The electrical sheet resistance between living cells grown on planar electronic contacts of semiconductors or metals is a crucial parameter for bioelectronic devices. It determines the strength of electrical signal transduction from cells to chips and from chips to cells. We measured the sheet resistance by applying ac voltage to oxidized silicon chips and by imaging the voltage change across the attached cell membrane with a fluorescent voltage-sensitive dye. The phase map of voltage change was fitted with a planar core-coat conductor model using the sheet resistance as a free parameter. For nerve cells from rat brain on polylysine as well as for HEK 293 cells and MDCK cells on fibronectin we find a similar sheet resistance of 10 MOhm. Taking into account the independently measured distance of 50 nm between chip and membrane for these cells, we obtain a specific resistance of 50 Ohmcm that is indistinguishable from bulk electrolyte. On the other hand, the sheet resistance for erythrocytes on polylysine is far higher, at around 1.5 GOhm. Considering the distance of 10 nm, the specific resistance in the narrow cleft is enhanced to 1500 Ohmcm. We find this novel optical method to be a convenient tool to optimize the interface between cells and chips for bioelectronic devices.
Fig. 3. Fluorescence lock-in imaging of erythrocyte ghost. (a) Fluorescence image of erythrocyte ghost attached to a silicon chip with poly-L-lysine and stained with BNBIQ. (b,c) Measured phase and amplitude map of the transfer functions {hJM,hFM} at f=25kHz. (d,e) Phase and amplitude profile across the attached ghost along the horizontal bar shown in (a). The area contact model is fitted to the data with a sheet resistance rJ = 1500MOhm (solid line) and plotted also for 500MOhm (dashed line). (f,g) Theoretical phase and amplitude maps.
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