Epidermal Strain Sensor Research Articles

Fast and Facile Liquid Metal Printing via Projection Lithography for Highly Stretchable Electronic Circuits.

Soft electronic circuits are crucial for wearable electronics, biomedical technologies, and soft robotics, requiring soft conductive materials with high conductivity, high strain limit, and stable electrical performance under deformation. Liquid metals (LMs) have become attractive candidates with high conductivity and fluidic compliance, while effective manufacturing methods are demanded. Digital light processing (DLP)-based projection lithography is a high-resolution and high-throughput printing technique for primarily polymers and some metals. If LMs can be printed with DLP as well, the entire soft devices can be fabricated by one printer in a streamlined and highly efficient process. Herein, fast and facile DLP-based LM printing is achieved. Simply with 5-10 s of patterned ultraviolet (UV)-light exposure, a highly conductive and stretchable pattern can be printed using a photo-crosslinkable LM particle ink. The printed eutectic gallium indium traces feature high resolution (≈20 µm), conductivity (3 × 106 S m-1), stretchability (≈2500%), and excellent stability (consistent performance at different deformation). Various patterns are printed in diverse material systems for broad applications including stretchable displays, epidermal strain sensors, heaters, humidity sensors, conformal electrodes for electrography, and multi-layer actuators. The facile and scalable process, excellent performance, and diverse applications ensure its broad impact on soft electronic manufacturing.

3D Printing of Electrically Conductive and Degradable Hydrogel for Epidermal Strain Sensor

3D Printing of Electrically Conductive and Degradable Hydrogel for Epidermal Strain Sensor

Seconds Timescale Synthesis of Highly Stretchable Antibacterial Hydrogel for Skin Wound Closure and Epidermal Strain Sensor.

Effective wound healing is critical for patient care, and the development of novel wound dressing materials that promote healing, prevent infection, and are user-friendly is of great importance, particularly in the context of point-of-care testing (POCT). This study reports the synthesis of a hydrogel material that can be produced in less than 10 s and possesses antibacterial activity against both gram-negative and gram-positive microorganisms, as well as the ability to inhibit the growth of eukaryotic cells, such as yeast. The hydrogel is formed wholly based on covalent-like hydrogen bonding interactions and exhibits excellent mechanical properties, with the ability to stretch up to more than 600% of its initial length. Furthermore, the hydrogel demonstrates ultra-fast self-healing properties, with fractures capable of being repaired within 10 s. This hydrogel can promote skin wound healing, with the added advantage of functioning as a strain sensor that generates an electrical signal in response to physical deformation. The strain sensor composed of a rubber shell realizes fast and responsive strain sensing. The findings suggest that this hydrogel has promising applications in the field of POCT for wound care, providing a new avenue for improved patient outcomes.

3D printing of electrically conductive and degradable hydrogel for epidermal strain sensor

Owing to superior electrical conductivity, stretchability, and biocompatibility, electrically conductive hydrogels have been widely applied in flexible wearable strain sensors. Generally, conductive fillers are required to be modified to enhance the dispersion stability in hydrogels, increasing the complexity of the experiment. Additionally, due to the intrinsic chemical and physical crosslinking networks, traditional hydrogels are not degradable, resulting in severe environmental pollution problems. Herein, we designed an electrically conductive and degradable hydrogel for the epidermal strain sensor through a facile digital light processing (DLP) three-dimensional (3D) printing technology. A three-step strategy of ultraviolet (UV) curing, ion sputtering, and water spraying was innovatively developed for alleviating the complexity and difficulty in preparing electronic-based hydrogels, and a mono-functional resin curing system was employed to endow hydrogels with promising degradability. This work opens a new path for the preparation of electrically conductive hydrogels and addresses the issues of environmental pollution caused by electronic waste.

Cross-Links-Entanglements Integrated Networks Contributing to Highly Resilient, Soft, and Self-Adhesive Elastomers with Low Hysteresis for Green Wearable Electronics.

Green wearable electronics are attracting increasing attention to eliminate harmful byproducts generated by traditional devices. Although various degradable materials have been explored for green wearable electronics, the development of degradable elastomers with integrated characteristics of low modulus, self-adhesion, high resilient, and low hysteresis remains challenging. In this work, a degradable elastomer poly(1,8-octanediol-co-citrate-co-caprolactone) (POCL) is reported, in which a loosely cross-linked network contains plenty of entangled flexible chains. The coexistence of covalent cross-links and entanglements of long polymer chains endows the elastomer with good resilience and low hysteresis, in addition to low modulus and self-adhesion. Taking advantage of the unique mechanical properties, epidermal strain sensors based on the POCL elastomer were prepared, which exhibited good adhesion to human skin, high sensitivity, high response rate, and excellent fatigue resistance. We also fabricated stretchable electroluminescent devices using this degradable elastomer and demonstrated the recyclability of the nondegradable materials in the electronic device.

Conductive and eco-friendly gluten/MXene composite organohydrogels for flexible, adhesive, and low-temperature tolerant epidermal strain sensors

Conductive hydrogels have drawn tremendous attention in the field of artificial intelligence, health monitoring, and soft robotics. However, combining hydrogels with conductivity, self-adhesiveness, low-temperature tolerance, biocompatibility, and biodegradability in green and low-cost ways remains a great challenge. Hence, the conductive MXene nanosheets were introduced into gluten networks by simple cooking inspired processes and solvents exchanged with glycerol/water binary solution to fabricate conductive organohydrogels (GMOHx). The GMOHx were conductive (5 ×10−4 S/m), flexible, adhesive to various surfaces (a max lap-shear strength of ~ 45.9 KPa), low-temperature tolerant (conductive and flexible under −20 °C), biocompatible, and easily degradable (degraded in neutral proteinase dispersion within 2 days), which can be applied as epidermal strain sensors to detect both large (e.g., elbow, wrist, and knee bending) and subtle (e.g., swallowing) scale human motions with accurate and long-term stable signals even under sub-zero temperatures.

Anti-freezing, water-retaining, conductive, and strain-sensitive hemicellulose/polypyrrole composite hydrogels for flexible sensors

High-performance conductive hydrogels are desired for the development of flexible wearable sensors. However, it remains a challenge to simultaneously realize high stretchable, freezing tolerance, and attractive conductivity through a simple approach. In this work, we present stretchable, water retention, anti-freezing, and conductive hemicellulose/polypyrrole (H/PPY) composite hydrogels for application in epidermal strain sensors via freezing-thawing method. The composite hydrogels were obtained using hemicellulose as a hydrophilic carrier and polypyrrole as a conductive matrix. The freezing and thawing temperatures of the hydrogels decreased and the conductivity increased benefiting from the introduction of glycerol and NaCl. The hydrogels had good mechanical properties, remodeling ability and water retention after storage for 7 days. In addition, the strain sensors assembled by hydrogels accurately monitored the signals of human motions with good strain sensing durability. It is expected that the as-prepared hydrogels have the potential to be applied in the fields of wearable devices.

An adhesive and self-healable hydrogel with high stretchability and compressibility for human motion detection

Hydrogel strain sensors have attracted tremendous interest in artificial intelligence and human health monitoring for their large stretchability, good biocompatibility and self-healing capability. However, most epidermal strain sensors suffer from a poor device-skin interface, resulting in unreliability in the signal recording. Here, we prepared a self-adhesive, stretchable and compressible hydrogel consisting of dynamic cross-linked multi-walled carbon nanotubes , polyvinyl alcohol and polyacrylamide. The hydrogel formed a stable device-skin interface without any fixation assistance and thus allowed stable human motion signal recording from walking to facial expression. The hydrogel sensors exhibited a large sensing range of 500 % with a high gauge factor of 4.02 at 300–500 % tensile strain and a compressibility up to 70 %. In addition, the hydrogel is self-healable, and the conductivity of the hydrogel can be restored within 730 ms. At last, the hydrogel sensors were applied in human motion detection from vital throat vibration to walking, which paves the way for their practical applications in health monitoring and human-machine interactions.

Noninvasive Flow Monitoring in Simple Flow Phantom Using Resistive Strain Sensors.

In this paper, we introduce a monitoring method for flow expansion and contraction in a simple flow phantom based on electrical resistance changes in an epidermal strain sensor attached to the phantom. The flow phantom was fabricated to have a nonflat surface and small modulus that are analogous to human skin. The epidermal sensors made of polydopamine and polyvinyl alcohol show sufficient linearity (R = 0.9969), reproducibility, and self-adhesion properties, as well as high sensitivity to small modulus measurements (<1% tensile strain). Pulsatile flow monitoring experiments were performed by placing the epidermal sensor on the flow phantom and measuring the relative changes in resistance by the heartbeat. Experiments were conducted for three types of vessel diameters (1.5, 2, and 3 mm). In each of the experiments, the vessels were divided into Top, Middle, and Bottom positions. Experiments for each position show that the relative changes in resistance increase proportionally with the diameter of the vessel. The vessels located close to the epidermal layer have greater relative electrical changes. The results were analyzed using the Bernoulli equation and hoop stress formula. This study demonstrates the feasibility of a noninvasive flow monitoring method using a novel resistive strain sensor.

Bioinspired, Microstructured Silk Fibroin Adhesives for Flexible Skin Sensors.

Wearable epidermal sensors are of great importance to the next generation of personalized healthcare. The adhesion between the flexible sensor and skin surface is critical for obtaining accurate, reliable, and stable signals. Herein we present a facile approach to fabricate a microstructured, natural silk fibroin protein-based adhesive for achieving highly conformal, comfortable, adjustable, and biocompatible adhesion on the skin surface. The microstructured fibroin adhesive (MSFA) exhibits reliable and stable bonding force on skin surfaces, even under humid or wet conditions, and can be easily peeled off from the skin without causing significant pain. Such an MSFA can greatly improve the sensitivity and reusability of epidermal strain sensors because of its conformal and tunable adhesion on skin surfaces. The MFSA has a great potential to be applied as a functional adhesive for various epidermal electronic sensors in the era of personalized healthcare.

Biomimetic epidermal sensors assembled from polydopamine-modified reduced graphene oxide/polyvinyl alcohol hydrogels for the real-time monitoring of human motions.

Conductive hydrogel-based epidermal strain sensors can generate repeatable electrical changes upon mechanical deformations for indication of the skin's physiological condition. However, this remains challenging for many conductive hydrogel sensors due to biomechanical mismatch with skin tissues and an unstable resistance variation response, resulting in non-conformable deformations with the epidermis and dermis, and consequently generating inaccurate monitoring of human movements. Herein, a conductive hydrogel that highly matches the skin is fabricated from dynamically hydrogen-bonded nanocrystallites of polydopamine-modified reduced graphene oxide (PDA-rGO) nanosheets composited with polyvinyl alcohol, namely the PDA-rGO/PVA hydrogel. PDA-rGO provides a large number of dynamic hydrogen-bonding interactions in the hydrogel, resulting in a skin-matching modulus (78 kPa) and stretchability. Moreover, the resultant hydrogel possesses excellent cytocompatibility and conductivity (0.87 S m-1), high sensitivity (gauge factor of compression: 20) at low strain and outstanding linearity at high strain as well as a stable resistance variation response. These desirable properties enable the application of the PDA-rGO/PVA hydrogel as a skin-friendly wearable sensor for real-time and accurate detection of both large-scale joint movements and tiny physiological signals, including the bending and relaxing of fingers, the wrist, elbow and knee joints, and wrist pulse and swallowing. Moreover, this hydrogel is integrated into a 2D sensor array that monitors strains or pressures in two dimensions, which is promising for electronic skin, biosensors, human-machine interfaces, and wearable electronic devices.

High-performance epidermal strain sensor based on macro-defect graphene foams

Three-dimensional (3D) epidermal sensing components featuring broad working range, excellent sensitivity and high signal-to-noise ratio (SNR), have attracted great interest for the fast developing wearable electronic systems. Herein, an efficient optimization of the sensitivity of graphene foams (GrF) based strain sensors is reported by making macroporous structure in the framework. The macron-scale hole makes the GrF more fragile points and leads to the resistance change signal more detectable under micro-deformation. The analysis of the comparative experiments shows that the macroporous structure could significantly increase the sensitivity of the 3D graphene-elastomer composite, endowing the piezoresistive sensor with a maximum gauge factor (GF) as high as 30 (about 10 times of pure GrF device) and a wide sensing range up to 20 %. In addition, the stretchable and biocompatible strain sensor can be easily attached on human skin as the wearable electronics, which shows ultrahigh SNR in real-time detection of human motion and typical acoustic waves. The rational design of macroporous structure provides an efficient, economical, and environmentally friendly strategy for electromechanical properties optimization, which may be valuable for designing and fabricating other high performance smart electronics.

High-Strength, Self-Adhesive, and Strain-Sensitive Chitosan/Poly(acrylic acid) Double-Network Nanocomposite Hydrogels Fabricated by Salt-Soaking Strategy for Flexible Sensors.

As a promising functional material, hydrogels have attracted extensive attention, especially in flexible wearable sensor fields, but it remains a great challenge to facilely integrate excellent mechanical properties, self-adhesion, and strain sensitivity into a single hydrogel. In this work, we present high in strength, stretchable, conformable, and self-adhesive chitosan/poly(acrylic acid) double-network nanocomposite hydrogels for application in epidermal strain sensor via in situ polymerization of acrylic acid in chitosan acid aqueous solution with tannic acid-coated cellulose nanocrystal (TA@CNC) acting as nanofillers to reinforce tensile properties, followed by a soaking process in a saturated NaCl solution to cross-link chitosan chains. With addition of a small amount of TA@CNC, the double-network nanocomposite hydrogels became highly adhesive and mechanically compliant, which were critical factors for the development of conformable and resilient wearable epidermal sensors. The salt-soaking process was applied to cross-link chitosan chains by shielded electrostatic repulsions between positively charged amino groups, drastically enhancing the mechanical properties of the hydrogels. The obtained double-network nanocomposite hydrogels exhibited excellent tunable mechanical properties that could be conveniently tailored with fracture stress and fracture strain ranging from 0.39 to 1.2 MPa and 370 to 800%, respectively. Besides, the hydrogels could be tightly attached onto diverse substrates, including wood, glass, plastic, polytetrafluoroethylene, glass, metal, and skin, demonstrating high adhesion strength and compliant adhesion behavior. In addition, benefiting from the abundant free ions from strong electrolytes, the flexible hydrogel sensors demonstrated stable conductivity and strain sensitivity, which could monitor both large human motions and subtle motions. Furthermore, the antibacterial property originating from chitosan made the hydrogels suitable for wearable epidermal sensors. The facile soaking strategy proposed in this work would be promising in fabricating high-strength multifunctional conductive hydrogels used for wearable epidermal devices.

High-Performance Liquid Alloy Patterning of Epidermal Strain Sensors for Local Fine Skin Movement Monitoring.

Nowadays, stretchable/epidermal electronics based on liquid alloys has attracted more and more attention, and various processing techniques have subsequently been developed to demonstrate diverse applications never seen before. However, to fully exploit its potential advantages, epidermal electronics is still searching for a technique meeting all demands on resolution, pattern complexity, and operational flexibility. In this study, we propose a technique that allows for complex and high-density patterns on thin stretchable substrates by combining ultraviolet laser patterning of a modified water-soluble mask, atomized spray deposition of liquid alloys on a flexible temporary substrate, lift-off by water dissolving, and finally, component integration and encapsulation. With this new technique, it was possible to make epidermal precision strain sensors with liquid alloy patterns of high density, which were capable of monitoring fine local skin movements such as the detailed process of wrinkle formation as well as the overall motion of the body part. In addition, this process is highly efficient and well controllable, with high potential for possible industrial automation and massive production.

Highly stretchable and ultrathin nanopaper composites for epidermal strain sensors

Multifunctional electronics are attracting great interest with the increasing demand and fast development of wearable electronic devices. Here, we describe an epidermal strain sensor based on an all-carbon conductive network made from multi-walled carbon nanotubes (MWCNTs) impregnated with poly(dimethyl siloxane) (PDMS) matrix through a vacuum filtration process. An ultrasonication treatment was performed to complete the penetration of PDMS resin in seconds. The entangled and overlapped MWCNT network largely enhances the electrical conductivity (1430 S m−1), uniformity (remaining stable on different layers), reliable sensing range (up to 80% strain), and cyclic stability of the strain sensor. The homogeneous dispersion of MWCNTs within the PDMS matrix leads to a strong interaction between the two phases and greatly improves the mechanical stability (ca. 160% strain at fracture). The flexible, reversible and ultrathin (<100 μm) film can be directly attached on human skin as epidermal strain sensors for high accuracy and real-time human motion detection.

A compliant, self-adhesive and self-healing wearable hydrogel as epidermal strain sensor

A compliant, self-adhesive and self-healing epidermal strain sensor that shows excellent sensing performance at ultra-low and high strain.

Ultrathin epidermal strain sensor based on an elastomer nanosheet with an inkjet-printed conductive polymer

To minimize the interference that skin-contact strain sensors cause natural skin deformation, physical conformability to the epidermal structure is critical. Here, we developed an ultrathin strain sensor made from poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) inkjet-printed on a polystyrene–polybutadiene–polystyrene (SBS) nanosheet. The sensor, whose total thickness and gauge factor were ∼1 µm and 0.73 ± 0.10, respectively, deeply conformed to the epidermal structure and successfully detected the small skin strain (∼2%) while interfering minimally with the natural deformation of the skin. Such an epidermal strain sensor will open a new avenue for precisely detecting the motion of human skin and artificial soft-robotic skin.

PDMS-Based Elastomer Tuned Soft, Stretchable, and Sticky for Epidermal Electronics.

A highly soft, stretchable, and sticky polydimethylsiloxane-based elastomer is achieved by adding small fractions of an amine-based polymer. The modified elastomer is tuned with a simple mixing step and shows good processability for microstructure fabrication. The modified elastomer shows excellent compatibility with an epidermal strain sensor on human skin.