TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Materials: (I) comprises at least one oxide of a group III-V element, and may be modified by hydrolysis products of tri- and/or di-alkoxysilanes.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Materials: (II) is at least one water-soluble compound that, after being released in the solution or gas phase, increases the porosity of the composite, particularly a polyol, polycarboxylic acid, urea derivative or carbohydrate.    TECHNOLOGY FOCUS - BIOLOGY - Preferred Cells: These are eukaryotic or prokaryotic, e.g. bacteria, algae, yeast, fungi, or plant or animal tissue fragments.Preferred Composite: (II) is 5-50 weight% and (III) 0.1-50, preferably about 10, wt.%. (A) may also include nutrients for (III).Preparation: An alcoholic silica nanosol is prepared by acid or alkaline hydrolysis of a tetraalkoxysilane, optionally modified by co-hydrolysis of metal alkoxide or halide, or tri- or di-alkoxysilane. The alcohol in the sol is replaced by water, particularly while passing a current of inert gas through at below 20 degrees C, then a homogeneous mixture prepared from the now entirely aqueous sol (containing at least one (II) and optionally nutrients) and an aqueous cell suspension. Gel formation is induced by neutralization, heating or adding fluoride, then the gel comminuted and dried to give a glass-like porous powder. Alternatively, to produce a layered composite, the homogeneous mixture (ungelled) is used to coat a material, by dipping, spraying or pouring, then dried. This method is particularly used to coat porous filler particles for bioreactors, or biofilters. EXAMPLE - A mixture of tetraethoxysilane (100 ml), ethanol (400 ml) and 0.01 M hydrochloric acid (200 ml) was stirred for 20 hours at room temperature, then 600 ml of the product treated with 450 ml water, while passing air through it. The flow of air was continued until the final volume was 600 ml. The resulting aqueous mixture (100 ml) was mixed with dispersion of bakers' yeast (2 g) in 1% aqueous Tween 80 (20 ml) and sorbitol (1 g; activator), then neutralized with 0.25% aqueous ammonia and stirred to form a gel. When this gel (0.5 g yeast equivalent) was tested for fermentation of 2% aqueous glucose solution (100 ml) at 30 degrees C, 15% conversion of glucose was reached after 3 hours; compared with about 5% conversion when no activator was added.