Current research areas include microfluidics and medical devices. Our microfluidics research focuses on the mechanisms and applications of cell biomechanics and micro-mechanical cell separation.
Microfluidic ratchet
We are currently investigating a microfluidic ratchet that exploits the deformation of individual cells through microscale funnel constrictions. The threshold pressure required to transport single cells through such constrictions is greater against the direction of taper than along the direction of taper. This physical asymmetry combined with an oscillatory excitation can enable selective and irreversible transport of individual cells in low Reynolds number flow. We devised a microfluidic device to measure the pressure asymmetry across various geometries of funnel constrictions. Using a chain of funnel constrictions, we find that oscillatory pressure enables ratcheting transport when the pressure amplitude and oscillation period exceeds the threshold required to transport single cells.
Deformation of a single cell through microscale funnel constrictions in forward and reverse directions. Key parameters of the microstructure include the funnel pore size and half-angle of the funnel taper.
Forward and reverse threshold pressures required to deform a single Mouse Lymphoma Cell through a 10 degree funnel constriction. The model curves are fitted based on a cell cortical tension of 750 pN/um.
Relevant Publications
Q. Guo, S.M. McFaul, H. Ma, Deterministic microfluidic ratchet based on the deformation of individual cells, Physical Review E, Vol. 83, 051910, 2011.
Chromatographic cell separation
Liquid chromatography is a classical technique for separating components of a mixture based on their ability to transit through a column of porous material. This technique has not been extended to the separation of eukaryotic cells because of the lack of column materials that can impart sufficiently distinct flow velocities to different cell phenotypes. We present a microfluidic technique for chromatographic separation of cells based on a dynamic microstructure. Distinct flow velocities are imparted to different cells based on their mechanical properties, including size and deformability, without chemical modification. Potential applications of this mechanism include the capture of viable circulating tumour cells and the purification of mesenchymal stem cells.
The operating principle of our chromatographic device. The floor of the separation channel is periodically raised and lowered as cells traverse the channel.
(A) When the channel floor is raised, small and deformable cells are able to traverse the channel while large and rigid cells are forced into traps in the textured ceiling.
(B) When the channel floor is lowered, trapped cells are released into the main floor and all cells flow unimpeded.
Video sequence of the selective capture of a mixture of polystyrene microspheres. A 10 micron sphere (circled) is flows past a trap and is captured while 6 micron spheres flow past unimpeded.
Relevant Publications
T. Gerhardt, S. Woo, H. Ma, Chromatographic behaviour of single cells in a microchannel with dynamic geometry, Lab on a Chip, Vol. 11, No. 11, pp.2731-2737, 2011.
Circulating Tumor Cells
Circulating tumor cells (CTCs) are malignant cells shed by a primary tumor. They circulate in the bloodstream and have the potential to establish metastases in anatomically distant tissues. The capture and enumeration of CTCs has gathered considerable attention because of the potential to use the number and status of these cells (1) as a prognostic marker for tailoring therapies to the needs of individual cancer patients, (2) as a rapid surrogate for evaluating the clinical benefit of new drugs, and (3) as a fluid biopsy of a particularly invasive subpopulation of the original solid tumor. The key challenge in working with CTCs has been their extreme rarity. Existing technologies overcome this problem by first enriching the concentration of CTCs by using the cell surface maker, EpCAM to select for cells with epithelial characteristics. The effectiveness of this selection technique has come into question since one of the key steps of invasion and metastasis is epithelial-to-mesenchymal transition where tumor cells initiate a phenotype switch that is accompanied by a loss of epithelial antigens (e.g. EpCAM). As a result, the aggressively metastatic CTCs are actually the most likely group to evade capture by current methods.
Based on microfluidic cell separation technologies developed by our group, we are developing mechanical means to enrich for CTCs from the blood of cancer patients. Since CTCs originate in the primary tumor, they are mechanically distinct from normal hematological cells. Our microfluidic device is designed to separate cells based on differences in cell size and cell rigidity with a high degree of selectivity. Our hypothesis is that after this mechanical selection step, the concentration of CTCs is sufficiently enriched to be identified and analyzed using various low-throughput methods. Key advantages of this approach over existing methods are the preservation of relevant CTCs that may have lost their expression for EpCAM due to epithelial-to-mesenchymal transition and the recovery of live CTCs.
Low Cost Malaria Detection Device
Malaria is one of the greatest challenges facing human health today. Globally, there are between 200 and 500 million cases of malaria per year, resulting in approximately 1 million deaths, the majority of which are small children. Malaria is caused by a microscopic parasite transmitted by mosquitoes to humans. Part of the lifecycle of this parasite is spent inside human red blood cells where it matures and multiplies. Presence of the parasite causes red blood cells to stiffen, impeding their circulating in blood capillaries and causing the infected cells to accumulate inside vital organs.
An important part of the development of new drugs and vaccines are the methods and technologies for evaluating the status of the parasite and its response to treatment. These types of evaluation tools are important both for early stage laboratory studies and for patient trials of new medicines. It has been previously shown that the stiffness of individual red blood cells can be used as an indicator of parasite growth. Current methods for measuring the stiffness of infected red blood cells, however, require technically demanding procedures that are performed by experienced personnel using specialized equipment. Based on technology developed by our group, we are developing microfluidic devices for measuring the stiffness of individual red blood cells in order to determine the status of the parasitic infection. Furthermore, we are developing method to automate such measurement processes in order to develop portable and low-cost evaluation devices. We have recently received funding from the Bill and Melinda Gates Foundation to pursue this work.