Caroline Multari

Broadband Electrical Detection of Individual Biological Cells

To resolve the dilemma of cell clogging or solution parasitics encountered by Coulter counters and to evolve a general-purpose electrical detection technique, we used broadband microwave measurements to overcome electrode polarization, ac dielectrophoresis to precisely place cells between narrowly spaced electrodes, and relatively wide microfluidic channels to prevent cell clogging. This unique combination of approaches resulted in reproducible sensing of single Jurkat and HEK cells, both live and dead, of different cultures at different times.

Assessment of Cytoplasm Conductivity by Nanosecond Pulsed Electric Fields

The aim of this paper is to propose a new method for the better assessment of cytoplasm conductivity, which is critical to the development of electroporation protocols as well as insight into fundamental mechanisms underlying electroporation. For this goal, we propose to use nanosecond electrical pulses to bypass the complication of membrane polarization and a single cell to avoid the complication of the application of the “mixing formulas.” Further, by suspending the cell in a low-conductivity medium, it is possible to force most of the sensing current through the cytoplasm for a more direct assessment of its conductivity. For proof of principle, the proposed technique was successfully demonstrated on a Jurkat cell by comparing the measured and modeled currents. The cytoplasm conductivity was best assessed at 0.32 S/m and it is in line with the literature. The cytoplasm conductivity plays a key role in the understanding of the basis mechanism of the electroporation phenomenon, and in particular, a large error in the cytoplasm conductivity determination could result in a correspondingly large error in predicting electroporation. Methods for a good estimation of such parameter become fundamental.

Broadband microchamber for electrical detection of live and dead biological cells

A novel broadband microchamber for electrical detection of live and dead biological cells was designed, fabricated and tested. The microchamber was formed between a gold coplanar waveguide fabricated on a quartz slide and the microfluidic channels fabricated in a polydimethylsiloxane cover. The coplanar waveguide allowed broadband impedance matching and efficient cell trapping. The microfluidic channels delivered single cells precisely. Tests on Jurkat cells in both time and frequency domains showed that live cells had lower resistance but higher capacitance than that of dead cells.

Improved broadband electrical detection of individual biological cells

Based on a homemade probe station on top of an inverted microscope for simultaneous microwave measurement and visual validation, broadband detection of live Jurkat cells was successfully extended from 2-3.5 GHz to 0.5-20 GHz with comparable sensitivity and reproducibility. With a carefully optimized coplanar waveguide, closely spaced microwave probes, and frequent calibrations, reference planes were established next to the microfluidic channel, which resulted in smooth and well-behaved scattering parameters without spurious resonances. From the measured scattering parameters, the extracted cytoplasm resistance of 190 kΩ was consistent with the previously reported value but validated over a much wider bandwidth.

Broadband single-cell detection with a coplanar series gap

Using a coplanar waveguide with a series gap in conjunction with dielectrophoresis trapping, consecutive S-parameter measurements between 0.5 and 20 GHz were quickly performed with and without a Jurkat cell trapped to compensate for a relatively noisy and drifting background. This allowed the small cytoplasm capacitance, on the order of 10 fF, to be reliably extracted. The extracted cytoplasm capacitance is within a factor of 2 of the previously reported value by using a shunt trap but is believed to be more accurate. The present technique can complement previously developed microwave and RF techniques in characterizing the capacitances and resistances of plasma and membrane for complete characterization of the electrical properties of a simple cell.

Reproducible broadband measurement for cytoplasm capacitance of a biological cell

Using a coplanar waveguide with a series gap in conjunction with dielectrophoresis trapping, consecutive S-parameter measurements between 0.5 and 20 GHz were quickly performed with and without a Jurkat cell trapped to compensate for a relatively noisy and drifting background. Based on sixteen measurements repeated on eight live cells and eight dead cells, differences in both return and insertion losses show two distinct distributions indicating either return loss or insertion loss alone can be used to distinguish a live cell from a dead one. Further, since the frequency dependence is generally linear or absent, discrete-frequency measurement (as opposed to sweep-frequency measurement) of return or insertion loss may suffice. If proven statistically by a much larger number of cells, this should greatly speed up the measurement to facilitate its eventual field use.