Star gels

In a typical synthesis, the corresponding precursor and calcium methoxyethoxide are dissolved in ethanol in an acidic medium (commonly HC1, 1N) under an argon atmosphere. The critical factors in obtaining crack-free star gels monoliths are the temperature and processing times, as indicated in Table 12.2. 

Tab. 12.2 Temperatures and processing times recommended for bioactive star gels fabrication...

Fig. 12.11 Transparent star gel monoliths prepared with calcium alkoxide (right) and without calcium (left).

Fig. 12.12 29Si CP-MAS NMR spectra of star gels obtained from Star A and Star B precursors. Trifunctional silicon centers were named with the conventional T notation, where T refers to (SiO) R Si(OR)3 units and n to the number of bridging oxygen atoms surrounding the central silicon atom. In both BSC materials, the T2, T1 and T° relative contribution was...

Fig. 12.14 Mechanical parameters calculated by nano-indenta-tion tests for a conventional sol—gel glass and two different bioactive star gels with analogous Ca/Si molar ratio.

Tab. 12.3 Fracture toughness (K c) values of a conventional sol-gel glass, star gels and those obtained for human cortical bone by different authors.

Currently, bioactive star gels have opened a new research field providing a range of novel bone degeneration treatment [111]. 

Sharp, K.G. and Michalczyk M.J. (1997) Star gels New hybrid network materials from polyfunctional single component precursors. Journal of Sol-Cel Science and Technology, 8, 541-546. 

Manzano, M., Arcos, D., Rodriguez Delgado, M., Ruiz, E., Gil, F.J. and Vallet-Regi, M. (2006) Bioactive star gels. Chemistry of Materials, 18, 5696-5703. 

Greene, M.E. (2007) Bone up on star gels — Research news. Materials Today, 10, 15. 

Finally, it should be mentioned that such intramolecular polycondensation is certainly an important phenomenon when a crowded structure forces the —Si(OR)3 groups to be close to each other. This is particularly the case for dendrimer precursors121,125 or related star gel precursors142,143 due to the proximity of the hydrolysable functions. 

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