Cell Impedance Probes

Peter Han, Geoff Zawtocki, Robert Halk

Major errors occur with biological conductivity measurements within a test cell due to electrode polarization at the interface between the electrode and the biological sample. In a test cell, the dependence of the electrode polarization upon frequency, current density, electrode geometry, and temperature makes it difficult to accurately measure th conductivity of a biological sample. To accurately measure conductivity changes in a biological sample as a function of temperature, the electrode polarization of the test cell must be determined as a function of temperature. The electrode polarization must then be subtracted from the impedance measured across the test cell to provide an accurate conductivity measurement of a sample.

To measure the conductivity of biological samples, a self contained unit is needed which can uniformly heat a sample and measure its conductivity independent of the polarization impedance.. For these measurements, one must take into account that the polarization impedance may change as a function of temperature.

At present, effective hyperthermia treatments require a detailed knowledge of temperature distributions throughout the treatment area. Temperature must be monitored quickly and accurately to ensure efficient and effective treatment. Unfortunately, temperature distributions are difficult to obtain within the ody. Accurate temperature measurements require the use of many invasive probes throughout the treatment region. The size of temperature probes makes it impossible to provide enough probes to the region to accurately measure its temperature distribution. Therefore, there is a need for a non-invasive or semi-invasive approach to detect histological changes within tissue to ensure efficient and effective treatment.

Tissue and cells exposed to hyperthermic temperatures show important irreversible histological changes that affect response to repeated hyperthermic treatments. Changes in tissue and cell structure appear to be detectable as changes in the conductivity of samples undergoing hyperthermic treatments. Significant differences in the conductivity of samples can be seen between a sample before and after an initial hyperthermic treatment. It is unclear as to the process by which the cell structure and the internal structure of samples are changed due to hyperthermic treatments. The ability to reate histological changes in tissues and cells to changes in conductivity would enable therapy to be monitored and modified by mapping conductivity changes throughout the treatment area. Ideally, this could provide a fairly non invasive and economical approach to implementing hyperthermia treatments.

SEM micrographs of fabricated probes

SEM micrograph of a fabricated cell probe. SEM of the single moving hinge device (previously described) from MCNC. Another view of the single moving electrode device. SEM of impedance measuring device with two identical, complementary electrodes (designed by Bob Halk). Each electrode has a fixed hinge assembly (middle) and a moving hinge. On the right and left side of the picture are push rods which are mechanically actuated by the two rectangular fixtures attached to them above and below. When pushed up, the two electrodes stand perpindicular to the substrate, with a sample to be placed in between them. The dark coils in between the electrodes is a resistive heating coil which serves to vary the impedance measuring temperature. The darker areas in the SEM are the polysilicon layers and the silicon substrate. The gold conductive layer is white in the SEM. The large white square near the bottom is one of the contact pads for the resistive heater. SEM of a released and actuated dual moving electrode device. Close-up SEM of one of the dual moving electrodes in its upright position. Both the free and fixed hinge are clearly visible. Super close up of a fixed hinge. Close view of an unreleased moveable hinge.

DESIGN layout for probes

Complete View This is a planview of the micro-electromechanical impedance testing device. The four large squares bordering the picture are gold electrical contact pads on a bed of polysilicon. The one different colored contact pad sits on an additional layer of polysilicon (this is to keep the wire connecting the movable electrode to its respective contact pad on the same level, so that there are no steep drop-offs between polysilicon layers, during manufacture, in order for the gold wire to be continuous upon deposition). There is a fixed electrode surrounded by a Polysilicon/oxide wall (to hold a test cell in place), a thermocouple junction connected to two contact pads, and a movable hinge/electrode assembly that allows the movable electrode to lie directly over the fixed electrode (when a sample is in place) at a perpendicular distance of 60 micrometers.

Movable Electrode/Free Hinge This is a blow-up and cross section of the movable electrode assembly and the free hinge assembly. The hinge is a double interlocking design, which allows rotation in two directions. The movable electrode is 60 micrometers in diameter. This electrode is designed to lift off from the substrate by mechanical manipulation of the round handle with a hole in it (so that no contact with the electrode has to be made), and lie directly over the fixed electrode when a sample is in place.

Fixed Hinge/Polysilicon-oxide wall This is a blow-up and cross section of the fixed hinge assembly and a polysilicon/oxide wall which serves to prevent the fixed hinge from rotating too much past 90 degrees (from the substrate).

Wire Connecting Movable Electrode to its Contact Pad This is a blow-up and cross section of the wire connecting the movable electrode to a contact pad. It was intentionally given a zig zag shape, so that the gold wire will not mechanically fail (due to bending stress) when the electrode is lifted off the surface of the substrate.

Free Hinge Assembly This is another blow-up and cross section of the free hinge assembly.

This design summary written and submitted byPeter.Han@dartmouth.edu

Other student designs

Christopher.G.Levey@ dartmouth.edu