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Wang, Ming; Rojas, Orlando J.; Ning, Like; Li, Yuehu; Niu, Xun; Shi, Xuetong; Qi, Haisong Liquid metal and Mxene enable self-healing soft electronics based on double networks of bacterial cellulose hydrogels

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Carbohydrate Polymers

DOI:

10.1016/j.carbpol.2022.120330 E-pub ahead of print: 01/02/2023

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Please cite the original version:

Wang, M., Rojas, O. J., Ning, L., Li, Y., Niu, X., Shi, X., & Qi, H. (2023). Liquid metal and Mxene enable self-

healing soft electronics based on double networks of bacterial cellulose hydrogels. Carbohydrate Polymers, 301,

[120330]. https://doi.org/10.1016/j.carbpol.2022.120330

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Carbohydrate Polymers 301 (2023) 120330

Available online 11 November 2022

Liquid metal and Mxene enable self-healing soft electronics based on double networks of bacterial cellulose hydrogels

Ming Wang

^a^,^b^,^c^,^d

, Orlando J. Rojas

^b^,^c^,^d^,^e^,^*

, Like Ning

^b^,^c^,^d^,^f

, Yuehu Li

^a

, Xun Niu

^b^,^c^,^d

, Xuetong Shi

^b^,^c^,^d

, Haisong Qi

^a^,^*

aState Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510641, China

bDepartment of Chemical and Biological Engineering, University of British Columbia, 2360, East Mall, Vancouver, BC V6T 1Z3, Canada

cDepartment of Chemistry, University of British Columbia, 2360, East Mall, Vancouver, BC V6T 1Z3, Canada

dDepartment of Wood Science, University of British Columbia, 2360, East Mall, Vancouver, BC V6T 1Z3, Canada

eDepartment of Bioproducts and Biosystems, School of Chemical Engineering, Aalto University, FI-00076 Aalto, Finland

fCollege of Chemical Engineering, Nanjing Forestry University, Longpan Road 159, Nanjing 210037, China

A R T I C L E I N F O Keywords:

Electroactive hydrogels Liquid metals Ti3C2Tx MXene Bacterial nanocellulose Self-healing Force mapping

A B S T R A C T

Liquid metal (LM) nanodroplets and MXene nanosheets are integrated with sulfonated bacterial nanocellulose (BNC) and acrylic acid (AA). Upon fast sonication, AA polymerization leads to a crosslinked composite hydrogel in which BNC exfoliates Mxene, forming organized conductive pathways. Soft conducting properties are achieved in the presence of colloidally stable core-shell LM nanodroplets. Due to the unique gelation mechanism and the effect of Mxene, the hydrogels spontaneously undergo surface wrinkling, which improves their electrical sensitivity (GF =8.09). The hydrogels are further shown to display interfacial adhesion to a variety of surfaces, ultra-elasticity (tailorable elongation, from 1000 % to 3200 %), indentation resistance and self-healing capabilities. Such properties are demonstrated in wearable, force mapping, multi-sensing and patternable electroluminescence devices.

1. Introduction

Self-healing and conductive hydrogels bearing viscoelasticity, epidermal compliance and electroactivity have drawn attention in recent years (Li, He, et al., 2021; Lo et al., 2021; Tringides et al., 2021;

Wang et al., 2022). These materials are designed to bridge the gap between biology and electronics, which are highly desirable in stretchable devices, soft robots and bionic technologies (Lee et al., 2021; Mannoor et al., 2013; Schroeder et al., 2017). Reliable stretchable electronics can be produced from self-healing conductive hydrogels that display electromechanical and multi-responsive functionalities (Xu et al., 2021; Zhu et al., 2018). For this purpose, conductive fillers have been added in 3D polymer networks (Ohm et al., 2021). Among them, 2D transition metal carbides/nitrides (MXene) have been considered for their impressive metalloid conductivity and chemical customizability (Lu et al., 2022;

Peng et al., 2021). Likewise, gallium-based room-temperature liquid metals (LM) can be added given their unique functions (Zhang, Zhong, et al., 2021; Zhang et al., 2015). The gelation of Mxene and LM leads LM/MXene-based hydrogels that display a behavior that relies on the

additive and synergistic functionalities of the components (Zhang et al., 2020).

LM hydrophilic nanodroplets (Li, Yan, et al., 2021; Ma et al., 2019;

Peng et al., 2019; Wang et al., 2018; Wang, Lai, et al., 2021; Xu et al., 2020) and MXene nanosheets (Ge et al., 2021; Huang et al., 2022) have been shown as single component systems. Unfortunately, most synthetic networks based on single LM or MXene so far still rely on the initiation of persulfates and toxic covalently bonded crosslinkers (MBAA). Their conductivity requires additional procedures with either ionic or electronic materials, resulting in hydrogels that are sub-optimal as far as their structure or electromechanical performance (Yu et al., 2022;

Zhang, Tang, et al., 2021). In addition, obtaining colloidally stable LM/

MXene nanoparticles in gel matrices is quite challenging, which is an important consideration as far as stress concentration and mechanical strength (Li et al., 2019; Peng et al., 2020; Seyedin et al., 2019). Here we consider bacterial nanocellulose as supporting material for LM/MXene nanoparticles. We further advance a microstructural design, critical to amplify or deflect external stimuli in smart hydrogels and sensing devices (Pang et al., 2015; Wang et al., 2019).

* Corresponding authors.

E-mail addresses: orlando.rojas@ubc.ca (O.J. Rojas), qihs@scut.edu.cn (H. Qi).

Contents lists available at ScienceDirect

Carbohydrate Polymers

journal homepage: www.elsevier.com/locate/carbpol

https://doi.org/10.1016/j.carbpol.2022.120330

Received 12 September 2022; Received in revised form 21 October 2022; Accepted 7 November 2022

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Carbohydrate Polymers 301 (2023) 120330

2 Recent microstructure designs have endowed electronic devices with deformability and reproducible sensing (Liu et al., 2015; Liu et al., 2017;

Song et al., 2017; Sunwoo et al., 2021; Won et al., 2019; Yu et al., 2017).

Inspired by human fingerprints, wrinkled hydrogels have been introduced (Kato et al., 2019). Structural designs has further led to electronic devices with excellent deformability and sensing coefficients. However, lengthy procedures, such as pre-stretching, surface oxidation, metal particle sputtering and solvent exchange, are required to obtain the wrinkled microstructures (Kato et al., 2020; Schedl et al., 2018; Wang et al., 2019; Yu et al., 2015).

In this study, we develop highly conductive LM/Mxene-based hydrogels by polymerization under sonication of acrylic acid (AA) in the presence of bacterial nanocellulose (BNC). The composite hydrogels are shown for their mechanical, self-healing, interfacial adhesion, and surface wrinkling properties. Considering their unique features, the introduced hydrogels (herein termed LMBP) are presented as a new option for next-generation electronic devices.

2. Results and discussion 2.1. LMBP hydrogel synthesis

The integration of LM/Mxene in BNC and PAA, forming a double network hydrogel (LMBP) is schematically presented in Fig. 1a. Firstly, multilayered MXene was obtained by LiF/HCl etching of the Al layer in Ti3AlC2 (Fig. 1b). X-ray photoelectron spectroscopy (XPS) showed the typical 460 and 455 eV peaks, which are attributed to Ti–O and Ti–C of MXene, confirming successful etching (Fig. 1f and Fig. S1) (Cao et al., 2019). The bacterial nanocellulose (BNC) was modified with sulfamic acid to install negatively charged groups (-OSO3−). The system showed fine (≈10 nm) and ultralong (≈11 μm) nanofibers (atomic force microscopy, AFM, Fig. 1c). Given its high electronegativity (zeta-potential of −106 mV, Fig. S2) and high-aspect-ratio, BNC formed complex physical interconnections with the other components, resulting in a stable ‘organic-inorganic’ hybrid system. The multilayer MXene (Ti₃C₂T_x) and bulk LM were dispersed in the gel prepolymer, forming a ternary system comprising spherical LM nanodroplets, multilayer

Fig. 1.a) Schematic description of the synthesis process of LMBP hydrogels. b) SEM image of Multilayered MXene. c) AFM image of sulfamic acid modified bacterial nanocellulose, BNC. d) BNC dispersed with single-layer MXene. e) TEM image of LM-BNC nanodroplets. f) XPS spectra of MXene and LMBP hydrogels, as indicated. g) FTIR of PAA, LM-PAA and LMBP. h) ¹H NMR spectra of produced hydrogel and PAA.

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MXene and single-layer MXene nanosheets suspended in BNC (scanning electron microscopy, SEM images in Fig. S3). LM and MXene were sonicated separately in the BNC suspension. The BNC-assisted interca- lation method effectively promoted exfoliation of MXene. The hydrogen bonding interaction with BNC stabilized the delaminated MXene (MXene/BNC, zeta-potential of −43 mV) and prevented agglomeration during the subsequent gelation.

AFM (Fig. 1d) and SEM (Fig. S4) images showed delaminated (exfoliated) MXene as ultrathin (≈2–3 nm) 2D structures. They are ex- pected to produce well-organized electron-conducting channels in the hydrogel. Colloidally stable LM nanodroplets (diameters ranging from tens of nanometers to a few microns) were formed in core–shell con- figurations (LM core encapsulated in BNC shells) and showed a zeta- potential of −66 mV (SEM and TEM images in Fig. 1e and Fig. S5).

The electrostatic repulsion prevented coalescence of the highly mobile LM phase. In addition, the hydrophilic shell prevented phase separation, which would otherwise result from the incompatibility between the LM and the polymer matrix.

2.2. Effect of sonication on LMBP development

The LMBP hydrogel was prepared by simple sonication of the LM/

MXene-BNC colloidal suspension in the presence of the AA monomer precursor. We note that the viscosity of the mixture gradually increased under ambient conditions, leading to a high elongational viscosity, Fig. S6. The system spontaneously gelled, within a few minutes. The gelation process was recorded by optical microscopy, which indicated LM nanodroplets homogeneously dispersed, immobilized and finally etched (Fig. S7). Notably, different to previously reported MXene-based hydrogels, no initiators nor cross-linking agents were introduced in this gel system (heating or UV irradiation were not applied either). Electron paramagnetic resonance tests (EPR, Fig. S8) showed the typical 1:2:2:1 ratio of peak signals, matching perfectly those of DMPO-OH radicals adducts (Han et al., 2011). The hydroxyl free radicals may generate from the reaction of LM/MXene, oxygen and water during sonication (Wan et al., 2021), which triggered free radical polymerization (Wang, Lai, et al., 2021; Wang, Feng, et al., 2021). The C––C vibration band at 1634 cm⁻¹of AA in the FT-IR spectra decreased with gelation, indicating an increased conversion (polymerization) of AA monomers (Fig. 1g) (Charles et al., 1987). The characteristic peaks of methylene ~1.01 ppm

and methyl ~2.30 ppm protons observed in the ¹H NMR spectrum further confirmed the formation of PAA (Ruiz-Muelle et al., 2019). The gray tone of the LM nanoparticles was gradually darkened upon gelation, which was the result of the erosion of LM by protons. The released Ga³⁺was bound to PAA and further cross-linked through the formation of Ga³⁺and -COO⁻-mediated complexation. The carboxyl peak of PAA was red-shifted (from 1718 to 1707 cm⁻¹), indicating the complexation of LM/MXene with the carboxyl groups. XPS spectra for LM collected from LMPB hydrogel showed the peaks of Ga 3d at 347.6 and 351.0 eV, confirming the formation of Ga³⁺(Xin et al., 2019).

2.3. MXene assisted gelation and self-healing

The gelation time, defined here as the time interval between the end of the sonication and the loss of fluidity, was shortened when we added small amounts of MXene (Video 1). To further investigate the role of MXene during gelation, thermal imaging (IR camera interfaced with an analysis system) was used to capture the changes in temperature and heat distribution. Thermal distribution images of LMBP prepolymer indicated local initial heating generated by the polymerization reaction.

With time, the heat front diffused throughout the system (Video 2 and Fig. S9). The later observation is typical of chain-initiation and growth in free radical polymerization (Spade & Volpert, 2001). Fig. 2a shows the real-time temperature changes of MXene/BNC-AA, LM-MXene/BNC- AA and LM-MXene/BNC-AA after sonication in an ice bath for 5 min.

The MXene/BNC-AA mixture did not show temperature changes because MXene alone cannot initiate free radical polymerization and release heat. Fig. 2b showed a small temperature rise interval (△T1 = 7 ^◦C) and a slow heating rate (0.0141 ^◦C/s) of the LM-AA mixture, indicating a limited gelation process. Even though MXene was introduced at low concentrations, the mixture exhibited a strong and spon- taneous exothermic reaction, with a large increase in temperature (△T2 = 16 ^◦C). Also, an order of magnitude higher heating rate, 0.137 ^◦C/s, was observed accompanied by rapid gelation. These results indicate the gelation kinetics changed dramatically by the effect of MXene.

We propose that the rapid gelation activated by MXene is a result of (1) the intensification of the friction between AA molecules and the surface of LM and MXene, initiating a violent exothermic process; (2) the molecular chelation of MXene nanosheets and Ga³⁺metal cations from

Fig. 2. a) Infrared thermal imaging (thermograms) obtained from (top to bottom) Mxene-BNC-AA, LM-BNC-AA and LM/MXene-BNC-AA. b) Temperature change of aqueous AA solution mixed with MXene, LM and LM/MXene. c) Heat generation rate LMBP hydrogel with different MXene loadings. d) Freshly gelled LMBP hydrogel and its shape-adaptive performance, incubation time: 3 h. e) Moldability in the initial stage of gelation.

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4 LM, which accelerated the release of free radicals (Deng et al., 2019;

Huang et al., 2022); (3) the radical polymerization reaction and release of heat with the production of free radicals (Iwasaki & Yoshida, 2005);

(4) the formation of short-chain PAA that rapidly cross-linked Ga³⁺with the polar terminal groups (–F, –O and –OH) of MXene nanosheets.

The addition of different concentrations of MXene showed significant exothermic events (Fig. 2c). However, the system with MXene at concentrations >1.7 mg/ml gelled slowly (Fig. S10). Although MXene promoted gelation, it hindered the oxidative polymerization when used in excess (Ge et al., 2021).

Due to the high AA conversion, the freshly gelled LMBP hydrogel showed good mechanical strength, for instance, it withstood large loads (Fig. 2d). At the initial stage of gelation, trivalent Ga cations were released sparsely in the matrix and the material exhibited stretching hysteresis, due to the poor crosslinking density (Fig. S11). Moreover, the weakly crosslinked hydrogels spontaneously adopted the shape of the given container, indicating shape adaptability and the potential for molding into different architectures. The material was easily printed into different geometries via molding or extrusion (Fig. 2e).

2.4. Surface self-wrinkling and adhesion

Surface wrinkling plays an important role in increasing surface area, which promotes strong adhesion to a variety of surfaces, akin to the gecko-feet and moth antennas (Kato et al., 2019). Surface folding and patterning often requires special procedures, such as those involving electrophoresis, solvent-induced swelling and tensile recovery (Zou et al., 2019). Remarkably, and in contrast to most reported wrinkling systems, our LMBP hydrogels spontaneously underwent surface wrinkling upon gelation (Fig. 3a and Video 3). The surface of the container

and the volume of the prepolymer influenced the depth of fold formation, 30–60 % of the gel thickness (Fig. 3b). We note that the hydrogels containing only MXene or LM, synthesized via potassium persulfate (KPS) to initiate the polymerization, did not spontaneously form wrinkles, as is the case in most reported MXene or LM hydrogels (Hu et al., 2021; Li, Yan, et al., 2021; Peng et al., 2019).

The self-wrinkling behaviors of LMBP hydrogels can be explained by the local inhomogeneous swelling of adjacent regions and elastic modulus mismatch. This is caused by the different rates of free radical polymerization reactions on the confined surface and the cross-linking of LM/MXene, associated with the unique gelation mechanism induced by LM and MXene (Fig. 3c). Specifically, LM-BNC nanoparticles and MXene nanosheets are mobile before gelation (microscopy observation) given the movement and aggregation of particles. In the gel solution, where both LM and MXene are present, free radicals are preferentially released and trigger the polymerization reaction. The differential gelation rate between small adjacent regions leads to spatial differences in the elastic modulus of the gel matrix. Furthermore, LM/

MXene exert an anchoring effect on the polymer chains, causing the polymer molecular chains to contract and orient around LM/MXene (Ou et al., 2022). The hard MXene phase has a higher modulus and lower swelling rate compared to the soft polymer phase, contributing to the elastic modulus mismatch and self-wrinkling (Lu et al., 2017). Finally, we observed that the self-wrinkled hydrogels exhibited different wrinkling depth and distances, depending on the MXene content (Fig. 3d). An increased MXene loading led to increased wrinkling, indicating a facile way to modulate the extent of wrinkling.

Because of the formation of strong electrostatic interactions at the interface between the carboxyl group of PAA and the substrate, the LMBP hydrogels adhered strongly to various substrates, Fig. 3e (Gan

Fig. 3. a) Top and bottom layers of the self-wrinkling hydrogel. b) Different wrinkle depths observed by optical microscopy. c) Possible mechanism of surface wrinkling. d) Wrinkled surfaces for systems with given MXene content. e) Photos illustrating LMBP hydrogel adhering to various substrates. f) Values of the corresponding adhesive strength (shear mode).

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et al., 2019). A high adhesive shear strength was measured on wood, which is attributed to the large contact area and carboxyl hydroxyl hydrogen bonds (Fig. 3f). MXene and LM show regulatory effects on self- adhesive strength. The increased adhesion with MXene may be a result of the synergistic effect of the reducing and physical cross-linking of MXene (Fig. S12). The increased addition of MXene nanosheets, in particular, promoted a loosely hydrogel, resulting in exposed carboxyl groups. Meanwhile, the interlocking effect of MXene nanosheets with polymer molecular chains enhanced the adhesion (Wang et al., 2020).

Rheology and mechanical properties of LMBP.

The PAA/BNC double-network supported the stretchable hydrogel by physical entanglement. SEM images (Fig. S13) revealed a continuous polymer backbone with a 3D porous structure. Mapping images indicated a uniform distribution of S (BNC), Ti (MXene), Ga and In (LM) elements in the matrix, with no signs of aggregation. A small amount of LM nanoparticles packaged in the highly cross-linked BNC and PAA shells. Free deformation effectively dissipated energy under strain, producing a toughening effect, Fig. 4a.

Frequency sweep experiments in the elastic interval (strain of 1 %) showed G^′>G^′′, ω of 1–100 rad s⁻¹, which indicated that the hydrogels (3-h incubation time) behave as a solid (Fig. S14). LMBP hydrogels exhibited a strong ω-dependent modulus even with high LM content.

This was especially the case at high frequency, which is related to the presence of LM nanoparticles and the displacement of MXene nanosheets. The G^′and G^{′ ′} of a LMBP hydrogel formed after 48 h gelation time were slightly higher than those formed upon gelation for 24 h, which may be due to the continuous rupture of LMNPs and the release of Ga³⁺. In a strain amplitude scan, the G^′ and G^{′ ′}dropped rapidly and intersected at the critical strain region (~200 % strain), indicating a sol-

gel transition and the collapse of the bonds between polymer chains (dissociation of hydrogen and coordination bonds) leading to excess strain (Fig. 4b). G^′ and G^′′ were significantly enhanced with the increased LM content, demonstrating the impact of crosslinking degree on the mechanical properties.

As shown in the stress-strain curves, the tensile elasticity of LMBP hydrogel can be tuned mainly by the LM content (strain up to 3200 %– 800 %). The increased LM content caused a reduction of the tensile strain, which is attributed to the enhanced coordination bonding between LM or Ga³⁺and the polymer chains. However, excessive LM may cause defects in the matrix and give rise to the extreme crosslinking, resulting in uneven stress distribution during stretching. The 1 wt%

LMBP hydrogel showed a good set of properties, including high stretchability (2350 % strain), good toughness (2.77 MJ/m³) and low Young's modulus (72.97 KPa). MXene also acts as a physical crosslinking agent in the hydrogel, together with LM. As shown in Fig. 4e, MXene anchors the supramolecular network or accumulate, creating defects in the matrix, eventually reducing the strain (Hu et al., 2023).

High strain rates led to high stress and reduced strain, suggesting dynamic cross-linked networks with limited energy dissipation (Fig. S15).

The deformable EGaIn droplets dissipated energy during hydrogel stretching, making the hydrogel insensitive to notching. Fig. 4f shows a notched LMBP hydrogel that supported a 500 % strain with a slight decrease in maximum stress (only 6 %). This is because when exposed to external forces, the polymer network tightens along the force applied at the notch, transferring the stress to the LM nanoparticles (Hu et al., 2021). The flexibility allows the hydrogel to deform reversibly along the stress, thus redirecting the tear along stretch and preventing crack propagation, improving the fracture energy (Fig. S16). In addition, an

Fig. 4.a) Schematic diagram of LM nanodroplets under stretching. b) Strain scan (G^′, storage modulus, solid symbol) and (G^′′, loss modulus, open symbol) of LMBP hydrogels. c) Stretchability of 0.5–2.0 wt% LM hydrogels. d) Elastic modulus and toughness. e) Effect of MXene concentration on the mechanical strength. f) Notch insensitivity and notched hydrogel under stretching (inset). g) Mechanical strength of original hydrogel and after healing and optical micrographs of wounds (inset).

h) Cyclic G^′and G^′′. i) Electric resistance vs time during the cyclic breaking - healing. j) Electrical resistance upon circuit repair. k) Simplified hydrogel network to show the self-healing mechanism.

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6 increased stretching (strain amplitude of 300 %) did not show a significant drop in load, which indicated excellent strain recovery via energy dissipation (Fig. S17). LMBP hydrogels showed good elastic recovery at low strains, as evaluated by the load drop in cyclic tensile tests at a strain of 100 % (load drop of 8 %), 500 % (load drop of 17.6 %) and 1000 % (load drop of 38.6 %). We note a pronounced hysteresis loop in the first cycle, especially under large strains, which remained nearly unchanged in subsequent cycles. This is partially attributed to the irreversible energy dissipation pathway by means of polymer chains, LM and MXene combination or physical bonds dissociation. Owing to the dual network, the hydrogel exhibited good compressive toughness, fatigue resistance and recovery (Fig. S18).

2.5. Hydrogel self-healing

The LM/MXene-mediated dynamic interactions impart the LMBP hydrogel with remarkable self-healing capabilities. All hydrogel samples were incubated for 24 h, and gently cut and placed in contact, without pressing. The fractured hydrogel specimens self-healed after physical contact for 6 h at room temperature, with no need for external inter- vention. The healed wound withstood stretching (>1000 % strain), bending and loading (100-g metal block), Fig. S19. The tensile profiles

and direct observation of the wound showed a positive correlation between self-healing efficiency and healing time, as well as a rapid mechanical recovery, within 12 h (Fig. 4g). Meanwhile, we used a successive alternate step-strain scan to further investigate the rheological self-healing behavior (Fig. 4h). When the LMBP hydrogel was sub- jected to a critical amplitude strain of 200 %, the G^′ dropped with simultaneous increase in G^′′. Hence, the values of G^′and G^′′intersected, indicating a sol-gel state and the collapse of a 3D polymer network. They recovered immediately to the original values once the strain was reduced to 10 %.

The real-time resistance of LMBP hydrogel wires showed that, once contacted, the conductivity was immediately reinstated, within 0.2 s (see LED lights). The resistance dropped from infinity to the initial value during cutting - contacting processes, which was repeatable (Fig. 4i).

This property allowed the hydrogel to rapidly power a device in the circuit through autonomous repair, such as restoring the brightness of LED lights in a parallel circuit, Fig. 4j. The self-healing efficiency, both mechanical (≈87.3 % and ≈97.6 %) and electrical (≈100 %) were evaluated by the ratio of the strain and stress at the fracture and the resistance after contact to the initial value, respectively (Fig. S19). The mechanism of self-healing involved dynamic interactions between the polymer network derived from LM and MXene crosslinking. Fig. 4k

Fig. 5. Multiple sensors and a flexible electroluminescent device based on LMBP hydrogels. a) Resistance vs strain profile, △R: resistance difference, R0: original resistance. b) Signal stability upon 200 stretch-release cycles at 80 % strain. Sensing to c) finger bending, d) swallowing and e) winking. f) Sensing to specific writing.

g) Strain-dominated 3D force mapping. h) Patternable electroluminescent devices.

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shows the dissociation and reorganization of dynamic bonds in the hydrogel network, mainly based on hydrogen (between PAA and MXene, PAA and BNC, PAA and PAA) and ionic (between Ga³⁺and PAA) bonds. In addition, a few LM nanoparticles embedded in PAA, and BNC accelerated their interaction with MXene and PAA upon breakage. This may continuously produce free radicals and further initiate possible covalent bonding at the wound. This unique self-healing explains the high tensile strength of the hydrogel compared to its original value.

2.6. Demonstration as sensors and patternable displays

The high conductivity of MXene (211 Sm⁻¹for the MXene/BNC ink) make the LMBP hydrogels conductive. The conductivity increases with MXene content but in the 0.06 to 0.37 S/m range, comparable to other MXene-based hydrogels (Fig. S20, Table S1). This suggest the potential for applications in flexible electronics, as demonstrated next in human motion monitoring, handwriting recognition and haptic effects.

In a typical sensor design, a hydrogel strip in series with LED was integrated in a circuit, and the visual variation of LED brightness indicated a high strain sensitivity (Fig. 5a). LMBP hydrogels' gauge factor (GF, defined as the ratio of resistance changes to strain, ΔR/R0 to ε_{) was} used to quantitatively assess the sensitivity of the sensors, which was fitted to two linear regions. To be specific, the GF was 3.1 over a strain range of 200 % and it increased to 8.1 when exposed to larger strains.

The GF values measured under the same conditions for MXene-PAA hydrogels thermally initiated with ammonium persulfate were 1.67 and 5.02, indicating that the surface structure had a positive effect on sensitivity and signal-to-noise ratio. The strain sensitivity is mainly attributed to the changes in the electron transport paths between the MXene nanosheet (low strain), expansion of ion migration channels (large strain) and the extension of wrinkles upon stretching (Wang et al., 2019). Fig. 5b shows a high electrical signal repeatability during 200 consecutive cycles at a strain of 80 %.

We further take advantage of the fact that LMBP hydrogels are moldable, self-healing and stimuli responsive. For instance, we tested LMBP hydrogel as resistive motion sensor, e.g., to detect human activity at a high level of sensitivity, including finger motion, among others (Fig. 5c). Small skin displacement, during swallowing and winking was rapidly tracked from a highly repeatable resistance variation waveform, Fig. 5d, e. When sandwiched between two PET films, the hydrogel tracked the process of writing and followed the load and direction, indicating the possibility for handwriting security applications (Fig. 5f).

Notably, the self-adhesive nature of the LMBP hydrogel allows it to adhere directly and firmly to skin or other soft substrates, and it deforms synchronously with the skin or substrate due to the matching low modulus.

The LMBP-based sensors were resistant not only to water but other solvents, including ethanol and acetone. However, the electrical resistance depended on the solvent type, for instance the free ions in water produced higher conductivity compared to that in ethanol (Fig. S21).

A sensor grid of 9 ×9 pixels was constructed with the LMBP hydrogel to demonstrate force mapping, Fig. 5g and Fig. S22. For instance, gravity-driven tensile strain was generated around a ball dropped on a grid and the variation of the electrical resistance, following the orthogonal strips the ΔR/R0 values were collected and 3-dimensionally patterned in force mapping. The results accurately reproduced the deformation of the mesh, akin to the shape of the given item. We note that hydrogels made into thin sheets or those with no Mxene (namely, LBP) or hydrogels with low MXene content, showed high light transmittance (Fig. S23). Such property expands the use of the hydrogels to flexible displays. As shown in Fig. 5h and Video 4, we assembled a three- layer hydrogel-based flexible electroluminescent device based on LMBP (bottom) and LBP (top) hydrogels with ZnS:Gu in the middle. A stable electric field formed between the two layers under a high frequency AC voltage. As a result, the middle layer became bright given the luminescence cause from electron leap.

3. Conclusion

LM nanodroplets and MXene nanosheets were integrated with bacterial nanocellulose and PAA in a 3D hydrogel network formed by sonication. During the sonication, multilayered MXene was exfoliated in-situ, forming well-organized conductive pathways; meanwhile, the LM nanodroplets were stabilized by BNC and PAA, forming a hydrophilic shell that prevented macrophase separation. Due to the modulus mismatch between MXene and the surrounding soft polymer matrix, wrinkled microstructures were spontaneously induced on the surface of the hydrogel. In an acidic gel matrix, the deformed LM nanodroplets released Ga cations together with MXene on polar surfaces to form strong dynamic crosslinks in the dual network. This unique cross-linking combined with the inorganic nanoparticles provided excellent interfacial adhesion, ultra-stretchability (2350 %, 178 KPa), notch insensitivity, high self-healing efficiency and multi-sensory capabilities.

Considering its unique features, the synthesized LMBP hydrogel is introduced, as a proof-of-principle, for uses in soft electronics, including wearables, force mappings devices, multi-sensors and patternable electroluminescence systems.

4. Experimental methods 4.1. Chemicals and materials

Bacterial nanocellulose and LM (Ga: In =3:1, EGaIn) were provided by Hainan Yide Food Co., Ltd. (Hainan, China) and Shenyang Jiabei Trading Co., Ltd. (Shenyang, China) respectively. Ti₃AlC₂(97 %) pow- der, sulfamic acid (99 %), N, N-Dimethylformamide (DMF, 99.5 %), lithium fluoride (LiF, 40 %), HCl and AA were purchased from Sigma Aldrich (USA).

4.2. Preparation of anionic bacterial nanocellulose

Anionic bacterial nanocellulose suspension (1.05 wt% concentration, 1.4 mmol/g –OSO3−) was prepared following previous work (Wang, Feng, et al., 2021). Briefly, a mixture of freeze-dried bacterial cellulose, DMF and sulfamic acid was stirred for 2 h at 80 ^◦C. The products were washed with ethanol and dialyzed to remove impurities. Finally, the system was microfluidized (five passes) =using a high-pressure micro- fluidizer homogenization. For simplicity, this sulfamic acid-modified bacterial nanocellulose is thereafter referred to as BNC.

4.3. Synthesis of LMBP hydrogel

Hydrogels combining LM, MXene, BNC and PAA (thereafter termed as LMBP) included LM (0.5, 1.0, 1.5, 2.0, 3.0 wt% with respect to the final mixture) and multilayered MXene (0.425, 0.85, 1.275, 1.7 mg/ml with respect to the final mixture). The LM and MXene were added to the BNC suspension and sonicated (BRANSON, SFX550; power of 300 W with 50 % amplitude, pulse mode: 3 s on and 1 s off) in an ice bath for 2 min. The LMBP hydrogels were obtained by continuously sonicating the system in the presence of AA (25 wt% with respect to the final mixture).

The gelation process was carried out under ambient conditions.

Following a similar procedure, LBP hydrogels were also produced in the absence of MXene.

4.4. Force mapping and electroluminescence

A 9 ×9-pixel haptic sensing array was assembled by cross-aligned hydrogels. The electrical signals of each hydrogel wire, before and after force loading, were monitored by using a digital source meter (Keithley 2450), and then were collected as ΔR/R0 data. The load applied to each crossing point was defined as the absolute ΔR/R0

product of the two intersecting hydrogel strips.

The flexible electroluminescent device was structured into three

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8 layers, where the middle light-emitting layer (spin coating ZnS:Cu with PDMS) was sandwiched between transparent LBP and LMBP hydrogels.

The top or bottom layer can be tailored into shapes or letters to display given patterns or characters. The two wires of the output device contacted the top and bottom layers and switched the excitation mode (Yang & Yuan, 2019).

Supplementary data to this article can be found online, including XPS fine spectrum of F 1s and Ti 2p from delaminated MXene, Ga 3d and In 3d from LM; Zeta potential of sulfamic acid-mediated BNC, MXene/BNC and LM/BNC; SEM images of LM/MXene-BNC ternary mixture solution;

AFM and SEM images of single-layer MXene/BNC nanosheets; SEM images of core-shell structure LM-BNC nanodroplets; Digital photos of gelation process; Adhesion strength by tensile machine test; Optical micrographs of LM-BNC-AA mixture and after polymerization; EPR measurements of the LM/MXene-BNC-AA mixture after sonication;

Infrared thermal imaging map of the prepolymer during gelation; Digital photos of wrinkled hydrogel and LM/Mxene-BNC-AA mixture with 1.7 mg/ml MXene; Digital photos of freshly gelled hydrogel; Adhesion strength by tensile machine test; SEM image of the surface and 3D structure of LMBP hydrogel and its element mapping; Rheological testing of LMBP hydrogel; Typical strain stress curves; Continuous cyclic stretch-recovery test; Compression elasticity test; Digital photos of self- healing hydrogel and the healing efficiency; hydrogel as a liquid sensor;

Compression-dominated 3D force mapping; transmittance of hydrogel with low MXene content or its thin film. In addition, four videos are provided. Supplementary data to this article can be found online at doi:

10.1016/j.carbpol.2022.120330.

CRediT authorship contribution statement

Ming Wang: Conceptualization, Methodology, Investigation, Formal analysis, Data curation, Writing – original draft. Orlando J. Rojas:

Resources, Supervision, Funding acquisition, Writing – review & editing.

Like Ning: Methodology, Formal analysis. Yuehu Li: Investigation, Formal analysis. Xun Niu: Methodology, Formal analysis. Xuetong Shi:

Investigation, Formal analysis. Haisong Qi: Conceptualization, Re- sources, Supervision, Funding acquisition, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

No data was used for the research described in the article.

Acknowledgment

The authors are grateful for the financial support for this work by Guangdong Province Science Foundation (2017GC010429) and Science and Technology Program of Guangzhou (202002030329). M.W., O.J.R, L.N., X.N. and X.S. gratefully acknowledge the Canada Excellence Research Chair Program (CERC-2018-00006),Canada Foundation for Innovation (Project number 38623) and ">H2020-ERC-2017-Advanced Grant ‘BioELCell’ (788489).

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