Amino acid-derived Cu(II)-coordinated supramolecular hydrogel with tunable mechanics, self-healing, and underwater adhesion

Abstract

For the first time, an amino acid-derived polymeric supramolecular hydrogel, poly(Cu(lysine methacrylamide)₂-random-acrylamide) [poly(Cu(LysMAM)₂-r-AM)], is synthesized via radical polymerization of Cu(lysine methacrylamide)₂ (Cu(LysMAM)₂) and acrylamide (AM) at room temperature. The hydrogel exhibits remarkable multifunctionality by demonstrating rapid self-healing within 5 seconds at 25 °C for a 3 mm cut, along with excellent load-bearing capability with a tensile strength of 0.14 MPa and an ultimate strain exceeding 35%. It further shows responsiveness to Cu²⁺ ions, possesses inherent electrical conductivity, and displays strong underwater adhesion driven by hydrophobic aggregation, highlighting its advanced and versatile performance characteristics. Upon treatment with 8-hydroxyquinoline, the hydrogel disassembles into poly(LysMAM-r-AM), which further forms a nanogel in the presence of Cu²⁺ at pH 9.5. The polymer selectively senses different heavy metals such as cadmium, lead, zinc, mercury, and nickel, but forms a nanogel exclusively with copper. Notably, the incorporation of Cu²⁺ imparts conductivity to the hydrogel, allowing it to function as a pressure-sensitive material. The conductivity variation with applied pressure makes this hydrogel a promising candidate for flat-foot detection via shoe sensors. This innovative hydrogel platform, combining metal selectivity, self-healing, underwater adhesion, and conductivity, opens avenues for applications in healthcare, wearable sensors, and adhesives.

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Introduction

Hydrogels are 3-D polymeric networks formed through the chemical or physical crosslinking of hydrophilic macromolecules, capable of retaining large amounts of water while maintaining structural integrity.^1–4 Owing to their high water content, interconnected network architecture, and tunable viscoelastic properties, hydrogels exhibit unique characteristics, such as flexibility, stretchability, transparency, and ionic conductivity.^5,6 In recent years, the integration of dynamic interactions such as hydrogen bonding and metal–ligand coordination has further expanded the functionality of hydrogels, enabling properties such as self-healing, stimuli-responsiveness, and enhanced mechanical performance.⁷ As a result, hydrogels have emerged as versatile platforms for applications in tissue engineering, wearable sensors, and advanced biointegrated devices.^8,9

Despite extensive research on conventional hydrogels derived from petroleum-based polymers, such materials often suffer from key drawbacks, such as limited biodegradability, potential environmental hazards, and insufficient mechanical robustness.^10,11 This has shifted attention toward natural or biopolymer-based hydrogels, particularly those originating from amino acids, which offer renewable, eco-friendly, and biologically compatible alternatives.¹² Among these, lysine stands out due to its excellent metal ion coordination capabilities, especially with Cu²⁺ ions, which facilitates the formation of stable, metal–organic polymer networks with tailored functionalities.¹³ Metallo-supramolecular hydrogels have a porous structure with high surface area, which can be further tailored by controlling the selection of metal ions and organic ligands. These gels exhibit unique structural and functional properties such as stimuli-responsive behavior, self-healing abilities,¹⁴ and tunable mechanical properties,¹⁵ because of the dynamic and reversible nature of metal–ligand interactions,¹⁴ and are used for applications in diverse fields such as catalysis,¹⁶ drug delivery,¹⁷ sensing,¹⁸ biomedical devices and adaptive materials.¹⁹

Recent advances in conductive hydrogel systems have facilitated their integration into wearable sensors and electronic skin applications due to their exceptional flexibility, electrical conductivity, and biocompatibility.²⁰ In parallel, multifunctional conductive hydrogels exhibiting combined features such as self-healing, stretchability, strong adhesion, and dual-mode sensing capabilities have been increasingly reported, reflecting the rising demand for next-generation smart soft materials.²¹ Moreover, flexible pressure sensors developed from advanced materials, including MXene-based composites and conductive hydrogels, have demonstrated remarkable sensitivity along with the ability to enable real-time physiological monitoring.²² Despite these advancements, achieving a simple, cost-effective, and metal-coordinated hydrogel system with tunable mechanics, selective metal sensing, and pressure-responsive conductivity remains challenging.^23–25 To overcome these issues, researchers have explored the incorporation of metal ions into polymer-based hydrogels to enhance conductivity and mechanical performance.^20,24,26 Mahapatra et al. reported on introducing Cu²⁺ ions into a hydrogel network formed by the polymerization of acrylamide and acrylic acid.²⁷ This integration of metal coordination with the hydrogel matrix enhanced its mechanical properties, resulting in improved strength, toughness, and recoverability.²⁸ However, several challenges persist in these systems, including non-uniform dispersion of nanoparticles, which can lead to inconsistent properties, concerns over biocompatibility and toxicity, especially in silver and gold-based materials, high costs associated with noble metals like gold and structural instability or poor water resistance in some acrylamide-based hydrogels.^20,24,26 Copper-based hydrogels have emerged as a more affordable alternative that provides antibacterial and conductive benefits. Nonetheless, unresolved issues remain, such as improving the distribution of copper nanoparticles within the hydrogel matrix, maintaining the structural integrity of the composite, and controlling copper ion release to avoid adverse biological or environmental effects.

This work presents the development of a multifunctional metallo-supramolecular hydrogel constructed from an amino acid-based monomer. The material is prepared via free radical polymerization of Cu(lysine methacrylamide)₂ (Cu(LysMAM)₂) with acrylamide (AM), yielding a composite polymer network denoted as poly(Cu(LysMAM)₂-r-AM). The distribution of copper is uniform within the hydrogel matrix. This hydrogel system not only harnesses the bioactivity and metal-chelating properties of lysine but also displays several integrated features: pressure-sensitive conductivity through Cu²⁺ incorporation, rapid self-healing under mechanical stress, selective nanogel formation via re-coordination with copper ions, and strong adhesion in aqueous environments. Additionally, its structural reversibility in the presence of 8-hydroxyquinoline enables dynamic control of the hydrogel network. Poly(LysMAM-r-AM) exhibits selective metal responsiveness, showing reactivity toward Hg(II), Pb(II), Cd(II), Zn(II), and Ni(II), while exclusively forming nanogels with copper. The conductivity variation under applied pressure positions the hydrogel as a potential candidate for applications such as shoe sensors for detecting flat feet. These multifunctional attributes highlight the hydrogel's promise in areas such as healthcare, sensing, and wearable technologies. By combining environmentally friendly monomers with dynamic metal coordination chemistry, this work introduces a hydrogel platform that addresses several limitations of existing systems. The combination of properties such as responsiveness, conductivity, robustness, and the potential biocompatibility makes it a promising material for future applications in biosensing, underwater adhesion, and soft bioelectronic devices.

Experimental section

Materials

The monomers, acrylamide (79-06-1, AM, 99%), methacryloyl chloride (920-46-7, 97%), basic copper(II) carbonate (12069-69-1), L-lysine monohydrochloride (657-27-2), N,N,N′,N′-tetramethylethylenediamine (110-18-9, TMEDA, ≥99%), ammonium peroxodisulfate (7727-54-0, APS), sodium hydroxide, acetone, glacial acetic acid (64-19-7, >99%), thioacetamide (62-55-5, >99%), 8-hydroxyquinoline, CHCl₃ (>99%), Cu(NO₃)₂·3H₂O (13778-31-9, >99%), CdBr₂, (13464-92-1, >99%), Ni(NO₃)₂·6H₂O (3264-82-2, >99%), ZnCl₂ (7646-85-7, >99%), HgCl₂ (7487-94-7, >99%), PbCl₂ (7758-95-4, >99%) and D₂O (7789-20-0, >99.9%) were purchased from Sigma-Aldrich.

Synthesis of Cu(LysMAM)₂

ε-L-Lysinyl methacrylamide copper complex was synthesized using a procedure similar to that described previously.²⁹ Initially, the crosslinker Cu(LysMAM)₂ was synthesized as follows: in a 100 mL round-bottom flask, L-lysine monohydrochloride (2.5 g; 14 mmol) was first dissolved in 25 mL of water and then heated for 10 minutes at 90 °C in an oil bath. After that, basic copper(II) carbonate (1.65 g; 7.5 mmol) was gradually added to the mixture while being constantly stirred. The reaction was maintained under reflux at the same temperature for 2 h, followed by hot filtration. The clear filtrate was brought to room temperature, followed by the successive addition of 6 mL dry acetone and 7 mL of 2 M KOH. The resulting solution was then cooled in an ice-water bath and stirred for 20 minutes. A solution of methacryloyl chloride (1.33 mL; 16 mmol) in 6 mL of acetone was gradually introduced over 30 minutes at 0 °C, along with the simultaneous dropwise addition of 8 mL of 2 M KOH to maintain the pH around 8. The reaction mixture was stirred at 25 °C overnight. The use of low-temperature controlled dropwise addition and a mildly basic environment ensured immediate neutralization of any HCl formed and suppressed hydrolysis of the acid chloride. The entire reaction was performed under anhydrous conditions to further prevent interference from moisture. The bluish-violet compound, Cu(LysMAM)₂, was collected via filtration using a Buchner funnel, thoroughly washed with ice-cold water, methanol, and diethyl ether, and subsequently dried under vacuum at 40 °C for 24 hours (yield: 85%). Structural confirmation was achieved through ¹H NMR, ATR-IR, and UV-visible spectroscopy.

¹H NMR (600 MHz, D₂O, δ ppm): 0.5–1.8 ppm (lysine methyl proton, –CH₂–), 2–2.9 ppm (–CH₃– in acrylate unit), 3.1–3.4 ppm (iso-butyl –CH– and lysine methyl proton, –CH₂–), 5.5–6.0 ppm (active double bond protons, CH₂–), 7.8–7.9 ppm (–NH– proton of amide group).

ATR-IR (cm⁻¹) (Fig. S1a): 3400–3000, ν_N–H (amine, amide); 2925, ν_C–H (vinyl); 1616, ν_C=C; 1527, ν_C=O (amide); 1401, νcoo.

Gel synthesis

The synthesized Cu(LysMAM)₂, AM, APS and TEMED were mixed in water to form a homogeneous mixture at room temperature. The gel mixtures were then cast into polystyrene moulds and kept at room temperature for 1 min to ensure complete solidification. The water content, Cu(LysMAM)₂ molecular weight, and AM mixing ratio can all be adjusted to alter the mechanical behaviour.

Synthesis of poly(LysMAM-r-AM) (PLAM)

In a typical procedure, PLAM was synthesized by taking poly(Cu(LysMAM)₂-r-AM) (2.5 g, 10 mmol) in a glass tube and dissolving it in water (36 mL), and then a chloroform solution (36 mL) of 8-hydroxyquinoline (0.895 g, 12 mmol) was added. It was vortexed for 2 h using a vortex mixer. A green precipitate appeared in the chloroform phase and was separated by filtering the biphasic mixture through a G4 sintered glass funnel. Residual 8-hydroxyquinoline was eliminated by extracting the solution with chloroform three times. Following the removal of 8-hydroxyquinoline, the aqueous phase was freeze-dried to yield poly(LysMAM-r-AM). The yield was 1.8 g (90%). UV, ATR-IR, and ¹H NMR spectroscopy were used to confirm the structure. ¹H NMR (600 MHz, D2O, δ ppm): 0.5–1.8 ppm (lysine methyl proton, –CH₂–), 2–2.9 ppm (–CH₂– in between repeating unit), 3–3.6 ppm (iso-butyl –CH–), 6.7–7.0 ppm (NH₂– of acrylamide and NH₃⁺ of LysMAM), 7.6–7.78 ppm (–NH– proton of amide group).

ATR-IR (cm⁻¹): ν_N–H (amide); 3338, ν_C–H; 2200–3200, ν_C=O (amide I); 1605, δ_N–H (amide II); 1651.

Metal ion-induced aggregation of poly(LysMAM-r-AM)

Typically, a stock solution of poly(LysMAM-r-AM) (2 mL, 1 mg ml⁻¹) in water at pH 9.5 was first prepared. 2 mL aqueous solution of Cu(NO₃)₂·3H₂O (1.5 mM) was added to this PLAM solution. The solution was allowed to stand for 12 h. The Cu(II)-induced aggregation of poly(LysMAM-r-AM) was monitored by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), dynamic light scattering (DLS), circular dichroism (CD), and UV-vis absorption spectroscopy.

Self-healing study

Under an optical microscope: a poly(Cu(LysMAM)₂-r-AM) hydrogel was pasted on a glass slide. The film was placed on an optical microscope with a camera for image capture at a magnification of 10X. By using a sharp blade, a cut of approximately 3.12 mm was made on the surface of the film. The self-healing process is monitored at 25 °C for 10 minutes. For macrostructure analysis, hydrogel strips of 10 cm length were prepared (n = 5) and subjected to uniaxial tensile testing using the Universal Testing Machine. To assess self-healing performance, hydrogel specimens were precisely cut at the midpoint, gently brought back into contact without applying force, and allowed to heal at ambient temperature for 5 seconds. Tensile strength measurements for both the original and healed hydrogels were performed at a crosshead speed of 1 mm min⁻¹. Then the self-healing efficiency was evaluated by comparing the tensile strength of the healed sample to that of the uncut, original sample. The efficiency was calculated using the following formula:

Swelling study

The swelling behaviour was investigated by immersing the hydrogels in different solvents at room temperature until equilibrium swelling was achieved. The samples were then blotted, weighed, and the corresponding swelling ratios were calculated using the following equation
where W_t denotes the swollen weight at time t and W₀ represents the initial dry weight.

Adhesive study

To prepare the two glass slides, one end of each slide was tied on with a thread and fixed with black electrical insulation tape. At the free end of one slide, poly(Cu(LysMAM)₂-r-AM) hydrogel is placed, and then the second slide is positioned to form a half-sandwich structure. The assembly is analysed for the lap-shear test at 25 °C.

Ethical consent

All human participant experiments were performed in accordance with applicable institutional ethical guidelines, and informed consent was obtained from each participant prior to their involvement.

Statistical analysis

All experiment results were presented as the mean ± standard deviation. The diameter of the self-aggregated nanospheres for fifty independent measurements (n = 50) and pore sizes of the hydrogels were presented as the mean ± standard deviation for hundred independent measurements (n = 100) and a histogram was constructed using Origin 8.5 software, and the statistical significance was set at p < 0.05.

Characterization

The chemical structure was characterized by NMR (Bruker 600 MHz) and ATR-IR (PerkinElmer Spectrum 100). Surface charges and hydro dynamic radius were measured by a Zetasizer NANO ZS90. Morphological analyses were analyzed by FESEM (Zeiss Gemini SEM500) and TEM (JEOL JEM-2100PLUS). The nano metal binding study was performed using a UV-visible spectrophotometer (Shimadzu UV-2600). Microscopy images were taken with a fluorescence light microscope (DM2700M, Leica). The rheological study was characterized by an Anton Paar MCR 102. Mechanical testing, i.e. tensile strength, elongation, lap-shear and self-healing, was performed using a Universal Testing Machine (Servo Hydraulic System MTS 370.10) having a load cell of 5 kN with crosshead speed of 1 mm min⁻¹. Healing efficiency is calculated based on tensile strength recovery. All mechanical data were processed using the manufacturer's software TRIOS, and rheological data were processed using Origin 8.5 software.

Results and discussion

To eliminate the need for time-intensive protective group strategies, chromatographic purification, and toxic organic solvents, the crosslinker Cu(LysMAM)₂ was first prepared from L-lysine monohydrochloride using a convenient and eco-friendly approach (Scheme 1). Through the creation of its lysine copper complex, it included the selective protection of α-NH₂ and –COOH groups of L-lysine, leaving an unprotected ε-NH₂ group (Scheme S1). The ε-L-lysinyl methacrylamide copper complex was produced by coupling the lysine copper complex with methacryloyl chloride in a solution of water and acetone (Scheme 1 and Scheme S1). The successful coupling of the L-lysine copper complex with methacryloyl chloride is demonstrated by the emergence of new bands in the ¹H NMR and ATR-IR spectra. In the ¹H NMR spectrum (Fig. S1), peaks are observed at 0.5–1.8 ppm (lysine methyl proton, –CH₂–), 2–2.9 ppm (–CH₃– in acrylate unit), 3.1–3.4 ppm (iso-butyl –CH– and lysine methyl proton, –CH₂–), 5.5–6.0 ppm (active double bond protons, CH₂–), and 7.8–7.9 ppm (–NH– proton of amide group), and in the ATR-IR spectrum (Fig. S2(a)), peaks are observed at 1616 cm⁻¹ (C=C stretching), 1650 cm⁻¹ (C=O amide I) and 1527 cm⁻¹ (N–H bending), showing the successful synthesis of the Cu(LysMAM)₂ crosslinker. Additionally, energy dispersive X-ray spectroscopy (EDS) was performed to confirm not only the elemental composition but also the spatial distribution of Cu within the synthesized Cu(LysMAM)₂ crosslinker. The EDS spectrum (Fig. S3(a)) revealed high weight percentages of carbon (59.5%), copper (21.8%), and oxygen (10.8%), consistent with the expected coordination complex structure. More importantly, elemental mapping (Fig. S3(b–d)) demonstrates a uniform distribution of carbon, nitrogen, and copper throughout the sample, confirming that Cu²⁺ is homogeneously incorporated within the ligand framework rather than forming localized aggregates or phase-separated domains. This homogeneous distribution is crucial, as it ensures uniform coordination crosslinking in the resulting hydrogel network, thereby contributing to consistent mechanical properties and functional performance.

Scheme 1 Synthesis scheme of a random copolymer hydrogel of Cu(LysMAM)₂ from L-lysine monohydrochloride.

Ammonium peroxodisulfate (APS) with TEMED in water at room temperature rapidly triggered the free radical polymerization of Cu(LysMAM)₂ and AM, resulting in the supramolecular hydrogel poly(Cu(LysMAM)₂-r-AM) within 1 min (Scheme S2 and video S1, SI). The fast gelation resulted from the combined effect of accelerated radical generation by the APS/TEMED redox pair and simultaneous Cu²⁺–ligand coordination crosslinking. ATR-IR analysis confirmed the existence of characteristic functional groups in the resulting hydrogel (Fig. S2(b)). The ATR-IR spectroscopy of the poly(Cu(LysMAM)₂-r-AM) hydrogel with peaks at 3340, νN–H (amine, amide), 2922, νC–H (vinyl), and 1530, but not 1610, νC=C, confirms the successful polymerization of AM and Cu(LysMAM)₂.

In order to produce the Cu-free polymer poly(LysMAM-r-AM), a straightforward method¹³ has been adapted that involved removing copper from the copper complex using 8-hydroxyquinoline (Scheme S3).

¹H NMR, ATR-IR and UV spectroscopy were utilized to analyze the produced poly(LysMAM-r-AM) copolymer. All the expected signals are present in the poly(LysMAM-r-AM), according to the ¹H NMR spectrum (Fig. 1a): 0.5–1.8 ppm (lysine methyl proton, –CH₂–), 2–2.9 ppm (–CH₂– in between repeating unit), 3–3.6 ppm (iso-butyl –CH–), 6.7–7.0 ppm (NH₂– of acrylamide and NH₃⁺ of LysMAM), and 7.6–7.78 ppm (–NH– proton of amide group). Additionally, bands corresponding to the functional groups found in poly(LysMAM-r-AM) were seen in the ATR-IR spectrum (Fig. 1b): ν_N–H (amide); 3338 cm⁻¹, ν_C–H; 2200–3200 cm⁻¹, ν_C=O (amide I); 1605 cm⁻¹, δ_N–H (amide II); 1651 cm⁻¹. Bands at 280 nm in the UV-vis spectrum (Fig. 1c) were found to match to the CT between deprotonated N and Cu(II).¹³ Following deprotection, the CT band disappeared, which validates the deprotection of copper. The poly(Cu(LysMAM)₂-r-AM) hydrogel shows a reversible sol–gel transition by changing the pH as shown in Fig. 2a. At pH 1.5, a gel-to-sol transition of the Cu-containing hydrogel was observed. However, when the pH increased to 10.5 the sol goes back to its initial hydrogel form.

Fig. 1 (a) ¹H NMR spectra of poly(LysMAM-r-AM). (b) ATR-IR spectra of poly(Cu(LysMAM)₂-r-AM) and poly(LysMAM-r-AM). (c) UV absorption spectra of poly(Cu(LysMAM)₂-r-AM) and poly(LysMAM-r-AM).

Fig. 2 (a) Reversible gelation of poly(Cu(LysMAM)₂-r-AM) hydrogel via NaOH-induced formation and HCl-triggered disassembly. FESEM and TEM micrographs of poly(LysMAM-r-AM)Cu nanogel (b) and (c) respectively, (d) UV-vis absorption spectra of the hydrogel before and after Cu-complexation, and UV-vis absorption spectra of poly(LysMAM-r-AM) in the presence of different M(II) ions in aqueous solution at pH 9.5: (e) Cu(II) and (d) Cd(II), Ni(II), Zn(II), Hg(II) and Pb(II).

Poly(LysMAM-r-AM) stock solution (2 ml, 1 mg ml⁻¹) in water at pH 9.5 is produced initially. This poly(LysMAM-r-AM) solution was mixed with 2 ml of an aqueous solution of Cu(NO₃)₂·3H₂O (1.5 mM). The solution was left to stand for 12 h. UV-vis absorption spectroscopy, TEM, and FESEM were used to study the Cu(II)-induced aggregation of poly(LysMAM-r-AM). For this case, spherical aggregation (diameter 100–130 nm) was observed (Fig. 2b and c), although the UV-vis spectra revealed complexation between poly(LysMAM-r-AM) and Cu(II) (Fig. 2d). The absence of aggregation was indicated by the absence of distinctive charge-transfer bands in the UV-vis absorption spectra of poly(LysMAM-r-AM) at pH 9.5 in the presence of various divalent metal ions, such as Cd(II), Ni(II), Zn(II), Hg(II), and Pb(II) (Fig. 2e). This discovery raises the possibility of using PLAM hydrogel for selective Cu(II)-responsiveness or extraction from aqueous systems. This is due to Cu(II) exhibiting selective coordination with LysMAM units, resulting from its strong affinity for mixed N/O donor ligands, which form stable chelate crosslinks unlike other metal ions.

To determine the viscoelastic mechanical behavior of the synthesized hydrogel, i.e. poly(Cu(LysMAM)₂-r-AM), oscillatory rheological analysis was conducted. The frequency sweep test revealed that the storage modulus (G′) was consistently higher than the loss modulus (G″) across the entire angular frequency range (0.1 to 100 rad s⁻¹), indicating the solid-like behavior and elastic dominance of the hydrogel network. G′ increased steadily with frequency, reaching a value above 12 kPa at 100 rad s⁻¹, while G″ showed a plateau around 4 kPa before slightly decreasing at higher frequencies (Fig. 3a). This behavior confirms the creation of a robust three-dimensional crosslinked network within the hydrogel, reflecting its mechanical stability. To evaluate the effect of crosslinker concentration on mechanical performance, we performed a systematic optimization study by varying the weight percentage of Cu(LysMAM)₂ from 1 wt% to 5 wt%, while keeping the amounts of AM, APS, and TEMED constant. Rheological frequency sweep analysis revealed that the G′ increased progressively with higher Cu(LysMAM)₂ content, indicating improved crosslinking density and network strength. At 5 wt%, G′ exceeded 12 kPa, confirming the formation of a robust and elastic hydrogel network. These findings indicate that the mechanical rigidity and structural stability can be controlled by varying the concentration of the metal–ligand crosslinker. The hydrogel network is stabilized primarily by dynamic Cu²⁺-ligand coordination bonds, while secondary hydrogen-bonding interactions between amide and carboxyl groups contribute to local structural organization and support. Additionally, the mechanical robustness of the hydrogel under uniaxial stress was tested using a universal testing machine (UTM). The resulting stress–strain curves (Fig. 3b) revealed that the hydrogel exhibited a soft and elastic network, with a tensile strength of 0.14 MPa and an ultimate strain of more than 35%. These values confirm that our metallo supramolecular network combines both good stretchability and appreciable mechanical strength. Furthermore, the printable nature of the poly(Cu(LysMAM)₂-r-AM) hydrogel was demonstrated using a mold-based fabrication approach. The hydrogel precursor solution was poured into polystyrene molds of different geometries and allowed to undergo in situ gelation at room temperature (Fig. 3c). This simple, scalable method enabled the hydrogel to adopt complex three-dimensional shapes with high structural fidelity.

Fig. 3 (a) Rheological data of poly(Cu(LysMAM)₂-r-AM) hydrogel with different wt.% of Cu(LysMAM)₂, varying as 1, 2, 3, 4, and 5; (b) Tensile stress–strain curve of the poly(Cu(LysMAM)₂-r-AM) hydrogel; (c) Various 3D printed structures of poly(Cu(LysMAM)₂-r-AM) hydrogel; (d) Tensile stress–strain curve of poly(Cu(LysMAM)₂-r-AM) hydrogel before and after self-healing.

Self-healing and load bearing

In dynamic, high impact circumstances the noncovalent interaction ensures sustained performance and resilience by imposing self-healing capabilities that quickly restore functioning after damage. An optical microscope with a camera was used to study the healing process after a 3 mm cut was created at the film's surface for this purpose. At 25 °C and under typical climatic conditions, the crack fully heals on its own in 5 sec, as seen in Fig. 4a(i and ii). The self-healing is caused by dynamic, reversible metal–ligand cooperation between amide/amine groups and Cu²⁺ ions. When mechanical damage occurs, these contacts quickly reform, allowing for fusion at the split interface (Fig. 4(c)). To demonstrate the material's strength, it was cut in half, with one half dyed with Eosin-B for improved visibility (Fig. 4b(i and ii)), and then reassembled for self-healing for 5 sec (Fig. 4b(iii)). The stretchability of the healable material was also examined. Very effective self-healing after 2 min is demonstrated by the repaired material's ability to withstand being dragged more than five times its initial length without breaking, as seen in Fig. 4b(iv). The self-healing material's load-bearing capacity was also examined. A 50 g weight was easily carried by the healed sample, as seen in Fig. 4b(v and vii), and a 219 g weight resulted in a small elongation at the healed junction but no fracture (Fig. 4b(vi and viii)). The endurance and strength of the material's self-healing behaviour were further demonstrated by tensile testing measurement to assess our poly(Cu(LysMAM)₂-r-AM) hydrogel's mechanical recovery both before and after self-healing. The repaired samples’ mechanical behavior is similar to that of the pristine material in each cycle, according to the stress–strain curves (Fig. 3d). Excellent tensile strength was demonstrated by the healing efficiency, which was determined to be roughly 94% after the first cycle and continuously over 92% for three consecutive healing cycles, which verified that the material maintained its mechanical properties even after healing.

Fig. 4 (a) Optical microscope images showing the self-healing process of a 3 mm incision on the thin film poly(Cu(LysMAM)₂-r-AM) hydrogel within 5 seconds. (b) (i) Prior to self-healing, and (ii) following self-healing; samples were stained with Eosin B to enhance visual clarity. The self-healing efficiency was assessed through stretchability tests: (iii) before stretching and (iv) after stretching. The load-bearing capability under a 219 g and 50 g weight of the self-healed poly(Cu(LysMAM)₂-r-AM) hydrogel was also evaluated in (v) to (viii) respectively. (c) Schematic representation of the self-healing mechanism of the hydrogel.

Water-tolerant adhesion

Poly(Cu(LysMAM)₂-r-AM) hydrogel exhibits rapid and robust adhesion to a variety of surfaces like glass, metal, and plastics. To quantify this adhesion, lap-shear measurements were carried out on glass and plastic substrates. An adhesion failure force of 65 kPa was observed for the hydrogel on the glass surface, compared to 35 kPa on the plastic surface (Fig. 5a). The findings imply that a substantial amount of force is necessary to cause failure on the glass surface as well as in the plastic substrate, suggesting stronger adhesion of the hydrogel. The observed failure mode on both glass and plastic surfaces was cohesive (Fig. 5b), suggesting strong adhesion between the material and the substrate, with separation occurring due to internal failure within the material. The strong adhesion to glass can be attributed to hydrogen bonding and ionic interactions. These results suggest that the poly(Cu(LysMAM)₂-r-AM) hydrogel shows good adhesion in glass and plastic.

Fig. 5 (a) Lap-shear test results of the poly(Cu(LysMAM)₂-r-AM) hydrogel adhesive on glass and plastic substrates. (b) Optical microscopy images captured following adhesion failure.

The fascinating field of underwater adhesion addresses the difficulties of joining materials in moist environments, where the special characteristics of water, such as its polarity and surface tension, frequently obstruct adhesion. Scientists have been inspired to develop synthetic alternatives by the clever examples seen in nature, such as mussels that make specific adhesives to adhere to surfaces underwater. Effective underwater adhesives work by forming strong chemical bonds, pushing water away from surfaces, locking onto surface irregularities, or using flexible, reversible interactions. Innovations in adhesives that are crucial for applications in fields like robotics, medical devices, and marine building have been made possible by these natural techniques. The poly(Cu(LysMAM)₂-r-AM) gels demonstrated a fast bonding adhesive behaviour for magnetic beads in water at room temperature (Fig. 6a(i–iii) and video S2, SI), plastic caps in 10 s in flowing water at room temperature (Fig. 6b(i–iv) and video S3), and plastic caps in 10 s in salt water at 45 °C (Fig. 6c(i–iv) and video S4), all based on the solvent exchange strategy that induces hydrophobic aggregation. Fig. 6a(ii) illustrates how poly(Cu(LysMAM)₂-r-AM) gel showed a notable adhesive behaviour for a variety of materials in water, such as steel, glass, magnetic beads, and polytetrafluoroethylene (PTFE). Additionally, two plastic plates can be securely bonded together using the adhesive gels.

Fig. 6 (a) Adhesive behavior for magnetic beads in still water at room temperature of poly(Cu(LysMAM)₂-r-AM) hydrogel (i) to (iii); (b) adhesion performance on plastic surfaces at the second interval (10 s) under flowing water conditions (i) to (iv); (c) adhesive behavior for plastics in still water at 45 °C (i) to (iv).

Furthermore, to assess whether solvent uptake could undermine adhesion or mechanical performance, we measured gel swelling in deionised water, DMSO, toluene, and ethanol over 24 h. In all cases, the hydrogel exhibited negligible swelling, i.e. <5% weight change and maintained its original shape, confirming that solvent uptake does not compromise its structural integrity or adhesive function (Fig. S6). Additionally, to quantify any minimal swelling that might impact adhesion, lap-shear tests were conducted on hydrogel-bonded glass substrates both before swelling and after 24 h of swelling in water. The results show that the adhesive strength after swelling retains 80% of the original value before swelling, demonstrating excellent maintenance of underwater adhesion even after extended immersion (Fig. S7). A comparison table (Table S2) summarizing the tensile strength, self-healing efficiency, and adhesion behavior of our hydrogel with other reported systems is provided.

Conductivity and flat foot detector

The highly conductive poly(Cu(LysMAM)₂-r-AM) hydrogel material's electrical conductivity varies with change in applied force on it. This is based on the notion that compression causes the internal structure of the hydrogel to become denser, which shortens the distance between polymer chains and enhances the movement of ionic charge, leading to higher electrical conductivity. To assess the change in the material's conductivity, we have studied the change in conductivity due to a change in pressure by using a multimeter connected to two thin platinum electrodes, positioned at both ends of the small piece of hydrogel that is sliced into pieces measuring 2 cm by 2 cm by 1 cm. Fig. 7a shows the increase in conductivity studied with a steady load increase using a hydraulic press. The initial conductivity of the hydrogel was 4.0 S cm⁻¹, which increases to 28.0 S cm⁻¹ upon compression to 750 psi (Fig. 7a and Fig. S8).

Fig. 7 (a) Conductivity of the hydrogel with change in pressure, (b) change in resistance with pressure due to flat feet and normal feet, (c) flat foot detection method (i) to (v).

This property of change in conductivity with pressure on the poly(Cu(LysMAM)₂-r-AM) supramolecular gel material has been used in this work to study the detection of flat feet, a frequent condition marked by the collapse of the foot arch when bearing weight (Fig. 7c). A piece of 3 cm by 2 cm by 1 cm hydrogel is placed at the arch area of the foot in a insole slipper using both sides of adhesive tape. Then two thin platinum electrodes placed at each end of the hydrogel sheet were attached and linked to a data collection tool (multi-meter) to measure the resistance of the hydrogel (Fig. 7c(i)).

Because a flat arch builds more pressure on the hydrogel material, the resistance of the gel material decreases over a wide range (from 0.044 Ω to 0.013 Ω) (Fig. 7c(iii and v)) for a person with a flat foot (Fig. 7c(v)), whereas the resistance of the gel material decreases over a very small range (from 0.044 Ω to 0.040 Ω) (Fig. 7c(iii)) in the case of a person with a normal foot (Fig. 7c(ii)) because a normal arch places less pressure on the hydrogel material. The representative figure (Fig. 7b) compares the prevalence of flat and normal foot arches and how arch type correlates with reduced structural resistance in the feet. The change in resistance is low for individuals with normal feet, while flat foot consistently shows higher change in resistance compared to normal arches across both groups. The study has been done using the same method for 22 females and 25 males (Table S1) to check the cycle of the material's reversibility in its initial condition. The reversibility is represented in Fig. S9 up to 50 cycles, without any major variation in the cycle. The poly(Cu(LysMAM)₂-r-AM) hydrogel demonstrates pressure-dependent ionic conductivity arising from mobile Cu(II) coordination ions within the hydrated network. While the present work focuses on relative conductivity variation and proof-of-concept flat-foot detection, future studies will include detailed electrochemical impedance spectroscopy to precisely quantify ionic conduction, along with multi-point sensor array designs for weight-normalized, quantitative foot-pressure mapping.

Conclusions

In conclusion, the amino acid-derived polymeric supramolecular hydrogel, poly(Cu(LysMAM)₂-r-AM), demonstrates a substantial step forward in the creation of multifunctional materials for diverse applications. Its rapid gelation, selective Cu²⁺-responsiveness, and transformation into nanogels under specific conditions highlight its potential for precision in chemical detection and material engineering. The hydrogel's conductivity, which varies with pressure, positions it as an innovative material for wearable sensing technologies, such as shoe sensors for flat feet detection. Moreover, its remarkable self-healing and load-bearing properties ensure durability and resilience, while its underwater adhesive capability further extends its usability in challenging environments. Collectively, these exceptional properties position this hydrogel as a transformative material, paving the way for groundbreaking advancements across healthcare, sensing, and adhesive technologies. Its versatility opens a broad spectrum of possibilities, from next-generation wearable sensors and precision diagnostics to robust underwater adhesives and beyond. This material not only addresses current challenges but also inspires new frontiers in smart functional materials, fostering innovative solutions in diverse fields and laying the foundation for future interdisciplinary research and technological breakthroughs.

Author contributions

All authors contributed to the writing of the manuscript. All authors have given approval to the final version of the manuscript. Credit: Nishikanta Singh contributed to conceptualization, investigation, methodology, drafting the original manuscript, and reviewing and editing the final version. Koushik Mahata was involved in investigation, methodology, and manuscript review and editing. Durgesh Kumar Sinha participated in investigation, methodology, and reviewing and editing the manuscript. Sanjib Banerjee was responsible for conceptualization, securing funding, methodology development, project administration, resource provision, supervision, and manuscript review and editing.

Conflicts of interest

The authors declare that there are no conflicts of interest.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). The supplementary information file includes detailed synthetic procedures, ¹H NMR and ATR-IR characterization, EDS mapping, rheological analysis, swelling studies, lap-shear adhesion data, flat-foot detection studies, pressure-dependent resistance measurements, reversibility studies, and comparison with reported metal-ion and supramolecular hydrogels. See DOI: https://doi.org/10.1039/d5qm00812c.

Acknowledgements

This research was supported by Defence Research and Development Organisation (DRDO), Govt. of India (No. ERIP/ER/202311001/M/01/1850). The authors sincerely thank the Central Instrument Facility at IIT Bhilai for providing access to research instrumentation. NS and KM acknowledge the Ministry of Education (MoE), Government of India, for the fellowship assistance. DKS acknowledges Council of Scientific & Industrial Research (CSIR), Government of India, for his fellowship assistance.

We thank all the volunteers who participated in the flat-foot detection study for their time, cooperation, and valuable contribution to this research.

Notes and references

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