Recent advances in soft materials and system integration technologies have provided a unique opportunity to design various types of wearable flexible hybrid electronics (WFHE) for advanced human healthcare and human-machine interfaces. The hybrid integration of soft and biocompatible materials with miniaturized wireless wearable systems is undoubtedly an attractive prospect in the sense that the successful device performance requires high degrees of mechanical flexibility, sensing capability, and user-friendly simplicity. Here, the most up-to-date materials, sensors, and system-packaging technologies to develop advanced WFHE are provided. Details of mechanical, electrical, physicochemical, and biocompatible properties are discussed with integrated sensor applications in healthcare, energy, and environment. In addition, limitations of the current materials are discussed, as well as key challenges and the future direction of WFHE. Collectively, an all-inclusive review of the newly developed WFHE along with a summary of imperative requirements of material properties, sensor capabilities, electronics performance, and skin integrations is provided.

Advanced Soft Materials, Sensor Integrations, and Applications of Wearable Flexible Hybrid Electronics in Healthcare, Energy, and Environment.

The fibrillation of pulp fiber was attempted by two methods, a high-pressure homogenizer treatment and a grinder treatment. The grinder treatment resulted in the successful fibrillation of wood pulp fibers into nanofibers. The nanofibers demonstrate promising characteristics as reinforcement material for optically transparent composites. Due to the size effect, the nanofiber-reinforced composite retains the transparency of the matrix resin even at high fiber content such as 70 wt %. Since the nanofiber is an aggregate of semi-crystalline extended cellulose chains, its addition also contributes to a significant improvement in the thermal expansion properties of plastics while maintaining its ease of bending. Cellulose nanofibers have tremendous potential as a future resource since they are produced in a sustainable manner by plants, one of the most abundant organic resources on earth.

Optically transparent composites reinforced with plant fiber-based nanofibers(ABSTRACTS (MASTER THESIS FOR GRADUATE SCHOOL OF AGRICULTURE))

Flexible, stretchable, and bendable materials, including inorganic semiconductors, organic polymers, graphene, and transition metal dichalcogenides (TMDs), are attracting great attention in such areas as wearable electronics, biomedical technologies, foldable displays, and wearable point-of-care biosensors for healthcare. Among a broad range of layered TMDs, atomically thin layered molybdenum disulfide (MoS2) has been of particular interest, due to its exceptional electronic properties, including tunable bandgap and charge carrier mobility. MoS2 atomic layers can be used as a channel or a gate dielectric for fabricating atomically thin field-effect transistors (FETs) for electronic and optoelectronic devices. This review briefly introduces the processing and spectroscopic characterization of large-area MoS2 atomically thin layers. The review summarizes the different strategies in enhancing the charge carrier mobility and switching speed of MoS2 FETs by integrating high-κ dielectrics, encapsulating layers, and other 2D van der Waals layered materials into flexible MoS2 device structures. The photoluminescence (PL) of MoS2 atomic layers has, after chemical treatment, been dramatically improved to near-unity quantum yield. Ultraflexible and wearable active-matrix organic light-emitting diode (AM-OLED) displays and wafer-scale flexible resistive random-access memory (RRAM) arrays have been assembled using flexible MoS2 transistors. The review discusses the overall recent progress made in developing MoS2 based flexible FETs, OLED displays, nonvolatile memory (NVM) devices, piezoelectric nanogenerators (PNGs), and sensors for wearable electronic and optoelectronic devices. Finally, it outlines the perspectives and tremendous opportunities offered by a large family of atomically thin-layered TMDs.

Flexible Molybdenum Disulfide (MoS2) Atomic Layers for Wearable Electronics and Optoelectronics

High-performance flexible strain and pressure sensors are important components of the systems for human motion detection, human–machine interaction, soft robotics, electronic skin, etc., which are envisioned as the key technologies for applications in future human healthcare monitoring and artificial intelligence. In recent years, highly flexible and wearable strain/pressure sensors have been developed based on various materials/structures and transduction mechanisms. Piezoresistive three-dimensional (3D) monolithic conductive sponge, the resistance of which changes upon external pressure or stimuli, has emerged as a forefront material for flexible and wearable pressure sensor due to its excellent sensor performance, facile fabrication, and simple circuit integration. This review focuses on the rapid development of the piezoresistive pressure sensors based on 3D conductive sponges. Various piezoresistive conductive sponges are categorized into four different types and their material and structural charact...

Recent Advances in Flexible and Wearable Pressure Sensors Based on Piezoresistive 3D Monolithic Conductive Sponges.

Machines are systems that harness input power to extend or advance function. Fundamentally, machines are based on the integration of materials with mechanisms to accomplish tasks-such as generating motion or lifting an object. An emerging research paradigm is the design, synthesis, and integration of responsive materials within or as machines. Herein, a particular focus is the integration of responsive materials to enable robotic (machine) functions such as gripping, lifting, or motility (walking, crawling, swimming, and flying). Key functional considerations of responsive materials in machine implementations are response time, cyclability (frequency and ruggedness), sizing, payload capacity, amenability to mechanical programming, performance in extreme environments, and autonomy. This review summarizes the material transformation mechanisms, mechanical design, and robotic integration of responsive materials including shape memory alloys (SMAs), piezoelectrics, dielectric elastomer actuators (DEAs), ionic electroactive polymers (IEAPs), pneumatics and hydraulics systems, shape memory polymers (SMPs), hydrogels, and liquid crystalline elastomers (LCEs) and networks (LCNs). Structural and geometrical fabrication of these materials as wires, coils, films, tubes, cones, unimorphs, bimorphs, and printed elements enables differentiated mechanical responses and consistently enables and extends functional use.

Materials as Machines

It is a challenge to synthesize all-in-one molecular networks that are autonomously self-healable over a wide temperature range (from subzero to high), transparent, stretchable, and conductive. Her...

Autonomous Self-Healing, Antifreezing, and Transparent Conductive Elastomers

Underwater vibration detection is of great importance in personal safety, environmental protection, and military defense. Sealing layers are required in many underwater sensor architectures, leading to limited working-life and reduced sensitivity. Here, a flexible, superhydrophobic, and conductive tungsten disulfide (WS2) nanosheets-wrapped sponge (SCWS) is reported for the high-sensitivity detection of tiny vibration from the water surfaces and from the grounds. When the SCWS is immersed in water, a continuous layer of bubbles forms on its surfaces, providing the sensor with two special abilities. One is sealing-free feature due to the intrinsic water-repellent property of SCWS. The other is functioning as a vibration-sensitive medium to convert mechanical energy into electric signals through susceptible physical deformation of bubbles. Therefore, the SCWS can be used to precisely detect tiny vibration of water waves, and even sense those caused by human footsteps, demonstrating wide applications of this amphibious (water/ground) vibration sensor. Results of this study can initiate the exploration of superhydrophobic materials with elastic and conductive properties for underwater flexible electronic applications.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201700655

Superhydrophobic WS2-Nanosheet-Wrapped Sponges for Underwater Detection of Tiny Vibration.

A new type of supramolecular elastomer, photopolymerized from two rationally designed polymerizable deep eutectic solvent (PDES) monomers, was reported. The resultant network was highly transparent, intrinsically conductive, stretchable, and self-healable. In particular, the healing process can even take place in various organic solvents, simultaneously with enhanced adaptability.

Transparent conductive elastomers with excellent autonomous self-healing capability in harsh organic solvent environments

Conductive elastomers (CEs) with strong mechanical properties have been fabricated and used in flexible electronics. However, the development of CEs with both super-high mechanical strength and extreme stretchability remains challenging. This paper reports on the development of a series of mechanically strong and tough CEs based on photopolymerizable deep eutectic solvents (PDESs) with dense hydrogen bonding interactions. One of these CEs exhibits the highest reported mechanical performance for elastomers, with tensile strength, strain at break and toughness up to 31.21 MPa, 3645% and 615 MJ m−3, respectively. Notably, this CE tolerates puncture and can lift a 10.5 kg weight (9500 times its own weight) without any failure. In addition, the series of CEs also shows excellent transparency (>94% in the visible range), favorable conductivity (0.007–0.04 S m−1), tunable stretchability (strain from 22% to 6164%) and good self-healing capability (electrical healing efficiency of 99% within 0.26 s). In view of the comprehensive properties of the CEs, we demonstrate the practical suitability of CEs by including them in a strain sensor to detect human motion. Based on their extremely facile preparation process and practical functionalities, we believe that CEs will not only contribute new methods for the development of mechanically strong conductors, but also open up novel avenues for flexible electronics.

Kaili Zhang

Papers

Autonomous Self-Healing, Antifreezing, and Transparent Conductive Elastomers

Superhydrophobic WS2-Nanosheet-Wrapped Sponges for Underwater Detection of Tiny Vibration.

Transparent conductive elastomers with excellent autonomous self-healing capability in harsh organic solvent environments

Polymerizable deep eutectic solvent-based mechanically strong and ultra-stretchable conductive elastomers for detecting human motions

Green polymerizable deep eutectic solvent (PDES) type conductive paper for origami 3D circuits