**1. Introduction**

The main objective of three-dimensional (3D) printing is to print a living cell or to create a three-dimensional biomaterial's scaffold. This innovative technology allows the reproducible and also the automated fabrication of three-dimensional useful living tissues by depositing biomaterials layer by layer with an accurate positioning of cells. This method allows to make a three-dimensional object and an accurate as well as scalable geometries that are not suggested by any approaches like two-dimensional cell cultures [1].

The choice of applying these 3D functional living tissues comes from fundamental research [2]. Learning about the cell-biomaterial interface at the nanoscale stage is vital in accommodating flaws in tissues, nanoparticle-cell connections and organ defects [3], toxicological analysis or drug investigation [4], and transplantation in living objects [5]. Because of the rising complexity required for these tissues, 3D bioprinting is facing a lot of challenges in all of manufacturing areas. For example, the cell-encapsulated materials are commonly observable to chemical cross linkers for extended periods of time during storage prior to printing, which can harm the cells. At the time of deposition, the mechanical stress generated by the printing itself can result in damage and injury to cell functioning by cell shearing or extrusion [6]. The instant new printing tissue is fabricated, because of the small vascularity of printed material; limited nutrients are supplied in 3D construct [7].

Usually, the requirements for a suitable cell-containing dispensable biomaterial or bioink are generally biocompatibility, exhaustive, biomimicry, printability, and essential mechanical properties. This is the main cause for the huge number of the manufacturers of commercially accessible 3D bioprinters—particularly extrusionbased 3D bioprinter, where hydrogel bioinks are recommended [8]. Particularly, hydrogels are unquestionably the most comprehensive biomaterials applied as cell matrix in bioinks because they can be engaged as cell matrix and be modified to replace or mimic local tissue [9]. The physical and chemical characteristics of the hydrogels will verify the performance of the cells. Normally hydrogels are like as jelly-type materials, where the liquid component is water. Actually, hydrogels are just like water by weight, but practically any flow will not occur in the steady state because of the three-dimensional cross-linked polymer network inside the fluid, which provides them unique properties comparable to those of living tissues. Due to their different biocompatibility and printability, various hydrogels that support cell growth are associated with bioink fabrication, i.e., gelatin, agarose, polyethylene glycol (PEG)-diacrylate, and alginate that are commonly used as bioinks. While alginate is an anionic polysaccharide derived from brown seaweed and generally consists of two polymer blocks, (1-4)-linked β-d-mannuronate (M) and its C-5 epimer α-l-guluronate (G) residues, basically all are covalently linked. The main elements in the alginic acid polymer chain are the carboxylic acid group which allows cross-linking. This converts alginate from its liquid state to a semisolid gel state. Sodium alginate is mostly used as bioink in tissue engineering and cell culture because of biocompatibility, low-cost, and fast gelation. In **Figure 1**, the presence of calcium ions and ionic interactions between Ca2+ and COO<sup>−</sup> occur, and crosslinking of alginate polymers results.

Ionic cross-linking is a method where cells cause minimum damage. The crosslinking process happens moderately rapidly. Alginate has structural similarity to natural extracellular matrices that is why it has been applied widely in various biomedical applications as well as in the delivery of bioactive agents and wound healing. For cell encapsulation, alginate hydrogels are generally applied. The whole procedure is prepared by mixing cells in alginate solution, and after the mixing process, the alginate-cell mixture drops into a bath of calcium chloride solution. But in low concentrations (1–2%), due to low viscosity, the alginate solution is not printable. For increasing the viscosity, other materials like methylcellulose or gelatin can be mixed with alginate for preparing the printability. The structural correspondence of alginate to extracellular matrices creates a perfect biomaterial. Matrix stiffness is a functional determinant of stem cell differentiation, and alginate makes a potential material to manage stem cell growth. Alginate helps support the cell growth and also has a high versatility, extending to both in vivo and in vitro differentiation. For 3D bioprinting applications, for example, extrusion printing needs quick gelation. In this case alginate proposes high gelation procedures when combined with a

**111**

the cells attached to it [13].

*Extrusion-based 3D bioprinting process.*

**Figure 2.**

**2. Application of alginate**

*Importance of Alginate Bioink for 3D Bioprinting in Tissue Engineering and Regenerative…*

multivalent cation, permitting gels to build up and deposit at constant temperature. It is also applied to encapsulate cells. This allows it to be an effective tool in varying the release rate of drug and growth factor delivery. While alginate degradation rate can be somewhat controlled by altering the MW of the alginate, it is still slow and difficult to control. The stiffness and composition properties of alginate bioink can be tuned to direct the differentiation of stem cells. Sodium alginate is available naturally which is biodegradable, non-immunogenic linear, and nontoxic polysaccharide polymer; it consists of mannuronic and guluronic acids [10]. The cost is also low being a marine material which can be extracted from the brown algae cell walls, forming hydrogel in certain conditions. Because of these advantages, bioengineers and material scientists use alginate for the preparation of bioinks in tissue engineering and regenerative medicines. The tissue fabrication by 3D bioprinting [11] and sodium alginate applications and properties [12] is currently separately reviewed. In this study we discussed the applications of alginate (**Figure 2**) in 3D bioprinting and blending alginate with other polymers to improve the biomaterial interaction of

The requirement for alginate-based biomaterials in drug delivery and tissue engineering is huge. As stem cells play a progressively more major function in the area of regenerative medicine [14, 15], the arrangement and relation between alginate-based materials and stem cells have been exclusively emphasized. Investigated by in vitro implantation and in vitro cytotoxicity assay, alginate-based scaffolds and microcapsules have shown minimum or minor cytotoxicity [16, 17]. These in vitro results recommended tunable connections between the bio-composites and the multiple platelet releasate-derived bioagents for improving hematoma-like fracture repair. Also, a simple invasive performance for in situ remedial of the implant structures through injection was established in rat tail vertebrae applying microcomputed tomography. These results confirmed that alginate-based scaffolds

*DOI: http://dx.doi.org/10.5772/intechopen.90426*

**Figure 1.** *Cross-linking process of alginate.*

*Importance of Alginate Bioink for 3D Bioprinting in Tissue Engineering and Regenerative… DOI: http://dx.doi.org/10.5772/intechopen.90426*

**Figure 2.** *Extrusion-based 3D bioprinting process.*

*Alginates - Recent Uses of This Natural Polymer*

linking of alginate polymers results.

Usually, the requirements for a suitable cell-containing dispensable biomaterial or bioink are generally biocompatibility, exhaustive, biomimicry, printability, and essential mechanical properties. This is the main cause for the huge number of the manufacturers of commercially accessible 3D bioprinters—particularly extrusionbased 3D bioprinter, where hydrogel bioinks are recommended [8]. Particularly, hydrogels are unquestionably the most comprehensive biomaterials applied as cell matrix in bioinks because they can be engaged as cell matrix and be modified to replace or mimic local tissue [9]. The physical and chemical characteristics of the hydrogels will verify the performance of the cells. Normally hydrogels are like as jelly-type materials, where the liquid component is water. Actually, hydrogels are just like water by weight, but practically any flow will not occur in the steady state because of the three-dimensional cross-linked polymer network inside the fluid, which provides them unique properties comparable to those of living tissues. Due to their different biocompatibility and printability, various hydrogels that support cell growth are associated with bioink fabrication, i.e., gelatin, agarose, polyethylene glycol (PEG)-diacrylate, and alginate that are commonly used as bioinks. While alginate is an anionic polysaccharide derived from brown seaweed and generally consists of two polymer blocks, (1-4)-linked β-d-mannuronate (M) and its C-5 epimer α-l-guluronate (G) residues, basically all are covalently linked. The main elements in the alginic acid polymer chain are the carboxylic acid group which allows cross-linking. This converts alginate from its liquid state to a semisolid gel state. Sodium alginate is mostly used as bioink in tissue engineering and cell culture because of biocompatibility, low-cost, and fast gelation. In **Figure 1**, the presence of calcium ions and ionic interactions between Ca2+ and COO<sup>−</sup> occur, and cross-

Ionic cross-linking is a method where cells cause minimum damage. The crosslinking process happens moderately rapidly. Alginate has structural similarity to natural extracellular matrices that is why it has been applied widely in various biomedical applications as well as in the delivery of bioactive agents and wound healing. For cell encapsulation, alginate hydrogels are generally applied. The whole procedure is prepared by mixing cells in alginate solution, and after the mixing process, the alginate-cell mixture drops into a bath of calcium chloride solution. But in low concentrations (1–2%), due to low viscosity, the alginate solution is not printable. For increasing the viscosity, other materials like methylcellulose or gelatin can be mixed with alginate for preparing the printability. The structural correspondence of alginate to extracellular matrices creates a perfect biomaterial. Matrix stiffness is a functional determinant of stem cell differentiation, and alginate makes a potential material to manage stem cell growth. Alginate helps support the cell growth and also has a high versatility, extending to both in vivo and in vitro differentiation. For 3D bioprinting applications, for example, extrusion printing needs quick gelation. In this case alginate proposes high gelation procedures when combined with a

**110**

**Figure 1.**

*Cross-linking process of alginate.*

multivalent cation, permitting gels to build up and deposit at constant temperature. It is also applied to encapsulate cells. This allows it to be an effective tool in varying the release rate of drug and growth factor delivery. While alginate degradation rate can be somewhat controlled by altering the MW of the alginate, it is still slow and difficult to control. The stiffness and composition properties of alginate bioink can be tuned to direct the differentiation of stem cells. Sodium alginate is available naturally which is biodegradable, non-immunogenic linear, and nontoxic polysaccharide polymer; it consists of mannuronic and guluronic acids [10]. The cost is also low being a marine material which can be extracted from the brown algae cell walls, forming hydrogel in certain conditions. Because of these advantages, bioengineers and material scientists use alginate for the preparation of bioinks in tissue engineering and regenerative medicines. The tissue fabrication by 3D bioprinting [11] and sodium alginate applications and properties [12] is currently separately reviewed. In this study we discussed the applications of alginate (**Figure 2**) in 3D bioprinting and blending alginate with other polymers to improve the biomaterial interaction of the cells attached to it [13].
