Sol-gel based materials for biomedical applications

In the past decade, the encapsulation of enzymes inside inorganic sol-gel matrices has become a generic method to prepare efficient biocatalysts which are easy to recycle. In this review, the sol-gel processes useful for enzyme encapsulation, (mostly sol-gel silica) are outlined. Then, the most recent developments in the applications of such biocatalysts are presented, in particular regarding biosensors and chemical synthesis. Finally, a special attention is addressed to the types of interactions which are considered to prevail between the enzyme or the substrates and products, and the matrix, in these materials.

The sol-gel encapsulation of enzymes

The conjugation of inorganic materials with biological systems has attracted research interest, as a consequence of the availability of the sol–gel process, operating under experimental conditions compatible with biomolecules and whole cells. In Section 2, the merits and limitations of alkoxide-based sol–gel silica as an immobilization material are presented and discussed with particular attention to the confinement of living cells and cell clusters. Biosil technology is based on the encapsulation of whole cells by a sol–gel silica layer deposited on the cell surface using silica precursors in the gas phase. It is presented in Section 3, which describes some important Biosil features such as mechanical stability, versatility for porosity control with the aim of immunological protection, and maintenance of cell viability with free expression of cell functions. Some recent unpublished results are presented to substantiate the properties of Biosil materials for encapsulation of functional animal cells. Extension of the Biosil process to alginate microencapsulation is then presented in Section 4, with emphasis on biocompatibility within the perspective of cell grafts and therapy. Section 5 covers some clinical transplantations concerning grafts with allogenic pancreatic islets and adenovirus infected cells. The conclusions (Section 6) present some potential guidelines for future trends, specifically for whole cell immobilization by silica and, more in general, for advanced biomaterials.

Encapsulation of functional cells by sol–gel silica: actual progress and perspectives for cell therapy

E. coli bacteria were encapsulated within silica gels and aged at room temperature in the absence of nutrients. Their viability was studied as a function of time using different viability tests. The plate count technique gives the number of culturable bacteria that remain able to form colonies in the presence of a culture medium. Their metabolic activity toward glycolysis was followed by 14C titration and 13C NMR spectroscopy. Several sol–gel matrices were tested in order to improve the viability of the trapped bacteria. The best results were obtained when encapsulation is performed in the presence of glycerol showing that almost 50% of the bacteria were still able to form metabolites after one month of ageing. Moreover, this study demonstrates that a wide range of viability tests can be adapted for use with cells encapsulated in mineral matrices.

A sol–gel matrix to preserve the viability of encapsulated bacteria

Optimisation of membrane properties of alginate microcapsules is a key factor for the application of microencapsulation techniques to bioartificial organ elaboration. Coacervation and layer-by-layer processes involving additional biopolymers have been extensively studied. Recently, the use of silica as a membrane-forming agent was investigated. This approach was rendered possible by the development of biocompatible routes to silica formation. The composites exhibit enhanced mechanical and thermal stability as well as suitable diffusion properties. Moreover, encapsulated enzymes and cells retain their biological activities. Similarly, silica can be associated to many other biopolymers, opening a promising route for new biocomposites design and biotechnology applications.

Silica-alginate composites for microencapsulation.

Biocomposite layers of silica and various bone-relevant proteins such as collagen, gelatine and commercial collagen hydrolysate can be obtained from coatings of silica sols mixed with proteins in water/dioxane. Investigations into the mechanical and cell proliferation properties for different sol parameters (pH, solvent), type and concentration of proteins, annealing and crosslinking of the biocomposite layers revealed that such coatings are highly biocompatible with excellent mechanical properties.

Biocompatibility of Modified Silica-Protein Composite Layers

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Biocompatibility of Modified Silica-Protein Composite Layers