Epidermal Sensors Research Articles

Epidermal piezoresistive structure with deep learning-assisted data translation

Continued research on the epidermal electronic sensor aims to develop sophisticated platforms that reproduce key multimodal responses in human skin, with the ability to sense various external stimuli, such as pressure, shear, torsion, and touch. The development of such applications utilizes algorithmic interpretations to analyze the complex stimulus shape, magnitude, and various moduli of the epidermis, requiring multiple complex equations for the attached sensor. In this experiment, we integrate silicon piezoresistors with a customized deep learning data process to facilitate in the precise evaluation and assessment of various stimuli without the need for such complexities. With the ability to surpass conventional vanilla deep regression models, the customized regression and classification model is capable of predicting the magnitude of the external force, epidermal hardness and object shape with an average mean absolute percentage error and accuracy of <15 and 96.9%, respectively. The technical ability of the deep learning-aided sensor and the consequent accurate data process provide important foundations for the future sensory electronic system.

Chitosan-driven biocompatible hydrogel based on water-soluble polypyrrole for stable human-machine interfaces

Recently, wearable hydrogel sensors based on polypyrrole have shown considerable promise in the realms of human-machine interfaces. However, because of the water insolubility of pyrrole and polypyrrole, the preparation of polypyrrole conductive hydrogels with comprehensive properties by a simplified method remains a great challenge. Herein, the water-soluble polypyrrole was synthesized and the conformal CxPy conductive hydrogels were fabricated by the strategy of one-pot method of introducing chitosan and water-soluble polypyrrole into acrylamide matrix containing cucurbit[7]uril. The hydrogel exhibited good mechanical strength (215.48 kPa at the fracture strain of 2149.17 %), superior adhesion strength (~51.54 kPa), excellent conductivity (0.534 S m−1) and biocompatibility (The cell viability of NIH3T3 is 98.25 %). As a strain sensor, C10P5 hydrogel exhibited excellent stability during 1000 times cycles, which is suitable for epidermal sensors to monitor body motions and physiological signals. Therefore, the chitosan-driven conductive hydrogel would present prosperous prospects in the fields of human-machine interfaces.

Laser-Induced Graphene-Based Wearable Epidermal Ion-Selective Sensors for Noninvasive Multiplexed Sweat Analysis.

Wearable sweat sensors are a rapidly rising research area owing to their convenience for personal healthcare and disease diagnosis in a real-time and noninvasive manner. However, the fast and scalable fabrication of flexible electrodes remains a major challenge. Here, we develop a wearable epidermal sensor for multiplexed sweat analysis based on the laser-induced graphene (LIG) technique. This simple and mask-free technique allows the direct manufacturing of graphene electrode patterns on commercial polyimide foils. The resulting LIG devices can simultaneously monitor the pH, Na+, and K+ levels in sweat with the sensitivities of 51.5 mV/decade (pH), 45.4 mV/decade (Na+), and 43.3 mV/decade (K+), respectively. Good reproducibility, stability, and selectivity are also observed. On-body testing of the LIG-based sensor integrated with a flexible printed circuit board during stationary cycling demonstrates its capability for real-time sweat analysis. The concentrations of ions can be remotely and wirelessly transmitted to a custom-developed smartphone application during the period in which the sensor user performs physical activities. Owing to the unique advantages of LIG technique, including facile fabrication, mass production, and versatile, more physiological signals (glucose, uric acid, tyrosine, etc.) could be easily expanded into the LIG-based wearable sensors to reflect the health status or clinical needs of individuals.

Highly Stretchable, Ultra‐Soft, and Fast Self‐Healable Conductive Hydrogels Based on Polyaniline Nanoparticles for Sensitive Flexible Sensors

AbstractHerein, novel conductive composite hydrogels are developed with high stretchability, ultra‐softness, excellent conductivity, and good self‐healing ability. The hydrogels are formed in the water/glycerol binary solvent system, in which the polyaniline nanoparticles (PANI‐NPs) are incorporated into the poly(poly(ethylene glycol) methacrylate‐co‐acrylic acid) (P(PEG‐co‐AA)) scaffolds via the dynamically electrostatic interactions and hydrogen bonds. The PANI‐NPs serve as conductive fillers to assign conductivity to the hydrogel, while the enhanced interfacial interactions between the PANI‐NPs and P(PEG‐co‐AA) matrix endow the hydrogel with high stretchability (&gt;1000%), low modulus (≈6 kPa), excellent elasticity (η = 0.07, energy loss coefficient at 500% strain), and fast self‐healing ability (93.3% after 10 mins). Particularly, the desirable anti‐freezing property is achieved by introducing a binary solvent system. The composite hydrogel‐based sensors are proposed, with the states‐independent properties, low detection limit (0.5% strain and 25 Pa), highly linear dependence, and excellent anti‐fatigue performance (&gt;1000 cycles). In addition, during the practical wearable sensing tests, various external stimulus and human motions can be detected, including speaking, writing, joint movement, or even small water droplets, indicating the potential applications for the next generation of epidermal sensors.

Self-healing hydrogel with multiple dynamic interactions for multifunctional epidermal sensor

With the fast development of flexible sensors, various materials are used to construct sensors. However, it is still a challenge to prepare a hydrogel sensor with the integrated functions of good mechanical and sensing performance, and excellent adhesion and self-healing properties. Herein, an all-in-one hydrogel sensor TG2P3 was constructed through the rational design of intermolecular interactions containing tannic acid (TA), graphene oxide (GO), glycerol, poly(allylamine hydrochloride) (PAH), poly(acrylic acid) (PAA), and Poly(N-isopropylacrylamide) (PNIPAM). The electrostatic interactions between the polymer chains of PAH and PAA, multiple hydrogen bonds, π-π interactions between GO and TA, and cation-π interactions between PAH and the aromatic benzene rings, all significantly contributed to the tremendous mechanical strain (1420%) of the hydrogel. Meanwhile, TG2P3 hydrogel showed an impressive self-healing efficiency of 87%, owing to the feature of multiple dynamic interactions. The addition of glycerol endowed TG2P3 hydrogel with outstanding anti-freezing and anti-drying properties, allowing the retainment of a desirable self-healing efficiency of 25.7% even at −20 ℃, as well as a high water-retention rate at room temperature (97.9%) and at 60 ℃ (95.8%). Notably, a remarkable stretchability of 572% could be observed even after 7 days of storage. Furthermore, the TG2P3 hydrogel sensor exhibited an expressive gauge factor (GF = 1.3) within a wide strain range (300%), owing to the synergistic effect of high mechanical and electrochemical compliance. By taking advantage of PNIPAM moiety's thermosensitivity, TG2P3 hydrogel could sensitively detect temperature variations. Lastly, TG2P3 hydrogel successfully demonstrated the applications in human motion monitoring sensors, flexible touch keyboards, and fever indicators, showing tremendous potential in the next generation of wearable epidermal electronics.

Preparation and characterization of self-healable and wearable hydrogels with ultrasensitive sensing performances

Conductive hydrogels with multifunction have received significant attention in the field of wearable electronic devices, especially in strain sensors and pressure sensors. The high sensor sensitivity, flexibility and mechanical strength are the top three critical factors for manufacturing of trustworthy sensors for practical applications. However, the conductive hydrogels are subjected to some limitations due to the lack of self-repair ability when damaged by external force, the low electrical conductivity and weak mechanical strength. To address these issues, multifunctional conductive hydrogels incorporating MXene nanosheets and lignin amine-silver nanoparticles (LA-Ag NPs) were constructed via a dynamic redox system. Benefiting from the introduction of MXene and LA-Ag NPs, the hydrogels demonstrated strong adhesiveness, self-healing property and high conductivity. Notably, the epidermal sensors demonstrated high sensitivity over a broad range as both strain sensor (GF = 9.17) and pressure sensor (S = 0.33 kPa−1).

A Scalable Laser‐Centric Fabrication of an Epidermal Cardiopulmonary Patch

AbstractRecent advances in scalable fabrication methods based on printing technologies have improved the yield and lowered the cost of manufacturing epidermal sensors. However, modern technologies still require expensive multi‐bio‐ink raw materials. A laser‐centric fabrication method that realizes a cost‐effective, scalable, and streamlined fabrication process with easily used material and equipment is reported. The fabricated epidermal patch can quickly respond to the demands of personalized models. Here, the epidermal sensor patch is applied to continuous monitoring of the human cardiopulmonary system. The sensor performs conventional bio‐signal monitoring but is also extendible to other monitoring forms. The patch provides enhanced functions through its gas‐permeable and skin‐adhesive microporous layer and its stretchable, conformal, and biocompatible multimodal sensing layer. The utility of the proposed sensor patch in clinical diagnosis in a mobile healthcare environment is demonstrated in multiple bio‐signal‐morphology analyses.

Sweat-Resistant Silk Fibroin-Based Double Network Hydrogel Adhesives.

The adhesion between flexible epidermal sensors and human skin is essential for maintaining the stable functionality of the sensors. However, it is still challenging for epidermal electronic devices to achieve durable adhesion to the surface of the skin, especially under sweaty or humid conditions. Here, we report a silk fibroin-polyacrylamide (SF-PAAm) double network (DN) hydrogel adhesive with excellent biocompatibility, strong and durable adhesion on wet surfaces, and tunable adhesive properties. The hydrophilic PAAm network greatly improves the water retention capability of the DN hydrogel and reduces the β-sheet crystalline content of SF, leading to excellent adhesive properties of the hydrogel across a wide range of humidity. The SF-PAAm DN hydrogel adhesive can be readily integrated with different epidermal sensor arrays and performs very well in real-time on-body sweat sensing. The SF-PAAm DN hydrogels have great potential for application in various epidermal healthcare sensors as well as medical adhesives for other medical applications.

Water-Tolerant MXene Epidermal Sensors with High Sensitivity and Reliability for Healthcare Monitoring

Flexible and wearable pressure sensors have gained great popularity in consumer electronics due to their potential applications in human healthcare, E-skin, and artificial intelligence interactions. MXene is regarded as one of the most ideal candidate sensing materials due to its high electrical conductivity and controllable interlayer space. However, the easy-to-oxidize characteristic of MXene materials greatly restricts the sensitivity and reliability of sensor devices, especially in wet climates. Herein, a highly sensitive and waterproof flexible pressure sensor using a free-standing hydrophobic bacterial cellulose/Ti3C2Tx MXene (HBT) hybrid film as a sensing layer is fabricated by facile and effective nanocellulose intercalation and fluorine modification strategies. The obtained pressure sensor delivers high sensitivity (65.5 kPa-1), fast response (50 ms), wide linear sensing range (0.002-30 kPa) with a low detection limit of 0.57 Pa, and excellent repeatability over 50,000 cycles. Meanwhile, owing to the highly hydrophobic surface of the HTB film, the outstanding sensing features could be well retained, although immersed in water several times. Benefiting from the excellent sensing properties and water resistance, the HBT sensor serves as a wearable force sensor to monitor the full-range human physiological motions regardless of whether the conditions are normal or wet. This work provides a new pathway to design the MXene pressure sensor with high reliability and demonstrates the promising usage of HBT sensors in portable biomedical electronics.

Self-adhesive polyurethane via selective photo-polymerization for biocompatible epidermal soft sensor and thermal heater

Polyurethane (PU) is the most extensively used soft backbone substrate for wearable applications. However, even with biocompatibility and stretchability, PU-based wearable devices are unsuitable for epidermal devices because they lack of adhesive properties. To impart adhesive properties to PU, commercial adhesive additives are required or the mixing ratio of specific materials should be adjusted. Despite PU biocompatibility, additional additives cause skin damage because they are not usually biocompatible due to their toxic ingredient. This study introduces a simple method of fabricating self-adhesive polyurethane (SAPU) without specific adhesive additives for a biocompatible epidermal soft sensor and an attachable epidermal thermal heater. During the selective photo-polymerization of PU resin, the defective crosslinking networks between laser-scanned resin lines enhance the adhesive properties of PU. Adjusting photo-polymerization conditions results in self-adhesive polyurethane with high adhesiveness and low Young's modulus comparable to those of human skin. A mouse skin toxicity test confirms the biocompatibility and highly conformal contact of SAPU. SAPU is then further applied for the fabrication of a biocompatible epidermal soft sensor and an attachable epidermal thermal heater with silver nanowire network. The fabricated self-adhesive epidermal soft sensor successfully detects human motion and bio-signals, whereas the self-adhesive epidermal soft heater efficiently transfers heat to the epidermis due to its excellent conformal contact characteristics on the human skin.

Materials with Tunable Optical Properties for Wearable Epidermal Sensing in Health Monitoring.

Advances in wearable epidermal sensors have revolutionized the way that physiological signals are captured and measured for health monitoring. One major challenge is to convert physiological signals to easily readable signals in a convenient way. One possibility for wearable epidermal sensors is based on visible readouts. There are a range of materials whose optical properties can be tuned by parameters such as temperature, pH, light, and electric fields. Herein, this review covers and highlights a set of materials with tunable optical properties and their integration into wearable epidermal sensors for health monitoring. Specifically, the recent progress, fabrication, and applications of these materials for wearable epidermal sensors are summarized and discussed. Finally, the challenges and perspectives for the next generation wearable devices are proposed.

Ultra-stretchable, adhesive, and self-healing MXene/polyampholytes hydrogel as flexible and wearable epidermal sensors

The hydrogel-based epidermal sensors have attracted tremendous attention due to their versatile potential applications in soft robotics, personal healthcare monitoring, and electronic skins. However, it remains a critical challenge for hydrogel-based sensors to simultaneously achieve self-healing capability and adhesiveness for full-scale human motion biomonitoring. Herein, we proposed a multifunctional MXene/polyampholytes nanocomposite hydrogel. The main network of this hydrogel was formed by the one-step radical polymerization of cationic monomer and anionic monomer. The simultaneous strong and weak ionic bonds-based crosslinking inside this hydrogel endowed it outstanding stretchable, tough, self-healing, adhesive properties, while the introduction of MXene further guaranteed the excellent mechanical properties and conductivity of this hydrogel. Our further exploration of this MXene/polyampholytes in the application as wearable epidermal sensor strongly proved its durable accuracy and sensitivity for the detection of human motions. This epidermal sensor also could be assembled to the detector of the daily activity monitor of Attention-deficit hyperactivity disorder (ADHD) patients, which could be utilized to aid the ADHD treatment by remindering the patients to pay attention to their actions. Our work shed a new light on the development of eco-friendly wearable sensor for the personalized healthcare monitoring, human–machine interfaces, and artificial intelligence.

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.

Water-Saturated Ion Gel for Humidity-Independent High Precision Epidermal Ionic Temperature Sensor.

Although ion gels are attractive sensing materials for deformable epidermal sensors or implantable devices, their sensing performances are highly affected by environmental humidity change, so that their sensing reliability cannot be secured. This study proposes a new concept of maintaining the high‐precision temperature sensing performance of highly deformable ion gel sensors. In this approach, a hydrophobic ion gel sensing layer is kept water‐saturated by attaching a hydrogel layer, rather than attempting to completely block water penetration. This study performs experimental and theoretical investigation on water concentration in the ion gel, using the analysis of mass transportation at the interface of the ion gel and the hydrogel. By using the charge relaxation time of the ionic molecules, the temperature sensor is not affected by environmental humidity in the extreme range of humidity (30%–100%). This study demonstrates a highly deformable on‐skin temperature sensor which shows the same performance either in water or dry state and while exercising with large strains (ε = 50%).

192 An Early Innovation Report of an Epidermal Ventriculoperitoneal (VP) Shunt Pressure Sensor

Abstract Introduction Ventriculoperitoneal shunt insertion, a common procedure for hydrocephalus, has a revision rate of approximately 20–30% within the first year, and continues to increase with each consecutive year. In Leeds, the average work-up cost for suspected shunt malfunctions was £765/patient. Additionally, patients with malfunctioning shunts present with vague symptoms potentially contributing to delayed presentation, especially important in Low-Middle Income Countries (LMICs) where neurosurgical expertise is less accessible. To reduce investigative costs and delayed presentations, we sought to design an epidermal shunt reservoir sensor to detect shunt malfunction by measuring the reservoir pressure. Method Literature searches were conducted to identify a proof of concept and suitable materials to engineer a concept device based on a Failure Modes and Effects Analysis (FMEA) table with design specifications to assess relative clinical risks. Using the IDEAL-D Framework, we designed our low-fidelity prototype using Solidworks® software. Results Literature revealed several devices aimed to detect shunt malfunctions. For our concept device, our focus was an accurate sensor and reliable connectivity. Our prototype uses gold nanowires to achieve high sensitivity to pressure change, as small as 15mPa. Near-field communication (NFC) for connectivity has less power requirement and has rapid connectivity compared to Bluetooth. Estimated cost of the device is £500. Conclusions Our device, in Stage 0 of the IDEAL-D Framework, identified potential sensors and connectivity methods. We could utilise magnets to adhere to the reservoir with overlying hair. However, magnetic fields and atmospheric pressure could cause interference. Power source options still require exploring before benchtop testing for accuracy, interference, and feasibility.

Microporous Multiresonant Plasmonic Meshes by Hierarchical Micro-Nanoimprinting for Bio-Interfaced SERS Imaging and Nonlinear Nano-Optics.

Microporous mesh plasmonic devices have the potential to combine the biocompatibility of microporous polymeric meshes with the capabilities of plasmonic nanostructures to enhance nanoscale light-matter interactions for bio-interfaced optical sensing and actuation. However, scalable integration of dense and uniformly structured plasmonic hotspot arrays with microporous polymeric meshes remains challenging due to the processing incompatibility of conventional nanofabrication methods with flexible microporous substrates. Here, scalable nanofabrication of microporous multiresonant plasmonic meshes (MMPMs) is achieved via a hierarchical micro-/nanoimprint lithography approach using dissolvable polymeric templates. It is demonstrated that MMPMs can serve as broadband nonlinear nanoplasmonic devices to generate second-harmonic generation, third-harmonic generation, and upconversion photoluminescence signals with multiresonant plasmonic enhancement under fs pulse excitation. Moreover, MMPMs are employed and explored as bio-interfaced surface-enhanced Raman spectroscopy mesh sensors to enable in situ spatiotemporal molecular profiling of bacterial biofilm activity. Microporous mesh plasmonic devices open exciting avenues for bio-interfaced optical sensing and actuation applications, such as inflammation-free epidermal sensors in conformal contact with skin, combined tissue-engineering and biosensing scaffolds for in vitro 3D cell culture models, and minimally invasive implantable probes for long-term disease diagnostics and therapeutics.

Environment Adaptable Nanocomposite Organohydrogels for Multifunctional Epidermal Sensors

AbstractElectronic skins, as a revolution in artificial intelligence, have drawn intensive attention in smart prosthetic devices, wearable health monitors and intelligent robot manufacturing. Conductive hydrogels as building blocks have been highlighted in artificial skin research. Nevertheless, the main challenges of hydrogel‐based electronics are poor temperature tolerance, weak mechanical robustness and limited stretchability. Herein, an organohydrogel is fabricated with “soft and hard” synergistic networks by combining “soft” polyacrylamide (PAM) and catechol‐modified hyaluronic acid (HA‐CA) polymer network with “hard” Laponite nanoparticles. The obtained organohydrogels exhibit excellent environment‐adaptability, super stretchability (&gt;8000%), superior adhesion to various substrates, superfast self‐healing efficiency (&lt;10 s), excellent conductivity (63 mS m−1) even at −0 °C and good biocompatibility. These outstanding properties render the organohydrogels as epidermal flexible sensors. The fabricated sensor exhibits wide strain sensing range (0−300%), super sensitivity (gauge factor, GF = 8.38−133.94) which can effectively detect and discriminate various human activities, and maintain their functions at extremely cold conditions. This versatile organohydrogel offers a platform for practical application of flexible wearable devices in extreme environments.

Laser-Induced Graphene as Wearable Epidermal Ion-Selective Sensors for Noninvasive Multiplexed Sweat Analysis

Laser-Induced Graphene as Wearable Epidermal Ion-Selective Sensors for Noninvasive Multiplexed Sweat Analysis

Electrochemical Properties of Phytosynthesized Gold Nanoparticles for Electrosensing

Gold nanoparticles are widely used in electrosensing. The current trend is to phytosynthesize gold nanoparticles (phyto-AuNPs) on the basis of the “green” chemistry approach. Phyto-AuNPs are biologically and catalytically active, stable and biocompatible, which opens up broad perspectives in a variety of applications, including tactile, wearable (bio)sensors. However, the electrochemistry of phytosynthesized nanoparticles is not sufficiently studied. This work offers a comprehensive study of the electrochemical activity of phyto-AuNPs depending on the synthesis conditions. It was found that with an increase in the aliquot of the plant extract, its antioxidant activity (AOA) and pH, the electrochemical activity of phyto-AuNPs grows, which is reflected in the peak potential decrease and an increase in the peak current of phyto-AuNPs electrooxidation. It has been shown that AOA is an important parameter for obtaining phyto-AuNPs with desired properties. Electrodes modified with phyto-AuNPs have demonstrated better analytical characteristics than electrodes with citrate AuNPs in detecting uric and ascorbic acids under model conditions. The data about the phyto-AuNPs’ electrochemistry may be useful for creating highly effective epidermal sensors with good biocompatibility.

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.