  We demonstrate a method of concentrating and patterning of biological cells
on a chip, exploiting the confluence of electric and thermal fields, without
necessitating the use of any external heating or illuminating source. The
technique simply employs two parallel plate electrodes and an insulating layer
over the bottom electrode, with a drilled insulating layer for inducing
localized variations in the thermal field. A strong induced electric field, in
the process, penetrates through the narrow hole and generates highly
non-uniform heating, which in turn, results in gradients in electrical
properties and induces mobile charges to impose directional fluid flow. The
toroidal vortices, induced by secondary electrokinetic forces originating out
of temperature-dependent electrical property variations, transport the
suspended cells towards a hot-spot site of the chip, for rapid concentrating
and patterning into different shaped clusters based on pre-designed conditions,
without exceeding safe temperature limits that do not result in damage of
thermally labile biological samples. We characterize the efficacy of the cell
trapping process for two different biological entities, namely, Escherichia
coli bacteria and yeast cell. These results may be of profound importance
towards developing novel biomedical microdevices for drug discovery, antibiotic
resistance assessment and medical diagnostics.
