Material properties in unconfined compression of human nucleus pulposus, injectable hyaluronic acid-based hydrogels and tissue engineering scaffolds

Abstract

Surgical treatment for lower back pain related to degenerative disc disease commonly includes discectomy and spinal fusion. While surgical intervention may provide short-term pain relief, it results in altered biomechanics of the spine and may lead to further degenerative changes in adjacent segments. One non-fusion technique currently being investigated is nucleus pulposus (NP) support via either an injectable hydrogel or tissue engineered construct. A major challenge for either approach is to mimic the mechanical properties of native NP. Here we adopt an unconfined compression testing configuration to assess toe-region and linear-region modulus and Poisson's ratio, key functional parameters for NP replacement. Human NP, experimental biocompatible hydrogel formulations composed of hyaluronic acid (HA), PEG-g-chitosan, and gelatin, and conventional alginate and agarose gels were investigated as injectable NP replacements or tissue engineering scaffolds. Testing consisted of a stress-relaxation experiment of 5% strain increments followed by 5-min relaxation periods to a total of 25% strain. Human NP had an average linear-region modulus of 5.39 +/- 2.56 kPa and a Poisson's ratio of 0.62 +/- 0.15. The modulus and Poisson's ratio are important parameters for evaluating the design of implant materials and scaffolds. The synthetic HA-based hydrogels approximated NP well and may serve as suitable NP implant materials.

Fig. 1

Unconfined compression testing configuration consisting of optically translucent testing surface with space for mounting of digital camera

Fig. 2

a Schematic of stress-relaxation experiment, consisting of incremental steps of 5% strain followed by a 5-min relaxation period to a total of 25% strain. b Average equilibrium stress–strain curves produced using mean A and B values curve fit according to the equation c Representative linear regression for calculation of Poisson’s ratio

Fig. 3

Comparison of material properties in unconfined compression. NP nucleus pulposus, A–C three experimental hyaluronic acid-based hydrogels, alg 2.0% medium viscosity alginate, aga 2.0% med viscosity agarose. a Toe modulus calculated at 0% strain, b linear region modulus calculated at 20% strain, c Poisson’s ratio, d relaxation (%) *significantly different than human NP (P < 0.05)