Nanoscale Strategies for Enhancing the Performance of Adhesive Dry Electrodes for the Skin.

High-quality electrophysiological monitoring requires electrodes to maintain a compliant and stable skin contact. This necessitates low impedance, good skin compliance, and strong adhesion to ensure continuous and stable contact under dynamic conditions. In this context, adhesive epidermal dry electrodes are advancing rapidly, which is promising for long-term applications in clinical diagnosis, wearable health monitoring, and human-machine interfaces. However, challenges persist, as conventional technologies usually fall short of meeting the high standards required for electrophysiological electrodes. This Perspective discusses four key aspects for high-performance epidermal electrodes from an adhesive perspective: initial adhesion, water resistance, dynamic stability, and removal simplicity. We review recent nanoscale strategies addressing these issues, providing a comprehensive guideline to enhance the application performance of epidermal dry electrodes. Additionally, we explore key nanoscale strategies and their associated functions, future technology roadmaps, and prospects for dry adhesive epidermal electrodes.

Highly Adhesive and Stretchable Epidermal Electrode for Bimodal Recording Patch.

For numerous biological and human-machine applications, it is critical to have a stable electrophysiological interface to obtain reliable signals. To achieve this, epidermal electrodes should possess conductivity, stretchability, and adhesiveness. However, limited types of materials can simultaneously satisfy these requirements to provide satisfying recording performance. Here, we present a dry electromyography (EMG) electrode based on conductive polymers and tea polyphenol (CPT), which offers adhesiveness (0.51 N/cm), stretchability (157%), and low impedance (14 kΩ cm2 at 100 Hz). The adhesiveness of the electrode is attributed to the interaction between catechol groups and hydroxyls in the polymer blend. This adhesive electrode ensures stable EMG recording even in the presence of vibrations and provides signals with a high signal-to-noise ratio (>25 dB) for over 72 h. By integrating the CPT electrode with a liquid metal strain sensor, we have developed a bimodal rehabilitation monitoring patch (BRMP) for sports injuries. The patch utilizes Kinesio Tape as a substrate, which serves to accelerate rehabilitation. It also tackles the challenge of recording with knee braces by fitting snugly between the brace and the skin, due to its thin and stretchable design. CPT electrodes not only enable BRMP to assist clinicians in formulating effective rehabilitation plans and offer patients a more comfortable rehabilitation experience, but also hold promise for future applications in biological and human-machine interface domains.

Liquid Metal-Polymer Conductor-Based Conformal Cyborg Devices.

Gallium-based liquid metal (LM) exhibits exceptional properties such as high conductivity and biocompatibility, rendering it highly valuable for the development of conformal bioelectronics. When combined with polymers, liquid metal-polymer conductors (MPC) offer a versatile platform for fabricating conformal cyborg devices, enabling functions such as sensing, restoration, and augmentation within the human body. This review focuses on the synthesis, fabrication, and application of MPC-based cyborg devices. The synthesis of functional materials based on LM and the fabrication techniques for MPC-based devices are elucidated. The review provides a comprehensive overview of MPC-based cyborg devices, encompassing their applications in sensing diverse signals, therapeutic interventions, and augmentation. The objective of this review is to serve as a valuable resource that bridges the gap between the fabrication of MPC-based conformal devices and their potential biomedical applications.

Stretchable surface electromyography electrode array patch for tendon location and muscle injury prevention.

Surface electromyography (sEMG) can provide multiplexed information about muscle performance. If current sEMG electrodes are stretchable, arrayed, and able to be used multiple times, they would offer adequate high-quality data for continuous monitoring. The lack of these properties delays the widespread use of sEMG in clinics and in everyday life. Here, we address these constraints by design of an adhesive dry electrode using tannic acid, polyvinyl alcohol, and PEDOT:PSS (TPP). The TPP electrode offers superior stretchability (~200%) and adhesiveness (0.58 N/cm) compared to current electrodes, ensuring stable and long-term contact with the skin for recording (>20 dB; >5 days). In addition, we developed a metal-polymer electrode array patch (MEAP) comprising liquid metal (LM) circuits and TPP electrodes. The MEAP demonstrated better conformability than commercial arrays, resulting in higher signal-to-noise ratio and more stable recordings during muscle movements. Manufactured using scalable screen-printing, these MEAPs feature a completely stretchable material and array architecture, enabling real-time monitoring of muscle stress, fatigue, and tendon displacement. Their potential to reduce muscle and tendon injuries and enhance performance in daily exercise and professional sports holds great promise.

Electronic exoneuron based on liquid metal for the quantitative sensing of the augmented somatosensory system.

The increasing demands in augmented somatosensory have promoted quantitative sensing to be an emerging need for athletic training/performance evaluation and physical rehabilitation. Neurons for the somatosensory system in the human body can capture the information of movements in time but only qualitatively. This work presents an electronic Exo-neuron (EEN) that can spread throughout the limbs for realizing augmented somatosensory by recording both muscular activity and joint motion quantitatively without site constraints or drift instability, even in strenuous activities. Simply based on low-cost liquid metal and clinically used adhesive elastomer, the EEN could be easily fabricated in large areas for limbs. It is thin (~120 µm), soft, stretchable (>500%), and conformal and further shows wide applications in sports, rehabilitation, health care, and entertainment.

Stretchable Liquid Metal-Based Metal-Polymer Conductors for Fully Screen-Printed Biofuel Cells.

We reported a straightforward and low-cost method to fabricate stretchable biofuel cells by using liquid metal-based metal-polymer conductors. The liquid-metal-based metal-polymer conductors had a conductivity of 2.7 × 105 S/m and a stretchability larger than 200%, giving the biofuel cell good conformability to the skin. The glucose biofuel cells (BFCs) yielded a maximum power density as 14.11 µW/cm2 at 0.31 V with 0.2 mM glucose, while the lactate BFCs reached 31.00 µW/cm2 at 0.51 V with 15 mM lactate. The results of 24 h short circuit current density showed that, with enough biofuel, this patch could be used over the course of an entire day for wearable sensors.

Skin Electronics from Biocompatible In Situ Welding Enabled By Intrinsically Sticky Conductors.

Welding usually involves high temperatures, toxic solvents, or conditions not compatible with human bodies, which severely limit the fusion of electronics and human tissues. To achieve direct welding of electronics on human skin, the intrinsically sticky conductors that can simultaneously achieve metal-grade electrical conductivity (≈41 7000 S m-1 ), hydrogel-grade stretchability (>900% strain), and self-adhesiveness (1.8 N cm-1 ) are reported. The sticky conductors composed of gallium indium alloy and acrylate polymer adhesives have a surface-enriched structure, which can form instant mechanical and electrical connections with different surfaces through gentle pressure without involving conditions that may damage human tissues. Based on the sticky conductors, the in situ welding of electronics on the skin is realized. To demonstrate the feasibility of in situ welding, electronic tattoos are achieved for movement monitoring. Intrinsically sticky electrodes that can resist drying and simultaneously deform with the skin for electrophysiological measurement are also developed.

In Situ Deposition of Skin-Adhesive Liquid Metal Particles with Robust Wear Resistance for Epidermal Electronics.

Comfort and mechanical stability are vital for epidermal electronics in daily use. In situ deposition of circuitry without the protection of substrates or encapsulation can produce imperceptible, conformal, and permeable epidermal electronics. However, they are easily destroyed by daily wear because the binding force between deposited materials and skin is usually weak. Here, we in situ deposited skin-adhesive liquid metal particles (ALMP) to fabricate epidermal electronics with robust wear resistance. It represents the most wear-resistant in situ deposited epidermal electronic materials. It can withstand ∼1600 cm, 175 g loaded paper tape wearing by a standard abrasion wear tester. Stretchability, conformality, permeability, and thinness of the ALMP coating provide an imperceptible and comfortable wearing experience. Without degradation of electrical property caused by solvent evaporation, the dry ALMP coating possesses natural advantages over gel electrodes. In situ deposited ALMP is an ideal material for fabricating comfortable epidermal electronics.

Realization of Moisture-Resistive Perovskite Films for Highly Efficient Solar Cells Using Molecule Incorporation.

The development of highly crystalline perovskite films with large crystal grains and few surface defects is attractive to obtain high-performance perovskite solar cells (PSCs) with good device stability. Herein, we simultaneously improve the power conversion efficiency (PCE) and humid stability of the CH3NH3PbI3 (CH3NH3 = MA) device by incorporating small organic molecule IT-4F into the perovskite film and using a buffer layer of PFN-Br. The presence of IT-4F in the perovskite film can successfully improve crystallinity and enhance the grain size, leading to reduced trap states and longer lifetime of the charge carrier, and make the perovskite film hydrophobic. Meanwhile, as a buffer layer, PFN-Br can accelerate the separation of excitons and promote the transfer process of electrons from the active layer to the cathode. As a consequence, the PSCs exhibit a remarkably improved PCE of 20.55% with reduced device hysteresis. Moreover, the moisture-resistive film-based devices retain about 80% of their initial efficiency after 30 days of storage in relative humidity of 10-30% without encapsulation.

Printable Metal-Polymer Conductors for Highly Stretchable Bio-Devices.

Stretchable, biocompatible devices can bridge electronics and biology. However, most stretchable conductors for such devices are toxic, costly, and regularly break/degrade after several large deformations. Here we show printable, highly stretchable, and biocompatible metal-polymer conductors by casting and peeling off polymers from patterned liquid metal particles, forming surface-embedded metal in polymeric hosts. Our printable conductors present good stretchability (2,316 S/cm at a strain of 500%) and repeatability (ΔR/R <3% after 10,000 cycles), which can satisfy most electrical applications in extreme deformations. This strategy not only overcomes large surface tension of liquid metal but also avoids the undesirable sintering of its particles by stress in deformations, such that stretchable conductors can form on various substrates with high resolution (15 µm), high throughput (∼2,000 samples/hour), and low cost (one-quarter price of silver). We use these conductors for stretchable circuits, motion sensors, wearable glove keyboards, and electroporation of live cells.