Advancing peptide hydrogels for intervertebral disc repair

Lower back pain is often caused by degeneration of the intervertebral disc and has a significant socioeconomic cost. Current treatments are limited in terms of their clinical success. Nucleus augmentation is being investigated as a potential treatment for degenerated discs with the aim of restoring the biomechanical function of the disc. A nucleus augmentation material must be delivered minimally invasively, restore the mechanical properties and be biocompatible. Selfassembling peptides have been previously shown to form hydrogels with a range of potential mechanical properties and therefore can be designed to have material properties suitable for nucleus augmentation. The aim of this work was to build upon existing self-assembling peptides with an overall charge of +2 mixed with a glycosaminoglycan (GAG) for nucleus augmentation. By changing the terminal amino acids between glutamine and serine, three peptides were used to investigate the effect of hydrogen bonding on selfassembly. The glutamine amino acids were able to form more and stronger hydrogen bonds that reduced the critical concentration for self-assembly. These differences in self-assembly were shown to affect the hydrogel lifetimes under passive diffusion and during cyclic compression testing. Rheology was used to assess the effect of the terminal amino acids on the mechanical properties of the hydrogels as well as the effect of delivery down minimally invasive needles. The different peptide-GAG hydrogels resulted in a range of mechanical properties suitable for nucleus augmentation. Injection down a needle had little to no effect on the mechanical properties of the hydrogel. Electron microscopy was used to image the fibrous networks of the hydrogels in different states. Cryo-focused ion beam scanning electron microscopy was used to create a 3D image of the fibres. Finally, cytotoxicity assays were used to assess different components of the hydrogels. There was some slight cytotoxicity associated with the soluble components of the peptides, however the hydrogels were not cytotoxic as biomaterials. The slight cytotoxicity was reduced by changing the counterion. Overall, the effect of hydrogen bonding on self-assembly was controlled by using glutamine and serine amino acids and analysed using a variety of multidisciplinary techniques. The peptides met the criteria outlined for nucleus augmentation devices and present a realistic and viable option for a clinically translatable treatment.