Welcome to the Glass Age

49 and assemble as covalent bonds form a gel. The gel is usually heated to drive off the water, which leaves a nano porous glass of large surface area. As the sol-gel process does not involve melting, sodium, which lowers the glass melting temperature, is not required in the composition, so the bioactive glass compositions can be more simple [8]. Foamed sol-gel and gel-cast foam scaffolds both result in spherical pores connected by circular interconnects or pore windows. Those ‘windows’ are key for 3D bone ingrowth. Glass powders can be 3D-printed if they are mixed with a binder, or carrier gel, that can undergo shear thinning [9]. The grid-like 3D structure can provide high strength in compression, similar to that of dense bone, while maintaining pore channels suitable for vascularised bone ingrowth (Figure 3.6b). Direct comparison of bone regeneration in foam and 3D-printed scaffolds was performed in a rabbit model. The pore window (interconnect diameter or channel width) were matched, at just over 100 µm, which meant the foam had a much higher total porosity than the printed scaffold (Figure 3.6a,b). At 11 weeks in vivo, the foam scaffold had biodegraded and the defect filled with new bone (Figure 3.6c), while the 3D-printed scaffold remained (Figure 3.6d). However, the bone growing through the 3D-printed scaffold was of higher density and could therefore be considered of higher quality, while the rapidly formed bone in the defect with the foam implant was similar to that of the control. A degradation time of 11 weeks may be too rapid in a human patient, so the 3D-printed scaffolds may be preferred. Despite their promise, porous bioactive glass bone scaffolds have yet to make it to clinical use. Perhaps surprisingly, bioactive glass has had an even greater impact in consumer healthcare. The largest commercial use of bioactive glass, and perhaps any bioactive biomaterial, is in toothpaste designed to treat hypersensitivity of teeth (Figure 3.7a). While many of us know the sharp pain one can feel when biting into cold ice Figure 3.6. Comparing bone regeneration in a rabbit between foam and 3D-printed bioactive glass scaffolds of similar pore channel sizes; scanning electron microscopy images: (a) interconnected pores in the glass foam, scale bar = 100 µm, (b) pore channels in the 3D-printed scaffold, scale bar = 100 µm; (c,d) new bone formation at 10 weeks after implantation of (c) the foam and (d) the 3D-printed scaffold (scale bar = 1 mm). The red boxes show examples of new bone formation. Source: Modified from Shi et al. [10].