William Taube Navaraj

New materials and advances in making electronic skin for interactive robots

Flexible electronics has huge potential to bring revolution in robotics and prosthetics as well as to bring about the next big evolution in electronics industry. In robotics and related applications, it is expected to revolutionise the way with which machines interact with humans, real-world objects and the environment. For example, the conformable electronic or tactile skin on robot’s body, enabled by advances in flexible electronics, will allow safe robotic interaction during physical contact of robot with various objects. Developing a conformable, bendable and stretchable electronic system requires distributing electronics over large non-planar surfaces and movable components. The current research focus in this direction is marked by the use of novel materials or by the smart engineering of the traditional materials to develop new sensors, electronics on substrates that can be wrapped around curved surfaces. Attempts are being made to achieve flexibility/stretchability in e-skin while retaining a reliable operation. This review provides insight into various materials that have been used in the development of flexible electronics primarily for e-skin applications.

Stretchable wireless system for sweat pH monitoring.

Sensor-laden wearable systems that are capable of providing continuous measurement of key physiological parameters coupled with data storage, drug delivery and feedback therapy have attracted huge interest. Here we report a stretchable wireless system for sweat pH monitoring, which is able to withstand up to 53% uniaxial strain and more than 500 cycles to 30% strain. The stretchability of the pH sensor patch is provided by a pair of serpentine-shaped stretchable interconnects. The pH sensing electrode is made of graphite-polyurethane composite, which is suitable for biosensor application. The sensing patch validated through in-depth electrochemical studies, exhibits a pH sensitivity of 11.13 ± 5.8 mV/pH with a maximum response time of 8 s. Interference study of ions and analyte (Na+, K+ and glucose) in test solutions shows negligible influence on the pH sensor performance. The pH data can be wirelessly and continuously transmitted to smartphone through a stretchable radio-frequency-identification antenna, of which the radiating performance is stable under 20% strain, as proved by vector network analyzer measurement. To evaluate the full system, the pH value of a human sweat equivalent solution has been measured and wirelessly transmitted to a custom-developed smart phone App.

Nanowire FET Based Neural Element for Robotic Tactile Sensing Skin

This paper presents novel Neural Nanowire Field Effect Transistors (υ-NWFETs) based hardware-implementable neural network (HNN) approach for tactile data processing in electronic skin (e-skin). The viability of Si nanowires (NWs) as the active material for υ-NWFETs in HNN is explored through modeling and demonstrated by fabricating the first device. Using υ-NWFETs to realize HNNs is an interesting approach as by printing NWs on large area flexible substrates it will be possible to develop a bendable tactile skin with distributed neural elements (for local data processing, as in biological skin) in the backplane. The modeling and simulation of υ-NWFET based devices show that the overlapping areas between individual gates and the floating gate determines the initial synaptic weights of the neural network - thus validating the working of υ-NWFETs as the building block for HNN. The simulation has been further extended to υ-NWFET based circuits and neuronal computation system and this has been validated by interfacing it with a transparent tactile skin prototype (comprising of 6 × 6 ITO based capacitive tactile sensors array) integrated on the palm of a 3D printed robotic hand. In this regard, a tactile data coding system is presented to detect touch gesture and the direction of touch. Following these simulation studies, a four-gated υ-NWFET is fabricated with Pt/Ti metal stack for gates, source and drain, Ni floating gate, and Al2O3 high-k dielectric layer. The current-voltage characteristics of fabricated υ-NWFET devices confirm the dependence of turn-off voltages on the (synaptic) weight of each gate. The presented υ-NWFET approach is promising for a neuro-robotic tactile sensory system with distributed computing as well as numerous futuristic applications such as prosthetics, and electroceuticals.

Upper limb prosthetic control using toe gesture sensors

A novel scheme to control upper-limb prosthesis with toe gesture sensing system is presented in this paper. In the proposed system, copper/polymer stack capacitive touch sensors fabricated on a flexible substrate, interfaced with electronics and wireless transmitters forms a smart sensing insole. The scheme takes advantage of the user making various gestures with their left and right hallux digits in the form of a Morse code. The touch results in change in capacitance of the sensors from 56 ±2 pF to 75±3 pF, which is readout by an interface circuitry. This is transmitted wirelessly to a computing system attached to the prosthetic hand, which controls it resulting in various upper-limb prosthetic gestures or grasp patterns depending on the corresponding mapped Morse code. The differential current at the output of the capacitor is converted into voltage through an integrator based capacitance-voltage converter(CVC), fabricated with 0.18μm CMOS technology. The CVC is interfaced with off-the-shelf components. Details of the sensor, sensor interface and systems design, fabrication, validation, and overall functional assessment are presented in this work to show the potential of using toe gestures for upper-limb prosthetic control.

Device modelling of bendable MOS transistors

This paper presents the directions for computer aided design, modelling and simulation of bendable MOSFET transistors towards futuristic bendable ICs. In order to compensate the bending stress a generalised geometry variation is discussed. Based on drain-current and threshold-voltage parameters varying under the bending stress, a Verilog-A compact model is proposed and describes I-V characteristics of a MOSFET in a standard 0.18-μm CMOS technology. This model has been compiled into Cadence environment to predict value and orientation of the bending stress. The proposed model validates against macro-model simulation results, and agrees for both the electron and hole conduction. It has been found that there is significant performance advantage in process-induced uniaxial stressed n-MOSFET, exhibiting a smaller drain-current variation and thresh old voltage shift by monitoring the bending stress and changing the supply voltage.

At-Home Computer-Aided Myoelectric Training System for Wrist Prosthesis

Development of tools for rehabilitation and restoration of the movement after amputation can benefit from the real time interactive virtual animation model of the human hand. Here, we report a computer-aided training/learning system for wrist disarticulated amputees, using the open source integrated development environment called “Processing”. This work also presents the development of a low-cost surface Electro-MyoGraphic (sEMG) interface, which is an ideal tool for training and rehabilitation applications. The processed sEMG signals are encoded after digitization to control the animated hand. Experimental results demonstrate the effectiveness of the sEMG control system in acquiring sEMG signals for real-time control. Users have also the ability to connect their prostheses with the training system and observe its operation for a more explicit demonstration of movements.

Large-area self-assembly of silica microspheres/nanospheres by temperature-assisted dip-coating

This work reports a temperature-assisted dip-coating method for self-assembly of silica (SiO2) microspheres/nanospheres (SPs) as monolayers over large areas (∼cm2). The area over which self-assembled monolayers (SAMs) are formed can be controlled by tuning the suspension temperature (Ts), which allows precise control over the meniscus shape. Furthermore, the formation of periodic stripes of SAMs, with excellent dimensional control (stripe width and stripe-to-stripe spacing), is demonstrated using a suitable set of dip-coating parameters. These findings establish the role of Ts, and other parameters such as withdrawal speed (Vw), withdrawal angle (θw), and withdrawal step length (Lw). For Ts ranged between 25 and 80 °C, the morphological analysis of dip-coatings shows layered structures comprising of defective layers (25-60 °C), single layers (70 °C), and multilayers (>70 °C) owing to the variation of SP flux at the meniscus/substrate assembling interface. At Ts = 70 °C, there is an optimum Vw, approximately equal to the downshift speed of the meniscus (Vm = 1.3 μm/s), which allows the SAM formation over areas (2.25 cm2) roughly 10 times larger than reported in the literature using nanospheres. Finally, the large-area SAM is used to demonstrate the enhanced performance of antireflective coatings for photovoltaic cells and to create metal nanomesh for Si nanowire synthesis.

Ultra-thin chips for high-performance flexible electronics

Flexible electronics has significantly advanced over the last few years, as devices and circuits from nanoscale structures to printed thin films have started to appear. Simultaneously, the demand for high-performance electronics has also increased because flexible and compact integrated circuits are needed to obtain fully flexible electronic systems. It is challenging to obtain flexible and compact integrated circuits as the silicon based CMOS electronics, which is currently the industry standard for high-performance, is planar and the brittle nature of silicon makes bendability difficult. For this reason, the ultra-thin chips from silicon is gaining interest. This review provides an in-depth analysis of various approaches for obtaining ultra-thin chips from rigid silicon wafer. The comprehensive study presented here includes analysis of ultra-thin chips properties such as the electrical, thermal, optical and mechanical properties, stress modelling, and packaging techniques. The underpinning advances in areas such as sensing, computing, data storage, and energy have been discussed along with several emerging applications (e.g., wearable systems, m-Health, smart cities and Internet of Things etc.) they will enable. This paper is targeted to the readers working in the field of integrated circuits on thin and bendable silicon; but it can be of broad interest to everyone working in the field of flexible electronics.

Flexible pressure sensing system for tongue-based control of prosthetic hands

A novel system to control robotic/prosthetic limbs with a tongue-based scheme is presented in this paper. In the proposed system six off-the-self pressure sensors soldered on a flexible PCB and placed in the inner part of cuspides, central and lateral incisors of the lower arch, are interfaced with electronics and wireless transmitters to form the complete system. The control system sends the appropriate commands to the prosthesis when users apply force to one of the pressure sensors with their tongue. Each pressure sensor represents one gesture for the prosthesis. The touch with the tongue results in an increase of the read pressure by the sensor. If the pressure exceeds a predetermined threshold the prosthesis is activated via wireless transmission of data. To package the system in the mouth a protective encapsulation was made using PDMS. Different thicknesses and concentrations of PDMS were tested to determine the optimum trade-off between the minimum exerted force by the user that activates the prosthesis and the threshold below which the prosthesis is not active.

Simulation study of junctionless silicon nanoribbon FET for high-performance printable electronics

High-performance electronics on flexible substrates along with low-cost fabrication by printing has gained interest recently. For this purpose, the printing of inorganic semiconductors based micro/nanostructures such as nanowires etc. are being explored. However, due to thermal budget, the controlled selective source/drain doping needed to obtain transistors from such structure remains a bottleneck post transfer printing. This paper presents an attractive solution to address this challenge. The solution is based on junctionless FETs (JLFET), which do not require selective doping. Unlike conventional JLFETs, which use nanowires, the devices presented here are based on nanoribbons as this enable larger channel width and hence high drive current. Studied through simulation, the JLFETs presented here show high-performance with current high enough to drive micro-LED. The TCAD simulation has been carried out to study the effect of single and dual metal gate (top and bottom side) of JLFETs as well as that of doping and nanoribbon thickness on the electrical characteristics. The simulation results indicate that the proposed devices will be suitable for high performance printable electronics applications.