Hydrogels have been attracting increasing attention for application in wearable electronics, due to their intrinsic biomimetic features, highly tunable chemical-physical properties (mechanical, electrical, etc.), and excellent biocompatibility. Among many proposed varieties of hydrogels, conductive polymer-based hydrogels (CPHs) have emerged as a promising candidate for future wearable sensor designs, with capability of realizing desired features using different tuning strategies ranging from molecular design (with a low length scale of 10-10 m) to a micro-structural configuration (up to a length scale of 10-2 m). However, considerable challenges remain to be overcome, such as the limited strain sensing range due to the mechanical strength, the signal loss/instability caused by swelling/deswelling, the significant hysteresis of sensing signals, the de-hydration induced malfunctions, and the surface/interfacial failure during manufacturing/processing. This review aims to offer a targeted scan of recent advancements in CPH based wearable sensor technology, from the establishment of dedicated structure-property relationships in the lab to the advanced manufacturing routes for potential scale-up production. The application of CPHs in wearable sensors is also explored, with suggested new research avenues and prospects for CPHs in the future also included.

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Conductive polymer based hydrogels and their application in wearable sensors: a review.

Socio-economic aspects of hydrogen energy: An integrative review

A Flexible, Stretchable and Triboelectric Smart Sensor Based on Graphene Oxide and Polyacrylamide Hydrogel for High Precision Gait Recognition in Parkinsonian and Hemiplegic Patients

Metal‐Organic Framework Reinforced Highly Stretchable and Durable Conductive Hydrogel‐Based Triboelectric Nanogenerator for Biomotion Sensing and Wearable Human‐Machine Interfaces

Polymer-based triboelectric nanogenerators: materials, characterization, and applications

Comparative study on the contact-separation mode triboelectric nanogenerator

Hydrogels are three-dimensional polymeric networks that exhibit impressive flexibility through appropriate chemical modification. Hydrogels can be prepared from natural or synthetic polymers which have shown promise in forming different components of stretchable electronic devices such as sensors and energy harvesters. Extensive efforts have been dedicated to creating novel designs, syntheses, and applications of hydrogels. This review presents a classification of hydrogels and describes the main approaches used to develop hydrogels. Moreover, the recent application of hydrogels in creating sensors, including hydrogel-based strain, gas, humidity, pH, and temperature sensors. Next, the current application of hydrogels as soft electrodes for energy harvesters is discussed. In addition, low-temperature tolerant, ultraflexible, biocompatible, and stretchable energy harvesters are illustrated by considering different compositions of hydrogels. The key challenges associated with the creation of hydrogel-based sensors and energy harvesters have been identified and discussed. Finally, a guideline for overcoming the current issues with hydrogel development and applications is provided for future research. 

Recent Progress in Hydrogel-based Sensors and Energy Harvesters

To successfully insert a microprobe into the brain, it must be stiff enough to tolerate the penetration force. In contrast, due to the brain micromotion during the use of the microprobe (operation), the mechanical mismatch between the stiff microprobe and soft surrounding neural tissue leads to tissue damage and, ultimately, the failure of the microprobe. In this study, an innovative design is proposed to create a binary stiffness compliant neural microprobe. The microprobe and surrounding neural tissue are simulated to calculate the microprobe’s equivalent elastic moduli and critical buckling force. Moreover, the induced strain on the tissue by the brain longitudinal and lateral micromotions is investigated based on the finite element method. To evaluate the microprobe’s efficiency compared to existing ones, the simulation is replicated for a cylindrical microprobe with the same diameter and length, fabricated from polyimide and coated with a neuro-integrative material. Based on the obtained results, the proposed microprobe’s elastic modulus drops from ≈ 4.2 GPa during insertion to ≈ 40 kPa during operation, depending on the force/motion applied. The corresponding critical buckling force is ≈ 267 mN. The maximum strain on the tissue around the proposed microprobe is ≈ 13% and ≈ 69% less than that of the polyimide microprobe, completely bonded to the surrounding tissue, during the brain longitudinal and lateral motions, respectively. The microprobe is fabricated based on two-photon polymerisation technology, characterised, and inserted into a lamb brain to confirm the feasibility of the proposed design. The proposed microprobe’s stiffness changes independently of its material and the surrounding environment’s characteristics. 

A binary stiffness compliant neural microprobe

Utilising a flexible intracortical microprobe to record/stimulate neurons minimises the incompatibility between the implanted microprobe and the brain, reducing tissue damage due to the brain micromotion. Applying bio-dissolvable coating materials temporarily makes a flexible microprobe stiff to tolerate the penetration force during insertion. However, the inability to adjust the dissolving time after the microprobe contact with the cerebrospinal fluid may lead to inaccuracy in the microprobe positioning. Furthermore, since the dissolving process is irreversible, any subsequent positioning error cannot be corrected by re-stiffening the microprobe. The purpose of this study is to propose an intracortical microprobe that incorporates two compressible structures to make the microprobe both adaptive to the brain during operation and stiff during insertion. Applying a compressive force by an inserter compresses the two compressible structures completely, resulting in increasing the equivalent elastic modulus. Thus, instant switching between stiff and soft modes can be accomplished as many times as necessary to ensure high-accuracy positioning while causing minimal tissue damage. The equivalent elastic modulus of the microprobe during operation is ≈ 23 kPa, which is ≈ 42% less than the existing counterpart, resulting in ≈ 46% less maximum strain generated on the surrounding tissue under brain longitudinal motion. The self-stiffening microprobe and surrounding neural tissue are simulated during insertion and operation to confirm the efficiency of the design. Two-photon polymerisation technology is utilised to 3D print the proposed microprobe, which is experimentally validated and inserted into a lamb's brain without buckling. 

A self-stiffening compliant intracortical microprobe

Polyacrylamide-Lithium Chloride-MXene Based Conductive Hydrogel as Stretchable Strain Sensor and Triboelectric Nanogenerator

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