The use of conducting liquids with high electrical conductivity, such as eutectic gallium–indium (EGaIn), has great potential in electronics applications requiring stretchability and deformability beyond conventional flexible electronics relying on solid conductors. An advanced liquid metal thin‐line patterning process based on soft lithography and a compatible vertical integration technique are presented that enable size‐scalable and high‐density EGaIn‐based, soft microelectronic components and circuits. The advanced liquid metal thin‐line patterning process based on poly(dimethylsiloxane) (PDMS) substrates and soft lithography techniques allows for simultaneous patterning of uniform and residue‐free EGaIn lines with line width from single micrometers to several millimeters at room temperature and under ambient pressure. Using this fabrication technique, passive electronic components and circuits are investigated under elastic deformations using numerical and experimental approaches. In addition, soft through‐PDMS vias with high aspect ratio are demonstrated for multilayer interconnections in 2.5D and 3D integration approaches. To highlight the system‐level potential of the patterning technique, a chemical sensor based on an integrated LC resonance circuit with a microfluidic‐tunable interdigitated capacitor and a planar spiral inductor is fabricated and characterized. Finally, to show the flexibility and stretchability of the resulting electronics, circuits with embedded light emitting diodes (LEDs) are investigated under bending, twisting, and stretching deformations.

Innovations in soft material synthesis and fabrication technologies have led to the development of integrated soft electronic devices. Such soft devices offer opportunities to interact with biological cells, mimicking their soft environment. However, existing fabrication technologies cannot create the submicron-scale, soft transducers needed for healthcare and medical applications involving single cells. This work presents a nanofabrication strategy to create submicron-scale, all-soft electronic devices based on eutectic gallium-indium alloy (EGaIn) using a hybrid method utilizing electron-beam lithography and soft lithography. The hybrid lithography process is applied to a biphasic structure, comprising a metallic adhesion layer coated with EGaIn, to create soft nano/microstructures embedded in elastomeric materials. Submicron-scale EGaIn thin-film patterning with feature sizes as small as 180 nm and 1 μm line spacing was achieved, resulting in the highest resolution EGaIn patterning technique to date. The resulting soft and stretchable EGaIn patterns offer a currently unrivaled combination of resolution, electrical conductivity, and electronic/wiring density.

To mimic human touch sensing, robotics must be able to leverage multiple sensory inputs. Previously, to achieve both proximity and pressure sensing, most approaches have required using two separate sensors, each with their corresponding electronics, limiting the achievable density. More recently, sensors with multifunctional pressure and proximity capabilities have been realized at the cost of compromised pressure sensing. Presented here is a new design for a multifunctional interdigitated fringe field capacitive pressure sensor with a pyramid microstructured dielectric layer that has proximity‐sensing capabilities (noncontact mode) while also sensing pressure (contact mode) as strongly as an equivalent parallel plate capacitive sensor of the same size. In contact mode, both sensors have a response time of less than 20 ms and can respond to loads lighter than 0.5 Pa. Further, the interdigitated fringe field sensor can clearly distinguish between the two sensing modes, as well as between conductive and nonconductive materials in the noncontact mode. Finally, we use the interdigitated fringe field sensor to demonstrate both proximity and high‐sensitivity pressure sensing on a robotic gripper.

to healthcare. [5,6] Unlike conventional solid-state electronics, soft electronics can be lightweight, stretchable, and reconfigurable, with biocompatible characteristics for skin-mountable and wearable sensing electronics. [7,8] Thereby, flexible and stretchable characteristics are achieved by using either 2D or 3D compliant wave-like, solid metal patterns [9,10] or elastic conductors based on conductive nanomaterials embedded in a polymer matrix. [11,12] An alternative approach to realize all-soft microsystems is the use of intrinsically soft conductors, such as gallium-based liquid metal (eutectic gallium-indium alloy, EGaIn). EGaIn-based soft electronics benefits from its nontoxicity, mechanical stability (unlimited stretchability, but ultimately limited by the mechanical properties of the encasing material), thermal conductivity (κ = 26.6 W m −1 K −1 ), and electrical conductivity (σ = 3.4 × 10 6 S m −1 ). [13][14][15] The low melting temperature (M P < 15 °C) and negligible vapor pressure of EGaIn facilitate room-temperature and ambient pressure manufacturing processing. [13][14][15] Moreover, thanks to the formation of a thin oxide layer (t ≈ 1-3 nm) on the EGaIn surface under atmospheric oxygen level, EGaIn structures maintain their mechanical shapes, [16,17] allowing 2D/3D EGaIn patterns on a soft elastomeric substrate, such as poly(dimethylsiloxane) (PDMS).The moldable characteristic of EGaIn has enabled a broad range of patterning methods based on lithography-enabled stamping and stencil printing, injection, as well as additive and subtractive direct write/patterning processes, [18][19][20] as summarized in Table S1 in the Supporting Information. Thereby, printing using lithography-defined stencils [21][22][23][24] yields simple and high throughput EGaIn patterning on elastomeric substrates with small features of w (width) ≈200 µm/t (thickness) ≈50 µm using metal stencil films, [21] w ≈ 20 µm/t ≈ 2 µm using microfabricated metal stencil films, [22] and w ≈ 20 µm/t ≈ 10 µm using polymer stencil films. [23] Limitations of this approach are the relatively low resolution, rough EGaIn surface, and excessive EGaIn loss during the stencil lift-off process. SubtractiveThe use of intrinsically soft conductors, such as gallium-based liquid metal (eutectic gallium-indium alloy, EGaIn), has enabled bioinspired and skin-like soft electronics. Thereby, creating patterned, smooth, and uniform EGaIn thin films with high resolution and size scalability is one of the primary technical hurdles. Soft lithography using wetting/nonwetting surface modifications and 3D heterogeneous integration can address current EGaIn patterning challenges. This paper demonstrates multiscale and uniform EGaIn thin-film patterning by utilizing an additive stamping process for large-scale (mm-cm) soft electronics and a subtractive reverse stamping process for microscale (µm-mm) soft electronics. While EGaIn patterning based on stamping is regarded as the least reliable patterning technique, this paper highlights multiscale and uniform thin-fi...