William Taube Navaraj | University of Glasgow (original) (raw)

Papers by William Taube Navaraj

Advanced Science, Feb 13, 2019

based energy storage technologies have been explored meet the demand in these applications. [7,12... more based energy storage technologies have been explored meet the demand in these applications. [7,12-17] Currently, the electrochemical based energy storage is largely based on Li ions batteries (LIBs), sodium ion batteries, or zinc-air batteries. [13-15,18-21] In particular, LiBs offer high energy density (≈500 W h kg −1) and benefit from well-developed manufacturing processes. [14,18,19] But majority of them are not flexible and their weight, low power density, long charging time (1-2 h), heat generation, [22] environmental concerns, [18,19,22,23] etc. limit their use in applications such as wearable systems. These limitations have already caught the attention of the research community, as evident from various works on flexible/stretchable batteries [12,13,24,25] and supercapacitors (SCs) with long cycling stability. [26-28] In particular, the SCs offer excellent energy and power densities with low-cost of fabrication. [26-31] Further they offer rapid charging (minutes vs hours in LiBs), long life cycle, do not generate heat [26,30] and are generally environment friendly. [28,29] With flexible and stretchable form factors, SCs can also conform to curved surfaces. [28] Among various types of SCs, the electrochemical double layer capacitors (EDLCs) [28,29] based on carbon materials are the most promising [28-32] because of their long lifetime (more than 10 6 charging/discharging cycles), low environmental impact, ease of maintenance, and flexible form factors. [26,27,30] The performance of SCs is mainly governed by their structure, surface morphology, electrolytes, and the electrochemical and electrical properties of active electrodes. [26,30,33-35] For this reason, the choice of electrode materials and the electrolyte are critical. Since the energy storage (areal energy density, E A = C A V 2 /2) depends on the potential window (V) and the specific capacitance (e.g., areal capacitance C A), researchers have focused on the ways to improve these values. [35-38] For example, a variety of carbon-based structures (e.g., graphene foam, [35] reduced graphene oxide (rGO), [36] etc.) have been explored for EDLC fabrication. The choice of electrolyte is also important to increase the V and hence the energy density. [37] Table S1 in the Supporting Information provides a comparison of C A , and V for EDLCs developed with various active carbonbased materials. The low values of C A (<10 mF cm −2) reported in the majority of the SCs can be attributed to the lack of electroactive surface per unit area needed to store the charge at the electrode-electrolyte interface. [33,38] The electrodes with multilayer structures have been explored to overcome such issues Energy autonomy is critical for wearable and portable systems and to this end storage devices with high-energy density are needed. This work presents high-energy density flexible supercapacitors (SCs), showing three times the energy density than similar type of SCs reported in the literature. The graphene-graphite polyurethane (GPU) composite based SCs have maximum energy and power densities of 10.22 µWh cm −2 and 11.15 mW cm −2 , respectively, at a current density of 10 mA cm −2 and operating voltage of 2.25 V (considering the IR drop). The significant gain in the performance of SCs is due to excellent electroactive surface per unit area (surface roughness 97.6 nm) of GPU composite and high electrical conductivity (0.318 S cm −1). The fabricated SCs show stable response for more than 15 000 charging/ discharging cycles at current densities of 10 mA cm −2 and operating voltage of 2.5 V (without considering the IR drop). The developed SCs are tested as energy storage devices for wide applications, namely: a) solarpowered energy-packs to operate 84 light-emitting diodes (LEDs) for more than a minute and to drive the actuators of a prosthetic limb; b) powering high-torque motors; and c) wristband for wearable sensors.

IEEE Transactions on Robotics, Apr 1, 2021

Electronic skin (eSkin) with various types of sensors over large conformable substrates has recei... more Electronic skin (eSkin) with various types of sensors over large conformable substrates has received considerable interest in robotics. The continuous operation of large number of sensors and the readout electronics make it challenging to meet the energy requirements of eSkin. In this article, we present the first energy generating eSkin with intrinsic tactile sensing without any touch sensor. The eSkin comprises a distributed array of miniaturized solar cells and infrared light emitting diodes (IRLEDs) on soft elastomeric substrate. By innovatively reading the variations in the energy output of the solar cells and IRLEDs, the eSkin could sense multiple parameters (proximity, object location, edge detection, etc.). As a proof of concept, the eSkin has been attached to a 3-D-printed hand. With an energy surplus of 383.6 mW from the palm area alone, the eSkin could generate more than 100 W if present over the whole body (area ∼1.5 m 2). Further, with an industrial robot arm, the presented eSkin is shown to enable safe human−robot interaction. The novel paradigm presented in this article for the development of a flexible eSkin extends the application of solar cell from energy generation alone to simultaneously acting as touch sensors. Index Terms-Electronic skin (eSkin), energy harvesting, human−robot interaction (HRI), proximity sensing, solar cell, touch sensing. I. INTRODUCTION Electronic skin or "eSkin" has recently emerged as a novel platform for advances in robotics, prosthesis, health diagnostics, therapeutics, and monitoring [1]. It allows robots and prosthetic limbs to gather tactile information from large area contacts and to exploit the same to operate in unstructured environment or to improve human−robot interaction (HRI) [2]-[6]. Likewise, eSkin has been explored for measurement of vital health parameters and to provide reliable, effective, and, sometimes, life-saving functions [7]-[9]. With increasing number and type of sensors (pressure, temperature, texture, proximity, etc.) and electronics associated with them on large area eSkin [10]-[13], a stable power supply is critical for practical usage [11], [14]. Thus, a realistic and accessible power source is urgently needed for a next-generation of smart, stand-alone, always-on eSkins. This is a challenge as the continuous power supply through batteries is not practical because they add weight, are not flexible, and may require redesigning of robotics platform [15], [16]. Likewise, for applications requiring intimate integration of eSkin

Journal of Computational Electronics, Nov 23, 2015

Tactile feedback is important to enhance the ability of amputees to interact with various objects... more Tactile feedback is important to enhance the ability of amputees to interact with various objects. In this regard, this paper presents a 3D-printed prosthetic limb with integrated biomimetic tactile sensors coupled with a feedback actuation mechanism which is integrated to a myo-electric band on the residual limb. The static and dynamic tactile sensors on 3D-printed hand were realized by using a capacitive architecture in tandem with a piezoelectric structure. The capacitive sensing structure exhibited non-linear characteristics with varying sensitivities of 0.25 kPa−1 in low pressure range (<100 Pa) to 0.002 kPa−1 in high pressure while the piezo-structure exhibited a sensitivity of 2.28 kPa−1. Two different types of actuators are employed and integrated to the myo-electric band to offer static and dynamic tactile feedback in the residual limb area.

ACS Nano, Mar 5, 2019

Graphene has great potential for high-performance flexible electronics. Although studied for more... more Graphene has great potential for high-performance flexible electronics. Although studied for more than a decade, contacting graphene efficiently, especially for large-area, flexible electronics, is still a challenge. Here, by engineering the graphene-metal van der Waals (vdW) contact, we demonstrate that ultra-low contact resistance is achievable via a bottom-contact

ACS Applied Materials & Interfaces, Jan 8, 2018

This work reports a temperature-assisted dip-coating method for self-assembly of silica (SiO 2) m... more This work reports a temperature-assisted dip-coating method for self-assembly of silica (SiO 2) micro-/nano-spheres (SPs) as monolayers over large areas (~cm 2). The area over which selfassembled monolayers (SAMs) are formed can be controlled by tuning the suspension temperature (T s), which allows precise control over meniscus shape. Furthermore, the formation of periodic stripes of SAM, with excellent dimensional control (stripe width and stripe-to-stripe

IEEE Sensors Journal, Oct 1, 2018

This paper presents a flexible ultraviolet (UV) photodetector (PD) system based on zinc oxide (Zn... more This paper presents a flexible ultraviolet (UV) photodetector (PD) system based on zinc oxide (ZnO) nanowires (NWs) for wearable UV dosimetry. High-crystal quality ZnO NWs have been synthesized by chemical vapour transport (CVT) technique on c-plane sapphire substrates, and thereafter, transferred and aligned at pre-defined locations on a flexible substrate using dielectrophoresis (DEP). The accurate control over DEP parameters permitted the fabrication of large-area (wafer scale) arrays of ZnO NWs based UV PDs. Resulting PDs showed photocurrent-to-dark current ratios above 10 3 %, fast response times (<1 s), high sensitivity to different UV light intensities, and good stability under several UV/dark irradiation cycles. In addition, above PDs presented a robust response under extreme bending conditions, which is critical for their application in high-performance wearable UV dosimeters.

Applied Physics Letters, Jul 2, 2018

Advanced intelligent systems, Sep 20, 2019

Advanced Robotics, Nov 2, 2015

Flexible electronics has huge potential to bring revolution in robotics and prosthetics as well a... more 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.

Biosensors and Bioelectronics, Jun 1, 2018

In this paper we have carried out investigation of reliability of Via-bridge under mechanical ben... more In this paper we have carried out investigation of reliability of Via-bridge under mechanical bending for application in flexible electronics. Progresses in the flexible electronics field unlocked the fabrication of next-generation devices such as foldable mobile phones, wearables [1] , electronic skin (e-skin) [2] - [4] , textile electronics [5] , flexible energy harvesting/storage [2] , [5] and flexible memory devices [6] , [7] .

Harnessing technological advances to develop nature-inspired systems has led to many interesting ... more Harnessing technological advances to develop nature-inspired systems has led to many interesting solutions such as electronic skin (e-skin) with features mimicking human skin, as well as, imparting new functionalities beyond human skin’s sensory level [1]. The major focus of e-skin research so far has been on the development of various types of sensors (e.g. contact, pressure, temperature, humidity, etc.) and their integration on large-area and flexible substrates. In this regard, two key challenges lie in realizing largearea e-skin: (1) processing of a large amount of data distributed over large areas, and (2) powering a large array of sensors. As an example, an estimated 45k mechanoreceptors (MRs) will be needed in about 1.5 m2 area to develop human inspired e-skin for robots. These sensory receptors process tactile data locally and require significant energy. Accordingly, flexible distributed tactile data processing and energy harvesting solutions are needed for an effective e-skin. Photovoltaics have shown one of the best performance for generating energy per unit area and are a promising candidate for e-skin [2]. Likewise, a neuromimicking approach could help to acquire and process sensors data locally as it leads to a significant downstream reduction in the numbers of neurons transmitting stimuli in the early sensory pathways in humans [3]. In this work, we show our recent research on e-skin (Figure 1) addressing above challenges through the development of a nanowire (NW) based neural field effect transistor (ν-NWFET) as a basic building block for neural-mimicking data processing (Figure 1(a)) and an energy-autonomous e-skin achieved by integrating graphene based transparent touch sensors to photovoltaic cells (Figure 1(d)). The heterogeneous integration of various materials led to achieving such functionalities. Nanomaterials such as graphene and Si NWs are considered as good candidates for flexible electronics due to their excellent mechanical flexibility, printability in large-area as well as outstanding electrical performance. Here, we present a low-cost method to transfer and pattern single layer graphene on large-area flexible and transparent substrates, resulting in a co-planar interdigitated capacitive structure. In terms of the sensing performance, our sensors can detect minimum pressures down to 0.11 kPa with a uniform sensitivity of 4.3 Pa−1 along a broad pressure range. Thanks to the transparency of graphene, the integration of touch sensors atop a photovoltaic cell is possible, which paves a new way for energy-autonomous, flexible, and tactile e-skin (Figure 1(d)). Using ν-NWFET to realize hardware neural network is an interesting approach as by printing NWs on large area flexible substrates it will be possible to develop a flexible tactile e-skin with distributed neural elements (for local data processing, as in biological skin) in the backplane. Given the previously demonstrated metalassisted chemical etching NW synthesis method and contact printing for large-area assembling of NWs, the ν-NWFET presented here is promising for large-area and low-cost flexible electronics. Modeling, simulation and fabrication of ν-NWFET shows that the overlapping areas between individual gates and the floating gate determines the initial synaptic weights of the neural network. Further, proof-of-concept is shown by interfacing it with a transparent tactile e-skin prototype integrated on the palm of a 3D printed robotic hand and performing coding of touch gesture. The research finds place in numerous futuristic applications such as prosthetics, robotics and electroceuticals, and this presentation will show the interesting progress made in this direction

A novel scheme to control upper-limb prosthesis with toe gesture sensing system is presented in t... more 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 upperlimb 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 0.18-μm CMOS technology. The CVC is interfaced with offthe-shelf components. Details of the sensor, sensor interface and system's 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.

Advanced electronic materials, Feb 18, 2020

npj flexible electronics, Mar 14, 2018

Flexible electronics has significantly advanced over the last few years, as devices and circuits ... more 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.

Herein, development of a 3-D printed soft robotic tri-gripper embedded with tactile sensor array ... more Herein, development of a 3-D printed soft robotic tri-gripper embedded with tactile sensor array is presented. A facile fabrication strategy by 3D-printing thermoplastic polyurethane (TPU) was employed to fabricate the soft tri-gripper consisting of 9 capacitive tactile sensor-laden phalanges. The 3D-printed TPU itself was used as a sensory dielectric for the fabricated tri-gripper. The sensor and interconnect electrodes have been designed to have minimum cross-sensor capacitive coupling with stretchable interconnects to ensure robust integration. The designed sensors were patterned as copper electrodes on top of flexible polyimide film and embedded within the gripper during the 3D-printing process. The sensors were characterised and it exhibited a maximum sensitivity of 2.87 %/kPa. The gripper was tested for up to 100 cycles of compression and expansion. The developed sensory gripper finds application in industrial and agricultural robotics. A sample application in a fruit pick-and-drop task was demonstrated which makes use of both artificial vision and tactile sensing modalities.

High-performance electronics on flexible substrates along with low-cost fabrication by printing h... more 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.