Flexible antibacterial degradable bioelastomer nanocomposites for ultrasensitive human–machine interaction sensing enabled by machine learning

Flexible wearables have attracted extensive interests for personal human motion sensing, intelligent disease diagnosis, and multifunctional electronic skins. However, the reported flexible sensors, mostly exhibited narrow detection range, low sensitivity, limited degradability to aggravate environmental pollution from vast electronic wastes, and poor antibacterial performance to hardly improve skin discomfort and skin inflammation from bacterial growth under long‐term wearing. Herein, bioinspired from human skin featuring highly sensitive tactile sensation with spinous microstructures for amplifying sensing sensitivity between epidermis and dermis, a wearable antibacterial degradable electronics is prepared from degradable elastomeric substrate with MXene‐coated spinous microstructures templated from lotus leaf assembled with the interdigitated electrode. The degradable elastomer is facilely obtained with tunable modulus to match the modulus of human skin with improved hydrophilicity for rapid degradation. The as‐obtained sensor displays ultra‐low detection limit (0.2 Pa), higher sensitivity (up to 540.2 kPa−1), outstanding cycling stability (>23,000 cycles), a wide detection range, robust degradability, and excellent antibacterial capability. Facilitated by machine learning, the collected sensing signals from the integrated sensors on volunteer's fingers to the related American Sign Language are effectively recognized with an accuracy up to 99%, showing excellent potential in wireless human movement sensing and smart machine learning‐enabled human–machine interaction.


INTRODUCTION
[17][18][19] Nevertheless, the sensing sensitivity is usually very low owing to the relatively limited contact area variation and contact resistance change of the sensing layer with the electrode in the sensors under external pressure.Human skin is an important organ for information exchange between human body and the outside world Zihong Fu and Mingcheng Wang contributed equally to this work.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.© 2024 The Authors.Aggregate published by SCUT, AIEI and John Wiley & Sons Australia, Ltd. from the highly sensitive perception of external stimuli by human skin to determine the shape, texture and hardness of the surface of objects.A large number of tiny spinosum, distributed between epidermis and dermis, could work as stress concentration points and stress "amplifier" when skin is stressed, making the skin sensitive to external stimuli even at low force intensity. [20]Therefore, bioinspired from human skin with highly sensitive tactile sensation microstructure of epidermis (protection layer), spinous microstructures (sensing layer) and nerve conduction layer (signal transmitting layer), it is highly desirable for preparing the flexible electronic sensors that can simultaneously realize high sensitivity, wide detection range and robust cycling stability S C H E M E 1 (A) Molecular structure of poly(1,8-octanediol-co-Pluronic F127 citrate) (POPC) (R 1 = ─OH, 1,8-octanediol unit or Pluronic F127 unit, R 2 = ─H or citric acid unit).(B) The schematic of the microstructure of human skin and the preparation process of the skin bionic sensor.After two steps of templating from lotus leaf and further coating of MXene nanosheets, the degradable elastomeric film MXene/POPC/BKB with MXene-coated spinous surface microstructure is obtained.After the assembly of the interdigitated electrode by screen-printing of MXene ink and MXene/POPC/BKB film with MXenecoated spinous surface microstructure, the flexible skin bionic electronics is facilely fabricated.(C) Schematic illustration of the wearable personal healthcare monitoring and machine learning-enabled human-interactive sensing from the antibacterial degradable flexible electronics with reliable antibacterial capability and robust degradability.
23][24] Along with the continually increased demand for electronic devices, the electronic wastes generated during the aging process of enormous electronic devices, pose an increasing risk to the environment.The mostly reported polymer matrix used in wearable electronic sensors, including polydimethylsiloxane (PDMS), [25] polyurethane (PU), [26] polyimide (PI), [27] and Ecoflex, [28] are unable to be easily degraded under gentle conditions, inevitably resulting in vast electronic wastes and environmental pollution.[31] Therefore, there is a highly urgent need to fabricate the skin-inspired flexible wearable highly sensitive electronic sensors with reliable antibacterial capability, robust degradability, wide detection range and excellent cycling stability from biocompatible degradable polymer matrix for human-interactive sensing.
Herein, bioinspired from human skin with highly sensitive tactile sensation microstructure, an antibacterial degradable flexible electronics is facilely fabricated by efficiently assembling the degradable elastomeric film with MXene coating on the contained spinous surface microstructure templated from the lotus leaf, and the interdigitated pattern-contained electrode by screen-printing of MXene ink face-to-face (Scheme 1).The degradable elastomeric substrate POPC is facilely synthesized through melt polycondensation.After adding the antibacterial agent benzalkonium bromide (BKB) in the curing stage of POPC, the degradable antibacterial elastomeric substrate POPC/BKB was obtained.MXene, emerging two-dimensional conductive nanomaterial, has a wide range of applications in electromagnetic shielding, [32] gas detection, [33] energy storage, [34] water purification, [35] and photocatalysis [36] due to its abundant functional groups (─F, ═O, ─OH, etc.), superior electrical property and great hydrophilicity.Since the surface of the lotus leaf is distributed with spinous microstructures similar to the spinosum in human skin, it can be employed as a template to construct convex spinous surface microstructures for highly sensitive sensing.After ingeniously coating MXene nanosheets onto the lotus leaf-templated microstructures surface of the degradable antibacterial elastomeric substrate POPC/BKB, the MXene-coated convex spinous surface microstructure-contained degradable elastomeric film MXene/POPC/BKB with POPC/BKB substrate as spinous sensing microstructures and protective epidermis is successfully prepared.The as-assembled sensor illustrates a high sensitivity (540.2 kPa −1 ), superior cycling stability over 23,000 cycles, ultra-low sensing limit (0.2 Pa), reliable biocompatibility, robust degradability in pH = 7.4 phosphate buffered saline (PBS) solution and antibacterial capability.The sensor can be facilely attached onto human skin to sensitively detect various physical signals (blood pulse, finger flexion, jaw opening, etc.), and to easily diagnose different diseases including temporomandibular joint disorder (TMD).Furthermore, the flexible sensors could also be used to map spatial pressure distribution by integrating into an electronic skin, showing excellent prospects in sensitively human motion detection, facilitated disease diagnostic sensing and intelligent human-machine interface.

RESULTS AND DISCUSSION
The degradable elastomer POPC (Scheme 1A), is a rubberlike polyester elastomer, which was obtained by firstly one-pot melt polycondensation of Pluronic F127, citric acid and 1,8-octanediol to form POPC pre-polymer (pre-POPC), and then thermal cross-linking in the mold (Figures S1 and  S2). [37]The tensile stress-strain curves of POPC elastomers with various amounts of the added Pluronic F127 (10, 20, 30, 40 and 50 wt%) were demonstrated in Figure S3.The POPC elastomers possessed an increased Young's modulus and decreased elongation at break with increasing amounts of Pluronic F127 (Figure S3 and Table S1).Thus, POPC 30 (30 wt% Pluronic F127 added in POPC) was selected as the optimized elastomer matrix with the Young's modulus of 0.87 MPa for matching the human skin (0.5−1.95 MPa) and the elongation at break of 102.1% for withstanding large stretching and deformations. [38]As shown for Fourier-transform infrared spectroscopy (FTIR) in Figure S4, the characteristic peaks of ester group (C(═O)─O─C, ≈1189 cm −1 ), ester carbonyl group (C═O, ≈1734 cm −1 ), methyl and methylene groups (─CH 3 and ─CH 2 ─, 2800-3000 cm −1 ) and hydroxyl group (─OH, ≈3486 cm −1 ) could be observed respectively.Compared with that for POPC 0 elastomer, POPC 30 elastomer showed two new peaks at ≈953 cm −1 and ≈1105 cm −1 , which are the stretching vibration peaks of the ether bond (─C─O─C─), demonstrating that Pluronic F127 was successfully involved in the crosslinked elastomer network. [37]Meanwhile, the water contact angle of POPC 30 film was smaller than that for POPC 0 film due to the richness of ether bond and hydroxyl group, indicating that POPC 30 film had better hydrophilicity (Figure S5).The antibacterial POPC 30 /BKB elastomer was obtained by adding 0.1 wt% of BKB while curing the pre-POPC 30 .POPC 30 /BKB had the approximate mechanical properties with POPC 30 , indicating that the addition of antibacterial agent had little impact on the physical properties of the crosslinked elastomer matrix (Figure S3 and Table S1).Therefore, POPC 30 /BKB elastomer was employed to prepare the degradable antibacterial elastomer matrix for flexible electronic sensors.Figure S6 S7). [39,40]fter etching off the aluminum (Al) layer in the MAX phase (Ti 3 AlC 2 ) (Figure S8) with HCl-LiF etchant to form accordion-like multilayered Ti 3 C 2 T x with obvious gaps between the nanosheets (Figure 1A), [41] and the subsequent ultrasonication and centrifugation of the multilayered MXene, the ultrathin MXene nanosheets were separated from the unexfoliated MXene multilayers, which could be observed from Figure 1B and Figure S9.Compared to the MAX phase, the attenuation of the (104) characteristic peak at 39 • and the shift of the (002) peak to a smaller angle for MXene nanosheet, were mainly contributed from the embedding of the lithium ions and the etching of the Al layer, indicating the successful etching of MXene (Figure S10). [42]urthermore, the thickness of an MXene nanosheet was observed to be ≈3.4 nm by atomic force microscopy (AFM) (Figure 1C).By the experiment of X-ray photoelectron spectroscopy (XPS), the presence of Ti, F, O and C elements could be observed from MXene nanosheets (Figure S11).As shown in Scheme 1B, after the replication of the microstructures of lotus leaf featuring evenly distributed skin-like spinous microstructure (Figure 1D,E) with cured PDMS elastomer and the further replication of the concave microstructures of the above obtained PDMS matrix (Figure 1F) with POPC 30 /BKB, the convex spinous microstructure-contained POPC 30 /BKB elastomer matrix was obtained (Figure 1G).After coating MXene nanosheets, the MXene-coated convex spinous microstructure-contained MXene/POPC 30 /BKB film was obtained (Figure 1H,I).
MXene ink was obtained by adding ethylene glycol (EG, 20 vol% in final ink) and a small amount of delaminated MXene nanosheets (2 wt%) to the MXene precipitate consisting of residual MAX phase and multi-layer MXene produced during the preparation of delaminated MXene nanosheets to be further ground thoroughly.EG was added to improve the ink viscosity for further uniform printing.Figure S12a demonstrates that the MXene ink was a homogeneous mixture without phase separation and aggregation.The as-prepared MXene ink possessed a great viscous property and a lower loss factor at rest (Figure S12), which could be verified by the almost stand of the MXene ink dropped onto a glass surface with incline angle at 30 • for 30 min.A high zero-shear viscosity is necessary for MXene ink to be printed as interdigitated electrode without leaking.The zeroshear viscosity of MXene ink without EG reached 611.2 Pa s at 0.01 s −1 , compared to that for MXene nanosheet aqueous solution at only 61.4 Pa s (Figure S13), ascribing to the improved solid content of MXene ink with the presence of the MAX phase and multi-layer MXene particles in the precipitate.The zero-shear viscosity of MXene ink with EG reached 960.1 Pa s at 0.01 s −1 from the addition of EG with a higher viscosity (1.61 × 10 −2 Pa s) in comparison with water (8.9 × 10 −4 Pa s), indicating the greatly improved ink viscosity with the addition of EG for further uniform printing.[46] In addition, MXene nanosheets acted as conducting binders to connect residual MAX phase and multi-layer MXene particles, leading to the higher conductivity with the resistivity at 0.069 Ω cm and the maintenance of mechanical integrity of the as-obtained MXene ink.After screen-printing MXene ink onto the POPC 30 /BKB elastomer substrate, the interdigitated electrode could be obtained (Figure S15).Figures S16a and  S16b show the dimension of an interdigitated electrode pattern and a photograph of interdigitated electrode respectively, which is flexible and could be easily folded and twisted (Figure S16c,d). Figure S17a shows the SEM image of the interdigitated electrode by screen-printing of MXene ink.As shown in Figure S17b, the enlarged view of the circuits of the interdigitated electrode showed that the interdigitated electrode was consisted of single or few layers MXene nanosheets, multi-layer MXene particles and unetched MAX phase particles.In addition, elemental mapping analysis as shown in Figure S17c   The initial contact area between MXene/POPC 30 /BKB film with MXene-coated convex spinous microstructure and MXene ink-printed interdigitated electrode was extremely limited without external pressure loading because of the existence of the spinous microstructure, resulting in a relatively higher initial contact resistance (Figure S21).After external pressure loading, the obvious deformation of the convex spinous microstructure could be achieved even under a tiny pressure, leading to highly significant contact area change and contact resistance variation between MXene/POPC 30 /BKB film with MXene-coated convex spinous microstructure and the interdigitated patterncontained electrode by screen-printing of MXene ink.Therefore, the ingenious design of the skin bionic microstructure is essential to enhance the change of the contact area and the variation of the contact resistance for flexible sensor under external switchable pressure loading, and the initial lower contact area and initial larger contact resistance.The sensitivity S is proportionally influenced by the contact area change and the initial contact area between MXene/POPC 30 /BKB film with MXene-coated convex spinous microstructure and the interdigitated electrode (S ≈ (ΔA∕A 0 ) P ), detailed deduction could be found in Figure S21. [18,47,48]he sensing sensitivity of the sensor could be observed in Figure 2A: S 1 in the pressure range up to 11 kPa at 540.2 kPa −1 , S 2 in the pressure range of 11-80 kPa at 31.4 kPa −1 , and S 3 in the pressure range of 80-250 kPa at 3.8 kPa −1 .Meanwhile, the finite element analysis (FEA) could be used to simulate how the spinous microstructures caused the larger contact area change between the convex spinous microstructure-contained MXene/POPC 30 /BKB film and the electrode, resulting in a better sensing sensitivity.Figures S22a and S22b showed the stress distribution of the Von Mises stress field under pressure-unloaded and 200 kPa pressure-loaded conditions to the convex spinous microstructure, respectively.The contact area between the convex spinous microstructure-contained sensing film and the electrode increased along with the gradually increased external pressure loading.The initial contact area is extremely small without external pressure loading for flexible electronic sensors.The pressure was firstly concentrated around the tip of the convex spinous microstructure under external smaller pressure loading for flexible electronic sensors, causing a significantly increase of the contact area.Along with the further gradual increase of the loaded external pressure, the deformation of the convex spinous microstructure tended to be gradually saturated and the compression sensitivity slowly increased.Figure S22c shows the curve for the calculated contact area variation model as a function of external pressure loading, which is basically consistent with the trend of the sensing performance.
Figure S23a demonstrates the sensing performance for the sensor assembled from the MXene-coated flat MXene/POPC 30 /BKB film and the MXene ink-printed interdigitated electrode, demonstrating relatively lower sensing sensitivity (only the best sensitivity at 32.8 kPa −1 ) in comparison with that for the sensor (Figure 2A) assembled from MXene/POPC 30 /BKB film with MXene-coated convex spinous microstructure and the MXene ink-printed interdigitated electrode (the best sensitivity at 540.2 kPa −1 ).It is mainly attributed to the larger initial contact area and the limited contact area change, resulting in the smaller initial contact resistance and the limited contact resistance variation under external pressure loading, and finally the relatively lower sensing sensitivity.Therefore, compared to the flat sensing layer, the microstructure-contained sensing layer is vitally important and helpful to greatly improve the sensing sensitivity from the reduced initial contact area and the increased initial contact resistance, and the obviously increased contact area change by the microstructure-induced stress concentration at the beginning and the increased contact resistance change.
Figure S23b illustrates the sensing performance for the sensor assembled from MXene/POPC 30 /BKB film with MXene-coated convex spinous microstructure and the MXene ink-printed two conductive stripes-contained electrode only with the highest sensitivity at 13.5 kPa −1 from the limited contact area change and the relatively lower contact resistance variation.Figure S23c shows the sensing performance for the sensor assembled face-to-face by two MXene-coated convex spinous microstructure-contained MXene/POPC 30 /BKB films with the maximum sensitivity at only 0.87 kPa −1 mainly due to their higher initial contact area.Figure S23d shows the sensing performance of the sensor assembled from MXene/POPC 30 /BKB film with MXene-coated convex spinous microstructure and the MXene ink-printed conductive planar electrode with a lower sensitivity of 0.18 kPa −1 because of the large initial contact area and relativity small initial contact resistance between MXene/POPC 30 /BKB film with MXene-coated convex spinous microstructure and the conductive planar electrode.In comparison, the flexible sensor assembled from MXene/POPC 30 /BKB film with MXene-coated convex spinous microstructure and the interdigitated electrode, exhibited the highest sensitivity from the smaller initial contact area and the larger initial contact resistance, the higher contact area changes and the obvious contact resistance variation (Figure S23e).
The sensing responses of the sensor at different pressures were shown in Figure 2B. Figure 2C shows the sensing performance to external pressure loading with different compression speeds under the loading−unloading cycles, indicating the stable sensing performance under various compression frequencies.As shown in Figure 2D, Figures S24  and S25, the assembled flexible electronic sensor possessed rapid response time (16 ms)/recovery time (17 ms), and the output sensing signal was well matched with the input pressure waves, ensuring a reliable real-time sensing performance for healthcare detection.Figure S26 showed the sensing responses of the flexible electronic sensor to 1.5 kPa pressure at various temperatures and the sensing responses of the flexible electronic sensor to different pressures at 25 • C. The sensing responses of the flexible electronic sensor is sensitive to external different pressures, while it is relative stable to external temperature change in experimental temperature range.In addition, Figure 2E is the I-V curves of the flexible electronic sensor, indicating a good ohmic contact behavior between the interdigitated electrode and MXene/POPC 30 /BKB film with MXene-coated convex spinous microstructure.Figure 2F exhibited the stable cycling sensing performance with more than 23,000 compression cycles for reliable human healthcare monitoring.
The as-prepared sensor, featuring fast response and high sensitivity, could be employed for highly sensitive monitoring of various human motions.Figure 3A demonstrates that the sensor could identify the tiny pressure (≈0.2 Pa) generated by placing an ≈2 mg white sesame seed onto the sensor.The sensor could sensitively monitor the ultralow external pressure with the experimental detection limit at ≈0.2 Pa (≈0.02 mN), which is much smaller than the low detection limit (<1 mN) of Merkel cells in the human skin. [12]Figure S27 and Table S2 showed the sensing performance comparison with the reported different pressure sensors in the detection limit, the highest sensitivity and the cycling stability, demonstrat-ing the excellent sensing performances of the flexible sensor in this work.After placing a stereo near the sensor and playing the word "sensor", the flexible electronic sensor could detect the small vibrations caused by the sound of the stereo, which was recorded as the corresponding characteristic peaks (Figure 3B).
The blood pulse at the human wrist can reflect the health condition, and the corresponding pulse peaks are helpful for demonstrating the heart rate and arterial conditions, which is important for disease diagnostic sensing and prevention.After attaching the flexible electronic sensor at the wrist artery of a healthy male adult (Figure 3C), the regular waveform of the artery was recorded at approximately 82 beats per minute (bpm), which is within the adults' normal range (60-100 bpm). [49]The detailed pulse waveform with systolic (P 1 ), diastolic (P 2 ) and reflex (P 3 ) peaks could be observed simultaneously.The radial artery augmentation index (AI r = P 2 /P 1 ) is an accessible and reliable method to evaluate the arterial stiffness and is a powerful approach to early clinical diagnosis of atherosclerosis.The subject's AI r could be calculated to be ≈0.55,which is within the normal range (statistical values around 0.55) of healthy adults in their mid-twenties. [50]In addition, the upstroke time is the rise time of arterial systole, which is less than 180 ms in healthy subjects. [51]The average upstroke time was calculated to be 140 ms for the subject, indicating a healthy cardiovascular status.The stiffness index (SI) is also used as an important parameter to determine cardiovascular disease and is defined as SI = H/PTT, where H is height and PTT (pulse transit time) is the time difference between P 1 and P 3 .From Figure 3C, the SI of the subject (height of 1.72 m) was calculated to be 3.7 m s −1 , which is within the reported range (more than 10 m s −1 for the patients with cardiovascular disease) for adults. [52]he flexible electronic sensor is also capable of measuring small pressures triggered by airflow (Figure 3D), such as the airflow by squeezing the rubber suction bulb.
Furthermore, the flexible electronics could also be employed to detect the movement of joints.After attaching the flexible electronics onto the knee (Figure 3E) and the finger (Figure 3F), the flexible electronic sensor could sensitively monitor the joint flexion, which is vitally significant for monitoring rehabilitation condition.[55] After attaching the flexible electronic sensors onto the mandibular condyle of a healthy subject and a TMD patient respectively, the different sensing performances could be observed during the opening and the closing of mandibles.The stable signals could be observed in Figure 3G for the healthy subject, while the abnormal sensing performance was observed in Figure 3H for the TMD patient due to the restricted mandibular opening and condylar displacement.It is helpful to determine whether a subject has temporomandibular joint disorder from the sensing performance, suggesting a promising application for the wearable clinical sensing and medical diagnosis.As shown in Figure 3I, the Morse codes such as "sensor" could be represented by touching the sensor with different time (including "Di-" and "Da⋅"), indicating that the sensor could be used in the field of secure communication.As shown in Figure 4A-E, the sensors were assembled into an electronic skin (e-skin), which was coated onto the back of the hand to simulate the tactile response of human skin and pressure distribution mapping.Figures 4A, 4B and 4C showed the schematic and the photograph of the interdigitated electrodes, and the schematic of the as-assembled e-skin with 3 × 3 pixels respectively.The pressure distribution mapping could be accurately obtained after touching the corresponding sensor of the e-skin (Figure 4D,E).As demonstrated in Figure 4F,G, the flexible electronics fixed onto the surface of an electronic piano screen, was reversibly touched by the finger of a manipulator to simulate the action of playing the piano of human, which was wirelessly controlled by a volunteer's finger straightening and bending after wearing a wireless somatosensory glove.After integrated with a wireless transmitter, the sensing signal from monitoring the reversible screen touching could be wirelessly captured by a mobile client (Figure 4H,I), exhibiting great potential in smart electronic skin, wireless medical diagnostics and intelligent human-machine interfacing.
As a forefront technology in artificial intelligence, machine learning was extensively employed for the efficient and speedy identification of diverse human activities (such as gesture recognition) to advance the realization of intelligent human-machine interaction.As shown in Figures 5A  and 5B, when the fingers of the volunteer's left hand attached with flexible electronic sensors on each finger respectively, demonstrated different American Sign Languages (including different letters, such as H, U, M, A, N), the corresponding sensing signals to the demonstrated different American Sign Languages could be observed (Figure 5B).The convolutional neural network (CNN) algorithm was employed to classify and recognize the input sensing signals (80% of the data for training and 20% for testing) for the demonstrated five different American Sign Languages collected as in Figure 5C.The loss function and learning accuracy were shown in Figure 5D, demonstrating the high robust classification accuracy of the proposed CNN model after 150 training epochs.The confusion matrix was shown in Figure 5E, displaying the achieved recognition accuracy of up to 99%.The biocompatibility of the sensor is vitally prerequisite for attaching the sensor directly onto the human skin, which was investigated by evaluating the cytotoxicity of the sensor by culturing mouse fibroblast L929 cells with the sensor extract for 24 and 72 h.The live/dead cell staining was performed using calcein acetoxymethyl ester (Calcein-AM) staining and PI staining, to observe the cell growth under confocal laser scanning microscope (CLSM).As demonstrated in Figure 6A, normal fusiform morphologies could be observed for the L929 cells in the sensor group, displaying a similar proliferation trend with the control group and the good biocompatibility of the sensor.To further quantitatively evaluate the cell proliferation after cultured for 24 and 72 h, the Cell Counting Kit-8 (CCK-8) test was performed. [56]Compared to the control group, there was no significant decline of the relative growth rates (RGR) with the cell cultured with the sensor (Figure 6B), indicating that the flexible electronics had excellent biocompatibility and could be safely used as a wearable device to be attached with human skin.
In addition, the long-term contact and wearing of the flexible electronics with human skin will probably increase the risk of skin discomfort, bacterial growth and even itching and inflammation.Therefore, it is highly vital for the flexible electronic sensors to possess the antibacterial capability.To prepare an antibacterial elastomer substrate, a trace amount of antibacterial reagent benzalkonium bromide (BKB) was added into the pre-POPC during the elastomer preparation.The antibacterial capability of the flexible electronic sensor against Escherichia coli (E.coli) (Gram-negative bacterial) and Staphylococcus aureus (S. aureus) (Gram-positive bacterial) was evaluated by the conventional colony-forming cell assay.As shown in Figure 6C,D colonies were survived after culturing with the antibacterial flexible electronic sensor, compared to that for the blank group.The bactericidal ratio of the sensor against S. aureus and E. coli were 94.8% and 98.5%, respectively, demonstrating an excellent antibacterial performance of the sensor (Figure S28).The great antibacterial performance of the sensor was originated from the cationic antibacterial reagent benzalkonium bromide, a hydrosoluble quaternary ammonium salt-based antibacterial agent.59] As displayed in Figure 7A, the degradable flexible electronic sensor, placed in a PBS solution with pH = 7.4, was gradually swollen after water adsorption and gradually degraded.The weight loss of the degradable sensor was observed to be almost 100% after 49 days in PBS solution of pH = 7.4, while only ≈25% of weight loss could be obtained for the POPC 0 /BKB film in the same period (Figure 7B).Firstly, the water molecules were adsorbed into the hydrophilic elastomer POPC 30 , leading to the swelling of the hydrophilic elastomer along with the falling off of MXene/POPC 30 /BKB film with MXene nanosheets-coated convex spinosum-like microstructures.Along with the dissolution of the unreacted monomers and the hydrolysis of polymer ester bonds, a small decrease in mass for the hydrophilic elastomer could be achieved.[62][63] The rapid degradation of the hydrophilic elastomer POPC 30 was attributed to the introduction of Pluronic F127 in POPC 30 , which could increase the hydrophilicity of the elastomer and reduce the cross-linking density of the elastomer (Table S3).

CONCLUSION
In summary, bioinspired from human skin with highly sensitive spinosum microstructures, a flexible antibacterial degradable ultrasensitive electronics was facilely fabricated by assembling the MXene nanosheets-coated convex spinosum-like microstructures-contained POPC 30 /BKB film with approximate Young's modulus matching to human skin templated from the lotus leaf and MXene ink-printed interdigitated electrode face-to-face.After melt polycondensation with Pluronic F127, 1,8-octanediol and citric acid, the degradable bioelastomer POPC was successfully prepared with tunable modulus for matching with the modulus of human skin to endow comfortable wearing, robust degradability and reliable antibacterial performance with the help of BKB.MXene ink for screen printing interdigitated electrode, was delicately obtained by ingenious introduction of ethylene glycol (EG) and MXene nanosheets into the left precipitate during MXene nanosheets solution preparation, demonstrating variable viscosity, reliable oxidation resistance and suitable conductivity.The as-prepared sensor exhibited higher sensitivity (540.2 kPa −1 ), broad sensing range (250 kPa), ultralow detection limit (0.2 Pa) and robust cycling stability (over 23,000 cycles) for detecting various human physical signals (such as blood pulse, finger flexion and jaw opening), and easily diagnosing various diseases (such as cardiovascular diseases and temporomandibular joint disorder).
With the assistance of machine learning, the sensing signals obtained from the sensors coated onto the fingers of the volunteer to different American Sign Languages can be recognized efficiently and quickly with up to 99% recognition accuracy.Furthermore, the flexible sensor possessed reliable biocompatibility, robust degradability in pH = 7.4 PBS solution and efficient antibacterial capability, which could be assembled into an electronic skin for realizing wireless intelligent human-machine interaction, showing excellent potential in sensitive human motions monitoring, wireless disease diagnostic sensing and smart touchable electronic skins for the promising development of a new generation of green and intelligent wearable electronic devices.

Preparation of pre-POPC
1,8-octanediol and citric acid (the molar ratio of 1,8octanediol and citric acid: 1.1:1.0)were placed in a 250 mL round bottom flask.Pluronic F127 was added and stirred to form a homogeneous mixture, and then the mixture was exposed to a constant flow of argon gas. [37]The mixture was heated to 160 • C and stirred continuously until it was completely melted, and then stirred continuously at 140 • C for 2 h to form a prepolymer with a certain viscosity.The prepolymer is dissolved in anhydrous ethanol to form a homogeneous solution for further storing.

Preparation of the convex spinous microstructure-contained POPC 30 /BKB elastomer
Configure the mixture of PDMS prepolymer and curing agent with a ratio of 5:1, and then degas in a vacuum oven for 20 min to remove the air bubbles at room temperature for subsequent using.Cut the dried lotus leaf (keep away from the leaf's veins) into 3 cm × 3 cm shape and fix it on the tetrafluoroethylene mold.The PDMS mixture was evenly poured on the lotus leaf and cured at 70 • C for 2 h.The cured PDMS membrane with the concave microstructures of the lotus leaf was peeled off and could be used for the further replication.After that, 0.1 wt% of benzalkonium bromide was added in the POPC 30 prepolymer and then the air bubbles were removed in vacuum oven for 20 min at room temperature to obtain POPC 30 /BKB prepolymer, which was uniformly poured on the PDMS template and cured under vacuum at 80 • C for 5 days, and peeled off to obtain the convex spinous microstructure-contained POPC 30 /BKB elastomer.

Synthesis of MXene nanosheets
MXene (Ti 3 C 2 T x ) was produced by etching MAX phase (Ti 3 AlC 2 ) by LiF/HCl method.Firstly, adding 1 g LiF to a tetrafluoroethylene vessel containing 20 mL of hydrochloric acid and stirred for 30 min.Next, 1 g Ti 3 AlC 2 was gradually mixed to the above solution and stirred magnetically at 35 • C for 24 h.The obtained solution was repeatedly centrifuged at 3500 rpm and washed with deionized water until the pH was greater than 6.To obtain a homogeneous MXene nanosheets solution, the precipitate obtained by centrifugation was sonicated on an ultrasonic cleaner for 40 min, and finally centrifuged at 3500 rpm for 1 h.

Fabrication of interdigitated electrode
The interdigitated electrode was obtained by screen-printing of MXene ink on POPC 30 /BKB substrate with patterned stainless steel wire mesh as a template.The left precipitate after centrifugation during the preparation of the MXene nanosheets solution mainly consisted of residual MAX phase and multi-layer MXene.A few delaminated nanosheets (2 wt%) and an appropriate amount (20 vol% in final ink) of ethylene glycol (EG) were added to the above precipitate, which was fully polished to obtain a uniform and viscous MXene ink (solid content: 22 wt%).The prepared MXene ink was spread on the screen stencil with the interdigitated electrode pattern, slid evenly on the screen with a squeegee, and dried at room temperature to obtain the interdigitated electrode.

Fabrication of the flexible electronic sensor
First, the convex spinous microstructure-contained POPC 30 /BKB elastomer was cut into 1 cm × 1 cm square shape, and coating a certain amount of MXene on the surface to form MXene/POPC 30 /BKB film with MXene-coated convex spinous microstructure.Then, the MXene/POPC 30 /BKB film with MXene-coated convex spinous microstructure is assembled face-to-face with the MXene ink-printed interdigitated electrode, and two conducting wires are used to connect the interdigitated electrodes to obtain the flexible electronic sensor.

Cytotoxicity test and live/dead double-staining experiment of flexible electronic sensor
The CCK-8 assay was used to assess the cytotoxicity of the prepared sensors.First, the sensor was sterilized and soaked in 15 mL Minimum Essential Medium (MEM) for 24 h and filtered using 0.22 µm filter to obtain the extracts.L929 cells at logarithmic growth stage were inoculated into 96-well plates, and 100 µL of medium was added to each well in the blank group, and the experimental group was replaced with an equal amount of extract and incubated for 24 and 72 h.After that, 10% CCK-8-contained 100 µL medium was added to each well, and the absorbance at 450 nm were detected by enzyme marker.The steps of cytotoxicity assay were repeated for 24 and 72 h.Dye dilution solution was prepared with Calcein-AM staining and PI staining, and 1 mL dye dilution solution was applied to each culture dish and cultured for 15 min with avoiding light and then photographed and recorded under a CLSM.
The relative growth rate was calculated from the equation below: relative growth rate (%) = (OD of experimental group − OD of the background) ∕ (OD of contrastive group − OD of the background) × 100 (%)

Antibacterial activity assessment of flexible electronic sensor
The antibacterial experiment evaluated the bacterial inhibitory effect of the flexible electronic sensor on E. coli (ATCC25922) and S. aureus (ATCC29213).The samples were cut into small pieces and placed in bacterial culture tubes, and 2 mL of diluted E. coli bacterial solution was added to co-culture for 4 h.Take 100 µL of the co-culture solution and spread it evenly on Luria-Bertani (LB) solid medium.Photograph and record the number of colonies after 18 h incubation.S. aureus was treated in the same way.
The bactericidal ratio was calculated from the equation below: bactericidal ratio (%) = (1 − number of colonies in the experimental group∕number of colonies in the control group ) × 100 (%)

Characterization and measurements
X-ray diffraction patterns (XRD), nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), and differential scanning calorimetry (DSC) were acquired from Rigaku D/Max 2500, Bruker AVANCE III, Thermo ESCALAB 250 and Mettler DSC1, respectively.The functional groups of the POPC 0 and POPC 30 film were measured by Fourier transform infrared spectra (FTIR, Nicolet Nexus 670).The morphologies of samples were observed using a scanning electron microscope (SEM, Hitachi S-4800), a transmission electron microscope (TEM, Hitachi HT7700) and an atomic force microscope (AFM, Bruker multimode 8).The contact angles of water droplets were measured by an SL200 A Contact Angle Analyzer (China) to evaluate the hydrophilicity of the POPC 0 and POPC 30 film.The piezoresistive properties of the pressure sensors were timely obtained by an electrochemical workstation (CHI660E, Shanghai Chenhua Inc.).The signed informed consent by the volunteer was firstly obtained for further human motions monitoring.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflicts of interest.

D ATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.

F
I G U R E 1 (A) Scanning electron microscope (SEM) image of the accordion-like Ti 3 C 2 T x .(B) SEM image and (C) AFM image of MXene nanosheet.(D) The photograph of a lotus leaf taken from Beijing Botanical Garden.(E) Typical top view SEM image of a lotus leaf.(F) SEM image of the concave microstructures of the obtained PDMS matrix after the first replication.(G) Typical top view SEM image of the spinosum-like convex spinous microstructurecontained POPC 30 /BKB elastomer matrix.(H,I) SEM images of MXene/POPC 30 /BKB film with MXene-coated convex spinous microstructure.
indicated the presence of Ti, F, O and C elements, demonstrating that the MXene ink was printed onto the substrate.Finally, the flexible electronics was assembled from MXene/POPC 30 /BKB film with MXene-coated convex spinous microstructure and MXene ink-printed interdigitated electrode face-to-face (Scheme 1B).As shown in FigureS18, the flexible electronics exhibited different sensing responses with different amounts of the coated MXene on the convex spinous microstructurecontained MXene/POPC 30 /BKB film.With the increasing MXene content, the sensing responses firstly increased and then decreased.The optimal MXene loading was selected as 0.35 mg for the maximum sensing response.The convex spinous microstructure-contained MXene/POPC 30 /BKB film with different amounts of the loaded MXene were shown in FiguresS19, S20, and Figure1H,I.The efficient conductive network pathway could not be formed with limited MXene nanosheets coating on the convex spinous microstructure-contained MXene/POPC 30 /BKB film (FigureS19).In contrast, as shown in FigureS20, too much amounts of the loaded MXene could cover up the convex spinous microstructure on the MXene/POPC 30 /BKB film, resulting in the relatively lower specific surface area and the limited contact area change under external pressure.The efficient conductive pathway could be obtained at the optimal MXene coating on MXene/POPC 30 /BKB film with the convex spinous microstructure (Figure1H,I).

F I G U R E 2
Sensing performance of the flexible electronic sensors.(A) The sensing sensitivity under the external pressure loading.The sensing responses to (B) various pressures and to (C) different frequency during the pressure loading−unloading cycles.(D) The response time of the flexible electronic sensor.(E) I−V curves of the sensor under different applied pressures.(F) The cycling durability under external pressure loading.

F I G U R E 3
The flexible electronics for human activities detection.(A) The detection limit of the sensor.Inset: the photograph of a white sesame seed placed on the flexible electronics.(B) The sensing response of the sensor for sensing the word "sensor" played by a stereo.Inset: schematic diagram of the sensor to detect the sound vibration generated by a stereo.(C) The sensing performance of the flexible electronics coated onto the wrist of a volunteer for the detection of the wrist pulse.Inset: photograph of the sensor coated onto the wrist and the enlarged waveform of blood pulse.(D) The sensing performance of the flexible electronics for sensing different pressures generated by squeezing the rubber suction bulb.Inset: photograph of the flexible electronics for sensing various pressures by squeezing the rubber suction bulb.(E) The sensing performance of the flexible electronics for sensing the reversible knee flexion.Inset: photograph of the sensor attached onto the knee for sensing the reversible knee flexion.(F) The sensing performance of the sensor to different finger bending angles.Inset: photographs of the sensors fixed onto the finger with various bending angles.The sensing performances of the sensor for the detection of the mandibular condyle movements for (G) normal people and (H) temporomandibular disorder (TMD) patient during opening and closing the mandibles.Inset: photographs of the sensors attached onto the related mandibular condyles.(I) The sensing signal of Morse code for "sensor".

F I G U R E 4
Flexible electronic sensors assembled for multifunctional electronic skins and human-machine interaction.(A) Schematic diagram and (B) photograph of the assembled interdigitated electrodes.(C) Schematic diagram of the electronic skin.(D,E) Photographs of the electronic skin fixed onto the back of the hand and touched by finger.Inset: the corresponding pressure distribution mapping for finger touching.(F,G) Reversible finger touching of the manipulator on the sensor attached onto the screen, wirelessly controlled by a volunteer's finger straightening and bending after wearing a wireless somatosensory glove.(H) Setup for wireless signal transmission.(I) Sensing responses wirelessly captured by a mobile client.F I G U R E 5 (A) The schematic for the fingers of the volunteer's left hand attached with flexible sensors on each finger respectively to display different American Sign Languages.(B) The corresponding sensing signals for the demonstrated different American Sign Languages (including different letters, such as "H", "U", "M", "A" and "N").(C) The detailed architecture of the constructed CNN model.(D) The loss function and the learning accuracy in 150 training epochs.(E) Confusion matrix for the recognition of different American Sign Languages.
, only very few bacteria F I G U R E 6 (A) Fluorescence microscope images of L929 cells after co-cultured with the flexible electronic sensor for 24 and 72 h, respectively (Green fluorescence represented the Calcein-AM stained living cells and red fluorescence represented the propidium iodide (PI) stained dead cells).(B) Relative growth rate values of L929 cells co-cultured with the flexible electronic sensor for 24 and 72 h.In vitro antibacterial activities of the flexible sensor against (C) E.coli and (D) S. aureus.Photographs of the survival bacteria clones on agar plates, respectively.

F I G U R E 7
(A) Photographs of degradation of the sensor placed in a PBS solution with pH = 7.4 for 49 days.(B) The residual mass of the degradable flexible electronic sensor and the POPC 0 /BKB film with the same size in a PBS solution at pH = 7.4 for 49 days, respectively.

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C K N O W L E D G E M E N T SThis work was financially supported by the National Natural Science Foundation of China (52222303, 51973008), Joint Project of BRC-BC (Biomedical Translational Engineering Research Center of BUCT-CJFH) (XK2022-03), and the Fundamental Research Funds for the Central Universities.The informed consent with signature was obtained from the volunteer for the human activity experiments.