Compliant Piezoresistive e-Skin for force and location sensing
Hussein, Zakareya Elmo
Electronic skin (e-skin) is an emerging field with an increasing number of applications such as biomedical, wearables sports and health monitoring, virtual and augmented reality. Less obvious use-cases can also be found in smart tyres for self-driving cars, structural integrity monitoring, and environmental pressure mapping. The e-skin market is growing at a fast pace, with a CAGR of 38.7% from its current worth of around ~$500 million. To date, it has been impossible to integrate the variety and density of mechanoreceptors found in human skin in a compliant form-factor with a similar Young’s modulus and other mechanical properties. Recent developments that attempt to combine non-tactile sensory inputs (e.g. temperature, humidity) with tactile ones sacrifice tactile feedback. A domain that has been labelled a grand challenge is that of robotic dextrous manipulation. Indeed, the attempt to input tactile data into machines is the birthplace of e-skin. No better is the challenge illustrated than with the evolutionary marvel of the human hand, and how our nervous system and brain use real-time feedback, control, and actuation to manipulate the world around us. Tactile inputs are force inputs. Thus, measuring force is chosen as the focus of this dissertation, which aims to tackle challenges in making e-skin ubiquitous, with a focus on robotics applications. The first two parts of this dissertation review the various sensing modalities which can be used to measure forces in e-skin, including capacitive, piezoresistive, electromagnetic induction, magnetic field detection, optical, piezoelectric, and triboelectric. Their respective strengths, weaknesses, and complementarity are weighed to justify the selection of the piezoresistive modality for static and low-frequency sensing. Furthermore, a broad literature review is given on piezoresistive sensing methods with a focus on traditional strain gauges and piezoresistors, liquid metal, percolation dominated polymer composites, tortuous strain gauges, crack-enhanced strain gauges, contact-resistance based sensors, and quantum tunnelling dominated polymer composites. These are also weighed with specific focus on their potential for compliance, mass-production, and repeatability to select two which are studied in more detail. The third part goes through the various characterisation tools and techniques used in this thesis. The fourth part of the dissertation addresses sensitivity, stretchability, robustness, and mass-producibility of a flexible thin film strain gauge sensor on a PDMS substrate. Attention is paid to selecting gold as the metal with the best properties by observing its behaviour under tensile strain. Controlling metal deposition thickness using sputtering leads to out-of-plane buckling causing wrinkles to form, which enables minor stretching. This in addition to a thin spin-coated substrate and surface-modified PDMS adhesion layer, yield a highly sensitive and compliant strain gauge that can be wrapped around most surfaces, the sensitivity of which can be increased by inducing microcracks in the metal layer. The use of shadow masks enables mass-production of strain gauges with 100% yield. However, an attempt to use diaphragm strain gauges in robotics applications fails due to hysteresis, creep, and fragility of the sensors. Subsequent electromechanical characterization reveals that although the gauge factor is high at ~1447, the stretchability of the substrate is only ~0.74%, causing catastrophic failure due to crack propagation at low strains. The three following parts of this thesis focus on the design and fabrication of an e-skin system by screen printing a novel magnetite micro-particle based quantum tunnelling material. It describes the design process for a glove made from the e-skin, which itself is produced using the tunnelling ink as well as other composite inks containing conductive fillers. A sensor design is also produced which measures magnitude of pressure and its location in real-time while minimising wiring for the overall system, so that it can be placed on each phalange of a dextrous humanoid hand or end-effector. The e-skin itself is compliant, highly sensitive, with IoT-ready wireless readout electronics. Its practical capabilities as an array as well as its robustness are demonstrated. A variety of different encapsulating materials are compared, their data analysed, and the sensor is successfully tested under seawater (a harsh environment). Complex grasping and manipulation of various objects is also demonstrated. Finally, the sensors are compared to existing industrial force-sensitive resistors (FSRs) with gold-standard dead-weight testing, concluding that they can measure ~20 times less force with the same repeatability while additionally providing information on location. This makes the sensors useful for many other real-world applications.