Bioelectronics stickers that interface the human epidermis and collect electrophysiological data will constitute important tools in the future of healthcare. Rapid progress is enabled by novel fabrication methods for adhesive electronics patches that are soft, stretchable and conform to the human skin. Yet, the ultimate functionality of such systems still depends on rigid components such as silicon chips and the largest rigid component on these systems is usually the battery. In this work, we demonstrate a quickly deployable, untethered, battery-free, ultrathin (~5 μm) passive "electronic tattoo" that interfaces with the human skin for acquisition and transmission of physiological data. We show that the ultrathin film adapts well with the human skin, and allows an excellent signal to noise ratio, better than the gold-standard Ag/AgCl electrodes. To supply the required energy, we rely on a wireless power transfer (WPT) system, using a printed stretchable Ag-In-Ga coil, as well as printed biopotential acquisition electrodes. The tag is interfaced with data acquisition and communication electronics. This constitutes a "data-by-request" system. By approaching the scanning device to the applied tattoo, the patient's electrophysiological data is read and stored to the caregiver device. The WPT device can provide more than 300 mW of measured power if it is transferred over the skin or 100 mW if it is implanted under the skin. As a case study, we transferred this temporary tattoo to the human skin and interfaced it with an electrocardiogram (ECG) device, which could send the volunteer's heartbeat rate in real-time via Bluetooth. Surface biopotentials collected from the human epidermis contain important information about human physiology, such as muscular, heart and brain activities. This includes electromyography (EMG) 1 , Electrocardiography (ECG) 2 , and Electroencephalography (EEG) 3 , among others. The collected data has applications in health monitoring (EMG, ECG, EEG), control of prosthetics 4 or novel forms of wearable human-machine interfaces (EMG) 5,6. Wearable stickers that interface the human epidermis and acquire biopotentials for electrophysiological monitoring can be potentially transformative in digital health, since they would eventually allow a fully wireless and hassle-free data collection from the human body. Unlike traditional "wearable" technology that is composed of several rigid components, these stickers are required to be soft, flexible and stretchable. In this way, they are able to follow the dynamic morphology of the skin and remain attached to the skin during natural human movements. An ideal biomonitoring sticker is as well thin, imperceptible, comfortable and untethered. This can be also in the form of an electrical bandage or a "temporary tattoo" which bonds strongly to the human skin and acquires and transmits the information. During the last five years, some reports on fabrication and applications of ultrathin stretchable electronic films, also called epidermal electronics 7 or electronic...

Abstract-In this paper the equivalent impedance of resonator arrays for wireless power transfer systems is obtained in closed-form from a continued fraction expression. Using the theory of difference equations, the continued fraction is described as the general term of a complex sequence defined by recurrence, and its convergence is analyzed. It is shown that the equivalent impedance can be easily found in closed-form in terms of the system parameters. In this way, the obtained closed-form expressions may help electrical engineers to quickly predict the behaviour of a system with the changes of its parameters. Some numerical examples of the theoretical results are given and discussed. Finally, the analytical formulae obtained in this work are validated with measurements and a good agreement is observed.

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This paper studies the power transfer characteristics of a resonator array for inductive power transfer by means of the accurate analytical solution of its circuit model. Through the mathematical inversion of a tridiagonal matrix, it is possible to obtain closed-form expressions for the current in each resonator and consequently expressions for the power transfer and efficiency of the system. The method can be applied to a resonator array powering a load at the end of the array or a receiver facing the array at any position. With the expressions obtained, it is possible not only to achieve a better understanding of the power transfer characteristics in resonator arrays but also to obtain the conditions for maximum power transfer or maximum efficiency, for several conditions and parameters of the system. A prototype of a stranded-wire resonator array powered by a resonant inverter, capable of delivering power to a load from 65 W to 90 W with efficiency values between 63% and 88%, was built in order not only to validate the expressions obtained but also to show their practical applicability and demonstrate that these arrays can be used for higher power transfer applications.