On-skin Sensor Research Articles

Nanoporous Carbon Aerogels for Laser-Printed Wearable Sensors

Laser-induced porous carbon materials have received growing attention as electrodes in micro-supercapacitors, electronics, and biosensors. Herein, resorcinol-formaldehyde (RF) aerogel is used for laser carbonization and activation to produce a nanoporous black conductive surface with high porosity and a specific surface area of 703 m2·g–1 as well as high absorbance to infrared light (∼98%). Furthermore, inspired by the photoreception behavior of octopus arms, a “thunder motif” nanoporous carbon aerogel-based resistance-type sensor is able to be laser-printed on paper-reinforced RF aerogel, which shows high integration of temperature (0.19%·°C–1), strain (gauge factor 16.7), humidity, and especially infrared light (0.12%·mW–1, which is not common for a normal on-skin sensor) sensing in one device and better performance than a serpentine layout sensor. With its good flexibility and biodegradability, this multifunctional sensor can serve as an on-skin wearable sensor that can sense gesture changes and breathing.

All-Fabric Piezoionic Sensor for Simultaneous Sensing of Static and Dynamic Pressures

The development of flexible and textile-based wearable pressure sensors has provided the opportunity of continuous and real time measurement of human physiological and biomechanical signals during daily activities. Pressure sensors are transducers that convert an exerted compression stress into a detectable electrical signal. Different transduction mechanisms have been introduced so far including triboelectricity, transistivity, capacitance, piezoelectricity, and piezoresistivity. Piezoresistive pressure sensors are the most widely used type due to the simplicity of their structure, and the wide range of materials that can be selected along with low-cost fabrication methods, and easy read-out system required for signal extraction.A vast majority of piezoresistive sensors developed so far are on-skin sensors developed to detect subtle pressures (1 Pa-10 kPa) for touchpads and electronic skin applications. However, to sense physiological signals such as pulse, respiration, and phonation the sensor range of detection must fall within medium range of 10 kPa to 100 kPa. As expected, for larger-scale human motion detection such as sleep posture and footwear evaluation, the sensor must be able to sense compression stresses larger than 100 kPa. This wide range of detection required by the piezoresistive pressure sensors is one of the important challenges in designing these sensors.Many of the piezoresistive sensors function based on employing the composite of conductive additives in an elastomer as an active layer. The functionality and sensitivity of these sensors are highly limited by the poor bulk mechanical properties of the elastomer in addition to unbreathability and the complications arising from the skin-sensor interface. Textile-based sensors overcome the issues regarding the elastomer sensors to a good extend. These sensors are mainly developed through coating fibers by conductive inks or intrinsically conductive polymers (ICPs). However, these sensors suffer from major drawbacks. First, the high conductivity of the conductive coatings leads to shortening in signals upon the application of a small amount of pressure. These sensors can respond either to static or dynamic pressures and once being pressed by a pressure, completely lose their sensitivity to further pressure exertions which resembles a connection/disconnection mode of performance. Second, the sensors need to be used in tight-fitting clothing to be able to capture signals which makes it quite uncomfortable and hinders the widespread adoption of the device in society.Here, we introduce an all-fabric piezoionic pressure sensor, called “PressION”, that successfully overcomes the mentioned drawbacks. We gained significant wide range of pressure detection (several kPa to larger than 100 kPa) through tunning the conductivity of the active layer by coating the fabric with an ion-conductive polysiloxane polymer. This optimal conductivity of the fabric makes the sensor capbale of simultaneously responding to both static and dynamic pressures. To overcome challenge of tight-fitting clothing, we took advantage of the fact that even with loose-fitting garments, there are still some parts of the garment which are pressed due to the exerted pressure by the body limbs over the torso or against an external surface such as bed/chair or even a subtle pressure exerted by a blanket over the body. The success of PressION was confirmed by data acquisition from different body locations (Fig1a-h). All these characteristics makes PressION an ideal candidate to be embedded in daily garments for future biomedical and psychological studies. -Figure Caption- A variety of physiological signals can be extracted by gently placing PressION on different locations of the body: (a) artery pulse from the face; (b) phonation from throat; (c) joint motion at the elbow or knee; (d) heartbeat from the chest while lying face down on a bed; (e) steps from the sensor used as an insole in footwear; and respiration from the sensor placed on the (f) back, (g) side or (h) front of the body. Figure 1

Self-reconfigurable high-weight-per-volume-gelatin films for all-solution-processed on-skin electronics with ultra-conformal contact

Although conventional skin-attachable electronics exhibit good functionalities, their direct attachment (without any adhesive) to human skin with sufficient conformal contact is challenging. Herein, all-solution-processed on-skin electronics based on self-reconfigurable high-weight-per- volume-gelatin (HWVG) film constructed using an effective, biocompatible water absorption-evaporation technique are demonstrated. Completely conformal contact of self-reconfigurable HWVG films is realized by rapidly inducing anisotropic swelling in the perpendicular direction and covering any curvature on the skin without spatial gap or void after shrinking. A sufficiently thin HWVG film (~2 um) exhibited higher adhesion owing to van der Waals force and the carboxylic acid and amine groups in HWVG film form cross-linkages through intermolecular bonds with human skin. Self-reconfigurable HWVG films with high biocompatibility are optimized to afford a superior efficiency of 87.83 % at a concentration of 20 % (w/v) and a storage modulus of 1822 MPa at 36.5 °C. Furthermore, functional nanoelectrodes consisting of self-reconfigurable silver nanowires/HWVG films for high-performance on-skin sensors allowing the detection of sensitive motion and electrophysiological signals, as well as an armband-type sensor system incorporated with a smartphone for health-care monitoring are demonstrated. Outstanding performances, including stability, reliability, flexibility, re-usability, biocompatibility, and permeability of on-skin electronics based on HWVG films can open-up a prospective route to realizing breathable human-machine interfaces based on biocompatible materials and processes.

Conductive PEDOT:PSS on surface-functionalized chitosan biopolymers for stretchable skin-like electronics

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a promising alternative transparent electrode to replace conventional indium tix oxide (ITO) for flexible and stretchable electronics. For their applications in optoelectronic devices, realizing both high conductivity and transmittance for the films is of great necessity as a suitable high performance transparent electrode. Here, we demonstrate simultaneously enhanced electrical and optical properties of PEDOT:PSS films prepared on chitosan bio-substrates by using an organic surface modifier, 11-aminoundecanoic acid (11-AA). The sheet resistance of PEDOT:PSS films decreases from 1120.8 to 292.8 Ω/sq with an increase in a transmittance from 75.9 to 80.4% by 11-AA treatment on the chitosan films. The functional groups of 11-AA effectively enhance the adhesion property at the interface between the chitosan substrate and PEDOT:PSS by forming strong interfacial bondings and decrease insulating PSS from PEDOT:PSS films. The wearable heater devices and on-skin sensors based on the 11-AA-treated PEDOT:PSS on the chitosan bio-substrates are successfully fabricated, showing the excellent thermal and sensing performances. The 11-AA surface-modification approach for highly conductive PEDOT:PSS on chitosan bio-substrates presents a great potential for applications toward transparent, flexible and stretchable electronics.

Multifunctional Ti3C2Tx MXene Composite Hydrogels with Strain Sensitivity toward Absorption-Dominated Electromagnetic-Interference Shielding.

The fast development of terahertz technologies demands high-performance electromagnetic interference (EMI) shielding materials to create safe electromagnetic environments. Despite tremendous breakthroughs in achieving superb shielding efficiency (SE), conventional shielding materials have high reflectivity and cannot be re-edited or recycled once formed, resulting in detrimental secondary electromagnetic pollution and poor adaptability. Herein, a hydrogel-type shielding material incorporating MXene and poly(acrylic acid) is fabricated through a biomineralization-inspired assembly route. The composite hydrogel exhibits excellent stretchability and recyclability, favorable shape adaptability and adhesiveness, and fast self-healing capability, demonstrating great application flexibility and reliability. More interestingly, the shielding performance of the hydrogel shows absorption-dominated feature due to the combination of the porous structure, moderate conductivity, and internal water-rich environment. High EMI SE of 45.3 dB and broad effective absorption bandwidth (0.2-2.0 THz) with excellent refection loss of 23.2 dB can be simultaneously achieved in an extremely thin hydrogel (0.13 mm). Furthermore, such hydrogel demonstrates sensitive deformation responses and can be used as an on-skin sensor. This work provides not only an alternative strategy for designing next-generation EMI shielding material but also a highly efficient and convenient method for fabricating MXene composite on macroscopic scales.

On-skin graphene electrodes for large area electrophysiological monitoring and human-machine interfaces

On-skin sensors capable of capturing human electrophysiological signals are valuable for a variety of areas, such as wearable health monitoring, prosthesis, human machine interfaces (HMI). Extensive research efforts have been devoted to developing on-skin electrodes and they offer potential advantages over conventional rigid electrodes. However, those approaches usually have limitations as they require sophisticated and costly fabrication processes to produce only a small number of electrodes. Here we report the reduced graphene oxide (rGO) electrodes prepared by simultaneous room temperature reduction and patterning process for scalable fabrication of a large number of on-skin electrodes. Due to the low thickness and adhesive substrate adopted, the rGO electrodes can form conformal contact with skin even during deformation, leading to reduced impedance in skin-electrode interface. The rGO electrodes have been successfully adopted for the measurements of electromyography (EMG), electrooculogram (EOG)and electroencephalogram (EEG) signals with high fidelity. And the implementation of a HMI task is demonstrated through the recognition of five typical human hand grasping gestures via a multi-channel rGO electrode array for the control of a dexterous robotic hand to mimic the gestures. The rGO electrodes presented in this study are promising for practical applications in healthcare and HMI.

A Biomimetic Interface with High Adhesion, Tailorable Modulus for On-Skin Sensors, and Low-Power Actuators

Recently, stimuli-responsive sensors and actuators have drawn significant attention as they are promising in a wide scope of applications, including wearable devices and soft robotics. However, the...

A wireless body area sensor network based on stretchable passive tags

A body area sensor network (bodyNET) is a collection of networked sensors that can be used to monitor human physiological signals. For its application in next-generation personalized healthcare systems, seamless hybridization of stretchable on-skin sensors and rigid silicon readout circuits is required. Here, we report a bodyNET composed of chip-free and battery-free stretchable on-skin sensor tags that are wirelessly linked to flexible readout circuits attached to textiles. Our design offers a conformal skin-mimicking interface by removing all direct contacts between rigid components and the human body. Therefore, this design addresses the mechanical incompatibility issue between soft on-skin devices and rigid high-performance silicon electronics. Additionally, we introduce an unconventional radiofrequency identification technology where wireless sensors are deliberately detuned to increase the tolerance of strain-induced changes in electronic properties. Finally, we show that our soft bodyNET system can be used to simultaneously and continuously analyse a person’s pulse, breath and body movement. By integrating wireless stretchable on-skin sensor tags and flexible readout circuits attached to textiles using an unconventional radiofrequency identification design, a body area sensor network can be created that can continuously analyse a person’s pulse, breathing and body movement.

Soft sensors form a network

A wearable wireless sensor network for personalized healthcare can be created through the indirect integration of soft on-skin sensors and rigid in-clothes circuits.

Hygroscopic Auxetic On-Skin Sensors for Easy-to-Handle Repeated Daily Use.

Despite the advance of on-skin sensors over the last decade, a sensor that solves simultaneously the critical issues for using in everyday life, such as stable performance in various environments, use over a long period of time, and repeated use by easy handling, has not yet been achieved. Here, we introduce an auxetic hygroscopic sensor that simultaneously meets all of the conditions. The auxetic structure with a negative Poisson's ratio matches with deformation of the skin in ankles; hence, a conformal contact between the sensor and the skin could be maintained during repeated movements. Sweat was absorbed in the auxetic electrode made of a hydrogel pattern coated with Ag nanowires and evaporated quickly; such hygroscopic characteristic led to excellent breathability. An electrocardiogram sensor and a haptic device were fabricated according to the proposed design for a sensor electrode. The sensors provide stable detecting performance in various environments, such as exercising, submersion in water, exposure to concentrated salt water, and continuous wearing for long time (7 days). Also, the sensors could be manually attached repeatedly without degrading the performance. This study provides new structural insights for on-skin sensors and presents future research directions.

Inflammation-free, gas-permeable, lightweight, stretchable on-skin electronics with nanomeshes.

Thin-film electronic devices can be integrated with skin for health monitoring and/or for interfacing with machines. Minimal invasiveness is highly desirable when applying wearable electronics directly onto human skin. However, manufacturing such on-skin electronics on planar substrates results in limited gas permeability. Therefore, it is necessary to systematically investigate their long-term physiological and psychological effects. As a demonstration of substrate-free electronics, here we show the successful fabrication of inflammation-free, highly gas-permeable, ultrathin, lightweight and stretchable sensors that can be directly laminated onto human skin for long periods of time, realized with a conductive nanomesh structure. A one-week skin patch test revealed that the risk of inflammation caused by on-skin sensors can be significantly suppressed by using the nanomesh sensors. Furthermore, a wireless system that can detect touch, temperature and pressure is successfully demonstrated using a nanomesh with excellent mechanical durability. In addition, electromyogram recordings were successfully taken with minimal discomfort to the user.

IC-integrated flexible shear-stress sensor skin

This paper reports the successful development of the first IC-integrated flexible MEMS shear-stress sensor skin. The sensor skin is 1 cm wide, 2 cm long, and 70 μm thick. It contains 16 shear-stress sensors, which are arranged in a 1-D array, with on-skin sensor bias, signal-conditioning, and multiplexing circuitry. We further demonstrated the application of the sensor skin by packaging it on a semicylindrical aluminum block and testing it in a subsonic wind tunnel. In our experiment, the sensor skin has successfully identified both the leading-edge flow separation and stagnation points with the on-skin circuitry. The integration of IC with MEMS sensor skin has significantly simplified implementation procedures and improved system reliability.