Synthesis and characterization of injectable bioadhesive hydrogels for nucleus pulposus replacement and repair of the damaged intervertebral disc.

Bioadhesive polymers are natural or synthetic materials that can be used for soft tissue repair. The aim of this investigation was to develop an injectable, bioadhesive hydrogel with the potential to serve as a synthetic replacement for the nucleus pulposus of the intervertebral disc or as an annulus closure material. Branched copolymers of poly(N-isopropylacrylamide) (PNIPAAm) and poly(ethylene glycol) (PEG) were blended with poly(ethylene imine) (PEI). This three component injectable system can form a precipitated gel at physiological temperature due to the phase transition of PNIPAAm. The injection of glutaraldehyde into the gel core will adhere the implant to the surrounding tissues. (1)H NMR results indicated the successful physical incorporation of PEI into the PNIPAAm-PEG network by blending. In addition, the covalent crosslinking between the amine functionalities on the PEI and the aldehyde functionalities on the glutaraldehyde was verified using FTIR difference spectroscopy. Mechanical characterization of these blends showed a significant increase (p < 0.05) in compressive modulus following glutaraldehyde injection. The in vitro bioadhesive force studies with porcine skin showed a significant increase (p < 0.05) in the mean maximum force of detachment for PNIPAAm-PEG/PEI gels when glutaraldehyde was injected into the gel core. The results of this study indicate that the reactivity between amines and aldehyde functionalities can be exploited to impart bioadhesive properties to PNIPAAm-PEG/PEI copolymers.