Cardiovascular diseases are the leading cause of death globally. Noninvasive, accurate, and continuous cardiovascular monitoring can enable the preemptive detection of heart diseases and timely intervention to prevent serious cardiac complications. However, unobtrusive, ambulatory, and comprehensive cardiac monitoring is still a challenge as conventional electronics are rigid, heavy, or consume too much power for long‐term measurement. This work presents a thin (200 µm), stretchable (20%), lightweight (2.5 g), wireless, and low‐power (<3 mW) cardiac monitoring device that conforms to the human chest like a temporary tattoo sticker, correspondingly known as an e‐tattoo. This chest e‐tattoo features dual‐mode electro‐mechanical sensing—bio‐electric cardiac signals via electrocardiography and mechanical cardiac rhythm via seismocardiography. A unique peripheral synchronization strategy between the two sensors enables the measurement of systolic time intervals like the pre‐ejection period and the left ventricular ejection time with high accuracy (error = −0.44 ± 8.74 ms) while consuming very low power. The e‐tattoo is validated against clinically approved gold‐standard instruments on five human subjects. The good wearability and low power consumption of this e‐tattoo permit 24‐h continuous ambulatory monitoring.

Due to the limitations of standard wet Silver/Silver Chloride (Ag/AgCl) hydrogel electrodes and the growing demand for long-term high fidelity surface electromyography (EMG) recording, dry epidermal electrodes are of great interest. Evaluating the usability and signal fidelity of dry epidermal electrodes could help determine the extent of potential applications using EMG electrodes. We collected EMG signals over eight days from the right rectus femoris of seven subjects using single-use dry epidermal electrodes and traditional Ag/AgCl electrodes while covered and uncovered during dynamic movements (leg extension, sit-to-stand, and treadmill walking at 0.75 m/s and 1.30 m/s). We quantified signal fidelity using signal-to-noise ratio (SNR); signal-to-motion ratio (SMR); and a metric we previously developed, the Signal Quality Index, which considers that better EMG signal quality requires both good signal-to-noise ratio and good signal-to-motion ratio. Wear patterns over the eight days degraded EMG signal quality. Uncovered epidermal electrodes that remained intact and maintained good adhesion to the skin had signal-to-noise ratios, signal-to-motion ratios, and Signal Quality Index values that were above the acceptable thresholds for limited dynamic lower limb movements (leg extension and sit-to-stand). This indicated that dry epidermal electrodes could provide good signal quality across all subjects for five days for these movements. For walking, the signal-to-noise ratios of the uncovered epidermal electrodes were still above the acceptable threshold, but signal-to-motion ratios and the Signal Quality Index values were far below the acceptable thresholds. The signal quality of the epidermal electrodes that showed no visible wear was stable over five days. As expected, covering the epidermal electrodes improved signal quality, but only for limited dynamic lower limb movements. Overall, single-use dry epidermal electrodes were able to maintain high signal quality for long-term EMG recording during limited dynamic lower limb movements, but further improvement is needed to reduce motion artifacts for whole body dynamic movements such as walking.

Alternatives to conventional wet Silver/Silver Chloride (Ag/AgCl) hydrogel electrodes could greatly improve the ability to record long-term surface electromyography (sEMG). This paper presents preliminary evaluation of dry epidermal electronic system (EES) electrodes over 8 days measurements. We tested 3 subjects and recorded sEMG signals of right rectus femoris during static and dynamic tasks from epidermal electrodes and traditional Ag/AgCl electrodes. For epidermal electrodes that remained intact and maintained adhesion to the skin, signal-to-noise ratio (SNR) values were greater than 16.9 dB and signal-to-motion ratio (SMR) values were greater than 13.1 dB, which were both above the accepted thresholds for good signal quality (SNR > 15 dB and SMR > 12dB). We also developed a new metric, the Signal Quality Index, SQI to help determine whether higher SNR or higher SMR were necessarily indicative of better EMG signal quality. Our results indicated that the single use dry epidermal electrodes were able to maintain high signal quality over 8 days and suggested that dry epidermal electrodes have the potential ability for long-term sEMG recording.

Seismocardiography (SCG) is a measure of chest vibration associated with heartbeats. While skin soft electronic tattoos (e‐tattoos) have been widely reported for electrocardiogram (ECG) sensing, wearable SCG sensors are still based on either rigid accelerometers or non‐stretchable piezoelectric membranes. This work reports an ultrathin and stretchable SCG sensing e‐tattoo based on the filamentary serpentine mesh of 28‐µm‐thick piezoelectric polymer, polyvinylidene fluoride (PVDF). 3D digital image correlation (DIC) is used to map chest vibration to identify the best location to mount the e‐tattoo and to investigate the effects of substrate stiffness. As piezoelectric sensors easily suffer from motion artifacts, motion artifacts are effectively reduced by performing subtraction between a pair of identical SCG tattoos placed adjacent to each other. Integrating the soft SCG sensor with a pair of soft gold electrodes on a single e‐tattoo platform forms a soft electro‐mechano‐acoustic cardiovascular (EMAC) sensing tattoo, which can perform synchronous ECG and SCG measurements and extract various cardiac time intervals including systolic time interval (STI). Using the EMAC tattoo, strong correlations between STI and the systolic/diastolic blood pressures, are found, which may provide a simple way to estimate blood pressure continuously and noninvasively using one chest‐mounted e‐tattoo.

Unlike wafer-based rigid electronics, soft electronics have many unique advantages including thinness, flexibility, stretchability, conformability, lightweight, large area, as well as low cost. As a result, they have demonstrated many emerging capabilities in healthcare devices, soft robotics, and human-machine interface. Instead of conventional microfabrication, there is an evergrowing interest in the freeform or digital manufacture of soft electronics. This review provides a survey for a cost- and timeeffective subtractive manufacturing process called the cut-and-paste method. It employs a mechanical cutter plotter to form patterns on various electronically functional membranes such as sheets of metals, functional polymers, and even two-dimensional (2D) materials, supported by a temporary tape. The patterned membranes can then be pasted on soft substrates such as medical tapes or even human skin. This process is completely dry and desktop. It does not involve any rigid wafers and is hence capable of making large-area electronics. The process can be repeated to integrate multiple materials on a single substrate. Integrated circuits (ICs) and rigid components can be added through a cut-solder-paste process. Multilayer devices can also be fabricated through lamination. We therefore advocate that the cut-and-paste method is a very versatile approach for the rapid prototyping of soft electronics for various applications.

Epidermal electronic system is a class of hair thin, skin soft, stretchable sensors and electronics capable of continuous and long-term physiological sensing and clinical therapy when applied on human skin. The high cost of manpower, materials, and photolithographic facilities associated with its manufacture limit the availability of disposable epidermal electronics. We have invented a cost and time effective, completely dry, benchtop "cut-and-paste" method for the green, freeform and portable manufacture of epidermal electronics within minutes. We have applied the "cut-and-paste" method to manufacture epidermal electrodes, hydration and temperature sensors, conformable power-efficient heaters, as well as cuffless continuous blood pressure monitors out of metal thin films, two-dimensional (2D) materials, and piezoelectric polymer sheets. For demonstration purpose, we will discuss three examples of "cut-and-pasted" epidermal electronic systems in this paper. The first will be submicron thick, transparent epidermal graphene electrodes that can be directly transferred to human skin like a temporary transfer tattoo and can measure electrocardiogram (ECG) with signal-to-noise ratio and motion artifacts on par with conventional gel electrodes. The second will be a chest patch which houses both electrodes and pressure sensors for the synchronous measurements of ECG and seismocardiogram (SCG) such that beat-to-beat blood pressure can be inferred from the time interval between the R peak of the ECG and the AC peak of the SCG. The last example will be a highly conformable, low power consumption epidermal heater for thermal therapy.

Epidermal electronics are sensors with mechanical properties matching human epidermis. Their manufacturing process includes photolithography and dry and wet etching within clean room facilities. The high cost of manpower, materials, photo masks, and facilities greatly hinders the commercialization potential of disposable epidermal electronics. In contrast, an embodiment of the invention includes a low cost, high throughput, bench top "cut and paste" method to complete the freeform manufacture of epidermal sensor system (ESS) in minutes. This versatile method works for many types of thin metal and polymeric sheets and is compatible with many tattoo adhesives or medical tapes. The resultant ESS is highly multimaterial and multifunctional and may measure ECG, EMG, skin temperature, skin hydration, as well as respiratory rate. Also, a stretchable planar coil made of serpentine ribbons can be used as a wireless strain gauge and/or a near field communication (NFC) antenna. Other embodiments are described herein.

Epidermal electronics is a class of noninvasive and unobstructive skin-mounted, tattoo-like sensors and electronics capable of vital sign monitoring and establishing human-machine interface. The high cost of manpower, materials, vacuum equipment, and photolithographic facilities associated with its manufacture greatly hinders the widespread use of disposable epidermal electronics. Here we report a cost and time effective, completely dry, benchtop “cut-and-paste” method for the freeform and portable manufacture of multiparametric epidermal sensor systems (ESS) within minutes. This versatile method works for all types of thin metal and polymeric sheets and is compatible with any tattoo adhesives or medical tapes. The resulting ESS are multimaterial and multifunctional and have been demonstrated to noninvasively but accurately measure electrophysiological signals, skin temperature, skin hydration, as well as respiratory rate. In addition, planar stretchable coils exploiting double-stranded serpentine design have been successfully applied as wireless, passive epidermal strain sensors.

The electrical activities of the brain and heart have been recorded and analyzed for diverse clinical and pathological purposes. To construct an implantable system for monitoring the electrical activity effectively, flexible and stretchable electrode arrays that are capable of making conformal contacts on the curvilinear, soft, and dynamic surfaces of the target organs have been extensively researched. Among many strategies, the most representative approach is to fabricate electrode arrays on plastic substrates to achieve more intimate and conformal contact with the target organs. Further optimizations are along with the development of ultrathin and stretchable electronics. Advanced structural modifications, such as thinning the overall profile or applying a mesh-like electrode network, have shown the greatly enhanced conformability and deformability of the device, providing improved signal-to-noise ratios (SNRs). Furthermore, brittle but high-performance silicon transistors have been successfully incorporated in flexible forms by virtue of mechanics-based active electronics designs, enabling the construction of high-density arrays comprising hundreds of multiplexed electrodes that can be individually addressed by only a few external wires. This review summarizes these strategies and describes their strengths and weaknesses, and it suggests possible technologies for nextgeneration electrode arrays.

Strategies for bio-integrated electronics must overcome challenges associated with the mismatch between the hard, planar surfaces of brittle semiconductor wafers and the soft, curvilinear tissues of timely dynamic biological systems. This paper discusses the successful structural design, fabrication and bio-integration of tissue-like electronics that can intimately conform to biological tissues and deform with the tissues in a mechanically invisible manner. Examples including epidermal electronics, instrumented balloon catheters, and smart finger tubes will be discussed to manifest our multidisciplinary research at the boundary of electronics, mechanics, materials, and biomedicine.