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 preparation of water repellent textiles by coating with different modified silica sols has been investigated. For this, pure and with 3-glycidoxypropyl triethoxysilane co-condensed silica sols were modified by three types of additives: alkyltrialkoxysilanes, polysiloxane derivatives and a fluorine containing silane. Hydrophobic properties of the coated fabrics of polyamide and of polyester mixed with cotton were determined using contact angle measurements. The hydrophobicity increases with increasing concentration of the alkylsilane additive in the silica sol and the length of the alkyl chain but with high additive concentrations plateau values in hydrophobicity were reached. Analogously textile coatings with high hydrophobicity were gained using hydrophobic polysiloxane or fluorine containing silicon compounds. The comparison of the different variants reveals that high wash-out stabilities were reached only by silica sols containing fluorine compoundsand hexadecylsilane additives. Therefore long-chain alkyltrialkoxysilane compounds could be used as substitutes for fluorine compounds for the surface modification of textiles in some practical applications.

Modified Silica Sol Coatings for Water-Repellent Textiles

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

The use of papers in packaging and development of novel technological applications for paper substrates largely depends on the control of the hydrophilic properties of the cellulose fibres and improvement of the water-repellent properties. This review provides an actual summary of available literature on theoretical concepts and practical methods to improve the hydrophobicity of cellulose fibres and paper webs. In the first part, the interaction of water with cellulose fibres and paper webs is described at different levels ranging from the molecular scale over the micro- to macroscale fibre properties towards the interactions with porous substrates. The concepts for hydrophobicity and superhydrophobicity applied to cellulose fibres are reviewed, considering the surface chemistry and topographical features. In the second part, current techniques for hydrophobization based on sizing or direct fibre surface functionalization are described. Besides traditional sizing procedures, novelties in nanoparticle applications as hydrophobic sizing agent are reviewed. Novel trends in physical, chemical and nanotechnological fibre surface modifications or surface coatings are reviewed to turn the wetting properties into the superhydrophobic regime. The main future concern in controlling cellulose wettability lies in the development of sustainable modification techniques based on renewable resources such as biopolymers and green chemistry.

Wetting and hydrophobic modification of cellulose surfaces for paper applications

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

Leaching stability is important when refining paper or textile surfaces with sol-gel silica layers that contain incorporated dyes or other components. First studies show a simultaneous increase in dye and Si concentration in the wash-out solution, indicating that the gradual dissolution of the complete composite layer plays an important role in the leaching process. The leaching stability of silica layers containing incorporated model dyes Aniline Yellow and Acid Orange 10 on different flexible substrates (cellulose paper, polyamide and polyester fabrics) could be improved in a number of different ways: (i) pretreating the substrate with coupling agents such as epoxyalkyl-silanes, (ii) fixing the dye within the silica layer by mixing it with alumina or alkylammonium compounds, and (iii) chemically fixing the dye by bonding it covalently to the silica matrix.

Improvement in the Leaching Behavior of Dye-Doped Modified Silica Layers Coated onto Paper or Textiles

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Biocompatibility of Modified Silica-Protein Composite Layers

Improvement in the Leaching Behavior of Dye-Doped Modified Silica Layers Coated onto Paper or Textiles