We report an unconventional approach for high-resolution, reconfigurable 3D printing using liquid metals for stretchable, 3D integrations. A minimum line width of 1.9 μm can be reliably formed using direct printing, and printed patterns can be reconfigured into diverse 3D structures with maintaining pristine resolutions. This reconfiguration can be performed multiple times, and it also generates a thin oxide interface that can be effective in preventing the spontaneous penetration of gallium atoms into different metal layers while preserving electrical properties under ambient conditions. Moreover, these free-standing features can be encapsulated with stretchable, conformal passivations. We demonstrate applications in the form of a reconfigurable antenna, which is tunable by changing geometeries, and reversibly movable interconnections used as mechanical switches. The free-standing 3D structure of electrodes is also advantageous for minimizing the number and space between interconnections, which is important for achieving higher integrations, as demonstrated in an array of microLEDs.

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Here we demonstrate an unconventional fabrication of highly transparent supercapacitors and electrodes using random networks of nanostructured metallic glass nanotroughs for their integrations as wirelessly rechargeable and invisible, skin heat patches.Transparent supercapacitors with fine conductive patterns were printed using an electrohydrodynamic jet-printing. Also, transparent and stretchable electrodes, for wireless antennas, heaters and interconnects, were formed using random network based on nanostructured CuZr nanotroughs and Ag nanowires with superb optoelectronic properties (sheet resistance of 3.0 Ω/sq at transmittance of 91.1%). Their full integrations, as an invisible heat patch on skin, enabled the wireless recharge of supercapacitors and the functions of heaters for thermal therapy of skin tissue. The demonstration of this transparent thermotherapy patch to control the blood perfusion level and hydration rate of skin suggests a promising strategy toward next-generation wearable electronics.

Human‐interactive displays involve the interfacing of a stimuli‐responsive sensor with a human‐readable response. Human‐readable responses include the five recognized senses, i.e., sight (vision), hearing (audition), taste (gustation), smell (olfaction), and touch (somatosensation). Vision is considered to be the most informative human stimulus so that the visualization of electrical, thermal, and mechanical data is important for various applications. Herein, the fabrication of human‐interactive displays is demonstrated in which active‐matrix arrays of pressure‐sensitive Si transistors with air dielectric layers are fully integrated with pixels of organic light‐emitting diodes (OLEDs). In this way, the luminance of the individual OLED pixels can be increased locally by pressing the display, and the luminance is dependent on the magnitude of the applied pressure. Furthermore, the air dielectric layer of transistors provides outstanding electrical properties, including high transconductance and negligible hysteresis. 3D integration of these transistors with dual‐side emissive OLED pixels is also demonstrated. Local pressing increases the light intensity of OLED pixel and then the underlaid Si channel can absorb this light successively to generate additional photocurrents from the pressure‐sensitive transistor, further enhancing its sensitivity. This human‐interactive display can visualize tactile pressure directly, suggesting the substantial promise as next generation displays for intelligent human‐machine interfacing.

Existing tactile stimulation technologies powered by small actuators offer low-resolution stimuli compared to the enormous mechanoreceptor density of human skin. Arrays of soft pneumatic actuators initially show promise as small-resolution (1- to 3-mm diameter), highly conformable tactile display strategies yet ultimately fail because of their need for valves bulkier than the actuators themselves. In this paper, we demonstrate an array of individually addressable, soft fluidic actuators that operate without electromechanical valves. We achieve this by using microscale combustion and localized thermal flame quenching. Precisely, liquid metal electrodes produce sparks to ignite fuel lean methane–oxygen mixtures in a 5-mm diameter, 2-mm tall silicone cylinder. The exothermic reaction quickly pressurizes the cylinder, displacing a silicone membrane up to 6 mm in under 1 ms. This device has an estimated free-inflation instantaneous stroke power of 3 W. The maximum reported operational frequency of these cylinders is 1.2 kHz with average displacements of ∼100 µm. We demonstrate that, at these small scales, the wall-quenching flame behavior also allows operation of a 3 × 3 array of 3-mm diameter cylinders with 4-mm pitch. Though we primarily present our device as a tactile display technology, it is a platform microactuator technology with application beyond this one.

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The next generation of optoelectronics with unique 3D architectures has been explored extensively over the past decade as the key to moving beyond the inherent planarity associated with conventional, photolithography-based microfabrication

We use magnetohydrodynamic levitation as a means to create a soft, elastomeric, solenoid-driven pump (ESP). We present a theoretical framework and fabrication of a pump designed to address the unique challenges of soft robotics, maintaining pumping performance under deformation. Using a permanent magnet as a piston and ferrofluid as a liquid seal, we model and construct a deformable displacement pump. The magnet is driven back and forth along the length of a flexible core tube by a series of solenoids made of thin conductive wire. The magnet piston is kept concentric within the tube by Maxwell stresses within the ferrofluid and magnetohydrodynamic levitation, as viscous lift pressure is created due to its forward velocity. The centering of the magnet reduces shear stresses during pumping and improves efficiency. We provide a predictive model and capture the transient nonlinear dynamics of the magnet during operation, leading to a parametric performance curve characterizing the ESP, enabling goal-driven design. In our experimental validation, we report a shut-off pressure of 2 to 8 kPa and run-out flow rate of 50 to 320 mL⋅min −1 , while subject to deformation of its own length scale, drawing a total of 0.17 W. This performance leads to the highest reported duty point (i.e., pressure and flow rate provided under load) for a pump that operates under deformation of its own length scale. We then integrate the pump into an elastomeric chassis and squeeze it through a tortuous pathway while providing continuous fluid pressure and flow rate; the vehicle then emerges at the other end and propels itself swimming.