Design and synthesis of proteoglycan analogues for tissue repair and regeneration

This thesis is concerned with the design and synthesis of a novel, injectable proteoglycan analogue for tissue repair. This is of particular relevance to the restoration of disc height to a degraded nucleus pulposus of the intervertebral disc. The focus is on the use of sulfonate monomers as proteoglycan analogues, in particular sodium 2-acrylamido-2-methylpropane sulfonic acid and the potassium salt of 3-sulfopropyl acrylate. For most biomedical applications, synthetic hydrogels need to show dimensional stability to changes in pH, osmolarity, and temperature. This is readily achieved by neutral structures however ionic sulfonate containing hydrogels are responsive to environmental change which renders them difficult to manage in most tissue replacement applications. In this case osmotic responsiveness rather than stability is desirable. Therefore sulfonate based materials possess advantageous properties. This is a result of the sulfonate becoming an ideal surrogate for the sulfate group present within the structure of natural proteoglycans. This thesis reports polymerisation studies based on the production of a redox initiated copolymer system capable of polymerising in situ within a timescale of circa. 5-7 minutes. The rheological properties, osmotic drive, and residual monomer content of successful compositions is analysed. Properties are adapted to mimic those of the target natural tissue. The adaptation of the material for use as an injectable intra-ocular lens, with hyaluronic acid as an interpenetrate is reported. The synthesis of a radiopaque macromer to allow visibility of the repair system once in situ is investigated and discussed. The results presented in this thesis describe a suitable proteoglycan tissue analogue which is injectable, biomimetic, osmotically responsive and mechanically stable in its desired application

Bioactive Nanocomposites for Tissue Repair and Regeneration: A Review

This review presents scientific findings concerning the use of bioactive nanocomposites in the field of tissue repair and regeneration. Bioactivity is the ability of a material to incite a specific biological reaction, usually at the boundary of the material. Nanocomposites have been shown to be ideal bioactive materials due the many biological interfaces and structures operating at the nanoscale. This has resulted in many researchers investigating nanocomposites for use in bioapplications. Nanocomposites encompass a number of different structures, incorporating organic-inorganic, inorganic-inorganic and bioinorganic nanomaterials and based upon ceramic, metallic or polymeric materials. This enables a wide range of properties to be incorporated into nanocomposite materials, such as magnetic properties, MR imaging contrast or drug delivery, and even a combination of these properties. Much of the classical research was focused on bone regeneration, however, recent advances have enabled further use in soft tissue body sites too. Despite recent technological advances, more research is needed to further understand the long-term biocompatibility impact of the use of nanoparticles within the human body

Using click chemistry to dial up the modulus of doubly crosslinked microgels through precise control of microgel building block functionalisation

Doubly crosslinked microgels (DX MGs) are hydrogels constructed by covalently interlinked vinyl-functionalised microgel particles. Until now it has not been possible to precisely control the extent of vinyl functionalisation of the microgel (MG) particles which act as the colloidal building blocks for hydrogel assembly. Furthermore, the range of DX MGs prepared to date has been modest. This study addresses both of these challenges by constructing a new class of DX MG using MG particles that were vinyl functionalised by copper catalysed azide–alkyne cycloaddition (CuAAC). Here, poly(2-vinylpyridine-co-propargyl acrylate) (PVP-PA) MG particles were prepared and vinyl functionalised by CuAAC using azidopropyl methacrylate (AZPMA) to give PVP-PAx–PMAy MGs. Values for the extent of vinyl functionalisation (y) were varied from 0 to 7.0 mol% in a precisely controlled manner. Concentrated dispersions were transformed from fluids to physical gels at pH values of less than or equal to 3.0 and covalently inter-linked to give PVP-PAx–PMAy DX MGs by free-radical coupling of the vinyl groups of neighbouring particles. The storage modulus of the DX MGs varied linearly with both MG concentration and the value for y. Not only did the new DX MGs studied here enable precise control of MG functionalisation and DX MG mechanical properties, they also showed evidence of colloidal crystallinity which may lead to future photonic gel applications. Our CuAAC-based approach should be versatile and is expected to enable a range of new DX MGs to be prepared

Factors Affecting Peptide Interactions with Surface-Bound Microgels

Effects of electrostatics and peptide size on peptide interactions with surface-bound microgels were investigated with ellipsometry, confocal microscopy, and atomic force microscopy (AFM). Results show that binding of cationic poly-l-lysine (pLys) to anionic, covalently immobilized, poly(ethyl acrylate-co-methacrylic acid) microgels increased with increasing peptide net charge and microgel charge density. Furthermore, peptide release was facilitated by decreasing either microgel or peptide charge density. Analogously, increasing ionic strength facilitated peptide release for short peptides. As a result of peptide binding, the surface-bound microgels displayed pronounced deswelling and increased mechanical rigidity, the latter quantified by quantitative nanomechanical mapping. While short pLys was found to penetrate the entire microgel network and to result in almost complete charge neutralization, larger peptides were partially excluded from the microgel network, forming an outer peptide layer on the microgels. As a result of this difference, microgel flattening was more influenced by the lower Mw peptide than the higher. Peptide-induced deswelling was found to be lower for higher Mw pLys, the latter effect not observed for the corresponding microgels in the dispersed state. While the effects of electrostatics on peptide loading and release were similar to those observed for dispersed microgels, there were thus considerable effects of the underlying surface on peptide-induced microgel deswelling, which need to be considered in the design of surface-bound microgels as carriers of peptide loads, for example, in drug delivery or in functionalized biomaterials