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While research in passive flexible circuits for wireless power transfer (WPT) such as coils and resonators continues to advance, limitations in their power handling and low efficiency have hindered the realization of efficient all-printed high-power wearable WPT receivers. Here, we propose a screen-printed textile-based 6.78 MHz resonant inductive WPT system using planar inductors with concealed metal–insulator–metal tuning capacitors. A printed voltage doubler rectifier based on silicon carbide diodes is designed and integrated with the coils, showing a power conversion efficiency of 80%–90% for 2–40 W inputs over a wide load range. Compared to prior wearable WPT receivers, it offers an order of magnitude improvement in power handling along with higher efficiency (approaching 60%) while using all-printed passives and a compact rectifier. The coils exhibit a simulated specific absorption rate under 0.4 W/kg for 25 W received power, and under 21 $^{\circ }$C increase in the coils' temperature for a 15 W DC output. Additional fabric shielding is investigated, reducing harmonics emissions by up to 17 dB. We finally demonstrate a wirelessly-powered textile-based carbon-silver Joule heater, capable of reaching up to 60 $^{\circ }$C at 2 cm separation from the transmitter, as a wearable application, which can only be wireless-powered using the proposed system.

We present a wirelessly powered ultraviolet-C (UVC) radiation-based disinfecting bandage for sterilization and treatment in chronic wound care and management. The bandage contains embedded low-power UV light-emitting diodes (LEDs) in the 265 to 285 nm range with the light emission controlled via a microcontroller. An inductive coil is seamlessly concealed in the fabric bandage and coupled with a rectifier circuit to enable 6.78 MHz wireless power transfer (WPT). The maximum WPT efficiency of the coils is 83% in free space and 75% on the body at a coupling distance of 4.5 cm. Measurements show that the UVC LEDs are emitting radiant power of about 0.6 mW and 6.8 mW with and without fabric bandage, respectively, when wirelessly powered. The ability of the bandage to inactivate microorganisms was examined in a laboratory which shows that the system can effectively eradicate Gram-negative bacteria, Pseudoalteromonas sp. D41 strain, on surfaces in six hours. The proposed smart bandage system is low-cost, battery-free, flexible and can be easily mounted on the human body and, therefore, shows great promise for the treatment of persistent infections in chronic wound care.

Packaging of flexible electronics is essential for e-textile applications to reduce degradation of performance caused by mechanical stress and environmental effects and to increase durability. Conformal coatings for packaging have the advantage of reducing rigidity and can be seamlessly integrated into fabrics. Vacuum forming is a technique for packaging electronic devices with thermoplastic films of various thicknesses providing uniform coating. Polyurethane is a widely used thermoplastic material in e-textile and can be easily processed by vacuum forming for packaging. In this article, a detailed explanation of the working of Formech 450DT vacuum former is discussed for packaging small electronic chip for e-textile application with thermoplastic polyurethane (TPU). Two types of flexible circuits were packaged: a carbon monoxide (CO) gas sensor and a series of resistors on flexible PCB. The packaged CO flexible sensor and series resistors endured 5.3 times and 1.7 times, respectively, more bending cycles than unpackaged flexible electronic filament samples. For the washing cycles, the packaged flexible strips with CO sensor and series resistors endured 1.5 times.

Cities are monitored by sparsely positioned high-cost reference stations that fail to capture local variations. Although these stations must be ubiquitous to achieve high spatio-temporal resolutions, the required capital expenditure makes that infeasible. Here, low-cost IoT devices come into prominence; however, non-disposable and often non-rechargeable batteries they have pose a huge risk for the environment. The projected numbers of required IoT devices will also yield to heavy network traffic, thereby crippling the RF spectrum. To tackle these problems and ensure a more sustainable IoT, cities must be monitored with fewer devices extracting highly granular data in a self-sufficient manner. Hence, this paper introduces a network architecture with energy harvesting low-cost mobile sensors mounted on bikes and unmanned aerial vehicles, underpinned by key enabling technologies. Based on the experience gained through real-world trials, a detailed overview of the technical challenges encountered when using low-cost sensors and the requirements for achieving high spatio-temporal resolutions in the 3D space are highlighted. Finally, to show the capability of the envisioned architecture in distributed sensing, a case study on air quality monitoring investigating the variations in particulate and gaseous pollutant dispersion during the first lockdown of the COVID.19 pandemic is presented. The results showed that using mobile sensors is as accurate as using stationary ones with the potential of reducing device numbers, leading to a more sustainable IoT.

Triboelectric nanogenerators (TENG) are one of the most promising candidates for powering wearable and portable devices. Example TENGs have demonstrated flexibility, light weight, biocompatibility, versatility and good performance. Textiles are a potential substrate onto, or into, which wearable technology is increasingly being incorporated but supplying power remains an enduring challenge. TENGs are a potential textile based mechanical energy harvesting power supply and there has been an increasing effort to combine TENGs with fabrics. A significant challenge exists in the integration without losing the performance of the TENG or the original properties (appearance, breathability, washability, and durability) and feel of the textile. Various approaches towards the realisation of textile-based TENGs (T-TENGs) have been demonstrated. Depending on its structure, T-TENGs can be divided into two main types, fabric-based TENG and fibre-based TENG. The fabric-based TENG is composed of conventional and/or modified fabrics, which serve as a substrate and/or a triboelectric material. The fibre-based TENG is fabricated as a single fibre or a collection of interlaced fibres. This paper provides an up to date review of the progress in the research of T-TENGs. The paper covers the basic operating principles, possible operation modes, textile manufacturing methods, material selections, T-TENG fabrication process, surface modification and structural designs. Issues, such as standardised measurement parameters, the challenges and limitations of T-TENG are discussed.

Abstract

This article reviews materials developed to enable energy harvesting from textiles. It includes energy harvesting from mechanical, thermal, and light sources, and covers materials employed into yarns that can be woven into the textile and films that are deposited onto the surface of the textile. The textile places challenging constraints on the materials, for example, by limiting processing temperatures to typically less than 150°C and presenting a rough, inconsistent surface profile. Example materials include a screen-printable low-temperature composite lead zirconate titanate polymer film and poly(vinylidene fluoride) polymer fibers, both of which have been shown to harvest mechanical energy from textiles. Thermoelectric solutions demonstrated thus far are limited and challenging to implement within a textile. Photovoltaic solutions include organic and dye-sensitized solar cells fabricated into functionalized yarns and as films spray-coated onto textiles. While numerous suitable example materials and textile devices have been demonstrated, work is still needed to develop these into practical energy-harvesting solutions.

This paper details the enhancements in the dielectric and piezoelectric properties of a low-temperature screen-printable piezoelectric nano-composite film on flexible plastic and textile substrates. These enhancements involved adding silver nano particles to the nano-composite material and using an additional cold isostatic pressing (CIP) post-processing procedure. These developments have resulted in a 18% increase in the free-standing piezoelectric charge coefficient d33to a value of 98 pC/N. The increase in the dielectric constant of the piezoelectric film has, however, resulted in a decrease in the peak output voltage of the composite film. The potential for this material to be used to harvest mechanical energy from a variety of textiles under compressive and bending forces has been evaluated theoretically and experimentally. The maximum energy density of the enhanced piezoelectric material under 800N compressive force was found to be 34J/m3on a Kermel textile. The maximum energy density of the enhanced piezoelectric material under bending was found to be 14.3J/m3on a cotton textile. These results agree very favourably with the theoretical predictions. For a 10x10cm piezoelectric element 100µm thick this equates to 38μJ and 14.3μJ of energy generated per mechanical action respectively which is a potentially useful amount of energy.

We present comprehensive experimental and theoretical work on tunnel-barrier rectifiers comprising double (Nb2O5-Al2O3) and triple (Ta2O5-Nb2O5-Al2O3) insulator configurations engineered to enhance low-voltage nonlinearity. The key rectifier properties, asymmetry, nonlinearity and responsivity have been assessed from current-voltage measurements. A superior low-voltage asymmetry (12 at 0.1 V) and responsivity (5 A/W at 0.2 V) for MIIIM rectifiers have been observed. The results demonstrate enhanced rectification by atomically multi-layering tunnel barriers in cascaded and non-cascaded MIIIM arrangements, for inclusion in optical nantennas.

A novel textile-based triboelectric nanogenerator (TENG) with woven structure operating in freestanding triboelectric-layer mode, defined as a woven-TENG, has been developed. Whereas most woven-structured TENGs operate in contact-separation mode and consist of one type of triboelectric material, this woven-TENG comprises woven electrodes and woven strips of positive and negative triboelectric material, which form a checker-like pattern over the electrodes with matching periodicity. The implementation of the positive and negative triboelectric material significantly improves the performance of the woven-TENG. Furthermore, in contrast to the conventional grating-structured and woven-structured TENG, which are designed to operate only in one moving direction, this new design also allows the woven-TENG to harvest energy from all-planar directions of movement. The woven-TENG with woven strips of nylon and polytetrafluoroethylenevinyl (PTFE) fabric can generate a root mean square (RMS) open-circuit voltage of 62.9 V, an RMS short-circuit current of 1.77 µA and a maximum RMS power of 34.8 µW at a load resistance of 50 MΩ, a mechanical oscillation of 2 Hz and a contact force of 5 N. This corresponds to a maximum RMS power density of 5.43 mW/m2.

This paper reports a novel design of textile-based triboelectric nanogenerator (TENG) with alternate grated strips of positive and negative triboelectric material operating in freestanding triboelectric-layer mode, defined as a pnG-TENG. Whereas most grating-structured TENGs operating in a freestanding triboelectric-layer mode comprise gratings of one type of triboelectric material separated by air gaps, this design presents a replacement of the air gaps by another triboelectric material with the opposite polarity to the existing triboelectric material. This is predicted to increase performance by increasing the contact area of the generator. The pnG-TENG with 10 gratings of nylon fabric and PVC heat transfer vinyl delivers an RMS open-circuit voltage of 136 V, an RMS short-circuit current of 2.68 μA and a maximum RMS power of 125 μW at a load resistance of 50 MΩ, a mechanical oscillation of 2 Hz and a contact force of 5 N. This corresponds to a maximum RMS power density of 38.8 mW/m2, which is 1.94 and 6.43 times greater than the power generated by the TENG with a single triboelectric material and the TENG with no gratings, respectively.

We present comprehensive experimental and theoretical work on tunnel-barrier rectifiers comprising double (Nb2O5-Al2O3) and triple (Ta2O5-Nb2O5-Al2O3) insulator configurations engineered to enhance low-voltage nonlinearity. The key rectifier properties, asymmetry, nonlinearity and responsivity have been assessed from current-voltage measurements. A superior low-voltage asymmetry (12 at 0.1 V) and responsivity (5 A/W at 0.2 V) for MIIIM rectifiers have been observed. The results demonstrate enhanced rectification by atomically multi-layering tunnel barriers in cascaded and non-cascaded MIIIM arrangements, for inclusion in optical nantennas.