## 5 Three-Dimensional Bioprinting of the Cornea  
### Advances in 3D Bioprinting Technology  
Three-dimensional (3D) bioprinting has revolutionized corneal regeneration by enabling precise deposition of cells and biomaterials to replicate the native cornea's structure and function. Figure 2 illustrates a conceptualized stepwise process for 3D bioprinting of corneal constructs, integrating insights from recent advancements in stem cell integration, bioink preparation, and tissue maturation as described in the literature [15; 26]. Among the leading techniques, extrusion-based bioprinting is widely used for its ability to handle highly viscous bioinks, which support high cell density and structural integrity. However, challenges in balancing cell viability with material stiffness remain, necessitating innovations in nozzle design and bioink formulations to optimize extrusion parameters, particularly under varying conditions such as microgravity [15; 64]. These advancements are critical for improving the reproducibility and functionality of bioprinted corneal constructs [64].  
Laser-assisted bioprinting offers unparalleled precision, making it ideal for creating intricate micro-architectures. This method enables the deposition of individual cells and bioinks, achieving excellent cell survival and precise stromal alignment [52; 65]. However, its high cost and complexity limit scalability, highlighting the need for further technological improvements to enhance accessibility. Drop-on-demand bioprinting complements these approaches by offering precise droplet placement and uniform bioink distribution, supporting scalability while maintaining cellular organization [51; 66].  
Stereolithography-based techniques have made significant advances in producing mechanically stable and optically transparent corneal stromal constructs with precise collagen alignment. Incorporating bioactive molecules, such as growth factors and cytokines, promotes natural tissue healing and epithelial regeneration, enhancing the functionality of these constructs [67; 68]. Hybrid scaffolds in multi-material bioprinting further improve structural fidelity and functionality, advancing the potential for clinical applications [69].  
Microgravity environments provide unique advantages for 3D bioprinting, as the absence of gravity reduces sedimentation and deformation in bioinks, enabling the precise deposition of cells and materials. Techniques like extrusion-based and laser-assisted bio-printing perform particularly well in microgravity, facilitating the creation of intricate and functional constructs directly in space. These advancements reduce reliance on Earth-based resupply during long-term missions, addressing key logistical challenges [63; 70].  
Innovations in bioink formulations continue to expand the potential of 3D bioprinting. UNION bioinks, for example, offer tunable mechanical properties and high biocompatibility, supporting the creation of corneal constructs that replicate the layered architecture of the native tissue [51; 52]. The incorporation of bioactive molecules, such as hyaluronic acid and VEGF inhibitors, enhances these bioinks by supporting cell differentiation and preventing neovascularization, which are critical for corneal clarity and function [68; 71].  
Despite these advancements, challenges remain in achieving long-term optical clarity, structural stability, and host tissue integration of bioprinted constructs. Ongoing refinements in bioprinting techniques, bioink formulations, and tissue maturation processes will be essential for transitioning this technology into clinical use [26; 64]. An operating microscope, precise instruments, skilled surgical personnel, and the availability of appropriate anesthetic will complicate the use of this procedure. Also, follow-up of any surgical procedure will require the use of a slit lamp or similar optical device.