(Nanowerk Spotlight) Wearable bioelectronic devices have emerged as a promising approach for monitoring health and delivering targeted therapies. These devices can capture physiological signals from organs like the heart, muscles, and brain to detect disease or control prosthetics. They can also deliver electrical stimulation to the skin, muscles, and nerves to regenerate damaged tissues or regulate muscle movements.
However, creating wearable electrodes that conform to the soft, curved, and dynamic surfaces of the human body has proven challenging. Conventional electrodes often struggle to maintain reliable contact with the skin, especially during movement. This can lead to inconsistent performance, discomfort for the wearer, and even skin irritation. Moreover, the “one-size-fits-all” approach of pre-made electrodes fails to account for the wide variability in individual physiology and anatomy.
To overcome these hurdles, scientists have explored various materials and fabrication strategies for epidermal electronics. Conductive textiles using metal-coated fibers, nanoparticles, and polymers have enabled flexible electrodes that integrate with clothing. Ultra-thin electronic tattoos incorporating metals, carbon nanotubes, and graphene have achieved intimate contact with the skin. Printable conductive hydrogels have also shown promise, leveraging the skin-like mechanical properties and high water content of polymer networks.
Despite progress, significant limitations remain. Textile electrodes exhibit poor skin adhesion and struggle with rough, curved, and dynamic surfaces. Electronic tattoos have largely focused on biopotential recording rather than electrical stimulation, partially due to challenges with Joule heating in ultra-thin conductive patterns. Conductive hydrogels often require complex manufacturing involving cytotoxic reagents, UV light, high temperatures, or high voltages, hindering rapid and scalable production.
Building upon these prior efforts, a research team led by Professor Hani Naguib at the University of Toronto has developed a novel 3D printable conductive hydrogel that can be directly extruded onto the skin to form customized electrodes in situ.
As reported in Advanced Functional Materials (“Conductive Bio-based Hydrogel for Wearable Electrodes via Direct Ink Writing on Skin”), their “Printable, Adhesive, Integrative on-skin, and Naturally sourced hydrogel with tunable properTy” (PAINT) leverages the biocompatibility and cross-linking mechanisms of bio-based polymers to enable facile fabrication of epidermal electronics.
Schematic illustration of the PAINT hydrogel network utilizing ionic bonds and hydrogen bonds that can reform after deformation and rupture. To reinforce the physically cross-linked PAINT hydrogel, covalent bonds are established between PDA before mechanically mixing the constituents to form the PAINT hydrogel precursor ink. (Image: reproduced with permission from Wiley-VCH Verlag)
The PAINT hydrogel combines carboxymethyl cellulose for mechanical stability, polydopamine for adhesion and conductivity, the conductive polymer PEDOT:PSS for charge transport, and phytic acid as a biocompatible cross-linker. Simple mechanical mixing yields a shear-thinning ink that rapidly solidifies into a flexible electrode after extrusion through a nozzle. Using a custom handheld 3D printer, the researchers demonstrated high-resolution patterning of conductive hydrogels directly onto skin and other surfaces.
Mechanical testing revealed that the printed PAINT hydrogels could withstand the stretching, compression, and bending experienced by the skin during natural movements. The incorporation of polydopamine enhanced adhesion to the skin and various other substrates. Crucially, the in situ gelation process allowed the hydrogel to establish maximal contact with the rough topography of the skin, thereby reducing the interfacial impedance at the bioelectronic interface.
To evaluate the performance of the PAINT electrodes, the scientists conducted electrocardiography and electromyography recordings on human subjects. Compared to standard silver/silver chloride gel electrodes, the 3D printed hydrogel electrodes achieved up to 88% higher signal-to-noise ratios when measuring electrical activity from arm muscles. This improved sensitivity was attributed to the intimate skin integration and combined electronic/ionic conductivity of the PAINT hydrogel.
The researchers also explored the use of their customizable electrodes for delivering functional electrical stimulation to paralyzed facial muscles. Whereas conventional stimulation electrodes often activate unintended regions due to poor spatial resolution, the PAINT electrodes could be precisely patterned to target specific muscles around the eye. This resulted in more natural eye closure with 36% less current required, highlighting the potential for enhanced comfort and efficacy in electrotherapeutic applications.
Looking ahead, the PAINT hydrogel could enable a new paradigm in personalized epidermal electronics. The ability to 3D print flexible and conductive materials directly onto the skin opens up exciting possibilities for biopotential monitoring, electrotherapy, and human-machine interfaces. With further development, this approach could lead to wearable devices that are optimally tailored to individual physiology, empowering patients and clinicians with responsive and adaptive therapies.
While challenges remain in terms of long-term stability, reproducibility, and integration with wireless data transmission, the work of Naguib and colleagues represents a meaningful step towards next-generation bioelectronics that can seamlessly merge with the human body.
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