Highly Stretchable Conductor by Self-Assembling and Mechanical Sintering of a 2D Liquid Metal on a 3D Polydopamine-Modified Polyurethane Sponge

Design and 3D Printing of Stretchable Conductor with High Dynamic Stability

As an indispensable part of wearable devices and mechanical arms, stretchable conductors have received extensive attention in recent years. The design of a high-dynamic-stability, stretchable conductor is the key technology to ensure the normal transmission of electrical signals and electrical energy of wearable devices under large mechanical deformation, which has always been an important research topic domestically and abroad. In this paper, a stretchable conductor with a linear bunch structure is designed and prepared by combining numerical modeling and simulation with 3D printing technology. The stretchable conductor consists of a 3D-printed bunch-structured equiwall elastic insulating resin tube and internally filled free-deformable liquid metal. This conductor has a very high conductivity exceeding 10⁴ S cm^-1, good stretchability with an elongation at break exceeding 50%, and great tensile stability, with a relative change in resistance of only about 1% at 50% tensile strain. Finally, this paper demonstrates it as a headphone cable (transmitting electrical signals) and a mobile phone charging wire (transmitting electrical energy), which proves its good mechanical and electrical properties and shows good application potential.

Liquid Metal Patterning and Unique Properties for Next-Generation Soft Electronics

Room-temperature liquid metal (LM)-based electronics is expected to bring advancements in future soft electronics owing to its conductivity, conformability, stretchability, and biocompatibility. However, various difficulties arise when patterning LM because of its rheological features such as fluidity and surface tension. Numerous attempts are made to overcome these difficulties, resulting in various LM-patterning methods. An appropriate choice of patterning method based on comprehensive understanding is necessary to fully utilize the unique properties. Therefore, the authors aim to provide thorough knowledge about patterning methods and unique properties for LM-based future soft electronics. First, essential considerations for LM-patterning are investigated. Then, LM-patterning methods-serial-patterning, parallel-patterning, intermetallic bond-assisted patterning, and molding/microfluidic injection-are categorized and investigated. Finally, perspectives on LM-based soft electronics with unique properties are provided. They include outstanding features of LM such as conformability, biocompatibility, permeability, restorability, and recyclability. Also, they include perspectives on future LM-based soft electronics in various areas such as radio frequency electronics, soft robots, and heterogeneous catalyst. LM-based soft devices are expected to permeate the daily lives if patterning methods and the aforementioned features are analyzed and utilized.

Maple Leaf Inspired Conductive Fiber with Hierarchical Wrinkles for Highly Stretchable and Integratable Electronics

Stretchable and durable conductors are significant to the development of wearable devices, robots, human-machine interfaces, and other artificial intelligence products. However, the desirable strain-insensitive conductivity and low hysteresis are restricted by the failure of stretchable structures and mismatch of mechanical properties (rigid conductive layer and elastic core substrate) under large deformation. Here, based on the principles of fractal geometry, a stretchable conductive fiber with hierarchical wrinkles inspired by the unique shape of the maple leaf was fabricated by combining surface modification, interfacial polymerization, and improved prestrain finishing methods to break through this dilemma. The shape and size of wrinkles predicted by buckling analysis via the finite element method fit well with that of actual wrinkles (30-80 μm of macro wrinkles and 4-6 μm of micro wrinkles) on the fabricated fiber. Such hierarchically wrinkled conductive fiber (HWCF) exhibited not only excellent strain-insensitive conductivity denoted by the relative resistance change ΔR/R₀ = 0.66 with R₀ the original resistance and ΔR the change of resistance after the concrete strain reaching up to 600%, but also low hysteresis (0.04) calculated by the difference in area between stretching and releasing curve of the ΔR/R₀ strain under 300% strain and long-term durability (>1000 stretching-releasing cycles). Furthermore, the elastic conductive fiber with such a bionic structure design can be applied as highly stretchable electrical circuits for illumination and monitors for the human motion under large strains through tiny and rapid resistance changes as well. Such a smart biomimetic material holds great prospects in the field of stretchable electronics.

Influence of microstructural alterations of liquid metal and its interfacial interactions with rubber on multifunctional properties of soft composite materials

Liquid metal (LM)-based polymer composites are currently new breakthrough and emerging classes of soft multifunctional materials (SMMs) having immense transformative potential for soft technological applications. Currently, room-temperature LMs, mostly eutectic gallium‑indium and Galinstan alloys are used to integrate with soft polymer due to their outstanding properties such as high conductivity, fluidity, low adhesion, high surface tension, low cytotoxicity, etc. The microstructural alterations and interfacial interactions controlling the efficient integration of LMs with rubber are the most critical aspects for successful implementation of multifunctionality in the resulting material. In this review article, a fundamental understanding of microstructural alterations of LMs to the formation of well-defined percolating networks inside an insulating rubber matrix has been established by exploiting several existing theoretical and experimental studies. Furthermore, effects of the chemical modifications of an LM surface and its interfacial interactions on the compatibility between solid rubber and fluid filler phase have been discussed. The presence of thin oxide layer on the LM surface and the effects and challenges it poses to the adequate functionalization of these materials have been discussed. Plausible applications of SMMs in different soft matter technologies, like soft robotics, flexible electronics, soft actuators, sensors, etc. have been provided. Finally, the current technical challenges and further prospective to the development of SMMs using non‑silicone rubbers have been critically discussed. This review is anticipated to infuse a new impetus to the associated research communities for the development of next generation SMMs.

Construction of liquid metal-based soft microfluidic sensors via soft lithography

Background

Liquid metal (LM) can be integrated into microfluidic channel, bringing new functionalities of microfluidics and opening a new window for soft microfluidic electronics, due to the superior advantages of the conductivity and deformability of LMs. However, patterning the LMs into microfluidic channels requires either selective surface wetting or complex fabrication process.

Results

In this work, we develop a method to pattern the LMs onto the soft elastomer via soft lithographic process for fabrication of soft microfluidic sensors without the surface modification, bulky facilities, and complicated processes. The combination of the interfacial hydrogen bond and surface tension enables the LM patterns transfer to the soft elastomer. The transferred LM patterns with an ellipse-like cross-section further improve the stability under the mechanical deformation. Three proof-of-concept experiments were conducted to demonstrate the utilization of this method for development of thermochromic sensors, self-powered capacity sensors and flexible biosensor for glucose detection.

Conclusions

In summary, the proposed method offers a new patterning method to obtain soft microfluidic sensors and brings new possibilities for microfluidics-related wearable devices.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01471-0.

Synthesis of a grape-like conductive carbon black/Ag hybrid as the conductive filler for soft silicone rubber

Conductive silicone rubber (CSR) is an outstanding stretchable conductive composite due to its excellent mechanical properties and stable conductivity. In this paper, silver nanoparticles were deposited on carbon black (CB) through a reduction reaction. The uniform dispersion of silver particles on the surface of CB as well as the grape-like branch structure of hybrid particles was formed by the condensation reaction of the hydroxyl groups of CB with (3-mercaptopropyl) trimethoxysilane (KH-590), along with the interattraction between sulfhydryl groups of KH-590 and silver ions. This sulfhydryl modified conductive carbon black/Ag hybrid filler (SMCB@Ag) avoided the high processing viscosity of CSR caused by the hydroxyl groups of CB. The percolation threshold of CSR made from SMCB@Ag was 5.5 wt% according to the percolation equation. With the addition amount of SMCB@Ag increasing to 10 wt%, the conductivity of CSR increased from 10^-5 to about 10¹. Moreover, the conductivity of this CSR showed excellent stability with extension of storage time and increase of stretching-recovery cycles.

Tailorable, Lightweight and Superelastic Liquid Metal Monoliths for Multifunctional Electromagnetic Interference Shielding

A confined thermal expansion strategy to fabricate liquid metal (LM)-based monoliths with continuous LM network at ultra-low content. The results show a strong integration advantage of LM-based monoliths in density, mechanical strength, electromagnetic interference shielding effectiveness, and near field shielding effectiveness, as well as multi-functions such as magnetic actuation.

ABSTRACT

Liquid metal (LM) has become an emerging material paradigm in the electromagnetic interference shielding field owing to its excellent electrical conductivity. However, the processing of lightweight bulk LM composites with finite package without leakage is still a great challenge, due to high surface tension and pump-out issues of LM. Here, a novel confined thermal expansion strategy based on expandable microsphere (EM) is proposed to develop a new class of LM-based monoliths with 3D continuous conductive network. The EM/LM monolith (EM/LMm) presents outstanding performance of lightweight like metallic aerogel (0.104 g cm⁻¹), high strength (3.43 MPa), super elasticity (90% strain), as well as excellent tailor ability and recyclability, rely on its unique gas-filled closed-cellular structure and refined LM network. Moreover, the assembled highly conducting EM/LMm exhibits a recorded shielding effectiveness (98.7 dB) over a broad frequency range of 8.2–40 GHz among reported LM-based composites at an ultra-low content of LM, and demonstrates excellent electromagnetic sealing capacity in practical electronics. The ternary EM/LM/Ni monoliths fabricated by the same approach could be promising universal design principles for multifunctional LM composites, and applicable in magnetic responsive actuator. [Image: see text]

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s40820-021-00766-5.

Highly Stretchable and Conductive Carbon Fiber/Polyurethane Conductive Films Featuring Interlocking Interfaces

Stretchable conductors are essential assembly units of next-generation flexible electronics, requiring excellent conductivity and stretchability simultaneously. However, poor interfacial adhesion between conductive fillers and polymer matrixes often triggers the relative slippage and dislocation of the conductive network, deteriorating the final conductivity. Herein, we constructed interlocking interfaces in a polyurethane (PU) conductive composite by introducing brush-like carbon fibers (CFs) with laterally grown zinc oxide nanowires (ZnO NWs). The ZnO NW-enabled construction of the functional interfaces integrated the CFs tightly with the polymer matrix to greatly improve the interfacial adhesion and suppress the sliding displacement of conductive fillers upon external load, contributing to excellent mechanical strength and conductive stability. Specifically, the combination of high mechanical strength (7.19 MPa) and stable conductivity (26.3 S/m under 100% strain, approaching 30 S/m of the initial conductivity) was demonstrated for the brush-like CF/PU film. Finally, the application potential of the novel stretchable conductor as a thermal therapy unit and connecting wire in a flexible circuit was explored successfully under complex dynamic deformations. Accordingly, this inspiring result creatively combines the interface geometry with conductive stability, and offers a facile and effective route to prepare excellent stretchable conductors, which can be easily applied to other conductive composites.

Superelastic, Fatigue-Resistant, and Flame-Retardant Spongy Conductor for Human Motion Detection against a Harsh High-Temperature Condition

The construction of wearable piezoresistive sensors with high elasticity, large gauge factor, and excellent durability in a harsh high-temperature environment is highly desired yet challenging. Here, a lightweight, superelastic, and fatigue-resistant spongy conductor was fabricated via a sponge-constrained network assembly, during which highly conductive graphene and flame-retardant montmorillonite were alternatively deposited on a three-dimensional melamine scaffold. The as-obtained spongy conductor exhibited a highly deformation-tolerant conductivity up to 80% strain and excellent fatigue resistance of 10,000 compressive cycles at 70% strain. As a result, the spongy conductor can readily work as a piezoresistive sensor and exhibited a high gauge factor value of ∼2.3 in a strain range of 60-80% and excellent durability under 60% strain for 10,000 cycles without sacrificing its piezoresistive performance. Additionally, the piezoresistive sensor showed great thermal stability up to 250 °C for more than 7 days and sufficient flame-retardant performance for at least 20 s. This lightweight, superelastic, and flame-retardant spongy conductor reveals tremendous potential in human motion detection against a harsh high-temperature environment.

Metal-Containing Polydopamine Nanomaterials: Catalysis, Energy, and Theranostics

Polydopamine (PDA) is a major type of artificial melanin material with many interesting properties such as antioxidant activity, free-radical scavenging, high photothermal conversion efficiency, and strong metal-ion chelation. The high affinity of PDA to a wide range of metals/metal ions has offered a new class of functional metal-containing polydopamine (MPDA) nanomaterials with promising functions and extensive applications. Understanding and controlling the metal coordination environment is vital to achieve desirable functions for which such materials can be exploited. MPDA nanomaterials with metal/metal ions as the active functions are reviewed, including their synthesis and metal coordination environment and their applications in catalysis, batteries, solar cells, capacitors, medical imaging, cancer therapy, antifouling, and antibacterial coating. The current trends, limitations, and future directions of this area are also explored.