# Current Biomedical Applications of 3D Printing and Additive Manufacturing
## 3 Biocompatible 3D Printing Materials
### Bioinks  
Many bioinks have been developed from hydrogels, which are very well established as suitable materials for 3D cell cultures [40; 41; 42]; they have very good biocompatibility and are highly tunable regarding their physical, mechanical and biological properties [43; 42; 44]. Hydrogels commonly show non-Newtonian, shear thinning behavior, which makes them suitable for extrusion bioprinting. Unmodified, however, they often have limitations regarding their print characteristics, which were described by [PERSON] et al. [32]. Hydrogels, which make the best cell culture scaffolds, often have low viscosities prior to crosslinking, resulting in poor shape fidelity following extrusion-based printing, and limiting their capacity to form larger structures without features collapsing [45; 46; 32]. This has encouraged the development of both novel bioinks and new techniques for cell-seeded biofabrication in recent years [47]. Table 2 summarizes frequently used bioinks and their applications, while a number of specific recent examples are further detailed. Biopolymer hydrogels that have been used for bioprinting include, but are not limited to, alginate, agarose, cellulose, collagen, fibrin, gelatin, gellan gum and hyaluronic acid. Alginate is a negatively charged polysaccharide derived from brown algae, and is one of the most commonly used hydrogels in both tissue engineering and bioprinting [62; 63]. It crosslinks upon the addition of divalent cations, and can be functionalized by the addition of arginine-glycine-aspartate (RGD) moieties, which are responsible for cells binding to the extracellular matrix [64], facilitating interaction with the scaffold. Alginate is also frequently blended with other biopolymers to modify both its printing and biological properties. A recent study incorporated hydroxyapatite (HAp) into alginate to produce a bioink for production of a calcified cartilage matrix [49]. The addition of HAp into alginate caused a reduction in \begin{table}
\begin{tabular}{c c c} \hline \hline
**Bioink** & **Cell Type/Tissue** & **Reference** \\ \hline Alginate & Chondrocytes/Cartilage & [48; 49] \\  & hMSCs & [50; 51] \\  & Chondrocytes & \\ Collagen & Hepatocytes/Liver & [52] \\   Fibrin & hMPCs/Skeletal Muscle & [53] \\  & HUVECs/Vascular structures & \\   Gelatin & hMSCs/Bone & [36; 54; 55] \\  & hMSCs/Cartilage & \\   Gellan gum & Chondrocytes/Cartilage & [47] \\  & Osteoblasts/Bone & \\   Hyaluronic & Fibroblasts & [56] \\   acid & & hMSCs & \\   PEG & Fibroblasts & [57; 58] \\   Tissue-derived & SCAP/Dentin & [59; 60; 61] \\   ECM & Kidney & [59; 61] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Commonly used bioinks and their recent tissue engineering applications (PEG: polyethylene glycol; ECM: extracellular matrix; hMSCs: human mesenchymal stem cells; hMPCs: human muscle progenitor cells; HUVECs: human umbilical vein endothelial cells; PAVICs: porcine aortic valve interstitial cells; SCAP: stem cells from the apical papilla). glucosaminoglycan (GAG) secretion of chondrocytes, an increase in production of Col II and an increase in calcified cartilage markers Col-X, ALP and Alizarin red staining [49]. The addition of sodium citrate allowed for a more-homogenous distribution of Hap, and also improved print characteristics by preventing clogging of the needle [49]. Gelatin is another natural biopolymer that has long been used in cell culture. Unmodified, it undergoes a thermoreversible sol-gel transition, but the speed of gelation is generally considered too slow to ensure shape fidelity. To overcome this, many groups have used gelatin methacrylate (GelMA) to enable UV crosslinking [65]. This has shown very good biocompatibility, degradability and is also a relatively inexpensive bioink [66; 67]. A study by [PERSON] et al. used GelMA functionalized with vascular endothelial growth factor (VEGF) moieties to bioprint spatially defined vascular structures (with HUVECs) within a bone-like (GelMA with silicate nanoparticles and hMSCs) construct [54].