3D‐Integrated and Multifunctional All‐Soft Physical Microsystems Based on Liquid Metal for Electronic Skin Applications

Abstract

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...

Gallium‐Based Thin Films for Wearable Human Motion Sensors

“…The gallium films can be used to produce precise, repeatable, and durable sensing devices. To date, a large variety of sensors based on liquid metal conductors have been proposed to measure finger joint movements . However, most of these demonstrations were qualitative, restricted to single or mm‐thick devices, or performed on mock‐up hand models.…”

Multiscale and Uniform Liquid Metal Thin‐Film Patterning Based on Soft Lithography for 3D Heterogeneous Integrated Soft Microsystems: Additive Stamping and Subtractive Reverse Stamping

“…Recently, we demonstrated high resolution, uniform, and residue‐free EGaIn thin‐line patterning ranging from single micrometer to millimeter scales by utilizing a chemical surface modification and residue transfer process in a reverse stamping approach to enable high‐density, soft passive electronics . Building on our previous work, this paper presents multiscale and uniform EGaIn thin‐film patterning by utilizing an additive stamping process for large‐scale (mm–cm) soft electronics and the subtractive reverse stamping process for microscale (µm–mm) soft electronics. By combining these complementary patterning techniques using 3D heterogeneous integration, fabricated and optimized soft electronic components built with different patterning processes can be integrated to form high‐density and multifunctional soft microsystems.…”

Wearable and Stretchable Strain Sensors: Materials, Sensing Mechanisms, and Applications

“…The dominant working principle of the liquid metal-based stretchable strain sensors is the geometrical effect. [167][168][169][170][171] In the following sections, it will be shown that the contribution of this mechanism to the overall sensing performance of nanomaterials-based stretchable sensors may not be significant.…”