## 5 Stereolithography-Based Bioprinting  
In stereolithography-based bioprinters, UV light is used to cure layers of photopolymer, stacks of which form the 3D object (Table 3). The biggest advantage of stereolithography in general and stereolithography-based bioprinting in particular is its very high resolution. Other advantages include high cell concentrations and no problem of nozzle clogging. The preparation of three-dimensional biodegradable poly(ethylene glycol)/poly(d,l-lactide) hydrogel structures using stereolithography at high resolutions was shown [46]. Cell-encapsulated hydrogels were also shown to be 3D-printed using stereolithography [48]. Cell-attachable and visible-light cross-linkable bioinks, based on gelatin methacryloyl (GelMA) with eosin Y (EY) photoin initiation, for stereolithography three-dimensional (3D) bioprinting were developed and used to print cell-laden hydrogels [13]. However, there are many disadvantages of this method. The biggest disadvantage is that only photocurable bioinks can be used. Another disadvantage is that the cells will get exposed to harmful UV light, which affects the cell viability [44].  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Properties**} & **Laser-Based** & **Inglei** & **EHD letting-Based** & **Acoustic** & **Microwave** & **Extusion-Based** & **Stereolithography-Based** \\  & **Bioprinting** & **Bioprinting** & **Bioprinting** & **Bioprinting** & **Bioprinting** & **Bioprinting** & **Bioprinting** \\ \hline Bioink viscosity & 1-300 mP2*s & 3-12 mP2*s & 1-1000 mP2*s & NA & 1-200 mP2*s & \(\sim\)600 kPa*s & \(\sim\)5 Pa*s \\ \hline Cell density & 10\({}^{6}\) cells/mL & 10\({}^{6}\) cells/mL & 10\({}^{6}\) cells/mL & 10\({}^{6}\) cells/mL & 10\({}^{6}\) cells/mL & 10\({}^{6}\) cells/mL & 10\({}^{6}\) cells/mL & \(>\)10\({}^{6}\) cells/mL \\ \hline Speed & 200–1600 mm/s & 10,000 droplets per second & 10-500 mm/s & 10,000 droplets per second & 100 droplets per second & 10-50 \(\mu\)m/s & High \\ \hline Resolution & 50\(\mu\)m & 50\(\mu\)m & 100 nm & 37 \(\mu\)m & 100 \(\mu\)m & 200 nm-6 \(\mu\)m \\ \hline Accuracy & High & Medium & Low & Medium & Medium & Low & High \\ \hline Cell viability & \(>\)95\% & \(>\)80\% & \(>\)80\% & \(>\)90\% & \(>\)80\% & 40-96\% & 25-88\% \\ \hline Structural integrity & Low & Low & High & Low & Low-medium & High & Medium-high \\ \hline Solability & Low & High & High & Medium & High & High & Medium-high \\ \hline Cost & High & Low & High & Medium-high & Medium & Low-medium & Medium \\ \hline \hline \end{tabular}
\end{table}
Table 4: A comparison of various bioprinting techniques as tabulated by [PERSON] et al. [51].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Category**} & \multicolumn{2}{c}{**Extusion-Based Bioprinting**} & \multicolumn{2}{c}{**Stereolithography-Based Bioprinting**} \\ \hline Trigger difference & Pneumatic pressure or mechanical force & & Light (usually UV) irradiation \\ \hline \multirow{3}{*}{Advantages} & 1. Scalability & 1. & Highest resolution \\  & 2. & High-viscosity bioink & 2. & Reduced printing time \\  & 3. & High cell concentration & 1. & Nozzle clogging \\ \hline \multirow{3}{*}{Disadvantages} & 1. & Lowest resolution & 2. & Photopolymerizable bioinks or bioinks containing UV-activated photo initiated damage of cells \\  & 2. & Nozzle clogging & 3. & UV irradiation damage of DNA and promotion of cell lysis \\  & 3. & & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: The advantages and disadvantages of extrusion-based bioprinting and stereolithography-based bioprinting. UV—ultraviolet.