Bioactive Glass-Ceramic Scaffolds from Novel ‘Inorganic Gel Casting’ and Sinter-Crystallization

Traditionally, bioactive glasses have been used to fill and restore bone defects. More recently, this category of biomaterials has become an emerging research field for bone tissue engineering applications. Here, we review and discuss current knowledge on porous bone tissue engineering scaffolds on the basis of melt-derived bioactive silicate glass compositions and relevant composite structures. Starting with an excerpt on the history of bioactive glasses, as well as on fundamental requirements for bone tissue engineering scaffolds, a detailed overview on recent developments of bioactive glass and glass-ceramic scaffolds will be given, including a summary of common fabrication methods and a discussion on the microstructural-mechanical properties of scaffolds in relation to human bone (structure-property and structure-function relationship). In addition, ion release effects of bioactive glasses concerning osteogenic and angiogenic responses are addressed. Finally, areas of future research are highlighted in this review.

A 3D scaffold has been developed that has the potential to fulfil the criteria for an ideal scaffold for bone tissue engineering. Sol-gel derived bioactive glasses of the 70S30C (70 mol% SiO2, 30 mol% CaO) composition have been foamed to produce 3D bioactive scaffolds with hierarchical interconnected pore morphologies similar to trabecular bone. The scaffolds consist of a hierarchical pore network with macropores in excess of 500 microm connected by pore windows with diameters in excess of 100 microm, which is thought to be the minimum pore diameter required for tissue ingrowth and vasularisation in the human body. The scaffolds also have textural porosity in the mesopore range (10-20 nm). The scaffolds were sintered at 600, 700, 800 and 1000 degrees C. As sintering temperature was increased to 800 degrees C the compressive strength increased from 0.34 to 2.26 MPa due to a thickening of the pore walls and a reduction in the textural porosity. The compressive strength is in the range of that of trabecular bone (2-12 MPa). Importantly, the modal interconnected pore diameter (98 microm) was still suitable for tissue engineering applications and bioactivity is maintained. Bioactive glass foam scaffolds sintered at 800 degrees C for 2 h fulfill the criteria for an ideal scaffold for tissue engineering applications.