1 Introduction

Conductive organohydrogels have garnered extensive attention in the broad fields of flexible sensors [1,2,3,4,5], wearable displays [6,7,8,9,10,11,12], and electronic skin [13,14,15,16,17], due to their unique properties, biomimetic structures and biocompatibility. Typically, the conductive hydrogels are prepared by either compositing with conductive fillers [18,19,20,21,22,23] or incorporating ionic pendant groups [24, 25] and salts [26,27,28]. Conductive hydrogels combine the unique advantages of conductive materials and conventional hydrogels, which exhibit good electronic properties, tunable mechanical flexibility and easy processing ability. However, the conventional conductive hydrogels have partial limitations, such as weak mechanical properties, unstable conductivity, and inhomogeneity, which strictly hindered its development. Some strategies are exploited to solve those limitations. For instance the imitation of snake scale structure [29], in which the micro-size poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) islands are percolatively connected endows a high stretchability of the bi-layered film. In addition, the silk nanofibers that are an excellent substrate for loading conductive materials, have been proposed to design high strength silk fibroin hydrogels with respect to the construction of a functional network under a multi-level structure from the molecular level to the micro- and macro-scales [30].

Recently, various efforts have been made to increase the strength and toughness of hydrogels, including the entanglement between polymer chains [31] and carbon materials [32,33,34] into nanofibers [35,36,37,38,39]. For example, the incorporation of carbon nanotubes and graphene into hydrogels could improve the tensile fracture strength, elongation rate, and compression strength of hydrogel through hydrogen bonding between oxygenated functional groups and the hydroxyl groups of polyvinyl alcohol (PVA) molecular chains. Unfortunately, the most crucial problem of carbon materials is their easy aggregation, which limits the efficiency of the hydrogel matrix and the applications of hydrogels. Additionally, the nanofibers can dynamically interact with the polymer network to modulate the interfacial interactions, which simultaneously enhance the strength and toughness of the composite hydrogels. Likewise, the conductive materials with many hydrogen bonds [40,41,42,43] have also been widely used to promote the mechanical properties of conductive hydrogels.

To date, the network structure and high hydrophilicity of hydrogels can enable them to hold large amounts of water, excellent conductivity, high stretchability, good viscoelasticity, self-adhesive and biocompatibility, along with anti-microbial activity using some particular polymers and polymer networks. For instance, the multifunctional hydrogel that was cross-linked by tannic acid (TA) via the intermolecular interactions between silk fibroin (SF) and TA was effectively constructed with high extensibility, underwater adhesivity, biocompatibility and antibiotic properties [44]. For the hydrogel sensor applications, the antifreeze ability, self-healing performance, ultraviolet filter capacity and high transparency are also critical factors. However, most traditional hydrogels use pure water as a solvent, resulting in hydrogels to be frozen into a hard solid below 0 °C [45]. Additionally, the key drawback of hydrogels is prone to lose water seriously in a dry environment, which significantly restricts the practical sensor application. In recent years, inspired by the antifreeze mechanism in nature, the binary solvents, zwitterionic penetrants or inorganic salts have been used to inhibit the formation of ice crystals or reduce the freezing point of water in the hydrogel, which successfully endows hydrogel sensor with moisturizing properties and antifreeze mechanisms [46, 47]. Although the polyvinyl alcohol–silk nanofiber–graphite carbon nitride nanosheet (PVA–SNF–CN) organohydrogel has excellent frost resistance, it does not have the characteristics of adhesion and self-healing. Moreover, the PVA–SNF–CN organohydrogel is opaquely attached to human skin, which has a negative impact on aesthetics [48]. Furthermore, the self-healing performance of the hydrogel with silk fibroin, polyvinyl alcohol and borax was tested. The hydrogel was observed to heal itself swiftly after being cut off, in which its electrical conductivity restored to the original level even after ten damage–recovery cycles [49]. Ning et al. incorporated tannic acid-coated hydroxyapatite nanowires (TA@HAP NWs) into the hydrogel. This nano-reinforced filler improved its mechanical properties and endowed the hydrogel with ultraviolet shielding ability. This feature reflects and absorbs short ultraviolet rays as well as transmits visible light. However, this hydrogel does not have self-healing ability and adhesion [50]. Subsequently, Li and his co-workers proposed a strict and transparent nanocomposite hydrogel based on a glycerin–water binary solvent system. The transparency of the hydrogel reached as high as 97%, whereas it had no self-healing ability [51].

Herein, we develop a novel polyvinyl alcohol–silk nanofiber–tannic acid (PVA–SNF@TA) organohydrogel with ultrahigh stretchable, self-healable, transparent and capable of shielding ultraviolet radiation. In this design, the tannic acid coated on the surface of silk nanofibers can endow the hydrogel with both excellent adhesive capability and tensile strength/strain. In addition, adding an appropriate amount of borax solution causes the hydrogel to have a self-healing ability. In practical applications, the flexible strain sensor made of PVA–SNF@TA organohydrogel has good sensing ability, anti-fatigue properties and excellent sensitivity, which accurately detects different movements of the human body. Furthermore, the flexible pressure sensor made of cylindrical PVA–SNF@TA organohydrogel can monitor other motion states (walking, running) of the human body. During repetitive movement, the organohydrogel pressure sensor still maintains excellent sensing stability. It is foreseeable that the our fabrication strategy can provide new ideas for guiding the design of next-generation multifunctional conductive organic hydrogel.

2 Experimental sections

2.1 Materials

Silk was purchased from Zhejiang Deqing Xinlian Silk Co., Ltd. Urea, sodium hydroxide (NaOH), polyvinyl alcohol-1799 (PVA), Tris–hydrochloric acid buffer (1 M, pH 9.5), ethylene glycol (EG), tannic acid (TA), sodium chloride (NaCl) and sodium carbonate (Na2CO3) were purchased from Aladdin Industrial Corporation (Shanghai, China). All reagents used in the experiment were analytical grade and used directly without further purification. Deionized water was prepared in all the experiments.

2.2 Preparation of SNF@TA suspension

Based on the degumming method in the previous literature, the original natural silk fiber was boiled twice in 0.5 wt% Na2CO3 solutions, each time for 30 min. The silk fiber was further washed with deionized water three times and then dried at 60 °C for 24 h to prepare SNF (Fig. 1a). Through mixing NaOH/urea/H2O (100 g) with a mass ratio of 7:12:81, the suspension was frozen in the refrigerator for 12 h at − 18 °C. The degummed silk fibers were cut into short threads with scissors. Afterwards, the staple fibers were dispersed in a mixed solvent system and continued to be frozen. It was further stirred violently for 10 min every 12 h for a total of four times. Then, the silk pulp mixture was separated by high-speed centrifugation, continuously washed and diluted with deionized water, and centrifuged to ensure the pH < 8. After 20 min of centrifugal separation (4000 rpm), the SNF dispersion was obtained as a transparent off-white liquid. The SNF distribution was condensed to 0.25% (mass fraction) and stored at 4 °C before use. The average diameter of SNF was measured to be 200 ± 10 nm (Fig. S1a).

Fig. 1
figure 1

The preparation process of the SNF@TA organohydrogel sensor. Schematic illustration of the preparation processes of a SNF gel and b SNF@TA. c The SEM images of SNFs. d The SEM images of SNF@TA. e The FT-IR spectra of TA, SNFs, and SNF@TA. f Schematic diagram of the strain sensor. g Schematic diagram of the pressure sensor

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The preparation route of silk nanofiber–tannic acid (SNF@TA) suspension is shown in Fig. 1b. According to the method in the literature [52], first, the mass ratio of SNF: TA = 1:3 was added to the sample bottle and stirred to make the tannic acid mix evenly. Then, the pH of the mixture was adjusted to 8 with 1 M Tris–HCl buffer (pH 9.5) and stirred at room temperature for 6 h. The prepared SNF@TA suspension was also preserved at 4 °C before use. The average diameter of SNF@TA was measured to be 210 ± 15 nm (Fig. S1b).

2.3 Design and preparation of PVA–SNF@TA organohydrogel

Multi-functional PVA–SNF@TA organohydrogel was prepared by a facile one-pot method. 1.3 g EG was added to the SNF@TA suspension with different mass, then kept the total mass of 5.5 g after dripping water, and continued to add 0.258 g NaCl to the mixed suspension. Next, 1 g PVA was further added to the suspension, after stirring at room temperature for 30 min, the mixture was heat to 95 °C until PVA was entirely dissolved. 500 μL (0.06 mol·L−1) borax solution was added with a liquid transfer gun. After heating and stirring for another 1 h, the mixed solution was transferred into a glass petri dish and put it in the refrigerator at − 18 °C for 12 h. The multifunctional PVA–SNF@TA organohydrogel was obtained by thawing the frozen organohydrogel at room temperature for 6 h.

For comparison, 1 g PVA and 0.258 g NaCl were dissolved in 5.5 g water, and then PVA hydrogel was obtained through the same experimental procedure. PVA hydrogel is noted as PVA-W, differ from the pure PVA organohydrogel of PVA. PVA organohydrogel with 0.1% SNF@TA is noted as PVA-0.1% SNF@TA. The same definition was applied to PVA-0.2% SNF@TA, PVA-0.3% SNF@TA and PVA-0.4% SNF@TA. The detailed composition of multifunctional PVA–SNF@TA organohydrogel is listed in Table S1. Among them, the PVA-0.3% SNF@TA organohydrogel was selected for characterization and testing of cold resistance, transparent self-healing and sensing properties.

2.4 Mechanical properties test of PVA–SNF@TA organohydrogel

The multifunctional PVA–SNF@TA organohydrogel was cut into a rectangle (40 mm length, 7 mm width and 1 mm thickness) for tensile testing. The mechanical properties of multifunctional PVA–SNF@TA organohydrogel were tested using a universal testing machine (Instron 3367, USA) at room temperature. The rectangular samples of organohydrogel were tested at a fixed tensile rate of 20 mm min−1.

2.5 Characterization test of PVA–SNF@TA organohydrogel

To prevent damage of the test equipment, the multifunctional PVA–SNF@TA organohydrogel should be pretreated. The specific operation was as follows: immersing the PVA–SNF@TA organohydrogel into a large amount of deionized water for 48 h to replace the ethylene glycol inner the organohydrogel network system. Deionized water should be replaced periodically throughout the replacement process, followed by freeze-drying treatment. The scanning electron microscope (SEM) was used to observe the surface and cross-sectional morphologies of the organohydrogel. The functional groups of the organohydrogel were determined using a Fourier transform infrared spectroscopy (FTIR) spectrometer. Before measurement, the organohydrogel was crushed into powder, and dispersed in KBr powder to make thin slices for testing. The X-ray diffraction (XRD) measurement of the organohydrogel was measured on the two-dimensional wide-angle X-ray diffractometer equipped with a D8 diffraction instrument (Bruck, Germany) at 40 kV and 40 mA.

2.6 Cold resistance test of PVA–SNF@TA organohydrogel

Differential scanning calorimetry (DSC) was used to study the cold resistance of PVA-W and PVA-0.3% SNF@TA, which was cooled from 25 to − 70 °C at a rate of 10 °C min−1 in a nitrogen atmosphere. The specific test experiment of cold resistance was designed: the two organohydrogel of PVA-W and PVA-0.3% SNF@TA were cut into strips and placed at − 18 °C for 24 h.

2.7 Transparency and ultraviolet filtration test of PVA–SNF@TA organohydrogel

The transmittance measurement was determined by ultraviolet–visible spectroscopy. The multifunctional PVA-0.3% SNF@TA organohydrogel was cut into strips with a thickness of 1 mm and placed in a colorimetric plate with a scanning wavelength range of 200–800 nm.

2.8 Self-healing ability test of PVA–SNF@TA organohydrogel

Real-time monitoring experiment of electrical conductivity during self-healing: strips of organohydrogel conductors were connected to the Keithley 2400 data source table to monitor the initial resistance, the real-time resistance after fracture, and the real-time resistance during re-contact. The cylindrical PVA-0.3% SNF@TA organohydrogel was cut into three parts. The middle part was stained with rhodamine B, and then the cylindrical organohydrogel was contacted automatically. The self-healing of organohydrogel was observed after 5 h in the oven at 25 °C.

2.9 Preparation and performance test of the strain sensor

After the PVA-0.3% SNF@TA organohydrogel was kept at rest for 6 h, a flexible strain sensor was prepared by using the organohydrogel as a conductor, which was fixed at both ends by using two copper foils with wires (Fig. 1f). A flexible strain sensor was directly attached to the skin of volunteers to detect human motion. The copper foil was attached to both ends of the organohydrogel sensor for adhesion and reinforcement to ensure a smooth test process. The sensing performance of the flexible strain sensor was measured by the data source table (Keithley 2400). When volunteers (a healthy 24-year-old male) performed a series of activities, such as smiling, bending wrists and stretching legs, the corresponding signals could be detected under different activity states.

2.10 Preparation and performance test of the pressure sensor

For preparation of PVA-0.3% SNF@TA organohydrogel, the gel solution was poured into a syringe of 5 ml, and the cylindrical organohydrogel was obtained by freezing and thawing. After placing at room temperature for 12 h, the pressure sensor was obtained by cutting the cylinder into a cylinder with 1 cm height, wrapping the cylinder with two square iron blocks, and sealing it with glass tape (Fig. 1g). The flexible pressure sensor was installed directly on the bottom of the volunteer's shoes and fixed with duct tape to detect human motion. The sensing performance of the flexible pressure sensor was recorded by an ion source meter (Keithley 2400). When the volunteer (a 24-year-old healthy male) engaged in a series of activities, such as walking and running, the corresponding signals could be detected under different activity states.

3 Results and discussion

3.1 Structural characterization of SNF@TA suspension

The modification of SNF was prepared by adding tannic acid to SNF suspension. Under the condition of weak alkali, the tannic acid will polymerize and deposit on the surface of SNF, resulting in the color change of SNFs suspension from white to yellow. The SNF@TA was rich in functional phenolic hydroxyl groups. At the same concentration of SNF, the SNF@TA exhibited no significant aggregation within the length scale tested, indicating the stability of the colloids. This may be attributed to the inability of small amounts of TA to interact with silk proteins and cause coalescence (Fig. 1c, d). In Fig. 1e, compared with SNF, the infrared spectrum of SNF@TA had more obvious stretching peaks at 872 cm−1, corresponding to the bending vibration of benzene ring C=C. The peaks at 1537 and 1449 cm−1 belong to the aromatic C–C [53]. This means that tannic acid was successfully coated on the surface of silk nanofiber.

3.2 Mechanism and mechanical properties of the organohydrogel

Figure 2a explains in detail the synthesis process and internal interaction of multifunctional organic hydrogel. Typically, the biocompatible PVA was selected as the matrix, and the tannic acid-modified SNF@TA was used as the reinforcing material to increase hydrogel’s tensile strength and strain. The molecular chains of silk nanofiber contain a variety of functional groups, including amides, carboxyl and amino groups. After coating with tannic acid, the phenolic hydroxyl groups on the surface of SNF@TA further formed reversible hydrogen bonds with the hydroxyl groups on the PVA molecular chains. Subsequently, the borax was ionized to yield borate ion and then chelated with the hydroxyl group of the PVA molecular chain [54]. This chelation recovered automatically after damage and proved that the hydrogel had a self-healing effect.

Fig. 2
figure 2

Mechanism and mechanical properties of PVA–SNF@TA organohydrogel. a Schematic diagram of internal interactions for PVA–SNF@TA organohydrogels. b Image of PVA-0.3% SNF@TA hydrogel under stretching and knotting. c Compressed image of PVA-0.3% SNF@TA hydrogel. d Tensile curves of PVA–SNF@TA organohydrogel and pure PVA hydrogel as reference. e Tensile strength and strain of hydrogel containing different SNF@TA. f The toughness of hydrogel containing different SNF@TA

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The multifunctional PVA–SNF@TA organohydrogels that were prepared with high strength, elasticity and compressibility. Figure 2b illustrates the tensile behavior of multifunctional PVA–SNF@TA organohydrogel under various conditions. The PVA-0.3% SNF@TA organohydrogel demonstrated the good ability to withstand sizeable tensile deformation. Figure 2c illustrates the shape recovery of the cylindrical organohydrogel. Impressively, the organohydrogel quickly returned to its original shape after being pressed into a thin sheet by a finger.

The one-dimensional silk nanofiber and PVA molecular chains can form hydrogen bonds to enhance the mechanical properties of the hydrogel. As shown in Fig. 2d, f, quantitatively, the stress–strain curves of multifunctional PVA–SNF@TA organohydrogels with different SNF@TA contents were used to evaluate their mechanical properties. Obviousely, all PVA–SNF@TA organohydrogels exerted better tensile strength and immense tensile strain than that of pure PVA hydrogels (Fig. 2d). Generally, the tensile strength and strain of PVA–SNF@TA organohydrogels increased with rising the SNFs content. However, when the content of SNF@TA exceeded 0.3%, the tensile strength and strain of PVA–SNF@TA organohydrogels decreased. The tensile strength of the hydrogel increased from 108.313 to 365.727 kPa, and the tensile strain increased from 218.907 to 343.225% (Fig. 2e). Matched with pure PVA hydrogel, the toughness of PVA-0.3% SNF@TA hydrogel was nearly 5.5 times higher than that of pure PVA hydrogel (Fig. 2f).

3.3 Structure and morphology of PVA–SNF@TA organohydrogel

Multi-functional PVA–SNF@TA organohydrogel had more robust mechanical properties than that of pure PVA organohydrogel, which was ascribed to the microstructure of organohydrogel and the dynamic hydrogen bond interaction between PVA and SNF@TA. In Fig. 3a, the broad peaks located at 3415, 2921 and 1442 cm−1 were attributed to the tensile vibration of hydroxyl groups, the tensile pulse of –CH2– and bending vibration of –CH2–, respectively [55]. When the SNF@TA had added to PVA organohydrogel, the peaks at 3415 and 1637 cm−1 were broadened, indicating the formation of a hydrogen bond between PVA and SNF@TA. In addition, there were no new characteristic peaks in the FTIR spectra of PVA, PVA-0.1% SNF@TA, PVA-0.2% SNF@TA, PVA-0.3% SNF@TA and PVA-0.4% SNF@TA. This might be attributed to the low content of SNF@TA [56]. As shown in Figs. S2 and 3b, the XRD spectra of PVA–SNF@TA organohydrogel were almost the same, and no new characteristic diffraction peaks appeared, implying that the addition of SNF@TA will not affect the structure of PVA–SNF@TA organohydrogel. This data was also the verification of PVA–SNF@TA's FTIR spectrum data.

Fig. 3
figure 3

The structure characterization and SEM images of PVA–SNF@TA organohydrogel. a The FT-IR spectra of PVA–SNF@TA organohydrogel. b The XRD pattern of PVA–SNF@TA organohydrogel. SEM images of PVA (c), PVA-0.1% SNF@TA (d), PVA-0.2% SNF@TA (e), PVA-0.3% SNF@TA (f) and PVA-0.4% SNF@TA (g). h Enlarged SEM image of selected zone in (f)

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Figure 3c–h shows the SEM images of different multifunctional PVA–SNF@TA organohydrogel. As seen in Fig. 3c, the pure PVA organohydrogel remained flat and without a trace of nanofibers. After the addition of SNF@TA, some holes appear on the surface of the organohydrogel, and hints of silk nanofiber (Fig. 3d–g) could be observed. As the content of SNF@TA increased, the pores on the surface organohydrogel also increased. Once the range of SNF@TA reached 0.4%, the surface organohydrogel would be filled with holes, and silk nanofiber could be seen to penetrate the networks (Fig. 3h). The porous structure of the organohydrogel was very beneficial in increasing the tensile strain of organohydrogel. Fig. S3 is the cross-sectional SEM image of PVA-0.3% SNF@TA organohydrogel under different magnifications. Obviously, the cross section of PVA-0.3% SNF@TA organohydrogel was a layered stack structure. Additionally, the silk nanofiber connected two-layered facilities (Fig. S3b) for the organohydrogel to afford greater external force under stress, which contributed to increase its tensile strength.

3.4 Cold resistance, adhesion, ultraviolet filtering, and self-healing properties of organohydrogel

Generally, when the ambient temperature drops below zero, almost all water-based organohydrogel will be frozen and lost flexibility, which seriously hinders their application in flexible sensors [43]. Herein, the multifunctional cold resistance organohydrogels based on a binary solvent system of ethylene glycol and water were prepared. We used pure water PVA-W organohydrogel as the control sample. Figure 4a, b clearly shows the significant difference between the PVA-W organohydrogel and PVA-0.3% SNF@TA organohydrogel at low temperatures. After the two pieces of PVA-W and PVA-0.3% SNF@TA were placed at − 18 °C for 24 h, the PVA-W changed from transparent to white and lost flexibility. Nevertheless, the PVA-0.3% SNF@TA still maintained flexible and twisted properties, confirming high mechanical stability.

Fig. 4
figure 4

Cold resistance, transparency and UV filtering of PVA–SNF@TA organohydrogel. a The appearance of PVA-W organohydrogel after freezing. b The appearance of PVA-0.3% SNF@TA organohydrogel after freezing. c The image of PVA-0.3% SNF@TA organohydrogel lighting up the LED bulb. d UV–Vis transmittance spectra of PVA-0.3% SNF@TA organohydrogel. e The demonstration of PVA-0.3% SNF@TA organohydrogel used as ultraviolet electronic skin

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In addition, the PVA-0.3% SNF@TA organohydrogel was used as conductor to brightly light up light-emitting diodes (LED) bulbs (Fig. 4c) either at room temperature or after resting at − 18 °C for 24 h. Then, the cold resistance of PVA-W and PVA-0.3% SNF@TA was further evaluated by using DSC. As shown in Fig. S4, the pure water-based organohydrogel PVA-W had an apparent phase transition peak around − 25 °C, which was caused by the freezing of free water in PVA-W. Admirably, there was no phase transition of PVA-0.3% SNF@TA from 25 to − 70 °C, indicating that the organohydrogel had excellent cold resistance ability. Therefore, the above data shows that the PVA-0.3% SNF@TA organohydrogel retains considerable flexibility and conductivity at low temperatures (− 18 °C), indicating the potential to be used in the field of low-temperature flexible sensing. Fig. S5a demonstrates that PVA-0.3% SNF@TA organohydrogel can adhere to different material surfaces, including steel, wood, polytetrafluoroethylene, glass and plastic. The organohydrogels could be directly attached to the gloved fingers or skin’s surface, while still withstanding the bending motions (Fig. S5b, c) without needing adhesive tape.

Moreover, the transparency is another key feature of hydrogel electronic skin sensors. The excellent transparency allows people to observe the inside of the electronic skin sensor. However, transparent hydrogels usually do not have the function of reflecting ultraviolet rays, which greatly limits their application in high altitude areas and deep space exploration. Therefore, a conductive hydrogel that has both transparency and ultraviolet (UV) protection can greatly expand its application fields. The transparency of 1-mm-thick PVA-0.3% SNF@TA reached 91% in the visible range (Fig. 4d). However, its transparency would quickly drop to around 20% when the wavelength dropped below 400 nm. This suggests the PVA-0.3% SNF@TA organohydrogel have an excellent UV filtering ability. As shown in Fig. 4e, we demonstrated an organohydrogel electronic skin flexible sensor. A volunteer arranged the organohydrogel sample on the back of his hand. The organohydrogel electronic skin flexible sensor exhibited aesthetic qualities (as the organic hydrogel was barely visible due to its transparency), which was capable of filtering ultraviolet rays.

Satisfactorily, the PVA-0.3% SNF@TA organohydrogel also had the capacity to self-repair. The organohydrogel cylinder was cut into three parts, and the middle part of the cylinder was stained with rhodamine B for easy observation. As seen in Fig. S6, the macroscopic self-healing process of the organohydrogel was directly observed. After automatic contact and storage at room temperature for 5 h, the organohydrogel repaired itself as well as withstanding extensive bending without re-fracture. Furthermore, the self-healing process will not affect the conductivity of the organohydrogel. In Fig. S7a, the PVA-0.3% SNF@TA organohydrogel was connected in series with the LED lamp. When the organohydrogel was cut in half, the LED lamp went out instantly. After contacting the two broken parts, the LED light immediately lit up again, wherein the brightness of the LED light was hardly lost. The resistance of organohydrogel was monitored in real-time. It was found that from fracture to contact, the opposition could be restored within hundreds of milliseconds (Fig. S7b). This means that within a few hundred milliseconds, the conductive network inside the PVA-0.3% SNF@TA organohydrogel has been recovered. The self-healing process of PVA-0.3% SNF@TA organohydrogel involved two types of dynamic interactions: on the one hand, the hydroxyl group of the PVA molecular chain chelates with borate radical to promote the self-healing process, and on the other hand, the stable hydrogen bond interaction between SNF@TA and PVA is formed in a very short period time.

3.5 Performance of strain sensor for human motion monitoring

Organohydrogel also relied on ion migration to achieve electrical conductivity. The stretching of the organohydrogel will narrow the ion channel and increase the migration path and resistance. According to this property, the organohydrogel could monitor the shape changes of the device. As shown in Fig. 5a, the relative resistance of PVA–SNF@TA organohydrogel increased with raising the strain. Within the strain ranging from 0 to 100%, the sensitivity factor of multifunctional PVA–SNF@TA organohydrogel is 1.094 and its linear correlation coefficient is 0.9739. The sensing ability of organohydrogel sensors under different deformations was further studied. In Fig. 5b, c, the organohydrogel sensor exhibited sensitive and stable strain sensing characteristics when monitoring both small deformations (5, 10 and 15%) and large deformations (20, 30 and 50%). This shows that the organohydrogel strain sensor can accurately identify different degrees of deformations. In addition, the cyclic tensile stability of the organohydrogel sensor was further tested. As seen in Fig. 5d, the output signal of the organohydrogel sensor remained stable after 1000 times of continuous stretching, only accompanied by a slight upward signal fluctuation. This result verifies that the organohydrogel flexible sensor has excellent fatigue resistance and durability, which is very important for the organohydrogel sensor to achieve stable and reliable signal output in practical applications.

Fig. 5
figure 5

The sensing performance of the PVA–SNF@TA strain sensor. a Dynamic stretched sensitivity of organohydrogel sensor in real-time. b Real-time relative resistance variation at small strains (5, 10 and 15%). c Real-time relative resistance variation at large strains (20, 30 and 50%). d Time-dependent relative resistance change (ΔR/R0) within 1000 cycles at the strain of 15%. e The relative resistance change curve of the finger bending at different angles (0, 45, 90, 30, 60 and 90°). f–h Curves of relative resistance changes in bending of finger joints, wrist and elbow joints. i–l Curves of relative resistance change in bending swallowing, smiling, nictitating and frowning

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The prepared multifunctional PVA–SNF@TA organohydrogel has high strength, self-adhesion, self-healing, antifreeze, transparency and UV filtration, which can be utilized as a strain sensor to monitor human motions. To this end, the PVA–SNF@TA organohydrogel flexible strain sensor was prepared to adhere to the skin of various parts of the human body to monitor the movements without tape. Moreover, the flexible strain sensor was closely be adhered on the skin to accurately test the sensing performance. Such organohydrogel can monitor various human movements, such as fingers, wrist bending and elbow extension at different bending angles. As seen in Fig. 5e–h, the organohydrogel flexible sensor was able to collect the movement signals of human limbs in real-time, which also demonstrates excellent signal repetitive stability. In addition, once the human stem returns to the initial state, the ΔR/R0 value of the organohydrogel could also be restored to the initial value.

Noticeably, the organohydrogel sensor can monitor the minimal deformations with high accuracy. The PVA–SNF@TA organohydrogel sensors attached to the face and throat to detect subtle deformations, such as swallowing, smiling, winking and frowning motions (Fig. 5i–l). However, it is hard to control that each movement was completely consistent. These inconspicuous differences in the signals further reflect the excellent accuracy of the flexible strain sensor. Therefore, the above experimental results prove that the PVA–SNF@TA organohydrogel sensor displays high strain sensitivity and great potential in human motion monitoring.

3.6 Human motion monitoring performance of the pressure sensor

A flexible pressure sensor based on cylindrical PVA–SNF@TA organohydrogels was also fabricated. As shown in Fig. 6a, a cylindrical organohydrogel with a height of 2 cm was placed on the table and repeatedly trampled by a volunteer with a weight of 65 kg. The cylindrical organohydrogel was pressed into thin slices under great gravity but not broken. After gravity was removed, the pressed sheet recovered back to a cylindrical shape, indicating that the stable organohydrogel has the potential to be operated for a long time. Typically, the organohydrogel samples, wires, square iron sheets and tape were assembled into flexible pressure sensors. A flexible pressure sensor was attached to the sole or under the finger to monitor its movement. In Fig. 6b–d, the flexible pressure sensor responded in real-time and outputted stable and repeatable signals when the volunteer performed different motion states (finger pressing, walking, and running). It is worth noting that there is no noticeable shift in the relative resistance change of ΔR/R0 during repeated motion, which indicates that the PVA–SNF@TA organohydrogel pressure sensor has excellent sensing stability. Therefore, considering those outstanding compression performances, we believe our PVA–SNF@TA organohydrogel pressure sensor will have great applications in smart sensors and wearable electronics.

Fig. 6
figure 6

The sensing performance of the PVA–SNF@TA pressure sensor. a The demonstration of compression resistance of cylindrical PVA–SNF@TA organohydrogel. b–d The relative resistance change curve of the PVA–SNF@TA organohydrogel pressure sensor in the state of finger pressing, walking and running

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4 Conclusions

In summary, we develop a PVA–SNF@TA organohydrogel with adhesion capabilities, self-healing, antifreeze, transparency, and UV resistance ability. The excellent performance of PVA–SNF@TA organohydrogel is attributed to (1) the reversible hydrogen bond formed by the catechol functional group on the surface of SNF@TA and the hydroxyl group on the PVA molecular chain; (2) the chelation reaction between borate ion and the hydroxyl group on the PVA molecular chain; (3) the organohydrogel own structure (surface pores). In contrast, the excellent antifreeze performance of organohydrogel is originated from the interaction between ethylene glycol and water molecules. The flexible strain sensor made of PVA–SNF@TA organohydrogel sheet has excellent adhesiveness and maintains excellent signal repeat stability when measuring human movements. It also presents superior accuracy by monitoring small deformations. Moreover, the flexible pressure sensor made of a PVA–SNF@TA organohydrogel cylinder can respond and repeat the measurement of different human motion states in real time, and have stable sensing ability. Based on the above advantages, these two flexible sensors show great potential for development in human motion detection.