Effect of Physicochemical Surface Properties of Silicon-Substituted Hydroxyapatite on Angiogenesis

Synthetic hydroxyapatite (HA) is a widely studied bioceramic for bone tissue engineering (BTE) due to its similarity to the mineral component of bone. As bone mineral contains various ionic substitutions that play a crucial role in bone metabolism, the bioactivity of HA can be improved by adding small amounts of physiologically relevant ions into its crystal structure, with silicate-substituted HA (Si-HA) showing particularly promising results. Nevertheless, it remains unclear how distinct material characteristics influence the bioactivity due to the intertwined nature of surface properties. A coculture methodology was optimized and applied for in vitro quantification of the biological response. Initially, HA and Si-HA samples were produced and characterized. To compare the bioactivity of the samples, a method was developed to measure interactions in an increasingly complex environment, first including fibronectin (FN) adsorption and subsequently cell adhesion in mono and coculture using primary human osteoblasts (hOBs) and human dermal microvascular endothelial cells (HDMECs), with and without FN precoating. An experimental set-up was designed to assess to what extent different surface features of the samples contribute to the induced biological response. An 8-nm gold sputter coating was applied to eradicate the electrochemical differences and polishing and abrading was used to reduce the differences in surface topographies. Overall, 1.25 wt% Si-HA exhibited most nanoscale variations in surface potential. In terms of bioactivity, 1.25 wt% Si-HA samples induced the highest osteoblast attachment and vessel formation. Additionally, in vitro vessel formation was established on Si-HA surfaces using a hOB:HDMEC cell ratio of 70:30 and a methodology was established that enabled the assessment of the relative effect of topographical and electrochemical features induced by silicon substitution in the HA lattice on their bioactivity. It was found that the difference in the amount of protein attached to HA and 1.25 wt% Si-HA after 2 h was affected by topographical differences. Conversely, electrochemical differences induced different vessel-like structure formation in coculture with a FN precoating. Without an FN precoating, both topographical and electrochemical differences dictated the differences in angiogenic response. Overall, 1.25 wt% Si-HA surface features appear to induce the most favorable protein adsorption and cell adhesion in mono and coculture with and without FN precoating.