The development of pNIPAM-Laponite® hydrogels for dental applications

Abstract

The impact of chemical composition on the mechanical and physical properties of Laponite® crosslinked pNIPAM hydrogels was investigated, and the implications of these changes on cell viability and behaviour were explored. A range of crosslink density was used, and two different hydrophilic comonomers (HEMA and HPMA) were incorporated into the reaction mixture at different ratios. Additionally, water dispersed hydroxyapatite nanoparticles (HAPna) was added to the hydrogel at a concentration of 0.5mg/ml, to promote osteogenic differentiation of hMSCs. Different compositions of novel pNIPAM based Laponite® hydrogels, were synthesised and characterised through this thesis. Structural properties of the hydrogels were studied using SEM, and the influence of chemical composition on pore size was elucidated. The implications of structural changes on a host of properties including mechanical robustness and other physical behaviour such as swelling and deswelling ratios were observed. The physical water state, water diffusivity and water/polymer interactions within the hydrogels were explored by monitoring dehydration and rehydration behaviour, using ATR-FTIR. Cell viability within a range of chemical compositions of the hydrogels, was performed using Alamar blue assay, and histological and immunohistochemistry investigations were used to evaluate type of matrix deposition within the hydrogels. Crosslink density was found to be a crucial factor to control, where changes in crosslink density influenced physical and mechanical properties of the hydrogels. Our findings showed increased crosslink density, lead to stiffer hydrogels as a result of a smaller pore size. Smaller pore size was also found to retard dehydration and rehydration rates. Furthermore, the free water fraction was reduced due to a decrease in pore size when crosslink density increased. In addition, cell viability varied as a result of using different crosslink density. Incorporation of HEMA as a comonomer increased the pore size of Laponite® crosslinked pNIPAM hydrogels and hydrogel stiffness was reduced. The hydrophilicity of the hydrogel was increased when HEMA was added, and accordingly the dehydration rate decreased and rehydration rate increased, as the HEMA ratio was increased in systematic manner. Toxicity to hMSCs was observed needing further investigation to fully understand the mechanism. Adding HPMA increased median pore size and decreased the mechanical stiffness of L-pNIPAM-co-HPMA hydrogels. The influence of HPMA on the hydrophilicity of the hydrogel was explored using water contact angle measurements. Dehydration and rehydration observations showed the influence of conflicting factors, including pore size, surface area of hydrogel droplet and hydrophilicity to affect dehydration and rehydration rates. L-pNIPAM90-co-HPMA10 (1) showed a significant increase (P<0.05) in cell viability of hMSCs when compared to L-pNIPAM (1). In addition, matrix deposition resembling that of cells undergoing osteogenic differentiation was promoted within L-pNIPAM90-co-HPMA10 (1). When HAPna was added to L-pNIPAM90-co-HPMA10 (1) the mechanical properties were enhanced, the pore size was reduced and further increases of matrix deposition resembling that of cells undergoing osteogenic differentiation were observed.