Degenerative disc disease (DDD) is caused by the damage or dehydration of the nucleus pulposus, which reduces the hydrostatic pressure on the internal surface of the annulus fibrosis. This results in abnormal compressive stress on  the intervertebral disc, causing tears, cracks and fissures in the annular tissues after repeated loads. This can manifest in back pain as the result of the nucleus migrating through the annulus and impinging on nerve roots. The current treatments for DDD provide only temporary relief of back pain with no restoration of biomechanical function. Investigators have begun to realize the potential benefits of replacing the damaged or dehydrated nucleus with a synthetic material, such as a hydrogel.

Using an in situ forming hydrogel could have important clinical consequences because it can be injected non-invasively using a small gage needle. One way to achieve in situ gel formation is using a thermo-responsive polymer that forms a free flowing solution in water at ambient temperatures and a gel at body temperature, such as N-isopropylacrylamide (PNIPAAm). Below the PNIPAAm LCST (ranging from 29-34ºC), PNIPAAm is hydrophilic, allowing it to form a miscible solution with water. Above this transition temperature, the polymer becomes hydrophobic, so the water and polymer separate, forming a compact gel. PNIPAAm-PEG copolymers that were either all branched or all tethered indicated trends toward higher swelling ratios and showed static mechanical behavior in the range suitable for restoring biomechanical function.