Strong and Tough Conductive Hydrogel with High Sensitivity via Self-Assembly-Induced Bridge Crosslinking

doi:10.21203/rs.3.rs-2749647/v1

Download PDF

Article

Strong and Tough Conductive Hydrogel with High Sensitivity via Self-Assembly-Induced Bridge Crosslinking

https://doi.org/10.21203/rs.3.rs-2749647/v1

This work is licensed under a CC BY 4.0 License

Version 1

posted

You are reading this older preprint version

Read the latest preprint version →

Conductive hydrogels possess remarkable potential for applications in soft electronics and robotics, owing to their unique combination of high electrical conductivity, stretchability, and impressive self-healing capabilities. Nonetheless, the limited strength and toughness of these hydrogels have traditionally impeded their practical implementation. Inspired by the hierarchical architecture of high-performance biological composites found in Nature, this study successfully fabricates a novel type of strong and tough conductive hydrogel through self-assembly-induced bridge crosslinking of MgB₂ nanosheets and polyvinyl alcohol (PVA) hydrogels. By combining the micro- to nano-level hierarchical lamellar structures of PVA hydrogels with the robust molecular-level B-O covalent bonds, the resulting conductive hydrogel exhibits an exceptional strength of 8.58 to 32.7 MPa and a high toughness of 27.56 to 123.3 MJ/m³. Moreover, the hydrogel demonstrates exceptional sensitivity (with a response/relaxation time of 20ms and a detection lower limit of ~ 1Pa) under external deformation, thanks to its nanoscale MgB₂ nanosheets/PVA lamellar structure and extremely low compressive modulus. These unique characteristics enable the conductive hydrogel to exhibit superior performance in advanced soft sensing applications, particularly in non-contact speaking detection. This study represents a major breakthrough, introducing a new class of conductive hydrogel that integrates exceptional strength, toughness, and sensitivity, thereby opening up exciting possibilities for the development of high-performance conductive hydrogels.

Physical sciences/Engineering/Mechanical engineering

Physical sciences/Materials science/Soft materials/Gels and hydrogels

Conductive hydrogel

hierarchical structure

bridge crosslinking

strong and tough

Electrically conductive hydrogels are highly versatile materials with significant potential for use in biomedicine^1–3, energy storage⁴, soft robotics^5,6, and soft electronics^7,8. However, their practical applications require a careful balance of properties, including high sensitivity to external deformation, self-healing capability, and mechanical robustness, which includes high strength, toughness, and stretchability^{9, 10}. Currently, conductive hydrogels are created by incorporating various conductive phases, such as nanomaterials (e.g., graphene¹¹, CNT¹², MXene^13–15, and Ag nanoparticles¹⁶), conductive ions (e.g., metal ions¹⁷ and ionic liquid¹⁸), and conductive polymers (e.g., PEDOT:PSS¹⁹, PANI²⁰, and PPy²¹), into the hydrogel matrix. Unfortunately, the weak molecular-level bond between conductive phases and hydrogel matrices, coupled with the disordered structural distribution, significantly limit their mechanical properties. The resulting conductive hydrogels exhibit feeble strength (less than 1 MPa) and fragility, making them susceptible to fracture^17,22. Therefore, developing conductive hydrogels with simultaneous outstanding strength, toughness, and sensitivity remains a significant challenge.

Natural materials in biology, such as bones and muscles, exhibit an exceptional combination of strength, toughness, and message-passing ability owing to their unique structural and molecular-level properties. For instance, the hierarchical organization of muscles provides both strength and flexibility, allowing them to withstand mechanical stress and contract and relax. Additionally, the interactions between proteins and other components of muscle tissue contribute to their overall strength and conductivity^23,24. Inspired by nature, various structural and molecular technologies have been developed for designing hydrogels for enhanced mechanical properties. Structural strategies, including freeze casting to create hierarchical structures^{17, 25}, stretch training to align polyvinyl alcohol (PVA) fibers²⁶, and salting out to induce chain aggregation^{27, 28}, have been explored. Molecular engineering approaches, including creating covalent bonds²⁹, composite networks³⁰, and nanocrystalline domains through annealing and eutectic solvents^{31, 32}, have shown great potential in enhancing the mechanical strength of hydrogels. However, most of these engineering methods are top-down approaches that have limitations in creating strong bonding between hierarchical units post-gelation³³. To overcome this, bottom-up approaches inspired by nature should be explored. Self-assembly is a group of bottom-up approaches for constructing natural or bioinspired composites with ordered structures, generally utilizing physical driving forces^34,35. During self-assembly, the basic units gradually aggregate and become bulk materials with strong interface bonding. For example, nanosheets with a two-dimensional structure can be easily aligned in solution under the unidirectional evaporation force. In addition, for hydroxyl-containing polymers, dehydration reactions occur between the hydroxyl groups of these soluble molecules, along with water evaporation on the air-liquid interface. The evaporation-induced assembly drives the organic components to crystallize layer-by-layer to form micro/nano lamellar structures^36–38. By mimicking the bottom-up self-assembly growth of biological composites in nature and introducing strong bonds in the formation of hierarchical hydrogels, stretchable and conductive hydrogels that are strong and tough could be promising.

To connect the structural and molecular elements involved in the bottom-up self-assembly, we propose to use MgB₂ as the electrically conductive phases in PVA hydrogels. MgB₂ has a crystal structure comprising alternating layers of Mg and B elements. Recent studies have revealed that creating B-O covalent bonds can significantly enhance the mechanical properties of materials that contain hydroxyl^39,40. Furthermore, MgB₂ can release ions and electrons in water slowly that can improve electrical conductivity ⁴¹. In order to reveal more borate ions that will enhance both mechanical strength and electrical conductivity, we have first exfoliated MgB₂ crystals into nanosheets with more B-OH species. Then, using a simple self-assembly-induced boron-bridging method (Fig. 1a), we have prepared MgB₂ nanosheet conductive PVA hydrogels (HLMN-hydrogel) with a hierarchical lamellar microstructure. This conductive hydrogel has impressive strength (8.58 to 32.7 MPa) and toughness (27.56 to 123.3MJ/m³) due to its hierarchical lamellar structure and strong interlayered B-O covalent bonds. Additionally, the nanoscale lamellar structure is highly sensitive to deformation pressure, and the HLMN-hydrogel displays superior sensitivity with a response/relaxation time of 20ms and a detection lower limit as low as 1Pa, making them suitable for a wide range of applications in various fields.

The preparation process of the hierarchical lamellar MgB₂ nanosheets conductive hydrogel (HLMN-hydrogel) involves several steps. First, we exfoliated the MgB₂ nanosheets from MgB₂ nanoparticles using a chelation reaction-based selective demetallization method. As shown from Fig. 1b, MgB₂ has a crystal structure comprising of alternating layers of Mg and B elements. The Mg²⁺ ions in the MgB₂ crystal structure can be selectively extracted by the ethylene diamine tetraacetic acid (EDTA) during the chelation reaction, resulting in a Mg-deficient crystal structure. Due to the layered crystal structures, the Mg-deficient layers can be easily delaminated through high-energy ultrasonic stirring, and therefore, the MgB₂ nanoparticles can be exfoliated into MgB₂ nanosheets. We can identify the platelet-like 2D nanosheets microstructure of the exfoliated MgB₂ with about 500 nm in length and multilayers in thickness from the transmission electron microscopy (TEM) images in Fig. 1c. The reduction of Mg element in MgB₂ nanosheets was demonstrated by X-ray photoelectron spectroscopy (XPS) (Fig. 1e). The B1s XPS spectra of the MgB₂ nanoparticles and nanosheets exhibited apparent differences, with the B1s peak for MgB₂ nanosheets resolved into three peaks at approximately 187, 187.7, and 188.4 eV (Fig. 1e). The broader peak at 188.4 eV for MgB₂ nanosheets can be attributed to the increased numbers of B-OH species⁴². Elemental analysis of the MgB₂ nanosheets using Energy-dispersive spectroscopy under the TEM (TEM-EDS) confirmed the composition of Mg and B elements (Fig. 1f). These boron-rich MgB₂ nanosheets have great potential for enhancing the mechanical properties of hydrogels or composites containing hydroxyl groups through molecular and structural engineering.

Secondly, we prepared the MgB₂ nanosheets (0.16wt.%) and PVA (4%wt.%) solution for the evaporation-assisted self-assembly process to achieve a lamellar arrangement of the PVA and MgB₂ nanosheets layers (Fig. 1a). We aim to maximize the MgB₂ nanosheet content in the hydrogel system for crosslinking and conductivity enhancement, while ensuring good flowability for self-assembly. As demonstrated in Supplementary Fig. 3, extensive experiments showed that increasing the concentration of either PVA or MgB₂ nanosheets will cause the solution to gel too quickly, hindering self-assembly. Next, a low-temperature thermal annealing process is utilized. The purpose of this process is to form PVA crystalline domains within the PVA layers, and create covalent bonding bridges between the hydroxyl groups in the PVA layers and either the boron in the MgB₂ nanosheets or the free borate ions in the materials system (Fig. 1a). Finally, the HLMN-hydrogel is obtained by swelling the annealed sample in water until equilibrium is achieved. The HLMN-hydrogel has a hierarchical lamellar microstructure with nanoscale alternating PVA nanolayers and MgB₂ nanosheets layers, as shown in Fig. 1g. Specifically, the hydrogel consists of the following hierarchical structures and molecular bonds: (1) PVA chains form lamellar aggregation (micro and nanoscale) through evaporation self-assembly and crystalline domains (nano/molecular scale) via thermal annealing. (2) MgB₂ nanosheets exhibit nanoscale lamellar alignment between the PVA layers through evaporation self-assembly. (3) The boron of the MgB₂ nanosheets and released borate ions in the material system are bridged with the PVA layer interface by forming strong B-O covalent bonding (molecular scale) to enhance interfacial interactions.

To investigate the benefits of using MgB₂ nanosheets, we prepared different hydrogels: freeze-thawed MgB₂ nanosheets hydrogels (Ft-MN-hydrogel), pure PVA hydrogels through self-assembly (PVA-hydrogel), and conductive self-assembled PVA hydrogels enhanced by MgB₂ nanoparticles (MP-hydrogel). The microstructures of these hydrogels were examined in Fig. 2a. The Ft-MN-hydrogels had a random porous microstructure, resulting from the non-directional thermal gradient in the freeze-thaw process. In contrast, the PVA hydrogels showed a well-ordered lamellar microstructure, highlighting the importance of self-assembly in constructing such microstructure. However, the MP-hydrogels had a less ordered structure than the PVA hydrogels due to the spherical shape of the MgB₂ nanoparticles, which disrupted the lay-by-lay evaporation and formation of PVA layers during self-assembly. The perfectly lamellar microstructure of the HLMN-hydrogels was attributed to the 2D platelet-like shape of the MgB₂ nanosheets, which facilitated their alignment in between the PVA layers and assisted the formation of the PVA layers during the layer-by-layer evaporation process. Furthermore, the increased interfacial surface area of the MgB₂ nanosheets revealed more B elements to create more B-O covalent bonding with PVA chains, which can lead to enhanced mechanical properties, as discussed in subsequent paragraphs.

Tensile test had been done for all hydrogels as shown in Fig. 2b,c. The HLMN-hydrogel exhibited significantly higher mechanical properties compared to the comparing groups. Its tensile strength (8.58 ± 1.34 MPa) was 11 times that of PVA-hydrogel (0.78 ± 0.04 MPa), 2.7 times that of MP-hydrogel (3.17 ± 0.56 MPa), and 66 times that of Ft-MN-hydrogel (0.13 ± 0.02 MPa) (Fig. 2b,c). The toughness of HLMN-hydrogel (27.56 ± 7.48 MJ/m³) was nearly 20 times that of PVA-hydrogel (1.37 ± 0.21 MJ/m³), 3.7 times that of MP-hydrogel (7.45 ± 1.88 MJ/m³), and 120 times that of Ft-MN-hydrogel (0.23 ± 0.05 MJ/m³) (Fig. 2b,c). Through modulating the water contents, the mechanical performances of the HLMN-hydrogels could be further enhanced (Supplementary Fig. 8). Remarkably, the mechanical performances of HLMN-hydrogel were beyond those of most hydrogels (Fig. 2f and Supplementary Table 1). A tiny cord of HLMN-hydrogel (≈ 0.3 mm × 3 mm) was able to bear a load of 2 kg, as shown in Fig. 2d. Furthermore, HLMN-hydrogel displayed great shape recovery capability and decent hysteresis performance (Fig. 2e). These results demonstrate the advantages of our facile assembly approach for fabricating strong and tough hydrogels.

We investigated the underlying mechanisms responsible for the remarkable mechanical properties of the HLMN-hydrogel. Small angle X-ray scattering (SAXS) tests were conducted on both Ft-MN-hydrogels and HLMN-hydrogels (Fig. 3a). The uniform circular pattern of the Ft-MN-hydrogel indicated its isotropic microstructure, while the scattered flattened circular or elliptical pattern of the HLMN-hydrogel indicated its anisotropic lamellar microstructure. SEM images and fracture cross-section analysis of the HLMN-hydrogels (Fig. 3b-d) confirmed the anisotropic lamellar microstructure. The uneven cross-section fracture surface of the PVA layers indicated their ability to deflect the fracture paths and dissipate more energy. The pull-out of the MgB₂ nanosheets at the fracture surface can lead to additional energy dissipation. To effectively enhance the pull-out effect, sufficient bonding between the MgB₂ and PVA layers is required. As discussed previously, the B-OH bonds in MgB₂ can create covalent bonding with the hydroxyl group in the PVA chains. The existence of the B-OH bonds on the surface of MgB₂ nanosheets was confirmed from the XPS results in Fig. 1e. The appearance of B-O-C bonds (1125 cm^− 1) and O-B-O bonds (710 cm^− 1) ^{39, 43, 44} in the FTIR results of HLMN-hydrogels confirmed the creation of strong covalent boronic ester (B-O-C) bonds in between the MgB₂ nanosheets and PVA layers (Fig. 3e). Therefore, besides the lamellar microstructure, molecular strengthening mechanisms contributed from the MgB₂ nanosheets also played critical roles in the strengthening. DSC analysis showed the appearance of more crystalline domains in the HLMN-hydrogels, indicating the effect of annealing in further enhancing their mechanical properties (Supplementary Fig. 10). Overall, the exceptional mechanical performance of HLMN-hydrogels was attributed to their hierarchical structures and molecular bonds, which include PVA lamellar aggregation and crystal domains, lamellar alignment of MgB₂ nanosheets, and covalent bonding bridges between PVA chains and B-OH species of MgB₂ nanosheets and borate ions (Fig. 3f). Additionally, HLMN-hydrogel displayed good self-healing capability attributed to borate ions that migrate into the crack and bridge the PVA chains on both sides (Supplementary Fig. 13).

The conductive HLMN-hydrogel not only exhibits outstanding mechanical performance, but also exceptional response sensitivity and low detection limit. Specifically, the pressure sensor can respond to a slight pressure of ~ 1 Pa, equivalent to a small flower weighing 56 mg on an area of 5 cm² (Fig. 4a). The response/relaxation time of HLMN-hydrogel is remarkably low at ≈ 20 ms, even faster than that of human skin (≈ 30–50 ms)⁴⁵ and electronic skin (Supplementary Table 2). Additionally, the gauge factor (GF) of HLMN-hydrogel increases from 0 to 4.43 as the stretch strain increases from 0 to 800% (Fig. 4b). In contrast, control studies of MP-hydrogel and Ft-MN-hydrogel showed poorer response time, relaxation time, and GF compared to HLMN-hydrogel (Supplementary Figs. 14 and 15). The electrical stability of HLMN-hydrogel is retained after 1500 tensile cycles, indicating its potential as a robust mechanical sensor (Fig. 4c).

The HLMN-hydrogel displays electrical conductivity by facilitating the movement of both electrons (through MgB₂ nanosheets) and ions (such as Mg²⁺ and B-species). The presence of conducting ions was confirmed through inductive coupled plasma emission spectrometer (ICP) analysis of the hydrogels (Supplementary Fig. 9,). In a temperature-resistance experiment (Supplementary Fig. 17), the resistance of the hydrogel decreased with an increase in temperature, indicating an increased movement of ions due to the elevated temperature, and further validating the existence of ionic conducting mechanism of the HLMN-hydrogel.

The exceptional sensing ability of the HLMN-hydrogel is due to its ordered nanoscale lamellar conductive PVA-MgB₂ layers and extremely low compressive modulus (1.86 ± 0.10 kPa) as shown in Supplementary Fig. 18. The HLMN-hydrogel's low compressive modulus allows for noticeable nanoscale deformation even under minimal force. This leads to a reduction in layer gaps which accelerates the transportation of Mg²⁺, B-species ions, and electrons for enhanced conductivity (Fig. 4e). Significantly, the nanoscale lamellar microstructures lead to a significant increase in contact areas between the multiple conductive PVA-MgB₂ layers, creating a more efficient conducting path for electrons and ions. This change in contact area is particularly noticeable and can persist even after the pressure is removed (Fig. 4a). To confirm this, a NaCl-PVA hydrogel was prepared by soaking a self-assembled PVA hydrogel with hierarchical lamellar structures into a high concentration NaCl solution (Supplementary Fig. 19). In the NaCl-PVA hydrogel, Na⁺ and Cl⁻ ions are distributed between the PVA lamellar layers but cannot crosslink with PVA resulting in non-conductive PVA layers. When pressure is applied, the movement of Na⁺ and Cl⁻ ions should be increased and leaded to a decrease in electrical resistance. However, the non-conductive PVA layers also come into contact simultaneously, impeding the ion transportation largely and therefore, leading to an overall increase in electrical resistance eventually. Supplementary Fig. 19 shows that the resistance of the NaCl-PVA hydrogel starts to rise upon pressure, and the reaction time is longer than that of the HLMN-hydrogel, confirming the critical role of conductive PVA-MgB₂ nanosheets' contact in the HLMN-hydrogel's ultrafast reaction time.

Notably and interestingly, our HLMN-hydrogel demonstrated remarkable non-contact speaking sensing ability due to its exceptional response sensitivity and low detection limit (Fig. 4d and Supplementary Movie 1). The HLMN-hydrogel produced stable and distinguishable signals when receiving spoken words such as "NUS," "mechanical," and "engineering." The signal for "mechanical engineering" represented the combined signal of "mechanical" and "engineering," demonstrating the accuracy and stability of the HLMN-hydrogel (Fig. 4d). Moreover, by utilizing the five output channels, the HLMN-hydrogel can be fabricated into an electronic glove to distinguish different unique signs, including "stop," "great," "victory," and "ok" (Fig. 4f). Additionally, the HLMN-hydrogel can act as a handwriting-sensing device, detecting distinguishable and highly repeatable waveforms for the words "ME" and "OK" (Supplementary Fig. 20). These experiments demonstrate the superior sensing capabilities and versatile applications of the HLMN-hydrogel.

The HLMN-hydrogel and its device, with outstanding sensing capability, displayed tremendous potential in consumer electronics, including speaking detection, virtual recreational gaming, sign language translation, remote control of the surgical robot, and rehabilitation tool for patients with hand disease, etc. For non-contact speaking detection, we displayed a simple application example. The signals of verbal instructions, such as “blue”, “red”, “yellow”, “green” and “turn off” were received by HLMN-hydrogel. The collected quantities of signals were used for the training of a neural network algorithm self-built using Python (Supplementary Fig. 21). Afterward, the speaking & commands transfer was achieved by a microcontroller (Fig. 5a). Different verbal instructions of “blue”, “red”, “yellow”, “green” and “turn off” exhibited accurate corresponding commands of turning up the blue, red, yellow, or green LED lights and turning off the lights (Fig. 5b and Supplementary Movie 2). Therefore, the fabricated hydrogels with the non-contact speaking sensing capabilities realized the ability to “listen” and to “understand”, by themselves. The results showed that HLMN-hydrogel possesses high sensitivity, accuracy, stability, and great potential as sensors and consumer electronics.

In summary, we have introduced a novel electrically conductive hydrogel, which utilizes MgB₂ nanosheets to induce a unique hierarchical lamellar structure. The resulting HLMN-hydrogel exhibited exceptional mechanical properties, including strength (8.58 to 32.7 MPa) and toughness (27.56 to 123.3MJ/m³), as well as remarkable sensing capabilities with a reaction time of 20 ms and a detection lower limit of ~ 1Pa. By incorporating lamellar layers and interface crosslinking bridges, the mechanical performance of HLMN-hydrogel exceeded that of recently reported tough hydrogels. Moreover, HLMN-hydrogel demonstrated exceptional non-contact speaking sensing ability, due to its unique nanoscale layered structure, enabling accurate and stable detection of verbal commands. These features make HLMN-hydrogel an attractive material for flexible electronics, e-skins, soft robotics, energy and biomedical applications. Overall, our hierarchical design strategy provides an effective approach for developing robust and functional hydrogels with advanced capabilities.

Materials

Polyvinyl alcohol (PVA, 99+% hydrolyzed, Mw 89000 ~ 98000), NaOH, ethylene diamine tetraacetic acid (EDTA) were purchased from Sigma. MgB₂ powder was brought from Xiaohuang nanotech, shanghai.

Synthesis of MgB₂ Nanosheets

The reaction solution was obtained by adding EDTA (12.89g), NaOH (3.95g) into 90mL H₂O. Then, the pH of the reaction solution was adjusted to 10. MgB₂ powders (2g) exfoliation happened when added to the reaction solution (25℃). After 3 days of exfoliation, the suspension was washed at least 5 times using deionized water and centrifuged to separate the powders. The collected precipitation was dispersed in water and delaminated under probe sonication for 30 mins. After that, the dispersion was centrifuged for 10 min at 10000 rpm, followed by freeze-drying and annealing at 750℃ to obtain MgB₂ nanosheets.

Fabrication of the HLMN-hydrogel, PVA hydrogel, MP-hydrogel, Ft-MN-hydrogel, and NaCl-PVA hydrogel

First, the fabricated MgB₂ nanosheets were ultrasonic dispersed in alkaline water (pH = 9). 4wt.% PVA solution was obtained after adding PVA into the mixture in 90 ℃ water bath for 1 hour, and the weight ratio of MgB₂ nanosheets and PVA was 4%. The mixed solution was then poured into a container for evaporation-assisted self-assembly. The use of alkaline water was for stabilizing the boronic esters bonds. The obtained lamellar dry gel was then thermally annealed at 60 ℃ for 30 mins to produce PVA crystalline domains and crosslinking bridges. HLMN-hydrogel was obtained by soaking the annealed gel in deionized water till equilibrium. The thickness of the sample can be controlled by the evaporation time, with 10 hours required for a 0.4 mm film and 5 days for a 5 mm hydrogel. To prepare PVA hydrogel, a 4%wt.% PVA solution was subjected to evaporation-assisted self-assembly and low-temperature annealing at 60 ℃, followed by soaking in water until equilibrium. For MP-hydrogel, MgB₂ powders passing through the 400-mesh screen were mixed with PVA solution, and the same self-assembling, annealing, and swelling process was carried out. Ft-MN-hydrogel was prepared through repeated freeze thawing approach. The freeze thawing solution was same as the solution for the preparation of HLMN-hydrogel. NaCl-PVA hydrogel was fabricated through soaking the dry PVA-hydrogel with hierarchical layers in a NaCl solution (0.1g/mL) until equilibrium.

Characterization

The microstructure and element mapping of MgB₂ nanosheets were characterized on a transmission electron microscope (TEM, JEM-2100F, Japan). X-ray photoelectric spectroscopy (XPS, ESCAlab250 Thermal Scientific, US) was used to determine the element composition of the materials, while X-ray diffraction (D8 Advance, Bruker) was used to determine the phase of the materials. The microstructure images of hydrogel were obtained by a scanning electron microscope (SEM, HITACHI S-4800). The hydrogel samples for microstructure observation were freeze-dried first, and then were frozen in liquid nitrogen for cross-section preparation. The bonding of the hydrogel was analysed by Fourier transform infrared spectroscopy (FTIR, Thermal Scientific, US). Small-angle X-ray scattering (SAXS) measurements were performed on a Xenocs Xeuss 2.0 system by placing the cross-section of the hydrogel perpendicular to the X-ray beam. The water contents of HLMN-hydrogel were estimated by comparing the mass difference between the samples in the water-containing state and dried state.

DSC measurement

Differential scanning calorimetry (DSC) were utilized to test crystalline domains at a rate of 20°C/min under an Argon atmosphere with a flow rate of 50 ml/min. Before testing, hydrogel slices were first immersed in cross-linking agent (50 vol.% glutaraldehyde: 5mL, 37 wt.% hydrochloric acid: 500µL, water: 50mL) to stabilize the amorphous chains. Then, excess hydrochloric acid was washed out from the stabilized hydrogels for 2 hours before they were dried in a 50°C blast oven overnight.

ICP test

The ions concentration in the reaction mixture was tested using Inductive Coupled Plasma Emission Spectrometer (ICP). First, MgB₂ nanosheets were ultrasonic dispersed in alkaline water (pH = 9) for 10 mins. Then, the dispersed solution was magnetic stirred for 1 hour at the 90 ℃ water bath. Finally, the supernatant was collected after centrifugation for ICP test. In stability test, the ions concentration in the water solution (20ml) after soaking the dry hydrogel (0.5g) for 3d and 7d was also investigated using ICP.

Mechanical performances

The mechanical performances of the hydrogels were evaluated by a universal tensile machine (Instron 5500) equipped with a maximum force of 1000 N loading cell. The dog bone-shaped hydrogel samples (width: 3 mm, length: 8 mm, thickness: 0.2 mm) were cut using a die cutter and tested at the tensile speed of 20mm/min. The toughness of the hydrogels was determined by calculating the area under the stress-strain curve. At least five samples were tested for each type of hydrogel. The fracture energy (Γ) of the samples was obtained by single-edge notch tension (SENT) test and calculated through following equation:

$$\varGamma =\frac{6Wc}{\sqrt{{1+\epsilon }_{0}}}$$

Where ε₀ represents the fracture strain of notched sample, c is the length of the notch, and W is the stretch energy calculated by integration of the nominal tensile strength versus strain of unnotched specimen with the same dimensions stretched to the ε₀ strain.

Self-healing

The HLMN-hydrogel was cut into two parts and then pre-healed for 2 minutes. The pre-healed samples were placed in an oven at 60 ℃ for 5 minutes to promote the formation of bonds and crystalline domains at the crack. The healed hydrogel was obtained by swelling the samples in water.

HLMN-hydrogel for Sensor Applications

The reaction/relaxation time and gauge factor of HLMN-hydrogel were explored using Keithley DMM6500 in the two-probe mode. For non-contact speaking detection, the hydrogel was encapsulated between PDMS film and PE thin film (Fig. 4d). Two copper-pad electrodes were fixed on the PDMS film to ensure stable contact between hydrogels and Keithley DMM6500. For electronic glove application, the hydrogels were encapsulated with PDMS and attached to each finger joint of the glove. The end of the hydrogels was firmly attached to the glove by cyanoacrylate glue. The five hydrogel sensors were connected with the NodeMCU development board ESP-32S. For the handwriting sensor, the hydrogel was encapsulated into a dragon skin frame (Supplementary Fig. 20). A PDMS film was put on the hydrogel to protect it from mechanical failure during the writing.

HLMN-hydrogel for Speaking & Commands Recognition

The collected speaking signals were processed in a self-built neuron network algorithm with Python. The speaking & commands transfer was achieved by a development board (NodeMCU, ESP-32S), which is compatible with the Arduino programming environment.

Acknowledgements

This research is supported by the MOE AcRF Tier 1 Grant (Project No. WBS A-0009123-01-00).

Author contributions

T.L. conceived, planned, and performed the experiments, analyzed data, and drafted the manuscript. H.Q., Y.Z. and X.D. helped in electronic experiments and material analysis. X.G. and M.Z. optimized the mechanism analysis. X.L. provided reagents and revised the paper. W.Z. initiated, designed, and supervised the study and revised the manuscript. All authors discussed results and commented on the manuscript.

Zhang W, et al. Catechol-functionalized hydrogels: biomimetic design, adhesion mechanism, and biomedical applications. Chem. Soc. Rev. 49, 433-464 (2020).
Hu Q, et al. Inhibition of post-surgery tumour recurrence via a hydrogel releasing CAR-T cells and anti-PDL1-conjugated platelets. Nat. Biomed. Eng. 5, 1038-1047 (2021).
Ji X, et al. Epigenetic Remodeling Hydrogel Patches for Multidrug-Resistant Triple-Negative Breast Cancer. Adv. Mater. 33, 2100949 (2021).
Guo Y, Bae J, Fang Z, Li P, Zhao F, Yu G. Hydrogels and Hydrogel-Derived Materials for Energy and Water Sustainability. Chem. Rev. 120, 7642-7707 (2020).
Xue P, et al. Near-Infrared Light-Driven Shape-Morphing of Programmable Anisotropic Hydrogels Enabled by MXene Nanosheets. Angew. Chem. Int. Ed. Engl 60, 3390-3396 (2021).
Na H, Kang Y-W, Park CS, Jung S, Kim H-Y, Sun J-Y. Hydrogel-based strong and fast actuators by electroosmotic turgor pressure. Science 376, 301-307 (2022).
Ohm Y, Pan C, Ford MJ, Huang X, Liao J, Majidi C. An electrically conductive silver–polyacrylamide–alginate hydrogel composite for soft electronics. Nat. Electron. 4, 185-192 (2021).
Su G, et al. Balancing the mechanical, electronic, and self-healing properties in conductive self-healing hydrogel for wearable sensor applications. Mater. Horiz. 8, 1795-1804 (2021).
Zhou Y, et al. Highly stretchable, elastic, and ionic conductive hydrogel for artificial soft electronics. Adv. Funct. Mater. 29, 1806220 (2019).
Lei Z, Wu P. A supramolecular biomimetic skin combining a wide spectrum of mechanical properties and multiple sensory capabilities. Nat. Commun. 9, 1134 (2018).
Gan D, et al. Graphene Oxide-Templated Conductive and Redox-Active Nanosheets Incorporated Hydrogels for Adhesive Bioelectronics. Adv. Funct. Mater. 30, 1907678 (2020).
Shin SR, et al. A Bioactive Carbon Nanotube-Based Ink for Printing 2D and 3D Flexible Electronics. Adv. Mater. 28, 3280-3289 (2016).
Lee KH, et al. Muscle Fatigue Sensor Based on Ti3C2Tx MXene Hydrogel. Small Methods 5, 2100819 (2021).
Li X, et al. Healable, Degradable, and Conductive MXene Nanocomposite Hydrogel for Multifunctional Epidermal Sensors. ACS Nano 15, 7765-7773 (2021).
Luo X, Zhu L, Wang Y-C, Li J, Nie J, Wang ZL. A Flexible Multifunctional Triboelectric Nanogenerator Based on MXene/PVA Hydrogel. Adv. Funct. Mater. 31, 2104928 (2021).
Zhao Y, et al. Skin-Inspired Antibacterial Conductive Hydrogels for Epidermal Sensors and Diabetic Foot Wound Dressings. Adv. Funct. Mater. 29, 1901474 (2019).
Dong X, Guo X, Liu Q, Zhao Y, Qi H, Zhai W. Strong and Tough Conductive Organo‐Hydrogels via Freeze‐Casting Assisted Solution Substitution. Adv. Funct. Mater. 32, 2203610 (2022).
Wang J, et al. Ultra-High Electrical Conductivity in Filler-Free Polymeric Hydrogels Toward Thermoelectrics and Electromagnetic Interference Shielding. Adv. Mater. 34, 2109904 (2022).
Shen Z, et al. High-Stretchability, Ultralow-Hysteresis ConductingPolymer Hydrogel Strain Sensors for Soft Machines. Adv. Mater., e2203650 (2022).
Fang Y, et al. Self-healable and recyclable polyurethane-polyaniline hydrogel toward flexible strain sensor. Compos. B. Eng. 219, 108965 (2021).
Tang S, et al. 2-Methylimidazole assisted synthesis of nanocrystalline shell reinforced PPy hydrogel with high mechanical and electrochemical performance. Chem. Eng. J. 430, 133033 (2022).
Zhao X, Chen X, Yuk H, Lin S, Liu X, Parada G. Soft Materials by Design: Unconventional Polymer Networks Give Extreme Properties. Chem. Rev. 121, 4309-4372 (2021).
Dong R, Ma PX, Guo B. Conductive biomaterials for muscle tissue engineering. Biomaterials 229, 119584 (2020).
Kuo IY, Ehrlich BE. Signaling in muscle contraction. Cold Spring Harb Perspect Biol 7, a006023 (2015).
Hua M, et al. Strong tough hydrogels via the synergy of freeze-casting and salting out. Nature 590, 594-599 (2021).
Lin S, Liu J, Liu X, Zhao X. Muscle-like fatigue-resistant hydrogels by mechanical training. Proc. Natl. Acad. Sci. 116, 10244-10249 (2019).
Cui W, et al. Strong Tough Conductive Hydrogels via the Synergy of Ion‐Induced Cross‐Linking and Salting‐Out. Adv. Funct. Mater. 32, 2204823 (2022).
Hua M, Wu S, Jin Y, Zhao Y, Yao B, He X. Tough-Hydrogel Reinforced Low-Tortuosity Conductive Networks for Stretchable and High-Performance Supercapacitors. Adv. Mater. 33, 2100983 (2021).
Chen M, Wu Y, Chen B, Tucker AM, Jagota A, Yang S. Fast, strong, and reversible adhesives with dynamic covalent bonds for potential use in wound dressing. Proc. Natl. Acad. Sci. 119, e2203074119 (2022).
Xu X, Jerca VV, Hoogenboom R. Bioinspired double network hydrogels: from covalent double network hydrogels via hybrid double network hydrogels to physical double network hydrogels. Mater. Horiz. 8, 1173-1188 (2021).
Lin S, et al. Anti-fatigue-fracture hydrogels. Sci. Adv. 5, eaau8528 (2019).
Zhang H, Tang N, Yu X, Li M-H, Hu J. Strong and Tough Physical Eutectogels Regulated by the Spatiotemporal Expression of Non-Covalent Interactions. Adv. Funct.l Mater. 32, 2206305 (2022).
Dong Y, Ramey-Ward AN, Salaita K. Programmable Mechanically Active Hydrogel-Based Materials. Adv. Mater. 33, e2006600 (2021).
Wang Y, et al. The predominant role of collagen in the nucleation, growth, structure and orientation of bone apatite. Nat. Mater. 11, 724-733 (2012).
Whitesides GM, Grzybowski B. Self-Assembly at All Scales. Science 295, 2418-2421 (2002).
Ghoshal S, Denner P, Stapf S, Mattea C. Structural and dynamical heterogeneities in PVA films induced by evaporation during the formation process. Chem. Phy. Lett. 515, 231-234 (2011).
Zhu H, et al. Nacre-like composite films with a conductive interconnected network consisting of graphene oxide, polyvinyl alcohol and single-walled carbon nanotubes. Materials & Design 175, 107783 (2019).
Huang Z, Yang G, Zhang J, Gray S, Xie Z. Dual-layer membranes with a thin film hydrophilic MOF/PVA nanocomposite for enhanced antiwetting property in membrane distillation. Desalination 518, 115268 (2021).
Wan S, et al. High-strength scalable MXene films through bridging-induced densification. Science 374, 96-99 (2021).
Chen K, et al. Graphene oxide bulk material reinforced by heterophase platelets with multiscale interface crosslinking. Nat. Mater. 21, 1121-1129 (2022).
Kochkodan V, Darwish NB, Hilal N. The chemistry of boron in water. Boron separation processes, 35-63 (2015).
Vasquez RP, Jung CU, Park M-S, Kim H-J, Kim JY, Lee S-I. X-ray photoemission study ofMgB2. Phys. Rev. B 64, (2001).
Karaahmet O, Cicek B. Effect of mechanically modification process on boron carbide synthesis from polymeric precursor method. Ceram. Int. 48, 11940-11952 (2022).
Yanase I, Ogawara R, Kobayashi H. Synthesis of boron carbide powder from polyvinyl borate precursor. Mater. Lett. 63, 91-93 (2009).
Wu W, Wen X, Wang ZL. Taxel-Addressable Matrix of Vertical-Nanowire Piezotronic Transistors for Active and Adaptive Tactile Imaging. Science 340, 952-957 (2013).

There is NO Competing Interest.

MovieS1Noncontactspeakingdetection.mp4
Supplementary Movie 1
MovieS2Command.mp4
Supplementary Movie 2
Graphicabstract.docx
Supplementaryinformation.docx

Download PDF

Version 1

posted

You are reading this older preprint version

Read the latest preprint version →

Strong and Tough Conductive Hydrogel with High Sensitivity via Self-Assembly-Induced Bridge Crosslinking

Status:

Version 1

Abstract

Figures

Introduction

Results And Discussion

Methods

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1