We have developed a hydraulic displacement amplification mechanism (HDAM) and studied its dynamic response when combined with a piezoelectric actuator. The HDAM consists of an incompressible fluid sealed in a microcavity by two largely deformable polydimethylsiloxane (PDMS) membranes. The geometry with input and output surfaces having different cross-sectional areas creates amplification. By combining the HDAM with micro-actuators, we can amplify the input displacement generated by the actuators, which is useful for applications requiring large deformation, such as tactile displays. We achieved a mechanism offering up to 18-fold displacement amplification for static actuation and 12-fold for 55 Hz dynamic actuation.

A novel biphasic sub-micrometer Au–AuGa₂/liquid metal framework, consisting of solid nanoparticles encapsulating liquid metal (LM) droplets, is introduced. By utilizing oxide-free galvanic replacement of a Ga-alloy LM with alkaline KAuBr₄, the controllable process of gold-based encapsulation of individual sub-micrometer LM droplets capped with polyvinylpyrrolidone, lysozyme or sodium alginate is demonstrated. The morphology, structure and composition of the encapsulated droplets are characterized in-depth via scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. For the first time, the existence of two kinetic regimes leading to the controllable synthesis of either intermetallic AuGa₂ or Au nanoparticles to encapsulate LM droplets is highlighted via the study of the evolution of the pH of the reaction medium. The as-formed biphasic sub-micrometer Au–LM frameworks demonstrate moderate stability in an aqueous environment and formidable structural and chemical stability in ambient air at room temperature. The encapsulation process can be adapted to the micro-patterning of LM droplet surfaces to produce conductive Au-encapsulated LM droplets then sintered to recover LM lines, while the sub-micrometer Au-LM droplets can be reintegrated onto larger LM surfaces and volumes to create multi-metallic and multi-scale biphasic structures with controllable electrical properties. The adaptability of the encapsulation process to the macro-, micro- and nanoscale of liquid metal features with a wide range of surfactants could in principle allow its use in various systems and applications.

Stretchable strain sensors that use a liquid metal (eutectic gallium–indium alloy; E-GaIn) and flexible silicone rubber (Ecoflex) as the support and adhesive layers, respectively, are demonstrated. The flexibility of Ecoflex and the deformability of E-GaIn enable the sensors to be stretched by 100%. Ecoflex gel has sufficiently large adhesion force to skin, even though the adhesion force is smaller than that for commercially available adhesives. This enables the sensor to be used for non-invasive monitoring of human motion. The mechanical and electrical properties of the sensor are experimentally evaluated. The effectiveness of the proposed sensors is demonstrated by monitoring joint movements, facial expressions, and respiration.

Tactile displays provoke tactile sensations by artificially stimulating tactile receptors. While many types of tactile displays have been developed, electrotactile displays that exploit electric stimulation can be designed to be thin, light, flexible and thus, wearable. However, the high voltages required to stimulate tactile receptors and limited varieties of possible sensations pose problems. In our previous work, we developed an electrotactile display using a micro-needle electrode array that can drastically reduce the required voltage by penetrating through the high-impedance stratum corneum painlessly, but displaying various tactile sensations was still a challenge. In this work, we demonstrate presentation of tactile sensation of different roughness to the subjects, which is enabled by the arrangement of the electrodes; the needle electrodes are on the fingertip and the ground electrode is on the fingernail. With this arrangement, the display can stimulate the tactile receptors that are located not only in the shallow regions of the finger but also those in the deep regions. It was experimentally revealed that the required voltage was further reduced compared to previous devices and that the roughness presented by the display was controlled by the pulse frequency and the switching time, or the stimulation flow rate. The proposed electrotactile display is readily applicable as a new wearable haptic device for advanced information communication technology.