## 5 Three-Dimensional Bioprinting of the Cornea
### Stem Cell Integration and Tissue Engineering  
The integration of stem cells in 3D bioprinting has driven significant progress in replicating the cornea's complex structure and function. Corneal keratocytes encapsulated within collagen-based bioinks have shown high viability, contributing to the biomechanical stability, transparency, and refractive properties of bioprinted constructs. These advances underscore the importance of viable cellular components in mimicking native corneal tissue effectively [51; 52].  
Limbal stem cells (LSCs) play a pivotal role in regenerating the corneal epithelium, particularly under pathological conditions. Found at the limbus, LSCs are uniquely suited  
Figure 2: The 3D bioprinting pathway for corneal repair in space. This conceptual diagram illustrates the stepwise process, starting from astronaut-derived stem cell collection (Step 1) to the preparation of bioinks (Step 2), bioprinting in microgravity (Step 3), tissue maturation in bioreactors (Step 4), and culminating in on-demand corneal repair (Step 5). Each stage highlights the innovative integration of biotechnology to address the challenges of ocular injuries during spaceflight. Created with BioRender.com.  
to repairing and maintaining the corneal surface. Techniques for ex vivo expansion of LSCs on biocompatible scaffolds, such as amniotic membranes or hydrogels, have demonstrated success in restoring epithelial integrity and reducing complications like neovascularization, making them essential for addressing limbal stem cell deficiency [72].  
Embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) offer broader regenerative potential by differentiating into the primary cell types of the cornea--keratocytes, epithelial cells, and endothelial cells. Co-culture systems pairing these cells with engineered scaffolds or bioinks mimicking the extracellular matrix (ECM) provide necessary structural and biochemical support. However, challenges such as teratoma formation remain significant [73; 74].  
Advanced bioinks, like UNION bioinks, have further enhanced the mechanical properties and biocompatibility of bioprinted corneal constructs. These bioinks replicate the organized collagen fibrils within the corneal stroma, ensuring structural integrity and optical clarity. Embedding corneal epithelial cells within these bioinks has improved epithelial regeneration and cell viability, addressing challenges like maintaining epithelial integrity during extended space missions [25; 51].  
Biomimetic scaffolds have emerged as critical tools for creating environments conducive to cellular adhesion, proliferation, and differentiation. Hybrid scaffolds combining natural polymers, such as collagen and hyaluronic acid, with synthetic materials like polyethylene glycol (PEG), offer a balance of mechanical strength and transparency. These scaffolds facilitate the organized growth of stem cells into functional tissues, closely replicating the native corneal architecture [74; 75].  
Dynamic bioreactors have revolutionized tissue engineering by replicating physiological cues essential for in situ tissue maturation. These systems guide stem cell differentiation and extracellular matrix remodeling by simulating natural biomechanical environments. Advancements in microfluidic bioreactors, which replicate corneal biomechanical properties, have further improved the functional integration of bioprinted constructs, making them particularly valuable for long-duration space missions [51; 75].  
Despite these innovations, achieving fully functional, multi-layered corneal constructs remains challenging. Transparent replacements require precise alignment of stromal collagen and integration of functional epithelial and endothelial layers. Addressing issues like cell-biomaterial interactions, uniform stem cell distribution in bioinks, and potential immune responses are critical areas for continued research [26; 65].