A Wearable Transient Pressure Sensor Made with MXene Nanosheets

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Cite This: Nano Lett. XXXX, XXX, XXX−XXX

A Wearable Transient Pressure Sensor Made with MXene Nanosheets for Sensitive Broad-Range Human−Machine Interfacing Ying Guo,†,§ Mengjuan Zhong,† Zhiwei Fang,‡ Pengbo Wan,*,† and Guihua Yu*,‡ †

State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas, Austin, Texas 78712, United States § State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China Downloaded via SWINBURNE UNIV OF TECHNOLOGY on February 3, 2019 at 21:13:37 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: Flexible and degradable pressure sensors have received tremendous attention for potential use in transient electronic skins, flexible displays, and intelligent robotics due to their portability, real-time sensing performance, flexibility, and decreased electronic waste and environmental impact. However, it remains a critical challenge to simultaneously achieve a high sensitivity, broad sensing range (up to 30 kPa), fast response, long-term durability, and robust environmental degradability to achieve fullscale biomonitoring and decreased electronic waste. MXenes, which are two-dimensional layered structures with a large specific surface area and high conductivity, are widely employed in electrochemical energy devices. Here, we present a highly sensitive, flexible, and degradable pressure sensor fabricated by sandwiching porous MXene-impregnated tissue paper between a biodegradable polylactic acid (PLA) thin sheet and an interdigitated electrode-coated PLA thin sheet. The flexible pressure sensor exhibits high sensitivity with a low detection limit (10.2 Pa), broad range (up to 30 kPa), fast response (11 ms), low power consumption (10−8 W), great reproducibility over 10 000 cycles, and excellent degradability. It can also be used to predict the potential health status of patients and act as an electronic skin (E-skin) for mapping tactile stimuli, suggesting potential in personal healthcare monitoring, clinical diagnosis, and nextgeneration artificial skins. KEYWORDS: Flexible electronic sensors, electronic skins, MXene, degradability, broad-range healthcare monitoring

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movements (such as arm bending, coughing, and joint motion). Flexible wearable pressure sensors are mostly prepared from a combination of conducting nanostructured material networks and flexible plastic substrates. Recently, remarkable nanostructured materials have been used to fabricate various flexible pressure sensors due to their excellent mechanical flexibility and electrical conductivity, such as metal nanoparticles,19 carbon nanotubes,20,21 metal nanowires, polymer nanofibers,22 carbonized nanofibers, graphene,23 and their nanocomposites.24−26 MXenes, which are two-dimensional layered nanomaterials with large specific surface areas, excellent hydrophilicity, and high conductivity, are widely used in electrochemical energy storage,27 transparent electrode materials,28 and nanocomposites.29,30 Despite their large specific surface area and strong conductivity, hierarchical MXene composite meshes have rarely been employed in fabricating flexible pressure sensors.31,32 Moreover, microstructured plastic or rubber substrates and thermosetting resins without easy

lexible wearable pressure sensors have attracted considerable attention for various potential applications, including electronic skins,1,2 human−machine interfaces,3,4 flexible touchable displays,5 and intelligent robotics, due to their reliable portability, excellent real-time electrical sensing performance regarding personal activities without interrupting or restricting the motions of the individual wearing the sensor, high integration potential, flexibility, and ease of processing. To date, most of the reported pressure sensors have realized excellent sensing performance by utilizing various sensing mechanisms, including piezoresistive,6−10 capacitive,11−13 piezoelectric,14−16 and triboelectric effects.17,18 Among these mechanisms, flexible piezoresistive pressure sensors, consisting of compatible substrates and a flexible conductive material network, have attracted considerable attention due to a reliable piezoelectric effect being enabled by a force-induced current or resistance change. Other advantages include a simple device assembly, relatively low energy consumption, and excellent sensing sensitivity. However, it is difficult to simultaneously obtain a high sensitivity, broad sensing range, long-term durability, and robust reproducibility to satisfy the requirements for full-scale medical monitoring from small deformations (such as breathing, pulse, and vocalization) to large © XXXX American Chemical Society

Received: November 8, 2018 Revised: January 16, 2019 Published: January 18, 2019 A

DOI: 10.1021/acs.nanolett.8b04514 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic illustration of the fabrication procedure of flexible wearable transient pressure sensors with MXene nanosheets. (b) TEM image of the MXene nanosheets. (c) AFM image of MXene nanosheets deposited on a mica plate. (d) SEM image of MXene/tissue paper. (e) The enlarged SEM image of an MXene/tissue paper fiber. (f, g) Photographs of the flexible wearable transient pressure sensor.

degradability, and reliable elasticity. A biodegradable PLA sheet and a PLA sheet patterned with an interdigitated conductive electrode are then sandwiched between them. The as-prepared wearable pressure sensor achieves high sensitivity, a low detection limit (10.2 Pa), ultrasensitive loading sensing of a sugar granule (2.3 mg), fast response (11 ms), low power consumption (10−8 W), and excellent reproducibility over 10 000 cycles and is biodegradable The fabricated pressure sensor could be attached to human skin to perform various broad-range medical monitoring tasks ranging from small deformations to large movements. The pressure sensor is also able to input Morse code by touching the sensor device surface; this capability could be used to predict the potential health status of patients with early stage Parkinson’s disease, which was simulated using imitated static tremors. Additionally, the sensor arrays were successfully assembled and used to detect various tactile signals and map spatial pressure distributions, suggesting potential applications in personal healthcare biomonitoring, clinical diagnosis, artificial intelligence, and next-generation artificial skins and wearable electronics. Figure 1a shows the facile device fabrication process. First, conductive MXene (Ti3C2Tx) nanosheets were obtained from the selective etching of the precursor (Figure S1a). The resulting colloidal solution contained MXene sheets with a lateral size of a few micrometers (Figure S1). Through the well-exfoliated ultrathin MXene sheets, the pores of the anodized aluminum oxide membrane were observed (Figure S1d). The associated size histogram (Figure S1e) indicates a mean lateral length () of 1.54 μm, and a clear slice

environmental degradability have been used for the preparation of wearable pressure sensors via a laborious and complicated fabrication process. High amounts of electronic wastes (e-wastes; 25 × 106 tons per year) and serious environmental pollution resulting from their released toxic materials have occurred with the increasing demand for consumer electronic devices due to the designed survivability and slow degradation of the devices in the environment.33 Thus, there is an urgent need for the development of degradable electronic devices that use naturally degradable sensing materials and substrates that degrade into biologically and environmentally friendly end products on limited time scales while also exceeding desirable device lifetimes. Recently, some “transient” electronic components have been developed with solvent-soluble active and passive materials, such as transistors,34−36 batteries,37,38 energy harvesters,39 and functional circuits for power harvesting.40 Most biodegradable polymers [e.g., silk,41 polylactic-co-glycolic acid (PLGA),42 polycaprolactone (PCL)43 and polylactic acid (PLA)]44 are used as substrates or encapsulation layers. It is still a substantial challenge to develop “transient” pressure sensors with high sensitivity, broad sensing range, environmental degradability, fast response, and reliable compatibility. Here, we demonstrate a facile approach for the simple, costeffective, and versatile fabrication of a flexible wearable transient pressure sensor with MXene nanosheets for highly sensitive broad-range human−machine interfacing (Figure 1). The flexible wearable transient pressure sensor is fabricated by impregnating MXene nanosheets into tissue papers (MXene/ tissue paper) with a porous structure, recyclability, low cost, B

DOI: 10.1021/acs.nanolett.8b04514 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 2. (a) Current response to increased pressures under loading and unloading. Inset: The current response in the low-pressure range of 89− 536 Pa. (b) Current response to increased pressures under loading and unloading at 23, 44, and 89 Pa. (c) Sensing sensitivity of the flexible pressure sensor to pressure. (d) The durability test under a pressure of 3036 Pa. Inset: The detailed durability performance under a pressure of 3036 Pa. (e) Response time of the pressure sensor. (f) I−V curves of the sensor with various applied pressures.

MXene/tissue paper. Figure S6 shows the lateral view of the whole device from the cross-section of the overall device. Figure 1f,g show the optical images of the assembled device with excellent flexibility. The sensing mechanism is due to tunable changes in contact resistance between the porous MXene/tissue paper and the interdigitated electrodes under external pressure (Figures 1a and S7). External pressure will cause a small compressive deformation of the porous soft MXene/tissue paper, leading to more contact and more conductive paths between the MXene and the interdigitated electrodes (Figure S7) under a fixed voltage of 0.01 V, thus causing an increase in the current. Upon unloading, both the PLA and MXene/tissue paper returned to their original shapes, reducing the contact area between the interdigitated electrodes and MXene and resulting in a reduced current (Figure S7). To ensure the mechanical flexibility and durability of the interdigitated electrodes on the PLA film, a serpentine electrode printed on the PLA film with a length of 20 mm was bent 1000 times to a radius of curvature of 5 mm with a linear motor, and the conductivity of the electrode did not obviously change (Figure S8). To evaluate the sensing performance of the pressure sensors, the flexible pressure sensor was connected with an electrical sensing analyzer by conductive copper wires, which were placed onto a force meter platform for real-time sensing recording. The sensitivity (S) of the pressure sensors is defined as S = δ(ΔI/I0)/δP, where ΔI is the relative change in the current, I0 is the current of the sensor without loading, and P is the pressure under loading. Figure 2a,b shows representative current plots (ΔI/I0) of the sensor under different external pressures. Figure S9 shows the sensing performance with the standard deviation for Figure 2a. A total of five samples were measured to obtain the average and representative sensing performance with a standard deviation of approximately 5%. Under each pressure, the sensor has a

structure is observed in the transmission electron microscope (TEM) image (Figure 1b). The thickness of the flake is approximately 4.2 nm according to the atomic force microscopy (AFM) measurement (Figure 1c), which is in good agreement with a previous report.45 The unique 2D structure endows the MXene sheets with the capability to assemble into flexible functional films or to serve as nanofillers for functional polymer nanocomposites. The successful preparation of MXene nanosheets is confirmed by the shift of the (002) X-ray diffraction peak to a smaller angle and the greatly weakened peak at 39°, as shown in Figure S2a. A Raman spectrum of a typical sample is shown in Figure S2b; the peaks at 198 and 717 cm−1 are assigned to the out-of-plane vibrations of Ti and C atoms, respectively, while the peaks at 284, 366, and 624 cm−1 are assigned to the in-plane (shear) modes of Ti, C, and surface functional group atoms, respectively.46 The XPS spectra (Figure S2c) indicate that the samples are mainly composed of C, Ti, O, and F. Thin MXene/tissue paper films (0.6 cm × 0.8 cm rectangles) with different sheet resistances were obtained from the impregnation of MXene nanosheets by immersing the tissue papers into MXene nanosheet solutions with different concentrations (Figure S3). The sheet resistance decreased from 56.5 MΩ/□ to 6.7 kΩ/□ for the different concentrations of MXene solution used when preparing the MXene/ tissue paper films. A concentration of 2.5 mg/mL was selected as the optimal concentration due to the low sensor energy consumption, low sheet resistance, low cost, and reproducibility. The MXene/tissue paper was then placed onto the PLA film with interdigitated electrodes (Figure S4), and this was followed by covering it with a thin sheet of black PLA (Figure 1a). By comparing the scanning electron microscopy (SEM) images of the tissue paper in Figure S5a,b, the uniform coating of MXene onto degradable porous tissue paper can be observed in Figure 1d,e, which shows SEM images of the C

DOI: 10.1021/acs.nanolett.8b04514 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 3. (a) Sensing performance of the pressure sensor attached to the arm muscle for reliable detection of radial muscle contraction resulting from reversibly making a fist. Inset: Photograph of the sensor attached onto the arm muscle while reversibly making a fist. (b) Sensing performance of the pressure sensor mounted onto the cheek for real-time sensing of occlusion. Inset: Photograph of the sensor attached onto the cheek. (c) Sensing performance of the pressure sensor adhered onto a human throat for the timely sensing of swallowing. Inset: Photograph of the sensor attached onto a human throat. (d) Sensing performance of the pressure sensor coated on the wrist for sensing the wrist pulse. Inset: Photograph of the sensor attached onto the wrist and palm. (e) The enlarged waveform of the blood pulse. (f) Current response of the sensor upon loading a sugar granule (2.3 mg). Inset: Photograph of a sugar granule (2.3 mg) loaded onto the sensor. (g) Sensing performance of the sensor under imitated knocking of early stage Parkinson’s disease featuring a static tremor frequency of 5 Hz. Inset: Photograph of the imitated knocking of the sensor. (h) The enlarged sensing performance of the imitated knocking. (j) Sensing performance of Morse code for “MXene” produced by touching the sensors.

good, stable sensing response and a large range of pressure detection (23 Pa−30 kPa). As shown in Figure 2c, the sensitivities of the flexible pressure sensors are composed of three regions: S1 in the low-pressure range (23 to 982 Pa), S2 in the pressure range from 982 to 10 kPa, and S3 in the pressure range from 10 to 30 kPa. The corresponding sensitivities of S1, S2, and S3 are 0.55, 3.81, and 2.52 kPa−1, respectively. In addition, the effects of different paper types on the sensing performance were explored. A filter paper substrate was employed for assembling the pressure sensor. The photographs of the filter paper after being impregnated with MXene nanosheets (MXene/filter paper), and the SEM images are shown in Figure S10a−c. The comparison of the sensing performance of the MXene/filter-paper-based pressure sensor and the MXene/tissue-paper-based pressure sensor (Figure S10d) showed that the MXene/tissue-paper-based pressure sensor performed significantly better than the MXene/filterpaper-based pressure sensor, which is due to the relatively rough surface of the tissue paper. The two-dimensional (2D)

view and three-dimensional (3D) topology of the two papers are shown in Figure S11 from a Sensofar 3D optical profiler. As shown in the 3D view in Figure S11, the surface undulation range of the tissue paper (−55.4 to +55.4 μm) is significantly larger than the range of the filter paper (−27.8 to +27.8 μm), indicating better surface roughness of the tissue paper.47 Furthermore, the different sensing sensitivities of the MXene/ tissue-paper-based pressure sensor with different thicknesses of the PLA encapsulation layer are demonstrated in Figure S12, indicating that the sensor with a thinner PLA encapsulation layer possesses better sensing sensitivities, which agrees well with the reported literature.48 Therefore, the MXene/tissuepaper-based pressure sensor with a 40 μm PLA encapsulation layer could be the optimal candidate for further sensitive broad-range human−machine interfacing. Figure 2d shows the reproducible sensing performance of the MXene/tissue-paper-based pressure sensor. After 10 000 compressive loading/unloading tests between 0 and 3036 Pa, the sensing response was clearly retained, indicating the potential for robust and stable human motion biomonitoring. D

DOI: 10.1021/acs.nanolett.8b04514 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 4. (a) Photograph of the E-skin assembled from the MXene/tissue-paper-based sensors with a size of 4 pixels × 4 pixels and (b) schematic illustration of the E-skin assembled from the MXene/tissue-paper-based sensors with a size of 4 pixels × 4 pixels. (c) Photograph of two fingers touching the E-skin and (d) the corresponding pressure distribution mapping from the sensing responses. (e) Photograph of three fingers touching the E-skin and (d) the corresponding pressure distribution mapping from the sensing responses. (g) Photographs of the MXene/tissue-paper-based sensors attached to a robot femorotibial joint with the arm undergoing uplift cycles along with the hand touching the sensor for (h) remote touching monitoring by wireless connection of the sensing response to a mobile phone. (i) The corresponding real-time sensing responses received wirelessly by the mobile phone for monitoring the reversible arm uplift cycles along with the reversible touching of the sensor surfaces and (j) the related resistance changes.

based pressure sensor under a pressure of 3036 Pa, ensuring a real-time sensing response to instant human−machine interfacing. As shown in Figure S15, no obvious effect of the different compressing strain rates on the sensing performance of the MXene/tissue-paper-based pressure sensor was observed at 3036 Pa. As shown in Figure 2f, the I−V curves indicated

Moreover, the current sensing responses matched well with the input pressure waves under a pressure of 3036 Pa (Figure S13), indicating that the output sensing signals can be accurately tracked and reflected the input loading forces. In addition, Figures 2e and S14 show the response time (11 ms) and the recovery time (25 ms) of the MXene/tissue-paperE

DOI: 10.1021/acs.nanolett.8b04514 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 5. (a) Photographs of the MXene/tissue paper sensor placed in a 0.5 M NaOH solution for different times. Photographs and SEM images of the MXene/tissue paper sensor (b, c) before and (d, e) after degradation.

at an operation voltage of 0.01 V with a current on the order of 10−6 A, corresponding to an energy consumption of approximately 10−8 W. As shown in Figure 3f, a sugar granule (2.3 mg) with a bottom area of 1.5 mm × 1.5 mm was loaded onto the surface of the pressure sensor to obtain the experimental detection limit, which was found to be 10.2 Pa. In addition, the pressure sensors were capable of monitoring low-frequency changes for potential disease diagnosis. For example, early stage Parkinson’s disease (PD), a slow and gradual neurodegenerative disease, features static tremors at a frequency of 4−6 Hz. Thus, it is vitally important to detect static tremor signs for potentially early stage disease diagnosis. Knocking on the pressure sensor at a relevant characteristic frequency could simulate early stage Parkinson’s disease with static tremors (Figure 3g). The enlarged view of the imitated knocking on the pressure sensor with the static tremor frequency at 5 Hz is demonstrated in Figure 3h, implying its potential application for the healthcare prediction of early stage Parkinson’s disease.50 As shown in Figure 3i, Morse code was input by touching the pressure sensor surface, demonstrating the output of the sensing curves of the current responses with five corresponding characters, such as “MXene”. Wearable pressure sensors could be assembled as smart artificial E-skins for detecting various tactile signals and mapping spatial pressure distribution (Figure 4a−f). Panels a and b of Figure 4 show a photograph of the E-skin assembled from the MXene/tissue-paper-based sensor arrays (each is 6 mm × 8 mm) with 4 × 4 pixels on a PLA substrate and a schematic illustration of the E-skin, respectively. When fingers touched the sensor surface, the contact points touched by the fingers were recognized by collecting the sensing response of each pixel and plotting the sensing response map with the pressure distribution and location. Figure 4c−f shows that the E-skin could differentiate the position and number of fingers that were pressing the sensing area very well. When two fingers touched the specific sensing E-skin surface, the E-skin gave sensing responses, as shown in Figure 4c,d. When three fingers pressed the E-skin, the pressure distribution and location were identified from the sensing response contrast mapping (Figure 4e,f). These results indicate that the E-skins assembled from the MXene/tissue-paper-based sensor arrays could be used for simultaneous spatial mapping of touchable pressure distributions and locations. Moreover, the flexible MXene/tissue-

that the pressure-sensitive sensing response of the sensor was stable at different external pressures and that the resistance (slope of I−V curves) was constant under each pressure. Due to the high sensitivity over a broad sensing range and flexibility of the MXene/tissue-paper-based pressure sensor, it is a reliable candidate for real-time detection of full-range human motions. The samples were attached to the skin with the help of medical tape for all human body interactions. The MXene/tissue-paper-based pressure sensor was attached to an arm muscle and was employed to reliably detect the radial muscle contraction through reversibly making a fist (Figure 3a), which is extremely useful for physical training and even potentially for curing muscle damage. After making a fist, the current signal increased along with the compression of the sensor. Figure 3b shows that a sensor was attached to a cheek for real-time sensing of occlusion, one of the physiological motions of humans. Upon occlusion, the sensing current signal timely increased, demonstrating that increased conductivity could be obtained from the improved contact and increased number of conductive paths between the MXene and the interdigitated electrodes under external pressure. In addition, our pressure sensor could also be employed to detect cycles of bending, torsion, and touching, demonstrating excellent and stable sensing performance (Figure S16a−c). Figure S17 shows the sensing performances of the device at different radii of curvature. Furthermore, our pressure sensor was also attached onto a neck for real-time detection of swallowing and speaking, demonstrating varied current signals under the external pressure arising from swallowing (Figure 3c). The MXene/tissue-paper-based pressure sensor could be used for the timely monitoring of the physical force of the wrist pulse for potential disease diagnosis (Figure 3d,e). Figure 3d displays the reproducible and regular waveforms of a wrist pulse with a periodic beating of 80 beats per minute when the pressure sensor was fixed onto the wrist compared to the background sensing response level of the pressure sensor attached to the palm. Figure 3e shows the enlarged wrist pulse waveforms with the characteristic systolic peak (P1) and diastolic peak (P2). The radial augmentation index (P2/P1), an important value for characterizing arterial stiffness, is calculated to be 0.74. This value is in good accordance with the literature reference for a 25-year-old healthy female in a normal state (height of 160 cm).49 The pressure sensor could operate well F

DOI: 10.1021/acs.nanolett.8b04514 Nano Lett. XXXX, XXX, XXX−XXX

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sheet and a PLA sheet patterned with an interdigitated conductive electrode. The assembled flexible transient pressure sensors exhibited high sensitivity, a low detection limit (10.2 Pa), fast response (11 ms), low power consumption (10−8 W), excellent reproducibility over 10 000 cycles, reliable biocompatibility, and environmentally friendly degradation. The pressure sensor could be attached to human skin to obtain a variety of broad-range biomonitoring data that ranged from small deformations to large movements. Moreover, the pressure sensor could be used to input Morse code by touching the sensor device surface. E-skins were successfully assembled from the sensor arrays to detect various tactile signals and to map spatial pressure distributions. Furthermore, the pressure sensor could be connected with a wireless transmitter for wireless sensing of a human−machine interface. The MXene-coated tissue meshes with the features of high conductivity, large specific surface area, reliable conductive paths, and tunable contact area under external pressure could be attributed to the excellent sensing performance of the pressure sensor. This approach provides new ways to assemble flexible wearable transient pressure sensors with high sensitivity, reproducibility, wireless capability, and degradability for degradable electronic devices, next-generation artificial skins, human−machine interfacing devices, and personal healthcare biomonitoring devices.

paper-based pressure sensor could be attached to the femorotibial joint of a robot with the integration of a wireless transmitter for wireless sensitive detection of a reversible arm uplift cycle along with reversible touching of its hand to the pressure sensor (Figure 4g). The sensing response could be wirelessly sent to a mobile phone for remote touch monitoring (Figure 4h). Figure 4i,j shows the real-time wireless sensing performance of the MXene/tissue-paper-based pressure sensor for monitoring the reversible arm uplift cycle for reversible touching of the hand of the robot to the sensor, displaying a repeated and real-time resistance decrease resulting from the larger robot motions. Table S1 shows the comparison of the sensing performances of our devices with other devices. This indicates that the wearable MXene/tissue-paper-based pressure sensor can wirelessly monitor human healthcare and human− machine interfacing with excellent reproducibility, timely wireless communication, and stability. To study the degradable performance of the sensors, the sensor device was placed into PBS solution and NaOH solution dyed with rhodamine at 37 °C, and the resulting mass changes were measured. Figure 5a shows photographs of the MXene/tissue paper sensor placed in 0.5 M NaOH solution for different times. The photographs indicate that after 14 days, the sensor was degraded due to the degradation of the PLA and the tissue paper. Photographs and SEM images of the MXene/tissue paper sensor before and after 14 days are shown in Figure 5, demonstrating that after 14 days, the sensor was degraded due to the high degradability of the PLA and the tissue paper. The color and microstructure of the MXene/ tissue paper changed after 14 days (Figure 5a−e), indicating that the MXene/tissue paper was degraded. The mass loss observed for the sensors incubated in PBS solution and NaOH solution was 23% and 68%, respectively (Figure S18). These results fully demonstrate that our sensor was degradable and could not cause any severe harm to the environment. The flexible wearable transient pressure sensor with the MXene nanosheets was employed for highly sensitive broadrange human−machine interfacing and human motion biomonitoring. The biocompatibility of the flexible pressure sensor was evaluated by performing a toxicity test for direct contact of the sensor with the human body. Silver paste possesses antibacterial properties, and an ideal application is in smart wound dressings. Meanwhile, the low content of silver paste assures the reliable high proliferation of cells and good biocompatibility.51,52 To assess the toxicity of the sensor, we performed an in vitro cytotoxicity test using L929 cells cultured for 24, 48, and 72 h. As shown in Figure S19a−d, L929 cells were cultured in the extract substrates of the tissue culture plate (TCP), PLA electrode, MXene/tissue paper, and the sensor for 72 h and showed a spindle-shaped morphology, indicating that the cells were in a healthy growth state. When the L929 cells were cultured for 24, 48, and 72 h, the relative growth rate (RGR) values were not significantly different from the tissue culture plates, indicating that the sensor is not cytotoxic to L929 cells (Figure S19e). These results indicate that the sensor could be used as a human-friendly wearable device for human−machine interfacing. In summary, a flexible wearable transient pressure sensor with an MXene mesh was fabricated for highly sensitive, reproducible, wireless, degradable, and broad-range (up to 30 kPa) human−machine interfacing. The flexible pressure sensor was assembled by impregnating MXene nanosheets into tissue papers and sandwiching them between a biodegradable PLA

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b04514. Details on materials, synthesis, and materials characterization: X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscope, scanning electron microscopy images, and the piezoresistive properties of the samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhiwei Fang: 0000-0001-8826-8834 Pengbo Wan: 0000-0001-8178-4262 Guihua Yu: 0000-0002-3253-0749 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.W. acknowledges financial support from the National Natural Science Foundation of China, the Beijing Natural Science Foundation (grant no. 2152023), the National Key Research and Development Project (grant no. 2016YFC0801302), the Beijing Talent Fund (grant no. 2016000021223ZK34), and the Fundamental Research Funds for the Central Universities. G.Y. acknowledges financial support from the Sloan Research Fellowship and the Welch Foundation award no. F-1861. G

DOI: 10.1021/acs.nanolett.8b04514 Nano Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.nanolett.8b04514 Nano Lett. XXXX, XXX, XXX−XXX