Liquid Metal‐Based Soft Materials for Self‐Healing Flexible Electronics

Innovations in soft materials have advanced the development of implantable devices for pressure monitoring, but fabrication and integration challenges remain, such as limited patterning resolution and poor scalability, hindering their miniaturization and wireless sensing capabilities. This study introduces methods and advantageous features of incorporating liquid metal into microfabriated, wireless soft implantable pressure sensors and wearable readout systems for large-area pressure mapping and adaptive implantation with autonomous folding and self-healing capabilities. Eutectic gallium-indium, a type of liquid metal, serves as both the deformable electrode for capacitive sensors and a low-resistance conductor for inductors. It is integrated into a thin-film, battery-free, inductive-capacitive wireless sensing platform. Scalable wireless sensor arrays are created through microfabrication for large-area pressure mapping. The wireless pressure sensor is also integrated with soft ferromagnetic and self-healing layers in cuff-type sensors to allow for autonomous folding in response to external magnets, eliminating the need for suturing. In addition, a miniaturized wearable readout system integrated into medical gloves enables wireless and real-time pressure monitoring. The presented wireless soft pressure-sensing method with large-area mapping and secure implantation capabilities offers alternatives to conventional medical tools for intraoperative monitoring and examinations.

Liquid metals have attracted attention as functional components for moldable electronics, such as soft flexible connectors, wires or conductive ink. The relatively high surface tension (> 400 mN m−1) and the fact that liquid metals do not readily wet ceramic or oxide surfaces have led to devising unique techniques to spread the liquid and mold its shape. These techniques include surface modification, electrowetting and vacuum filling of channels. This work presents an injection technique based on pressurized fountain pen lithography with glass nanopipettes developed to directly pattern liquid metal on flat hard substrates. The liquid metals were eutectic alloys of Gallium, including Gallium-Indium (EGaIn), Gallium-Indium-Zinc and Gallium-Indium-Tin. The nanopipettes were coated internally with gold, acting as a sacrificial layer and facilitating the wetting of the pipette down to its pore, with an inner diameter of ~ 100–300 nm. By applying hydrodynamic pressure to the connected end of the pipette, the metal was extruded through the pore, forming long continuous (> 3 mm) and narrow (~ 1–15 µm) metal lines on silicon oxide and gold surfaces at room temperature and ambient conditions. With this robust platform, it is possible to pattern liquid metals on a variety of substrates and geometries down to the micron range.

Low‐melting point metals and alloys, as emerging 3D printing ink, have attracted more and more attention especially for flexible or intelligent conductors and electronics. However, achieving 3D continuity of liquid metal structures or patterns using a common method is still challenging and worth pursuing. In this study, a generalized method is proposed for 3D printing structures made of low melting gallium‐based alloys with diverse melting points, which should greatly expand their applications. The mechanism for shaping liquid metal inks relies on the combination actions between electrocapillarity and oxidation, which significantly reduces the high surface tension as well provides solid frame for the inner liquid metals. Taken galinstan as an example, the printing conditions, including the viscosity of hydrogel, positive voltage (0–15 V), calcium chloride concentration, and velocity (the movement speed of the printing head) are explored to optimize the process. The printed galinstan is characterized to observe the morphology and elemental analysis of the surface oxide layer. During the process, the chemical reagents are all safe and non‐toxic, which is in line with the green product requirements. This printing method shall have broad application prospect in flexible electronics, biosensors, biomedical engineering, contrast agent in vivo, and other fields.

The focus of this chapter is to review the literature on flexible and stretchable electronics fabricated using gallium-based liquid metal alloys. Over the past few decades, gallium-based liquid metal alloys have gathered growing interest due to their application in various soft electronic applications as conductive yet fluidic metals that can be patterned at room temperature. Gallium-based liquid metal alloys have a melting point at or near room temperature and offer a low toxicity as opposed to the conventional liquid metal – mercury, in addition to having negligible vapor pressure and low viscosity. The surface of these metals reacts rapidly with oxygen in air to form a thin surface oxide ‘skin’ that allows these liquids to be deposited in several useful stable shapes despite the destabilizing effects of a large surface tension and gravity. There are several methods to pattern these liquid metals – traditionally using lithography-based subtractive approaches, and unconventionally using 3D printing and other additive techniques. Gallium-based liquid metal alloys have been used for fabricating various electronic structures such as ultrastretchable and self-healing wires, interconnects, and stretchable antennas. Integrating these components, many electronic components have been built such as optical and soft memory devices, reconfigurable circuits, soft diodes, microfluidic pumps, soft strain sensors, capacitors, and circuit boards.

Electrical and mechanical integration approaches are essential for emerging hybrid electronics that must robustly bond rigid electrical components with flexible circuits and substrates. However, flexible polymeric substrates and circuits cannot withstand the high temperatures used in traditional electronic processing. This constraint requires new strategies to create flexible materials that simultaneously achieve high electrical conductivity, strong adhesion, and processibility at low temperature. Here, an electrically conductive adhesive is introduced that is flexible, electrically conductive (up to 3.25×105 S m−1) without sintering or high temperature post‐processing, and strongly adhesive to various materials common to flexible and stretchable circuits (fracture energy 350 <Gc < 700 J m−2). This is achieved through a multiphase soft composite consisting of an elastomeric and adhesive epoxy network with dispersed liquid metal droplets that are bridged by silver flakes, which form a flexible and conductive percolated network. These inks can be processed through masked deposition and direct ink writing at room temperature. This enables soft conductive wiring and robust integration of rigid components onto flexible substrates to create hybrid electronics for emerging applications in soft electronics, soft robotics, and multifunctional systems.

This paper proposes a low-cost soft pressure sensor. The pressure sensor is fabricated by 3D printing. The material uses soft silicone with ductility and skin-friendly properties, which can better restore the original shape after extrusion. And when it comes into contact with the skin, it will not cause skin allergies because o-f the time. The conductive cotton yarn is used as the main wire inside the sensor (the sensor size is about 17.5mmx13.5mmx4mm). Users feel comforted when using this soft material, increasing measurement accuracy. In addition, to monitor the sensor's resistance range and deformation it needs to be tested by the CNC lathe status after the sensor is completed.

Flexible electronics have inspired many novel and very important applications in recent years and various flexible electronic devices such as diodes, transistors, circuits, sensors, and radiofrequency (RF) passive devices including antennas and inductors have been reported. However, the lack of a high-performance RF resonator is one of the key bottlenecks to implement flexible wireless electronics. In this study, for the first time, a novel ultra-flexible structured film bulk acoustic resonator (FBAR) is proposed. The flexible FBAR is fabricated on a flexible polyimide substrate using piezoelectric thin film aluminum nitride (AlN) for acoustic wave excitation. Both the shear wave and longitudinal wave can be excited under the surface interdigital electrodes configuration we proposed. In the case of the thickness extension mode, a flexible resonator with a working frequency as high as of 5.2325 GHz has been realized. The resonators stay fully functional under bending status and after repeated bending and re-flattening operations. This flexible high-frequency resonator will serve as a key building block for the future flexible wireless electronics, greatly expanding the application scope of flexible electronics.

Saline based microfluidic soft pressure sensor utilizing a three-dimensional focused electric field for motion and healthcare monitoring

Several gallium-based liquid metal alloys are liquid at room temperature. As 'liquid', such alloys have a low viscosity and a high surface tension while as 'metal', they have high thermal and electrical conductivities, similar to mercury. However, unlike mercury, these liquid metal alloys have low toxicity and a negligible vapor pressure, rendering them much safer. In comparison to mercury, the distinguishing feature of these alloys is the rapid formation of a self-limiting atomically thin layer of gallium oxide over their surface when exposed to oxygen. This oxide layer changes many physical and chemical properties of gallium alloys, including their interfacial and rheological properties, which can be employed and modulated for various applications in microfluidics. Injecting liquid metal into microfluidic structures has been extensively used to pattern and encapsulate highly deformable and reconfigurable electronic devices including electrodes, sensors, antennas, and interconnects. Likewise, the unique features of liquid metals have been employed for fabricating miniaturized microfluidic components including pumps, valves, heaters, and electrodes. In this review, we discuss liquid metal enabled microfluidic components, and highlight their desirable attributes including simple fabrication, facile integration, stretchability, reconfigurability, and low power consumption, with promising applications for highly integrated microfluidic systems.

This article reviews properties and characteristics of soft ionic polymer–metal nanocomposites as soft biomimetic multifunctional distributed robotic materials, soft actuators, soft sensors, soft transducers and energy harvesters, and artificial muscles. After presenting some fundamental properties of biomimetic distributed nanosensing and nanoactuation of ionic polymer–metal composites, the discussion extends to some recent advances in manufacturing techniques, 3D fabrication of these soft actuators and sensors and some recent modeling and simulations, sensing and transduction, and product development. The relationships between the properties and their connection to the characteristics of soft actuators and sensors are also discussed.

Aiming at monitoring of lower limb movement, a MEMS integrated chip packaged in a sensing suit and a thin film pressure sensor packaged in a sensing insole are used to collect lower limb motion information. However, it was found that there was a huge error in the measurement of pressure when using the film-based pressure sensors to monitor the foot pressure in the lower limb movement. Hence, this paper describes a method for calibration of soft pressure sensors and characterizes the pressure through the proposed approach. The results show that there is a nonlinear relationship between the input (pressure) and the output (inverse value of the electrical resistance); the fitting error is about 5.5%; the hysteresis is around 4.2%;the zero drift is about 0.11%; the repeatability error is nearly 10.7% and the uncertainty of the sensor is about 14.64%. Therefore, it suggests that there often are notable repeatability errors and sensor uncertainty in the soft thin film pressure sensors, which would affect the accuracy in the monitoring of lower limb movements.

Textile-based pressure sensor arrays: A novel scalable manufacturing technique

Gait analysis plays an important role in various applications such as health care, clinical rehabilitation, sport training and pedestrian navigation. In order to monitor the human gait, an interesting approach is to analyze the foot plantar pressure distribution between the foot and the ground. In recent years, the emergence of flexible, soft and lightweight sensors facilitates the rapid technological advances in in-shoe foot pressure measurements, thereby especially carbon nanotubes-based sensors provide an outstanding solution for the implementation of flexible, soft pressure sensors in foot pressure distribution analysis. This chapter focuses on the design and implementation of multiwalled carbon nanotubes (CNT)/polydimethylsil-oxane (PDMS) based nanocomposite pressure sensors for the analysis of the foot pressure distribution. The sensor is durable, stable and shows sensitivity of 3.3 k\({\Omega }\)/kPa and hysteresis smaller than 3.64% with maximum detectable pressure up to 217 kPa, which is suitable for the measurement of human foot pressure. The proposed sensor has been implemented in a flexible in-sole, which is designed based on normal arch foot anatomy. A total of 12 sensors are distributed in the heel, lateral back foot, midfoot and front foot. The foot pressure distribution for different persons while walking and standing using nanocomposite sensor based in-sole were investigated by measuring the changing in resistance of the pressure sensors, when pressure applied on it. It shows that foot pressure distribution is higher in the fore foot and the heel while person standing in normal position. While walking, initially the foot pressure is in the heel and then transferred to the entire foot and finally it is concentrated on the fore foot.KeywordsCarbon nanotubesPolydimethylsiloxaneNanocompositesPressure sensorsScreen printingSolution mixingGait analysis

The development of flexible electronics (FEs) has rapidly accelerated in numerous fields due to their exceptional deformability, bending, and stretchability. Room‐temperature gallium‐based liquid metals (LMs) are considered as efficient conductive materials for FEs due to their outstanding electrical conductivity and intrinsic flexibility. Recently, 3D printing has become a promising technique for fabricating FEs. However, the poor printability due to high surface tension and fluidity offers huge challenges in the 3D printing of LMs. This review summarizes the effective strategies to address these challenges. It primarily focuses on three points: 1) how to improve the printability of LM and its wettability with the substrate, 2) how to select the appropriate printing method to improve the printing speed and ensure the resolution of printing structure, and 3) how to provide perfect encapsulation for LM‐based FEs with 3D printing. Following a brief introduction, the mainstream printing technologies and recent developments in the 3D printing of LMs are provided, with an emphasis on the selection of printing method, improvement of printability, encapsulation, and conductivity activation. Then, the revolutionary changes attained after 3D printing of LMs are specifically focused upon. Finally, opinions and potential directions for this thriving discipline are explored.

Dry liquid metals stabilized by silica particles: Synthesis and application in photothermoelectric power generation