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Atopic dermatitis (AD) is one of the most common skin disorders, affecting nearly one-fifth of children and adolescents worldwide, and currently, the only method of monitoring the condition is through an in-person visual examination by a clinician. This method of assessment poses an inherent risk of subjectivity and can be restrictive to patients who do not have access to or cannot visit hospitals. Advances in digital sensing technologies can serve as a foundation for the development of a new generation of e-health devices that provide accurate and empirical evaluation of the condition to patients worldwide. The goal of this review is to study the past, present, and future of AD monitoring. First, current medical practices such as biopsy, tape stripping and blood serum are discussed with their merits and demerits. Then, alternative digital methods of medical evaluation are highlighted with the focus on non-invasive monitoring using biomarkers of AD—TEWL, skin permittivity, elasticity, and pruritus. Finally, possible future technologies are showcased such as radio frequency reflectometry and optical spectroscopy along with a short discussion to provoke research into improving the current techniques and employing the new ones to develop an AD monitoring device, which could eventually facilitate medical diagnosis.

Ambient energy harvesting has great potential to contribute to sustainable development and address growing environmental challenges. Converting waste energy from energy-intensive processes and systems (e.g., combustion engines and furnaces) is crucial to reducing their environmental impact and achieving net-zero emissions. Compact energy harvesters will also be key to powering the exponentially growing smart devices ecosystem that is part of the Internet of Things, thus enabling futuristic applications that can improve our quality of life (e.g., smart homes, smart cities, smart manufacturing, and smart healthcare). To achieve these goals, innovative materials are needed to efficiently convert ambient energy into electricity through various physical mechanisms, such as the photovoltaic effect, thermoelectricity, piezoelectricity, triboelectricity, and electromagnetic power transfer. By bringing together the perspectives of experts in various types of energy harvesting materials, this Roadmap provides extensive insights into recent advances and present challenges in the field. Additionally, the Roadmap analyzes the key performance metrics of these technologies in relation to their ultimate energy conversion limits. Building on these insights, the Roadmap outlines promising directions for future research to fully harness the potential of energy harvesting materials for green energy anytime, anywhere.

This paper presents a holistic and authoritative review of the role of microwave technologies in enabling a new generation of wearable devices. A human-centric Internet of Things (IoT) covering remote healthcare, distributed sensing, and consumer electronics, calls for high-performance wearable devices integrated into clothing, which require interdisciplinary research efforts to emerge. Microwaves, the “interconnect” of wireless networks, can enable, rather than solely connect, the next generation of autonomous, sustainable, and wearable-friendly electronics. First, enabling technologies including wireless power transmission and RF energy harvesting, backscattering and passive communication, RFID, and electromagnetic sensing are reviewed. We then discuss the key integration platforms, covering smart fabrics and electronic textiles, additive manufacturing, printed electronics, natively-flexible and organic RF semiconductors, and fully-integrated CMOS systems, where opportunities for hybrid integration are highlighted. The emerging research trends, from mmWave 6G, RF sensing and imaging, to healthcare applications including neural implants, drug delivery, and safety upon exposure to microwaves are revisited and discussed, presenting a future roadmap for interdisciplinary research towards sustainable and reliable next-generation wearables.

The dc power collected by radio frequency energy harvesting (RFEH) absorbing surfaces is limited by their physical aperture. Here, a compact, conformable, and ultrathin sub-1 GHz large-area RFEH surface is proposed based on electrically small ( ka=0.58 ) “wire-type” rectenna elements, with an effective area exceeding its physical aperture size. Using optimized large-signal complex source tuning, the rectifiers achieve up to 36% measured power conversion efficiency (PCE) at 20 dBm. The proposed 15 cm-diameter six-element array generates a 27.5 μ W at 1 V output from a 0.17 μ W/cm 2 incident power density across a 40 k Ω load. Owing to its series connection, the voltage output is boosted, and the load resistance dependence is suppressed enabling over 60% of the maximum RF-to-dc PCE to be preserved for loads between 7 and 100 k Ω , with an area-normalized figure-of-merit nearly 100% higher than previous arrays. The proposed array is integrated with a commercial DC-DC converter and demonstrated powering a Bluetooth low energy (BLE) wireless sensor node (WSN) from an unprecedented incident power density of 0.25 μ W/cm 2 . A practical demonstration using a commercial 3 W, 915 MHz Powercast source is presented, showing an 11 m operation range with a 1 V output and illustrating the impact of polarization mismatch on the proposed array.

Near-field radiative Wireless Power Transfer (WPT)has the advantage of high-power WPT. In this paper, an all textile high-power WPT receiver is realized for the first time, using an embroidered coil and a printed tuning capacitor on a textile substrate. The proposed coil is demonstrated receiving over 3 W DC power at 6.78 MHz with a 23% end-to-end efficiency. A low Specific Absorption Rate (SAR) of 0.37 W/kg is observed, through full-wave simulation, for up to 10 W high frequency input to the on-body coil, demonstrating its suitability for wearable applications.

While a range of Simultaneous Wireless Information and Power Transfer (SWIPT) rectennas were proposed for wearables, the human-proximity effects on a complex-conjugate matching network-free rectenna remain mostly unknown. This paper reports for the first time the impact of human proximity on a complex conjugate rectenna's impedance. The impedance of a dual-mode SWIPT microstrip/dipole rectenna is measured experimentally for varying separation from the body and on different body positions. The measured input impedance is used to simulate the rectifier's RF-to-DC efficiency using a non-linear model showing the effects of the power level, load, and frequency on the DC output for different on-body impedances.

While far-field wireless power transfer (WPT)promises a long range of operation, it falls short on the DC powerwhich can be provided. On the other, hand resonant near-fieldWPT suffers from a short wireless range. In this work, a dual mode high/low-power near/far-field rectenna is demonstrated for the first time based on a flexible rectifier and a textile antenna. Based on a high-efficiency single-series rectifier, the far-field rectenna achieves a peak efficiency of 42.7% from an ultra low power density of 0.43 μW/cm2, with a 1 V DC output from4 μW/cm2. A high-power 6.78 MHz bridge rectifier is designed and implemented on a flexible textile-based substrate with a10 dB return loss for inputs above 15 dBm. As a near-field receiver, the antenna could receive 3.75 W DC power at 5 cm from the transmitting coil, with an end-to-end efficiency of 30%inclusive of rectifier, coils, and power amplifier losses. Based on the proposed dual-mode antenna, textile-based wearables could be wirelessly powered dynamically using resonant near-field and radiative near-field WPT.

While millimeter-wave (mmWave) technologies are often associated with costly applications using large arrays, several inexpensive implementations promise mmWave connectivity closer to the user in Body Area Networks (BANs) applications. Here, we evaluate the potential for multi-mode mmWave links for information and power transfer applications. The co-existence of off-body radiative and on-body wave-guiding mechanisms is experimentally investigated based on state-of-the-art transmission lines and antennas. First, a body-to-body link with at least -50 dB channel gain is demonstrated based on wide-beam microstrip and reflector-backed broadband antennas. Co-existence is then studied experimentally by measuring the coupling between the off-body communication/power transfer antenna and a wearable Single Wire Transmission Line (SWTL), with an ultra-low on-body attenuation of below -0.8 dB/cm around 28 GHz. Less than -40 dB coupling is demonstrated for clearances as low as 1 cm between the antenna and SWTL. The measured results indicate that co-located textile-based antennas and transmission lines can enable multi-mode high-performance body-centric mmWave networks, and highlight the need for interference-countering mechanisms in future high-density BANs.

Anticipated by 2035, a trillion wirelessly connected Internet of Everything (IoE) devices will be deployed. Therefore, sustainable and low-cost antennas are crucial for enabling an environmentally-friendly IoE. We present the first non-metallic carbon-based microstrip patch antenna implemented on textiles for wearable applications. The antenna is fabricated on a standard felt/woven-polyester fabric substrate using stencil casting and demonstrated for S-band applications. The antenna maintains a wide 10% fractional bandwidth from 3.16 to 3.49 GHz and achieves at least 30% radiation efficiency, 10.4 dBi measured directivity, and 6.1 dBi measured gain. It is demonstrated that non-metallic carbon-based antennas are suitable for future wearable IoE applications.

Textile-based antennas are typically realized using conductive fabrics adhered to thick non-woven fabrics such as felt, which are not suited for additive manufacturing due to their roughness. This paper presents screen printing on thin and breathable polyurethane films as a method for realizing e-textile antennas without altering the feel of the fabric, demonstrated through a microstrip antenna on standard felt fabric. The fabricated proximity-coupled patch maintains at least 100 MHz (4\%) bandwidth from 2.46 to 2.56~GHz regardless of bending and human proximity, 6.6~dBi measured gain, and 69\% radiation efficiency. The robustness of the proposed antenna is demonstrated, for the first time, through the measured radiation properties, showing that the antenna maintains its peak gain and radiation patterns after more than 10,000 bending cycles.

Wearable radio frequency (RF) energy harvesting is highly dependent on the distance from the source and human-caused RF shadowing. Therefore, energy storage devices integrated with rectennas are of paramount importance to overcome this intermittency. In this paper, the use of carbon-based e-textile supercapacitors for storing the RF-DC converted power for powering body area networks nodes is investigated. A voltage doubler sub-1 GHz flexible rectifier, whose peak power conversion efficiency (PCE) approaches 80% is coupled to a two-cell 15.5 mF textile-based supercapacitor operating up to 4 V DC. Owing to the rectifier's low optimum load resistance and high DC Voltage output, the average charging PCE of the rectifier-supercapacitor module reaches 31% for a 9.5 dBm input. Time-varying s-parameter measurements are performed to compare the time-averaged matching as opposed to instantaneous measurements using a resistive load, where the textile supercapacitor exhibits a similar response to a commercial supercapacitor. Finally, the RF-charged textile supercapacitor is demonstrated, for the first time, powering a microcontroller and Bluetooth transmitter with an average power consumption of 350 uW for up to 102 s, following 40 s of charging at 9.5 dBm, demonstrating its suitability for RF-powered body area networks applications.

The broad spectrum available at millimeter-wave (mmWave) bands has attracted significant interest for a breadth of applications, with 5G communications being the main commercial drive for mmWave networks. Wireless power transmission and harvesting at mmWave bands have attracted significant attention due to the potential for minimizing the harvesting antenna size, allowing for more compact rectennas. For a fixed antenna size, the received power increases with frequency. Nevertheless, several challenges lie in realizing high efficiency antennas and rectifiers at mmWave bands. This paper reviews the recent advances in mmWave rectenna design at a component- and system-level. Low-cost antennas and components for mmWave power harvesting, such as high efficiency scalable rectifiers on polymers and high radiation efficiency antennas on textiles, are reviewed. Both the antenna and rectifier can be realized using low-cost fabrication methods such as additively-manufactured circuits and packages, in addition to digital integrated circuits (ICs) for the rectifiers. Finally, this paper provides an overview of future antenna design challenges and research directions for mmWave power harvesting.

Wireless power transfer (WPT) and Radio Frequency (RF) energy harvesting (EH) are increasingly seen as an enabling technology for power-autonomous Internet of Things (IoT) [1, 2]. RFEH and WPT are a particularly attractive power source for flexible, printed, and e-textile systems due to their compatibility with standard fabrication processes, abolishing the need for specific materials and transducers. This work provides an overview on flexible WPT at multiple frequencies (6.78 MHz to 26 GHz) and techniques from near-field WPT to Millimetre-wave WPT and RFEH.

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.

Energy harvesting powered devices have the potential for widespread use in buildings. The most prevalent ambient energy source available in buildings is light, which is normally harvested using photovoltaic devices. The light to be harvested can be from both natural and artificial sources and a range of different types of solar cells are available to suit differing light sources and intensities, and as such must be selected to suit the type of light to be encountered. For use inside buildings it is desirable to choose a device which will operate efficiently under artificial lighting as many locations have limited or no natural light, however, the nature of artificial sources is changing over time from incandescent sources, through fluorescent lights, with LED lights currently attracting significant interest due to energy savings. This paper presents an investigation into the selection of solar cell type for a range of artificial lighting sources and illumination levels.

This paper compares performance of a piezoelectric vibration energy harvester, in terms of output power, under different types of vibration. The most common method used to characterize a vibration energy harvester is to excite the energy harvester under a sinusoidal vibration of its resonant frequency and measure the voltage across certain electrical loads. However, in practical applications, the vibration spectrum usually contains multiple peaks at different frequencies. In this research, a piezoelectric energy harvester was tested under vibration taken from a PZL-SW4 helicopter and the charging rate was compared with that when excited with sinusoidal vibration at the resonant frequency.

This paper describes a novel airflow energy harvester for wireless sensing applications. The energy harvester consists of a wing that is attached to a cantilever spring. A set of permanent magnets is fixed on the wing and a coil is attached to the base of the energy harvester. The wing oscillates when the airflow blows over it, which causes the magnetic flux cutting the coil to change and generates electrical power. The device has dimensions of 14.1cm×10cm×5.5cm. Experimental results have shown that the energy harvester can operate at wind speeds as low as 1.5m•s-1 with a corresponding electrical output power of 20µW. When the airflow speed is between 2 and 4m•s-1, which are typical values measured in the duct of an office building, the output power is between 90 and 573µW, which is sufficient for periodic sensing and wireless transmission.

In this paper the performance in terms of resonant frequency and output power of cantilever-based vibration energy harvesters under different mechanical axial loads will be compared providing a comprehensive assessment of this method as a way of increasing the operational frequency range through tuning. Two types of axial loads were used in the test, i.e. tensile and compressive force. To eliminate the effect of any other mechanical loads on performance of the energy harvester and minimize external damping, a contactless force, i.e. magnetic force, was applied to the cantilever. It is found that although resonant frequency tuning by applying compressive force is more sensitive than tuning by tensile force, the reduction in Q-factor under compressive loads makes it less useful than tuning by applying tensile loads. Axial compressive loads can, however, be used to achieve a bi-stable structure which can also be a solution to wideband energy harvesters.

A vibration energy scavenger, manufactured entirely by thick-film construction, has been developed to power autonomous subsystems in an embedded health and useage system. The device is constrained to a 2mm thickness and has been designed for a specific helicopter application. The resulting power output is capable of powering an ‘off-the-shelf’ microcontroller based system.

This paper reports a self powered smart tag consisting of a vibration energy harvester, the power conditioning circuit, sensors and a RF transmitter for wireless structural health monitoring (SHM) in aeronautical applications. The smart tag has dimensions similar to a credit card and aims to be integrated into the surface of the aircraft. The smart tag is powered by an integrated bimorph piezoelectric generator that extracts energy from ambient vibrations at the frequency of 67Hz and the amplitude of 0.4g (3.9ms-2). The generator is fabricated using thick film printing technology. Experimentally, the generator produced the maximum RMS output power of 181uW which is sufficient to enable periodic sensing and transmission. Description of the smart tag as well as the experimental results of the piezoelectric generator are presented.

This paper proposes, for the first time, an automated energy harvester design flow which is based on a single HDL software platform that can be used to model, simulate, configure and optimise energy harvester systems. A demonstrator prototype incorporating an electromagnetic mechanical-vibration-based micro-generator and a limited number of library models has been developed and a design case study has been carried out. Experimental measurements have validated the simulation results which show that the outcome from the design flow can improve the energy harvesting efficiency by 75%.

This paper presents a piezoelectric vibration energy harvester fabricated using screen printing. The formulation of a novel tungsten based polymer ink enables the deposition of an inertial mass enabling the entire structure to be fabricated by the printing process. The devices have been designed for use on an aircraft Health and Usage Monitoring System (HUMS). Initial devices produce a peak power of 117?w at 6.9m/s2 and 70Hz with an optimum load of 140kW and an output voltage of 2.9V.

This paper presents a tunable electromagnetic vibration-based micro-generator with closed loop frequency tuning. Frequency tuning is realized by applying an axial tensile force to the micro-generator. A closed-loop frequency tuning system has been developed to control the tuning process so that the generator always operates at the ambient vibration frequency to make the entire process automatic. Experimentally the resonant frequency has been successfully tuned from 67.6 to 98 Hz when various axial tensile forces were applied to the structure. The generator produced a power of 61.6 to 156.6 μW over the tuning range when excited at vibrations of 0.588 m/s2.

Studies into electroporation have grown rapidly in biotechnology and medicine in recent years. This paper presents the design and construction of a low cost programmable electroporation system for biological applications. The system consists of a control module, a pulse generation circuit and a high voltage switch using a power MOSFET. The programmable electroporation has been designed, developed and tested. Using a standard commercial electroporation cuvette, it is possible to generate electric fields of 100 to 1000V/cm with programmed pulse lengths of 10?sec to 20msec. The system was evaluated with Hela cells and propidium dye to evaluate transfection rates under a variety of electroporation conditions. Initial results showed that the electroporation system achieved a peak cell transfection efficiency of 48.74% at 600V/cm with pulse lengths of 10 ms.

This paper compares two thick-film actuation technologies and assesses their suitability for use within micromachined devices. The materials are a piezoelectric composition, which exhibits a d33 coefficient of the order of 130pC/N, and a magnetostrictive material which possess a magnetostriction of 4.4ppm at a magnetic field of 11.5kA/m. Examples of their application in micromachined devices are also given.

This paper presents a process for fabricating diaphragms, suitable for use in micromachined pumps within the field of micro-fluidics. To enable the actuation of a thick-film deposited magnetostrictive material, actuation coils are fabricated onto the non-chamber face of the diaphragms. These actuation coils are fabricated using a multilayer deposition approach compatible with micromachining processes. The selection of diaphragm and actuation region dimensions has been achieved through the use of finite element modeling. The paper also describes the fabrication process used to realize the structures.

Radio frequency energy harvesting (RFEH) and wireless power transfer (WPT) are increasingly seen as a method of enabling sustainable computing, as opposed to mechanical or solar\ud EH WPT does not require special materials or resonators and can be implemented using low-cost\ud conductors and standard semiconductor devices. This work revisits the simplest antenna design,\ud the wire monopole to demonstrate the lowest-footprint, lowest-cost rectifying antenna (rectenna)\ud based on a single Schottky diode. The antenna is fabricated using a single Litz-wire silk-coated\ud thread, embroidered into a standard textile substrate. The rectifier is fabricated on a compact lowcost flexible printed circuit board (PCB) using ultra-thin polyimide copper laminates to accommodate low-footprint surface mount components. The antenna maintains its bandwidth across the\ud 868/915 MHz license-free band on- and off-body with only −4.7 dB degradation in total efficiency in\ud human proximity. The rectenna achieves up to 55% RF to DC efficiency with 1.8 V DC output, at 1\ud mW of RF power, demonstrating its suitability as a power-supply unit for ultra-low power e-textile\ud nodes.