45S5 Bioglass® is a class A bioactive glass (45S5BG - 45 wt.% SiO2, 24.5 wt.% CaO, 24.5 wt.% Na2O and 6 wt.% P2O5 glass forming system) [1] capable of inherently bonding to human bone [2] and soft tissue [3] in physiological media. The proposed mechanism [4, 5] for bond formation includes five material-based sequences and six steps on the cellular level, whereby the release of soluble ion species from the glass and the formation of a surface apatite play a major role: the release of sodium and calcium leads to an increase of local pH and thus a highly porous silica gel layer on top of the glass (an ideal adsorptive for growth factors and stem cells) is formed by hydrolysis; released water-soluble ion species are genetically active on the cellular level and mitosis of osteoprogenitor cells [6] and angiogenesis are stimulated [7]; in vicinity of glass surface a supersaturation with respect to carbonate hydroxyapatite is being built up and crystallization takes place. The formation and crystallization of this defective hydroxy-carbonate surface layer (very similar to inorganic parts in human bone) is a prerequisite for bioactivity and bonding to tissue [8]. It is recognized that the described mechanism relays on ion-exchange with the surrounding medium by dissolution and precipitation mechanisms.
Although several studies have indicated enhanced bioactivity of bioactive glasses on the nanoscale (see e.g. [9-13]), the currently applied bioglass particulates (from conventional glass melting routes) typically exhibit particle sizes of several tens to hundred micrometers. Up to now, bottom-up processing routes (e.g. sol-gel processing [14] and flame spray pyrolysis[15]) are used for the production of nanoscaled bioglasses (nBG). Given the potential disadvantages of bottom-up synthesis routes (e.g. toxic precursors, costly, limited in composition), top-down processing by wet grinding in stirred media mills (under controlled conditions) offers an attractive alternative towards submicron (bioactive) materials [12; 16-22].
In the present communication, wet comminution of 45S5BG in stirred media mills is discussed. It was found that the in vitro bioactivity and biocompatibility of 45S5BG after processing in n-pentanol increased; in addition, the mass specific surface area increased by a factor of 30 with respect to the feed material. The resulting product exhibits a flake-like morphology. After processing in water, bioactivity is lost entirely and spongy porous products are obtained. The mechanisms leading to the different products are briefly discussed.