High-Adhesive Flexible Electrodes and Their Manufacture: A Review

All human activity is associated with the generation of electrical signals. These signals are collectively referred to as electrical physiology (EP) signals (e.g., electrocardiogram, electroencephalogram, electromyography, electrooculography, etc.), which can be recorded by electrodes. EP electrodes are not only widely used in the study of primary diseases and clinical practice, but also have potential applications in wearable electronics, human–computer interface, and intelligent robots. Various technologies are required to achieve such goals. Among these technologies, adhesion and stretchable electrode technology is a key component for the rapid development of high-performance sensors. In recent years, remarkable efforts have been made in the development of flexible and high-adhesive EP recording systems and preparation technologies. Regarding these advancements, this review outlines the design strategies and related materials for flexible and adhesive EP electrodes, and briefly summarizes their related manufacturing techniques.

Self-Patterned Stretchable Electrode Based on Silver Nanowire Bundle Mesh Developed by Liquid Bridge Evaporation

A new strategy is required to realize a low-cost stretchable electrode while realizing high stretchability, conductivity, and manufacturability. In this study, we fabricated a self-patterned stretchable electrode using a simple and scalable process. The stretchable electrode is composed of a bridged square-shaped (BSS) AgNW bundle mesh developed by liquid bridge evaporation and a stretchable polymer matrix patterned with a microcavity array. Owing to the BSS structure and microcavity array, which effectively concentrate the applied strain on the deformable square region of the BSS structure under tensile stretching, the stretchable electrode exhibits high stretchability with a low ΔR/R0 of 10.3 at a strain of 40%. Furthermore, by exploiting the self-patterning ability—attributable to the difference in the ability to form liquid bridges according to the distance between microstructures—we successfully demonstrated a stretchable AgNW bundle mesh with complex patterns without using additional patterning processes. In particular, stretchable electrodes were fabricated by spray coating and bar coating, which are widely used in industry for low-cost mass production. We believe that this study significantly contributes to the commercialization of stretchable electronics while achieving high performance and complex patterns, such as stretchable displays and electronic skin.

A stretchable and adhesive ionic conductor based on polyacrylic acid and deep eutectic solvents

Hydrogels are a widely used ionic conductor in on-skin electronic and iontronic devices. However, hydrogels dehydrate in the open air and freeze at low temperatures, limiting their real applications when they are attached on skin or exposed to low temperatures. Polymer-ionic liquid gels can overcome these two obstacles, but synthetic ionic liquids are expensive and toxic. In this work, we present an ionic conductor based on polyacrylic acid (PAAc) and deep eutectic solvents (DESs) that well addresses the aforementioned challenges. We polymerize acrylic acid in DESs to get the PAAc–DES gel, which exhibits excellent stretchability (> 1000%), high electrical conductivity (1.26 mS cm−1), high adhesion to the skin (~ 100 N m−1), as well as good anti-drying and anti-freezing properties. We also demonstrate that the PAAc-DES gel can be used as an on-skin electrode to record the surface electromyographic signal with high signal quality, or as a transparent stretchable electrode in iontronic devices that can work at –20 °C. We believe that the PAAc–DES gels are an ideal candidate as epidermal electrodes or transparent stretchable electrodes.

A method for three-dimensional single-cell chronic electrophysiology from developing brain organoids

Human induced pluripotent stem cell-derived brain organoids have shown great potential for studies of human brain development and neurological disorders. However, quantifying the evolution and development of electrical functions in brain organoids is currently limited by measurement techniques that cannot provide long-term stable three-dimensional (3D) bioelectrical interfaces with brain organoids during development. Here, we report a cyborg brain organoid platform, in which 2D progenitor or stem cell sheets can fold "tissue-like" stretchable mesh nanoelectronics through organogenesis, distributing stretchable electrode arrays across 3D organoids. The tissue-wide integrated stretchable electrode arrays show no interruption to neuronal differentiation, adapt to the volume and morphological changes during organogenesis, and provide long-term stable electrical contacts with neurons within brain organoids during development. The seamless and non-invasive coupling of electrodes to neurons enables a 6-month continuous recording of the same brain organoids and captures the emergence of single-cell action potentials from early-stage brain organoid development.

Ultra-robust stretchable electrode for e-skin: in situ assembly using a nanofiber scaffold and liquid metal to mimic water-to-net interaction

The development of stretchable electronics will thrive on the novel interface structure to solve the stretchability-conductivity dilemma, which is still a great challenge. Herein, we report a nano-liquid metal (LM)-based high-robust stretchable electrode (NHSE) with a self-adaptable interface that mimics water-to-net interaction. Based on in situ assembly of electrospun elastic nanofibers scaffold and electrosprayed LM nanoparticles, the NHSE exhibits an extremely low sheet resistance of 52 mΩ/□. It is not only insensitive to a large degree of mechanical stretching (i.e., 350% electrical resistance change upon 570% elongation), but also immune to cyclic deformation (i.e., 5% electrical resistance increase after 100,000 stretching cycles with 100% elongation). These key properties are far more superior to the state-of-the-art reports. Its robustness and stability are verified under diverse circumstances, including long-term exposure in air (420 days), cyclic washing (30,000 times), and resilience against mechanical damages. The combination of conductivity, stretchability and durability makes the NHSE a promising conductor/electrode solution to flexible/stretchable electronics for applications such as wearable on-body physiological signal detection.

Armadillo-inspired Micro-foldable Metal Electrode with Negligible Resistance Change under Large Stretchability

Stretchable electrodes are one of the essential building blocks for stretchable electronics. However, most of reported stretchable electrode show considerable conductivity decrease under large strechability. Until now, it is still...

Aligned wave-like elastomer fibers with robust conductive layers via electroless deposition for stretchable electrode applications

The robust conductive layer formed on wave-like fibers by electroless deposition endows electrodes with high stretchability and low resistance.