cellulosic support for cartilage cell growth [76]. Besides, cellulose nanocrystals are attracting attention for their applications in tissue engineering [77] while nanoscale cellulose materials are being studied as well [78].
3.4.1.2 Chitosan
Owing to its wide availability and biocompatibility, chitosan has been extensively investigated for role in wound healing, burn treatment, ophthalmology, drug delivery and as artificial skin [79]. It has the advantage of the flexibility of chemical modification at the –NH2 group at C2 position, compared to cellulose (–OH at C2) [13]. Such modifications can often lead to improved biodegradability and hydrophilic nature.
Chitosan is an effective agent for wound healing and preparation of artificial skin. For the development of artificial skin, it is usually mixed with another agent. Reports like detailed chitosan–gelatin [80, 81], chitosan–gelatin–hyaluronic acid [82], or nanocomposites– chitosan [83] based chitosan composites as artificial skin due to their flexibility, antimicrobial properties, accelerated wound healing due to gas and liquid exchange through the matrix, and favourable mechanical properties. Wound infections result due to a disturbed host–bacteria equilibria in tissue, with the balance in favour of bacteria [84].
Due to its anti-microbial role and the hydrating ability of its hydrogels, chitosan composites were also tested as curative agents for wounds. This demonstrates a broad variety of antimicrobial and anti-fungal activities, while the efficacy and mode of action differs. However, fungi are more susceptible than bacteria in many cases [85]. In particular, chitosan–Zn complex has been found to be active against many microbes [86]. The lower molecular weight fractions were found to have greater activity [79]. Consequently, there are various commercial wound dressings based on chitin or chitosan as the chief biomolecule [87].
Chitosan–gelatin biocomposites with nanophase hydroxyapatite are investigated in tissue engineering applications for development of biological substitutes for implantation [79]. Tissue architecture includes functionalized scaffolds that have stable binding matrix for attachment and proliferation of cells and to maintain their functions; they are then implanted in the target organism. Additionally, porous scaffolds reported based on chitosan which were both flexible and broadly applicable [88, 89].
3.4.1.3 Alginate
Alginate, a polymer commonly isolated from brown algae, due to its abundance and low cost, finds applications in food and pharmacies. However, the recent advances in the field of tissue growth, therapeutics and wound cure are limited by the fact that it does not interfere with cells, making it possible to function [90]. Tissue engineering being an interdisciplinary and varied field, the matrix needs to satisfy a lot of criteria for the growth of different cells with varied attachment, nutrient requirements and growth conditions. Alginate successfully utilized as a scaffold for bone fibroblasts [91–93], liver cells [94–97], skin [98, 99], cartilage [100–103], muscle [104, 105], dental [106], neural [107], and spinal cord [108]. Furthermore, alginate dressings are also used for wound care, because of their excellent absorption capabilities, anti-microbial properties, and the ability to provide a moist environment by soaking up the exudates and forming a non-adhering gel [109–111].
3.4.2 Food Applications
3.4.2.1 Cellulose
Native cellulose is a water-insoluble polymer which has very limited applications. A few niche applications are to use it as a filler (to increase the bulk) and to filter beverages, where it is the processing aid. The majority of the native cellulose acts as raw material for its partially depolymerized forms, such as microcrystalline cellulose (MCC), and powdered cellulose. Highly pure cellulose is further used for producing cellulose ethers [112]. Most of these ethers have wide applications as food additives. For example, MCC has a broad variety of uses in candy bars mixtures and confectionary, batters, processed meat products, chocolate drinks, dressings and icings, fillings, high fiber drinks puffed snacks, sauces, low fat cream and ice cream products [113]. In general, MCC provides stability, reproducible flow rate, improves water uptake and accelerates dispersion and disintegration [114]. Cellulose ethers are used especially for baking (shape retention, gluten replacement, reduced staling during storage), fillings (reduces boil-out during heating, predictable product consistency, shape retention), extruded products (example, French fries), soups and sauces (reduced boil over, thermal stability), confectionary, meat products and pet food [115]. Ethyl cellulose is used as a thickener for hydrophobic materials (oils, alcohols, flavorings), controlled release applications (tablet coatings, encapsulation of minerals and vitamins), binder (in granulation processes) and flavor masking (for bitter active molecules) [116]. HPC is a non-ionic ether which is soluble in organic solvents and finds uses in foam-based, dairy free whipping products [117]. HPMC and MC have common properties, and are interchangeable with one another [118].
3.4.2.2 Chitosan
It is considered an excellent emulsifier for stabilizing oil without any surfactant in water emulsions. The chitosan amphiphilicity plays a significant role of stabilization in acidic conditions. This reduces the interfacial stress and facilitates emulsion formation. The capability to emulsify depends on molecular weight and deacetylation degree (DD); i.e., lower DD (~60%) and higher DD (~80%) are more effective than medium DD (~65–77%) [119]. In view of its emulsification ability, a chitosan–soybean oil emulsion has been used for coating eggs, improving their shelf life [120]; another study has reported that lemon oil–chitosan coating increased the shelf life of strawberries [121].
Also, chitosan is used to interact with cell surfaces of both gram-positive and gram-negative bacteria that are capable of releasing certain proteinaceous components from the cell. This gives chitosan an anti-microbial activity, useful for preservation of processed foods [119]. The existence of amine (–NH2) and hydroxyl (–OH) groups allow new functional groups to be introduced that can change the characteristics. A modified chitosan showed increased activity against E. coli than native chitosan [122]. Chitosan composites have also been investigated as food coatings and edible films; however, they are frequently mixed with other bio-actives to improve water vapor or oxygen barrier properties. It has also been used for encapsulation and delivery of essential oils, vitamins, flavonoids, plant extracts, probiotics, and polyphenols [119], in the form of microparticle and nanoparticles emulsions. A niche application of chitosan is enzyme immobilization. A report investigated alternate ways to immobilize enzymes using chitin-activated by formaldehyde [123]. A similar work reported immobilization of crude seal gastric proteases, with properties similar to rennin [124]. Enzyme immobilized chitosan investigated as biosensors for glucose [125], choline [126], pathogens [127], and polyphenolics [128]. Wastewater treatment from food industries is another area where chitosan is very effective, as it adsorbs a large variety of water impurities; it is utilized for bioconversion of phenolics, dye, and small ions (Zn2+, Cu2+) removal [119].
3.4.2.3 Alginates
Alginate is used as food additive in the form of its salts (Na, K, NH4, Ca, propan-1,2-diol). Because of its emulsifying nature, sodium alginate is used as a can-sealing compound and stabilizer. Alginate propylene glycol used for salad dressings and butter is an emulsion stabilizer [129].
With its gelling ability, alginates are used for encapsulation and immobilization of volatile molecules like fats and flavors. Alginate is also ideal for entrapment of enzymes and probiotics in supplements, such that an adequate number of beneficial microbes are delivered directly to the human gut. Alginates can increase the survival chances of the microbes during food storage as well as the extreme conditions in stomach and small intestine. Alginates have the innate ability to form transparent gels at room temperatures. With the current focus on replacing nonbiodegradable or non-recyclable food packaging, alginates can have wide applications in food coatings. The anti-microbial activity can also help in their use as coatings for vegetables, fruits, fish and meat products to act as a barrier against surface spoilage. Alginates also adsorb heavy metal