Description
The objective of this research was to first fundamentally understand the significance and contribution of the microstructure of the nucleus pulposus of the intervertebral disc on the functionality and mechanical behavior of the nucleus and the entire disc. In turn, this understanding was used to synthesize a surrogate composite material that mimics the characteristics of the natural nucleus pulposus. The motivation for these specific objectives is the association of lower back pain and the degeneration of the intervertebral disc, which is a major societal and economic problem in industrialized societies. Of specific focus to this research is the nucleus pulposus due to its vital physiological and mechanical functionalities to the entire motion segment. The nucleus consists of short, discontinuous collagen and elastin fibers (~15%) that are suspended in aggrecan matrix with 80% of its wet-weight as water. Previously, nucleus pulposus replacement attempts have not considered the inhomogeneity and anisotropy of the native nucleus pulposus, and therefore do not accurately mimic the biomechanics of the intervertebral disc. Therefore, a compilation of the range nucleus pulposus reported data was presented. Past literature has reported an aggregate modulus, Ha, that spans 3 orders magnitudes, from 16–6000 kPa. Additionally, the elastic modulus, E, spanned 3.25–202 kPa. The investigation into this discrepancy was proceeded by constructing new synthetic materials consisting of alginate/polyacrylamide hydrogels with the inclusion of chopped Eglass fibers. The viscoelastic properties were then tested and compared to the results of those published for the natural nucleus pulposus. To construct hydrogels with mechanical characteristics the spanned the natural nucleus discrepancy, these alginate/polyacrylamide hydrogels were cross-linked by five different multivalent cations. Characterization of these hydrogels was performed by dynamic submersion compression testing, with a frequency sweep ranging from 0.1 to 10 Hz. It was found that the inclusion of E-glass fibers increased the storage modulus by 79% on average across the frequency sweep. The results of this research can be used to transform the design methodologies for the next generation of disc replacement technologies by more accurately mimicking the natural functioning of the intervertebral disc.
