Auditory brainstem implants (ABIs) provide sound awareness to deaf individuals who are not candidates for the cochlear implant. The ABI electrode array rests on the surface of the cochlear nucleus (CN) in the brainstem and delivers multichannel electrical stimulation. The complex anatomy and physiology of the CN, together with poor spatial selectivity of electrical stimulation and inherent stiffness of contemporary multichannel arrays, leads to only modest auditory outcomes among ABI users. Here, we hypothesized that a soft ABI could enhance biomechanical compatibility with the curved CN surface. We developed implantable ABIs that are compatible with surgical handling, conform to the curvature of the CN after placement, and deliver efficient electrical stimulation. The soft ABI array design relies on precise microstructuring of plastic-metal-plastic multilayers to enable mechanical compliance, patterning, and electrical function. We fabricated soft ABIs to the scale of mouse and human CN and validated them in vitro. Experiments in mice demonstrated that these implants reliably evoked auditory neural activity over 1 month in vivo. Evaluation in human cadaveric models confirmed compatibility after insertion using an endoscopic-assisted craniotomy surgery, ease of array positioning, and robustness and reliability of the soft electrodes. This neurotechnology offers an opportunity to treat deafness in patients who are not candidates for the cochlear implant, and the design and manufacturing principles are broadly applicable to implantable soft bioelectronics throughout the central and peripheral nervous system.
At low pH, underwater adherence between poly(N,N-dimethylacrylamide) hydrogels and poly(acrylic acid) brushes is due to the formation of multiple hydrogen bonds. The effect of the key parameters controlling the formation of these interactions (contact time and composition of the hydrogel) was investigated with a contact mechanics test using a flat probe. We specifically quantified the loss of adherence during the progressive swelling to equilibrium of the gels and, at fixed contact time, found a significant decrease in adherence between the preparation state and the swelling equilibrium even in the case of relatively low dilution factors. This adherence loss was attributed to the slowdown of the kinetics of formation of multiple H-bond interactions as the gel approached its equilibrated state. In both limiting conditions the energy of adherence scaled with the polymer concentration, independent of the crosslinks density of the gel, suggesting that the Lake and Thomas amplification factor is not relevant for these weak bonds.
In the exciting race to design and engineer biointegrated and body‐like electronic systems, many efforts concentrate on the integration of hydrogels in electronic assemblies. The versatility of hydrogels chemistry combined with their tissue‐mimicking properties inspires numerous demonstrations of hydrogel‐based touch panels, robots, and sensors over the years. However, their long‐term integration in a thin and functional electronic assembly remains a challenge: their sensitivity to both air‐drying and water swelling leads to important volume change of the network that is incompatible with the cohesion of a multilayer system, and has irreversible impact on the electronic properties of the assembly. To tackle this issue, proposed is a method to fabricate a hydrogel–elastomer micrometric bilayer with a stable interface, using of a low‐swelling type of hydrogel, i.e., poly(2‐hydroxyethyl methacrylate) and silicone rubber. The bilayer can sustain multiple hydration/dehydration cycles without delamination and can be kept for several months in its dry configuration. Combined with soft metallization technology, the bilayer can be readily integrated into a soft electronic circuit thereby opening a technological route for microfabricated, on‐demand morphing systems.
A new strategy for the fabrication of micropatterns of surface-attached hydrogels with well-controlled chemistry is reported. The "grafting onto" approach is preferred to the "grafting from" approach. It consists of cross-linking and grafting preformed and functionalized polymer chains through thiol-ene click chemistry. The advantage is a very good control without adding initiators. A powerful consequence of thiol-ene click reaction by UV irradiation is the facile fabrication of micropatterned hydrogel thin films by photolithography. It is achieved either with photomasks using common UV lamp or without photomasks by direct drawing due to laser technology. Our versatile approach allows the fabrication of various chemical polymer networks on various solid substrates. It is demonstrated here with silicon wafers, glass and gold surfaces as substrates, and two responsive hydrogels, poly(N-isopropylacrylamide) for its responsiveness to temperature and poly(acrylic acid) for its pH-sensitivity. We also demonstrate the fabrication of stable hydrogel multilayers (or stacked layers) in which each elementary layer height can widely range from a few nanometers to several micrometers, providing an additional degree of freedom to the internal architecture of hydrogel patterns. This facile route for the synthesis of micrometer-resolute hydrogel patterns with tailored architecture and multiresponsive properties should have a strong impact.
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