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Recent developments in telemedicine have caused significant interest in the prolonged monitoring of bioelectric signals. This drives the search for easy-to-use, biocompatible, and environmentally friendly alternatives to conventional resistive wet electrodes. Here we demonstrate the use of Coconut-Oil and Carbon Black based stretchable dry electrodes to monitor electrophysiological signals without the need for conductive gels. The developed material is embedded into an elastomer matrix, exhibits a specific resistance ρ of 33.2 12.3 Ω m, high conformability, and a stretchability up to 1500±%. The realised epidermal electrodes were used to record Electrocardiographic (ECG) signals in a 3-lead configuration and compared to commercial wet electrodes. Even after being elongated by 100 % for 100 stretch/release cycles, a reliable recording of the QRS-complex is demonstrated without the need for any contact enhancing or skin irritating substances, proving its potential use in long term ECG monitoring applications.

Capacitive non-contact imaging of electric fields and potentials with micro-metre resolution can provide relevant insights into material characterisation, structural analysis, electrostatic charge imaging and bio-sensing applications. However, scanning electric potential microscopes have been confined to rigid and single-probe devices, making them slow, prone to mechanical damage and complex to fabricate. In this work, we present the design and characterisation of a novel 5-element flexible array of electric potential probes with spatial resolution down to 20μm to speed up the scanning time. This was achieved by combining flexible thin-film probes for active guarding and shielding with state-of-the art discrete conditioning circuits. The potential of this approach is showcased by using the fabricated array to image latent fingerprints deposited on an insulating surface by contact electrification, obtain the surface topography of conductive samples and to visualise local dielectric variations.

Measuring electric potentials using encapsulated flexible probes is advantageous especially when the signal source is comprised of soft and irregular organic tissues, such as the human brain or muscle fibers. However, the capacitive nature of encapsulated probes demands circuits with ultra-high input impedance and low 1/f noise, which are not trivially achieved using flexible transistors. Here, we show an a-IGZO based capacitive bootstrapped cascode buffer where the input impedance is actively controlled to allow capacitive electric potential measurements. By exploring different biasing schemes of the cascode element, we show a relative gain of 55 dB and a bandwidth between 450 Hz and 19 kHz.

Non-contact imaging of electric potentials with micro-metre resolution can provide relevant insights in material characterisation, electrostatic charge imaging and bio-sensing applications. However, scanning electric potential microscopes have been confined to rigid and single-probe devices, making them slow, prone to mechanical damage and complex to fabricate. In this work, we present a novel 5-element flexible array of electric potential probes with spatial resolution down to $20 \mu \mathrm{m}$ which reduces the scanning time by a factor of 5 when compared to a single probe device. This was achieved by combining flexible thin-film probes for active guarding and shielding with state-of-the art discrete conditioning circuits. The potential of this approach is showcased by using the fabricated array to image latent fingerprints deposited on an insulating surface by contact electrification. This is the first example of a micro-scale array of electric potential sensors.

The zero potential technique is an efficient way to selectively condition passive resistive sensor arrays. However, for flexible wearable sensor systems, the currently utilised amplifiers are not desirable as they are normally based on rigid devices. We present a variation of the zero potential approach utilising flexible thin-film transimpedance amplifiers. This integrated amplifier is based on IGZO thin-film transistors (TFTs) and has a measured cut-off frequency of $\approx 4$ kHz, which is sufficient for most sensing applications. To understand the performance when conditioning a grid of sensors, the flexible system was simulated and compared to the results obtained from rigid transimpedance amplifiers. While these rigid devices led to virtually no crosstalk between sensors, the flexible transimpedance amplifier maintained a selectivity factor of nearly 3. At the same time, the fully flexible IGZO amplifier can be seamlessly integrated with a flexible sensor array and has a footprint of only 1.6 mm2. This approach can enable easily manufactureable, fully flexible, and wearable sensor systems with a minimum number of active devices, avoiding active matrix structures and individual sensor conditioning while maintaining a certain level of sensitivity.

In order to develop and evaluate the extent to which itching affects a person's daily life, it is useful to develop automated means to recognise the action of scratching. We present an investigation of sensors and algorithms to realise a wearable scratch detection device. We collected a dataset, where each user wore 4 inertial measurement unit (IMU) sensors and one electric potential sensor (EPS). Data were collected from nine users, where each user followed a 40-min protocol, which involved scratching different parts of head, shoulder, and leg, as well as other activities such as walking, drinking water, brushing teeth, and typing to a computer. The dataset contained 813 scratching instances and 5 h 15 min of recorded data. We investigated the trade-offs between the number of devices worn (comfort) and accuracy. We trained the k-NN and random forest algorithms by using between 1 and 5 features per channel. We concluded that a scratch could be detected with 80.7% accuracy by using the random forest algorithm on hand coordinates, which required four devices. However, an f1 score of 70% could be achieved with k-NN with IMU and EPS data, which only required one device. Moreover, the fusion of IMU data with EPS data improved the accuracy and reduced the deviation between the folds. This expanded the state-of-the-art method by opening up new trade-offs between accuracy and comfort for future research

Textiles enhanced with thin-film flexible sensors are well-suited for unobtrusive monitoring of skin parameters due to the sensors&rsquo, high conformability. These sensors can be damaged if they are attached to the surface of the textile, also affecting the textiles&rsquo, aesthetics and feel. We investigate the effect of embedding flexible temperature sensors within textile yarns, which adds a layer of protection to the sensor. Industrial yarn manufacturing techniques including knit braiding, braiding, and double covering were utilised to identify an appropriate incorporation technique. The thermal time constants recorded by all three sensing yarns was &lt, 10 s. Simultaneously, effective sensitivity only decreased by a maximum of 14% compared to the uncovered sensor. This is due to the sensor being positioned within the yarn instead of being in direct contact with the measured surface. These sensor yarns were not affected by bending and produced repeatable measurements. The double covering method was observed to have the least impact on the sensors&rsquo, performance due to the yarn&rsquo, s smaller dimensions. Finally, a sensing yarn was incorporated in an armband and used to measure changes in skin temperature. The demonstrated textile integration techniques for flexible sensors using industrial yarn manufacturing processes enable large-scale smart textile fabrication.

Conductivity is a fundamental property of materials; in particular the precise quantification of electrical sheet resistance is essential for the development of electronic thin-film devices. Conventionally, resistive probes are used to perform the corresponding measurements. However, non-invasive methods are more desirable as they minimize the required sample preparation, as well as geometrical influences. Existing non-contact conductivity measurements mostly rely on the transmission of electromagnetic waves through conductive thin-films. Hence, they only characterize translucent samples at high frequencies. We present an alternative technique based on the attenuation of low frequency 10 kHz electric fields. The approach is used to quantify the field-effect induced conductivity variation of a flexible Indium-Gallium-Zinc-Oxide semiconductor film. A custom-built high-impedance electrometer is used to capacitively measure the attenuation of an alternating electric field passing through the film. The obtained data is discussed and related to the absolute conductivity with the aid of two bespoke models describing the impedance mismatch between the sample and its surroundings. The sheet conductivity is modulated and measured from 660 nS to 116 µS while conventional DC current/voltage measurements serve as a reference. Both methods show a high degree of correlation to the reference measurements. Unlike techniques based on light, the low frequency signal used here resembles quasi-static characteristics, and enables the direct measurement of DC parameters.

Safety requirements and physical constraints often prohibit contacting high voltage terminals. This limits the options for monitoring and maintaining high voltage machinery and power distribution grids. We present a non-contact AC (alternating current) voltage and frequency measurement system with 600 mm of operation range to overcome this issue. The method relies on measuring the electric potential of an AC voltage source using a single capacitive electrometer. Simultaneously, the distance from the AC source is measured using time-of-flight sensors. By combining the data from both sensors, AC voltages can be measured accurately without the need for any galvanic contact. The system is packaged in a handheld battery powered form factor. Measurements of voltages between 25 VRMS and 250 VRMS and distances from 25 mm to 600 mm demonstrated accuracy within 4%. Furthermore, frequencies between 5 Hz to 500 Hz were measured with 10 mHz of resolution. The presented technique can be used as a test and measurement instrument where the source terminals are inaccessible or as a wearable safety device in high-voltage environments.

In this talk, we show how acoustic metamaterials can be used to build the acoustic equivalent of optical devices. We demonstrate two key devices: (1) an acoustic prism, used to send the different notes in a melody towards different directions, and (2) an auto-zoom lens, used to send sound to a moving target. We conclude, discussing potential applications and limitations.

In this paper, Low Earth Orbit radiation, temperature and magnetic field conditions are mimicked to investigate the suitability of flexible Indium Gallium Zinc Oxide transistors for lightweight space wearables. Such wearable devices could be incorporated into spacesuits as unobtrusive radiation detectors or physiological monitors. Due to the harsh environment to which these space wearables would be exposed, they have to be able to withstand high radiation doses, low temperatures and magnetic fields. For this reason, the impacts of high energetic electron irradiation with fluences up to 1E12 e−/cm2, low operating temperatures down to 78K and magnetic fields up to 11mT are investigated. This simulates 278 h in Low Earth Orbit. The threshold voltage and mobility of transistors that were exposed to e- irradiation are found to shift by +(0.09 +- 0.05)V and (0.6 +- 0.5) cm 2 V−1 s−1. Subsequent low temperature exposure resulted in additional shifts of +0.38V and -5.95 cm2 V−1 s−1 for the same parameters. These values are larger than the ones obtained from non irradiated reference samples. In addition, the performance of the devices was observed not to be significantly affected by the magnetic fields. Finally, a Cascode amplifier presenting a voltage gain of 10.3 dB and a cutoff frequency of 1.2 kHz is demonstrated after the sample had been irradiate, cooled down and exposed to the magnetic fields. If these notions are considered during the systems’ design, these devices can be used to unobtrusively integrate sensor systems into space suits.

A biocompatible inexpensive strain sensor constituting of an elastomer filled with natural coconut oil (CNO) and carbon black (CB) is presented here. Strain sensors are widely utilized for applications in human activity recognition, health monitoring, and soft robotics. Given that these sensors are envisioned to be present in a plethora of fields, it is important that they are low cost, reliable, biocompatible, and eco-friendly. This work demonstrates that CNO can be used to create conductive percolation network in elastomers, without the necessity for harmful chemicals or expensive machinery. The sensor has a gauge factor of 0.77 ± 0.01, and the sensing material has a porous morphology filled with an oily suspension formed of CNO and CB. Results indicate that the liquid filled porous structure can improve the reliability of these resistive strain sensors in comparison to sensors fabricated utilizing commonly used non-polar solvents such as heptane. Consequently, the sensor demonstrates a hysteresis of only 2.41% at 200% strain over 250 stretch/release cycles. Finally, to demonstrate the potential of this fabrication technique, a functionalized glove is developed and used to detect wrist motion. These easily manufacturable and cost-effective sensors enable wearable on-skin ergonomic intervention systems with minimal impact on the environment.

Acoustic cameras are a common approach to solving the challenge of sound localisation in non-destructive testing. The required microphone arrays, however, are often cumbersome: a large number of sensors is needed to achieve the required accuracy and their mutual distances need to be maximized to increase spatial resolution. Current acoustic cameras are not easily portable. In this work, we explore whether acoustic metamaterials can be used to create compact acoustic cameras, complete of autozoom objectives. We present our proof-of-concept prototype, comprising 8 MEMS microphones, using beamforming algorithms now sufficiently simple to run on a Raspberry Pi. We discuss how the performance changes when passing from a single lens to a two-lenses objective. Finally, we highlight the perspectives, the limitations and some key user scenarios for this approach.

Virtual and Augmented Reality (VR and AR) are increasingly becoming a consumer commodity, where sound plays a central role, usually delivered through headphones. Headphones, however, do not allow direct interactions between users outside the virtual world: other solutions may be necessary. In this work, we present a different approach: a sound projector, which can deliver spatial sound at a distance by forming a beam of audible sound. Our device consists of a portable speaker and two metamaterial lenses of different focal length, arranged in a telescope. We test our device by delivering signals between 5 and 6 kHz to moving people, tracked by a camera. We benchmark its performance with measurements and a pilot study, with human participants. Finally, we connect our projector to a VR headset and show how it can be used to deliver messages in VR. In this work, we discuss the limitations and perspectives of similar metamaterial-based delivery systems.

The detailed measurement and characterization of strain-induced performance variations in flexible InGaZnO thin-film transistors (IGZO TFTs) resulted in a SPICE TFT model able to simulate tensile and compressive bending. This model was used to evaluate a new concept, namely, the active compensation of strain-induced performance variations in pixel driving circuits for bendable active matrix arrays. The designed circuits can compensate the mobility and threshold voltage shifts in IGZO TFTs induced by bending. In a single TFT, a drain current of 1mA varies by $83~\mu \text{A}$ per percent of mechanical strain. The most effective compensation circuit design, comprising one additional TFT and two resistors, reduces the driving current variation to $1.1~\mu \text{A}$ per percent of strain. The compensation circuit requires no additional control signals and increases the power consumption by only $235~\mu \text{W}$ (corresponds to 4.7%). Finally, switching operation is possible for frequencies up to 200 kHz. This opens a way toward the fabrication of flexible displays with constant brightness even when bent.

One of the most valuable contributions robotics can offer is support to daily human activities, yet rigid robots often fail to comply with safety regulations in the proximity of humans. Soft robotics takes inspiration from living organisms' ability to adapt to their environment using flexible structures. These systems have to generate mechanical forces and simultaneously sense their environment. We developed a soft gripper with integrated sensing microstructures by monolithically 3D printing the structure. The rubber gripper mimics the versatile sensing and actuation abilities of living organisms. This is done using stereolithographic printing technology, rubber material, and resistive, pressure sensitive EGaIn microchannels. Printed microscale pressure sensing cavities are filled with liquid metal and act as resistive pressure sensors. They imitate human haptic perception and provide a sensitivity of 0.5% kPa−1. Simultaneously, a soft-robotic actuator design, which is derived from pneumatic networks, delivers a force of 2.5 N with 16 kPa of actuating pressure and an average efficiency of 0.56 mW kPa−1. Monolithically 3D printed systems promise numerous advantages since the compliance matching between multi-modal capillary sensing networks and actuators enables scale production of smart soft manipulators. Potential applications include collaborative manufacturing and medical support systems such as exoskeletons.

Electric potential sensors (EPS) are classified as capacitive sensors with the ability to measure small variations in electric potential or electric field remotely and accurately. Here we show how a low cost single chip version of EPS can be integrated into a wearable device such as smart watch to provide relevant information about habitual movements specifically, hair touching and scratching as well as leg movement. This new modality could be used in consumer care product research such as studying the quality of shampoos and to study restless leg syndrome remotely without the need of wearing additional sensors. In both scenarios, a single sensor was worn on the wrist, similar to a smart watch, with the sensing electrode pointing away from the body (i.e. no skin contact).

Hand-written fabrication techniques offer new ways of developing customizable, biodegradable and low-cost electronic systems. In this work, a new level of complexity is demonstrated for hand-written electronics by fabricating passive components, circuits and a sensorsystem on paper. The system comprises a pencil-written graphite force-sensitive-resistor, a pencil-drawn RC-filter, a pen-written half-wave rectifier, and a commercial front-end voltage amplifier. The sensor system exhibits a linear response for pressures up to 1.2 kPa, and a sensitivity of 51 mV kPa-1 . Furthermore, the electrical and mechanical performance of the single components and circuits is studied. Diodes fabricated through pen-written deposition of silver and nickel contacts on amorphous Indium-Gallium-Zinc-Oxide coated paper show rectification ratios up to 1:8. Tensile and compressive bending measurements applied to all pencil-written components for radii down to 0.1 mm indicate minor influence of strain. Similar results are obtained for circuits created from these individual components. Diodes and half-wave rectifiers show a stable behavior when bent to a radius of 5 mm. The presented techniques can enable the development of flexible and eco-friendly wearables and sensors for consumer and healthcare applications, and are an effective way for school-pupils to explore the world of electronics.