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Circulating Tumor Cell Filter

The filtration mechanism

The idea of filtering cancer cells from blood is not new. Previous attempts used static filters, or perhaps a graduated array of filters of decreasing size, and struggled with clogging. Simply put, cancer cells or other large bodies in the bloodstream would become ensnared in the filter and a backlog would ensue. To address this concern, we developed a mechanism that allowed us to adjust the filter's pore size. By periodically expanding the filter and releasing the captured cells, we effectively restore the mechanism to its original state and eliminate clogging based failures.

To build this adjustable filter, we borrowed an idea from the microfluidic valve. Just like a valve we use a two-layer construction, forming a thin diaphragm between the top and bottom layers that can be moved up and down with a differential pressure. This allows us to adjust the height of the top channel, the critical dimension that determines whether a cell passes through or gets captured. Unlike in valves, we don't want the top channel to seal completely, so we added a center fin to act as a mechanical stop, limiting the deflection of the membrane.

While the device is operating, the filter will assume one of two positions:

  • Constricted: in which the diaphragm is raised into the center fin, and the resulting channel is large enough to allow blood cells through but small enough to capture cancer cells
  • Relaxed: in which the diaphragm is lowered, and all cells (including those captured earlier) can flow through

A) A CAD mockup of the two layer device. B) The actual profile of a master mold I made in the cleanroom, measured using interferometry.

Next, we'll look at how this mechanism was implemented in a full microfluidic device to filter millions of cells.