We report on the structural transformations of Bioglass ® during thermal treatments. Just after the glassy transition, at 550 °C, a glassy phase separation occurs at 580 °C, with the appearance of one silicate-and one phosphate-rich phase. It is followed by the crystallization of the major phase Na 2 CaSi 2 O 6 , from 610 to 700 °C and of the secondary phase, silicorhenanite, at 800 °C. The latter evolves from the phosphate-rich glassy phase, which is still present after the first crystallization. In order to control the processing of glass-ceramic products from Bioglass ® , crystallization kinetics were studied via differential scanning calorimetry measurements in the range of 620-700 °C and temperature-timetransformation curves were established.

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In order to evaluate whether human osteoblastic cells differentiate normally on hydroxyapatite, we have compared the adhesion, proliferation, and differentiation of human trabecular (HT) osteoblastic cells on synthetic-dense hydroxyapatite and on standard plastic culture. We show here that initial HT cell attachment was 4-fold lower on hydroxyapatite than on plastic after 4 h of culture, and that normal cell attachment on hydroxyapatite was restored after 18 h of culture. HT cell proliferation was similar on the two substrates at 2-8 days of culture, but was lower on hydroxyapatite compared to plastic after 15 and 28 days of culture, as evaluated by DNA synthesis or cell number. HT cells cultured on both substrates produced an abundant extracellular matrix which immunostained for Type I collagen. The levels of carboxyterminal propeptide of Type I procollagen (P1CP) in the medium were lower in HT cell cultures on hydroxyapatite than on plastic. In addition, (3H)-proline incorporation into matrix proteins and the mean thickness of matrix layers were 52% and 26% lower, respectively, on hydroxyapatite compared to plastic after 4 weeks of culture, indicating that the total collagenous matrix synthesized by HT cells was lower on hydroxyapatite. However, (3H)-proline and calcium uptake expressed per cell was higher on hydroxyapatite than on plastic. The results show that human osteoblastic cells attach, proliferate, and differentiate on dense hydroxyapatite with a sequence similar to that of plastic. However, the growth of human osteoblastic cells is lower on hydroxyapatite in long-term culture, which results in a reduced amount of extracellular matrix, although matrix production per cell may be increased.

Polycrystalline hydroxyapatite (HAP) ceramics were densified by hot pressing. The effects of thermal treatments and of a sintering additive (Na3PO4) on the microstructure, flexural strength and fracture toughness were investigated. Hot pressing without additive resulted in dense HAP having a Small average grain size (below 0.5 pm). Spontaneous microcracking of the material was also noted. This originated from the thermal expansion anisotropy of HAP crystals. The presence of the sintering aid promotes grain growth. Dense materials exhibited mechanical properties depending on the microstructure. The highest values obtained were 137 M Pa and 1.2 M Pa~/~ for the flexural strength and fracture toughness, respectively. A decrease of both strength and toughness was observed with increasing average grain size. This behaviour is attributed to the weakening of the grain boundaries by either the development of initial microcracking or the Na3P04 addition. It is concluded that hot pressing is very useful to elaborate dense HAP having good mechanical characteristics.