Energetically Autonomous, Wearable, and Multifunctional Sensor

Recent Progress on Flexible Self-Powered Tactile Sensing Platforms for Health Monitoring and Robotics

Over the past decades, tactile sensing technology has made significant advances in the fields of health monitoring and robotics. Compared to conventional sensors, self-powered tactile sensors do not require an external power source to drive, which makes the entire system more flexible and lightweight. Therefore, they are excellent candidates for mimicking the tactile perception functions for wearable health monitoring and ideal electronic skin (e-skin) for intelligent robots. Herein, the working principles, materials, and device fabrication strategies of various self-powered tactile sensing platforms are introduced first. Then their applications in health monitoring and robotics are presented. Finally, the future prospects of self-powered tactile sensing systems are discussed.

Studies on the Platinum Thick Film Sensor Conformally Written by Laser Micro-Cladding: Formability, Microstructure, and Performance

In this paper, laser micro-cladding technology (LMC) was conducted to prepare high-temperature Pt thick film sensors in situ. The formability, microstructure, sintering mechanism, and electrical properties of the LMCed Pt thick films were first studied systematically. Results indicated that with the increase of laser power density, the sintering degree of the Pt thick film increased obviously, improving its adhesion strength and reducing its resistivity. However, when the laser power density exceeded the threshold, holes or grooves were formed in the Pt film, leading to the degeneration of its properties. A Pt thick film with good adhesion strength, excellent conductive networks, and the minimum resistivity (46 ± 2 μΩ·cm) was obtained at a laser power density of 1.37 × 10⁶ W·cm^-2. Then, Pt thick film temperature sensors (including Pt thermal resistance temperature (RTD) and Pt-Pt10%Rh thermocouple sensors) were conformally prepared by LMC. Their temperature-sensing performance became stable after the initial high-temperature calibration, with a linearity of 0.9985 for the RTD with a TCR of 2.46 × 10^-3/°C (at 920 °C) and a linearity of 0.9905 for the thermocouple with a Seebeck coefficient of 9.7 μV/°C, both of which are better than that made by direct DC magnetron sputtering deposition. Therefore, this work provides a novel feasible way to conformally integrate high-performance Pt film sensors in situ.

Highly sensitive determination of paracetamol, uric acid, dopamine, and catechol based on flexible plastic electrochemical sensors

Flexible sensing is an alternative to traditional sensing and possesses good flexibility and wearability. Intrinsically conductive polymers, particularly poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), have received significant attention due to their high mechanical flexibility and good biocompatibility. Here, we report the design of highly conductive and electrochemically active PEDOT:PSS-coated plastic substrate electrodes by combining N-doped graphene (NG) or S-doped graphene (SG) with methanesulfonic acid-treated PEDOT:PSS (denoted as NG-f-MSA-PEDOT:PSS/PET and SG-f-MSA-PEDOT:PSS/PET) by a simple drop-coating method. At room temperature, the NG-f-MSA-PEDOT:PSS/PET electrode demonstrated the lowest detection limits of 17.09, 33.84, 28.30, and 44.96 nM for paracetamol, uric acid, dopamine, and catechol (S/N = 3), respectively. The NG-f-MSA-PEDOT:PSS/PET electrode had good anti-interference ability and reproducibility without employing expensive noble metals and requiring much effort to polish the surface of traditional glass carbon electrodes. Most importantly, this film electrode could maintain a stable electrochemical response under different bending and crease states and had excellent mechanical stability and flexibility.

Wearable Sensors Based on Solid-Phase Molecular Self-Assembly: Moisture-Strain Dual Responsiveness Facilitated Extremely High and Damage-Resistant Sensitivity

Wearable sensing technologies have gained increasing interest in biomedical fields because they are convenient and could efficiently monitor health conditions by detecting various physiological signals in real time. However, common film sensors often neglect body moisture and enhance the sensitivity by enhancing the conductive dopants and self-healing ability. We report in this work a supramolecular film sensor based on solid-phase molecular self-assembly (SPMSA), which smartly utilizes the body moisture to enhance the sensitivity for human-machine interaction. The carbon nanotube (CNT)-doped SPMSA film is able to capture environmental moisture quickly. Upon contact to human skin, the moisture not only promotes the junction between CNTs but also contributes to the conductivity. As a result, the sensitivity can be enhanced 4 times. In this way, we are able to obtain the highest sensitivity of 700% with the lowest CNT doping rate of 0.5%. Furthermore, the current sensor displays damage-inert sensing performance. In the presence of a hole of up to 50% of the film area, the sensitivity remains unaffected due to the decreases in the absolute conductivity of the film sensor before and after a trigger to the same extent. In this way, we have developed a new principle in the design of a film sensor for human-machine interaction, which releases the sensor from focus on promoting conductivity and self-healing materials.

Tuning Strain Sensor Performance via Programmed Thin-Film Crack Evolution

Stretchable strain sensors with well-controlled sensitivity and stretchability are crucial for applications ranging from large deformation monitoring to subtle vibration detection. Here, based on single-metal material on the elastomer and one-pot evaporation fabrication method, we realize controlled strain sensor performance via a novel programable cracking technology. Specifically, through elastomeric substrate surface chemistry modification, the microcrack generation and morphology evolution of the strain sensing layer is controlled. This process allows for fine tunability of the cracked film morphology, resulting in strain sensing devices with a sensitivity gauge factor of over 10 000 and stretchability up to 100%. Devices with a frequency response up to 5.2 Hz and stability higher than 1000 cycles are reported. The reported strain sensors, tracking both subtle and drastic mechanical deformations, are demonstrated in healthcare devices, human-machine interaction, and smart-home applications.

Noninterference Wearable Strain Sensor: Near-Zero Temperature Coefficient of Resistance Nanoparticle Arrays with Thermal Expansion and Transport Engineering

In this study, non-temperature interference strain gauge sensors, which are only sensitive to strain but not temperature, are developed by engineering the properties and structure from a material perspective. The environmental interference from temperature fluctuations is successfully eliminated by controlling the charge transport in nanoparticles with thermally expandable polymer substrates. Notably, the negative temperature coefficient of resistance (TCR), which originates from the hopping transport in nanoparticle arrays, is compensated by the positive TCR of the effective surface thermal expansion with anchoring effects. This strategy successfully controls the TCR from negative to positive. A near-zero TCR (NZTCR), less than 1.0 × 10^-6 K^-1, is achieved through precisely controlled expansion. Various characterization methods and finite element and transport simulations are conducted to investigate the correlated electrical, mechanical, and thermal properties of the materials and elucidate the compensated NZTCR mechanism. With this strategy, an all-solution-processed, transparent, highly sensitive, and noninterference strain sensor is fabricated with a gauge factor higher than 5000 at 1% strain, as demonstrated by pulse and motion sensing, as well as the noninterference property under variable-temperature conditions. It is envisaged that the sensor developed herein is applicable to multifunctional wearable sensors or e-skins for artificial skin or robots.

Highly Sensitive and Durable Sea-Urchin-Shaped Silver Nanoparticles Strain Sensors for Human-Activity Monitoring

High-performance strain sensors, composed of various artificial sensing materials on/in stretchable substrates, show great promise for applications in flexible electronic devices. Here, we demonstrated a highly sensitive and durable strain sensor consisting of a ribbon of close-packed sea-urchin-shaped silver nanoparticles (SUSNs) sandwiched between two layers of poly(dimethylsiloxane) (PDMS). Each of SUSNs possesses high-density and spherically distributed sharp spines over the body, which promotes electron transduction and further improves signal detection. This SUSN-based sensor possesses a desirable integration of high sensitivity (a gauge factor of 60) and large stretchability (up to 25%) at tensile sensing, broadening its application in wearable devices. Moreover, it also shows fast response (48 ms), good reproducibility, and long-term stability (>2500 cycles at 20% strain). It can also be used to detect compressing (sensitivity up to 31.5) and folding-type bending deformations. The sensing mechanism, the resistance of the sensors varying as the deformation load, results from the inter-spine contacts change and the microcracks evolution caused by variation in the gap between SUSNs. The sensor's sensitivity at different degrees of strain was also achieved by controlling the width of the close-packed SUSNs ribbon. For practical demonstration, the SUSN-based sensors could be used as wearable devices for monitoring human activities ranging from subtle deformations to substantial movements.

Fabrication and ex vivo evaluation of activated carbon-Pt microparticle based glutamate biosensor

As one of the most abundant neurotransmitters in the brain and the spinal cord, glutamate plays many important roles in the nervous system. Precise information about the level of glutamate in the extracellular space of living brain tissue may provide new insights on fundamental understanding of the role of glutamate in neurological disorders as well as neurophysiological phenomena. Electrochemical sensor has emerged as a promising solution that can satisfy the requirement for highly reliable and continuous monitoring method with good spatiotemporal resolution for characterization of extracellular glutamate concentration. Recently, we published a method to create a simple printable glutamate biosensor using platinum nanoparticles. In this work, we introduce an even simpler and lower cost conductive polymer composite using commercially available activated carbon with platinum microparticles to easily fabricate highly sensitive glutamate biosensor using direct ink writing method. The fabricated biosensors are functionality superior than previously reported with the sensitivity of 5.73 ± 0.078 nA μM^-1 mm^-2, detection limit of 0.03 μM, response time less than or equal to 1 s, and a linear range from 1 μM up to 925 μM. In this study, we utilize astrocyte cell culture to demonstrate our biosensor's ability to monitor glutamate uptake process. We also demonstrate direct measurement of glutamate release from optogenetic stimulation in mouse primary visual cortex (V1) brain slices.

Transparent Body-Attachable Multifunctional Pressure, Thermal, and Proximity Sensor and Heater

A multifunctional sensor capable of simultaneous sensing of temperature, pressure, and proximity has been developed. This transparent and body-attachable device is also capable of providing heat under low voltage. The multi-sensor consists of metal fibers fabricated by electrospinning and electroplating. The device comprises randomly deposited metal fibers, which not only provide heating but also perform as thermal and proximity sensors, and orderly aligned metal fibers that act as a pressure sensor. The sensor is fabricated by weaving straight rectangular electrodes on a transparent substrate (a matrix). The sensitivity is readily enhanced by installing numerous matrices that facilitate higher sensing resolution. The convective heat transfer coefficient of the heater is h = 0.014 W·cm⁻²·°C⁻¹. The temperature coefficient of resistivity (TCR) and pressure sensitivity (η_P) are 0.038 °C⁻¹ and 5.3 × 10⁻³ kPa⁻¹, respectively. The superior sensitivity of the device is confirmed via quantitative comparison with similar devices. This multifunctional device also has a superior convective heat transfer coefficient than do other heaters reported in the literature.

Wearable, Stretchable, Transparent All-in-One Soft Sensor Formed from Supersonically Sprayed Silver Nanowires

The demand for wearable, stretchable soft electronics for human-machine interface applications continues to grow given the potential of these devices in humanoid robotics, prosthetics, and health-monitoring devices. We demonstrate fabrication of multifunctional sensors with simultaneous temperature-, pressure-, proximity-, and strain (or bending)-sensing capabilities, combined with heating and UV-protection features. These multifunctional sensors are flexible, light, and transparent and are thus body-attachable. Silver nanowires are supersonically sprayed on a large-scale transparent and flexible roll-to-roll substrate. The junctions between nanowires are physically fused by a strong impact resulting from supersonic spraying, which promotes adhesion and efficient deposition of the nanowire network. Accordingly, nanowires are strongly interconnected, facilitating efficient propagation of electric signals through the fused nanowire network, which allows simultaneous operation of such sensors while maintaining significant transparency. These multifunctional sensors are mechanically durable and retain long-term stability. A theoretical discussion is provided to explain the respective mechanisms of heating and proximity, pressure, and strain sensing.

High-Performance Flexible Ultraviolet Photodetectors Based on AZO/ZnO/PVK/PEDOT:PSS Heterostructures Integrated on Human Hair

Flexible optoelectronics is an emerging research field that has attracted a great deal of interest in recent years due to the special functions and potential applications of these devices in flexible image sensors, optical computing, energy conversion devices, the Internet of Things, and other technologies. Here, we examine the high-performance ultraviolet (UV) photodetectors using AZO/ZnO nanorods/PVK/PEDOT:PSS heterostructures integrated on human hair. Due to the precise interfacial energy-level alignment among all layers and superior mechanical characteristics of human hair, the as-obtained photodetector shows a fast response time, high photoresponsivity, and excellent flexibility. According to integrate 7 heterostructures as 7 display pixels, the flexible UV-image sensor has superior device performance and outstanding flexibility and can produce vivid and accurate images of Arabic numerals from 0 to 9. Different combinations of the two heterostructures can also be used to achieve flexible photon-triggered logic functions, including AND, OR, and NAND gates. Our findings indicate the possibility of using human hair as a fiber-shaped flexible substrate and will allow the use of hair-based hierarchical heterostructures as building blocks to create exciting opportunities for next-generation high-performance, multifunctional, low-cost, and flexible optoelectronic devices.

Multilayered Ag NP-PEDOT-Paper Composite Device for Human-Machine Interfacing

Flexible pressure sensors have attracted increasing interest because of their potential applications on wearable sensing devices for human-machine interface connections, but challenges regarding material cost, fabrication robustness, signal transduction, sensitivity improvement, detection range, and operation convenience still need to be overcome. Herein, with a simple, low-cost, and scalable approach, a flexible and wearable pressure-sensing device fabricated by utilizing filter paper as the solid support, poly(3,4-ethylenedioxythiophene) to enhance conductivity, and silver nanoparticles to provide a rougher surface is introduced. Sandwiching and laminating composite material layers with two thermoplastic polypropylene films lead to robust integration of sensing devices, where assembling four layers of composite materials results in the best sensitivity toward applied pressure. This practical pressure-sensing device possessing properties such as high sensitivity of 0.119 kPa^-1, high durability of 2000 operation cycles, and an ultralow energy consumption level of 10^-5 W is a promising candidate for contriving point-of-care wearable electronic devices and applying it to human-machine interface connections.

Body Compatible Thermometer Based on Green Electrolytes

With the use of coordinated complexes between aliphatic diols and calcium chloride (CaCl₂) as green electrolytes, a body compatible, ecofriendly and low-cost thermometer is successfully developed. This particular conductive liquid possesses unique features of ultrafast response and high sensitivity against temperature change. The influences of CaCl₂ concentration and the category of aliphatic diols on conductivity change reveal that the thermal sensing abilities of such green electrolytes are positively relevant to the viscosity change along with temperature change. Owing to the advantages of stability, reliability, and security, the thermometer can implement long-term and continuous temperature monitoring, which can fully meet the requirements of application of medical monitors, diagnostics, and therapies. Moreover, the inherent advantages of thermometers, including satisfactory biocompatibility and nontoxicity, afford great promise for applications in invasive and inflammatory devices.