Table 2.

A summary of natural polymers, advantages, disadvantages, crosslinkers cross-linking agents or factors used and their effect.

Polymer	Advantages	Disadvantages	Cross-Linking Agent or Factor	Effect	Ref
Chitosan	Antioxidant, Antifungal, Anti-inflammatory, Antibacterial Non-toxic Cost-effective Easy structure modification Thermal and chemical stability Responsive to external stimuli A polycationic character that promotes fast gelling in the basic pH of normal tissues	Relatively poor mechanical and barrier properties Naturally brittle Low lipophilicity for emulsions	Vanillin N,N/-methylene bis-acrylamide (MBA) Poly (N-isopropyl acrylamide	Improves the balance of chitosan between affinity and insolubility in oil due to the hydrophobic methoxyphenyl group in the vanillin aromatic ring. Assisted in the adsorption of Cr6+ ions from its water solution. High antibacterial activity cotton fabrics	[80,82,136,158,159]
Gelatin	Non-immunogenic Non-toxic Amphoteric Non-carcinogenic Good cell adhesion, proliferation, and differentiation due to many binding sites	Thermosensitive	2-chloro-1-methylpyridinium iodide (CMPI)	Activation of carboxylic acid sodium salt under heterogeneous reaction with high water uptake ability, reasonable biodegradability, and excellent cytocompatibility	[83,85,157]
Alginate	Non-toxic Non-immunogenic Good adhesion Thickening and stabilizing Gel-forming and film-forming Fiber spinning Hydrophilic Cost-effective Acidic environment neutralizer Excellent hemostatic properties	Weak mechanical strength Scarcity of efficient sites for cell adhesion, thus, poor cell attachment and proliferation. Alginate gels shrink at low pH Difficulties in sterilization, handling, and storage Difficult to control the release of alginate encapsulated material due to its porosity, permeability, and degradation	Calcium ions Sodium ions	Alginate hydrogel changed weight by 10% in pure water and 90% in an isotonic solution Selective binding to G sequences of alginate and form heat-stable three-dimensional gel networks High quantity water absorption due to ion exchange	[81,83,84,89,160,161]
Collagen	High antigenicity	Ethical and cultural issues Inconsistency Low mechanical strength Fast degradation rate Potential toxicity due to residual catalysts or initiators	Polypropyleneimine-octa-amine dendrimers	Supports adhesion and proliferation of human corneal epithelial cells without encouraging cellular toxicity	[92,146,157,162]
Cellulose	Pure A high degree of porosity Good tensile strength Low immunogenicity High relative permeability to gases and liquids High retention and ion exchange capacity	Insoluble in most solvents	Citric acid	Formation of carboxylic bridges between cellulose fibril chains, thus preventing cellulose condensation during drying Improved rehydration ability, porosity, wettability, and water swelling rate	[95,96,163]
Hyaluronic acid	Non-immunogenic	Usefulness degraded by hyaluronidase	N-(3-Dimethylaminopropyl)-N’-ethyl carbodiimide hydrochloride (EDC) 2-chloro-1-methylpyrinium iodide (CMPI)	Faster degradation rate and smoother surfaces, lower cytotoxicity for corneal endothelial cells, and minimal inflammatory cell infiltration or foreign body reaction after implantation Facilitates intra- and inter-molecular ester bond formation between the carboxyl and hydroxyl groups of hyaluronic and exhibits better resistance against hydrolytic degradation	[131]
Fibrin	Abundant and simple Resistance to degradation Fast isolation from the patient’s blood. Promotes expression of proinflammatory cytokines, cell migration, cell adhesion, and phagocytosis in monocytes, macrophages, and neutrophils.	Risk of infection transmission	Transglutaminase 2 (TG2)	Enhances proinflammatory activity to surface adhered fibrinogen	[164,165]