Corneal stromal defects are one of the main corneal wounds which cost the healthcare system over $2 billion per year . Conventional standards of care for corneal stromal defects, including the use of cyanoacrylate glue, tissue grafting, or corneal transplantation, have significant drawbacks. For instance, cyanoacrylate glue is associated with cytotoxicity, lack of transparency, rough and irregular surface, difficult handling, and lack of biointegration with the corneal stromal tissue. Tissue grafting and Figure 1. (a, b) Representative slit lamp and (c) optical coherence tomography (OCT) images of GelMA-based bioadhesive after applying to deep corneal stromal defects in a rabbit model. Bioadhesive remained intact and stayed completely attached to the cornea over the 4-week follow up. The hydrogels were photocrosslinked in situ using 20 %(w/v) GelMA and 4 min light exposure time. corneal transplantation require donor tissue and advanced surgical skills and equipment and suffers from a shortage of the donor cornea worldwide . To overcome these challenges, here we engineered a highly biocompatible and and transparent adhesive hydrogel for corneal reconstruction using a naturally derived polymer, gelatin which is a partially hydrolyzed form of collagen with similar bioactivity. Materials and Methods All chemicals were purchased at analytical grade and used without further purification. GelCORE (gelatin for corneal regeneration) adhesive was synthesized through the methacrylation of porcine skin gelatin (Sigma) with methacrylic anhydride (Sigma), according to a procedure described previously . Hydrogels were photopolymerized using Eosin Y (0.1 mM) as a photoinitiator, Triethanolamine (1.5 %(w/v)) as a co-initiator and N-vinylcaprolactam (1 %(w/v)) as a co-monomer. The hydrogel prepolymer solution containing 20 %(w/v) GelCORE and photoinitiators, were mixed gently and photopolymerized for 4 min using a Food and Drug Administration (FDA) approved visible light source, FocalSeal (Genzyme Biosurgery, Inc., 450-550 nm). Results and Discussion Our results demonstrated that physical properties and adhesion strength of the engineered bioadhesives could be tuned by changing the total polymer concentration, and visible light exposure time. Furthermore, following American Society for Testing and Materials (ASTM) standard tests, the bioengineered hydrogels showed superior adhesive properties (i.e. adhesion strength, burst pressure, and lap shear) compared to commercial surgical adhesives, Evicel ® and CoSEAL®. In vitro cell studies also showed that engineered hydrogels were cytocompatible with corneal keratocytes (corneal fibroblast cells) and promoted cell integration after application. Finally, we applied the hydrogel precursor to the half-thickness corneal stromal defects in a rabbit model and photopolymerized it with visible light for 4 min to form a highly adhesive hydrogel (Figure 1). In vivo experiments in rabbits showed that adhesive hydrogels could effectively seal corneal defects and form a transparent adhesive gel with a smooth surface (Figure 1). In situ photopolymerization of bioadhesives facilitated easy delivery to the cornea, and allowed for curing of the bioadhesive exactly according to the required geometry of the tissue to be sealed, which is an advantage over pre-formed materials, as e.g., scaffolds or sheets. Conclusion In this study, we synthesized photocrosslinkable gelatin-based hydrogels that possessed superior physical and adhesion properties compared to commercially-available alternatives. In addition, the adhesive hydrogels showed high cytocompatibility in vitro using keratocytes. In vivo experiments in rabbits showed that the adhesive hydrogels could effectively seal corneal defects. Overall, our results proved that the bioengineered GelCORE hydrogels may constitute an effective strategy to be used for corneal sealing and regeneration.