Recent advances in microfluidics and the lab-on-a-chip field have created enabling technology platforms for effective sorting and analysis of biological cells. Among those, the platforms based on electrokinetic principles are drawing increasing attention because of their label-free and cost-effective nature. Nevertheless, these electrokinetic-based platforms suffer from issues such as challenging electrofluidic integration, electrode fouling, contamination, bubble formation, and limited field penetration into the sample flow, all mainly linked to the use of thin-film surface microelectrodes and their 2D profile. Although these issues could be addressed by bulk microelectrodes in the form of metal or doped silicon blocks acting as fluidic channel sidewalls, an electrical wiring l...[ Read more ]

Recent advances in microfluidics and the lab-on-a-chip field have created enabling technology platforms for effective sorting and analysis of biological cells. Among those, the platforms based on electrokinetic principles are drawing increasing attention because of their label-free and cost-effective nature. Nevertheless, these electrokinetic-based platforms suffer from issues such as challenging electrofluidic integration, electrode fouling, contamination, bubble formation, and limited field penetration into the sample flow, all mainly linked to the use of thin-film surface microelectrodes and their 2D profile. Although these issues could be addressed by bulk microelectrodes in the form of metal or doped silicon blocks acting as fluidic channel sidewalls, an electrical wiring layer is often required to interconnect the spatially distributed blocks. More importantly, the linear sidewalls of such blocks prohibit bulk microelectrodes from projecting a genuine 3D profile, the presence of which could not only ease the overall electrofluidic integration but also couple the electric and hydrodynamic flow fields in ways that are beneficial to cell sorting.

This work introduces silicon blocks featuring nonlinear sidewalls and demonstrates their utility as building blocks for the construction of unique bulk microelectrode designs with greatly simplified electrofluidic integration and effective penetration of field into the sample flow. Specifically, an interdigitated comb array with built-in fluidic pores along the array digits is demonstrated for the selective isolation of colorectal tumor cells from lymphocytes under dielectrophoretic forces. The design, because of its 3D profile, can sort cells at a high loading density (10⁷ cells mL^-1) and yet still achieve a rate of cancer-cell recovery 82% and an efficiency of blood-cell removal 99%. Another design, which contains a trifurcation at the downstream of a flow channel that is defined between a pair of bulk microelectrodes is demonstrated for blood plasma extraction in a continuous-flow under the combined effect of electrothermal drag and dielectrophoretic forces. Plasma from 1.5 μL of whole blood diluted to 4% hematocrit in a high-conductivity buffer (1.5 S m^-1) is extracted in a continuous flow at a fraction of 70%; the extracted plasma is nearly 99% pure. The plasmas obtained using this device and using conventional centrifugation and sedimentation are of comparable quality, with negligible hemolysis and protein denaturation due to thermal gradients. The device achieves high target-molecule recovery efficiency, delivering 90% of the proteins detected in the plasma obtained using sedimentation. The clinical utility of the extracted plasma is further validated on the detection of prostate-specific antigen at clinically relevant levels. Lastly, a single-cell impedance flow cytometer is presented for the first time based on bulk microelectrodes featuring linear sidewalls, thus relieving the burden of alignment that comes with thin-film surface microelectrodes and their opposing arrangement for a maximum sensitivity. Using the cytometer, colorectal cancer cells and blood cells can be rapidly and accurately identified based on their impedance signatures. An improved design with a potentially enhanced sensitivity is also described based on silicon blocks featuring nonlinear sidewalls.