1 Introduction

Over the past decades, stretchable electronics have received widespread attention in such electronics systems as human–machine interfaces [1,2,3,4], motion detection [5,6,7], and electronic skin [8,9,10]. Conductive hydrogels are flexible, cross-linked materials integrating polymer matrixes with electrically conductive fillers. Owing to their good biocompatibility and adjustable mechanical properties, conductive hydrogels have been regarded as a promising candidate for wearable sensors [11,12,13,14].

Polyacrylic acid (PAA) [15], polyacrylamide (PAM) [16, 17], and polyvinyl alcohol (PVA) [18, 19] have been widely used as the hydrogel polymer matrixes due to their hydrophilic, malleable, and processable properties. PVA is featured with abundant hydrophilic side groups, good biocompatibility, and eco-friendliness. Differed from PAA and PAM, PVA is a relatively inexpensive polymer, and moreover, it is commercially available and therefore is more convenient to construct the hydrogel without considering problems of polymerization residues [20, 21]. Nevertheless, PVA suffers from poor mechanical strength, which limits its practical applications. Improving the performances of materials by constructing specific structures is a typical approach [22,23,24]. Designing a double network (DN) structure, inspired by the hybrid structure of human skin which is composed of interlocked flexible chains and rigid chains, has become a popular strategy to improve the mechanical properties of the hydrogel [25,26,27]. In the DN structure, the flexible chains enhance the tensile property of the gel, while the rigid chains are capable of improving the mechanical strength [12, 28]. Abouzeid and co-workers [29] fabricated a DN-structured hydrogel consisting of TEMPO-oxidized cellulose nanofiber (TOCN) and PVA, which had potential applications for 3D printing, but the elongation was unsatisfactory. Jing et al. [28] prepared a flexible hydrogel with excellent sensitivity by introducing cellulose nanofibril (CNF) into PVA, and it was capable of detecting various human motions, but the fracture strength was low. Li et al. [30] fabricated a DN hydrogel via PVA and poly (acrylamide-co-acrylic acid) (PMAA) which could accurately and stably monitor human body movement, but the detection range was not wide enough. Therefore, it is essential to find suitable materials to build a DN hydrogel that is flexible yet tough with a wide detection range.

Conductive fillers including inorganic carbon materials (for example, graphene (GR) [31, 32], carbon nanotubes (CNTs) [33], carbon black (CB) [34]), conductive polymers (such as polypyrrole (PPy) [35], polyaniline (PANI) [36]), and metals [37,38,39] are usually used to construct the conductive path in the hydrogel matrix. In order to prepare a conductive hydrogel with excellent mechanical properties and desirable sensitivity towards wearable devices, the conductive fillers are expected to possess both high electrical conductivity and good dispersion in the hydrogel. However, each of these abovementioned conductive fillers has its own flaws. For example, CNT and GR are mostly composed of hydrophobic carbon and therefore difficult to disperse well in hydrophilic hydrogels. PPy and PANI are rigid chains and prone to produce polymerization residues, which leads to decreased mechanical performance of the hydrogel [35, 36]. Metals such as silver or gold nanowires easily slide or break upon stretching [40, 41]. Recently, MXene, the thriving family of transition mental carbides/nitrides or carbonitrides, has emerged as a promising conductive nanofiller for conductive hydrogels due to its 2D morphology structure, hydrophilicity, and mental-like conductivity [42,43,44].

Herein, a facile freezing–thawing method has been proposed to construct a purely physically cross-linked MXene-composited polyvinyl alcohol/sodium carboxymethylcellulose (PVA/CMC) double network hydrogel which is further reinforced by tannic acid (TA). In this PCTM (short for PVA/CMC/TA/MXene hydrogel), PVA, which is biocompatible and tractable [38], serves as the flexible backbone. CMC, a derivative of cellulose and an anionic polysaccharide that is biodegradable and environmentally benign [45, 46], mainly functions as the rigid subnetwork skeleton in the PCTM, and naturally occurring TA is introduced with the purpose to further enhance the mechanical properties of the hydrogel via tight hydrogen bonds between TA and the double network of PVA/CMC. MXene is utilized to build the conductive path. The abundant hydrophilic functional groups of MXene help to achieve a uniform distribution in the hydrogel, which is beneficial for achieving high sensitivity and wide operation window [47]. The interactions among PVA and CMC, TA, and MXene greatly help dissipate the energy within the multiple cross-linking structure, leading to an improved mechanical strength of PCTM. Compared with other complicated ways to prepare conductive hydrogels, the freezing–thawing method is much easier to achieve [48]; moreover, the purely physically cross-linked hydrogel constructed with eco-friendly compositions can be degraded without producing any toxic outgrowth, which helps to alleviate the increasingly serious environmental problem. The unique multiple synergetic networks of PCTM impart promising mechanical strength (a fracture tensile strength of 1.8 MPa at a fracture strain of 740%) and high sensitivity with a wide detection window (a gauge factor of 2.9 at a strain range of 0–700%) as well as long-term durability over 3000 continuous cycles. Moreover, the PCTM is able to detect both large and small scales of human motion and to distinguish voices as flexible wearable sensors. As a proof of the concept, a sensor array was fabricated for the real-time detecting distribution of the weight or pressure, suggesting its promising potentials for electronic skin, human–machine interaction, and soft robot applications.

2 Experimental

2.1 Materials

Polyvinyl alcohol (PVA, 98.0–99% hydrolyzed) and lithium fluoride (LiF) were obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. Tannic acid (TA, 99%) was purchased from Shanghai Titan Scientific Co., Ltd. Titanium aluminum carbide (Ti3AlC2, 400 mesh) was supplied by 11 Technology Co., Ltd (China). Sodium carboxymethylcellulose (CMC, AR, pH are 6.5–8.0, the sodium content is 6.5–8.0%) was purchased from Jiangtian Chemical Co., Ltd (China). All chemicals and solvents were used as received without additional purification.

2.2 Synthesis of MXene

MXene (Ti3C2Tx) was prepared via selective etching of aluminum from Ti3AlC2. Firstly, 1.98 g of LiF powder was added into 20 mL of 9 M HCl solution, and then 2 g of Ti3AlC2 powder was slowly added to the above solution under the magnetic stirring (300 rpm). The mixture was then heated at 35 ºC for 24 h. Finally, the product was washed with deionized water and centrifuged at 4000 rpm repeatedly until pH of the solution was higher than 6. MXene powders were prepared by lyophilizing MXene aqueous dispersion for 48 h.

2.3 Preparation of PCTM hydrogel

In a typical experiment, PVA (4.5 g), CMC (0.5, 1.0, 1.5 g), TA (0.5, 0.75, 1.0 g), and MXene (0, 0.05, 0.1, 0.15 g) were dissolved in 35 mL of deionized (DI) water at 95 ºC to obtain a homogeneous solution by the mechanical stirring (200 rpm) for 2 h using a simple silicone oil-bath method. After it cooled down to room temperature, the sol was poured into a mold and subjected to several times (1, 2, 3, 4) of freezing–thawing cycle (frozen at − 20 ºC for 6 h and then thawed at room temperature for 1 h) to get the hydrogel with desired shapes. Note that in the hydrogel PVA/CMCx/TAy/MXenez-N (PTCM for short), x, y, z, and N represent the weight ratio of the PVA to CMC, PVA to TA, PVA to MXene, and the times of the freezing–thawing cycles, respectively. For example, the PVA/CMC9/TA9/MXene45-3 hydrogel, denoted as PC9T9M45-3 for short, means that the weight ratio of PVA to CMC is 9, PVA to TA 9, and PVA to MXene 45, and the freezing–thawing times is 3.

2.4 Characterization

The hydrogels were freeze-dried and gold sputtered before observation. The field emission high-resolution scanning electron microscope (FE-SEM) was performed on an Apreo of FEI Company (USA). Fourier transform infrared spectroscopy (FT-IR) studies of dried PVA, PVA/CMC, and PVA/CMC/TA/MXene (PCTM) were conducted using VECTOR22 type from Bruker, Germany. The crystalline structure of the samples was examined by X-ray diffraction (XRD, Bruker D8 Advance). The electrical conductivity of the hydrogels (σ, in S·m−1) was measured using RIGOL DM3068 digital multimeter according to the equation σ = L/(R·A), where L (m) is the length of the PCTM, R (Ω) is the electrical resistance, and A (m2) is the cross-section area of PCTM. Thermogravimetric curves were recorded by NETZSCH STA449F5 synchronous thermal analyzer. The analysis was performed with 10 ± 0.1 mg of hydrogel in platinum pans under nitrogen atmosphere at a gas flow rate of 5 mL·min−1. The experiments was heated from ambient temperature to 800 °C at a heating rate of 20 °C·min−1. A porosity analyzer (Autosorb iQ, USA) was used to analyze the texture of the dried hydrogel by N2 physisorption. Before testing, the sample was degassed in a sample tube at a degassing temperature of 90 °C for 8 h under vacuum and then measured by analyzing the amount of N2 gas adsorbed with the relative vapor pressure (P/P0) ranging from 0.05 to 0.995 at − 196 °C. N2 adsorption isotherm was used to determine the Brunauer–Emmett–Teller (BET) specific surface area of the PCTM. The Barrett–Joyner–Halenda (BJH) method was used to determine the pore size distribution.

2.5 Mechanical and rheological properties

The tensile and compressive mechanical properties of PCTM hydrogels were tested on a Uniaxial Materials Testing System (LIGAO, HF-9002) equipped with 10 kg load cell at room temperature. Tensile test was conducted using a dumbbell-shaped (20 mm × 4 mm × 2 mm) PCTM specimen at 30 mm·min−1. Uniaxial cyclic compressive test was conducted using a cylindrical-shaped PCTM specimen (20 mm in height, 12.5 mm in diameter) at a compressive rate of 30 mm·min−1. The toughness was obtained by integrating the area under the stress–strain curve. The elastic modulus was calculated from the slope of the stress–strain curve. The rheological behavior of hydrogels was studied by a dynamic rheometer (MARS 60, HAAKE). Oscillatory frequency sweep measurements were performed at shear frequency in the range of 0.1–10 Hz at a strain of 1% to determine the storage (G′) and loss modulus (G″) of the hydrogels. Continuous flow experiment gives information of the viscosity of the hydrogel at shear rates ranging from 0.1 to 10 s−1.

2.6 Electrical tests

PCTM was sandwiched between two polyimide layers and connected with the copper wire for assembling of the strain sensor (20 mm × 4 mm × 2 mm). The sensor was connected with a computer and the Uniaxial Materials Testing System. Electrical signals of the strain sensor were recorded by RIGOL DM3068 digital multimeter at 1 V.

2.7 Real-time pressure monitoring

The multiplexed circuit of the device consisted of pressure-regulated resistors (RP1 − RP9) made from PCTM and regular resistors (R1 − R9, 1 kΩ). The circuit was encapsulated with insulating PVC and connected to a Single Chip Micyoco (SCM, Arduino MEGA 2560) for the signal processing and a computer for outputting the signal. The resistance changes induced by different weights (10, 20, 50, and 100 g) on the same site or different sites were read out by the SCM and graphically shown on the computer that prewrote programs.

3 Results and discussion

3.1 Fabrication of PCTM

PCTM was synthesized via a facile freezing–thawing method, which started by dissolving both PVA and CMC in water and heating at 95 ºC for 2 h as shown in Fig. 1a. TA and MXene were subsequently added to PVA/CMC, resulting in PVA/CMC/TA/MXene sol, i.e., the precursor for the hydrogel. Finally, the sol was converted to the PCTM hydrogel after several cycles of freezing–thawing process wherein the PVA polymer chains crystallized and therefore enhanced the mechanical properties of the hydrogel [49, 50]. Figure 1b shows the corresponding digital photographs of the sols and their schematic structure. PVA and CMC were readily dissolved in water due to their hydrophilic properties. TA, a natural polyphenolic chemical composed of five digallic acid units attached to one central glucose core, shows high affinity to both PVA and CMC, which could be evidenced by the homogeneous solution of PVA/CMC/TA. The conductive path in the hydrogel was constructed with MXene which was prepared via selective etching of aluminum from Ti3AlC2 (Max) as illustrated in Fig. S1. The multilayered and delaminated structure of MXene was observed by SEM images in Fig. S2. The successful preparation of MXene was proved in Fig. S3 by the new (002) peak locating at 6º and disappearance of the (104) peak at 39º as compared with Max. Different from other hydrophobic conductive fillers such as carbon nanotube [51] and graphene [52], MXene sheets were dispersed well in the aqueous sol thanks to abundant hydrophilic functional groups (-OH, -O, etc.), forming a uniform multiple conductive network in the hydrogel [53,54,55,56]. Figure 1c shows the enlarged hydrogel network of PCTM, and the water content of PC4.5T6M45-3 composite hydrogel was measured to be 81% (Fig. S4), suggesting rich hydrophilic groups in the hydrogel network, and the practical weight percent of MXene is 1.7% measured from TGA analysis (Fig. S5). The nanocrystals of PVA formed during the freezing–thawing cycles act as physical cross-links and therefore endow PCTM with excellent mechanical strength [57]. The influence of freezing–thawing times N on mechanical properties of PCTM was explored. The fracture strength and fracture strain initially increased with increasing the freezing–thawing times and reached to a maximum when N is 3 (Fig. S6), indicating enhanced physical cross-linking densities [58, 59]. The rich cross-links existing in PCTM significantly increase the intermolecular and intramolecular interactions and ensure the integrity of the hydrogel, which could be proved by the fact that after immersed in water at room temperature for 3 weeks, the hydrogel still remained intact (Fig. S7). However, when N was higher than 3, for example, 4, the decrease of the fracture strain occurs due to the over-crystallization of PVA and phase separation caused by too many freezing–thawing cycles [60]. The chemical structure of the PVA, PVA/CMC, and PVA/CMC/TA/MXene hydrogel is characterized by FT-IR in Fig. S8. For PVA, a wide and dull absorption peak locating at 3336 cm−1 was attributed to –OH [48]. Due to the rich -OH groups of PVA/CMC and the hydroxyl groups of TA, the hydrogen bonds can significantly further enhance the mechanical property of the hydrogel. PVA/CMC shows three sharp peaks at 1643, 1423, and 1029 cm−1, which are attributed to vibrational bands of C = O, C-H bonds, and C–O–C stretching of carboxylate of CMC [49]. In PVA/CMC/TA/MXene, i.e., PCTM, the new peak at 1540 cm−1 is attributed to the aromatics ring [50, 61], indicating the existence of TA. Moreover, the -OH peak shifts to a lower wavenumber of 3264 cm−1, and the stretching vibration of C = O blue shifts to 1712 cm−1, suggesting the newly formed strong hydrogen bonding in the hydrogel [62].

Fig. 1
figure 1

Preparation of the PCTM hydrogel. a Schematic and b the digital photographs of preparing the PCTM hydrogel. c The proposed hydrogel network of PCTM

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3.2 Morphology of PCTM

The digital photographs of the hydrogels with different components are shown in Fig. S9. The PVA and PVA/CMC hydrogels were white with slight difference, while the PVA/CMC/TA was brown due to the addition of TA, and the PVA/CMC/TA/MXene was black because of the introduction of MXene. The homogeneous color of all the hydrogels reveals the excellent compatibility among each component. In the freeze-dried PCTM, the surface exhibits a porous cross-linked structure (Fig. 2a). SBET calculated from the N2 adsorption isotherm (Fig. S10a) was 0.020 cm3/g, and the total pore volume (Vp) was 0.020 cm3/g. and. SBET was determined to be 13.0 m2/g, The pore size distribution curve using BJH model (Fig. S10b) shows that the average pore width (dp) of PCTM is 6.0 nm [23]. The layered sheets and abundant hydrophilic functional groups on the surface facilitate the uniform dispersion of MXene in network of the hydrogel [63], which could be evidenced by the evenly distributed Ti and F elements in the elemental mapping (Fig. 2b). The cross-sectional profiles in Fig. 2c, d indicate that the hydrogel has a highly porous structure.

Fig. 2
figure 2

Surface and cross-sectional profiles of the freeze-dried PCTM. a SEM images of the surface, b the elemental mapping of C, O, F, and Ti (the inset shows magnified cross-linked microstructures), and d–e cross-sectional profile of PCTM hydrogel

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3.3 Mechanical properties of PCTM

Mechanical properties are essential for hydrogels for practical applications. Figure 3a shows that PCTM does not break up even a weight of 200 g was hanging on the hydrogel. Moreover, PCTM could withstand a variety of tensile deformation such as knotted stretching or twisted stretching (Fig. 3b), suggesting the promising ductility and flexibility. The effect of the content of different components on the mechanical strength of the hydrogels was explored as shown in Fig. S11. The CMC mainly serves as the subnetwork skeleton in the hydrogel [11, 12]. As the content of CMC increases, the DN structure of hydrogel gradually forms, and the elongation at break of the hydrogel is increased. When the weight ratio of PVA to CMC is 4.5:1, the elongation at break reaches the highest, with a little decrease in the tensile strength. It should be noted that the introduction of TA further remarkably improved the tensile strength of the hydrogel. The reinforcing mechanism is that rich hydrogen groups in TA can form tight hydrogen bonds with the DN structure of PVA/CMC [64, 65]. By further increasing the content of CMC and TA, the hydrogel becomes rigid, and the maximum fracture stress and tensile length decreased. This phenomenon may be explained by strain concentration caused by the excessive CMC and TA [66]. MXene has negligible effect on the mechanical strength of the hydrogel since the stress–strain curves of the hydrogels with and without MXene almost overlapped. By comparison, PC4.5T6M45-3 exhibits the optimum mechanical properties. As compared with pure PVA and PVA/CMC hydrogel, PC4.5T6M45-3 hydrogel shows remarkably enhanced mechanical performance in terms of the fracture tensile strain and fracture strength (Fig. 3c). The tensile strength of PC4.5T6M45-3 reached as high as 1.8 MPa, compared with only 0.5 MPa for the pure PVA. Meanwhile, the toughness and elastic modulus were significantly improved to 6.24 MJ·m−3 and 2.4 kPa, respectively (Fig. S12), as compared with 0.98 MJ·m−3 and 0.44 kPa of PVA. Modest modulus and toughness comparable to those of human skin are highly required for wearable strain sensors. The toughness and elastic modulus of the PC4.5T6M45-3 are approximately equal to those of soft tissues in human body [67]; therefore, it is able to match the process of human movement. In order to further evaluate the tensile strength of the PC4.5T6M45-3 hydrogel, the hydrogel was stretched from 100 to 700%. The closed stress–strain curves and the hysteresis loops at different strain revealed that the PC4.5T6M45-3 was capable of returning to its primary state via the energy dissipation process by the physical cross-linking (i.e., hydrogen bonds and nanocrystals) (Fig. 3d). The PC4.5T6M45-3 remained stable after 100 cycles of stretching (Fig. 3e), demonstrating its excellent anti-fatigue performance. Meanwhile, the elastic property via compressive loading–unloading process of the PC4.5T6M45-3 hydrogel was evidenced by the photographs in Fig. S13. The compressive stability of the PC4.5T6M45-3 is examined in the cycling test which shows that PC4.5T6M45-3 recovers to its original state without any rupture. The compressive stress–strain curve also has a closed hysteresis after 100 cycles (Fig. S14), suggesting the energy-dissipative, viscoelastic, and highly compressive behavior. The rheological analysis of PC4.5T6M45-3 was conducted in the frequency range of 0.1–10 Hz. G′ was always higher than G″ (Fig. 3f), and both values of the PC4.5T6M45-3 were larger than those of pure PVA hydrogel, indicating the enhanced solid-state elastic characteristics [68] (Fig. 3g). The continuous flow experiment gives information on the viscosity of the PC4.5T6M45-3 at increased shear rate from 0.1 to 10 s−1 (Fig. 3h), which reveals that the hydrogel exhibits excellent shear-thinning behavior.

Fig. 3
figure 3

Mechanical properties of PCTM hydrogels. Photographs showing a loaded and b knotted-stretched and knotted-twisted PCTM. c Tensile stress–strain curves of pure PVA, PVA/CMC4.5, and PC4.5T6M45-3. d Closed stress–strain curves of PC4.5T6M45-3 at different tensile strains (100–700%). e Loading–unloading curves of the PCTM at a strain of 600% for 100 cycles. f–g Frequency sweep test for pure PVA and PC4.5T6M45-3. h The viscosity of the PC4.5T6M45-3 hydrogel at increased shear rates by continuous flow experiment

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3.4 Electrical performance of the hydrogel

The electrical behavior of PC4.5T6M45-3 hydrogel was also investigated to evaluate their potential applications for wearable electronics. The PC4.5T6M45-3 hydrogel was connected to a closed electrical circuit with a red LED to indicate the electrical conductivity variation upon the hydrogel was stretched or pressed. The LED became brighter as the hydrogel was pressed and became darker when stretched (Fig. S15 and Video S1, S2), implying a piezoresistive strain dependency of the hydrogel. The conductivity of the PC4.5T6M45-3 hydrogel is mainly contributed to MXene loadings. When the weight ratio of PVA to MXene was 90, 45, and 30, the conductivities were 0.63, 1.3, and 1.5 S/m. A sensing mechanism is proposed in Fig. 4a. When the PC4.5T6M45-3 hydrogel was stretched, the conductive network structure underwent tensile deformation and ruptured, and the distance between MXene nanosheets was increased, resulting in increased electrical resistance [44, 69]. The sensing performance of the PC4.5T6M45-3 hydrogel as strain sensors was evaluated. The sensitivity of the strain sensor is represented by gauge factor (GF), which is defined by GF = δ(ΔR/R0)/δε, where ε is the strain, ΔR and R0 correspond to the resistance change and the resistance without the strain, respectively. GF reaches the maximum when the weight ratio of PVA to MXene was 45 (Fig. S16). It is speculated that when MXene nanosheets were redundant, the stretching tension could not successfully separate MXene nanosheets from each other, and therefore, the conductive pathways could not be disrupted, resulting in diminished sensitivity. A high GF of 2.9 was obtained in a wide strain range of 0–700% for PC4.5T6M45-3 fitted from the relative resistance change–strain curve (Fig. 4b), which is attributed to the uniform distribution of MXene. Loaded with a fast step strain of 10%, the response and recovery times of PC4.5T6M45-3 were measured to be 66 ms and 102 ms, respectively (Fig. 4c), which were much shorter compared with the other reported values [70,71,72,73]. Figure 4d, e show the strain-sensing behavior of the PC4.5T6M45-3 strain sensor at low frequencies (0.06–0.25 Hz) and high frequencies (0.5–2.3 Hz), respectively. The frequency-independent sensing behavior ensures a precise and reliable detection for human motion. Low detection limit and broad detection range are also important for practical applications. The sensor can detect both tiny (1–10%) (Fig. 4f) and large (100–400%) (Fig. 4g) strain with excellent reproducibility in five repetitive loading–unloading cycles. Moreover, the hydrogel exhibited long-term stability (3000 extension-release cycles at a strain of 10%), which is attributed to the integrity of the network during the stretching (Fig. 4h). GF, the linearity range, and the response time were compared with other literature in Table S1, which further proves the higher sensitivity, wider operation range, and shorter response time of the hydrogel.

Fig. 4
figure 4

a Schematic illustration of conductive paths change in the PCTM hydrogel upon stretching. b Relative resistance change–strain curves of the PC4.5T6M45-3 hydrogel as a strain sensor. c Instant response of PC4.5T6M45-3 which shows fast response and recovery times of 66 and 102 ms, respectively. d Low (0.06–0.25 Hz) and e high stretch frequencies (0.5–2.3 Hz) under 10% strain. f–g Relative resistance changes under different cyclic stretching–releasing strains (small strain of 1–10%, large strain of 100–400%). h PC4.5T6M45-3 strain sensor with 3000 loading–unloading cycles (10% strain, 2.3 Hz)

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In order to demonstrate the potential application for human motion detection, PCTM was employed to monitor both large and minute human motions. The PCTM sensor was attached to the knee (Fig. 5a) and elbow (Fig. 5b), respectively. ΔR/R0 changes regularly according to different deformations, and the sensor generates continuous different characteristic electrical signals under both fast and slow strain changes. Figure 5c shows that the PCTM sensor responses rapidly upon deformation when attached to the joint of the finger, and ΔR/R0 increases with increasing the bending angle from 0º to 120º and returns to 0 when it is completely recovered, demonstrating the reliability of the PCTM sensor. More interestingly, the PCTM sensor can also detect subtle deformations from speaking (Fig. 5d) and air blowing (Fig. 5f).

Fig. 5
figure 5

PCTM for wearable sensor applications. a Knee and b elbow bending signal monitoring. c Relative resistance changes of PCTM sensor attached to the finger upon increasing the bending angle from 0 to 120º. Voice detection for different words d “hydrogel” and e “sensor.” f Signals of tiny vibrations of blowing air

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Furthermore, to explore the potential application of the flexible PCTM sensor for real-time monitoring of the magnitude of the force, a checkerboard-like sensor was constructed, where a checkerboard-like panel was connected with printed circuit board (PCB) and a computer (Fig. 6). The sensor detected the weight, and the signal was processed by PCB and graphically shown by the computer. Depending on the brightness of the corresponding panel, the specific weight can be clearly distinguished (10 g, 20 g, 50 g, 100 g, I–IV, Video S3). Moreover, each unit can be controlled to recognize different weights individually (Fig. S17 and Video S4), demonstrating potential prospects for electronic skin, human–machine interaction, and soft robot applications.

Fig. 6
figure 6

PCTM for pressure visualization application. The brightness indicates different weights placed on the test point of the checkerboard-like panel

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4 Conclusion

In summary, we have developed a facile and eco-friendly method to fabricate PCTM conductive composite hydrogels. The PC4.5T6M45-3 hydrogels showed excellent mechanical properties (a fracture strain up to 740%, a fracture tensile strength up to 1.8 MPa) and high sensitivity (a GF of 2.9) with a wide operation window (0–700%). In addition, the assembled strain sensor exhibited excellent cycle stability after 3000 cycles and is able to detect distribution of weight or pressure in real time. More importantly, the biocompatibility property ensured that the assembly sensor could contact with the human body directly and would not harm the environment. All these merits enable the hydrogels an ideal candidate for wearable devices for monitoring of human physiological activity and in soft intelligent robots.