Low-cost, sensitive and flexible NH3 sensors are increasingly needed for health monitoring and food freshness assessment. Carboxyl groups (-COOH) enable ammonia (NH3) adsorption and porous film benefit the gas sensing performance. Therefore, an active flexible porous film based on Polystyrene (PS)/polystyrene-block-poly (acrylic acid) (PS-b-PAA) was designed via water-in-oil emulsion templating and used as a selective NH3 sensor in this work. The PS-b-PAA acts as the sensing medium, with its amphiphilicity facilitating pore distribution and abundant -COOH exposure. The PS provides a flexible framework considering the PS-b-PAA's difficulty in forming a flexible film. This film shows a response of 2.3 toward 50 ppm NH3, excellent deformation-interference immunity by maintaining this response after the sensor chip bent up to 114.59 degrees and 1500 cycles at 44.07 degrees, high selectivity in various mixtures, long-term stability over 2 months. The low-cost, room-temperature, and scalable fabrication strategy yields a film with high selectivity, long-term stability, and robust flexibility, highlighting its practical potential. A wireless equipment was developed for simulated exhaled breath and food spoilage monitoring. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) confirmed the NH3 response stems from Br & Oslash;nsted acid-base interactions. Density functional theory (DFT) calculations attributed the selectivity to strong binding between NH3 and -COOH. This work marks the first application of PS-b-PAA-a material combining reactive functional groups with inherent amphiphilicity that facilitates porous structure formation-in chemiresistive gas sensing, thereby opening up the use of amphiphilic block copolymers with functional groups in this field.

Continuous blood pressure (BP) monitoring remains a clinical imperative with the global prevalence of cardiovascular diseases. Despite the development of flexible electronics for on-skin sensing of hemodynamic signals, the reliability of continuous BP tracking is still limited by the poor skin-device coupling at anatomical concavities (e.g., wrist), which leads to motion artifacts and compromised long-term wearability. Herein, we designed a breathable, full-module shape-adaptive pulse sensing wristband leveraging the foam structure of the shape memory polymer (SMP). The novel device demonstrates superior pressure sensing performance, achieving ultrahigh sensitivity (>2289.31 kPa(-1)) within a wide range (0-700 kPa), as well as ultralow detection limit (0.16 Pa) and response/recovery time (1.64 ms/3.42 ms), which enables the recording of arterial pulses in high fidelity. Through processing the pulse waveform with a machine learning-based algorithm, continuous BP monitoring was achieved, with 97%/90% accuracy for systolic/diastolic BP (SBP/DBP) prediction, surpassing the clinical standard for BP measurement. Moreover, the wristband's full-module breathability and the stress-free skin-sensor interface ensure long-term wearability without skin irritation for up to 24 h. Significantly, the incorporation of SMPs dramatically optimize the bioelectronic interface for epidermal systems. And the as-developed pulse sensing wristband holds great potential for population-scale cardiovascular health management.

Cardiovascular diseases are a serious threat to humans. Arterial pulse monitoring using wearable electronics could help to assess the cardiac conditions of the wearer, which further reduces the possibility of a sudden lethal heart attack. However, pulse sensors are usually tightly bonded to the wrist during pulse monitoring. This scenario brings great discomfort to the wearer, but also causes unreliable pulse recording due to the susceptibility of motions and pressure sensing nonlinearity with high preload. To this end, a 3D-printed iontronic pressure sensor with high sensitivity (11.65 pF kPa-1) and ultrawide linearity range (150 kPa) was developed, which could monitor a fingertip pulse wave in a "plug-and-play" manner. The wide linearity range enabled the sensor to accurately record the fingertip pulse with variable applied preload, which dramatically improved the reliability of practical pulse sensing. The sensor was applied for monitoring the pulse of patients with cardiovascular diseases, and the correlation between disease type and characteristic pulse waveforms was analyzed. The superior pulse monitoring performance, as well as unprecedented operational convenience, highlights the great potential of the as-prepared pulse sensor in wearable health monitoring.

High-performance ion sensing is of paramount importance for advancing physiological and environmental monitoring systems. Despite the development of various ion sensing strategies, paradigms with superior performance and minimal energy consumption remain to be pursued. Herein, we present a self-powered ion sensor based on the heterostructure of laser-induced graphene (LIG) and indium tin oxide (ITO). Leveraging the ionovoltaic effect of LIG nanochannels and the work function alignment at the LIG/ITO interface, the sensor could spontaneously generate a voltage spike upon the addition of saline solution, with the spike amplitude increasing steadily with ion concentration over a wide range between 10-3 M to 2 M for a variety of electrolytes. The working principles of the ionovoltaic sensor are comprehensively investigated through multiple comparative performance analyses and surface characterizations. Furthermore, the sensor could be applied to measure the salinity of human sweat and seawater, showcasing great versatility for physiological and environmental sensing applications. The superior ion sensing performance, self-power operation, combined with the scalable fabrication via laser direct writing (LDW), endows the novel ion sensor with great potential for the integration into nextgeneration smart electronics.

High-temperature operation and poor selectivity are two main shortcomings of metal oxide (MOX)-based gas sensors. Gas selectivity is an important but complex issue. In this study, alcohol selectivity at room temperature was studied systematically from two dimensions: carbon chain lengths and isomers. A room temperature- operated alcohol sensor based on TiO2 burr-like nanorods was developed. Its responses to alcohols increase as their carbon chain lengths increase, and decrease in the order of primary, secondary, and tertiary alcohols. Overall, a new relationship between the gas molecular structure and response is reported here: the more slender the gas molecule, the higher the response. A novel sensing mechanism is reported here by studying the anomalous p-type responses to the alcohols at room temperature and a temperature-driven p-n response transition. The mechanisms for all the above results were studied experimentally and theoretically by characterizing the reaction path using Fourier transform infrared spectroscopy and analyzing adsorption characteristics by density functional theory calculations. The anomalous p-type response is caused by the surface adsorption of the alcohols. The adsorption energies, electronegativity, and steric hindrance contribute to the gas selectivity together. Besides the fundamental research, a homemade portable system for on-field n-butanol concentration monitoring was built. This study provides new insights into the gas selectivity and development of room temperature- operated MOX-based gas sensors.

Respiratory monitoring has evolved into a critical non-invasive diagnostic tool for managing chronic respiratory pathologies. Despite the progress in the design of strain and humidity sensors for the acquisition of the respiratory patterns, the current systems suffer from significant functional redundancy in the detection of airflow dynamics and humidity oscillations during ventilation cycles, which renders a limited diagnostic specificity in respiratory monitoring. Herein, we developed a strain/humidity bimodal sensor based on a graphene-cellulose nanofibril (graphene-CNF) composite film. The sensor demonstrates superior strain sensing (GF = similar to 380 within 0 %-20 % strain) and humidity sensing (0.58 RH-1 within 5 %-90 % RH) performance, as well as unique moisture-triggered self-healing properties (with a healing efficiency of 47.3 % and healing time of 40 s). The multimodal functionality enables versatile applications spanning skin-attachable physiological sensing, environmental monitoring, and proximity-responsive human-machine interfaces. Specifically for respiratory monitoring, the sensor achieves concurrent acquisition of ventilatory waveforms and the hydration status of the exhaled gas, which facilitates the systemic physiological assessment. The as-developed system not only makes substantial optimization in respiratory monitoring mechanism, but also shows great potential in wearable diagnostics, environmental monitoring, and smart interactive devices.

Electronic skins (e-skins) are desired to perceive both the intensity and spatial distribution of applied pressures. Despite the continuous progress in the design of high-performance individual pressure sensors, the acquisition of the locational information of pressures still mostly relies on the preparation of high-density sensor matrixes within e-skins, which dramatically increases device complexity and hinders the straightforward human-machine interaction. In recent years, the integration of optical pressure visualization units within e-skins has been raised as an alternative strategy for obtaining pressure distribution. By utilizing pressure-induced light emission and color change, the applied pressure could be visualized directly through the distinct optical signals, eliminating the necessity of additional data processing and display modules. In this perspective, the main strategies to achieve pressure visualization in e-skins are introduced, including their mechanism, device layout, materials, and applications. The challenges and prospects of this emerging field are also discussed.

As a common gas-sensing material, TiO2 is limited by poor gas selectivity and humidity immunity like many other metal oxides. Here, an ultra-selective and humidity-resistant room temperature-operated NO2 sensor is developed using black TiO2 for the first time. Compared with conventional white TiO2, black TiO2 significantly enhances NO2 selectivity and humidity resistance by enhancing the NO2 response for ≥10 times while simultaneously suppressing responses to 7 common interfering gases and H2O adsorption. A wireless portable equipment is developed that exhibits the capabilities of identifying NO2 from a mixed atmosphere and recognizing environmental differences in practical scenarios. These improvements and different response types are attributed to the regulation of H2O adsorption: H2O layers are formed on white TiO2, leading to dominant ion-proton conductivity and blocked gas-solid interactions, while H2O adsorption is suppressed and hydroxyl groups are formed on black TiO2, enhancing NO2 response. This study not only promotes a significant advancement and proves the feasibility of using common metal oxides for high-performance NO2 detection at room temperature in a mixed environment irrespective of ambient humidity, but also offers valuable insights into the sensing mechanisms.

Nanostructures significantly affect the performance of metal oxide semiconductor (MOS) based gas sensors. Herein, we proposed a method of inducing sulfur -doping (S -doping) in the synthesis to control the morphology of MoO 3 nanoflakes. The MoO 3 nanoflakes prepared with increased S -doping have morphologies with decreased thickness, increased aspect ratio, increased surface area and increased surface chemisorbed oxygens, which improved sensing properties including higher response, better selectivity to ethanol and lower critical temperatures for the temperature -dependent dual selectivity. The response to 500 ppm ethanol at 350 degrees C was improved by 3 -fold as compared to the MoO 3 obtained without S -doping. The sensors exhibited a temperature -dependent dual selectivity to isopropanol (IPA) and ethanol. The critical temperatures exhibited a decreasing trend for the gas sensors made of MoO 3 which are obtained with increasing S -doping. The feasibility of inducing S -doping in the preparation to modify the morphology of MoO 3 nanoflakes and using it to enhance the gas sensing performance are reported for the first time. It should have a chance to be widely spread into the applications for which higher aspect ratio is beneficial, such as various sensors and photocatalysis.

Flexible strain sensor arrays hold great promise in on-skin monitoring of human signals and activities. Despite the development of strain-sensitive materials and patterning technologies for improved performance and device integration, the metal film serving as interconnects is always vulnerable upon stretch, which hinders the operation under large strains. Herein, a novel strategy is developed for achieving stretch-tolerant interconnects within a sensor array. Through introducing a high-modulus capping layer for the deposition of Ag interconnects, followed by silanization-assisted lamination onto the stretchable substrate where strain-sensitive graphene patches are inkjet-printed, the deformation of Ag interconnects is largely suppressed upon the global strain of the device, and a high working range of 40% strain is achieved. Moreover, the chemical bonding between the capping layer and the stretchable substrate ensures a stable contact between the electrode and the sensitive layer under vigorous bending. The as-prepared sensor array demonstrates high sensitivity (gauge factor (GF) > 100) within a wide range (18%), and could reliably monitor various physiological signals and human activities. A machine learning-assisted wearable gesture recognition system is developed based on the sensor array and a convolutional neural network (CNN), which could distinguish from 10 defined gestures with 100% accuracy after 14 training processes. The facile and effective strategy could be universally applied for metal interconnects protection under stretch, and dramatically facilitate the design of smart flexible electronics.

Carbon nanotubes (CNTs) and graphene have commonly been applied as the sensitive layer of strain sensors. However, the buckling deformation of CNTs and the crack generation of graphene usually leads to an unsatisfactory strain sensing performance. In this work, we developed a universal strategy to prepare welded CNT–graphene hybrids with tunable compositions and a tunable bonding strength between components by the in situ reduction of CNT–graphene oxide (GO) hybrid by thermal annealing. The stiffness of the hybrid film could be tailored by both initial CNT/GO dosage and annealing temperature, through which its electromechanical behaviors could also be defined. The strain sensor based on the CNT–graphene hybrid could be applied to collect epidermal bio-signals by both capturing the faint skin deformation from wrist pulse and recording the large deformations from joint bending, which has great potential in health monitoring, motion sensing and human–machine interfacing.

Mimicking the function of human skin is highly desired for electronic skins (e-skins) to perceive the tactile stimuli by both their intensity and spatial location. The common strategy using pixelated pressure sensor arrays and display panels greatly increases the device complexity and compromises the portability of e-skins. Herein, we tackled this challenge by developing a user-interactive iontronic skin that simultaneously achieves electrical pressure sensing and on-site, nonpixelated pressure mapping visualization. By merging the electrochromic and iontronic pressure sensing units into an integrated multilayer device, the interlayer charge transfer is regulated by applied pressure, which induces both color shifting and a capacitance change. The iontronic skin could visualize the trajectory of dynamic forces and reveal both the intensity and spatial information on various human activities. The integration of dual-mode pressure responsivity, together with the scalable fabrication and explicit signal output, makes the iontronic skin highly promising in biosignal monitoring and human-machine interaction.

Arteriosclerosis, which appears as a hardened and narrowed artery with plaque buildup, is the primary cause of various cardiovascular diseases such as stroke. Arteriosclerosis is often evaluated by clinically measuring the pulse wave velocity (PWV) using a two-point approach that requires bulky medical equipment and a skilled operator. Although wearable photoplethysmographic sensors for PWV monitoring are developed in recent years, likewise, this technique is often based on two-point measurement, and the signal can easily be interfered with by natural light. Herein, a single-point strategy is reported based on stable fingertip pulse monitoring using a flexible iontronic pressure sensor for heart-fingertip PWV (hfPWV) measurement. The iontronic sensor exhibits a high pressure-resolution on the order of 0.1 Pa over a wide linearity range, allowing the capture of characteristic peaks of fingertip pulse waves. The forward and reflected waves of the pulse are extracted and the time difference between the two waves is computed for hfPWV measurement using Hiroshi's method. Furthermore, a hfPWV-based model is established for arteriosclerosis evaluation with an accuracy comparable to that of existing clinical criteria, and the validity of the model is verified clinically. The work provides a reliable technique that can be used in wearable arteriosclerosis assessment systems.

This work presents the unique advantage of textiles in constructing piezocatalytic platforms. The mesoporous and intertwined textile could amplify the striking force of water flow, by which piezocatalysis under low-frequency water flow is achieved.

Gassensors with multiselectivity are drawing increasing attentionand sensors with temperature-tuned dual selectivity have been developed.Because Au nanoparticles (NPs) can lower the activation barrier forsensing reactions, herein, Au NP decoration was used to expand theselectivity range of a ZnO nanotetrapod (NTP)-based sensor. A temperature-tunedtriselectivity sensor based on Au NP-ZnO NTPs was developed.The sensor exhibited enhanced selectivity for formaldehyde when theoperation temperature was <200 degrees C, ethanol when the operationtemperature was between 200 and 340 degrees C, and acetone when theoperation temperature was >400 degrees C. Additionally, Au NP decorationincreased the magnitude of response and decreased the optimal detectiontemperature of the sensor. This work demonstrates that Au NP decorationis an efficient approach for improving the performance of semiconductor-basedgas sensors with temperature-tunable selectivity.

A one-step liquid phase precipitation method (LPD) was developed to synthesis burr-like TiO2 nanostructures at room temperature. Nanotubes, nanorods, nanotetrapods and nano-urchins with bur-surface were obtained. Growth mechanism was studied by comparing TiO2 obtained with different growth time. The growth of the TiO2 is accompanied with etching of the inner ZnO which resulted in a hollow nanostructure and assembly of TiO2 on both the outside and the inner side of the previously obtained hollow structures. The room-temperature operation, mass production, low cost and universality of this synthesis method could show potential in commercial production of TiO2 nanostructures.

Wrinkling is a well-known phenomenon observed at various length scales for diverse materials. Despite a variety of research studies focusing on investigating wrinkling mechanisms and utilizing them to create surface patterns recently, wrinkling also brings about the loss of specific functional properties that were initially endowed to the thin films. For instance, wrinkling may result in the degradation of electrical and optical properties of the film; and the wrinkled films are easily delaminated. Therefore, it is still meaningful to find ways for the suppression of wrinkling on the thin film, although the relevant works are far less than those that utilize the surface instability to achieve certain structures. In this review, the approaches to restrain the emergence of wrinkles will be introduced. Following the introduction part, numerical analysis for wrinkle generation will be first discussed, by which the key parameters determining wrinkle initiation and morphology will be provided. Then, wrinkle suppression strategies by tailoring these parameters will be introduced in the following sections. This review aims to provide useful guidance for future research on alleviating surface fluctuations and achieving desired device functionality.

Interfacial solar evaporators (ISEs) for seawater desalination have garnered enormous attention in recent decades due to global water scarcity. Despite the progress in the energy conversion efficiency and production rate of ISE, the poor portability of large-area ISE during transportation as well as the clogging of water transport pathways by precipitated salts during operation remain grand challenges for its fielded applications. Here, we designed an ISE with high energy conversion efficiency and shape morphing capability by integrating carbon nanotube (CNT) fillers with a light-responsive shape memory polymer (SMP, cross-linked polycyclooctene (cPCO)). Utilizing the shape memory effect, our ISE can be folded to an origami with 1/9 of its original size to save space for transportation and allow for on-demand unfolding upon sunlight irradiation when deployed in service. In addition, the ISE is equipped with a real-time clogging monitoring function by measuring the capacitance of the electric double layer (EDL) formed at the evaporator/seawater nanointerface. Due to its good energy conversion efficiency, high portability, and clogging monitoring capability, we envisage our ISE as a promising selection in solar evaporation technologies.

Integrating electronics in clothing is a significant milestone of wearable technology. The priority is to develop flexible interconnector with great washability, durability and comfort wearability. In this work, we reported an air-permeable and machine-washable Cu/Ni interconnector based on highly flexible fibrous polyimide (FPI) membrane. Benefiting from the porous merit of FPI membrane and robust parylene encapsulation, the novel interconnector presents excellent conductivity (14 mO/sq), decent air-permeability (11 KPa*s/m) and superior electromechanical stability after abrasion of 50,000 cycles, bending of 10,000 cycles, and machine washing of 50 times. Moreover, assemblies made from the FPI-based circuit board not only show desirable lifespan in harsh environments, including seawater, ice, boiled water, and heavy rain, but also can be woven into fabrics for broad wearable applications. The work presents a facile and effective method to fabricate highly flexible, comfortable and durable interconnector, guaranteeing the reliability for long-term wearable application and showing great possibility for high-throughput production.

Flexible stimuli-responsive materials are deformable, stretchable, light-weight, and desirable for smart personal protective equipment (PPE), and as the primarily functional components in the wearable system which spontaneously respond to surrounding variations. These materials enable the traditional PPE who provide passive protection to be smart with the abilities of sensing, actuating, surface changing and self-healing, which enhances the protection and reduces the unintentional occupation injuries. This article presents a critical review of the structure, properties, fundamental mechanisms and current development of flexible stimuli-responsive materials and their potential/present applications to smart PPE, covering strain, pressure, temperature, and gas sensors, biopotential electrodes, exosystems, switchable wetting surfaces and biosafety masks. Scientific and practical challenges along with critical issues and opportunities are also discussed.