Devices that look like tiny diving boards are a launching platform for
research that could improve detergents and advance understanding of disease.
Rice Univ. researcher Sibani Lisa Biswal and Kai-Wei Liu, a graduate student
in Biswal’s lab who recently earned her doctorate at Rice, used
microcantilevers as ultrasensitive measuring devices to study how lipid
bilayers interact with surfactants.
Their results were reported online in Analytical
Lipid bilayers are membranes that surround the cells of every living
organism. Along with specific membrane proteins, they act as gatekeepers that
allow ions, proteins, and other essential molecules to pass into the cell. Individual
lipid molecules in the bilayer have a hydrophilic head and two hydrophobic
tails. They naturally aggregate into two-layered sheets, with the heads pointed
out and the water-avoiding tails pointed inward.
Liu and Biswal, an assistant professor in chemical and biomolecular
engineering, described in a previous paper how to attach lipid bilayers to
microcantilevers, which have traditionally been used as analytical biosensors.
A protective coating on the thin gold layer makes the top of the “diving
board” inert, so the membranes attach themselves to and spread out over
the silicon dioxide bottom. The exchange of energy as the membrane meets the
solid platform changes the surface tension and bends the cantilever enough to
be measured by a laser sensor. Any change to the membrane will alter the bend,
which can be measured with nanometer resolution, Biswal said.
In the new work, the researchers introduced varying concentrations of
lysolipids to the supported lipid bilayers. Lysolipids are surfactants, compounds
that lower the surface tension of liquids and can act as detergents, among
other things. Like the molecules that make up lipid bilayers, lysolipid
molecules have a hydrophilic head but only one hydrophobic tail.
Liu and Biswal found that in low concentrations, lysolipid molecules wedged
themselves into the bilayer as their water-hating tails cozied up to the
membrane’s hydrophobic inner ring; this changed the surface tension on the
All of these forces can be measured, Biswal said. “The cantilever
naturally wants to bend with whatever force the membrane puts on it,” she
In high concentrations, lysolipid monomers form micelles, rings of molecules
that interact with the membranes and disrupt the hydrophobic interactions that
keep them together.
Depending on their strength, the micelles can either weaken the membranes by
pulling lipid molecules away or destroy the membranes completely.
That is precisely what you want a detergent to do to a stain, and the new
technique would be very useful for fine-tuning cleaning agents, Biswal said.
“A vast amount of research has gone into detergency,” she said.
“There are a lot of detergencies based on enzymes, the biomolecules that
cleave peptide bonds. A lot of stains are organic molecules. If you can cleave
them, you can clean surfaces much better.”
Biswal sees other potential for the technique. “We’re interested in
using this as a general platform for looking at small molecules,” she said.
Liu is pursuing one such path. She is studying how hepatitis C peptides
behave in the presence of a microcantilever-mounted membrane. “This could
be a way to probe how viruses are able to enter cell membranes or disrupt
proteins on their surfaces,” she said.
Biswal suggested that carbon-60 atoms might also be a good subject. “We
don’t know enough about how nanomaterials interact with cell membranes, and
since buckyballs are naturally hydrophobic, they might be interesting to