This review is devoted to the most recent developments (2000)(2001)(2002)(2003)(2004)(2005) of sol-gel materials at the interface with biology. In the context of bioencapsulation in mineral hosts, novel synthetic approaches have been designed, allowing the immobilization of numerous proteins, enzymes and immune molecules as well as poly-saccharides, phospholipids and nucleic acids. These efforts have led to the development of new biosensors and bioreactors. A similar trend was also observed for whole cell encapsulation, survival periods over several weeks now being achieved. This has opened the possibility of designing hybrid hosts for cell-based biosensing and bioproduction, ultimately allowing the development of artificial organs. Indeed, applications of sol-gel processes are not restricted to bioencapsulation, as demonstrated by recent progress in drug release systems and bioactive materials. Finally, the considerable efforts devoted to the biomimetic elaboration of mineral structures suggest that they might be the key for future development of improved sol-gel materials for bio-applications.

The influence of poly(lysine) and poly(arginine) on silicate solutions and silica sols has been investigated. In both cases, solid formation could be observed that appeared to depend on the polymer chain length. Polyelectrolytes induced gelation of diluted sodium silicate solutions. Studies of the silicic acid content using the colorimetric molybdosilicate method suggested that polymers act as gelating agents of silica oligomers via electrostatic interactions that favor condensation. In the case of silica sols, quasi-elastic light scattering measurements indicate that particles are first adsorbed on the polymer chain to form aggregates that then flocculate in the presence of additional peptides. Structural characterizations of the solids obtained in both cases were consistent with the proposed models. These results are discussed in the frame of biogenic silica formation at the protein/silica interface.

The encapsulation of enzymes within silica gels has been extensively studied during the past decade for the design of biosensors and bioreactors. Yeast spores and bacteria have also been recently immobilized within silica gels where they retain their enzymatic activity, but the problem of the long-term viability of whole cells in an inorganic matrix has never been fully addressed. It is a real challenge for the development of sol-gel processes. Generic tests have been performed to check the viability of Escherichia coli bacteria in silica gels. Surprisingly, more bacteria remain culturable in the gel than in an aqueous suspension. The metabolic activity of the bacteria towards glycolysis decreases slowly, but half of the bacteria are still viable after one month. When confined within a mineral environment, bacteria do not form colonies. The exchange of chemical signals between isolated bacteria rather than aggregates can then be studied, a point that could be very important for 'quorum sensing'.

A wide variety of biomolecules, ranging over proteins, enzymes, antibodies and even whole cells, have been embedded within sol-gel glasses. They retain their bioactivity and remain accessible to external reagents by diffusion through the porous silica. Sol-gel glasses can be cast into desired shapes and are optically transparent, so it is possible to couple optics and bioactivity to make photonic devices and biosensors. The high specificity and sensitivity of enzymes and antibodies allows the detection of traces of chemicals. Entrapped living cells can be used for the production of metabolites, the realization of immunoassays and even for cell transplantation.

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We present a simple one-pot crystallization method, inspired by biological conditions, for the synthesis of hydroxyapatite (Ca5(PO4)3OH) nanocrystals. The reaction proceeds via NH3 vapor diffusion into a CaCl2−NaH2PO4 mixed solution that is free of any organic additives. The advantage of relying on acidic calcium-phosphate precursors here is, first, that the reaction can be performed at room temperature within a short time and without direct pH control and, second, that it does not produce any secondary phases or byproduct. Furthermore, the addition of NaHCO3 to the salt solution or the introduction of (NH4)2CO3 instead of NH3 lead, respectively, to the precipitation of B- or A-type carbonate-apatite phases according to the FT-IR data. Multinuclear solid state NMR studies and especially 13C CP experiments allow an in-depth characterization showing the presence of A/B substitutions in carbonated samples as well and indicate a close similarity to deproteinated bovine compact bone. A precipitation mechanism accounting for the precipitation of mainly A- or B-type carbonated apatite under the respective experimental conditions is proposed.

The dynamic behavior of nanoscale mesoporous oxide materials exposed to aqueous solutions under biologically relevant conditions is shown to be highly dependent on composition, porosity, and calcination temperature. Dynamic processes were followed as a function of exposure on thin oxide films amenable to environmental ellipsometry porosimetry for the analysis of mechanical strength and pore size distributions as a function of exposure. Additionally, X-ray photoelectron spectroscopy was used for the elucidation of compositional changes as a function of exposure. Combined, this approach gives the first detailed, quantitative information of the degradation of nanoscale oxide materials under biologically relevant conditions. This approach also shows the utility of using film geometry as a convenient model system for the study of dynamic properties, as films are amenable to sensitive ellipsometric characterization. Pure silica films underwent a rapid degradation, occurring on the time scale of hours, while silica films mixed with 10% or less of zirconia or alumina were significantly more stable. These mixed metal oxide films showed structural changes on two time scales, undergoing a rapid partial degradation followed by a stabilization of the structure as the composition of the films evolved toward a depleted silica state. The time scales of these two processes were on the order of hours and days, respectively, and could be tuned by varying the composition and the calcination temperature of the films. These time scales are especially relevant to the culture and growth of mammalian cells and for drug release applications. Titania materials were shown to be stable under all conditions studied, making them suitable candidates for applications where the scaffold functions as a permanent support. These results yield unprecedented levels of detail on the kinetics of degradation and the dynamic structural and compositional changes occurring in these nanostructured materials.

The viability of bacteria in the presence of sol-gel reagents has been studied in order to define the best experimental conditions for the sol-gel encapsulation of E. coli. The b-galactosidase activity of these bacteria, trapped in sol-gel silica matrices, was then analyzed. Two routes, using alkoxide and aqueous precursors, have been used and compared. It appears that the aqueous route is less damaging than the alkoxide one. Moreover the aqueous silica matrix appears to slow down the lysis of cell membranes when bacteria are aged without nutrient.