Written by Sumaiya Sayeed, '19.5
Edited by Lauren Anderson, '21
In a sample of many types of cells amidst a body of fluid, it can be difficult to capture the tiny objects for diagnostic purposes. What if we could squeeze these cells through tiny pores? Researchers have done just that.
As centuries of biology have taught us that different types of cells maintain various properties, and as the hard work by biologists have been able to identify molecular pathways and protein expression unique to certain cell types, the field of diagnostics harnessing these differences has emerged. The most widespread example of this is using blood samples to detect circulating tumor cells (Pantel, Speicher 2016). While theoretically these differences between cells should be enough to isolate desired cell types, practical challenges impede these processes. For instance, what if there is one circulating tumor cell for every billion red blood cells?
Until now, the main mode of capturing rare cells is through engineering systems that specifically trap particular cells. Because all cells display unique proteins on their membrane that identify them, researchers have developed specific antibodies, or particles that attach to desired proteins to capture cells. These antibodies are engineered to be attached to a small magnetic bead. Thus, the cells of interest should thus be attached to these beads, which can then be attracted toward a magnet for capture. The system, while ingenious and used in a variety of applications, is limited by its reliance on a high population of cells. (Jain et. al. 2013)
Recently, focus has shifted from protein biomarkers on cells to cellular mechanical properties. Particularly, cellular stiffness may prove to be an effective property that can differentiate between rare cells and the surrounding cells in clinical samples. Researchers in the University of Miami (Aljaghtham et. al. 2019) used numerical simulations to model cell sorting using stiffness. In solid mechanics, more “stiff” is defined as requiring more force to change shape. By applying hydrodynamic forces on cells passing through a membrane, they can be separated based on whether they are soft or stiff. If the cells are soft, they can be deformed by the applied force to pass through a thin pore. If they are stiff, and thus unwilling to be squeezed or bent, they will become trapped in a membrane pore unless more force is exerted to move them.
Applying this concept to a device, researchers can design a channel with continuous flow with these membranes to separate cells. Force applied on the cells is varied by the velocity of the flow. As shown in the figure from the paper, as flow rate increases, more force is applied to the cell. At the low flow rate (4 L/min), there is not enough force to push the cell through the pore. In contrast, 8 L/min is sufficient to push the cell through the pore.
Technologies that utilize cellular mechanics rather than protein expression may serve as a new frontier for harnessing cell separation. Not only does mechanics enable capturing of cells, it enables their preservation, as the cells are not tagged with antibodies and magnetic beads. At the same time, much optimization is required before this can be used. What if a cell is has a small diameter and ends up passing through any membrane pore? What happens if cells begin to clog the membrane? While the process still is not ready for clinical usage, it offers new methods of separating cells that remain rare and untouched.
Aljaghtham, Mutabe S., et al. "Numerical simulations of cell flow and trapping within microfluidic channels for stiffness based cell isolation." Journal of biomechanics (2019).