to the synthesis of and cost-effective approach for developing pure metal oxides starch nanocrystals, many procedures have also been established to [91,92]. The preparation of nanosize anatase through the polymer- realize and characterize the nature of several polymer matrices in gel technique is far less complex than the hydrothermal technique which the particles have been ﬁlled [86,87]. Several research ac- [89,90]. In general, the polymer-gel technique occurs by mixing a tivities in starch-based nanocrystals have been conducted and are polymer in a metal added solution to achieve metal-polymer going on, although the expected commercial success is yet to be mixtures [94,95]. The resulting nanoparticle size and purity are documented [72,86]. Researchers studied the morphology and comparable to the products achieved by applying the hydrothermal
R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 279 Fig. 7. SEM images of starches in different magniﬁcations before (left) and after (right) hydrolysis from botanical sources [86]. technique [91]. There are various disadvantages with the hydro- it can function as an efﬁcient nanomaterial for many applications thermal approach when compared to polymer-gel technique. For [37]. It is due to the attractive physical as well as chemical char- instance, the hydrothermal approach presents trouble adjusting acteristics associated with ZnO nanoparticles, they are being used the reaction time and involves autoclave treatment [93,94]. The in several applications like biomedicine, UV absorption, photo- ﬁnished outputs must then be puriﬁed, unlike the polymer-gel catalysis, solar cells and photonic materials [98]. By taking the technique which yields reﬁned products [94]. High-resolution advantage of the photocatalytic activity of ZnO, the nanoparticles transmission electron microscopy (HR-TEM) is used as a charac- are applied for degradation of phenol and chlorinated phenols like terization technique and is adapted to produce leading information 2,4,6-trichlorophenol [99]. It is further adapted to carry out the on the atomic scale [90,94,95]. Fig. 8 shows HR-TEM image forTiO2 degradation of methylene blue, direct dyes, acid red, and ethyl vi- nanomaterial calcined at 400 C, which shows that the particle size olet, which are common organic pollutants in wastewater [93,99]. is of 12 nm [95,96]. The TiO2 nano-materials are a great deal of There has been a considerable deal of synthetic methods studied interest owing to their remarkable properties likeable, non-toxic for the synthesis of ZnO nanoparticles including solvothermal and low cost. However, the broadband gap of TiO2 (~3.2 eV) method, solegel, vapour-liquid-solid technique, pulsed-laser pro- limits to absorbing only UV photons [95e97]. In extension, TiO2 cess, thermal evaporation technique, template-assisted process, exhibits fast recombination of photo-induced electrons and holes molecular beam epitaxy technique, chemical vapour and deposi- leading to the reduction in photocatalytic activity [95e97]. In order tion, electrochemical process, and reverse micellar [98,99]. How- to overcome these limitations, there have been numerous synthetic ever, these synthetic processes have certain limitations such as methods established such as non-metal element doping, coupling high temperature (850e925 C), high pressure (100 MPa), long with semiconductors, noble metal loading, and speciﬁcally the reaction time (30 days), and utilization of toxic reagents (O3). recombination of TiO2 with newly emerging carbon-based nano- Therefore, improvement of sustainable and efﬁcient synthesis materials [91,95]. process is of great interest for the synthesis of nanoparticles. Currently, certain green preparation techniques for synthesis of 5.2. Zinc oxide (ZnO) and other nanomaterials compatible to bio- inorganic TiO2, and ZnO photo-active nanomaterials are also systems generated [94,98]. Owing to the unique benign environmental impacts and 6. Coupling of nanomaterials extensive applications in diverse ﬁelds, zinc oxide (ZnO) is another commonly used semiconductor component. ZnO presents a Each of these approaches delineates and elucidates the short- broadband gap which corresponds to 3.37 eV [98,99]. In addition to comings of the TiO2 nanomaterials by coupling TiO2 with metal this, ZnO shows high binding energy around 60 meV and therefore oxides or sulphides namely ZnO/TiO2, CuO/TiO2, SnO2/TiO2, CdS/
280 R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 Fig. 8. High-resolution transmission electron microscopy (HR-TEM) characterization for TiO2 nanoparticles calcined at 400 C [92]. TiO2, and ZnS/TiO2, which in turn will fulﬁl the electronehole pair 7. Nanoencapsulation and their green properties separation under both UV and visible light illumination [95,96]. A larger contact interface between titanium nanotubes (TNT) and A collection of nanoparticle-based systems produced from graphene is supposed to build up the photo-induced charge sepa- particular ingredients, sufﬁcient for encapsulation of micro- ration which will enhance the photocatalytic activity [96]. Analysts nutrients, are highlighted in Fig. 9 [100]. The exploration of tech- have been practising hydrogenation, metal and non-metal doping, nologies for nanoencapsulation is being carried out to provide and sensitizing with a small band gap semiconductor substance to safety for bioactive constituents namely vitamins, antioxidants, diminish bandgaps and promote the performance via usage of lipids and proteins [100]. The nanoencapsulation is mainly used for sunlight [96]. All these approaches have demonstrated advance- the production of food with functional properties [100,101]. Hence, ment in the photocatalytic activities [96]. Three types of hybrid this technology is expected to demonstrate promising advance- synthetic photo-active nanomaterials have been newly established ments in terms of nutrition as well as public health [100,101]. using green chemistry. They can be produced appropriately by Several types of nanocarriers for incorporation of nutraceuticals for using metal oxides or metal sulphides (ZnO, CdS) to gain photo- its subsequent application in food systems have been developed active ZnO-TNT and CdS-TiO2 composites as reported by the par [99,101]. et al. [97,100]. Graphene oxide (GO) and graphene (GR) are carbo- Recently, bio-based phase change material (bio-PCM) has been naceous substances which are completely suited to respond as effectively encapsulated in ultraﬁne ﬁbres by means of coaxial catalyst supports [98]. Graphene oxide can extend the dispersion of electrospinning approach [100]. Natural soy wax has been TiO2 catalyst and cut down the tendency of electronehole pair employed in form of the bio-PCM for thermal storage as well as recombination [98]. Both graphene oxide and graphene can be Polyurethane (PU) is applied in the role of the covering or shell hybridized with TiO2 to form a graphene-TiO2 (TiO2/GR) or gra- component for encapsulation [100e101]. The bio-PCM ﬁbres have phene oxide-TiO2 (TiO2/GO) nanocomposite, which can be been examined by various microscopy and spectroscopy techniques employed to split water and degrade wastewater pollutants [98]. [101]. The information reveals that coaxial electrospinning led to a TiO2 nanotubes with enhanced catalytic activity can be produced consistent ﬁbre morphology with a coreeshell conﬁguration along conveniently under deﬁnite conditions and in this area several re- with a homogeneous wax distribution across the core of the ﬁbres searches are going on for the more efﬁcacy [98,100,101]. Titanium [101]. Thermal study data conﬁrm that the enthalpy improves with nanotube (TNT) incorporates the traditional TiO2 nanoparticles with wax quantity [101]. The ﬁbrous structures displayed well balanced its retained exceptional properties like high conductivity, large thermal storage capacity as well as discharging characteristics for surface area, good mechanical as well as electrical properties, thermo-regulating functionality [101]. The thermal characteristics greater aspect ratio and a large contact interface [98,101]. To are uncharged after hundreds of heating-cooling evaluation cycles, improve TNT activity, it is suggested to increase the adsorption of showing that the composite ﬁbres possess excellent thermal reactant molecules and light owing to the enormous quantity of durability as well as stability [100,101]. In a further study, micro- active sites [102,103]. In extension, the huge aspect ratio of TNT encapsulation of vegetable-derived palmitic acid (PA) in bio-based nanostructure and a broader contact interface between TNT and GR polylactic acid (PLA) covering or shell by means of solvent evapo- are supposed to improve the photo-induced charge separation ration and oil-in-water emulsiﬁcation has been examined [102]. [103]. All the preceding properties lead to the enhancement of the Fourier transform infrared spectroscopy as well as scanning elec- photocatalytic activity for TiO2 nanomaterials [98]. tron microscopy has been performed to verify the effective
R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 281 encapsulation of palmitic acid in PLA shells [102]. Differential mm. Due to their distinct as well as optimum morphology and, scanning calorimetry has been accomplished to examine the ther- therefore, greater AV quantity, PVP, in the shape of nanoﬁbers, as mal characteristics, thermal stability, and core content material of well as WPC, of nanocapsules, were additionally preferred to the manufactured phase change materials microcapsules (micro- examine the AV durability against ultraviolet (UV) light condition. phase change materials) [102]. By means of many parametric tests, Fourier transform infrared (FTIR) spectroscopy revealed the effec- the impact of phase change materials as well as solvent quantity, oil tive encapsulation of AV in the biopolymer matrices, showing both phase-to-aqueous phase proportion, in addition to surfactant together encapsulates an excellent chemical interaction with the description together with the content on the morphology, particle bioactive ingredient [108,109]. Ultraviolet-visible (UVevis) spec- dimensions, and consequently thermal characteristics of the phase troscopy demonstrated that, although PVP nanoﬁbers provided an change materials microcapsules have been studied [102,103]. Pre- unsatisfactory impact on the AV degradation throughout UV light cise Experimental results revealed that PVA is an excellent emul- exposure (~10% of stability after 5 h), WPC nanobeads provided siﬁer [103]. Furthermore, there additionally persisted an outstanding safeguard (stability of >95% after 6 h) [102,108]. This appropriate PVA content to minimize the typical dimensions of has been attributed to favourable interactions between WPC along microphase change materials [103]. If the PVA content crossed with the hydrophilic components of AV in addition to the intrinsic beyond this quantity, the emulsiﬁer molecules are likely to produce UV-blocking as well as oxygen barrier features offered by the micelles among by themselves [104,105]. This resulted in the protein [102,108]. adhesion of very small microspheres on the top surface of micro- phase change materials along with larger sized micro-phase change 8. Nature inspired hydrogels materials [102,104]. SEM microstructures demonstrate the micro - phase change materials comprise of 0.4, 0.6, as well as 0.8 g of Hydrogels belong to a class of polymeric materials with hydro- palmitic acid although maintaining a speciﬁed PLA amount (i.e. philic features, which are of great importance in many different 1.2 g). It is found that the micro-phase change materials shapes, and areas [103,104]. Since ﬁve decades, hydrogels are being used surface morphologies are practically unaffected if the PA content because of their attractive physic-chemical as well as biological greater than before [102]. Moreover, as the micro-phase change characteristics [103]. Such properties are mainly due to the materials exhibited their sphericity, certain irregular surface mor- arrangement of the three-dimensional network, which in turn phologies with the existence of small microspheres are noticed forms the hydrogel structure [103]. The most relevant character- [100e105]. istics of hydrogels are undoubtedly the ability to absorb and retain a The utilization of electrohydrodynamic preparation method considerable amount of aqueous liquid [104]. This results from the (EHDP) to encapsulate natural aloe Vera (AV, Aloe barbadensis hydrophilic cross-linked network, which can retain three- Miller) making use of together the artiﬁcial polymers, i.e., poly dimensional structure even in the swollen state without disinte- vinylpyrrolidone (PVP) as well as poly(vinyl alcohol) (PVOH), and gration [104]. This feature is due to the cross-linked points that also naturally produced polymers, i.e., barley starch (BS), whey hold the polymeric chains together, that form the network. In most proteins concentrate (WPC), and also maltodextrin [106,107]. The of the cases, chemical or physical pathways drive the cross-linking AV leaf juice was employed in form of water-based solvent for process during gel formation [103]. The product that results due to EHDP, therefore the prepared biopolymer solution characteristics chemical cross-linking process is called as chemical hydrogels. were examined to ﬁgure out their inﬂuence on the procedure [106]. These chemical hydrogels conduct the formation of irreversible The morphological evaluation demonstrated in the previous sec- covalent bonds among the polymeric chains of the hydrogel tions are depends on effective preparation situation, nature artiﬁ- [103,104]. On the other hand, in physically cross-linked hydrogels cial polymers (primarily created ﬁbre-like arrangements) [106]. called as physical hydrogels, the polymeric chains are held by Typical dimensions ranged from one hundred nm to above three physical interaction which may be electrostatic, hydrogen bonding, Fig. 9. Colloid based delivery systems for encapsulation, protection and delivery of functional food constituents and deliver functional food ingredients at a targeted location with respect to dependency on hydrophilicity or hydrophobicity [100].
282 R.K. Mishra et al. / Journal of Science: Advanced Materials and Devices 3 (2018) 263e288 van der Waals force of attraction and physical entanglement branched polymers, which form semi-interpenetrating network [103,104]. Owing to the reversible character, physical hydrogels can could be subjected to separation from its constituent polymer be disintegrated due to impairment of physical interactions network. The separation process could be achieved in the absence responsible for their crosslinking [103,104]. Changes in the external of breakage of chemical bonds [105]. In general terms, the basic medium such as pH, ionic strength and electric ﬁeld can disinte- reaction system for the radical polymerization of starch is grate the physical hydrogels [104]. Despite the differences between composed of starch (raw or modiﬁed), a catalyst (responsible for the pathways that form chemical and physical hydrogels, it is the radical formation), the monomers with reactive functional possible to obtain hydrogels in several formats such as spheres, groups and a crosslinker molecule [such as N,N-methylene- cylinders, ﬁlms and membranes [103,104]. In addition to this, bisacrylamide (MBA)]. Using raw or chemically modiﬁed starch, it hydrogels can also be produced in macro, micro and nano-scale is possible to apply different techniques to form hydrogel networks dimensions. In the last two decades, the applications of hydrogels from the reaction system. The technique utilized can be chosen are focussed not only on liquid absorption/retention. The potential according to the hydrogel destination [71,103,104]. It is very of this promising class of materials is now applied in very varied important to take the technique into account as it affects consid- industrial, technological and biotechnological sectors. This scenario erably the ﬁnal properties of the hydrogel [71,103,104]. A very is due to the formation of a new class of hydrogels called as smart positive aspect of this methodology is the possibility of preparing hydrogels, which can provide different responses (e.g. volume, starch-based hydrogels in the presence of organic polymers and porosity and mechanical changes) according to external and in- inorganic compounds [105]. Numerous papers have described the ternal stimuli [103,104]. In addition to such an evolution, the use of incorporation of inorganic clays, magnetic and metallic nano- different classes of polymers considerably enhanced their proper- particles, hydroxyapatite cellulose whiskers and waste residues in ties and applicability [104]. Biopolymers (such as polysaccharides the starch-based hydrogel formulation, resulting in composite and proteins) possess unique and desirable physicochemical and materials. In most of the cases, these hydrogel composites show biological properties (i.e. Biocompatibility, biodegradability, non- superior features compared with the conventional starch-based toxicity and some biological activities) that stimulate their use in hydrogels, including the applicability and the capacity to respond preparation of different materials [103,104]. For example, hydrogels to external stimuli [105]. Fig. 10 shows an illustrative scheme to prepared from biopolymers (mainly polysaccharides) have found produce a starch-based hydrogel nanocomposite developed. In this great applicability as biomaterials. Furthermore, the interesting case, cellulose nano-whiskers are inserted during the hydrogel properties of polysaccharides comes from their structure, which, in formulation. general, has a wide variety of functional moieties (eCOOH, eOH, The main advantages of hydrogels include (i) liquid uptake ca- eNH2, eNHOCCH3 and eOSO3H) that can be crosslinked by reac- pacity (swelling), (ii) network like morphology, (iii) molecular tion with a coupling agent or that allow the insertion of cross- structure and (iv) mechanical properties. There are a huge number linkable groups or polymeric chains in the polysaccharide back- of qualitative and quantitative techniques, which can be applied for bone [105,106]. The preparation of polysaccharide-based hydrogels characterization of these properties [107]. As already mentioned, could also be carried out by polyelectrolyte complexation among most of the starch-based hydrogel properties are directly con- macromolecules with functional groups having opposite electric nected with two main factors: (i) the polymers combined with charges [106]. As discussed earlier, starch shows all the desirable starch and (ii) the method applied to form the hydrogels [103,104]. features required for preparing chemical and physical hydrogels These factors are known to affect the number of crosslinking points, [103,105]. Number of reports and review articles have described the porosity, and distribution of hydrophilic groups inside or at the preparation of starch-based hydrogels using several methodologies surface of the hydrogel matrix. Generally, these three aspects [105,106]. In general, the main strategy adopted to prepare starch- determine all the properties of hydrogel and it is difﬁcult to based chemical hydrogels is based on the reaction of the hydroxyl establish how they are interconnected [103,104]. The liquid uptake moieties in the starch backbone with bi- or multifunctional com- capacity, for example, depends on the hydrophilicity of the starch- pounds that work as coupling agents. To date, the compounds most based hydrogel matrix. From this characteristic, another important commonly used are glutaraldehyde and epichlorohydrin [104]. aspect arises the capacity of liquid retention by the hydrogel ma- Despite the production of interesting hydrogels by this strategy, the trix. Several parameters control the change in either swelling or de- use of coupling agents is not encouraged when the aim is to prepare swelling like temperature, pH, salinity as well as the ionic strength hydrogels for biomedical uses [106,107]. This is because; the of the medium. The starch-based hydrogels prepared from hydro- coupling agents might show some level of toxicity, which decreases philic polymers/monomers, in a general way, possess the capacity the range of applicability of the chemical hydrogels obtained [107]. for absorption and ﬂuid retention, which classify these hydrogels as An efﬁcient strategy usually adopted to avoid the undesirable issue superabsorbent [104]. Raw starch is not so hydrophilic owing to its of the toxicity of the crosslinker is to perform radical reactions of granular structure and for this reason, the association of starch with unsaturated monomers with starch or starch-based macro- more hydrophilic polymers is required in order to prepare mate- monomers containing carbonecarbon double bonds. In most cases, rials with a high liquid uptake capacity [106]. In the ﬁeld of polymer superabsorbent hydrogels result from this method [103,104]. The science, hydrogels have evolved into materials with outstanding copolymerization and simultaneous crosslinking of starch with features and many potential applications, from soil conditioners vinyl-functionalized monomers have been reported [104e106]. and hygienic products to tissue engineering, drug delivery systems, Generally, starch is chemically modiﬁed to add reactive vinyl and imprinted polymers [103e107]. Therefore, it is not surprising groups to its backbone, and such groups allow crosslinking among that reports on hydrogels and hydrogel composites are still hot the starch chains or/and crosslinking by grafting with other poly- topics in the ﬁeld of material science [104e106]. In terms of mers or monomers. Most of the hydrogels prepared using this hydrogel composites, regarded as those containing micro- and methodology show semi-interpenetrating network characteristics nano-sized particles in their formulation, the addition of the rein- [104,105]. IUPAC deﬁnes a semi-IPN as a polymeric material forcement phase is generally performed to improve thermal, me- comprising at least one network and at least one linear or branched chanical and optical properties, water uptake capacity, release rate polymer, which in turn is characterized by the penetration of both of solutes, response to external stimuli and degradation rate among on a molecular scale. Semi-interpenetrating network is distinct as other properties, tailoring for speciﬁc applications [104,105]. Many compared to an interpenetrating network. This is since the linear or different reinforcing phases have been studied, including mineral