Stability and Mechanical Properties of Injectable Hydrogels for Nucleus Pulposus Replacement
Authors:   , Maier Elyse Anne , Lowman Anthony M. 
Published: July 2024

Many people, young and old, are affected by lower back pain in their daily lives. Missed workdays and the strain on healthcare are prominent societal issues that stem from this cause. A large percentage of those suffering's pain can be said to originate from a degenerating disc in their lumbar spine. The intervertebral disc is made up of the Nucleus Pulposus (NP), a soft gel-like materiaL, which is surrounded and restrained by the Annulus Fibrosus (AF). Degeneration of this disc, which can lead to pain, loss of mobility, and ultimately highly invasive surgery, begins with the degeneration of the NP. Nucleus replacement is an attractive option because of its percutaneous approach that is researched for use in early stages of degeneration, where an intact or minimally damaged annulus is present. It involves removing the degenerated nucleus so that an implant material can be injected into the cavity. In this work, a novel system is proposed for a hydrogel that can be injected into the de-nucleated cavity as a viscous solution and fill it completely to form a solid hydrogel implant that has the ability to restore the normal biomechanics of the spine. A system of 150/PEG, 20% PVA/PVP, and 65% H20 was found to have desirable mechanical properties for NP replacement, where a range of 0.05- 1.5 MPa is suitable for the application. Furthermore, the modulus could be increased through freeze/thawing. The incorporation of PEG was found to favorably affect the solubility of the solutions and enabled gelling during cooling because it was found to expel water from solution as the temperature was lowered. These gels were equilibrated to the osmotic pressure of a healthy NP using a 0.12 MPa osmotic PEG solution and were found to be stable for the duration of the 35 days of observation. Lastly, it was desired to determine if the 0.12 MPa osmotic PEG solution accurately replicated the environment of the spine. Samples were observed for 48 hours at 37 C in 0.12 MPa PEG, PHS, and 20 wt% Dextran solutions as well as in de-nucleated cavities of calf spines. Of these solutions, 0.12 MPa PEG solution was found to most closely replicate the environment of the spine. Furthermore, the implants' responses to the natural tissue were analyzed qualitatively and were found to maintain excellent contact with the surrounding tissue with no warping or shriveling observed. After 48 hours at 37 C, 0.12 MPa PEG conditioning drove the freeze/thaw cycled samples to an approximate compressive modulus of 0.65 MPa and the non-freeze/thaw cycled samples to approximately 0.25 MPa, both calculated between 10 and 20% strain. Through the incorporation of PEG into the hydrogel network, an injectable implant that exhibited desirable mechanical properties stabilityilty was found. The addition of not nol only achieved the desired modulus, but also allowed highly concentrated solutions to be created through autoclaving at high temperature, which promogellingllng upon cooling. This changesolubilityilty through changes in temperature and the addition of PEG achieved the goal of an implant that could be injected as a solution and form a solid cohesive hydrogel within the annular restraint, which alleviates concerns of expulsion through the annulus.