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