Ex situ synergistic reinforcement of a MOF-based supramolecular polymer enables tough, highly flexible, and responsive artificial epidermis-inspired hydrogels

Abstract

Metal–organic frameworks (MOFs) have numerous applications, such as energy storage, medical delivery, and wastewater remediation. Nevertheless, their applications to conductive hydrogels are restricted due to their limited dispersion, and aggregation within the network leading to limited stretchability, susceptibility to damage during cyclic activities, and reduction in the lifetime of the sensor. To overcome these issues, the selection of a suitable solvent medium, surfactant, and surface chemistry of the polymer chain in the hydrogel network are the primary factors. Herein, we created a flexible and tough, poly(lauryl methacrylate-acrylamide)@cobalt-manganese metal organic framework supramolecular composite hydrogel. Cetyltrimethylammonium bromide (CTAB), a cationic surfactant, and polymer chains facilitated the uniform distribution of the Co-Mn-MOFs. Hydrophobic interaction along with the other supramolecular interactions leads to synergistic reinforcement of the MOFs leading to uniform dispersion of the MOFs. The subsequently produced Co-Mn-MOF-based supramolecular hydrogels display remarkable anti-fatigue resistance, ultra-stretchability (1655%), toughness (447 kJ m⁻³), high conductivity of 0.33 S m⁻¹, and a strain detection range from minute to large (0.5–700%) with a gauge factor of 9.47, and can be sculpted in diverse 3d-like designs. A sensor was developed and utilized to detect various human actions, and varied levels of strain and pressure, and was applied as an artificial epidermis. We believe that this approach has potential for creating wearable strain sensors, artificial skin, and MOF-based smart electronics.

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1. Introduction

The field of soft and flexible electronics has garnered significant attention from a wide range of industries, such as wearable electronics,^1,2 energy storage,³ artificial intelligence,^4,5 soft robotics,^6,7 and others. Sensors with skin-like properties in soft and flexible electronics have shown potential for various applications, such as humanoid robotics,⁸ human–machine interfaces,⁹ and personalized health monitoring.¹⁰ These sensors can convert external stimuli, such as temperature changes, humidity, and mechanical signals, into detectable electrical signals.^11,12 Recently, researchers have created skin-like materials with high sensitivity by blending conductive components, including metal particles,¹³ carbon nanomaterials, and conductive polymers into flexible polymer matrices.¹⁴ Nevertheless, the complex deformations of natural skin are challenging to adapt, as these strain sensors are less resilient and flexible than human skin.^15,16 Consequently, there is a reduction in detection range and a decrease in sensing stability. Designing highly flexible and elastic sensors with exceptional fatigue resistance is crucial to developing intelligent wearable applications. Conductive hydrogels, known for their remarkable biocompatibility and conductivity, offer a suitable framework for developing strain sensors that mimic the properties of human skin.¹⁷ Nevertheless, hydrogel-based wearable sensors have been shown to be impractical for real-world applications due to their insufficient toughness, limited stretchability, and inadequate fatigue resistance.¹⁸ These defects lead to limitations in the sensing range and reduced durability. A recent objective has been to produce highly durable and flexible hydrogels by integrating carefully engineered network structures, such as macromolecular micro-particles with reversible non-covalent interactions,¹⁹ including ionic bonding,²⁰ hydrogen bonding,²¹ electrostatic interaction,²² and hydrophobic interactions.²³ Various types of hydrogels, including double-network, hydrophobically associated,²⁴ ionically cross-linked, and composite hydrogels,²⁵ have been created to address these challenges. Composite hydrogels have exceptional elongation properties and can be further enhanced by introducing conductive nanoparticles to boost conductivity.²⁶ Electrostatic attraction or hydrogen bonding with nanoparticles predominantly drives the hydrogel network's performance. Conversely, composite hydrogels exhibit substantial hysteresis and low mechanical strength due to the presence of water in the hydrogel network, which results in fragile connections. Remarkable mechanical and electrical characteristics make hydrophobic materials, including graphene,²⁷ carbon nanotubes,²⁸ nanomaterials, and MOFs, be used extensively in developing composite hydrogels for wearable electronic devices, and strain and pressure sensors.²⁹

MOFs are porous crystalline materials that form through the self-assembly of metal ions and organic bridging ligands. Their porous patterns are well-defined and periodic, resulting in substantially larger surface areas than traditional porous materials. MOFs provide numerous chemically reactive sites, adjustable pore properties, and excellent thermal stability.³⁰ These materials have garnered interest in various applications, such as gas storage and separation,³¹ adsorption,³² chemical sensing,³³ catalysis,³⁴ battery technology,³⁵ and hydrogel strain sensors.³⁶ The poor solubility of MOFs in water and their aggregation in hydrogel synthesis can be resolved by boiling water. However, the MOF structure and interactions with polymer chains may be influenced by heating. An alternative method is to create durable and flexible MOF-polymer composites, which can be achieved by incorporating MOFs into polymer substrates. This technique overcomes the drawbacks of simple blending, including particle aggregation and insufficient particle–polymer matrix interactions.³⁷ Nevertheless, the overall performance of MOF-polymer composites can be impacted by particle aggregation and poor particle–polymer interactions resulting from the straightforward blending of these materials. Establishing a standardized and straightforward method for preparing ex situ MOF-based composite hydrogels is essential to enhance the mechanical performance of wearable electronic devices, such as strain, pressure, and skin-like sensors, thereby reducing the effort.

The present investigation produced hydrophobically cross-linked cobalt manganese (Co-Mn-MOFs-based) supramolecular hydrogels that exhibited exceptional elastic properties, outstanding repeatability, and fatigue resistance. The Co-Mn-MOFs were synergistically reinforced into poly(LM-AAM) hydrogels with the help of CTAB and magnetic stirring. CTAB (cationic surfactant) enhances the uniform dispersion of Co-Mn-MOFs in the hydrogel network due to development of strong supramolecular interactions involving hydrophobic cross-linking, hydrogen bonding, and electrostatic interaction within the material. This led to improved mechanical properties and conductivity. The [poly(LM-AAM)@Co-Mn-MOFs] supramolecular hydrogels exhibited exceptional conductivity, flexibility, fatigue resistance, stretchability, and ultra-robustness. We utilized the composite hydrogels to create strain sensors, which display exceptional sensitivity as a strain/pressure sensor, possessing a broad range of detection capabilities and outstanding resistance to fatigue. The technology recognizes large-strain human movements (such as flexing of the finger, wrist, elbow, punch making, hand closing, and opening) and small-strain movements (such as writing, speaking, and drawing IUPAC structures) without lag. The sensor is highly responsive and reliable, similar to the mechanical and sensory properties of human skin with exceptional sensitivity and stability making it well-suited for measuring a range of forces, such as those encountered by pressure and artificial skin.³⁸

2. Materials and methods

2.1. Materials

Lauryl methacrylate (LM), cetyltrimethylammoniumbromide (CTAB), ammonium persulfate (APS), tetramethylethylenediamine (TEMED), and lithium chloride (LiCl) were purchased from Sigma Aldrich. Acrylamide (AAM) was obtained from Acros, and the previously prepared cobalt manganese metal–organic frameworks (Co-Mn-MOFs) were used.³⁸ All the chemicals were used without further purification. Double-deionized water (DW) was utilized throughout the entire experimental work.

2.2. Fabrication of hydrophobically associated hydrogel

The current study used the free radical micellar crosslinking copolymerization of AAM, LM, and CTAB in aqueous solutions to synthesize poly(LM-AAM@Co-Mn-MOFs) supramolecular hydrogels. Initially, 0.4 g of CTAB and 0.25 g of LiCl were dissolved in 10 ml of DW in five different beakers at 25 °C by magnetic stirring at 350 rpm. After 15 minutes, different concentrations of Co-Mn-MOFs (1%, 2%, 3%, 4%, and 5%) were loaded into each beaker separately and the mixtures were stirred again for at least 5 hours as MOFs are crystalline in nature, and their full dispersion in the solution takes time. Each mixture was charged with 200 μl LM and 2 g of AAM, and after entire dissolution of each reagent, 0.05 g of APS and 20 μl of TEMED were added as a source of free radical initiator and accelerator, respectively. The homogeneous solution was then transferred to a mold with a 35 mm width and 80 mm length, and kept in an oven at 60 °C for 1 hour to complete the polymerization. The prepared hydrogels were peeled off from the mold and saved in airtight sample bags for further use. For the contrast study, all the chemicals were kept constant except Co-Mn-MOFs. The prepared samples were coded as MOF1, MOF2, MOF3, MOF4, and MOF5, where the number represents the % concentration of Co-Mn-MOFs. For a comparative study, a sample was synthesized without the addition of MOFs, named as MOF0.

2.3. Characterization

For characterizing the composite hydrogels, scanning electron microscopy (SEM, Jeol Japan) and Fourier transform infrared (FT-IR, Anton Paar, Germany) analysis were used. Before characterization, the samples were sliced and dried in an oven for 24 hours at 60 °C. SEM examined the hydrogel morphology, while FT-IR spectroscopy determined the critical functional groups present in the hydrogels in the 400–4000 cm⁻¹ range. A digital microscope (DM) was utilized to investigate the dispersion of MOFs in the respective supramolecular polymer hydrogel after its synthesis at a resolution of 600×.

2.4. Rheological analysis

The composite hydrogels were tested for viscoelastic behavior using an Anton Paar rheometer (Physica MCR 301) with a 25 mm diameter measurement probe. The rheological tests included an amplitude sweep investigation with strains ranging from 0.01 to 1000% at a constant frequency of 10 rad s⁻¹. The frequency sweep test was performed at frequencies ranging from 0.1 to 100 rad s⁻¹ maintaining the temperature at 25 °C. Eqn (1) was used to compute percentage increases and decreases in storage modulus.where X₂ is the critical strain of MOF4 and X₁ is the essential strain of MOF0.

2.5. Mechanical analysis

To examine the mechanical performance, the composite hydrogels were cut to known dimensions of 40 mm length, 10 mm width, and 1.09 mm thickness. A universal testing machine (UTM) with a capacity of 30 kN and a load cell of 500 N was used. During the analysis, the test rate was kept at 50 mm min⁻¹. Eqn (2) was used to calculate the area beneath the stress–strain curve and determine the toughness (U) of the hydrogels.The dissipation energy (ΔU) was calculated using eqn (3) and the area between the cyclic loading and unloading curves.where σ represents the stress or mechanical force applied to the hydrogel, and dε is the differential strain (change in length or deformation) divided by the materials initial length.

2.6. Electrical and strain sensing behavior

The strain-sensing properties of the prepared hydrogel were determined by an electrochemical workstation through AUTOLAB (Netherlands) using an electrode system. The composite hydrogel was cut at a length of 40 mm and width of 10 mm, and this hydrogel was placed between two copper electrodes and a potential of 1 V was applied. The hydrogel strips were put on the movement site for human motion monitoring and connected to the auto-lab. The movement of the human part causes a change in the hydrogel strip area, which produces a current signal detected by a potentiostat through chronoamperometry. These current signals were converted into resistance by using Ohm's law and subsequently converting them into relative resistance using eqn (4);where ΔR is the relative resistance, R₀ represents the initial resistance (no deformation), and R represents the resistance in real-time under specific applied strain (%).

The hydrogel electrical conductivity was measured using a Hioki LCR Meter (Hioki Japan) by applying eqn (5);

where ρ is the resistivity, R is the resistance, A is the cross-sectional area, and L is the length. The resistivity was converted into conductivity according to eqn (6),where A is the cross-sectional area and d represents the thickness of the hydrogel.

For strain/pressure sensitivity and human motion detection, the electrochemical workstation is used to record the applied strain and human movements in the form of current variations (ΔI/I₀) and subsequently converting them into relative resistance (ΔR/R₀). The strain gauge factor (GF) is defined and calculated according to eqn (7).

where R₀ and R are the initial and final resistances of the original and stretched hydrogels, respectively, and ε is the strain applied to the hydrogel.

2.7. A precursor for 3D printing

The synthesized composite MOF-based hydrogels were sculpted in different shapes utilizing different shapes molds. The precursor solution containing MOFs and other reagents was transferred to a 10 ml syringe and carefully poured into the molds of different shapes. These molds were then kept for the inside polymerization of the solution at 60 °C in an oven. The prepared 3D-like hydrogel shapes were investigated for strain sensing and human motion detection.

2.8. Informed consent

Informed consent was obtained for the experiments involving human participants.

3. Results and discussion

3.1. Synthesis of composite hydrogels

Hydrophobically cross-linked poly(LM-AAM)@Co-Mn-MOFs supramolecular hydrogels were created using a one-pot free radical polymerization method and ex situ micelle copolymerization of Co-Mn-MOFs, LM, and AAM, as depicted in Fig. 1. APS was used as a thermal initiator to start the polymerization reaction. As MOFs have poor solubility and remain undispersed in distilled water, micelles were created in an aqueous solution using a surfactant. The selection of surfactant is a crucial factor as it leads to the proper dispersion of MOFs. Looking to the nature of Co-Mn-MOFs, cationic surfactant CTAB was selected as the micelle producing agent. The linker used in the synthesis of Co-Mn-MOFs is trimesic acid (TMA), which carries negatively charged carboxyl functional groups on its surface. This negatively charged functional group on the Co-Mn-MOFs can be readily involved in electrostatic interactions with the cationic surfactant as well as create hydrogen bonding with the polymer network. This cationic surfactant can also provide room for producing hydrophobic interactions with the polymer and physical interaction that produces a stable and homogenous dispersion of Co-Mn-MOFs. However, in the absence of surfactant, Co-Mn-MOF could not disperse and produce a suspension with MOF particles settling down (Fig. 1). The micelles were stabilized by the addition of hydrophobic monomer LM, contributing to the hydrophobic association and ultimately producing a network structure by cross-linking numerous micelles to each other (Fig. 1). LiCl is added to impart ionic conductivity, making the hydrogel ideal for wearable strain-sensing applications. The addition of AAM further strengthens the hydrogels by producing a dual network by co-polymerization with the LM and creating a dynamic linkage through micelles. These links can provide the hydrogel with some intriguing characteristics. The produced composite hydrogels considerably lost energy under stress due to the reversible nature of these connections. It is clear that the various Co-Mn-MOF percentages successfully enhanced the hydrogel's mechanical performance through physical interactions with the (LM-AAM) polymer chains. However, due to the non-dispersive nature of Co-Mn-MOFs, the mechanical strength is negatively affected at high concentrations. In the current research, six samples were developed for extensively studying the composite hydrogels as MOF0 (control sample), MOF1 MOF2, MOF3, MOF4, and MOF5 based on varying the concentration of MOFs while keeping the other chemicals at a constant ratio. As the concentration of the MOFs was increased in the hydrogel matrix from 1% to 4%, the mechanical strength, conductivity, strain sensitivity, etc, were continuously increased, but this faced a decline in mechanical strength in the case of 5% MOFs. Therefore, in testing the composite hydrogels for applications, sample MOF4 was utilized.

Fig. 1 Synthesis of poly(LM-AAM)@Co-Mn-MOF composite hydrogels and the interactions involved in the hydrogel network.

The surface morphology of the composite hydrogel was examined using SEM, and the results are depicted in Fig. 2(a)–(c). The results demonstrate that MOF0 has a slightly rough surface with fissures visible in its structure. Adding 4% MOF significantly impacts the hydrogel surface morphology, increasing porosity and forming a well-patterned porous structure. The Co-Mn-MOFs are responsible for the change in surface morphology due to their increased porosity. The well-patterned structure results from the newly generated cross-linking points of the Co-Mn-MOFs. The hydrogels high porosity facilitates the conduction of ions and electrons, hence improving their conductivity. The surface of the composite hydrogel was further investigated with the DM to observe the difference between surfaces of MOF0 to MOF5. Fig. 2(d)–(f) shows the proper dispersion of MOFs in the supramolecular hydrogel network. In contrast, the dispersion of MOFs in MOF5 is improper leading to a decrease in the hydrogel mechanical properties and other characteristics.

Fig. 2 (a) SEM images of MOF0 at 5 μm; (b) SEM image of MOF1 at 5 μm; (c) SEM images of MOF4 at 50 μm; (d) DM image of MOF0 at 600× resolution; (e) DM image of MOF1 at 600× resolution; (f) DM image of MOF4 at 600× resolution; (g) DM image of MOF5 at 600× resolution.

The chemical structure of the hydrogels (MOF0, MOF1, and MOF4) was analyzed in the context of using FT-IR in the range of 400 cm⁻¹ to 4000 cm⁻¹ (Fig. 3(a)). The characteristic peaks at 3377 cm⁻¹ and 1671 cm⁻¹ of MOF0 are attributed to the stretching vibrations of N–H and C=O groups on AAM and LM in the composition of the control sample, respectively.³⁹ Notably, N–H stretching vibration is shifted from 3377 cm⁻¹ to 3350 cm⁻¹, and the C=O stretching vibration peak shifted from 1671 cm⁻¹ to 1645 cm⁻¹ in the case of MOF1 and MOF4, indicating the interaction of Co-Mn-MOFs with the N–H and C=O functional groups. Such a visible red and then blue shift confirms the formation of hydrogen bonding between the Co-Mn-MOFs and the AAM (NH) and LM (CO) functional groups. The only difference found in the spectra of MOF1 and MOF4 is the reduction in the peak intensity, which happens due to higher concentration of MOFs in the hydrogel composition. This higher reduction in the peak intensity represents the involvement of Co-Mn-MOFs in the formation of hydrogen bonding, which reduces the free movements of bonds and vibrations, thereby decreasing the peak intensity. This indicates the successful incorporation of Co-Mn-MOFs into the polymer chains in the hydrogel matrix. Similarly, the peaks at 2921 cm⁻¹ and 2850 cm⁻¹ show the stretching vibration of methylene groups (CH₃ and CH₂) of lauryl methacrylate. The peak at 1464 cm⁻¹ signifies the bending of C-H, and this region also contains carboxylate groups of the Co-Mn-MOF hydrogel. The peak at 3350 cm⁻¹ is about to disappear in MOF4, further confirming the incorporation of Co-Mn-MOFs into the hydrogel matrix. The existence of all the characteristic peaks confirms the successful preparation of hydrophobically associated composite hydrogels based on MOFs.¹¹

Fig. 3 (a) FT-IR spectra; (b)–(d) rheological investigation of the MOF4-based hydrogel's shear strain (%) and frequency sweep.

The impact of Co-Mn-MOFs on the hydrogels was investigated by analyzing their rheological properties. Amplitude sweep (AS) and frequency sweep (FS) experiments were conducted at room temperature to assess the hydrogels viscoelastic behavior. During the strain range of 0.01–1000% in AS, the storage modulus (G′) and loss modulus (G′′) were determined. As shown in Fig. 3(c), at low stresses (0.01–17%), G′ was greater than G′′, indicating the solid-like behavior of both the MOF0 and MOF4 composite hydrogels. However, as the strain increased, G′ declined, and G′′ increased, indicating a crossover point where the moduli inverted, and the hydrogel system began to behave more like a liquid. These findings demonstrate the characteristic viscoelastic behavior of the hydrogels. Additionally, Fig. 3(c) highlights the effect of Co-Mn-MOFs on the rheological characteristics of the hydrogels, with sample MOF4 showing a 154% increase in G′ and a 153% increase in G′′ compared to MOF0. This increase suggests the non-covalent interactions between the Co-Mn-MOFs, polymer chains, and surfactant. In the FS test, G′ and G′′ for samples MOF0 and MOF4 were examined over a frequency range of 0.1 to 100 rad s⁻¹ at a constant amplitude of 5% (Fig. 3(d)). The observation showed a continuous increase in G′ and G′′ at low frequencies (0.1–10 rad s⁻¹), indicating solid-like behavior, followed by a slight decrease at higher frequencies, suggesting viscoelastic behavior. Consistent with the strain sweep results, sample MOF4 exhibited greater G′ and G′′ values than MOF0, suggesting that Co-Mn-MOFs significantly reinforce and enhance the rheological properties of the hydrogels.

3.2. Mechanical properties of the hydrogels

One of the important parameters for smart flexible electronics and artificial epidermis skin-like sensors is their stretchability. The mechanical properties of the composite hydrogels were investigated both by manual operation in the laboratory and by UTM. The results are accurately illustrated in Fig. 4, which demonstrates the impressive ability of composite hydrogel MOF4 to maintain its structure and stretchability in various situations, such as knotting, twisting, crossover stretching, puncturing, and rolling tests, showcasing its exceptional flexibility and durability. The hydrogel structure durability is enhanced by numerous non-covalent binding sites responsible for these resistant attributes.

Fig. 4 Photographs of the MOF4 hydrogel in different forms: (a)–(c) stretchability in knot, twists, and cross-over (d) puncturing test, and (e) four times rolling around a pen.

The hydrogel's mechanical performance was further checked by different tests, including the tensile and cyclic tensile testing of continuous loading-unloading. Various measurements, such as fracture stress, fracture strain, elastic modulus (Young's modulus), toughness, and fracture energy, were estimated from the stress–strain curves. Fig. 5(a) shows that with the addition of Co-Mn-MOFs, the mechanical performance of the composite hydrogels increases with superior stretchability for MOF4, and then a sudden decrease was observed. The MOF4 hydrogel was found to have the maximum elasticity, exhibiting an impressive strain of up to 1655%. It can be deduced that adding Co-Mn-MOFs strengthens the hydrogels network due to the higher cross-linking density of Co-Mn-MOFs with the hydrogel network. Fig. 5(b) shows fracture strain and fracture stress values, with the higher being 1655% and 689 kPa in the case of MOF4, respectively. A rise in fracture strain and stress is due to the Co-Mn-MOF concentration, which enhances the physical cross-linking points between the polymer chain and the micelle via hydrogen bonding, resulting in enhanced mechanical properties. Similarly, the fracture energy increases from 18.81 kJ m⁻³ (MOF0) to 457.03 kJ m⁻³ (MOF4), as depicted in Fig. 5(c). However, when the Co-Mn-MOF content increased to 5%, the fracture energy dropped to 71.31 kJ m⁻³, suggesting that the hydrogel network shrank due to the uneven distribution of MOFs. This may be due to the agglomeration of MOFs in the hydrogel network. The elastic modulus (Young's modulus) varied with the MOF percentage, increasing steadily from 0.2558 kPa to 1.2149 kPa as the MOFs percentage increased from 0% to 4%. At 5% concentration, Young's modulus decreases to 1.11 kPa due to the accumulation of Co-Mn-MOFs inside the composite hydrogel (Fig. 5(c)). The toughness increases from 18.569 kJ m⁻³ to 446.563 kJ m⁻³ for MOF0 and MOF4, facing a sudden fall to 71.317 kJ m⁻³ for MOF5, as shown in Fig. 5(d). Cyclic tensile experiments were performed to investigate the energy dissipation mechanism in the hydrogels. Fig. 4(e) shows the consecutive fatigue state loading–unloading cycles for MOF4 at 300% strain without any resting periods between cycles. During the first cycle (Fig. 5(f)), the lost energy was 5.1 kJ m⁻³, indicating high resilience. The wasted energy during the second, third, fourth, and fifth cycles was 2.4 kJ m⁻³, 1.9 kJ m⁻³, 1.7 kJ m⁻³, and 1.1 kJ m⁻³, respectively. Similarly, Fig. 5(g) depicts consecutive cyclic tensile curves for MOF4 at 500% strain without resting periods between cycles. The results demonstrate that as the number of loading–unloading cycles increased from the first to the fifth cycle, the dissipation energy decreased steadily until it reached a stable value (Fig. 5(h)). During the first cycle, the lost energy was 9.1 kJ m⁻³, which is modest considering the stress of 127 kPa, indicating that the hydrogels is highly resilient. During the second and third cycles, the wasted energy decreased to 3.3 kJ m⁻³ and 3.2 kJ m⁻³, respectively. By the fourth and fifth cycles, the wasted energy was reduced to 2.9 kJ m⁻³ and 2.8 kJ m⁻³, respectively. This decrease is due to internal structural changes in the hydrogel network, where polymer chains begin to slide and elongate in the direction of applied stress, followed by the disruption of cross-linking networks (hydrophobic association and hydrogen bonds). A prominent hysteresis loop was observed during the first tensile cycle, indicating significant energy loss due to fracture in the polymer networks. When the tension was removed, the polymer networks were easily recreated. The hysteresis loops nearly overlapped from the second to the fifth cycle, indicating that the first cycle's structural changes were retained. Fig. 4(i) shows a comparative graph of dissipated energy for MOF4 at 300% and 500% strain, indicating better energy dissipation at 500% strain.

Fig. 5 Mechanical study of composite hydrogels: (a) tensile test of the hydrogels; (b)–(d) derived parameters from the tensile tests, fracture strain-stress, fracture energy, Young's modulus, and toughness; (e) and (f) fatigue resistance test of composite hydrogel MOF4 (cyclic loading-unloading of up to 5 cycles) and dissipated energy at 300% strain; (g) and (h) cyclic loading-unloading test of up to 5 cycles and dissipated energy at 500% strain; (i) comparative graph of dissipated energy at 300% and 500% strain.

3.3. Strain sensitivity

Stretchable electronics and skin like sensors require heightened sensitivity and enhanced reflex time. MOFs are one of the highly porous and crystalline materials and can greatly improve the sensitivity of the flexible medium. Co-Mn-MOFs are one of the excellent choices for improving strain sensitivity due to its highly porous, uniform, and crystalline nature resulting in superior conductivity and enhanced mechanical strength. Due to the improved conductivity (0.33 S m⁻¹) and mechanical strength of the MOF4 hydrogel, it was utilized for strain sensing applications. A specially made setup was used to observe strain sensitivity using a 2 V DC source connected in series with an LED (light emitting diode) and the hydrogel strip (Fig. 6). As the voltage was applied, Li⁺ and Cl⁻ ions in the hydrogels network started moving, displaying ionic conductivity and the LED lit up. Stretching of the hydrogels at 600% strain results in the enlargement of the conduction channels, which caused the LED to dim, showing ion movement. Fig. 6(a)–(c) shows the hydrogels outstanding strain sensitivity and reversibility when the stress was released, and it returned to its original length (0% strain), leading to an increase in LED brightness.²⁴Fig. 6(d) shows the conductivity of the prepared hydrogel at different concentrations, increasing from 0.172 S m⁻¹ to 0.4 S m⁻¹ with an increase in MOF concentration.

Fig. 6 Analysis of strain sensitivity of the composite hydrogel MOF4: (a)–(c) stretching effect to demonstrate the response of LED to various strains i.e. 0% and 600%; (d) conductivity; (e) response of the MOF4 hydrogel on a mobile screen by drawing structures and writing; (f) response towards a smart watch; (g) utilization as a metallic pen while performing a calculation on a phone calculator.

Due to this heightened conductivity, the composite hydrogel MOF4 was synthesized in a special type of mold to be worn on human fingers and check their interaction with a touch screen as shown in photographs Fig. 6(e) and (f). Multiple tasks were carried out, during which the MOF4 worked flawlessly, and no lagging in response was observed on mobile screens. This property enables the user to write and draw different alphabets and diagrams without harm or interruptions, showing that the composite hydrogel MOF4 can be used in smart artificial skin like technology. In addition, a metallic pen was made using the composite hydrogel MOF4 (Fig. 6(g)), and the results show that the screen starts responding to themetallic pen as the composite hydrogels is worn on its tip (Video S1, ESI†).

Due to exceptional overall performance, the composite hydrogel MOF4 shows great promise in strain sensing; therefore, a strain sensor was developed, and its sensing capabilities were evaluated. Initially, the sensor relative resistance change was measured under varying strains at both small and large levels, as shown in Fig. 7(a) and (b). The results show a noticeable rise in relative resistance when strain increased from 0.5% to 50% and subsequently from 100% to 700%. This suggests that as hydrogels are stretched; the cross-section decreases, displaying more resistance to electron flux and ions (Fig. 7(a)). Large (700%) and small (0.5%) strains are both effectively detected by the prepared composite hydrogels, which is much better than the previously reported literature.²⁴ The response and recovery times of the composite hydrogel MOF4 sensors were determined to be 0.1 s and 0.08 s, respectively (Fig. 7(d)), which is far better than the previously reported study.³⁹ The slow-fast stretching response of the hydrogels is shown in Fig. 7(e). It has been determined that the synthesized material can be utilized as a strain sensor in various wearable devices, soft electronics, and cyclic activities.⁴⁰

Fig. 7 (a) Small strain sensing performance from 10–50% strain (inset graph 0.5–1.5% strain); (b) large strain sensing performance from 100–700% strain; (c) response–recovery time of the strain sensor; (d) illustration of the strain sensitivity of the hydrogel system; (e) slow-fast stretching of the designed hydrogel; (f) gauge factor of the synthesized composite hydrogel sensor; (g) multiple cycle test (anti-fatigue resistance) at 100% strain.

The strain sensing performance of the composite hydrogel was further evaluated regarding strain gauge factor (GF), a crucial quantitative aspect for assessing the strain sensitivity of materials. As shown in Fig. 7(f), the applied strain immediately affects the GF, which has a value of 9.47 at 850% of applied strain. Fig. 7(f) shows a linear relationship between relative resistance and strain across different strain ranges. They exhibit a linear trend with ΔR/R₀ = 1.07543e + 0.00458e² with a regression coefficient (R²) value of 0.99 in the strain range of 0–850%. The hydrogel percentage strain causes the GF to increase, with the maximum being 9.47 at 850% strain.²⁴ It is crucial to notice that GF is directly related to the applied strain, as shown in Fig. 7(f), where the GF value is 1.62 (for 0–150% applied strain), 3.85 (for 151–350% strain), 6.08 (for 351–550% strain), and 9.47 (for 551–850% strain), showing the enhancement in the GF value as the applied strain increases. The obtained GF value at an applied strain of 0–850% is far better than the previously reported studies on strain sensors of this type^41,42 (Table 1). The multiple cycles of the prepared hydrogels were determined through continuous stretching and relaxing at a 100% strain for up to 10 minutes and performed 530 cycles to test the hydrogel anti-fatigue resistance (Fig. 7(g)). This suggests that the composite hydrogel MOF4 is highly reproducible and durable even after repeated stretching and releasing procedures.³⁹ Throughout the test, there were no appreciable current drops indicating that the composite hydrogel MOF4 is a best fit for cyclic activities.²⁴ All these results indicate the extremely sensitive nature of the composite hydrogel MOF4 suggesting its usage in smart skin like sensors and wearable technologies.

Table 1 A comparison of the primary performance indicators from the current research with those reported in recent studies of a similar kind

Sample	Gauge factor	Working range (%)	Response-recovery time (ms)	Stretchability (%)	Conductivity (S m^−1)	Ref.
PLMG hydrogels	4.21	800	200–99	1000	—	43
P(DA-AM AETAC)BM-MOFs	14.8	700	195–145	1588	1.3	38
XnG hydrogel	28.8	800	150–130	1100	0.21	44
TA@CNC-hydrogel	5.5	100	—	2900	—	45
HA hydrogel	3.54	500	198–300	1360	—	46
GG-reinforced conductive hydrogel	8.2	400	250–130	401.1	0.20	24
P(AAm-co-VI)/QCS-Fe^3+ ionic hydrogel	2.37	300	—	633	0.49	47
PAMC hydrogels	2.27	600	216–227	661.2	2.89	48
A-MWCNTs	9.2	500	110–130	1005	—	40
HPAAm/Cs-c-MWCNT hybrid hydrogel	1.65	100	180–151	3964	—	39
MXene/PHMP hydrogels	7.17	500	—	—	0.016	49
MA hydrogel	6.9	500	80–60	2102	0.20	50
P(LM-AAM)Co-Mn-MOF hydrogel	9.47	850	100–80	1655	0.33	This work

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3.4. Human motion monitoring

Due to outstanding sensing capabilities, compatibility with the human skin, and characteristics like flexibility, stretchability, and durability, the composite hydrogel MOF4 is appropriate for applications in human movement detection. A sensor was designed by cutting a strip of the composite hydrogel MOF4 measuring 40 × 10 × 1 mm³ and carefully positioned on specific human body parts, such as the finger, wrist, elbow, punch making, and on-hand (closing and opening) to track different human motions. The sensor was connected with a two-electrode system of a potentiostat to run chronoamperometry. Large human movements were detected by directly affixing the sensor with the volunteer's finger, wrist and elbow, and punch-making. The results in Fig. 8(a)–(e) demonstrate that it is possible to identify various body postures based on the signal waveforms of changes in resistance. The real-time response of the designed sensor to finger bending is illustrated in Fig. 8(a) and (b). Initially, the finger was bent at a single angle followed by bending at 0°, 30°, 60°, and 90°. The resistance stabilized at a specific value for each held angle when the hydrogel deformation remained constant. The hydrogel resistance progressively increased after bending the finger to various angles. When the wrist was bent and straightened, the resistance signal changed and returned to its initial value. It showed excellent repeatability and stability at bending angles of 0°, 60°, and 90° (Fig. 8(c)).⁵¹ Additionally, the sensor accurately responds to elbow movements at one-angle and the motion of punch making is illustrated in Fig. 8(d), and subsequent releasing and hand closing and opening were monitored in Fig. 8(e). The subsequent act of bending the sensor is shown in Video S2 (ESI†). The results assure that the composite hydrogel MOF4 is good at detecting large human motions.

Fig. 8 (a) A schematic illustration of sensing locations gives various types of human motion monitoring; (b) forefinger bending at one and different angles; (c) wrist bending at one and different angles; (d) elbow at one angle and punch making; (e) hand closing and opening; (f) response of the physiological applications of MOF4 hydrogels; (g)–(i) response of the hydrogel sensor to different speaking words; (j) response of the hydrogel sensor toward drinking lemonade; (k) response of the hydrogel sensor toward swallowing a sweet.

Sensors made with composite hydrogel MOF4 have high sensitivity, robustness, conductivity, and fast response-recovery time, making them valuable tools for monitoring small physiological human activities. In Fig. 8(f)–(k), the hydrogel sensor was affixed to the larynx to illustrate the vocal cord motion. The sensor's ability to identify and differentiate spoken words is demonstrated quite clearly in Fig. 8(f)–(i), and the sensor reacts accurately when the volunteer says the word MOFs, Nimra, University of Peshawar, and I Love University of Peshawar. The property of the hydrogel sensor to distinguish between various spoken words, and its repeatability by producing the same peaks when the word is repeated, make it an excellent fit for designing wearable technology for both small and large scale human motions. In addition, Fig. 8(j) and (k) illustrates that the sensor responded differently to drinking lemonade and swallowing sweets.

3.5. Real time writing and pressure sensor designed to track and analyze writing and different pressure levels

A sensor was designed by sandwiching the hydrogel between a two electrode setup to track and differentiate between the different words as they are written on the surface of the sensor (Fig. 9(a)–(e)). The composite hydrogel MOF4 sensor consistently responded to writing the alphabet, such as vowels in capital letters A, E, I, O, and U, as shown in Fig. 9(b). The sensor invariably responded to capital letters and could differentiate between them. Additionally, as shown in Fig. 9(c)–(e), the sensor reacted differently to drawing structures like cyclopropane, cyclohexane, and cyclobutane, as well as the linear structures of butene, butyne, and butane. The sensor also produced different responses to shapes like circles, triangles, and rectangles (Fig. 9(e)). Furthermore, in Fig. 9(f), the sensor demonstrates its sensitive behavior by successfully tracking the writing of constant symbols such as Ω (omega), ν (frequency), ρ (density), and γ (gamma). Similarly, the MOF4 hydrogel can serve as a sensitive pressure sensor. In Fig. 9(g), upon gently pressing the sensor by hand, it produces low intensity peaks, and this peak intensity gradually increased as the pressing force increased from gentle to hard confirming the pressure sensitivity of composite hydrogel MOF4. Similarly, Fig. 9(h) shows the response of muscle pressure while holding a heavy object, and the resistance goes up and down with the hand movement. Fig. 9(i) shows the up-down movement of the foot by pressing the composite hydrogel MOF4. These results confirm the versatility of composite hydrogels and their successful application in the electronic device industry.^24,52

Fig. 9 Writing and drawing abilities of composite hydrogel MOF4: (a) general representation of the writing sensor; (b) writing vowels; (c) and (d) drawing the IUPAC structure of cyclopropane, cyclohexane, cyclopentane and butene, butyne, and butane; (e) drawing different shapes (circle, triangle, and rectangle); (f) drawing symbols; (g) different pressure with a finger on the hydrogel; (h) muscle sensing pressure; (i) different pressure by foot.

3.6. Shape molding behavior of composite hydrogel MOF4

Shape adaptability and morphological flexibility is one of the demanding trends in smart wearable electronics. Shape molding hydrogels are used in actuators to mimic movements enabling lifelike motions and flexibility. These hydrogels play a significant role in artificial muscles and can be sculpted into different shapes of prosthetics. To test the shape adaptability of composite hydrogel MOF4, various shape molds were used and the composite hydrogel MOF4 successfully adopted that shape, as shown in Fig. 10. In order to demonstrate the complex shapes of MOF4 hydrogel, researchers created several objects using molds. However, in Fig. 10(a), the complex structures of the chemistry laboratory instruments and some aromatic structures were made using a mold that looks like 3d structures. The composite hydrogel was synthesized in an auxetic-like structure and utilized for strain sensing and human motion detection. Fig. 10(b) shows the response of the composite hydrogel MOF4 (auxetic like structure) towards small and moderate repeated stretching and subsequent relaxing in the horizontal direction. The auxetic-like structure of MOF4 undergoes elongation and subsequent recovery to the initial position after the strain is released, during which changes in the resistance are encountered and successfully detected by the sensor. Fig. 10(c) demonstrates that the composite hydrogel MOF4 molded into an auxetic-like structure can periodically detect the bending motion of a human finger. Fig. 10(d) demonstrates that the sensor consistently produced a similar response to the significant deformation during wrist bending activity. Under a state of rest, the sensor displayed minimal resistance. The subsequent act of bending the wrist elongates the sensor, resulting in increased resistance that indicated the level of activity exhibited by the volunteer. An intentional cut was made in the middle of the hydrogel molded into an auxetic-like structure during which the resistance increases with each successive cut (Fig. 10(e)). These results suggest that the composite hydrogels can be synthesized in various morphological forms with enhanced functionalities.⁵³

Fig. 10 Shape molding behavior of the composite hydrogel MOF4: (a) photographs of laboratory instruments and aromatic structures; (b) auxetic-like structure with small strain sensing and moderate strain sensing of the composite hydrogel; (c) auxetic-like structure hydrogels used to detect finger bending; (d) wrist bending detection capabilities of auxetic-like structure hydrogel; (e) cutting of the auxetic-like structure.

4. Conclusion

We have successfully dispersed MOFs in the solvent deionized water using cationic surfactant CTAB as a dispersion agent and prolonged magnetic stirring. The solution was utilized to produce supramolecular composite hydrogels based on Co-Mn-MOFs. CTAB functions as a dynamic linker in addition to acting as a dispersing unit, forming hydrophobic interaction between polymer chains and electrostatic interaction with the Co-Mn-MOFs in the hydrogel network leading to uniform dispersion of the MOFs. The supramolecular hydrogel shows improved mechanical characteristics with a notable toughness value of 446.563 kJ m⁻³, outstanding stretchability of approximately 1655%, and exceptional resilience against fatigue. Furthermore, the hydrogel's high conductivity made it sensitive, like artificial e-skin and strain sensors. The designed strain sensor demonstrated remarkable repeatability, a broad detection range (0.5–700% strain), and extraordinary sensitivity (GF = 9.47). The sensor, which resembles skin, has a lot of potential for monitoring multiple human activities and reacts to various pressures ranging from large-scale to minute. The novel methodology for the hydrogel synthesis described in this work establishes a strong basis for creating materials with comparable performance. This could be accomplished using diverse MOFs and hydrophobic fillers to enhance electrical gadgets. The unique combination of shape molding and strain sensitivity exhibited by the composite hydrogel enabled the development of wearable strain sensors. The flexibility in the morphology of the composite hydrogel offers significant opportunities for producing intricate and design-tailored strain-sensing components for monitoring human motion and creating intelligent devices like wearable electronics, sensors, and biomedical equipment.

Data availability

This work is original research, with primary research/new data available in the figures.

Conflicts of interest

The authors declare no conflict of interest to exist.

Acknowledgements

This research work was supported by Researchers Supporting Project Number (RSP2025R45) at King Saud University, Riyadh, Saudi Arabia. The authors wish to thank Researchers Supporting Project Number (RSP2025R45) at King Saud University Riyadh Saudi Arabia for financial support.

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Footnote

† Electronic supplementary information (ESI) available: Video 1: the screen response to a metallic pen when the composite hydrogel is worn on its tip. Video 2: the subsequent act of bending the sensor. See DOI: https://doi.org/10.1039/d4tc05163g

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