3D Printed Spring‐Type Electronics with Liquid Metals for Highly Stretchable Conductors and Inductive Strain/Pressure Sensors

Abstract

structures in two or three dimensions. This method enables the stretching of hard and non-stretchable materials (such as, metals [2,[30][31][32] and plastics [1,4,33] ). Deformable structures, such as serpentine, [2,34] mesh, [29,35] kirigami-shapes, [1,6,36] or helix/spring structures, [26,30,31,[37][38][39][40][41][42] can be fabricated by deposition, [2,35] lift-off, [36] laser cutting, [1,37] pre-stretching, [30,38,39] or molding methods. [40][41][42] In particular, the spring structure is stretchable and resilient, enabling electronic devices to be highly stretchable and durable. [42] In addition to stretchable conductors, [42] accurate strain and bending sensors [9] have been reported using inductance changes in coil structures. Devices with liquid metals have better cyclic electrical characteristics than those with rigid metallic materials such as, copper nanowires [40] and silver nanoparticles. [42] Further, a helix-structured device with a metal wire wrapped around a nylon fiber enabled the estimation of pressure by inductance changes in compression. [43] 3D printers have enabled the fabrication of complex 3D structures with structural deformation [44,45] or internal channels. [9,46,47] They can also utilize soft materials such as, elastomers and gels. Therefore, soft spring-type electronic devices have been developed with combinations of electronic materials as well. [9,47] For example, a spring-type device composed of silicone rubber and liquid metal as the core and shell, respectively, exhibited highly stable electrical characteristics in cyclic deformations. [9] However, achieving the desired deformable properties, such as ≈400% stretchability, is challenging when 3D objects are composed of only soft materials, because the resulting devices cannot maintain their structures owing to self-weight deformation. For instance, a spring structure cannot be maintained without a supporting rod inside the coil owing to the deformation caused by its weight. The presence of the supporting rod limits the extent of stretching because the radius of the coil decreases with stretching. Therefore, spring structures with soft material as the shell and liquid metal as the core have achieved almost 100% stretching. [9] Moreover, the supporting rod disturbs the compression of only the spring section. Hence, sensors that measure pressure based on the deformation of the spring structure have not been realized in soft materials.

Low-melting-point alloys integrated extrusion additive manufacturing

“…Figure 2(c) shows a scanned 3D model of the first author's head and an LMPA statue fabricated from it. As indicated in the literature, LMPA has excellent potential for future multi-functional applications, [28][29][30][31][32] including smart structures, electromagnetic shielding, biomedicine, thermal management, and energy harvesting. To the best of our knowledge, our study is the first to demonstrate the fabrication of complex lattice, tooth, and bone LMPA structures using EAM, which is a low-cost AM technique.…”

“…[27] The literature shows that LMPAs have excellent potential for future multi-functional applications. [28][29][30][31][32] However, the fabrication of good quality complex LMPA parts by 3D printing techniques remains challenging, and additive manufacturing LMPA is still in its infancy.…”

Stretchable conductors for stretchable field-effect transistors and functional circuits

“…Notably, a liquid metal-based conductor can also be developed by 3D printing. 435 For instance, Okutani et al printed the spring-type electronic devices that is a core canal filled with a 3D-printed elastomeric shell and LM. 435 The conductor with spring type can be stretched with little force (1.25 N) and has a negligible change in resistance up to 400% elongation and a 5.5% increase in resistance at 640% elongation.…”

3D Printing of Liquid Metals: Recent Advancements and Challenges

“…However, at high extrusion speed, the inner wall of the flexible wire is not consistent and is pushed up into a ball. [61,62]…”