Research Interests

Silicon-derived compounds, particularly silicones and silica, are known for their surface activity.  The research projects in our lab share the common mission of bringing concepts from synthetic chemistry into biological systems that are in need of new experimental approaches. Much of our current interest is concentrated on developing molecules that can control interfaces, particularly silicones, which are broadly used in a variety of biomaterial applications including wound dressings, and implantable devices.  Although various members of the group have expertise in biology, and in colloid and surface science, the heart of the group concentrates on synthetic chemistry. 

A. New Routes to Explicit, Functional Silicones

There are few effective reactions to create well-defined silicones of precise structure and molecular weight.  As a consequence, little is known about the structure surface activity relationships (or other physical properties).  We are approaching this problem from a few directions.
1. Piers-Rubinsztajn Reaction
The Piers-Rubinsztajn reaction is the 'condensation' that arises between a hydrosilane (R3SiH) and an alkoxysilane in the presence of B(C6F5)3. A siloxane bond is formed simulaneously with an alkane.

Generic
            Piers-Rubinsztajn Reaction

   a) Precise Silicones
We have undertaken a systematic studies to develop a series of covenient building blocks. The process occurs in high yield, with trivial workup and allow in a few steps the preparation of precise silicone structures  (no mixtures!) of intermediate molecular weight. The reaction is rather tolerant to fucntional groups, and the work has therefore been extended to silicone surfactants and copolymers with alkoxy-functional arylamines and ethers. 

Precise silicone synthesis
Piers-Rubinsztajn Surfactant
                        SynthesisPiers-Rubinsztajn with Arylamines


    d)
Foams
The alkane byproduct can be used to generate silicone foams, provided that the rate of bubble generation matches the rate of cure. 
Polymer Foam

Current projects: We are attempting to broaden the scope of the reaction to include silicone copolymers, the preparation of very high molecular weight precise silicones, and examining the ability of these materials to structure silicones and other media.


2. Click Chemistry
Since its description by Sharpless, the copper-catalyzed Huisgen cyclization (CuACC) of alkynes with azides has been broadly adopted as a means to link organic
moieties. Its use in biomedical applications is disadvantageous because of the potential biotoxicity of copper.
The process works very well without catalysts at elevated temperature (the Huisgen cyclization).  With lower electron density alkynes, the reaction occurs efficiently at temperatures below 60 ∘C, although a mixture of regioisomers is formed. We have demonstrated that the reaction will allow the preparation of a series of silicone surfactants based on small hydrophiles or on poly(ethylene glycol).  The process has also been extended to the preparation of silicone elastomers that do not need any type of catalyt present.
Click onto
            DMAD

Silicone-PEG copolymer

Current projects: The click process, particularly in the absence of catalysts, allows the preparation of silicones and silicone-copolymers comprised of different compounds that is currently possible. We are exploring this process to create silicone/biomolecule(biopolymer) composites and to examine the compatibility of such surfaces in vivo, including in the eye


3. Structured copolymers
    a) Polymerized Microemulsions
An unexpected outcome of the work described above was the ability to prepare bicontinuous microemulsions from high molecular weight silicones and HEMA (hydroxyethyl methacrylate).  Gyroidal structures with domain sizes between 10-30 nm formed. It was possible to indepently polymerize the silicone and water phases without losing the bicontinuous structure. Such polymers have interest as ophthalmic materials, particularly because oxygen, nutrients, and hydrophobic drugs can migrate through them.
Bicontinuous MicroemulsionMicroemulsion Phase Diagram

Current projects:
To control the demain size of these materials more precisely and examine their in vivo compatibility. 

B. Silica

Most of the earth's crust is comprised of silica in one form or another. Well defined amorphous silica structures are available using sol-gel chemsitry. Under appropriate conditions, it is possible to structure the silica to be macroporous and mesoporous. The development of new precursors for silica permits the incorporation of proteins into the silica structures, and we have demonstrated that these can be used for drug screening, in biodiagnostics, and as immobilized enzyme: the silica acts to protect the protein and prohibit it's escape, but allows both substrates and products to enter and leave the silica monolith.  Our current interst in theis area are related to encapsulating bacteriophage (a virus that infects bacteria) in silica as part of our work on Bioactive Paper.

Macroporous silica
Bar = 0.5 micron

The same types of processes can be used to prepare (atypically large) silica structures in silicone oil. This is surprising because the only source of (required) water comes from moisture in the air. The silica can act to structure the silicone both internally and at the interface.

Silica structures
            in silicone elastomers

Current projects: Our current research involves controlling the morphology of the silica, investigating the reinforcement provided by high surface area silicones, and developing these materials as delivery vehicles for biologically active molecules. 


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