## 3 Tissue Engineering Approach
### 3D Bioprinting  
To overcome the lack of a fine control of structural aspects, additive manufacturing (AM) techniques have been introduced into the tissue engineering area. AM is a process by which a digital model for a 3D object is assembled in a layer-by-layer manner [115]. AM advantages include the ability to fabricate complex geometries with multimaterial parts [115]. Among these techniques, 3D printing plays a key role in developing personalized treatments, surgical planning and the testing deployment of devices in realistic pathways due to its potential to fabricate patient-specific 3D structures in a cost-, time- and waste-effective manner [116]. In the medical field, the latest promising evolution of 3D printing is 3D bioprinting. 3D bioprinting is an innovative biofabrication strategy, which allows the precise positioning of non-living materials, as in 3D printing, and living materials in a prescribed 3D hierarchical organization [117]. During the bioprinting process, the bioinks, composed of cells \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Fabrication Technique** & **Scaffold Structure** & **Cell Type** & **Research Stage** & **Reference** \\ \hline Photolithography & Porous scaffold & \begin{tabular}{c} Mouse retinal progenitor \\ cells \\ \end{tabular} & In vitro & [110] \\ Photolithography & Thin film scaffold & \begin{tabular}{c} Mouse retinal progenitor \\ cells \\ \end{tabular} & In vitro & [111] \\ Photolithography & Porous scaffold & \begin{tabular}{c} Mouse retinal progenitor \\ cells \\ \end{tabular} & In vitro and in vivo & [112] \\ Photolithography + wet & Mesh-supported & \
\begin{tabular}{c} Human cell line \\ ARPE-19/H9-RPE \\ \end{tabular} & In vitro & [113] \\ Photolithography + wet & Mesh-supported &
\begin{tabular}{c} Human ESC-RPE cells \\ \end{tabular} & In vitro and in vivo (rat) & [8] \\ and ion etching & submicron membrane & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: A summary of retinal scaffolds fabricated by lithography techniques. This table highlights the fabrication technique, the structure of scaffold and the cell type used. RPE: retinal pigment epithelium; ESC: embryonic stem cells. embedded in biocompatible materials, are dispensed to form the functional desired structures. 3D bioprinting aims to create 3D bioengineered structures serving in regenerative medicine, pharmacokinetics and basic cell biology studies [117]. Such 3D bioprinted constructs can be then cultured in bioreactor systems to obtain mature functional tissues and organs [118]. A few examples of 3D bioprinted scaffolds are represented by synthetic skin to be transplanted onto patients with burn injuries, heart valve replication and bionic ears [119; 120; 121]. Currently, there are mainly three types of bioprinting systems (laser-based, inkjet-based and extrusion-based) characterized by a high deposition accuracy, stability and cell viability [117]. Laser-based technology exploits a pulsed laser source and an optical path to focus a laser on a target from which the bioink is printed and deposited onto a substrate. The target is composed of a glass slide, a metal slide and bioink. The laser is focused through the glass slide onto the metal slide inducing a vaporization of the metal-absorbing layer, thus resulting in the production of a jet of bioink. Many researchers have demonstrated that cells were highly viable after laser-based 3D bioprinting [122]. However, cell placement accuracy can be challenging and, in addition, this technology is expensive and relatively slow. On the contrary, inkjet-based bioprinting is an inexpensive and simple to use technology while offering a relatively high resolution and cell viability [122]. In inkjet bioprinting, small droplets of bioink are ejected from a nozzle through microheater or piezoelectric systems and then dispensed onto a substrate [122]. Inkjet bioprinting was investigated to seed RGCs and glial cells.