and host integration properties have been gaining attention [66, 67]. For example, a biomimetic scaffold consisting of a bioglass–collagen–hyaluronic acid–phosphatidylserine composite has been evaluated to enhance the adhesion, proliferation, and migration properties of human mesenchymal stem cells. In another study, hyaluronic acid, silk fibroin, and collagen combinations showed to be osteogenetic [66]. Hyaluronic acid-based materials are also used in hydrogel form to obtain high water content, oxygen, nutrients, and metabolites permeable scaffolds. For instance, Zanchetta et al. designed a hydrogel scaffold based on hyaluronic acid, chondroitin 6 sulfate, and dermatan sulfate with a promising osteogenesis-promoting property in rat models [71]. Hyaluronic acid is also considered as a promising candidate for central neural tissue engineering, because of its interconnected porous structure which facilitates the delivery of nutrition and penetration of cells, nerve fibers and blood vessels. In in vivo models, hyaluronic acid was demonstrated to be effective in reducing glial and peripheral scar formation and enhancing neural regeneration [72, 73]. The modulus of hyaluronic acid hydrogels was also reported to affect differentiation of neural progenitor cells: most of the neural progenitor cells cultured in hydrogels with mechanical properties comparable to those of neonatal brain tissue differentiated into neurons with extended long, branched processes, however neural progenitor cultured in stiffer hydrogels, with mechanical properties comparable to those of adult brain, mostly differentiated into astrocytes [74].
Alginate, also known as alginic acid, is a non-immunogenic, biocompatible polysaccharide obtained from kelp, brown algae, and some bacteria. It is possible to obtain stable hydrogels of alginate through the addition of divalent metal cations, such as Sr2+, Ca2+, and Ba2+, to aqueous alginate solution [40, 75]. However, pure alginate has been shown to have several drawbacks including poor efficiency for promoting cell adhesion and slow degradation kinetics in physiological conditions [76, 77]. Therefore, studies have been focused on enhancing its physicochemical properties and cytocompatibility. For instance, Sarker et al. developed an alginate-gelatin crosslinked hydrogel through covalent crosslinking of alginate di-aldehyde with gelatin and showed that this composite hydrogel supported cell attachment, spreading and proliferation of human dermal fibroblasts [76]. In another study, immobilization of the arginylglycylaspartic acid (RGD) peptide, the most common peptide motif responsible for cell adhesion to the extracellular matrix, to sodium alginate was found to promote cell adherence to the matrix, accelerate cardiac tissue regeneration while preventing cell apoptosis [78]. On the other hand, poor solubility and slow degradation rate of pure alginate are other limitations in the use of it in biomedical applications. Although mammals do not have enzymes to degrade alginate, via exchange reactions involving monovalent cations such as sodium ions, ionically cross-linked alginate gels can be dissolved by the liberation of the divalent ions cross-linking the gel into the surrounding media. However even if the gel dissolves, it may not be cleared from the body [79]. An attractive approach to increase the biodegradability of alginate includes oxidation of alginate chains, typically with sodium periodate without interfering with its gel-forming ability in the presence of divalent cations [80].
Dextran is an uncharged, linear homopolysaccharide synthesized from sucrose by several lactic acid bacteria including Streptococcus mutans, Leuconostoc mesenteroides, and Lactobacillus brevis. High water solubility, biocompatibility, biodegradability, non-immunogenicity, and non-antigenicity properties make dextran as a good candidate for biomedical applications, including tissue engineering and drug delivery. First, dextran can be biodegraded easily by the enzyme dextranase presents in several organs of the human body including liver, colon, spleen, and kidney. Dextran can also be metabolized by different bacteria residing in the human colon [81]. To be used in biomedical applications, native dextrans with high molecular weights hydrolyzed by partial depolymerization [82]. Porous dextran hydrogels have been produced through crosslinking reactions mediated by hydroxyl groups of α-1,6-linked d-glucose residues and numerous other chemical modifications yielding dextran derivatives have been also explored. Furthermore, researchers have used co-polymerization and surface grafting methods to improve the cell-adhesion property of dextran [40]. Noel et al. investigated the cell-selective response of extracellular peptides using dextran scaffolds and they showed that vinylsulfone-modified dextran tethered with the peptides RGD (Arg-Gly-Asp), YIGSR (Tyr-Ile-Gly-Ser-Arg), and SGIYR (Gly-Ile-Tyr-Arg) was able to improve cellular adhesion [83]. In another study Sun et al. modified dextran hydrogel by decreasing crosslinking density and therefore they improved the hydrogel properties including increased swelling, rapid disintegration, reduced rigidity, and increased vascular endothelial growth factor release capability. In addition, immobilization of defined angiogenic growth factors in that modified dextran macroporous scaffold was capable to induce a rapid proliferation of functional vasculature, in vivo [84].
Gellan gum is another bacterial polysaccharide used in biomedical applications. During fermentation, Sphingomonas strains including Sphingomonas elodea (ATCC31461), Sphingomonas paucimobilis NK2000, Sphingomonas paucimobilis E2 (DSM 6314), and Sphingomonas paucimobilis GS1 can produce this high molecular weight, anionic, and linear extracellular polysaccharide. Based on the number of acetyl groups, gellan gum can be classified as native gellan gum, deacetylated gellan gum and, deacetylated and clarified gellan gum [81, 85]. The native form of gellan is composed of acetyl and L-glyceryl groups bounded to glucose residue adjacent to glucuronic acid. In the fermentation broth, acetyl and L-glyceryl groups can be eliminated by hot alkaline hydrolysis to obtain a deacylated, linear, simple polysaccharide chain generically known as gellan gum. This deacetylation process leads to a change in the gellan form from a soft, flexible, thermally reversible structure to a more rigid, more brittle and more thermo-resistant structure. Whereas, to obtain clarified gellan gum, deacylated gellan is subjected to a clarification process by increasing the temperature of the fermentation broth to 95 °C. This heating process causes the killing of bacterial cells, getting rid of cell protein residues, and reducing the viscosity of the broth. The process is completed by filtration followed by alcohol precipitation [81]. Depending on the purity levels, there are different trades of gellan gum to be used in pharmaceutical applications, food or pharmaceutical industries or preparation of biological growth media for plant tissue cultures and microbial cultures [86]. With its high water-retaining, viscoelasticity, high biocompatibility, and biodegradability properties, gellan hydrogels can be used as scaffold materials. When injected into the defective tissue area, a hydrogel of gellan can suit the shape of the defect in its gel structure. This feature of gellan hydrogels makes them ideal to be used in regenerative medicine. Accordingly, gellan-based hydrogels have been extensively examined in the context of tissue engineering applications including disc regeneration, tendon repair, fibrocartilage tissue engineering, spinal cord repair, neoskin vascularization, and wound healing along with osteochondral and bone tissue regeneration [87–91]. However, the mechanical properties and processability of gellan gum are not satisfactory for tissue engineering: gellan gum hydrogels are mechanically weak, their high gelling temperature is unfavorable and specific attachment sites for anchorage-dependent cells are inadequate. However, chemical modification and functionalization through the incorporation of the multiple hydroxyl groups and the free carboxyl per repeating unit of gellan gum has been claimed to be used to optimize its physicochemical and biological properties [92].
Recently, bioprinting has emerged as a potentially revolutionizing method for personalized regenerative medicine [93]. Using the bioprinting method, it is possible to generate 3D tissues and organs as direct copies of patienťs organ parameters [94]. A bioink has to be printable, but also should provide the required elasticity and strength to mimick the mechanical properties of native tissues and maintain the original printed structure for a long time. Besides, a bioink has to be biocompatible and should not cause an inflammatory reaction, and it has to support attachment, proliferation, and differentiation of cells. Sometimes biodegradability is aimed; if it is the case, degradation products have to be evaluated for cytotoxicity [93, 95]. Natural polysaccharides, such as collagen, hyaluronic, acid, alginate, and different hydrogels, can be utilized as 3D bioprinting materials for the printing of various types of structures as scaffolds [96]. Furthermore, synthetic polymers such as polycaprolactone are used to obtain an optimal mechanical strength of the printed constructs [95]. Designing natural polysaccharide-based 3D-printed technologies/scaffolds needs detailed investigations
