Design of a microfluidic device for lymphatic biology
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The lymphatic system has three primary roles: transporting lipids, transporting immune cells, and maintaining fluid balance. Each one of these roles are influenced by the presence of flow. Inflammation increases lymph flow, lipid uptake is enhanced by flow, cancer cell migration increases in the presence of flow, and lymphatic permeability and lymphatic contractility respond to changes in flow. Flow is very important to lymphatic function, and yet, there are no in vitro models that incorporate both luminal (flow along cell lumen) and transmural (flow through cell lumina) flow for lymphatics. To address this need, a microfluidic device has been developed that can incorporate both of these types of flow. This is achieved by driving flow through a channel which creates a pressure gradient that drives fluid through a porous membrane into an adjacent channel. Following several design iterations, the device can be easily fabricated, imaged, and cells can grow and survive in it. Permeability experiments have been performed in static and flow, 0.175 mL/min (0.5 dyne/cm²), cases. The effective permeability of dextran in the static and flow cases was calculated to be 0.0083 μm/s and 2.05 μm/s respectively. While the effective permeability of bodipy in the static and flow cases was calculated to be 0.0053 μm/s and 2.57 μm/s respectively. The static values are similar to values obtained in a transwell study by Dixon et al. As mentioned, lipid uptake is increased in the presence of flow and these numbers suggest the same. In addition to permeability studies, experiments were performed with cancer cells suspended in a collagen gel. Two image processing techniques were used to quantify cancer cell migration. The first technique was used to calculate the number of cells present at the beginning of the experiment and the number of cells that were ever present during the experiment in that particular z slice. The static case yielded a cell flux of 15 additional cells. While the two flow cases, within interstitial flow range, had a flux of 24 and 40 cells. This suggests that flow increases migration in cancer cells and is in agreement with the literature. The second technique was used to show that the cells in the static and flow cases are similarly motile, but the flow case is more directed in the z direction towards the membrane. The future work for this device is quite extensive, but a strong foundation centered around basic capabilities like inducing flow, seeding cells, and imaging has been formed.