Soft Bioelectronic Stickers: Selection and Evaluation of Skin‐Interfacing Electrodes

Abstract

Surface biopotentials collected from the human epidermis contain important information about human physiology, such as muscular, heart, and brain activities. However, commercially available wearable biomonitoring devices are generally composed of rigid hardware incompatible with the mechanical compliance of soft human tissues. Thin‐film stretchable e‐skin circuits that can interface the human skin represent an excellent alternative for long‐term wearable biomonitoring. Here, a series of soft and stretchable electrodes are evaluated for their applicability in biopotential sensing. This includes conductive composites made of polydimethylsiloxane (PDMS) as a base substrate and conductive particles, i.e., carbon (cPDMS), silver (AgPDMS), anisotropic z‐axis conductors made with silver‐coated nickel particles (zPDMS), as well as a combination of a conductive tough hydrogel with PDMS, and finally ultrathin tattoo‐like adhesive poly(vinyl alcohol)‐coated films with stretchable biphasic Ag‐EGaIn electrodes. These electrodes are compared between themselves and against the gold‐standard Ag/AgCl and stainless steel electrodes, in order to assess relative performance in signal‐to‐noise ratio (SNR) during electrocardiography, and electrode‐skin impedance for a range of frequencies. Results show a direct relation between conformity of the electrode–skin interface and the SNR value. An all‐integrated biomonitoring patch with embedded processing and communication electronics, hydrogel electrodes, and a multilayer liquid metal circuit is presented for electromyography.

Fully organic compliant dry electrodes self-adhesive to skin for long-term motion-robust epidermal biopotential monitoring

“…They are different from other dry electrodes in literature. Nanocomposites with conductive nanofillers in the elastomer matrix can have high stretchability and high conductivity 41 , and they have been studied as dry electrodes for epidermal biopotential measurement 13,16,59,60 . However, the nanocomposite dry electrodes usually give rise to much higher electrode-skin impedance than the PWS electrode because the conductive nanofillers are the minority in the nanocomposites and their effective contact area to skin is thus actually very small 43 .…”

“…Although other stretchable PEDOT: PSS composites were used as the dry electrodes, they are not adhesive and thus give rise to high noise during body movement. Some soft adhesive electrodes were reported in literature 59,67 . For instance, ultrathin electrodes can be adhesive to skin.…”

High Resolution Soft and Stretchable Circuits with PVA/Liquid‐Metal Mediated Printing

“…Stretchable electronics have a wide range of applications in flexible displays, [ 1 ] sensors, [ 2–6 ] health monitoring devices, [ 7–10 ] structural electronics, [ 11,12 ] wearable motion sensing, [ 13 ] and e‐textiles; [ 14–17 ] reviews of these applications exist in the literature. [ 18 ] Such circuits should ideally remain functional under bending, folding, twisting, and tensile strain.…”

Soft mixed ionic–electronic conductive electrodes for noninvasive stimulation

“…The demand for noninvasive, stimulating, and recording wearable electronic devices is expanding, with broad applications including electrotherapeutics, biosensing, remote healthcare/wellness monitoring, wound healing, human machine interface systems, and sports medicine . These systems all require electrodes to facilitate charge transfer across the interface between the device and the skin.…”

“…On‐going research efforts seek to address this issue through improvements in the electrodes and/or the coupling with the skin. Flexible electrodes, which can adhere, conform, and stretch along with the skin, offer a means to bypass challenges of rigid electrodes . By matching the mechanical and thermal properties of the skin improved interfacial charge transfer may be achieved.…”