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Smart materials containing tiny channels, a thousandth the thickness of a
human hair could carry drug molecules direct to their target in the body and
release them when the material reaches body temperature. The heat-responsive
materials would allow drugs that are otherwise inactive when taken by mouth
to be administered without the patient getting the needle. Related materials
might also act as artificial enzymes, speeding up the conversion of the
trapped molecules into useful products.
Millicent Firestone at the Argonne National Laboratory's Advanced Photon
Source and her colleagues have designed several polymeric materials
containing these tiny water and membrane channels. Their studies using
BESSRC/XOR sector 12 are revealing how chemical fine-tuning of the polymer's
structure can change the size of the channels so that different molecules
can be held inside selectively for subsequent ejection.
The chemical driver behind the research is the possibility of certain
molecules self-assembling into more complex chemical structures without
further intervention by the chemist. This allows chemists to design a
building block that will then build itself into the desired material.
Firestone and her colleagues explain that this is one way of making
materials with a controlled structure on the nanoscale that respond to
different external stimuli, such as light, temperature, and pH. By
fine-tuning the chemistry of the building blocks it is then possible to make
such nanostructures with different response levels to the stimulus. For
instance, when a polymer called PEG is mixed with a fatty, phospholipid
molecule in water containing a surfactant (a soap-like molecule), the
building blocks rearrange themselves into microscopic bubbles known as
micelles. The resulting material is gel-like and doubly refracts light - it
is birefringent. By changing the length of the chemical side chains on the
PEG component it is possible to change the level of birefringence. Such
materials are of interest for researchers working in optoelectronics.
Firestone and her colleagues, however, have spotted the potential of the
related polymer N-isopropylacrylamide (PNIPAM) in making, not
microscopic bubbles, but channels in a similar mixture of ingredients. These
channels are essentially "endless" bubbles into which water and other
molecules can fit. While other researchers have found they can change the
size of micelles by tweaking the chemistry of the building blocks, the
Firestone team set out to make the size of their water channels tunable
using an external stimulus, such as a change in temperature. Other
researchers have already demonstrated that PNIPAM undergoes dramatic,
changes in its chemical conformation when the temperature rises, so it
seemed the perfect starting material for making a thermally-responsive
material.
Dan Hay, a post-doctoral research associate synthesised a series of new
materials in which a PNIPAM group was grafted on to the head of a
phospholipid. They used several spectroscopic techniques to demonstrate that
the chemicals had self-assembled into the structures they were expecting and
optical microscopy to confirm birefringence in these new materials. They
found that the mixture was a transparent gel at room temperature (22
Celsius), but when warmed to 32 Celsius it became an opaque fluid.
The researchers then turned to the beamline to obtain a detailed view of the
water channels using SAXS, small-angle x-ray scattering. They carried out
SAXS at different temperatures. At room temperature, the diffraction pattern
for the material carries three peaks, which the researchers explain is
likely due to a layered, or lamellar, structure composed of alternating
layers of the PNIPAM-based material and water layers. The SAXS also suggest
that the stacking of these layers is directional alluding to the
channel-like nature of the layering. When the gel is warmed above 32
Celsius, the diffraction pattern changes dramatically, indicating that the
layered structure collapses and the size of the water channels are
compressed.
See: Daniel N. T. Hay,1 Paul G. Rickert, 2 Sönke Seifert, 3 and Millicent A.
Firestone1 "Thermoresponsive Nanostructures by Self-Assembly of a
Poly(N-isopropylacrylamide)-Lipid Conjugate," J. Am. Chem. Soc. 126,
2290-2291 (2004).
Author affiliations: 1Materials Science Division, 2Chemistry Division,
3Advanced Photon Source Division, Argonne National Laboratory, Argonne, IL
60439, USA
This work was supported by the United States DOE-BES, Div. of Material
Sciences under Contract W-31-109-ENG-38 to the University of Chicago.
In Issue 76
Are films ferroelectric?
Discipline for gold nanocrystals
X-rays shed light on machinery of photosynthesis
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