Epidermal Sensors Research Articles

Touch-Based Stressless Cortisol Sensing.

Tracking fluctuations of the cortisol level is important in understanding the body's endocrine response to stress stimuli. Traditional cortisol sensing relies on centralized laboratory settings, while wearable cortisol sensors are limited to slow and complex assays. Here, a touch-based non-invasive molecularly imprinted polymer (MIP) electrochemical sensor for rapid, simple, and reliable stress-free detection of sweat cortisol is described. The sensor readily measures fingertip sweat cortisol via highly selective binding to the cortisol-imprinted electropolymerized polypyrrole coating. The MIP network is embedded with Prussian blue redox probes that offer direct electrical signaling of the binding event to realize sensitive label-free amperometric detection. Using a highly permeable sweat-wicking porous hydrogel, instantaneously secreted fingertip sweat can be conveniently and rapidly collected without any assistance. By eliminating time lags, such rapid (3.5min) fingertip assay enables the capture of sharp variations in cortisol levels, compared to previous methods. Such advantages are demonstrated by tracking cortisol response in short cold-pressor tests and throughout day-long circadian rhythm, along with gold-standard immunoassay validation. A stretchable epidermal MIP sensor is also described for directly tracking cortisol in exercise-induced sweat. The rapid touch-based cortisol sensor offers an attractive, accessible, stressless avenue for quantitative stress management.

Healable, Degradable, and Conductive MXene Nanocomposite Hydrogel for Multifunctional Epidermal Sensors

Conductive hydrogels have emerged as promising material candidates for epidermal sensors due to their similarity to biological tissues, good wearability, and high accuracy of information acquisition. However, it is difficult to simultaneously achieve conductive hydrogel-based epidermal sensors with reliable healability for long-term usage, robust mechanical property, environmental degradability for decreased electronic waste, and sensing capability of the physiological stimuli and the electrophysiological signals. Herein, we propose the synthesis strategy of a multifunctional epidermal sensor based on the highly stretchable, self-healing, degradable, and biocompatible nanocomposite hydrogel, which is fabricated from the conformal coating of a MXene (Ti3C2Tx) network by the hydrogel polymer networks involving poly(acrylic acid) and amorphous calcium carbonate. The epidermal sensor can be employed to sensitively detect human motions with the fast response time (20 ms) and to serve as electronic skins for wirelessly monitoring the electrophysiological signals (such as the electromyogram and electrocardiogram signals). Meanwhile, the multifunctional epidermal sensor could be degraded in phosphate buffered saline solution, which could not cause any pollution to the environment. This line of research work sheds light on the fabrication of the healable, degradable, and electrophysiological signal-sensitive conductive hydrogel-based epidermal sensors with potential applications in human-machine interactions, healthy diagnosis, and smart robot prosthesis devices.

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.

Smart Patch for Skin Temperature: Preliminary Study to Evaluate Psychometrics and Feasibility.

There is a need for continuous, non-invasive monitoring of biological data to assess health and wellbeing. Currently, many types of smart patches have been developed to continuously monitor body temperature, but few trials have been completed to evaluate psychometrics and feasibility for human subjects in real-life scenarios. The aim of this feasibility study was to evaluate the reliability, validity and usability of a smart patch measuring body temperature in healthy adults. The smart patch consisted of a fully integrated wearable wireless sensor with a multichannel temperature sensor, signal processing integrated circuit, wireless communication feature and a flexible battery. Thirty-five healthy adults were recruited for this test, carried out by wearing the patches on their upper chests for 24 h and checking their body temperature six times a day using infrared forehead thermometers as a gold standard for testing validity. Descriptive statistics, one-sampled and independent t-tests, Pearson’s correlation coefficients and Bland-Altman plot were examined for body temperatures between two measures. In addition, multiple linear regression, receiver operating characteristic (ROC) and qualitative content analysis were conducted. Among the 35 participants, 29 of them wore the patch for over 19 h (dropout rate: 17.14%). Mean body temperature measured by infrared forehead thermometers and smart patch ranged between 32.53 and 38.2 °C per person and were moderately correlated (r = 0.23–0.43) overall. Based on a Bland-Altman plot, approximately 94% of the measurements were located within one standard deviation (upper limit = 4.52, lower limit = −5.82). Most outliers were identified on the first measurement and were located below the lower limit. It is appropriate to use 37.5 °C in infrared forehead temperature as a cutoff to define febrile conditions. Users’ position while checking and ambient temperature and humidity are not affected to the smart patch body temperature. Overall, the participants showed high usability and satisfaction on the survey. Few participants reported discomfort due to limited daily activity, itchy skin or detaching concerns. In conclusion, epidermal electronic sensor technologies provide a promising method for continuously monitoring individuals’ body temperatures, even in real-life situations. Our study findings show the potential for smart patches to monitoring non-febrile condition in the community.

Printed Iontophoretic‐Integrated Wearable Microfluidic Sweat‐Sensing Patch for On‐Demand Point‐Of‐Care Sweat Analysis

AbstractIn recent years, wearable epidermal sweat sensors have received extensive attention owing to their great potential to provide personalized information on the health status of individuals at the molecular level. For on‐demand medical analysis of sweat in sedentary conditions, a cost‐effective wearable integrated platform combining sweat stimulation, sampling, transport, and analysis is highly desirable. In this work, a printed iontophoretic system integrated into a microfluidic sensing platform, which combines sweat induction, collection, and real‐time analysis of sweat‐ions into a single patch for on‐demand sweat monitoring on human subjects in stationary conditions is reported. The incorporation of microfluidics features facilitates sweat sampling, collection, and guiding through capillary effect. The multisensing sensor array exhibits sensitivity close to Nernstian behavior for sodium, potassium, and pH. The correlation between the concentrations of ions measured with the sweat patch and with ion chromatography analysis demonstrates the applicability of the system for real‐time point‐of‐care monitoring of the health status of individuals. Furthermore, the sweat patch electronic interface with wireless transmission enables real‐time data monitoring and storage over a cloud platform. This printed iontophoretic‐integrated fluidic sweat patch provides a cost‐effective solution for the on‐demand analysis of sweat components for healthcare applications.

Toward the Use of Temporary Tattoo Electrodes for Impedancemetric Respiration Monitoring and Other Electrophysiological Recordings on Skin.

The development of dry, ultra-conformable and unperceivable temporary tattoo electrodes (TTEs), based on the ink-jet printing of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) on top of commercially available temporary tattoo paper, has gained increasing attention as a new and promising technology for electrophysiological recordings on skin. In this work, we present a TTEs epidermal sensor for real time monitoring of respiration through transthoracic impedance measurements, exploiting a new design, based on the application of soft screen printed Ag ink and magnetic interlink, that guarantees a repositionable, long-term stable and robust interconnection of TTEs with external “docking” devices. The efficiency of the TTE and the proposed interconnection strategy under stretching (up to 10%) and over time (up to 96 h) has been verified on a dedicated experimental setup and on humans, fulfilling the proposed specific application of transthoracic impedance measurements. The proposed approach makes this technology suitable for large-scale production and suitable not only for the specific use case presented, but also for real time monitoring of different bio-electric signals, as demonstrated through specific proof of concept demonstrators.

Liquid Metal-Based Epidermal Flexible Sensor for Wireless Breath Monitoring and Diagnosis Enabled by Highly Sensitive SnS2 Nanosheets.

Real-time wireless respiratory monitoring and biomarker analysis provide an attractive vision for noninvasive telemedicine such as the timely prevention of respiratory arrest or for early diagnoses of chronic diseases. Lightweight, wearable respiratory sensors are in high demand as they meet the requirement of portability in digital healthcare management. Meanwhile, high-performance sensing material plays a crucial role for the precise sensing of specific markers in exhaled air, which represents a complex and rather humid environment. Here, we present a liquid metal-based flexible electrode coupled with SnS2 nanomaterials as a wearable gas-sensing device, with added Bluetooth capabilities for remote respiratory monitoring and diagnoses. The flexible epidermal device exhibits superior skin compatibility and high responsiveness (1092%/ppm), ultralow detection limits (1.32 ppb), and a good selectivity of NO gas at ppb-level concentrations. Taking advantage of the fast recovery kinetics of SnS2 responding to H2O molecules, it is possible to accurately distinguish between different respiratory patterns based on the amount of water vapor in the exhaled air. Furthermore, based on the different redox types of H2O and NO molecules, the electric signal is reversed once the exhaled NO concentration exceeds a certain threshold that may indicate the onset of conditions like asthma, thus providing an early warning system for potential lung diseases. Finally, by integrating the wearable device into a wireless cloud-based multichannel interface, we provide a proof-of-concept that our device could be used for the simultaneous remote monitoring of several patients with respiratory diseases, a crucial field in future digital healthcare management.

Satellite-Based Sensor for Environmental Heat-Stress Sweat Creatinine Monitoring: The Remote Artificial Intelligence-Assisted Epidermal Wearable Sensing for Health Evaluation.

Wearable human sweat sensors have offered a great prospect in epidermal detection for self-monitoring and health evaluation. These on-body epidermal sensors can be integrated with the Internet of Things (IoT) as augmented diagnostics tools for telehealth applications, especially for noninvasive health monitoring without using blood contents. One of many great benefits in utilizing sweat as biofluid is the capability of instantaneously continuous diagnosis during normal day-to-day activities. Here, we revealed a textile-based sweat sensor selective for perspired creatinine that is prepared by coating poly(vinyl alcohol) (PVA)-Cu2+-poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) and cuprous oxide nanoparticles on stretchable nylon, is equipped with heart rate monitoring and a satellite-communication device to locate wearers, and incorporates machine learning to predict the levels of environmental heat stress. Electrochemical impedance spectroscopy (EIS) was used to investigate different charge-transfer resistances of PVA and PEDOT:PSS with cuprous and cuprite ions induced by single-chain and ionic cross-linking. Furthermore, density function theory (DFT) studies predicted the catalytic binding of sweat creatinine with the sensing materials that occurred at thiophene rings. The hybrid sensor successfully achieved 96.3% selectivity efficacy toward the determination of creatinine contents from 0.4 to 960 μM in the presence of interfering species of glucose, urea, uric acid, and NaCl as well as retained 92.1% selectivity efficacy in the existence of unspecified human sweat interference. Ultimately, the hand-grip portable device can offer the great benefit of continuous health monitoring and provide the location of any wearer. This augmented telemedicine sensor may represent the first remote low-cost and artificial-intelligence-based sensing device selective for heat-stress sweat creatinine.

Self-healing, anti-freezing, adhesive and remoldable hydrogel sensor with ion-liquid metal dual conductivity for biomimetic skin

Flexible wearable sensors assembled from conductive hydrogels have received great attention due to their wide application in human-machine interfaces, medical and healthcare detection. However, the traditional conductive gel needs to be attached to the application surface with external force, and only respond to the single strain stimulus. Moreover, due to the existence of water, ordinary hydrogels cannot work at below zero temperatures, which severely limits the application of hydrogel-based flexible electronic devices. The flexible wearable epidermal sensors assembled by ultra-sensitive polyvinyl alcohol-tannic acid-eutectic gallium-indium (PVA-TA-EGaIn) hydrogels with adhesiveness, rapid self-healing, high electrical conductivity and mechanical properties, and temperature sensitivity. The rigid conductive hydrogel prepared by freeze-thaw cycles shows outstanding tensile/compressive strength (1.13 MPa/4.59 MPa) and toughness (1.9 MJ/m3), and reveals excellent fatigue resistance. It also exhibited high conductivity (3.63 S m−1) and strain sensitivity (gauge factor = 2.59). In addition, the hydrogels maintained good flexibility and conductivity at −10 °C. Besides, the PVA-TA-EGaIn hydrogels shows remoldability, which greatly prolongs the service life of the gel. The composite hydrogels show the potential of building the next generation of multifunctional hydrogel-based flexible wearable sensors in human motion monitoring, voice recognition and medical diagnosis.

Tannic Acid-Silver Dual Catalysis Induced Rapid Polymerization of Conductive Hydrogel Sensors with Excellent Stretchability, Self-Adhesion, and Strain-Sensitivity Properties.

The application of conductive hydrogels in intelligent biomimetic electronics is a hot topic in recent years, but it is still a great challenge to develop the conductive hydrogels through a rapid fabrication process at ambient temperature. In this work, a versatile poly(acrylamide) @cellulose nanocrystal/tannic acid-silver nanocomposite (NC) hydrogel integrated with excellent stretchability, repeatable self-adhesion, high strain sensitivity, and antibacterial property, was synthesized via radical polymerization within 30 s at ambient temperature. Notably, this rapid polymerization was realized through a tannic acid-silver (TA-Ag) mediated dynamic catalysis system that was capable of activating ammonium persulfate and then initiated the free-radical polymerization of the acrylamide monomer. Benefiting from the incorporation of TA-Ag metal ion nanocomplexes and cellulose nanocrystals, which acted as dynamic connecting bridges by hydrogen bonds to efficiently dissipate energy, the obtained NC hydrogels exhibited prominent tensile strain (up to 4000%), flexibility, self-recovery, and antifatigue properties. In addition, the hydrogels showed repeatable adhesiveness to different substrates (e.g., glass, wood, bone, metal, and skin) and significant antibacterial properties, which were merits for the hydrogels to be assembled into a flexible epidermal sensor for long-term human-machine interfacial contact without concerns about the use of external adhesive tapes and bacterial breeding. Moreover, the remarkable conductivity (σ ∼ 5.6 ms cm-1) and strain sensitivity (gauge factor = 1.02) allowed the flexible epidermal sensors to monitor various human motions in real time, including huge movement of deformations (e.g., wrist, elbow, neck, shoulder) and subtle motions. It is envisioned that this work would provide a promising strategy for the rapid preparation of conductive hydrogels in the application of flexible electronic skin, biomedical devices, and soft robotics.

Performance and Durability of Thread Antennas as Stretchable Epidermal UHF RFID Tags

Epidermal sensors based on battery-less Radiofrequency Identification (RFID) aim at collecting biophysical parameters with a high level of comfort for the user. This paper investigates the performance and durability of epidermal RFID tags, equipped with a self-tuning RFID IC, that are based either on copper wires or conductive yarns. The tags are deployed onto an ultra-thin stretchable and transparent substrate to achieve comformability to body discontinuities. A statistical analysis on volunteers showed that, in the whole UHF band (860-960 MHz), reliable read ranges of 1 m are easily achieved while up to 2 m can be reached in some favorable configurations. Both tags withstand wear, mechanical stress due to the movements of the body, sweating, and water. In particular, the tag made of conductive yarn lasts for more than 20 days. This new family of epidermal tags is moreover suitable for low-cost and large-scale manufacturing through the widely available machines used for wire-laying, bending, and shaping.

Double-Framed Thin Elastomer Devices.

Elastomers and, in particular, polydimethylsiloxane (PDMS) are widely adopted as biocompatible mechanically compliant substrates for soft and flexible micro-nanosystems in medicine, biology, and engineering. However, several applications require such low thicknesses (e.g., <100 μm) that make peeling-off critical because very thin elastomers become delicate and tend to exhibit strong adhesion with carriers. Moreover, microfabrication techniques such as photolithography use solvents which swell PDMS, introducing complexity and possible contamination, thus limiting industrial scalability and preventing many biomedical applications. Here, we combine low-adhesion and rectangular carrier substrates, adhesive Kapton frames, micromilling-defined shadow masks, and adhesive-neutralizing paper frames for enabling fast, easy, green, contaminant-free, and scalable manufacturing of thin elastomer devices, with both simplified peeling and handling. The accurate alignment between the frame and shadow masks can be further facilitated by micromilled marking lines on the back side of the low-adhesion carrier. As a proof of concept, we show epidermal sensors on a 50 μm-thick PDMS substrate for measuring strain, the skin bioimpedance and the heart rate. The proposed approach paves the way to a straightforward, green, and scalable fabrication of contaminant-free thin devices on elastomers for a wide variety of applications.

Experimental Assessment of Wireless Monitoring of Axilla Temperature by Means of Epidermal Battery-Less RFID Sensors.

Wireless epidermal devices (WEDs), based on UHF radio frequency identification (RFID), enable a contactless and noninvasive human body monitoring through sampling of health parameters directly on the skin. With reference to body temperature, this letter reports an experimental campaign aimed at assessing the degree of agreement of a battery-less plaster-like WED, placed in the armpit region, with a standard axilla thermocouple thermometer. A measurement campaign over 10 volunteers, for overall 120 temperature outcomes, revealed a good correlation among the instruments (Person's coefficient p = 0.78) and a difference of less than 0.6 °C in the 95% of the measured cases, provided that a user-calibration is applied. RFID-WED enables a noncontacting reading up to 20 cm and direct connectivity with a cloud architecture. Envisaged applications are the periodic monitoring in clinical and domestic scenarios, as well as the screening of restricted communities related to COVID-19 control and recovery.

Medium-distance affordable, flexible and wireless epidermal sensor for pH monitoring in sweat

In the last decade, wearable sensors have gained a key role on biomedical research field for reliable health state monitoring. A wide plethora of physics marker sensors is already commercially available, including activity tracker, heart rate devices, and fitness smartwatch. On the contrary, wearable and epidermal sensors for chemical biomarker monitoring in several biofluids are not ready yet. Herein, we report a wireless and flexible epidermal device for pH monitoring in sweat, fabricated by encompassing a screen-printed potentiometric sensor, an integrated circuit, and antenna embedded onto the same Kapton substrate. An iridium oxide film was electrodeposited onto the graphite working electrode providing the pH sensitive layer, while the integrated circuit board allows for data acquisition and storing. Furthermore, a radio frequency identification antenna surrounding the entire system enables data transmission to an external reader up to nearly 2 m in the most favourable case. The potentiometric sensor was firstly characterised by cyclic voltammetry experiments, then the iridium oxide electrodeposition procedure was optimised. Next, the sensor was tested toward pH detection in buffer solutions with a near-Nernstian response equal to −0.079 ± 0.002 V for unit of pH. Interference studies of common sweat ions, including Na+, K+ and Cl−, showed any influence on the pH sensor response. Finally, the integrated epidermal device was tested for real-time on-body pH sweat monitoring during a running activity. Data recorded for a running subject were wireless transmitted to an external receiver, showing a pH value close to 5.5, in agreement with value obtained by pH-meter reference measurement.

Ultrastretchable, Self‐Healable, and Wearable Epidermal Sensors Based on Ultralong Ag Nanowires Composited Binary‐Networked Hydrogels

AbstractStretchable and biocompatible flexible electronic devices are essential to meet the increasing demands of complex and multifunctional personal healthcare systems. To detect various external stimuli, noninvasively epidermal sensors with reliable and sustainable performances are desirable. Herein, ultrastretchable, self‐healable, and wearable epidermal sensors based on ultralong Ag nanowires (AgNWs) composited binary‐networked hydrogels are fabricated. The flexible hydrogel sensors can monitor dynamic strains in a wide range (4–3000%), realize high healing efficiency (94.3%) and strong adhesiveness, which is attributed to the strong covalent bond and reversible physical interaction structured binary‐network. The ultralong AgNWs network remains in direct contact under strain, ensures a rapid response to external stimuli. The strong interactions between polymer matrix and the nanowires endow the hydrogel sensors excellent sensitivity (gauge factor of 4.59) within a wide sensing range (0–850%). The cycling stability of the hydrogel sensors is further improved by the composition of AgNWs, presenting negligible degradation both on tension and compression. Based on the advantageous performances, the flexible stain sensors can differentiate complicated human motions and realize phonation recognition precisely, showing promising application in next‐generation wearable epidermal sensors with ultrabroad working range and high sensitivity.

72-LB: Novel Epidermal Adhesive Sensors to Enhance Continuous Glucose Measurement in Patients with Diabetes: The EASE Study

By providing a more comfortable and convenient glucose testing experience, non-invasive, wearable biosensors could bolster patients’ engagement with their diabetes mellitus (DM). We investigated the accuracy and acceptability of a novel, needle-free continuous glucose sensor. At 2 g and 5 x 4 mm, this sensor is ultra-light, thin, flexible, and self-adherent to the skin. A mild electrical current applied to the epidermis causes sodium ions, then interstitial fluid (ISF) glucose molecules, to migrate toward an electrode. The sensor gauges glucose by detecting its electrical charge strength. Four individuals with DM participated (age 42-61 years, mean 51.5 years; 1 patient with type 1 DM). Over 4 hours, we obtained 3 sensor and 3 glucometer readings concurrently at each of the following time points: following an 8-hour fast, and every 60 minutes after a standard 500-kcal, 75-g high-carbohydrate meal. The correlation coefficient of sensor and glucometer readings was 0.80 (p = 0.00023). Participants rated on a 1-5 scale, with 5 being the highest: 4.25 for comfort, and 4.0 for being receptive to long-term use of such a sensor. Two individuals denied discomfort; 1 felt tingling; and 1 noted a needle-like sensation. The function and patient-friendly design of a wearable glucose sensor may help substantially advance glucose testing, and in turn, improve clinical outcomes and quality of life. Disclosure E. Chao: None. E. De La Paz Andres: None. A. Barfidokht: None. J. Wang: None. Funding University of California, San Diego; National Institutes of Health (UL1TR001442)

Plasma-jet-induced programmable wettability on stretchable carbon nanotube films

Stretchable surfaces with site-selective and degree-controlled wettability are indispensable for various applications ranging from self-cleaning to droplet manipulation and drag reduction; however, superwettable surfaces or patterns with robust stretchability are seldom demonstrated to date. To address such challenging issue, herein, we proposed a facile approach to programmatically realize extreme wetting-contrast patterning of stretchable carbon nanotubes/polydimethylsiloxane (CNTs/PDMS) using atmospheric micro-plasma jet (μPJ). The μPJ had been used to engineer both superhydrophobicity and superhydrophilicity through exploiting surface hierarchical roughness under a shielding nitrogen gas and oxygenic functional groups in atmosphere, respectively, tailoring the water contact angles from ~160° to <10° ultimately. The as-prepared superhydrophobic, conductive surfaces could bear tensile strain even up to 100% and demonstrated no noticeable wettability deterioration after 1000 cyclic stretching from ε = 0% to ε = 50%. And they also presented a distinguished enduring stability in terms of exposure to air and even extreme acidic/alkaline aqueous droplets. As the randomly entangled CNTs film could respond to tensile strain, the superhydrophobic films had been successfully applied to water-repellent epidermal sensors. And hydrophilic patterns had been fabricated on the superhydrophobic surfaces as well. It can be envisioned that a variety of novel applications ranging from water/blood-repellent human-machine interfaces to in-plane microfluidic can be implemented via this powerful wettability regulation paradigm.

DNA-inspired anti-freezing wet-adhesion and tough hydrogel for sweaty skin sensor

Epidermal sensors via self-adhesive hydrogels offer great promise for the motion detection, health monitoring, and disease diagnosis. However, the self-adhesion of recent hydrogel sensors trends to be inefficient for the wet, oily and sweaty skin of human. Besides, it is also enormously challengeable to construct a robust adhesion system in both oil and water. Herein, a facile approach is presented to fabricate DNA-inspired anti-freezing, tough and wet-adhesive hydrogels assisted by nucleobase pairs. The hydrogels simultaneously demonstrate robust adhesion for various substrates and non-swelling behavior in the water and oil, as well as remarkable mechanical stability and adhesive behavior in a harsh environmental temperature from −20 °C to 80 °C. Furthermore, a hydrogel epidermal sensor is successfully fabricated and can effectively adhere to the wet skin of human for monitoring various human motions including speaking, nodding, and various joint bending. The multiple advantageous features would endow DNA-inspired hydrogels with considerable potential in promising applications in multiple solvents, such as flexible electronic devices, battery adhesives, soft robots.

Stretchable and Skin-Conformable Conductors Based on Polyurethane/Laser-Induced Graphene.

The conversion of various polymer substrates into laser-induced graphene (LIG) with a CO2 laser in ambient condition is recently emerging as a simple method for obtaining patterned porous graphene conductors, with a myriad of applications in sensing, actuation, and energy. In this paper, a method is presented for embedding porous LIG (LIG-P) or LIG fibers (LIG-F) into a thin (about 50 μm) and soft medical grade polyurethane (MPU) providing excellent conformal adhesion on skin, stretchability, and maximum breathability to boost the development of various unperceivable monitoring systems on skin. The effect of varying laser fluence and geometry of the laser scribing on the LIG micro–nanostructure morphology and on the electrical and electromechanical properties of LIG/MPU composites is investigated. A peculiar and distinct behavior is observed for either LIG-P or LIG-F. Excellent stretchability without permanent impairment of conductive properties is revealed up to 100% strain and retained after hundreds of cycles of stretching tests. A distinct piezoresistive behavior, with an average gauge factor of 40, opens the way to various potential strain/pressure sensing applications. A novel method based on laser scribing is then introduced for providing vertical interconnect access (VIA) into LIG/MPU conformable epidermal sensors. Such VIA enables stable connections to an external measurement device, as this represents a typical weakness of many epidermal devices so far. Three examples of minimally invasive LIG/MPU epidermal sensing proof of concepts are presented: as electrodes for electromyographic recording on limb and as piezoresistive sensors for touch and respiration detection on skin. Long-term wearability and functioning up to several days and under repeated stretching tests is demonstrated.

Study of Partially Transient Organic Epidermal Sensors.

In this study, an all-organic, partially transient epidermal sensor with functional poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) conjugated polymer printed onto a water-soluble polyethylene oxide (PEO) substrate is studied and presented. The sensor’s electronic properties were studied under static stress, dynamic load, and transient status. Electrode resistance remained approximately unchanged for up to 2% strain, and increased gradually within 6.5% strain under static stress. The electronic properties’ dependence on dynamic load showed a fast response time in the range of 0.05–3 Hz, and a reversible stretching threshold of 3% strain. A transiency study showed that the PEO substrate dissolved completely in water, while the PEDOT:PSS conjugated polymer electrode remained intact. The substrate-less, intrinsically soft PEDOT:PSS electrode formed perfect contact on human skin and stayed attached by Van der Waals force, and was demonstrated as a tattoolike epidermal sensor.