This Small Business Technology Transfer Phase I project seeks to develop a flexible, pressure-sensitive membrane comprising chemically functionalized multiwalled carbon nanotubes (f-MWCNTs) to address several issues that hamper the use of existing piezoresistive membrane materials: a) interfacial integrity, b) fatigue life related to the number of pressure application cycles, and c) cost effectiveness. A piezocomposite sensor converts mechanical energy into an electrical signal and shows reversibility of electromechanical response in the elastic region of deformation. Commercial piezoresistive composites based on lead zirconia titanate (PZT) or metallic fillers lack reliability over time because of brittleness and poor adhesion to the substrate. Piezoresistive composites incorporating conductive carbon black suffer from nonlinearity, non-uniformity, hysteresis and limited sensor life. While use of f-MWCNTs to form the conductive network will enable piezo-resistive behavior, their use will also convey multifunctional aspects to the membrane: improved fatigue life, physical strength, and thermal conductivity (reducing heat buildup that contributes to fatigue). The Phase I project will tackle the initial development phase of a piezo-resistive nanocomposite (PZ-NC) elastomeric membrane incorporating homogeneously dispersed f-MWCNTs as the conductive element. Ultimately, the PZ-NC membrane thus developed will be integrated into a Schottky diode array element for repetitive use, large-area pressure sensors.
The broader impact/commercial potential of this project extends to several distinct market verticals. Modern biometric analysis, such as fingerprinting, is accomplished with digital scanners, where images can be distorted by sweat or excess pressure. Alternatively, pressure sensitive membranes can be used for fingerprinting where a load of 0-5 N results in a significant change in resistance of the membrane. For this type of application the increase in horizontal conductivity with pressure and the gradient response of the conductivity are vital. The proposed technology of PZ-NC, if successful, will address these issues, and will significantly improve the accuracy of the latter technology, for example by lowering the False Acceptance Rate (FAR). Furthermore, elastomer seals and gaskets are routinely used in oil and gas operations. The ability to quickly and accurately measure pressure on sealing elements in difficult-to-reach locations is a critical and currently underserved need. Pressure sensitive seals, comprising PZ-NC materials such as those proposed herein, could relay pressure changes or overpressure conditions, giving operators an indication of impending danger or of required maintenance.
This Phase I STTR project was directed at the development of a piezoresistive elastomer nanocomposite, e.g. a thin polymer membrane whose electrical properties are altered by the application of an external force to the membrane. The project successfully developed a composition whose electrical resistance changes by up to five orders of magnitude upon the application of a compressive force as low as 25 N, analogous to a light touch with a finger. Development of this composition involved new technologies for the dispersion of nanomaterials within a polymer matrix to tune the response of a membrane fabricated from the composition. The project also successfully completed initial stages of integration of the membrane with a flexible electronics based measurement system to provide a means of extracting information from the membrane. The diode technology employed will ultimately allow a sensor with exquisite sensitivity both in terms of the force applied to the membrane as well as the geographic location of the applied force, down to a resolution of approximately 0.004". Due to its unique sensitivity and resolution, the technology developed in this Phase I effort has a multitude of potential applications in both consumer level devices and industrial and military sensors. Applications under development include a Braille reading device to assist blind and low vision individuals who are not Braille literate, and industrial and military pressure mapping sensors to provide accurate information about forces impinging upon the membrane in various scenarios.
