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Electronic skin (E-skin) capable of detecting various physical stimuli is required for monitoring external environments accurately. Here, we report an all-soft multiaxial force sensor based on liquid metal microchannel array for electronic skin applications. The proposed sensor is composed of stretchable elastomer and Galinstan, a eutectic gallium-indium alloy, providing a high mechanical flexibility and electro-mechanical durability. Liquid metal microchannel arrays are fabricated in multilayer and positioned along a dome structure to detect multi-directional forces, supported by numerical simulation results. By adjusting the height of the dome, we could control the response of the multiaxial sensor with respect to the deflection. As a demonstration of multiaxial force sensing, we were able to monitor the direction of multidirectional forces using a finger by the response of liquid metal microchannel arrays. This research could be applied to various fields including soft robotics, wearable devices, and smart prosthetics for artificial intelligent skin applications.

Despite various advantages and usefulness of semiconductor metal oxide gas sensors, low selectivity and humidity interference have limited their practical applications. In order to resolve these issues, we propose a new concept of a selective gas filtering structure that increases the gas selectivity and decreases the moisture interference of metal oxide gas sensors by coating metal organic frameworks (MOFs) on a microporous elastomer scaffold. Cu(BTC) with an excellent selective adsorption capacity for carbon monoxide (CO) compared to hydrogen (H

Capabilities for continuous monitoring of pressures and temperatures at critical skin interfaces can help to guide care strategies that minimize the potential for pressure injuries in hospitalized patients or in individuals confined to the bed. This paper introduces a soft, skin-mountable class of sensor system for this purpose. The design includes a pressure-responsive element based on membrane deflection and a battery-free, wireless mode of operation capable of multi-site measurements at strategic locations across the body. Such devices yield continuous, simultaneous readings of pressure and temperature in a sequential readout scheme from a pair of primary antennas mounted under the bedding and connected to a wireless reader and a multiplexer located at the bedside. Experimental evaluation of the sensor and the complete system includes benchtop measurements and numerical simulations of the key features. Clinical trials involving two hemiplegic patients and a tetraplegic patient demonstrate the feasibility, functionality and long-term stability of this technology in operating hospital settings.

A handheld confocal microscope using a rapid MEMS scanning mirror facilitates real-time optical biopsy for simple cancer diagnosis. Here we report a handheld confocal microscope catheter using high definition and high frame rate (HDHF) Lissajous scanning MEMS mirror. The broad resonant frequency region of the fast axis on the MEMS mirror with a low Q-factor facilitates the flexible selection of scanning frequencies. HDHF Lissajous scanning was achieved by selecting the scanning frequencies with high greatest common divisor (GCD) and high total lobe number. The MEMS mirror was fully packaged into a handheld configuration, which was coupled to a home-built confocal imaging system. The confocal microscope catheter allows fluorescence imaging of in vivo and ex vivo mouse tissues with 30 Hz frame rate and 95.4% fill factor at 256 × 256 pixels image, where the lateral resolution is 4.35 μm and the field-of-view (FOV) is 330 μm × 330 μm. This compact confocal microscope can provide diverse handheld microscopic applications for real-time, on-demand, and in vivo optical biopsy.

Description, Thermoforming is a versatile technique to fabricate stretchable and customizable 3DEs for a variety of wearable applications., Recently, three-dimensional electronics (3DE) is attracting huge interest owing to the increasing demands for seamless integration of electronic systems on 3D curvilinear surfaces. However, it is still challenging to fabricate 3DE with high customizability, conformability, and stretchability. Here, we present a fabrication method of 3DE based on predistorted pattern generation and thermoforming. Through this method, custom-designed 3DE is fabricated through the thermoforming process. The fabricated 3DE has high 3D conformability because the thermoforming process enables the complete replication of both the overall shape and the surface texture of the 3D mold. Furthermore, the usage of thermoplastic elastomer and a liquid metal–based conductive electrode allows for high thermoformability during the device fabrication as well as high stretchability during the device operation. We believe that this technology can enable a wide range of new functionalities and multiscale 3D morphologies in wearable electronics.

Capabilities for continuous monitoring of pressures and temperatures at critical skin interfaces can help to guide care strategies that minimize the potential for pressure injuries in hospitalized patients or in individuals confined to the bed. This paper introduces a soft, skin-mountable class of sensor system for this purpose. The design includes a pressure-responsive element based on membrane deflection and a battery-free, wireless mode of operation capable of multi-site measurements at strategic locations across the body. Such devices yield continuous, simultaneous readings of pressure and temperature in a sequential readout scheme from a pair of primary antennas mounted under the bedding and connected to a wireless reader and a multiplexer located at the bedside. Experimental evaluation of the sensor and the complete system includes benchtop measurements and numerical simulations of the key features. Clinical trials involving two hemiplegic patients and a tetraplegic patient demonstrate the feasibility, functionality and long-term stability of this technology in operating hospital settings., Uninterrupted monitoring of pressure and temperature at skin interfaces can help to minimize the potential for pressure injuries in hospitalized or bedridden patients. Here, the authors introduce a soft, skin-mountable sensor that can continuously provide readings via antennas mounted under bedding, and demonstrate the functionality and robustness of the devices on patients.

A handheld confocal microscope using a rapid MEMS scanning mirror facilitates real-time optical biopsy for simple cancer diagnosis. Here we report a handheld confocal microscope catheter using high definition and high frame rate (HDHF) Lissajous scanning MEMS mirror. The broad resonant frequency region of the fast axis on the MEMS mirror with a low Q-factor facilitates the flexible selection of scanning frequencies. HDHF Lissajous scanning was achieved by selecting the scanning frequencies with high greatest common divisor (GCD) and high total lobe number. The MEMS mirror was fully packaged into a handheld configuration, which was coupled to a home-built confocal imaging system. The confocal microscope catheter allows fluorescence imaging of in vivo and ex vivo mouse tissues with 30 Hz frame rate and 95.4% fill factor at 256 × 256 pixels image, where the lateral resolution is 4.35 μm and the field-of-view (FOV) is 330 μm × 330 μm. This compact confocal microscope can provide diverse handheld microscopic applications for real-time, on-demand, and in vivo optical biopsy.

Soft strain sensors have attracted significant attention in wearable human motion monitoring applications. However, there is still a huge challenge for decoupled measurement of multidirectional strains. In this study, we have developed a biaxial and stretchable strain sensor based on a carbon nanotube (CNT) film and a microdome array (MA)-patterned elastomeric substrate. The MA structures lead to generating localized and directional microcracks of CNT films within the intended regions under tensile strain. This mechanism allows a single sensing layer to act as a strain sensor capable of decoupling the biaxial strains into axial and transverse terms. The ratio of resistance change between two perpendicular axes is about 960% under an x-directional strain of 30%, demonstrating the biaxial decoupling capability. Also, the proposed strain sensor shows high stretchability and excellent long-term reliability under a cyclic loading test. Finally, wearable devices integrated with the strain sensor have been successfully utilized to monitor various human motions of the wrist, elbow, knee, and fingers by measuring joint bending and skin elongation.

In this research, an ultrathin, biocompatible, and flexible pressure sensor with a wide pressure range has been developed and applied in biomedical applications. The pressure sensing mechanism is based on the variation of contact resistance between an electrode and a three-dimensional microstructured polyimide/carbon nanotube composite film. The sensor has a thickness of about 31.3 μm, a maximum sensitivity of 41.0 MPa-1, and a sensing range of 10-500 kPa. Moreover, in situ temperature measurement by an integrated resistive temperature detector enables data correction for varying temperature conditions. In order to show the advantages of the fabricated sensor, it is attached to the human body and integrated with the surface of a radiofrequency ablation (RFA) needle with small radius of curvature. In the experiments, the proposed pressure sensor measured subtle pressure levels (pulse pressure) and high pressure levels (fingertip pressure) without losing conformal contact with the skin. In addition, when the pressure-sensor-integrated RFA needle was inserted into a bovine liver, successful detection of steam popping phenomenon was observed.

Wearable pressure sensors have been attracting great attention for a variety of practical applications, including electronic skin, smart textiles, and healthcare devices. However, it is still challenging to realize wearable pressure sensors with sufficient sensitivity and low hysteresis under small mechanical stimuli. Herein, we introduce simple, cost-effective, and sensitive capacitive pressure sensor based on porous Ecoflex-multiwalled carbon nanotube composite (PEMC) structures, which leads to enhancing the sensitivity (6.42 and 1.72 kPa

A biopsy needle with electrical impedance sensor array based on stainless steel microelectrodes (EIS needle) was developed for real-time four electrode measurement and multi-spot sensing of tissues during the biopsy process. The sensor performance was characterized by using saline solutions with various concentrations, which proved accurate, stable and reliable electrical impedance measurement. The capability of impedance-based tissue sensing was verified by the conductivity measurement of agarose hydrogel based phantom mimicking cancer tissue. Furthermore, multi-spot impedance sensing during needle insertion was demonstrated using porcine meat with muscle and fat layers, which exhibited a clear discrimination between different types of tissues. Also, the electrical impedance difference between normal and fatty livers of mouse model was measured by the EIS needle. We could successfully demonstrate that the EIS needle can provide localized and accurate characterization of biological tissues at the needle tip.

We developed a new method to directly fabricate 3D multiaxial force sensor using fused deposition modeling (FDM) 3D printing of functionalized nanocomposite filaments. Here, 3D cubic cross shaped force sensor is suggested to measure the forces from three axes (x, y and z). The sensor has two components – a structural part and a sensing part – both of which are concurrently fabricated by 3D printing with different functional filaments. The structural part is printed with thermoplastic polyurethane (TPU) filament and the sensing part is printed with carbon nanotube (CNT)/TPU nanocomposite filament with a piezoresistivity on the surface of the structural part. The resistances of the sensing part are measured in three axial directions; Rx, Ry, and Rz and the force applied on each axis is measured by the resistance change. The 3D-printed multiaxial force sensor could detect the sub-millimeter scale deflection and its corresponding force on each axis. According to the sensing principle, when Fz = 4 N was applied, Rz was decreased by 2% while only 0.2% resistance change of Ry was induced. In addition, a simultaneous resistance measurement system was developed for a real-time force sensing in three axes. With its customizability, rapid manufacturing, and economic feasibility, this manufacturing approach allows direct fabrication of multiaxial sensors without additional assembly or integration processes.

Wearable pressure sensors capable of sensitive, precise, and continuous measurement of physiological and physical signals have great potential for the monitoring of health status and the early diagnosis of diseases. This work introduces a 3D-printed rigid microbump-integrated liquid metal-based soft pressure sensor (3D-BLiPS) for wearable and health-monitoring applications. Using a 3D-printed master mold based on multimaterial fused deposition modeling, the fabrication of a liquid metal microchannel and the integration of a rigid microbump array above the microchannel are achieved in a one-step, direct process. The microbump array enhances the sensitivity of the pressure sensor (0.158 kPa-1 ) by locally concentrating the deformation of the microchannel with negligible hysteresis and a stable signal response under cyclic loading. The 3D-BLiPS also demonstrates excellent robustness to 10 000 cycles of multidirectional stretching/bending, changes in temperature, and immersion in water. Finally, these characteristics are suitable for a wide range of applications in health monitoring systems, including a wristband for the continuous monitoring of the epidermal pulse rate for cuffless blood pressure estimation and a wireless wearable device for the monitoring of body pressure using a multiple pressure sensor array for the prevention of pressure ulcers.

Flexible and wearable pressure sensors have attracted a tremendous amount of attention due to their wider applications in human interfaces and healthcare monitoring. However, achieving accurate pressure detection and stability against external stimuli (in particular, bending deformation) over a wide range of pressures from tactile to body weight levels is a great challenge. Here, we introduce an ultrawide-range, bending-insensitive, and flexible pressure sensor based on a carbon nanotube (CNT) network-coated thin porous elastomer sponge for use in human interface devices. The integration of the CNT networks into three-dimensional microporous elastomers provides high deformability and a large change in contact between the conductive CNT networks due to the presence of micropores, thereby improving the sensitivity compared with that obtained using CNT-embedded solid elastomers. As electrical pathways are continuously generated up to high compressive strain (∼80%), the pressure sensor shows an ultrawide pressure sensing range (10 Pa to 1.2 MPa) while maintaining favorable sensitivity (0.01-0.02 kPa

In this research, we introduce a strain-insensitive soft pressure sensor using 3D-printed microchannel mold and liquid metal for health monitoring applications. Our soft pressure sensor consists of elastomer and liquid metal for its intrinsic stretchable property. Also, serpentine geometric pattern of the microchannel enabled the sensor to be strain-insensitive for stable signal response and recovery in wearable applications. Various experiments for sensor characterizations were conducted and the study of FEM simulation verified the strain-insensitive behavior of serpentine pattern. As a health monitoring application, body pressure distribution was monitored using several pressure sensors attached on the clothes and the body pressures were compared according to various lying conditions.

This paper reports a wide range-sensitive, bending-insensitive pressure sensor based on a carbon nanotube (CNT)-coated porous elastomer for a wearable healthcare application. (1) Sensing performance was evaluated over ~1MPa, which includes whole human tactile and body weight levels. The sensor exhibits a very wide sensing range from ultralow to high pressure (50Pa-1MPa) with maintenance of favorable sensitivity and reliable dynamic responses. (2) To our knowledge this is the first to investigate bending-insensitivity of the CNTs-coated porous PDMS, which facilitates accurate detection of normal pressures even on curved surfaces like human skins. (3) Finally, we demonstrated a foot insole for real-time monitoring of foot plantar pressure distribution.

In this research, we introduce a wearable soft microfluidic pressure sensor using 3D-printed microbumps and liquid metal and its potential applications in health monitoring. Our pressure sensor fabricated with elastomer and liquid metal which provide stretchability has an enhanced sensitivity (0.3 kPa−1@100kPa) compared to other reported microfluidic pressure sensors [1] and a good recovery characteristic in pressure range to 100 kPa. It has an enough sensitivity to be applied in pressure monitoring applications. Blood pressure could be estimated through monitoring pulse rate and electrocardiogram (ECG) data simultaneously. Also, human body pressure distribution was monitored using multiple pressure sensors attached on the clothes when lying on the bed. Biosignals and human body pressure data achieved through the proposed pressure sensor would be applied to early diagnosis and prevention of diseases such as diabetes and bedsores.

In this work, we report a novel stretchable fabric heater based on silver nanowire, carbon nanotube composites. We propose improved spray coating method to develop a stretchable fabric heater with excellent mechanical properties and electrical properties. The combination of the carbon nanotubes (CNTs) with high structural stability and silver nanowires (Ag NWs) with high conductivity could highlight the advantage of each nanomaterials. The proposed fabric heater showed a stable temperature control even in the folded and rolled states. In addition, fabric heaters were applied in commercial cotton gloves successfully.

In this work, we report a highly sensitive and flexible capacitive pressure sensor based on a porous dielectric matrix containing multi-walled carbon nanotubes (MWNTs) as conductive fillers. Porous dielectric matrix containing conductive fillers has a high resolution because the conductive fillers increase dielectric constant of the matrix and make big difference of dielectric constant between micro-pore in the matrix and matrix itself. This difference leads to a high sensitivity. Additionally the output signal of the pressure sensor shows little variation under different environment disturbances, such as dynamic pressure inputs, temperatures, and different air humidity levels. Furthermore, a robot-arm system, with the ability to move a soft material, is constructed to demonstrate the practical applications of the pressure sensor by using a pressure feedback control.

Conventional flexible pressure sensors are not suitable for high-pressure applications due to their low saturation pressure. In this study, an ultra-wide range pressure sensor is designed based on the optimized microstructure of the polyimide/carbon nanotubes (PI/CNT) nanocomposite film. The sensing range of the pressure sensor is expanded by adopting polyimide (PI) with a high elastic modulus as a matrix material and its sensitivity is improved through functional sensing film with tip-flattened microdome arrays. As a result, the pressure sensor can measure a wide pressure range (≈ 0-3000 kPa) and possesses the sensitivity of ≈ 5.66 × 10-3 -0.23 × 10-3 kPa-1 with high reliability and durability up to 1000 cycles. The proposed sensor is integrated into the hand and foot pressure monitoring systems for workout monitoring. The representative values of the pressure distribution in the hands and feet during the powerlifting are acquired and analyzed through Pearson's correlation coefficient (PCC). The analyzed results suggest that the pressure sensor can provide useful real-time information for healthcare and sports performance monitoring.

There is a great demand for the development of self-powered physical sensors for wearable applications in recent years. However, it is still challenging to achieve self-powered sensors with a high stability, accuracy, and linearity. Here, a novel wearable self-powered pressure sensor based on the integration of a piezo-transmittance microporous elastomer (PTME) and a thin-film organic solar cell (OSC) is proposed. In contrast to the sensors based on other mechanisms such as piezoelectricity or triboelectricity, the proposed self-powered pressure sensor is cable of measuring static pressure continuously and stably, and utilizes the ambient light as the power source regardless of its intensity. The PTME shows the light transmittance changes by a gradual closure of micropores with compression in response to the applied pressure. This unique optical characteristics of the PTME enables the OSC to generate varying electrical current in response to the pressure. The proposed self-powered pressure sensor shows a high-performance with a sensitivity of 0.101/kPa, a linearity of R2 = 0.995, and fast and reversible response to the pressure up to ~100 kPa. As practical applications of the proposed sensor, a detection of flexion/extension of a human finger for the manipulation of a prosthetic robot finger and a wind detection for the continuous monitoring of the wind speed and direction have been demonstrated.

High-performance and low-power flexible Schottky diode-based hydrogen sensor was developed. The sensor was fabricated by releasing Si nanomembrane (SiNM) and transferring onto a plastic substrate. After the transfer, palladium (Pd) and aluminum (Al) were selectively deposited as a sensing material and an electrode, respectively. The top-down fabrication process of flexible Pd/SiNM diode H2 sensor is facile compared to other existing bottom-up fabricated flexible gas sensors while showing excellent H2 sensitivity (ΔI/I0 > 700–0.5% H2 concentrations) and fast response time (τ10–90 = 22 s) at room temperature. In addition, selectivity, humidity, and mechanical tests verify that the sensor has excellent reliability and robustness under various environments. The operating power consumption of the sensor is only in the nanowatt range, which indicates its potential applications in low-power portable and wearable electronics.

Radiofrequency ablation (RFA) method is an effective method in cancer surgery. However, due to its high temperature and pressure, explosive behavior called ‘steam popping’ was occurred during RFA. This phenomenon can cause recurrence and metastasis of the cancer. Though this is a critical problem, but steam popping still can be detected by the explosive sound. In this research, thin film based flexible pressure sensor with the range of 100–400 kPa was developed, and attached to the RFA needle to monitor the steam popping through pressure variation. Fabricated needle showed low interference from temperature variation, electromagnetic noise, and chemical environment noise. Proposed needle successfully monitor the steam popping phenomenon.

In this work, we developed micro-crack based stretchable strain sensors consist of polydimethylsiloxane (PDMS) with micro-dome structure arrays and carbon nanotube (CNT) film coated on its surface. Due to a stress concentration caused by micro-dome structure under a certain strain, micro-cracks of CNT film are generated and propagated more easily as compared with the case of a CNT film on the plane surface. In addition, because the micro-cracks are only positioned at designed region which is between micro-domes, electrical resistances in elongated and perpendicular directions are clearly different thanks to the novel mechanism. As results of these phenomena, strain decoupling is achieved and a sensitivity of the sensor can be modulated by adjusting the dimension of micro-dome arrays. We also conducted the numerical simulation based on finite element method (FEM) to verify the designed sensing mechanism and predict the performance of the sensor. This kind of work is important for the motion detecting sensor for acquiring decoupled information from complex signal. Finally, we fabricated a smart glove that could detect a biaxial strain applied on the back of a hand skin to demonstrate that this sensor could be used as wearable device.