General Data

Sol-gel technique includes hydrolysis and condensation of metal alkoxides in adequate solvents followed by thermal treatment. The biomaterial to be encapsulated is added to sol after partial hydrolysis of the alkoxides.

inorganic prepolymer

organic-inorganic prepolymer

M - metal (Si, Zr, Ti, Al, ...)
R - organic group ( CH₃, C₂H₅, C₃H₆NH₂, C₃H₆SH, CH=CH₂, ... )
R´- organic group ( CH₃, C₂H₅, i-C₃H₇ ...)

As the degree of cross-linking from polycondensation increases, the gel becomes viscous (b) and solidifies (c). The process continues during aging and porous matrix is formed around the biomaterial trapped inside (d).

Economy

Available precursors - metal alkoxides and oligomers. Silicon compounds and polymers, e.g. poly(dimethylsiloxane), are produced in large scale, e.g Wacker Chemie, Gelest

Mechanical Properties

Oxide layers are not elastic in comparison with those made of organic polymers. Inorganic and organic-inorganic matrices are characterized by measuring of porosity and specific surface in dry matter.

Encapsulation Mechanism

Physical entrapment of the biomaterial added into the prepolymer. Matrices prepared from precursors with amino or sulfur groups might be covalently bonded to the biomaterial.

Encapsulation Procedure

Typical laboratory procedure includes hydrolysis and polycondensation of tetraethoxysilane or tetramethoxysilane catalyzed with an acid. After partial evaporation of generated alcohol the prepolymer is cooled in an ice bath and the biological material is introduced in a suitable buffer.

Biocompatibility

The sol-gel process, originally developed for glass production, includes extremes of alcohol concentration, temperature and pH, which are detrimental to biomaterial. To avoid the denaturation of proteins the process was modified. The resulting silica gel is chemically inert and biocompatible. Its mechanical rigidity suppresses cell budding.

Shape

Inorganic gels with a biological material can be easily shaped by casting, molding and jet-cutting. Thin layers can be formed by dipping and dropping on a rotary disk. A method of encapsulating by vapor deposition of SiO₂ has been developed for very sensitive plant or animal cells.

Modifications

Chemical composition and physical properties of sol-gel matrices can be tailored. Chemical durability of the matrix is determined by metal. Composition of alkoxy groups and additives influence the rate of polymerization. Organosubstituents modify hydrofobicity. Porosity can be widely varied in large scale by water content and conditons of drying.

Applications

Sol-gel matrices offer advantages over organic polymers in easiness of their integration into sensors and transduction platforms. Their optical transparency makes them ideal for the construction of sensors based on measuring of absorbance or fluorescence.

Commercial product: Lipase immobilized in Sol-Gel-AK from Fluka, Buchs, CH.

List of possible applications:
- Reactions catalyzed by lipases in organic solvents
- Immobilization of Pichia pastoris catalyzing oxidation of benzyl alcohol in organic solvents
- Encapsulation of Pancreatic islets
- Non-toxic biosorbent of heavy metals
- Sensors: glucose, lactate, immunoassays, TNT, organophophate pesticides, NO, CO, H₂O₂ and phenols
- Permeable barriers for treating contaminated plumes from landfills
- Coatings of bone implants
- Bone replacement material

Legal Status

Silicone dioxide, silica aerogel, magnesium silicate, sodium aluminosilicate, aluminiumcalcium silicate and sodium methasilicate are listed on the list of materials GRAS by the US Food and Drug Administration (FDA). These substances are generally recognized as safe as Substances Migrating to Food from Paper and Paperboard Products ( § 182.90), Anticaking Agents (§ 182.2727), Table Salts and Ingredients in Baked Goods (§ 184.1769)

Literature

Ellerby L.M., Nishida R.C., Nishida F., Yamanaka S.A., Dunn B., Valentine J.S., Zink J.I.: Encapsulation of Proteins in Transparent Porous Silicate Glasses Prepared by the Sol-Gel Method. Science 255, 1113-1115 (1992).

Gill I. Pastor E., Ballesteros A.: Lipase-Silicone Biocomposites: Efficient and Versatile Immobilized Biocatalysts. J.Am. Chem.Soc. 121, 9487-9496 (1999)

Contacts and Links

Research Groups:
- J.D. Brennan, McMaster University, Hamilton, Ontario, Canada
- J. Livage, Université Paris VI, Paris, France
- D. Avnir, Hebrew University of Jerusalem, Jerusalem, Israel

For list of research groups working in the sol-gel science see http://www.solgel.com

Gabriela Kuncova
Institute of Chemical Process Fundamentals
Academy of Sciences of the Czech Republic
Rozvojova 135, 16502 Prague 6-Suchdol, Czech Republic
Phone: +420 2 20390243
Fax: +420 2 20920661,
kuncova@icpf.cas.cz

Dear Sirs:
My name is Dr Claudio Rottman and I am the Chief Chemist responsible for the R&D in the company that I work for Sol-Gel Technologies in Israel. Since I believe that you are interested in know about new microencapsulation products which are in stages of worldwide commercialization, I want to inform you about our product UV-Pearls which is a microencapsulation line of UV absorbers utilized by the cosmetic industry for sunscreens and daily care topical formulations.
This product was developed 6 years ago by my colleges and me and is exclusively distributed today by one of the biggest chemical companies in the world.
This technology is based on the sol-gel process, which has been adapted in order to obtain a microcapsule having a core-shell structure in which the shell is pure silica and the core is the active.
I hope that this information is useful for you and will be published in your web site.

Sincerely,
Claudio

Claudio Rottman, Ph.D.
Chief Chemist
Sol-Gel Technologies Ltd.
Phone: +972 2 9997661
Fax: +972 2 9997662
claudio.rottman@sol-gel.com