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The outmost layer of skin epidermis comprises of nanostructured corneocytes embedded in extracellular lipid matrix that endows the epidermis with permeative and protective properties. Inspired by such structure, here we produced multifunctional porous polymer films on hydrogels by a simple yet robust in situ interfacial precipitation polymerization of specific water-soluble monomers that become insoluble as they polymerize. This was applied on diverse hydrogel substrates, yielding unusually durable interfaces which exhibited epidermis-like characteristics. For example, the polymer films possess a thickness of 20 to 330 µm and comprise of interconnected nanoparticles tightly bonded to hydrogel surfaces, which structurally resemble the layer of interlocked corneocytes in the epidermis. Due to these unique structural features, the film exhibits an excellent permeability towards small molecules in the first instance but can then be tailored towards effective protection from excess water loss and for enhanced electrical resistivity. These polymer films provide a platform for the development of engineered materials for diverse applications.

Radio-Frequency (RF) induction heating is a versatile in-situ method for contactless heating of structures by utilizing either magnetic hysteresis loss or eddy-current loss mechanism. Achieving high heating efficiency without degrading mechanical properties is a major challenge. Herein, a RF induction compatible self-healing composite was developed by hybridizing iron oxides (Fe3O4) nanoparticles with carbon fibre veils (CFVs) in poly(ethylene-co-methacrylic acid) (EMAA), which could possess both high magnetic and electrical properties. Owing to the multiscale conductive networks built by Fe3O4 nanoparticles and CFVs, the electrical conductivity of the nanocomposite was found to be higher than the linear combination of the individual contributions, thus creating a synergistic improvement in electrical conductivity and heating efficiency. Furthermore, single lap shear test results demonstrated that the combination of Fe3O4 nanoparticles and CFVs could significantly improve the bonding strength of EMAA polymer. Therefore, the hybridization of magnetic particles with conductive fibres offers a promising technology for a wide range of applications, such as self-healing, reversable bonding, and multiple use bonded composites.

By drop-casting conductive inks onto the soft 3D printed surfaces, strain sensors of spatially-varying thickness mimicking the inherent surface undulation of the substrate are fabricated with a gauge factor of 151 over a linear strain range of ε = 97%.

Stretchable electronics that can elongate elastically as well as flex are crucial to a wide range of emerging technologies, such as wearable medical devices, electronic skin, and soft robotics. Critical to stretchable electronics is their ability to withstand large mechanical strain without failure while retaining their electrical conduction properties, a feat significantly beyond traditional metals and silicon-based semiconductors. Herein, we present a review of the recent advances in stretchable conductive polymer nanocomposites with exceptional stretchability and electrical properties, which have the potential to transform a wide range of applications, including wearable sensors for biophysical signals, stretchable conductors and electrodes, and deformable energy-harvesting and -storage devices. Critical to achieving these stretching properties are the judicious selection and hybridization of nanomaterials, novel microstructure designs, and facile fabrication processes, which are the focus of this Review. To highlight the potentials of conductive nanocomposites, a summary of some recent important applications is presented, including COVID-19 remote monitoring, connected health, electronic skin for augmented intelligence, and soft robotics. Finally, perspectives on future challenges and new research opportunities are also presented and discussed.

Ensuring sustainable energy development and reducing CO2 emissions are two major challenges that need to be overcome to deal with the global economic crisis and to alleviate climate change. Hot dry rock is a renewable energy resource with a huge potential. CO2-based enhanced geothermal systems (CO2-EGS) can achieve both heat extraction and CO2 storage in hot dry rock. To investigate the storage rate, a heat extraction experiment involving the alternating cyclic injection of water and supercritical CO2 was conducted. The ion contents of the fracture solution of the different injection cycles were measured, and the amount of CO2 mineralization storage was calculated. In addition, changes in the mineral composition of the fracture surface were observed and analysed. The results revealed that the mineral dissolution on the fracture surface was mainly feldspars, which possibly accompanied biotite dissolution. The precipitated minerals that stored the carbon were mainly calcite and dolomite, and possibly illite and cancrinite. Many blocky carbonate minerals were distributed on the flat surface of the fracture, while many rod-shaped carbonates were distributed in the concave holes of the microfractures. The research results reveal the CO2 storage mechanism and are significant for the CO2 storage prediction in the CO2-EGS.

Cryogenic fracturing with liquid nitrogen (LN2) offers the benefits of reducing the water consumption and adverse environmental impacts induced by water-based fracturing, as well as potentially enhancing the fracture complexity. We performed a series of laboratory experiments to explore the key mechanisms governing the breakdown pressures of shale during cryogenic fracturing. In this study, cylindrical shale samples were pre-conditioned by exposing a borehole to low-temperature LN2 for a certain time period, and then, the samples were fractured using gaseous N2 under triaxial stress and a high reservoir temperature. The effects of various key parameters on the breakdown pressure were investigated, including the duration of the low-temperature LN2 treatment, the confining pressure, the reservoir temperature, and the direction of the shale bedding relative to the borehole axis. The results demonstrate that the injection of low-temperature LN2 as a pre-fracturing fluid into a borehole can significantly reduce the breakdown pressure of the shale during subsequent nitrogen fracturing. This reduction in breakdown pressure can be further intensified by increasing the duration of the LN2 pre-conditioning. Without LN2 pre-conditioning, the breakdown pressure initially increases and then decreases with increasing reservoir temperature. When LN2 pre-conditioning is applied, the breakdown pressure keeps decreasing with increasing reservoir temperature. As the confining pressure increased, the breakdown pressure increased linearly in the tests with and without LN2 pre-conditioning. The experimental results demonstrate that LN2 pre-conditioning before N2 fracturing is a promising waterless fracturing technique that reduces the breakdown pressure and enhances the fracture complexity.

Critical to the applications of pseudocapacitors is the cycling performance of electrodes. Here, we present a new carbon fibre electrode enhanced by multilayer structure of vertical graphene (VG) and manganese dioxide (MnO2) to achieve a cycling performance at a very high areal capacitance (546.3 mF/cm2) that outperforms most of the reported pseudocapacitors in the literature. Particularly, this multilayer electrode is able to retain 99.3% of its initial capacitance after 10,000 cycles at scan rate of 200 mV/s. The retention ratio is significantly higher than the state-of-art value. In addition, this electrode shows excellent rate performance (75.4% capacitance retention for current density from 1 to 10 mA/cm2). This exceptionally high stability at high capacitance and rate performance originates from the multilayer electrode’s effectiveness in self-replenishing the degraded MnO2 in the outer layer. The microcracks and micro holes induced in the VG layer by electrical charge/discharge cycling enable the inner layer MnO2 to progressively participate in the redox reaction. Results from energy dispersive spectroscopy, X-ray photoelectron, and Raman spectroscopy confirm the self-replenishing mechanism responsible for the exceptionally stable performance. The new multilayer electrode designed herein provides a new route for improving the cycling performance of pseudocapacitors.

The interface between structural electrodes and solid electrolytes plays a key role in the electrical-mechanical properties of energy storage structures. Herein, we present a surface functionalization method to improve the ion conduction efficiency at the interface between a structural electrode and a solid electrolyte that consists of a bi-continuous network of epoxy and ionic liquid (IL). Composite supercapacitors made with this electrolyte and carbon fiber (CF) electrodes coated with manganese dioxide (MnO

Achieving moldable energy storage devices will make it possible to power wearable electronics by adapting to body contours for wear comfort. Herein, we present a new approach to create hierarchically structured electrodes that enable supercapacitors to retain their capacity under mechanical deformation. The electrodes are made by first growing vertical graphene nanosheets (VGNs) and then depositing manganese oxide (MnO2) on ductile nickel wires. Two such electrodes are made into a moldable supercapacitor using a solid-state electrolyte containing carboxymethylcellulose and sodium sulfate. This foldable supercapacitor achieves a high areal capacitance up to 56 mF cm−2, areal energy density of 7.7 μWh cm−2, and areal power density of 5 mWh cm−2. These exceptional properties originate from the synergy between VGNs and MnO2, where the highly porous VGNs serve two critical functions: a mechanically robust platform of large surface area allowing high mass loading deposition of pseudocapacitive MnO2 and an interconnected conductive network for efficient electron/ion transport. Fiber-shaped supercapacitors made from these electrodes can be molded into different shapes by bending and twisting with little performance loss. The promising results presented in this study provide a new route for fabricating high-performance moldable energy storage devices for wearable electronics and wireless electronic skins.

Wearable temperature sensors meeting the resolution of medical-grade thermometers are needed to continuously monitor skin temperature variations indicative of diseases and sports performance. Herein, we present a new technique for creating stretchable capacitive sensors with a temperature resolution of 0.14 °C and minimum cross-sensitivity to mechanical stretch. Two complementary strategies were employed to minimize the stretch-sensitivity and achieve high temperature resolution: (a) increasing the temperature sensitivity by using a dielectric core material of positive temperature coefficients of permittivity and thermal expansion, and (b) reducing the cross-interference of mechanical deformation by optimizing the kirigami pattern. The optimized kirigami-capacitive wearable temperature sensors with a thermoplastic polyurethane core sandwiched between electrodes made of silver nanowires exhibit a high temperature sensitivity of 0.007 °C−1 with an excellent linearity of 0.998. Computational modeling of kirigami geometry effects on strain and temperature sensitivities of capacitive sensors reveals that the optimized kirigami design can reduce the strain cross-sensitivity by a factor of 3125, which enables capacitive temperature sensors to achieve the resolution of point-of-care medical devices. The combined characteristics of high sensitivity, linearity, breathability, and stretchability make the new capacitive sensors a promising candidate for medical-grade wearable temperature sensors.

The direct deposition of thin metal films on highly elastic substrates is a major technique used to fabricate flexible electronics such as wearable sensors, stretchable transistors, and deformable displays. However, these devices suffer from low stretchability due to the limited ductility of the metal films. Herein, we present a new method of significantly extending the sensing range of metal film sensors by introducing nanofibers to control the growth of microcracks. This new technique overcomes the major limitation of flexible electronic devices' propensity to abrupt electrical failure caused by plastic deformation and long-channel cracks in thin metal films of low yield strain and ductility. The fibre-metal films were made by depositing metals on a flexible substrate coated with carbon nanofibers (CNFs). The resulting nanofiber-metal composite films were found to form short microcracks bridged by conductive CNFs, instead of long-channel cracks commonly found in metal films subjected to mechanical stretching. This led to an impressive increase in the sensing range, from 5% to 120% with an ultrahigh sensitivity of 417 and an excellent linearity of 97%, which are significantly higher than the reported values of microcrack-based strain sensors. A holey pattern analogous to the breathable medical tape was introduced into these strain sensors to endow them with good breathability and wearing comfort. The shape and alignment of the holes control the directional sensing capability of breathable strain sensors to detect complex human motions. The techniques presented herein offer a new route to develop next-generation flexible electronics using nanofiber-reinforced metal composite films.

Wearable temperature sensors with high sensitivity, linearity, and flexibility are required to meet the increasing demands for unobtrusive monitoring of temperature changes indicative of the onset of infections and diseases. Herein, we present a new method for engineering highly sensitive and flexible temperature sensors made by sandwiching a poly(3,4-ethylenedioxythiophene):polystyrene (PEDOT:PSS) sensing film between two poly(dimethylsiloxane) (PDMS) substrates. Pre-stretching the sensor to a certain strain can create stable microcracks in the sensing layer that bestow high senstivity and linearity. The average length and density of the microcracks, which dictate the sensor's temperature sensitivity, can be engineered by controlling three key processing parameters, incuding (a) pre-stretching strain, (b) sulfuric acid treatment time, and (c) surface roughness of the substrate. For a given acid treatment time and surface roughness condition, the density and average length of the microcracks increase pre-stretching strain. A smooth PDMS substrate tends to yield long and straight cracks in the PEDOT:PSS film, compared to shorter microcracks with higher density on rough surfaces. Crack density can be further increased via sulfuric acid treatment with an optimum duration of approximately 3 h. Prolonged treatment would result in weak adhesion between the PEDOT:PSS film and the PDMS substrate, which in turn reduces the microcrack density but increases the crack length. By optimizing the three design parameters we have designed a high performance PEDOT:PSS-PDMS sensor that provides a combined high temperature sensitivity of 0.042 °C-1 with an excellent linearity of 0.998 (from 30 to 55 °C), better than the highest temperature sensitivity of PEDOT:PSS based sensors reported in the literature. With a good optical transparency, high temperature sensitivity, excellent linearity, and high flexibility, this microcrack-based sensor is a very promising wearable temperature-sensing solution.

Quantitative information on the magnitudes and directions of multiple contacting forces is crucial for a wide range of applications including human-robot interaction, prosthetics, and bionic hands. Herein we report a highly stretchable sensor integrating capacitive and piezoresistive mechanisms that can simultaneously determine multiple forces. The sensor consists of three layers in a sandwich design. The two facesheets serve as both piezoresistive sensors and electrodes for the capacitive sensor, with the core being a porous structure made by using a simple sugar particle template technique to give them high stretchability. The two facesheets contain segregated conductive networks of silver nanowires (AgNWs) and carbon nanofibers (CNFs). By measuring the changes in the electrical resistance of the facesheets and the capacitance between the facesheets, three separate mechanical stimuli can be determined, including normal pressure, in-plane stretch, and transverse shear force. The newly developed multidirectional sensor offers a significant opportunity for the next generation of wearable sensors for human health monitoring and bionic skin for robots.

Microcracking mechanism is an effective technique for creating highly sensitive resistive strain sensors, but sensors enabled by this mechanism tend to possess a limited sensing range. Herein, we present a new method of broadening the sensing range by bridging the microcracks with conductive nano-sized fibres. The key sensing parameters such as sensitivity and linear range can be optimized by controlling the concentration of the conductive nanofibers and the plasma treatment duration of the supporting hyperelastic substrate. The experimental results reveal that our nanofiber-reinforced strain sensors, which remain highly transparent, are capable of remarkable sensitivity with a gauge factor between 22.7 and 278.1 with a linear range of 201% and 47%, respectively, significantly better than those previously reported in the literature for microcrack-based strain sensors. This exceptional improvement originates from the pulling out of the nanofibers without rupture, bridging the microcracks to prevent further growth and, thus, improving the stability of the sensors under repeated stretching. The nanofiber-reinforced strain sensors can detect both small (wrist pulse) and large (joint movements) mechanical deformations. The combined high sensitivity, linear range, and optical transparency enable this new type of sensor to be used as inconspicuous wearable devices for human motion detection and health monitoring.

Hydraulic fractures in enhanced geothermal system (EGS) are main channels for geothermal exploitation from hot dry rock. However, due to the in-situ stress and chemical effects, the hydraulic fractures will close gradually, which will cause rapid reduction of permeability and heat recovery efficiency in EGS. Hydraulic fracturing proppant treatment is effective to decrease fracture closing and maintain fluid circulation in geothermal exploitation. In this paper, a self-developed triaxial seepage test device was used to accommodate granite samples with single artificial fracture and study the effects of proppants on rock permeability and heat recovery efficiency during geothermal energy extraction. In the experiment, the rock sample temperature was between 100 and 280 °C, and the confining pressure was between 26 and 35 MPa. The experimental results showed that both permeability and heat recovery efficiency of samples decreased with the increase of rock temperature and confining pressure. The addition of proppants was unable to prevent the reduction of permeability and heat recovery efficiency in the samples with the increase of confining pressure and temperature, but could effectively reduce the negative influence. The further analysis reveals the pressure dissolution and water weakening effects on fracture were the main reason for the decrease of permeability and net heat extraction rate in EGS, which could give a better understanding on geothermal reservoir reconstruction.

Rapid and contactless on-demand debonding and rebonding of high-strength joints is a promising solution for quick attachment, repairs, disassembly, and recycling of structural systems. Herein we show that rapid and contactless on-demand debonding and rebonding can be achieved by incorporating ferrimagnetic iron oxide nanoparticles (Fe3O4) into poly(ethylene-methacrylic acid) (EMAA). When subjected to an external alternating magnetic field, the ferrimagnetic nanoparticles generate localized heat which melt the EMMAA, resulting in quick (

Wearable sensors are emerging as a new technology to detect physiological and biochemical markers for remote health monitoring. By measuring vital signs such as respiratory rate, body temperature, and blood oxygen level, wearable sensors offer tremendous potential for the noninvasive and early diagnosis of numerous diseases such as Covid‐19. Over the past decade, significant progress has been made to develop wearable sensors with high sensitivity, accuracy, flexibility, and stretchability, bringing to reality a new paradigm of remote health monitoring. In this review paper, the latest advances in wearable sensor systems that can measure vital signs at an accuracy level matching those of point‐of‐care tests are presented. In particular, the focus of this review is placed on wearable sensors for measuring respiratory behavior, body temperature, and blood oxygen level, which are identified as the critical signals for diagnosing and monitoring Covid‐19. Various designs based on different materials and working mechanisms are summarized. This review is concluded by identifying the remaining challenges and future opportunities for this emerging field., Wearable sensors have demonstrated grand potential for remote health monitoring recently, particularly after the Covid‐19 pandemic. The need for wearable telehealth systems capable of monitoring vital signs, such as respiratory rate, body temperature, and blood oxygen level has surged. In this review, the recent advances in creating these sensors and remaining challenges as well as prospects are presented.

Hybrid piezo–triboelectric nanogenerators constitute a new class of self-powered systems that exploit the synergy of piezoelectric and triboelectric mechanisms to improve energy harvesting efficencies and address the energy and power needs of portable and wearable electronic devices. The unique, synergistic electrical coupling mechanisms of piezoelectric and triboelectric effects increase the electric outputs and energy conversion efficiency of hybrid generators to beyond a linear summation of the contributions from individual triboelectric and piezoelectric mechanisms. Due to their large surface-area-to-volume ratios and outstanding mechanical, electronic and thermal properties, nanomaterials are favourable building blocks for constructing hybrid nanogenerators and represent a large family of flexible energy harvesting electronic structures and devices. Herein, we review the recent advances of hybrid piezo–triboelectric nanogenerators, with a particular focus on microstructure design, synergy mechanisms, and future research opportunities with significant potential for physiological monitoring, health care applications, transportation, and energy harvesting. The main strategies for improving electrical output performance are identified and examined, including novel nanostructures for increasing the contact area of the triboelectric pair, and nano-additives for enhancing the surface potential difference between the triboelectric pair and piezoelectric layers. Future applications and commercialization opportunities of these nanogenerators are also reviewed.

Achieving a switchable conductance in metallic nanowire networks is of great importance for applications in wearable electronics, selectors, information storage, and neuromorphic computing. However...