Our work with thin polymer film deposition is motivated by two desires: (a) to control electrokinetic flow by spatially patterning surface charge at glass-liquid interfaces, and (b) to enable electrophoretic separations of single-cell lysates on chip by eliminating cell-wall adhesion and facilitating optical manipulation of individual cells.
Electroosmotic flow (EOF) is generated at glass-liquid interfaces becuase the surface SiOH groups of glass will tend to become charged by losing or gaining protons depending on the pH of the fluid. Ions from the fluid preferentially concentrate near the wall (the so-called electrical double layer or Debye layer) to counteract this charge. If an electric field is applied to either end of a microchannel, the excess ion concentration in the Debye layer experiences a body force and moves toward one of the electrodes. In the steady-state, viscosity couples with this electrokinetic force to generate a uniform flow of fluid in a microchannel.
We are using laser-deposited thin films to
change the surface charge properties of glass
surfaces. Doing so enables techniques that
require electroosmotic flow for fluid manipulation
in one part of a microchip but the absence
of surface charge elsewhere on the same chip
(either for separation fidelity or cell manipulation).
These coatings are applied by covalently bonding
a self-assembling monolayer (SAM) to the silica
surface, followed by laser-induced polymerization
of acrylamide to the SAM. The acrylamide coating
is uncharged, and shields the charged silica surface
from the fluid. The area coated with the SAM retains
most of the original charge and thus supports electroosmotic
|Click for popup movie of cell solutions on uncoated microchannels.||Click for popup movie of cell solutions on coated microchannels.|
We have applied these coatings to facilitate electrophoretic separations of single-cell lysates on chip by eliminating cell-wall adhesion. Laser tweezers can be used to manipulate cells by generating strong, localized electromagnetic field gradients, but the forces generated on cells are weak (5 pN) and cannot be used to detach adhered cells from walls. Because of this, cell-wall adhesion must be eliminated. The movies at right show identical cell solutions on coated and uncoated channels. Cells adhere strongly to uncoated silica surfaces but do not adhere to coated surfaces, even after hours of exposure.
The separation at left was effected on silica surfaces coated with the SAM. While the SAM decreases the surface charge slightly and increases the hydrophobicity of the surface, separations on SAM-coated microchannels give similar results to those on uncoated channels.
B.J. Kirby, A.R. Wheeler, R.N. Zare, J.A. Fruetel, T.J. Shepodd "Programmable Modification of Cell Adhesion and Zeta Potential in Silica Microchips,"Lab On a Chip v3 pp5-10 (2003).
B.J. Kirby, A.R. Wheeler, T.J. Shepodd, J.A. Fruetel, E.F. Hasselbrink, R.N. Zare "A Laser-Polymerized Thin Film Surface Modification for Suppression of Electroosmotic Flow and Cell Adhesion in Silica Microchannels," in Micro Total Analysis Systems 2001, Kluwer Academic Publishers, 2001.
A.R. Wheeler, K. Morishima, B.J. Kirby, A. Leach, R.N. Zare "Neural Cell Analysis using Micellar Electrokinetic Chromotogrpahy," in Micro Total Analysis Systems 2001, Kluwer Academic Publishers, 2001.