Microsensors Fabricated from Piezoresistive Elements to be Used With Sensing Materials Which Change Thickness or Volume on Exposure to Analyte.

       The construction of rugged, cheap, reliable, and small chemical microsensors whose output signal can be expressed in terms of  DC conductivity is of current interest for constructing devices that can detect and identify chemical vapors alone or in a complex mixture.Ideally such sensors should be able to function in either a liquid or vapor environment. 

    We propose a chemical or biological microsensor device which directly measures the "swelling" of polymeric or biomolecule layers upon exposure to analytes.  The device can be used with composite materials as well as pure polymers and can also be used in either with either liquid or vapor phase analytes.  For large scale applications polymers can be dissolved in solvents and deposited on ceramic substrates by spin coating, drop deposition or by ink jet printer technologies.  In these systems the adhesion of the film to the ceramic is relatively strong and reliable over time.  Should difficult situations arise the surface of the (single component) ceramic can be derivatized for better adhesion. The lateral size of this film need only be a few microns; thus, applications using expensive materials as the sensing element are possible. 

    Using a simple mechanical approach mechanism, a piezoresistive cantilever (commercially available from Cantimer Corp., Menlo Park, CA) is brought into contact with the sensor film.  These cantilevers are only 100-200 microns long, and about 50 microns wide.  These cantilevers contain an internal channel of piezoresistive material, connected to two tiny external electrodes. The non-stressed resistance of these cantilevers is on the order of 2.3k ohms, but changes rapidly and measurably in response to any bending of the cantilever. In fact, these cantilevers are sensitive enough to measure bending strains of only a few tens of Å.

     Any swelling of the active material in contact with the cantilever tip will result in an immediate, easily measurable change in the cantilever channel resistance.  This change will be in exact proportion to the amount of the vertical swelling, a simple multi-meter is thus sufficient to record the sensing activity.  These cantilevers are fabricated using standard semiconductor techniques and are not subject to degradation due to chemical exposure except in all but the most extreme cases (such as HF exposure). 

Fig. 2.  Diagram of multiple sensor array integrated onto a single, tiny chip.

    A single sensor based on the above design would occupy only a tiny area.  Because of this small size, large numbers of Piezoresistive cantilever based sensors could be incorporated onto a single substrate (up to 100 on a 1 cm2 substrate).  Each could utilize a different active sensing material of either single polymer or composite.

    These sensors also differ from sensing devices based on vibrating cantilever technologies. Using these previous techniques, a small cantilever is driven into oscillation at one of its resonant frequencies using external circuitry.  The cantilever itself is coated with some active sensing material.  Adsorption of analyte molecules on the vibrating cantilever changes the frequency or amplitude of the vibration, and this change is sensed by internal or external electronic circuitry.  Sensors based on this technology require extensive electronic circuitry, both to drive the cantilever, and in to sense the change in cantilever frequency/amplitude upon analyte exposure.  In addition, fabricating arrays consisting of many, close packed vibration cantilevers is extremely difficult due to differences in cantilever resonant frequencies, and the proximity of the cantilevers themselves.  Finally, these vibrating cantilever sensing devices are highly subject to external vibration or movement, making fabrication of truly portable devices difficult.

Chemical Sensors

      We have tested these devices using common polymeric materials as the active sensing element.  These polymers include PVA, PEVA, and PIB.  The measured response of these sensors to a 50% saturated vapor of several organic analytes is indicated in the table below.

    All three polymer sensors responded to all of the various analytes, however a unique signature for each analyte is obtained by using the three sensors in combination.

Biological Sensors

    We have also used these sensors to detect the presence of certain biological molecules.  By forming a "sensing layer" comprised of thiolated single-strand DNA (SS DNA), we were able to measure the cantilever response to the presence of the complimentary DNA strand in solution.  For these experiments, 25-base SS DNA was used.  The sensors were able to discriminate between the exact complimentary strand (full response), and strands differing by a single base (50% response), and by 5 bases (10% no response). 

Detection of complimentary SS-DNA by piezoresistive microcantilever.

Response of Cantilever to DNA Strands