MECHANICAL PROPERTIES OF A PHOTOPOLYMERIZABLE HYDROGEL FOR INTERVERTEBRAL DISC REPLACEMENT

We report on the modeling and experimental validation of a photopolymerizable hydrogel for the replacement of the interior of the intervertebral disc (so called Nucleus Pulposus). The hydrogel is initially injected in its liquid form and then photopolymerized inside via a small catheter. The light necessary for the photopolymerization is constrained to a small light guide to keep the surgical procedure as minimally invasive as possible. During polymerization, the material's absorption and scattering coefficients change and directly influence local polymerization rates and hence the mechanical properties. Quantitative scattering and absorption values as well as monomer conversion rates of the hydrogel sample were validated using a Monte Carlo model for photopolymerization. By controlling the input light pattern, local material properties can be engineered, such as elastic modulus and swelling ratio to match the set of requirements for the implant. Experiments were conducted by polymerizing a hydrogel in a column-like volume using an optical fiber for light delivery. Quantitative scattering and absorption values as well as monomer conversion rates of the hydrogel sample were validated using a newly established Monte Carlo model for photopolymerization. The results were used to study and predict 3D polymerization patterns for different illumination configurations. Swelling ratio and elastic modulus were measured as a function of monomer conversion. Preliminary results on hydrogel fatigue tests in an in-vitro bovine disc will be shown.