Abstract:We introduce a new class of neural implants with the topology and compliance of dura mater, the protective membrane of the brain and spinal cord. These neural interfaces, which we called e-dura, achieve chronic bio-integration within the subdural space where they conform to the statics and dynamics of neural tissue. e-dura embeds interconnects, electrodes and chemotrodes that sustain millions of mechanical stretch cycles, electrical stimulation pulses, and chemical injections. These integrated modalities enable multiple neuroprosthetic applications. e-dura extracted cortical states in freely behaving animals for brain machine interface, and delivered electrochemical spinal neuromodulation that restored locomotion after paralyzing spinal cord injury. e-dura offers a novel platform for chronic multimodal neural interfaces in basic research and neuroprosthetics.3 Neuroprosthetic medicine is improving the lives of countless individuals. Cochlear implants restore hearing in deaf children, deep brain stimulation alleviates Parkinsonian symptoms, and spinal cord neuromodulation attenuates chronic neuropathic pain (1). These interventions rely on implants developed in the 1980s (2, 3). Since then, advances in electroceutical, pharmaceutical, and more recently optogenetic treatments triggered development of myriad neural interfaces that combine multiple modalities (4-9). However, the conversion of these sophisticated technologies into chronic implants mediating long-lasting functional benefits has yet to be achieved. A recurring challenge restricting chronic bio-integration is the substantial biomechanical mismatch between implants and neural tissues (10-13). Here, we introduce a new class of soft multimodal neural interfaces that achieve chronic bio-integration, and we demonstrate their long-term efficacy in clinically relevant applications. e-dura fabrication. We designed and engineered soft interfaces that mimic the topology and mechanical behavior of the dura mater (Fig. 1A-B). The implant, which we called electronic dura mater or e-dura, integrates a transparent silicone substrate (120µm in thickness), stretchable gold interconnects (35nm in thickness), soft electrodes coated with a novel platinum-silicone composite (300µm in diameter), and a compliant fluidic microchannel (100µm x 50µm in crosssection) (Fig. 1C-D, fig. S1-S2-S3). The interconnects and electrodes transmit electrical excitation and transfer electrophysiological signals.The microfluidic channel, termed chemotrode (14), delivers drugs locally (Fig. 1C, fig. S3). Microcracks in the gold interconnects (15) and the newly developed soft platinum-silicone composite electrodes confer exceptional stretchability to the entire implant (Fig. 1B, Movie S1). The patterning techniques of metallization and microfluidics support rapid manufacturing of customized neuroprostheses.4 e-dura implantation. Most implants used experimentally or clinically to assess and treat neurological disorders are placed above the dura mater (3,(16)(17)(18). The compliance of e-...

Electrical neuromodulation of lumbar segments improves motor control after spinal cord injury in animal models and humans. However, the physiological principles underlying the effect of this intervention remain poorly understood, which has limited this therapeutic approach to continuous stimulation applied to restricted spinal cord locations. Here, we developed novel stimulation protocols that reproduce the natural dynamics of motoneuron activation during locomotion. For this, we computed the spatiotemporal activation pattern of muscle synergies during locomotion in healthy rats. Computer simulations identified optimal electrode locations to target each synergy through the recruitment of proprioceptive feedback circuits. This framework steered the design of spatially selective spinal implants and real–time control software that modulate extensor versus flexor synergies with precise temporal resolution. Spatiotemporal neuromodulation therapies improved gait quality, weight–bearing capacities, endurance and skilled locomotion in multiple rodent models of spinal cord injury. These new concepts are directly translatable to strategies to improve motor control in humans.

Liquid metals, encapsulated in soft materials, have therefore attracted much attention in recent years [2a,8] to manufacture soft conductors with metallic conductivity, high stretchability and reconfigurability. [9] Gallium-based alloys, rather than toxic mercury, are widely used. The high surface tension and the passivating oxide skin that spontaneously forms on the surface of these liquids hinder their patterning using conventional techniques. Alternative methods focus on injection into channels, molding and printing for rapid manufacturing of highly conductive and stretchable metal networks but none of these patterning techniques offer high-resolution batch processing over large (wafer-scale) surface areas. [10] Based on these observations, we developed a new class of stretchable electronic conductors formed of biphasic solidliquid thin metal films. A bilayer metallization sequence starting with the sputtering of an alloying gold film followed by the thermal evaporation of liquid gallium (that displays a melting point of 29.8 °C [11] ) results in a heterogeneous film composed of clusters of the solid intermetallic alloy AuGa 2 and supercool liquid gallium forming a continuous network and dispersed bulges [11b,12] (Figure 1a-c). We designed and engineered the biphasic metallic films to be compatible with large-area and standard microfabrication. Figure 1d,e shows examples of fine patterns produced at wafer scale on elastomeric substrates. Multilayered stretchable circuits can be readily integrated by covalently bonding membranes hosting patterned biphasic conductors connected through soft vias. Figure 1e displays a 4 × 4 wafer-sized hybrid array of surface mounted light emitting diodes interconnected with a two-level network of biphasic solid-liquid conductors. The array withstood demanding multiaxial inflation cycles, constantly delivering power to the optoelectronic devices (Movie S1, Supporting Information).To prepare the stretchable biphasic solid-liquid thin metal films, a two-step process was developed in which liquid gallium was evaporated on a substrate preliminarily coated with a wetting and alloying thin film. We selected poly(dimethylsiloxane) (PDMS), a silicone, as the soft carrier substrate and a gold film sputtered on the PDMS as the alloying layer. However, our process is not limited to those materials ( Figure S1 and S2, Supporting Information). Non-noble metals may be used, provided the alloying thin film is not oxidized.The high surface tension of the liquid metal prevented the formation of an evaporated continuous liquid metal film on bare silicone substrates. Instead, the surface of the elastomer was covered with a nonconducting arrangement of liquid gallium microdroplets ( Figure S3, Supporting Information). In contrast, evaporating gallium on an alloying metal film, first deposited on the silicone surface, overcame the cohesive forces Stretchable conductors are the foundation of soft electronic circuits. [1] Manufacturing elastic wiring networks to distribute and carry electrical pote...

Emerging applications of the Internet of Things in healthcare, wellness, and gaming require continuous monitoring of the body and its environment, fueling the need for wearable devices able to maintain intimate, reliable, and unobtrusive contact with the human body. This translates in the necessity to develop soft and deformable electronics that match the body's mechanics and dynamics. In recent years, various strategies have been proposed to form stretchable circuits and more specifically elastic electrical conductors embedded in elastomeric substrate using either geometrical structuring of solid conductors or intrinsically stretchable materials. Gallium (Ga)-based liquid metals (LMs) are an emerging class of materials offering a particularly interesting set of properties for the design of intrinsically deformable conductors. They concomitantly offer the high electrical conductivity of metals with the ability of liquids to flow and reconfigure. The specific chemical and physical properties of Ga-based LMs differ fundamentally from those of solid conductors and need to be considered to successfully process and implement them into stretchable electronic devices. In this Account, we report on how the key physical and chemical properties of Ga-based LMs can be leveraged to enable repeatable manufacturing and precise patterning of stretchable LM conductors. A comprehensive understanding of the interplay between the LM, its receiving substrate chemistry and topography, and the environmental conditions is necessary to meet the reproducibility and reliability standards for large scale deployment in next-generation wearable systems. In oxidative environments, a solid oxide skin forms at the surface of the LM and provides enough stiffness to counterbalance surface tension, and prevent the LM from beading up to a spherical shape. We review techniques that advantageously harness the oxide skin to form metastable structures such as spraying, 3D printing, or channel injection. Next, we explore how controlling the environmental condition prevents the formation or removes the oxide skin, thereby allowing for selective wetting of Ga lyophilic surfaces. Representative examples include selective plating and physical vapor deposition. The wettability of LMs can be further tuned by engineering the surface chemistry and topology of the receiving substrate to form superlyophobic or superlyophilic surfaces. In particular, our group developed Ga-superlyophilic substrates by engineering the surface of silicone rubber with microstructures and a gold coating layer. Thermal evaporation of Ga on such engineered substrates allows for the formation of smooth LM films with micrometric thickness control and design freedom. The versatility of the available deposition techniques facilitates the implementation of LM conductors in a wide variety of wearable devices. We review various epidermal electronic systems using LM conductors as interconnects to carry power and information, transducers and sensors, antennas, and complex hybrid (soft-rigid) ele...

Several methods are proposed to manipulate and pattern liquid metal films into elastic conductors but all lack precise control over the film thickness and roughness, thereby limiting its uniformity, stability, and reproducibility. Here, an approach relying solely on wetting phenomena is proposed to produce smooth film of liquid gallium (Ga) on extended surface areas with controlled thickness and electrical properties. The surface chemistry and topography of silicone rubber (poly(dimethylsiloxane)) is engineered with microstructured pillars and gold precoating layer to produce Ga superlyophilic substrates. Physical vapor deposition of Ga on such substrates leads to the formation of smooth and homogeneous films by imbibition of the surface topography rather than coalescence and formation of Ga drops. By capillarity, Ga accumulates in between the pillars up to their top surface, forming a smooth film with a root mean square roughness (Rq) smaller than 100 nm. The wetting conditions and electromechanical properties of the resulting films are compared based on the selection of the microtexture patterns and a model of the film sheet resistance as a function of the texture geometrical parameters is established.

On page 4507, S. P. Lacour and co-workers present highly conductive and stretchable solid-liquid films that are formed by physical vapor deposition of gallium onto an alloying gold layer. The image shows patterns defined by lift-off on an elastomer membrane. The magnified view is a false-color scanning electron microscopy (SEM) image (×5000) of the surface of the films under 50% applied strain, showing the liquid Ga (blue-gray) flowing between the AuGa2 /Ga clusters (gold).

In article number https://doi.org/10.1002/advs.201800256, Arthur Hirsch and Stéphanie P. Lacour engineer the surface chemistry and topography of silicone rubber (PDMS) with micro‐structured pillars and a gold pre‐coating layer to produce gallium super‐lyophilic substrates. Physical vapor deposition of gallium leads to the formation of smooth and homogeneous films by imbibition of the surface topography rather than coalescence and formation of gallium drops. By capillarity, gallium accumulates in between the pillars up to their top surface, forming a smooth film.

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