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...

Soft energy storage devices, such as supercapacitors, are an essential component for powering integrated soft microsystems. However, conventional supercapacitors are mainly manufactured using hard/brittle materials that easily crack and eventually delaminate from the current collector by mechanical deformation. Therefore, to realize all-soft supercapacitors, the electrodes should be soft, stretchable, and highly conductive without compromising the electrochemical performance. This paper presents all-soft supercapacitors for integrated soft microsystems based on gallium–indium liquid metal (eutectic gallium–indium alloy, EGaIn) electrodes with integrated functionalized carbon nanotubes (CNTs). Oxygen functional groups on the surface of the CNTs ensure strong adhesion between the functionalized CNTs and the thin native oxide layer on the surface of EGaIn, which enables delamination-free soft and stretchable electrodes even under mechanical deformation. The electrochemical performances of fabricated all-soft supercapacitors in a parallel-plate arrangement were investigated without and with applied mechanical deformation. The fabricated supercapacitors exhibit areal capacitances as high as 12.4 mF cm–2 and show nearly unchanged performance under 30% applied strain. They maintain >95% of their original capacitance after >4200 charging and discharging cycles with a periodic applied strain of 30%. Finally, fabricated supercapacitors have been successfully integrated with a commercial light-emitting diode to demonstrate an integrated soft microsystem.

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.