Evolution of silicate bioglass particles as porous microspheres with a view towards orthobiologics

Although FDA approved and clinically utilised, research on 45S5 Bioglass® and S53P4 including other bioactive glasses continues in order to advance their applicability for a range of alternate applications. For example, rendering these particles porous would enable incorporation of varying biological payloads (i.e. cells, drugs and growth factors) and making them spherical would enhance their flow properties enabling delivery to target sites via minimally invasive injection procedures. This paper reports on the manufacture of solid (non-porous; SGMS) and highly porous microspheres (PGMS) with large external pores and fully interconnected porosity from bioactive silicate glass formulations (45S5 and S53P4) via a single stage flame spheroidisation process and their physicochemical properties including in vitro biological response. Morphological and physical characterisation of the SGMS and PGMS revealed interconnected porosity up to 65 ± 5%. Mass loss studies comparing between SGMS and PGMS revealed 1.5 times higher mass loss for the PGMS over 28 days. Also, in vitro bioactivity studies using simulated body fluid (SBF) revealed hydroxyapatite (HA) formation at earlier time point for PGMS compared to their SGMS counterparts (i.e day 1 for PGMS and day 3 for SGMS of 45S5). In addition, HA layers were also formed in cell culture media, with the exception of SGMS of 45S5, which revealed CaP formation with a ratio of 1.52-1.78. Direct cell seeding and indirect cell culture studies (via incubation with microsphere degradation products) revealed mouse 3T3 cells were able to grow and undergo osteogenic differentiation in vitro, confirming cytocompatibility of both 45S5 and S53P4 SGMS and PGMS. More importantly and especially for orthobiologic applications, cells were observed to have migrated within the pores of the PGMS. As such, the PGMS developed from these bioactive silicate glasses are highly promising candidate materials for orthobiologics and alternate applications requiring delivery of biologic payloads.