# Use of Anionic Polysaccharides in the Development of 3D Bioprinting Technology
## 3 3D Bioprinting Technology Concept  
Parallel to the many advances made in science and technology, 3D printing has been applied to many multidisciplinary fields due to its unique physical, chemical and biological properties as well as multifunctional material-based construction. In addition, 3D printing has extended applicability toward the fabrication of three-dimensional constructs containing living cells. This technology is expected to solve the problem of the conventional tissue engineering approach and the arduous task of controlling the position of individual cells. In the conventional tissue engineering approach, cell-laden constructs are prepared by pouring cell-suspending solution to porous scaffolds. In contrast, in 3D bioprinting, cell-laden constructs are prepared by localizing cell-suspending bioink using printers as intended. The existing bioprinting systems can be roughly classified into four types (Figure 4) [12; 66; 67; 68]: inkjet bioprinting (a), stereolithography bioprinting (b), micro-extrusion bioprinting (c), and laser-assisted bioprinting (d). Inkjet bioprinting system deposits droplets of bioink-containing cells on a substrate through an ejection from a small needle. Gelation of the deposited bioink should occur in-situ at the deposited position. The diameter of the droplets is within a range of several dozens to several hundred micrometers. An attractive point of inkjet bioprinting is a micrometer scale resolution (10-50 \(\upmu\)m). Figure 4: Schematic diagram of four common bioprinting systems. In contrast, a well-known limitation is the necessity of rapidly gellable bioinks with low viscosity (<10 mPa-s) [66]. It is ideal for bioprinting systems, to enable hydrogelation processes that are cytocompatible. Stereolithography bioprinting requires bioinks curable through photo-irradiation using a rastered laser beam or a dynamically projected light source. Cell-laden constructs are fabricated on the surface of a substrate submerged in the bioink. When it comes to the quality, resolution, and accuracy of manufactured parts, stereolithography bioprinting is the most promising system. The difficulty of this approach is that the same layer cannot be fabricated by multiple bioinks. Micro-extrusion bioprinting is the most intensively investigated. 3D constructs are obtained by continuous extrusion of bioink filament fed through a needle. The bioinks with the viscosity of 30 - 6\(\times\)10\({}^{7}\) mPa-s are applicable to this system [66]. In addition, in comparison with other bioprinting systems, micro-extrusion bioprinting enables one to prepare cell-laden constructs with higher cell density [12]. The existence of various applicable biomaterials and inexpensive printers are also attractive points of this system. Laser-assisted bioprinting is one of the most cutting-edge systems, which allows the printing of bioink inclusive of cells with a cell-level resolution. In this system, a pulsed laser beam is focused and scanned over a donor substrate that is coated with an absorbing layer and a layer of bioink. The resolution is in the range of 10-100 \(\upmu\)m. A few of the drawbacks of this printer is that it has a more complicated structure, it is relatively expensive, and irradiation of a laser beam can lead to cell damage [69]. Biomaterial Application of Alginate in 3D Bioprinting and Perspectives Using Other Polyuronic Acid Polysaccharides  
Alginate, as a biomaterial for tissue engineering applications, has a number of attractive features such as biocompatibility, low toxicity, relatively low cost, and mild and rapid hydrogenation by the addition of divalent cations such as Ca\({}^{2+}\)[70]. The mild and rapid hydrogenation is useful as a component of bioink for stabilizing it at the designated position. In a pioneer work by [PERSON] et al. for inkjet bioprinting, 3D cell-laden constructs were obtained by dropping sodium alginate solution containing cells into CaCl\({}_{2}\) solution from an inkjet nozzle [71; 72; 73].