Pda@mxene / clay / pnipam composite conductive hydrogel and preparation and application thereof
By using PDA@MXene/Clay/PNIPAM composite conductive hydrogel, the problem of mismatch between mechanical and signal conduction in traditional wearable devices is solved, achieving high mechanical performance and temperature sensitivity, making it suitable for flexible sensor applications.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- LANZHOU INSTITUTE OF CHEMICAL PHYSICS CHINESE ACADEMY OF SCIENCES
- Filing Date
- 2023-09-11
- Publication Date
- 2026-07-07
AI Technical Summary
The bulky sensing elements and rigid metal circuit components in traditional wearable devices lead to a mismatch between mechanics and signal transmission, limiting their application in the human body. Furthermore, the low mechanical strength of PNIPAM hydrogel makes it difficult to meet the requirements of flexible sensors.
The PDA@MXene/Clay/PNIPAM composite conductive hydrogel was used to improve the mechanical properties and electrical conductivity of the hydrogel through ion coordination crosslinking and hydrogen bonding interactions, combined with MXene nanosheets prepared by etching. The addition of self-adhesive PDA material improved dispersibility and adhesion.
It achieves high tensile strength, strong adhesion, strain sensitivity, and temperature sensitivity, with the elongation at break increased to 1723%. It exhibits excellent strain sensitivity over a wide strain range and a temperature sensitivity of 2.749℃⁻¹, making it suitable for a wide range of applications in flexible sensors.
Smart Images
Figure CN117089158B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to a composite conductive hydrogel, specifically to a PDA@MXene / Clay / PNIPAM composite conductive hydrogel and its preparation and application. Background Technology
[0002] Flexible strain sensors have attracted significant attention due to their immense application potential in wearable and implantable devices, soft robotics, electronic skin, motion monitoring, and medical diagnostics and healthcare. However, the integration of bulky sensing elements and rigid metal circuitry in traditional wearable devices can lead to mechanical and signal transmission mismatches between the device and biological tissue, limiting their widespread use in the human body. Ideal wearable implantable devices should possess excellent comprehensive characteristics, including mechanical stability, biocompatibility, and adjustable moisture balance, to better mimic human skin. Hydrogels, as flexible materials, offer advantages over traditional wearable devices due to their biomimetic structural properties and functional adjustability.
[0003] Polyisopropylacrylamide (PNIPAM), a typical thermosensitive polymer, undergoes a rapid phase transition upon temperature stimulation, and its response temperature is tunable. This property makes PNIPAM hydrogels potential for use as actuators. Furthermore, its critical solution temperature (LCST) is approximately 33°C, close to the temperature of the human body surface. PNIPAM hydrogels exhibit excellent temperature sensitivity and are therefore also used as matrix materials for temperature sensing. However, the relatively low mechanical strength of PNIPAM-based hydrogels remains a challenge that needs to be overcome.
[0004] Due to their high water content, hydrogels suffer from poor mechanical properties, hindering their application in wearable electronic devices for detecting human body signals. To improve the mechanical properties of hydrogels, incorporating nanomaterials to achieve nanocomposite toughening is a common method for preparing high-strength hydrogels. Among these, MXene is a class of metal carbide or metal nitride materials with a two-dimensional layered structure, resembling stacked potato chips, exhibiting excellent mechanical properties and outstanding electrical conductivity. Compared to pure PNIPAM hydrogels, MXene / PNIPAM hydrogels show significantly improved fracture strength and elongation at break. Summary of the Invention
[0005] The purpose of this invention is to provide a PDA@MXene / Clay / PNIPAM composite conductive hydrogel, its preparation and application. This composite conductive hydrogel has a dual crosslinking effect of ionic coordination crosslinking and hydrogen bonding interaction, and has high tensile strength, strong adhesion, strain sensitivity and temperature sensitivity.
[0006] To achieve the above objectives, the present invention provides a PDA@MXene / Clay / PNIPAM composite conductive hydrogel, which is obtained by polymerizing PDA@MXene, isopropylacrylamide, and clay under the action of an initiator and a accelerator; wherein the mass ratio of PDA, MXene, isopropylacrylamide, and clay is 0.23-0.92:14.5-58:2500:800.
[0007] Preferably, the PDA@MXene is obtained by reacting an MXene dispersion and dopamine in a Tris reagent.
[0008] Preferably, the concentration of the MXene dispersion is 0.5–2.0 mg / mL. The MXene dispersion is MXene dispersed in water.
[0009] Preferably, the MXene used is prepared by etching the MAX phase with LiF and hydrochloric acid.
[0010] Preferably, the isopropylacrylamide used is purified by recrystallization.
[0011] Another object of the present invention is to provide a method for preparing the composite conductive hydrogel, characterized in that the method comprises: polymerizing PDA@MXene, isopropylacrylamide and clay at 20°C under the action of an initiator and an accelerator.
[0012] Preferably, the initiator is selected from potassium persulfate; the promoter is selected from N,N,N,N-tetramethylethylenediamine.
[0013] Preferably, the amounts of PDA, MXene, isopropylacrylamide, clay, initiator, and accelerator are 0.23–0.92 mg: 14.5–58 mg: 2500 mg: 800 mg: 50 mg: 30 μL.
[0014] Preferably, the preparation of PDA@MXene is as follows: the MXene dispersion is sonicated, Tris reagent (to provide an alkaline environment for the polymerization of dopamine) is added and the dispersion is fully dispersed, and then dopamine is added and the reaction is fully carried out.
[0015] Preferably, the recrystallization purification of isopropylacrylamide is as follows: add n-hexane to isopropylacrylamide, heat to dissolve until clear, cool to room temperature, continue cooling at 0°C, filter the precipitated isopropylacrylamide crystals, wash with n-hexane at 0-10°C, and dry the filtered isopropylacrylamide crystals.
[0016] Another object of the present invention is to provide the application of the aforementioned composite conductive hydrogel in flexible sensors.
[0017] The PDA@MXene / Clay / PNIPAM composite conductive hydrogel of the present invention, its preparation and application, have the following advantages:
[0018] (1) The PDA@MXene / Clay / PNIPAM composite conductive hydrogel of the present invention has a dual cross-linking effect of ion coordination cross-linking and hydrogen bonding interaction. By introducing MXene nanosheets prepared by etching method, the hydrogel is endowed with excellent mechanical properties, high electrical conductivity and fast response time, exhibiting high tensile strength, strong adhesion, strain sensitivity and temperature sensitivity. In particular, it exhibits excellent mechanical properties, with an elongation at break of up to 1723%, which is twice that of pure PNIPAM hydrogel without PDA@MXene. It has broad application potential in the field of flexible sensing.
[0019] (2) The PDA@MXene / Clay / PNIPAM composite conductive hydrogel of the present invention exhibits excellent strain sensitivity in a large strain range (0.05 to 500%) and can be used to monitor body movement, including pulse, facial expression changes, and elbow and knee flexion.
[0020] (3) The PDA@MXene / Clay / PNIPAM composite conductive hydrogel of this invention utilizes polyisopropylacrylamide (PNIPAM) to construct a hydrogel with excellent temperature sensitivity. Due to the introduction of NIPAM, it can accurately convert external temperature changes into electrical signals in real time, with a negative relative resistance change, the magnitude of which increases with increasing temperature. This is because the molecular chains of the hydrogel shrink with increasing temperature, reducing the distance between conductive particles. The magnitude of the relative resistance change with increasing temperature reflects the temperature sensitivity of the sensor. It can be clearly observed that the relative resistance change shows an almost linear response, with a temperature sensitivity of 2.749℃. -1 ;
[0021] (4) The PDA@MXene / Clay / PNIPAM composite conductive hydrogel of the present invention introduces self-adhesive PDA material into the conductive hydrogel system. The terminal groups (-O, -F, -OH) and DA functional groups (-OH, -NH) on the surface of MXene can form hydrogen bonds. The M ions of MXene can form strong chelates with the -O of the catechol group in the polydopamine structure. PDA can modify the surface of MXene and improve the dispersibility of MXene in the hydrogel. Moreover, PDA enables strain sensors to be directly connected to human skin without the need for other additional tapes, thereby promoting the processing in actual monitoring applications.
[0022] (5) The PDA@MXene / Clay / PNIPAM composite conductive hydrogel of the present invention has non-uniformity and fixed cross-linking points in the PNIPAM hydrogel system due to the traditional chemical cross-linking agent N,N-methylenebisacrylamide (MBAA), which makes the resulting hydrogel easy to break and lacks tensile properties, and has some problems in the application of flexible sensors. Compared with the traditional chemical cross-linking agent, Clay, as a cross-linking agent, is mainly composed of minerals that exist in the human body, which shows excellent biocompatibility. Moreover, clay has a layered structure. During the polymerization process, the clay layers are peeled off. The peeled clay layers act as cross-linking agents with hydrogen bonds, ionic bonds and polymers, which can further improve the mechanical properties of the hydrogel and enhance the gelation rate. The polymerization process is simple and easy to operate. Attached Figure Description
[0023] Figure 1 Scanning electron microscope image of the PDA@MXene / Clay / PNIPAM composite conductive hydrogel prepared in Example 3 of this invention.
[0024] Figure 2 The conductivity results are for the composite hydrogels prepared in Examples 1-4 of this invention.
[0025] Figure 3 The mechanical properties of the composite hydrogels and pure PNIPAM hydrogels prepared in Examples 1-4 and Comparative Example 2 of this invention are shown in the test results.
[0026] Figure 4 The results show the adhesion performance test results of the composite hydrogel prepared in Example 3 of this invention.
[0027] Figure 5 The strain response test results are for the composite hydrogel prepared in Example 3 of this invention.
[0028] Figure 6 The results of human monitoring tests are for the composite hydrogel prepared in Example 3 of this invention.
[0029] Figure 7 The temperature response test results are for the composite hydrogel prepared in Example 3 of this invention. Detailed Implementation
[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.
[0031] The reagents used in the following examples and comparative examples are as follows:
[0032] MXene sheets are prepared by etching the raw material MAX phase from Jilin 11 Technology Co., Ltd. The etching reagents are lithium fluoride (LiF, purity ≥98%) from Shanghai Aladdin Biochemical Technology Co., Ltd. and hydrochloric acid from Chengdu Kelong Chemical Co., Ltd., China.
[0033] Dopamine (DA) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.
[0034] Tris (hydroxymethyl)aminomethane (Tris) was purchased from Chengdu Kelong Chemical Co., Ltd.
[0035] N-Isopropylacrylamide (NIPAM) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.
[0036] Potassium persulfate (KPS, K2S2O8) was purchased from Chengdu Kelong Chemical Co., Ltd.
[0037] N,N,N,N-Tetramethylethylenediamine (TEMED) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.
[0038] The clay was purchased from Rockwood Ltd., and is a synthetic spodumene clay.
[0039] Example 1
[0040] A PDA@MXene / Clay / PNIPAM composite conductive hydrogel is prepared by the following steps:
[0041] (1) Recrystallization purification of isopropylacrylamide (NIPAM)
[0042] Weigh 40g of isopropylacrylamide (NIPAM) powder and pour it into a 250mL single-necked flask. Then, add 200mL of n-hexane and dissolve it in a boiling water bath (70℃) with magnetic stirring until clear. Allow it to cool naturally to room temperature and continue cooling overnight in a refrigerator (0℃). Filter the cooled NIPAM crystals and wash with refrigerated n-hexane (0–10℃). Finally, vacuum dry the filtered NIPAM crystals in a 40℃ oven for 48 hours. Purify the purchased isopropylacrylamide by recrystallization to remove impurities such as the stabilizer MEHQ, thereby improving the purity and giving it better solubility and controllability.
[0043] (2) Preparation of PDA@MXene composite material
[0044] 0.3028 g of Tris base powder and 75 μL of HCl were quantitatively added to a 250 mL volumetric flask, and the pH was adjusted to 8.5 to obtain the Tris reagent.
[0045] Measure 29 mL of MXene dispersion (MXene dispersed in water) with a concentration of 0.5 mg / mL and sonicate for 30 min. Then add 0.46 mL of Tris reagent and stir for 1 h to ensure complete dispersion. Subsequently, add 0.23 mg of dopamine powder to the above solution and stir continuously for 4 h to ensure complete reaction, thereby obtaining PDA@MXene solution.
[0046] (3) Preparation of PDA@MXene / Clay / PNIPAM composite hydrogel
[0047] In an ice-water bath under a nitrogen atmosphere, 2.5 g of NIPAM powder was added to the PDA@MXene solution and stirred thoroughly until dissolved. Then, 0.8 g of clay, 0.05 g of KPS, and 30 μL of TEMED were added to the mixture, and the mixture was stirred for 20 min. Finally, the mixture was quickly poured into a mold and polymerized at 20 °C for 24 h.
[0048] The resulting PDA@MXene / Clay / PNIPAM composite hydrogel has an electrical conductivity of 0.094 S / m, a tensile breaking strength of 91 kPa, and an elongation at break of 1404%.
[0049] Example 2
[0050] A PDA@MXene / Clay / PNIPAM composite conductive hydrogel is prepared in a manner that is basically the same as in Example 1, except that:
[0051] In step (1), the concentration of the MXene dispersion used was 1.0 mg / mL; the amount of Tris reagent used was 0.92 mL; and the amount of dopamine powder used was 0.46 mg.
[0052] The resulting PDA@MXene / Clay / PNIPAM composite hydrogel has an electrical conductivity of 0.12 S / m, a tensile breaking strength of 112 kPa, and an elongation at break of 1527%.
[0053] Example 3
[0054] A PDA@MXene / Clay / PNIPAM composite conductive hydrogel is prepared in a manner that is basically the same as in Example 1, except that:
[0055] In step (1), the concentration of the MXene dispersion used was 1.5 mg / mL; the amount of Tris reagent used was 1.38 mL; and the amount of dopamine powder used was 0.69 mg.
[0056] The resulting PDA@MXene / Clay / PNIPAM composite hydrogel has an electrical conductivity of 0.13 S / m, a tensile breaking strength of 117 kPa, and an elongation at break of 1723%.
[0057] Example 4
[0058] A PDA@MXene / Clay / PNIPAM composite conductive hydrogel is prepared in a manner that is basically the same as in Example 1, except that:
[0059] In step (1), the concentration of the MXene dispersion used was 2.0 mg / mL; the amount of Tris reagent used was 1.84 mL; and the amount of dopamine powder used was 0.92 mg.
[0060] The resulting PDA@MXene / Clay / PNIPAM composite hydrogel has an electrical conductivity of 0.14 S / m, a tensile breaking strength of 80 kPa, and an elongation at break of 1261%.
[0061] Comparative Example 1
[0062] A Clay / PNIPAM composite hydrogel is prepared in a manner similar to that of Example 1, with the following differences:
[0063] PDA@MXene was not prepared; step (2) was not performed.
[0064] In step (3), the PDA@MXene solution is replaced with 29 mL of deionized water.
[0065] The resulting Clay / PNIPAM composite hydrogel has an electrical conductivity of 0.092 S / m, a tensile strength at break of 72 kPa, and an elongation at break of 862%.
[0066] Comparative Example 2
[0067] An MXene / Clay / PNIPAM composite hydrogel is prepared in a manner that is basically the same as in Example 1, except that:
[0068] PDA@MXene was not prepared; step (2) was not performed.
[0069] In step (3), the PDA@MXene solution is replaced with 29 mL of MXene dispersion with a concentration of 1.5 mg / mL.
[0070] The resulting MXene / Clay / PNIPAM composite hydrogel has an electrical conductivity of 0.13 S / m, a tensile breaking strength of 90 kPa, and an elongation at break of 1128%.
[0071] Comparative Example 3
[0072] A PDA / Clay / PNIPAM composite conductive hydrogel is prepared in a manner that is basically the same as in Example 1, except that:
[0073] In step (2), deionized water was used instead of MXene dispersion, and 1.38 mL of Tris reagent was added to 29 mL of deionized water; the amount of dopamine powder used was 0.69 mg.
[0074] In step (3), NIPAM powder is added to the PDA solution prepared in step (2).
[0075] The resulting PDA / Clay / PNIPAM composite hydrogel has an electrical conductivity of 0.091 S / m, a tensile breaking strength of 57 kPa, and an elongation at break of 885%.
[0076] Experimental Example 1: Surface morphology of the PDA@MXene / Clay / PNIPAM composite conductive hydrogel in Example 3 of this invention
[0077] Scanning electron microscopy (SEM) was used to obtain images of the PDA@MXene / Clay / PNIPAM composite conductive hydrogel prepared in this invention by freeze-drying pretreatment before observation. See [link to SEM image]. Figure 1 The figure shows a uniform and dense honeycomb network structure of the hydrogel.
[0078] Experiment Example 2: Conductivity Test
[0079] The conductivity of the composite hydrogels prepared by PDA@MXene / Clay / PNIPAM in Examples 1-4 and Comparative Examples 1-3 of this invention was tested. Specifically, the two ends of the hydrogel were first bonded together with conductive copper strips and copper wires, and then connected to a digital source meter to test its electrical signal.
[0080] The results are as follows Figure 2 As shown in Table 1, Figure 2 The graph shows the conductivity results of the composite hydrogels prepared in Examples 1 to 4 of this invention (the horizontal axis represents the concentration of MXene). It can be seen that the conductivity gradually increases with the increase of MXene concentration, reaching a maximum of 0.14 S / m.
[0081] Experiment Example 3 Mechanical Property Testing
[0082] The mechanical properties of the composite hydrogels prepared in Examples 1-4 and Comparative Examples 1-3 of this invention, as well as the pure PNIPAM hydrogel, were tested. Specifically, different samples were tested using a universal tensile testing machine to obtain mechanical data.
[0083] The results are as follows Figure 3 As shown in Table 1,Figure 3 The black line A represents pure PNIPAM hydrogel (Ph), the blue line represents MXene / Clay / PNIPAM hydrogel (Comparative Example 2), and the red line represents PDA@MXene / Clay / PNIPAM composite conductive hydrogel (Example 3), with their elongation at break and tensile strength increasing sequentially. Figure 3 Figure B shows the mechanical properties of the hydrogel at different MXene concentrations (0, 0.5 mg / mL, 1.0 mg / mL, 1.5 mg / mL, and 2.0 mg / mL). It can be seen that the mechanical properties first increase and then decrease with increasing MXene content. The decrease in the mechanical properties of the hydrogel as the MXene content continues to increase may be due to the aggregation of excessive MXene nanosheets, which reduces the relative sliding between the nanosheets and leads to a decrease in mechanical properties.
[0084] Table 1 shows the performance results of the composite hydrogels prepared in Examples 1-4 and Comparative Examples 1-3 of this invention.
[0085]
[0086] Experiment 4 Adhesion Performance Test
[0087] The adhesion properties of the composite hydrogel prepared in Example 3 of this invention were tested on metal, glass, polyvinyl chloride and wood. The results are as follows:
[0088] The results are as follows Figure 4 As shown, Figure 4 A, B, C, and D represent metal, glass, polyvinyl chloride, and wood, respectively. It can be seen that PDA@MXene / Clay / PNIPAM hydrogel has good adhesion and can adhere to different substrates.
[0089] Experimental Example 5: Strain Response Test
[0090] The composite hydrogel prepared in Example 3 of the present invention was subjected to strain response testing. Specifically, the two ends of the hydrogel were first bonded together with conductive copper strip and copper wire, and then connected to a universal tensile testing machine and a digital source meter. The electrical signal and mechanical properties were tested simultaneously.
[0091] The results are as follows Figure 5 As shown, Figure 5 A represents the response tests for slight and small strains. Figure 5 B represents the large strain response test, which shows that the PDA@MXene / Clay / PNIPAM hydrogel exhibits stable electrical signal changes and sufficiently high sensitivity over a wide strain range.
[0092] Experimental Example 6: Human Body Monitoring Test
[0093] The composite hydrogel prepared in Example 3 of this invention was subjected to human body monitoring test. Specifically, the two ends of the hydrogel were first bonded together using conductive copper strips and copper wires to connect the digital source meter to the human body monitoring site and test the motion signal.
[0094] The results are as follows Figure 6 As shown, Figure 6 A represents pulse monitoring, with a cycle of 72 beats per minute. -1 , Figure 6 B represents arm movement monitoring. It can be seen that the PDA@MXene / Clay / PNIPAM hydrogel can be used to explore the entire range of human activities and monitor subtle physiological signals and important joint movements.
[0095] Experiment Example 7 Temperature Response Test
[0096] The composite hydrogel prepared in Example 3 of this invention was subjected to human monitoring tests, as detailed below:
[0097] First, the two ends of the hydrogel were bonded together using conductive copper strips and copper wires. To prevent moisture evaporation, VHB tape was used to seal the surface of the hydrogel. Then, the PDA@MXene / Clay / PNIPAM hydrogel sensor was placed in water baths at 25, 30, 35, and 40°C in sequence, and the changes in sensor resistance were recorded using a digital source meter.
[0098] The results are as follows Figure 7 As shown, Figure 7 A represents the change in electrical signal at different temperatures. Figure 7 B represents the sensitivity result. Figure 7 C represents the electrical signal from a repeated experiment at 30℃. Figure 7 D represents the response time at different temperatures. It can be seen that the sensor has a high sensitivity of 2.7488℃. -1 It also exhibits repeatability in temperature response, with an average temperature response time of 64.8 ms.
[0099] Although the present invention has been described in detail through the preferred embodiments above, it should be understood that the above description should not be considered as a limitation of the present invention. Various modifications and substitutions to the present invention will be apparent to those skilled in the art after reading the above description. Therefore, the scope of protection of the present invention should be defined by the appended claims.
Claims
1. A PDA@MXene / Clay / PNIPAM composite conductive hydrogel, characterized in that, This composite conductive hydrogel was obtained through the following method: PDA@MXene was obtained by reacting MXene dispersion and dopamine in Tris reagent; wherein the mass ratio of Tris base powder in the MXene dispersion to Tris reagent was 14.5 : 0.56, 29 : 1.11, 43.5 : 1.67, or 58 : 2.
23. A composite conductive hydrogel was obtained by polymerizing PDA@MXene, isopropylacrylamide, and clay under the action of an initiator and a accelerator; wherein the mass ratio of dopamine, MXene, isopropylacrylamide, and clay was 0.23~0.92:14.5~58:2500:
800.
2. The composite conductive hydrogel according to claim 1, characterized in that, The MXene used was prepared by etching the MAX phase with LiF and hydrochloric acid.
3. The composite conductive hydrogel according to claim 1, characterized in that, The isopropylacrylamide used was purified by recrystallization.
4. The method for preparing the composite conductive hydrogel according to any one of claims 1-3, characterized in that, The method includes: PDA@MXene was obtained by reacting MXene dispersion and dopamine in Tris reagent; wherein the mass ratio of Tris base powder in the MXene dispersion to Tris reagent was 14.5 : 0.56, 29 : 1.11, 43.5 : 1.67, or 58 : 2.
23. A composite conductive hydrogel was obtained by polymerizing PDA@MXene, isopropylacrylamide, and clay under the action of an initiator and a accelerator; wherein the mass ratio of dopamine, MXene, isopropylacrylamide, and clay was 0.23~0.92:14.5~58:2500:
800.
5. The preparation method according to claim 4, characterized in that, The initiator is potassium persulfate; the accelerator is N,N,N,N-tetramethylethylenediamine.
6. The preparation method according to claim 4, characterized in that, The dosage of dopamine, MXene, isopropylacrylamide, clay, initiator and accelerator is 0.23~0.92mg: 14.5~58mg: 2500mg: 800mg: 50mg: 30 μL.
7. The preparation method according to claim 5, characterized in that, The preparation of PDA@MXene is as follows: the MXene dispersion is sonicated, Tris reagent is added and the dispersion is fully dispersed, and then dopamine is added and the reaction is fully carried out.
8. The preparation method according to claim 4, characterized in that, The isopropylacrylamide used was purified by recrystallization. The steps of recrystallization purification of isopropylacrylamide were as follows: hexane was added to isopropylacrylamide, heated to dissolve until clear, cooled to room temperature, and cooled further at 0°C. The precipitated isopropylacrylamide crystals were filtered and washed with hexane at 0~10°C. The filtered isopropylacrylamide crystals were dried.
9. The application of the composite conductive hydrogel as described in any one of claims 1-3 in flexible sensors.