**5. In-situ gelation-based biomaterials**

Recently, in-situ gelation-based, injectable hydrogels have gained attention in corneal tissue engineering because of several advantages over conventional solid implants. Injectable hydrogels can be delivered in a doctor's or ophthalmologist's clinic instead of an operating theater, since there is no need for surgical intervention. The risk of parasurgical infection, patient discomfort, or scar formation is reduced. The in-situ forming hydrogel can adapt to complex tissue cavities, mold, or irregular wounds with better integration [61]. It is easy to encapsulate therapeutic molecules such as peptides, drugs, or exosomes into the hydrogel for synergistic effect. The clinical potential of in-situ forming hydrogels for corneal regeneration has been reviewed by Poudel et al. [61].

In-situ composite hydrogels made from hydroxypropyl chitosan (HPCTS) and sodium alginate dialdehyde via Schiff 's base chemistry showed histocompatibility, nontoxicity, and biodegradability. The composite hydrogel incorporating corneal endothelial cells was tested in a rabbit cornea injury model and showed its ability to support the reconstruction of corneal endothelium [62]. An Avastin® containing in-situ forming PEG hydrogel was explored for sustained release of the drug to treat corneal neovascularization. The transparent PEG hydrogel was nontoxic to L-929 cells after 1 week of incubation. The in-situ forming hydrogel showed sustained release of Avastin for up to 14 days without any apparent hydrolysis of the drug molecule [63].

To improve the drug efficacy and increase ocular drug availability, another in-situ forming, thermoresponsive chitosan-gelatin-based hydrogel was developed. The fast-gelling hydrogel was fabricated using β-glycerophosphate disodium salt hydrate (β-GD) and genipin to encapsulate timolol maleate for sustained release. The chitosan gelatin solution was instilled into the lower conjunctival sac of rabbits' eyes to form a hydrogel that provided sustained release of timolol maleate for up to 24 h [64]. Chun et al. developed injectable in-situ forming hydrogel comprising collagen type I that was cross-linked using a multifunctional polyethylene glycol (PEG)-Nhydroxysuccinimide (NHS). When tested for its ability to heal corneal defects, the hydrogel allowed the migration of epithelial cells to form multilayer at the site of corneal stromal defects without inflammation [65].

The GelCORE bioadhesive hydrogel was developed for the sutureless repair of corneal injuries. The highly biocompatible, transparent hydrogel is cured in-situ by visible light cross-linking. In-vivo studies in rabbit corneas showed regeneration of the stroma and epithelium after excision of rabbit corneal tissues [58]. Bioorthogonally cross-linked hydrogels comprising hyaluronan and collagen showed improved mechanical strength for corneal repair. The in-situ forming hydrogel possesses low refractive index than the native cornea, showed excellent biocompatibility and epithelization in in-vitro and in-vivo studies of partial thickness defects [59].

Griffith and coworkers have recently explored the clinical potential of a fully synthetic hydrogel made from CLP-PEG that incorporated fibrinogen. The "LiQD Cornea" gelled spontaneously in situ at body temperature without the need for light exposure. The hydrogel stimulated the stable (over 1 year) regeneration of corneal epithelium, stroma, and nerves, serving as an alternative to a partial thickness anterior lamellar allografts (**Figure 2C**) [56]. For patients who have corneal perforations (**Figure 2A**), these are emergencies that require patching. The LiQD Cornea was tested and found to also be effective glue-fillers for patching perforated corneas in rabbits, mini-pigs (**Figure 2B** and **C**) [60], and in a cat model [66].

*Advances in Biomaterials for Corneal Regeneration DOI: http://dx.doi.org/10.5772/intechopen.106966*

**Figure 2.**

*LiQD hydrogel evaluation in rabbit and mini-pigs (A) Human cornea perforation. (B) Postsurgical perforated rabbit cornea containing LiQD cornea hydrogel at Day 0, 2, and 28. Perforation can be easily seen at Day 0. At Day 2, the air bubble introduced during surgery was prominent, indicating the perforation was sealed. Perforation was healed after 28 days of injecting LiQD cornea hydrogel. (C) Mini pig corneas showing presence of LiQD cornea, syngeneic graft, and unoperated cornea after 12 months of surgery. Reproduced from [60].*
