The relative influence of physical and chemical bonds to overall gel properties are explored in gelatin gels. Physical, chemical, chemical-physical, and physical-chemical gels are obtained by cooling the protein solution and/or by transglutaminase reaction. Each type of network is characterized by rheology and polarimetry. It is shown that the overall properties as well as the dynamics inside the gels are dependent upon the order of formation and on the relative amount of triple helices and covalent bonds. Enzyme hydrolysis of covalent gels is slower than that of physical gels, as confirmed by the kinetics of helix release and degradation. A scheme is proposed to explain the results at both the physicochemical and the molecular levels.

The release of molecules entrapped within biogels is dictated by diffusion laws. Innovative biogel architectures are conceived and tested to control small molecule delivery from gelatin gels. The ionic interactions modulate the release of small molecules. Alginate is then added to gelatin gels and further hydrolyzed; the influence of viscosity is discussed. Next, various mixed gels are compared, such as a gelatin-alginate IPN and the original architecture of an alginate gel entrapped in a gelatin gel with or without a polysaccharidase. The relative influence of ionic interactions and diffusional constraints on the delivery of small charged molecules is explored, and a solution for controlling diffusion is proposed for any situation.

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In some biological processes, two enzymes with antagonistic activities--the one creating a bond, the other destroying it--are involved in a reaction cycle. Several catalysts have the ability to modify the rheological properties of biological media participating in the production of a solid gel phase which later dissolves. Transglutaminase, catalyzing intermolecular protein cross-linking, is considered here as a reverse protease as far as the physical state of a proteic gel is concerned. A kinetic model including diffusion constraints and based on a protease/transglutaminase cycle interconverting insoluble gel and soluble proteolysis fragments showed that alternate sol/gel and gel/sol transitions could occur within such a system, generating transient gel phases. Then, ephemeral gels were obtained in vitro using an experimental system consisting of gelatin, transglutaminase, and thermolysin. Modulating the enzyme activity ratio allows us to "program" the global behavior: polymerization/solubilization cycle of a mixture containing at least one protein and two enzymes without any change in temperature or medium composition.