Optical Properties of Gelatin Methacrylate and Implications for In Situ Cross-Linking in Spinal Cord Lesions and EngineeredHydrogel Systems

Understanding how light interacts with biomaterials is critical not only for therapeutic delivery in vivo but also for enabling emerging light-based fabrication methods such as volumetric bioprinting. In particular, optical characterization of photosensitive hydrogels like Gelatin Methacrylate (GelMA) provides foundational data for additive manufacturing of complex, tissue-engineered constructs with spatially controlled architecture. GelMA is a promising candidate for use as an injectable hydrogel for spinal cord injury (SCI) repair. This is primarily due to its highly tunable properties, allowing it to mimic tissue. Additionally, GelMA can be optically crosslinked. Optical simulations of this material within the spinal cord can inform whether the cross-linking of this material is feasible and uniform within a particular injury with a chosen exposure paradigm. These simulations require the optical properties of the tissues and materials of interest. However, the optical properties of GelMA have not been studied. The optical properties of GelMA were measured using a double integrating sphere setup and modelled with inverse adding doubling. We measured the absorption coefficient ({micro}a [mm-1]), reduced scattering coefficient ({micro}s[mm-1]), and scattering anisotropy (g [unitless]) for GelMA hydrogels, and examined the effect on these properties when magnetically alignable microstructures were suspended in the hydrogel. We conducted Monte Carlo simulations using these optical properties to determine whether the GelMA hydrogels would be cross-linked under different illumination conditions within a representative spinal cord injury geometry. The depth of hydrogel cross-linking for this representative spinal cord injury geometry was experimentally validated. Overall, it was found that GelMA without rods has an {micro}a = 0.516 {+/-} 0.025 mm-1, with insignificant scattering at 450 nm, GelMA with rods has an {micro}a = 0.588 {+/-} 0.018 mm-1, {micro}s = 0.196 {+/-} 0.024 mm-1, and g = 0.906 {+/-} 0.014 at 450 nm. Additionally, it was observed that in fluid-filled lesions, external illumination is often sufficient to achieve uniform gel curing. In contrast, fibrotic lesions require an intraspinal approach to adequately deliver light throughout the lesion, although the dose distribution will be nonuniform. Finally, experimental results confirmed the simulation findings, showing that when exposing GelMA to 63.29 mW/cm2 of 450 nm light, curing is uniform within the first 4 mm of gel and worsens with depth. These findings support the conclusion that larger volumes of photosensitive GelMA can be feasibly cured within the porcine spinal cord, but an intraspinal exposure will likely be required within human spinal tissue. These findings not only support the feasibility of in situ photopolymerization for SCI repair, but also provide critical optical parameters and validation approaches relevant to bioprinting systems.