Fabrication of Bioactive β-Si3N4 Scaffolds by in-situ reactive foaming during alkali activation

Low-cost glass-ceramic scaffolds for bone repair remain an open challenge: despite decades of development since 45S5 Bioglass, such materials still exhibit inadequate mechanical and fracture performance. The recent demonstration of Si 3 N 4 biocompatibility has prompted extensive investigation of bioactive Si 3 N 4 based composite scaffolds, but the high sintering temperatures these require devitrify the bioactive glass phase and compromise the very bioactivity sought. Here we report an eco-sustainable alternative route in which β- Si 3 N 4 powder is chemically tailored by in-situ alkali activation and calcined under mild conditions (750-900°C), with pre-existing elemental Si impurity acting as the foaming agent. The resulting 750°C scaffolds combine a compressive strength of 6 MPa with a total porosity of 65 %, of which 46 ± 14 % of the pore volume resides in pores larger than 100 µm - the threshold associated with vascular invasion and bone ingrowth. XRD revealed the formation of combeite at 750°C and its decomposition at 900°C, and immersion in simulated body fluid confirmed the bioactivity of the combeite-bearing matrix. Calcination temperature had little effect on total porosity but markedly altered its distribution, identifying 750°C as the condition that preserves both the bioactive phase and the macropores architecture. This low-temperature route offers a green and accessible pathway to mechanically competent bioactive scaffolds.