Skin-mounted electrodes depend on something that sounds simple but is difficult to maintain: good contact with the skin.
Conductive gels can flow into the small ridges and irregularities of the skin surface, which helps keep electrode-skin impedance low. But those same fluid-like properties can create problems with spreading, drying and mechanical stability. Dry electrodes are easier to handle and tend to be more stable, but a solid material cannot always follow the microscopic topography of skin closely enough to maintain intimate contact.
Researchers at Pennsylvania State University recently approached this problem with a conductive bioadhesive for wearable bioelectronics that can switch between two mechanical states. The material can be printed and stored as a dry film, then softened at the point of use so it conforms more closely to skin.
Mechanical testing was part of figuring out whether that concept actually worked. The researchers used a CellScale UniVert to measure the tensile behaviour of the printed adhesive and its 90° peel adhesion directly on skin. Those measurements were then considered alongside conductivity, electrode-skin impedance, EEG recordings and pulse measurements.
Why a Conductive Bioadhesive for Wearable Bioelectronics Needs More Than Conductivity
Electrical conductivity is only one part of the problem for wearable bioelectronics.
An epidermal electrode also needs to stay in contact with a surface that is soft, textured and constantly moving. A material can have excellent conductivity on the bench and still perform poorly as an electrode interface if it lifts away from the skin, leaves microscopic gaps or becomes too stiff to follow deformation.
The researchers describe this as a trade-off between conformal wetting and mechanical stability. Wet conductive gels can make intimate contact with skin, but they can dehydrate or move during use. Dry soft electrodes remove the fluid phase and improve handling, though their solid state can limit how closely they conform to skin microtopography.
The idea behind the new conductive bioadhesive for wearable bioelectronics was not simply to make a stickier electrode. Instead, the researchers wanted the same material to behave differently during storage and during use.
That broader problem has appeared in other conductive material systems as well. We previously looked at wet conducting polymers with strong adhesion, where attachment to the substrate had to be considered alongside the electrical and mechanical behaviour of the polymer.
Building a Conductive Bioadhesive for Wearable Bioelectronics That Can Change State
The reactivatable conductive adhesive, or RCA, was built around a polyurethane-poly(acrylic acid) graft interpenetrating network, referred to as PU-PAA g-IPN.
PEDOT:PSS was added as the conductive phase. It forms conductive pathways through the material without becoming part of the covalently bonded PU-PAA network. PEDOT:PSS is common in research involving electroactive polymer materials, but here its concentration also became an important mechanical design variable.
The material was processed as a viscoelastic ink using direct ink extrusion printing. After drying, it became a mechanically stable, non-tacky film that could be handled and packaged.
A small amount of solvent changed that behaviour.
When the dry RCA was exposed to isopropyl alcohol, hydrogen-bond interactions within the polymer network reorganized and the material softened into a more gel-like state. It could then move into pores and irregularities at the skin surface rather than simply resting on top of them.
For a conductive bioadhesive for wearable bioelectronics, that change of state is useful because the properties needed during storage are not necessarily the properties needed against the skin.
Responsive adhesion has also appeared in quite different material systems. In another UniVert study, researchers investigated switchable underwater adhesion using pull-off testing, where changing the state of an adhesive interface altered its mechanical attachment to a surface.
Tuning a Conductive Bioadhesive for Wearable Bioelectronics Changes More Than Conductivity
The researchers prepared four RCA formulations with increasing amounts of PEDOT:PSS, labelled P2:A8, P3:A7, P4:A6 and P5:A5.
Conductivity increased steadily as more PEDOT:PSS was added, reaching approximately 3.5 S/cm in the P5:A5 formulation. That part is fairly intuitive. More conductive polymer created a more conductive composite.
The mechanical response was less straightforward.
As PEDOT:PSS content increased, the printed material became stiffer and lost some of its stretchability. Adhesion also changed. In the skin peel tests, formulations with higher PEDOT:PSS content generally showed lower adhesion.
So improving one property of the conductive bioadhesive for wearable bioelectronics started to work against others.
The researchers eventually selected P4:A6 as a compromise. It retained relatively high electrical conductivity while still providing the stretchability and adhesion they wanted for the electrode interface.
That balance is one of the more interesting parts of the study. It would have been difficult to pick the formulation from conductivity measurements alone.
How the UniVert Was Used to Test a Conductive Bioadhesive for Wearable Bioelectronics
Mechanical characterization was carried out using a CellScale UniVert. Two different tests were used because stretchability and adhesion answer different questions about how a skin-interfacing material will behave.
Tensile testing of the printed conductive adhesive
For the tensile measurements, the researchers extrusion-printed dogbone specimens with a 10 mm gauge length, 4 mm width and thickness of approximately 0.15 mm.
The specimens were stretched at a constant rate of 1 mm/s until failure. This tensile testing produced the stress-strain curves shown in the adapted Figure 3 below.
One thing that stands out is how much the response changed with formulation. The lower-PEDOT:PSS samples stretched several hundred percent before failure, while the highest concentration produced a much stiffer response and failed at substantially lower strain.
For a conductive bioadhesive for wearable bioelectronics, that matters because skin motion does not stop once an electrode is attached. The conductive layer has to tolerate deformation while maintaining both contact and electrical pathways.
How do you measure skin adhesion in a wearable electrode?
The researchers also performed 90° peel testing on both glass and human skin.
For the skin measurements, thin RCA strips were placed on a volunteer’s finger and activated in place using isopropyl alcohol. The bonded length was kept at 5 mm. The free end of the specimen was attached to the load cell and peeled at 1 mm/s while the test maintained a 90° peel angle.
This is a useful example of adhesive and sealant testing where the substrate is not a rigid engineering surface. Skin is compliant, textured and variable, which is exactly why direct adhesion measurements can be useful when designing epidermal devices.
For this conductive bioadhesive for wearable bioelectronics, the peel test provided information that the tensile test could not. A formulation might stretch well as a free specimen but still form a relatively weak interface with skin.
How Skin Adhesion Can Affect Electrode-Skin Impedance
The next question was whether the mechanical behaviour translated into a useful electrode interface.
The researchers compared the selected P4:A6 formulation with a commercial conductive EEG gel using both conventional Ag/AgCl electrodes and soft PEDOT:PSS-PU-CNT electrodes.
Measurements were made on the wrist and on the hairy scalp.
With the rigid Ag/AgCl electrodes, the RCA produced lower impedance than the commercial gel over much of the measured frequency range. The difference was most apparent at lower frequencies, including the range below approximately 100 Hz that is particularly relevant for many electrophysiological recordings.
A similar pattern appeared with the soft electrodes.
The adhesion measurements also showed substantially greater interfacial energy for the RCA than for the commercial conductive gel in the configurations the researchers tested.
For the conductive bioadhesive for wearable bioelectronics, this links the mechanical story back to electrical performance. The softened adhesive can move into the small features of the skin surface, increasing contact rather than leaving the interface dependent on a relatively smooth solid surface.
The data do not mean that adhesion alone determines electrode-skin impedance. Material conductivity, electrode architecture, surface conditions and other factors are involved. But in these experiments, stronger conformal contact and lower impedance appeared together.
From a Conductive Bioadhesive for Wearable Bioelectronics to EEG Recording
The material was eventually moved out of standalone mechanical and impedance testing and into electrophysiological recordings.
The researchers placed RCA-integrated electrodes over the occipital region of subjects with dense hair. During eyes-closed EEG recordings, the power spectral density showed a pronounced peak near 10 Hz, consistent with alpha activity.
When the subjects opened their eyes, that peak was strongly reduced.
The same general behaviour was observed with both rigid Ag/AgCl electrodes and the softer PPC electrodes. The researchers also used the RCA interface to record pulse waveforms over the radial artery.
This gives the mechanical testing a useful endpoint. The tensile and peel measurements were not being collected simply to describe another soft polymer. They were part of working through the properties of a conductive bioadhesive for wearable bioelectronics that eventually had to function while attached to a person.
The human measurements were proof-of-concept tests, so there is still a gap between these results and long-term clinical use. Questions such as repeated activation, prolonged wear and performance across a broader population would need their own testing.
Similar coupling between mechanical loading and device function appears in other soft biointerfaces. We previously followed a tissue-adhesive hydrogel optical fibre under repeated strain, where optical performance had to be considered alongside stretching and attachment to tissue.
Using the UniVert to Test a Conductive Bioadhesive for Wearable Bioelectronics
The UniVert is a uniaxial mechanical testing system used for materials that may require very different test configurations depending on the question being asked.
In this study, the same system was used first to stretch thin printed dogbone specimens to failure and then to measure 90° peel adhesion from skin. The specimen geometry, fixtures and loading protocol changed, but both experiments produced force and displacement data that could be used to understand the mechanical behaviour of the RCA.
That kind of flexibility is useful when working with soft polymers, thin films, hydrogels and adhesive interfaces. A researcher developing a conductive bioadhesive for wearable bioelectronics may need to know how far a material can stretch, how its stiffness changes with formulation, and how much force is required to separate it from a surface. Those are related questions, but they are not the same mechanical test.
The CellScale UniVert is used for controlled uniaxial testing across configurations including tension, compression and peel testing. In practice, the test setup is selected around the material and the mechanical property being measured rather than trying to force every specimen into one standard geometry.
Here, combining tensile and peel measurements gave the researchers two different views of the same conductive bioadhesive for wearable bioelectronics. One described the printed material itself. The other described what happened when that material met skin.
Citation
, , , et al. “ State-Programmable Conductive and Reactivatable Bio-Adhesive for High-Fidelity Epidermal Biointerfaces.” Small (2026): e75442. https://doi.org/10.1002/smll.75442
