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.

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...

of objects, as well as motions and gestures of the human body. [4,5] Moreover, the brain can interpret more sophisticated information to distinguish, for example, touch and pain, and eventually convey feelings and emotions. [4,5] Therefore, electronic skins (e-skins), which can mimic the capability of the sensory receptors of the human skin, can provide an improvement in the quality of life to people with skin damage or amputations by restoring these capabilities. [1,4,5] This motivation for e-skins has initiated the development of a variety of soft materials, [6,7] manufacturing methods, [8,9] and force detection schemes [10][11][12][13] based on electrical, thermal, and optical principles to provide a seamless link between humans and the digital world. [14,15] However, one major technical challenge in current skinmountable and wearable electronics is the mechanical mismatch between soft biological skins and tissues and conventional rigid and bulky electronic materials. [16][17][18] Furthermore, highdensity integration and multifunctional sensing capability are additional technical hurdles to mimic human skins. [19,20] Moreover, from a systems point of view, although discrete stretchable temperature, [21,22] pressure, [23,24] and strain [25,26] sensing elements have been investigated, they typically require rigid electronic components and/or printed circuit boards (PCBs) for both sensors and readout systems, which limits their ultimate usability for skin-mountable and wearable electronics and leads to hard-soft material interfacial failure. [16,27] The growing demand for skin-mountable electronics has resulted in significant research in the area of stretchable electronics, that is, electronics which maintain their electrical functionality even upon large mechanical deformations. [16][17][18] To achieve flexible and stretchable electronic characteristics, three fundamental approaches have been investigated. [28,29] The first approach is to create 2D or 3D wave-like solid metal patterns on a soft substrate that can endure large mechanical deformation. [30,31] Limitations of this approach are that the rigid wavy metal patterns limit the ultimate strain and lower the density of electronic components. The second approach is to fabricate highly elastic conductors by mixing conductive nanomaterials, for example, low-dimensional carbons, [32] nanowires, [33] and nanoparticles, [34] into polymer matrices or by coating them on a soft substrate. This approach enables inexpensive fabrication processes by printing inks for conductive circuits, but the relatively low resolution and low conductivity are still challenging This paper presents 3D-integrated and multifunctional all-soft physical microsystems, which are composed of a soft sensor, a soft interconnector, and a soft readout circuit. The microsystems utilize gallium-based liquid metal (eutectic gallium-indium alloy, EGaIn) and poly(dimethylsiloxane) (PDMS) and are fabricated using an advanced EGaIn thin-line patterning technique based on soft lithography. Combining the...

Lightweight, flexible, stretchable, and wireless sensing platforms have gained significant attention for personal healthcare and environmental monitoring applications. This paper introduces an all-soft (flexible and stretchable), battery-free, and wireless chemical microsystem using gallium-based liquid metal (eutectic gallium-indium alloy, EGaIn) and poly(dimethylsiloxane) (PDMS), fabricated using an advanced liquid metal thin-line patterning technique based on soft lithography. Considering its flexible, stretchable, and lightweight characteristics, the proposed sensing platform is well suited for wearable sensing applications either on the skin or on clothing. Using the microfluidic sensing platform, detection of liquid-phase and gas-phase volatile organic compounds (VOC) is demonstrated using the same design, which gives an opportunity to have the sensor operate under different working conditions and environments. In the case of liquid-phase chemical sensing, the wireless sensing performance and microfluidic capacitance tunability for different dielectric liquids are evaluated using analytical, numerical, and experimental approaches. In the case of gas-phase chemical sensing, PDMS is used both as a substrate and a sensing material. The gas sensing performance is evaluated and compared to a silicon-based, solid-state gas sensor with a PDMS sensing film.