April 14
@
4:00 PM
–
5:00 PM
Room 1610 Engineering Hall
Daniel Hammer
Alfred G. and Meta A. Ennis Professor of Bioengineering
University of Pennsylvania
Philadelphia, PA
A Tale of Two Motilities
In the spirit of Bird, Stewart and Lightfoot, we have been studying an important problem in transport phenomena, the biological and bio-inspired motility of cells.
Motility is important for the functioning of the immune system, mostly because immune recognition requires molecular transfer by direct cell to cell contact. We have been studying a fascinating form of cell motility in which T-lymphocytes can migrate against the direction of flow, much like a salmon can swim upstream. Upstream migration is solely due to interactions between a specific lymphocyte receptor, LFA-1, and its natural ligand; no other receptor can support upstream migration. Our lab has found that many actively motile cells in the immune system have the ability to migrate upstream. Using CRISPR-Cas9 deletion, we have identified several molecules in cells that are critical for upstream migration; deletion using Cas9 reverses the direction of migration. We have also made the first traction maps of forces exerted by upstream migrating cells. We found that during upstream migration, cells maintain their “architecture,” with active forces in the front and rear, but the magnitude of the forces greatly increases, allowing cells to exert sufficient traction to overcome the applied hydrodynamic forces.
Our laboratory is also interested in making synthetic cells, or protocells, that can mimic the behavior of biological cells. In collaboration with Daeyeon Lee at Penn, we have been making inert capsules using microfluidic assembly that can display motility in solution. By attaching urease to the surface of a capsule, we can drive autonomous motion of the capsule in a field of urea. We find that asymmetry of the capsule, in geometry or chemistry, or both, greatly enhances capsule motion. In gradients of urea, our capsules display negative phoresis (move down the gradient). We have preliminary results for the urease-driven motion of Janus capsules, made by microfluidic assembly from mixtures of phase-separating polymers, as a function of the geometry of the capsules.
In the end, we draw analogies between our biological and bio-inspired motile systems, ultimately finding they have little in common.