Wearable neuroelectronic interfaces enable electrical recording and stimulation of the nervous system directly through the skin. As medical devices, they could support long-term monitoring and non-invasive neuromodulation outside clinical settings, enabling earlier diagnosis, more personalized therapy, and improved management of chronic conditions. Nowadays, most skin electrodes are simple pads that rely on conductive gels. In wearable use, they are often uncomfortable and sensitive to motion, sweat, and drying, which causes impedance drift and unreliable signal quality. Improved skin interfaces (multielectrode arrays on soft substrates with stable electrical contact) could better conform to the body and make closed-loop operation feasible beyond controlled hospital settings. The objective is to advance wearable neuroelectronics by improving skin-electrode technologies for surface electrophysiology and by defining practical principles for wearable closed-loop operation in health applications.

A first strategy is a wet interface based on hydrogel-coated electrodes supported by nanoporous platinum-based substrates. Hydrogels provide a soft, hydrated ionic contact that lowers impedance, but can delaminate under hydration and strain. Nanoporous surfaces are manufactured by dealloying a co-sputtered PtIr-Al thin film, creating a porous morphology that strengthens hydrogel anchoring. Mechanical testing supports strong metal-hydrogel adhesion, while electrochemical characterization shows improved charge transfer and low impedance. When integrated into soft arrays, these electrodes enable transcutaneous spinal cord stimulation (tSCS) in mice with reproducible evoked electromyography (EMG) responses and location-dependent recruitment, demonstrating a spatially selective and conformable skin interface.

A second strategy is a dry interface based on glassy carbon (GC) microneedle-array electrodes (MAE), that can penetrate the stratum corneum and thus achieve sufficiently low-impedance skin contact without gels. GC is obtained by carbonizing polymer precursors and combines high hardness with bulk conductivity and electrochemical stability. An optimized wafer-level process produces high-aspect-ratio MAEs , which are integrated onto soft substrates via a dedicated release and transfer method. Characterization confirms skin penetration and reduced electrode-skin impedance compared to planar samples, and in vivo experiments show that the same interface supports both EMGs and tSCS in mice, enabling a high-resolution bidirectional skin interface.

At the system level, three wearable closed-loop projects in humans are used to extract practical constraints for translation: a step-synchronized vibrotactile feedback loop during walking, an EEG-guided stimulation loop adapted to the user's engagement during learning, and a multimodal artificial-pancreas controller explored computationally using glucose and heart rate with offline RL. The main lesson is that each closed-loop system is its own world: both device performance and user adoption depend on identifying the key requirements for that specific application, especially in terms of safety, comfort, and personalization.

Overall, this thesis provides interface- and system-level building blocks that move wearable neuroelectronics toward reliable closed-loop health technology, linking improved skin electrical contact with the constraints that matter when devices are worn and used in daily life.