|PhD, Chemical Engineering, 2009|
North Carolina State University
M.S., Chemical Engineering, 2005
North Carolina State University
B.S., Chemical Engineering, 2003
Clemson University, Clemson, South Carolina
Research Focus: Rapid Deposition of Live Cells and Large Particles Using “Convective-Sedimentation” Assembly
Convective assembly is a process by which micro- and nanoscale particles can be deposited into an ordered array on a substrate. We use a rapid deposition process that traps a droplet of the yeast cell suspension between two plates to create a meniscus (Figure 1). As evaporation occurs, the capillary forces between the individual cells pull them together, forming a close-packed coating. At the same time, additional cells are pulled toward the coating by convective fluid transport, thus allowing growth of the close-packed coating.
Figure 1: Schematic of the convective-sedimentation assembly process. The bottom slide remains in place while the top slide is pushed to the right by a linear motor at a rate vw. The arrows indicate the direction in which sedimentation, evaporation, and convection act on the cells.
Because yeast cells are so large (~5 µm), sedimentation is a factor in the deposition. This deposition technique, in which convective assembly takes place in the presence of sedimentation, is termed “convective-sedimentation” assembly. By inclining the entire apparatus, we can increase the uniformity of the coatings (Figure 2). Angling the device such that the cells sediment toward the three-phase contact line increases the coating uniformity, which can be seen in Figure 2 by the color variations in the images. We have developed a model that, with input of a number of experimental parameters, has been fitted with the data for the change in device inclination such that it can predict the effect that changing the angle between the slides will have on the coating length, thickness, and uniformity. This can be seen in the experimental and model thickness profiles presented in Figure 3. These profiles show that increasing the angle of forward inclination of the device increases the uniformity, as does decreasing the angle between the two slides.
Figure 2: Effect of device inclination on coating uniformity. Schematics of device alignment at (a) backward inclination, (b) no inclination, and (c) forward inclination. Corresponding images of coatings deposited at (d) backward inclination, (e) no inclination, and (f) forward inclination. All scale bars are 5 mm.
Figure 3: Coating thickness profiles from model (a, c, e) and experimental (b, d, f) data for no inclination (a & b), forward inclination of 20° (c & d), and backward inclination of 20°.
By mixing large latex spheres (~10 µm) with the yeast cells, we deposited composite coatings in a single step. The latex particles form a porous, protective coating over the yeast cells (Figure 4a). When the coating is exposed to growth media, the yeast cells proliferate, completely covering the latex particles (Figure 4b). Contaminants are then introduced and rest on top of the yeast cell layer (Figure 4c). Then, with exposure to a fluid flow, the contaminant, along with the top layer of cells, was removed from the coating (Figure 4d), leaving a clean coating that when exposed to growth media again, will proliferate as before. Of interest is using these coatings, with various cell types, as self-cleaning coatings with the cell shedding as an “artificial skin” along with any additional functionality present in the cell type.
Figure 4: Images and schematics demonstrating cell-particle coatings with rudimentary self-cleaning properties. (a) Initial coating. (b) Coating after exposure to growth media for 24 hours, allowing for significant cell proliferation. (c) Coating with fluorescent latex “contaminant”. (d) Coating following contaminant and cell removal with a stream of water. All scale bars are 50 µm.