Harnessing the Rheological Properties of Liquid Metals To Shape Soft Electronic Conductors for Wearable Applications.

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) electronic circuits. In addition, we highlight the limitations and challenges inherent to the use of Ga LM conductors that include electromigration, corrosion, solidification, and biocompatibility.

A Method to Form Smooth Films of Liquid Metal Supported by Elastomeric Substrate.

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.

Intrinsically Stretchable Biphasic (Solid-Liquid) Thin Metal Films.

Stretchable biphasic conductors are formed by physical vapor deposition of gallium onto an alloying metal film. The properties of the photolithography-compatible thin metal films are highlighted by low sheet resistance (0.5 Ω sq(-1) ) and large stretchability (400%). This novel approach to deposit and pattern liquid metals enables extremely robust, multilayer and soft circuits, sensors, and actuators.

Biphasic Metal Films: Intrinsically Stretchable Biphasic (Solid-Liquid) Thin Metal Films (Adv. Mater. 22/2016).

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).

Spatiotemporal neuromodulation therapies engaging muscle synergies improve motor control after spinal cord injury.

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 the therapeutic approach to continuous stimulation applied to restricted spinal cord locations. Here we developed 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 and flexor synergies with precise temporal resolution. Spatiotemporal neuromodulation therapies improved gait quality, weight-bearing capacity, endurance and skilled locomotion in several rodent models of spinal cord injury. These new concepts are directly translatable to strategies to improve motor control in humans.

Biomaterials. Electronic dura mater for long-term multimodal neural interfaces.

The mechanical mismatch between soft neural tissues and stiff neural implants hinders the long-term performance of implantable neuroprostheses. Here, we designed and fabricated soft neural implants with the shape and elasticity of dura mater, the protective membrane of the brain and spinal cord. The electronic dura mater, which we call 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. The soft implants extracted cortical states in freely behaving animals for brain-machine interface and delivered electrochemical spinal neuromodulation that restored locomotion after paralyzing spinal cord injury.