Researchers from Drexel University and Penn State University have developed a printable hydrogel electrode, reinforced with laser-induced graphene (LIG) and reduced graphene oxide (rGO), that combines an ultralow Young's modulus of 1.08 kPa with roughly 8,000% stretchability (able to stretch to about 80 times its original size) and strong wet adhesion - properties aimed at solving one of wearable biosensing's persistent problems: keeping an electrode in stable contact with skin through sweat, body hair and everyday movement.
The deposited gel on the skin. Image from: Scienc Advances
Continuous, simultaneous monitoring of signals from the heart, brain, muscles and eyes is valuable for both physical and mental health tracking, since these physiological signals are interrelated - anxiety, for instance, shows up as coordinated changes across the heart, sweat glands, muscles and lungs. But conventional metal- or gel-based bioelectrodes tend to lose performance on hairy skin, under sweating conditions, or simply from the mechanical mismatch between a stiff electrode and soft, stretchy skin during long-term wear.
Hydrogels have emerged as a promising fix, and bioinspired adhesion chemistries (catechol-based glues, ionic coordination, hydrogen-bonding networks) have improved how well they stick to skin. But combining ultrasoftness (soft enough to conform through hair), strong yet removable adhesion under wet conditions, mechanical robustness, and reusability in a single material has remained difficult.
The team's approach was to build the conductive component of the hydrogel from two complementary graphene derivatives. LIG, produced by laser irradiation of carbon-rich material, is highly porous and mechanically soft; rGO, formed by reducing graphene oxide, has a partially restored conjugated structure that gives it higher conductivity along with good dispersibility and mechanical stiffness. Combining porous, flexible LIG with stiffer, more conductive rGO - both generated in situ by the same laser-reduction step - gave the resulting hydrogel network softness, stretchability, toughness and conductivity simultaneously. Polydopamine (PDA) and tannic acid (TA) were added to provide strong wet-adhesive properties.
A key design feature is control over gelation time via the pH of the precursor solution: by adjusting pH, the researchers could keep the mixture liquid long enough to be dispensed from a syringe or printed into a custom shape, then trigger it to set solid on skin. This tunability - from approximately 1,300 seconds down to as little as 72 seconds, depending on pH - makes the gel, which the researchers term the RTLR gel (radically tuned LIG- and rGO-based hydrogel), compatible with extrusion-based fabrication methods such as 3D printing and injection, letting electrodes be custom-shaped and patterned for specific body locations or individual users.
The resulting hydrogel reached a toughness of 1240.7 kJ/m3 and an adhesive strength of 51.97 kPa, and adhered strongly to a range of test surfaces including copper, steel, glass and rubber. Its porous structure allows sweat to pass through the gel rather than pooling and dislodging the sensor, while its softness and adhesion together give it low skin contact impedance and a high signal-to-noise ratio across dry, wet and hairy skin conditions, with motion-artifact-free monitoring of electrophysiological signals. In durability testing, the gel could reportedly be peeled off and reapplied dozens of times without degrading its performance.
The researchers demonstrated the electrodes in several signal-acquisition tests: electrocardiogram (ECG) sensors placed on the wrist and chest maintained a stable signal through bending and stretching, and electro-oculography (EOG) sensors tracked blinking and eye-movement exercises with similarly consistent results. As a proof-of-concept application, the team built a sensor array to capture multiple physiological markers of anxiety - placing hydrogel electrodes on the eyelid, palm and wrist of a test subject to simultaneously monitor blinking, sweat (electrodermal activity) and heart rate while the subject listened alternately to relaxing music and irritating sounds. In a related test, a self-described arachnophobe volunteer was shown videos of spiders, and the sensors continuously recorded the rise in sweating and eye-blinking that accompanied the volunteer's mounting anxiety response. The platform is designed to concurrently capture ECG, EOG, electroencephalogram (EEG), electromyogram (EMG), electrodermal activity (EDA) and strain signals, with the researchers pointing to anxiety monitoring and nerve rehabilitation as target applications.
Abu Musa Abdullah, PhD, a post-doctoral researcher of mechanical engineering and mechanics in Drexel's Nick Howley College of Engineering and Computing and one of the research leaders, explained the motivation behind the design: "The ultimate goal of a biosensor is to be so reliable and comfortable that the wearer can go about their life without constantly noticing it's there. Taking this into consideration, we devised a formula for a hydrogel that can be printed in any shape or size and is adhesive enough to cling to skin even when its sweaty or hairy and durable enough to be reused."
On why the material's flexibility in shape and use case matters, Abdullah added: "This manufacturing flexibility enables us to fabricate customized electrodes in different shapes and patterns that conform closely to specific body locations or individual users. Rather than enabling an entirely new type of physiological measurement, our hydrogel makes existing measurements more reliable under challenging real-world conditions."
He also pointed to signal quality as the core problem the hydrogel addresses: "If a sensor is uncomfortable, rigid or loses contact with the skin during movement, it can introduce noise into the recorded signals and reduce the quality of the data. Our hydrogel is designed to conform closely to the skin, even on hairy or sweaty areas and during motion, helping maintain stable signal quality while improving comfort even after implementation for multiple cycles."
The team describes the results as a proof-of-concept and plans to continue optimizing the hydrogel for specific use cases, including exercise-monitoring sensors for physical training and rehabilitation. As Abdullah put it: "While further development is needed before commercialization, we believe this customizable, reusable approach represents a promising new direction for wearable bioelectronics compared with today's conventional electrodes."
