Skin-printable, Self-adhesive and Bio-based Hydrogel for the Bioelectronic Interface

Inspired by the customizability of injectable hydrogels in the medical field as cell cultures and engineered tissues, a conductive and 3D-printable hydrogel based on bio-polymers is developed to interface between the skin and medical devices during rehabilitation treatments of surface functional electrical stimulation (sFES). The sFES treatment involves delivering electric currents through the skin to induce muscle contraction, with clinical evidence supporting the ability of sFES treatments to restore voluntary motor functions in previously paralyzed patients. However, physiological differences in muscle distribution and body curvatures among individuals require sFES treatments to be personalized. Therefore, to increase the accessibility of sFES rehabilitations by enabling remote sessions and shape-configurable electrodes universal to all patients, the authors developed a novel material capable of being adapted to any shape and size, with self-adhesive and electrically conductive properties to ensure conformity to body morphology and to guarantee sFES signal stability. As the electrodes are single-use, the hydrogel is made from naturally occurring, biocompatible, and eco-friendly polymers that are easy to fabricate and process. Carboxymethyl cellulose is adopted as the matrix for its water solubility and mechanical properties, along with poly(3,4-ethylene-dioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the conductive additive and mussel-inspired polydopamine as the adhesive additive. A mild and biocompatible gelation method exploiting hydrogen bonding is implemented by adding phytic acid, with <i>in situ</i> formation of the extruded hydrogel taking place minutes after being 3D-printed onto the skin. The gelation process is examined with FTIR spectroscopy, which confirms the molecular interactions between bio-polymer components, scanning electron microscopy, which ensures the macroscopic morphology of the formed hydrogel, and rheological measurements over time, which characterizes the printability of the precursor ink. The pre-gel solution exhibits tailorable viscosity, allowing for facile processability and shape-configurability. Adhesive and electrical properties of the hydrogel are studied in contact with porcine skin over time. Although the self-adhesion of the hydrogel is two folds lower than commercial tissue adhesives, the interfacial electrical impedance is improved by an order of magnitude compared to commercial hydrogel electrodes. The sFES performance of the printable and conductive hydrogel is further assessed with a developed handheld 3D printer, a lightweight instrument to extrude the hydrogel ink in uniformly deposited layers with shape fidelity of mm-thick resolutions. Compared to conventional sFES electrodes of both hydrogel and metal-electrolyte gel systems, the printable hydrogel electrodes can induce muscle movement during sFES with better precision and accuracy, improving the efficiency by 10% when stimulating eye closure to the orbicularis oculi muscle. The printable and conductive hydrogel created for wearable purposes may be adapted for other applications involving bi-directional communications of electric signals across the bioelectronic interface, such as surface electromyography and surface electrocardiography.