Integrated Pressure/Temperature Sensor Array Based on Nickel Conductive Composite

Abstract

Implementation of electronic artificial skin has been widely studied, from basic concept to prototypes, for potential applications in robot engineering and prosthetic replacement. Electronic Artificial skin plays a key role of sensing external environment, such as pressure and temperature, and delivering transformed signals either to robot control or human nerve system. In order to truly mimicking human skin, artificial skin at least needs to contain both pressure and temperature sensing elements in an array format. In fact, a couple of trials have been attempted to integrate sensing both elements onto single skin. Combination of commercial temperature sensing chips with printed pressure sensitive resistor or assembly of separately fabricated sensor arrays of each type has been demonstrated. These hybrid type integration or assembly approach renders rather complicated processes and thus increases fabrication cost. For sensing elements, conductive composite materials have been commonly used, whose resistance changes as geometrical dimension changes with applied pressure or temperature. In most cases, the conductive composite materials have been used only for single type of sensing element, either pressure or temperature sensor. It is challenging to differentiate two type of sensing part in one substrate with single conductive composite material and to independently read out each signal. Therefore, there have been no reported researches on using single conducting composite materials to a multi-sensing device. In addition, the conductive composite materials were typically fabricated "on" either flexible or stretchable substrate only after readout active-matrix circuitry was fabricated on the substrate. Therefore, there can be limitation in selection of materials and device structure, and process incompatibility that can makes mass manufacturing of the active-matrix sensor arrays difficult. However, when the sensor arrays are separately fabricated by embedding the sensing elements in the substrate, they can be easily incorporated into passive-matrix system or can be simply laminated on the separately fabricated active-matrix circuitry, as in case of the electronic paper front-plane technology. In this thesis, a simple fabrication method of integrated pressure/temperature sensor arrays by embedding conductive nickel (Ni) particles in poly(dimethyloxane) (PDMS) medium for electronic artificial skin application will be elucidated. The pressure and temperature sensing parts are formed in one pixel but have different heights, which are implemented by introducing a corrugated structure to Ni/PDMS composite with a pre-patterned aluminum mold. Since Ni particles are ferromagnetic materials, Ni/PDMS mixture can be patterned by exposure to patterned magnetic fields. Magnetic field exposure helps both lateral patterning and vertical particle alignment, which directly improved sensitivity and linearity of the sensor. Independent and stable read-out signals for pressure and temperature sensors are successfully obtained even under repeated measurements. This technology has advantages of simple tuning for sensitivity and operation ranges by changing particle concentration and device physical dimension, easy scaling-up to large area by seamlessly bonding small arrays or using large-area magnetic field modulator, and potential implementation of the sensor frontplane for active-matrix backplane read-out circuitry. Electronic artificial skin passive-matrix system with about 10 ppi resolution with the integrated 16 by 16 pressure and 15 by 15 temperature sensor arrays have been finally demonstrated. Furthermore, a highly stretchable electrode with demonstration of a resolution sustaining lighting device by fully utilizing the magnetic patterning/aligning method will be also studied. This stretchable electrode based on conductive composite shows unique property that is negative strain-dependency in electrical resistance. Although cyclic behavior of pure nickel composite needs more improvement, nickel-based composite materials have excellent advantages over other materials in terms of simple patterning and in-situ embedding in the matrix. This novel technology would be one of the key enabling technology in implementing future stretchable electronic display devices.