Joule Heating-Induced Dispersion in Open Microfluidic Electrophoretic Cytometry.

While protein electrophoresis conducted in capillaries and microchannels offers high-resolution separations, such formats can be cumbersome to parallelize for single-cell analysis. One approach for realizing large numbers of concurrent separations is open microfluidics (i.e., no microchannels). In an open microfluidic device adapted for single-cell electrophoresis, we perform 100s to 1000s of simultaneous separations of endogenous proteins. The microscope slide-sized device contains cells isolated in microwells located in a ∼40 μm polyacrylamide gel. The gel supports protein electrophoresis after concurrent in situ chemical lysis of each isolated cell. During electrophoresis, Joule (or resistive) heating degrades separation performance. Joule heating effects are expected to be acute in open microfluidic devices, where a single, high-conductivity buffer expedites the transition from cell lysis to protein electrophoresis. Here, we test three key assertions. First, Joule heating substantially impacts analytical sensitivity due to diffusive losses of protein out of the open microfluidic electrophoretic (EP) cytometry device. Second, increased analyte diffusivity due to autothermal runaway Joule heating is a dominant mechanism that reduces separation resolution in EP cytometry. Finally, buffer exchange reduces diffusive losses and band broadening, even when handling single-cell lysate protein concentrations in an open device. We develop numerical simulations of Joule heating-enhanced diffusion during electrophoresis and observe ∼50% protein loss out of the gel, which is reduced using the buffer exchange. Informed by analytical model predictions of separation resolution (with Joule heating), we empirically demonstrate nearly fully resolved separations of proteins with molecular mass differences of just 4 kDa or 12% (GAPDH, 36 kDa; PS6, 32 kDa) in each of 129 single cells. The attained separation performance with buffer exchange is relevant to detection of currently unmeasurable protein isoforms responsible for cancer progression.