Metal-Ion-Coordinated Conductive Hydrogels for Strain Sensing from Coordination Design to Wearable Applications

Abstract

Conductive hydrogels have emerged as promising candidates for flexible strain sensors owing to their high water content, low elastic modulus, and intrinsic ionic conductivity. However, conventional hydrogel networks often suffer from an inherent trade-off among conductivity, mechanical robustness, and long-term stability, which limits their practical deployment in wearable sensing scenarios. The introduction of metal–ligand coordination bonds into hydrogel networks offers a versatile strategy to address these challenges: dynamic coordination cross-links can dissipate energy under deformation and reform upon unloading, thereby enhancing toughness, enabling self-healing, and contributing to ionic transport. This review focuses on metal-ion-coordinated conductive hydrogels designed for strain-sensing applications. Representative coordination systems based on Fe³⁺, Ca²⁺, Zn²⁺, Al³⁺, Cu²⁺, Ti⁴⁺, and Zr⁴⁺ are surveyed, with emphasis on their characteristic polymer matrices, ligand chemistries, and network-construction strategies. Key sensing-relevant properties—including ionic conductivity, mechanical stretchability, self-healing capability, interfacial adhesion, freezing resistance, and resistance to dehydration—are discussed in relation to coordination network design. Typical application demonstrations in large-deformation motion monitoring and subtle physiological signal detection are reviewed. Unlike existing reviews that survey conductive hydrogels broadly by conductive mechanism or sensor type, this review takes metal-ion coordination as the central organizing principle and systematically traces its influence across the full design chain—from ion–ligand coordination chemistry through network architecture to macroscopic sensing output. By comparatively analyzing seven representative metal-ion systems within a unified framework, this work aims to clarify how the choice of metal ion governs the interplay among conductivity, mechanical robustness, self-healing, and strain sensitivity—a perspective that has not yet been systematically addressed in prior reviews. Finally, current challenges—including the conductivity–mechanics coupling bottleneck, insufficient long-term stability, biosafety concerns for skin-contact deployment, the lack of standardized evaluation protocols, and device-integration barriers—are identified, and future directions for this field are outlined.

1. Introduction

1.1. Background of Flexible Strain Sensors

The rapid development of wearable electronics, electronic skins, human–machine interfaces, and health monitoring systems has created a growing demand for flexible strain-sensing materials [1,2,3]. The primary function of a strain sensor is to convert external mechanical deformation into detectable electrical signals; accordingly, the material requirements extend beyond electrical conductivity to include adequate flexibility, mechanical durability, and stable signal output [4]. Although conventional metal-foil and semiconductor strain gauges have been widely used in structural engineering, their inherent rigidity limits applicability in scenarios involving large deformation, conformal attachment to curved surfaces, and compatibility with biological tissues [5]. Against this background, soft-matter sensing strategies based on flexible polymer composites and ion-conducting gels have attracted increasing attention [6,7,8].

1.2. Advantages and Limitations of Conductive Hydrogels

Among the various flexible sensing materials, hydrogels stand out as a class of candidates with unique advantages, owing to their high water content, low elastic modulus, and favorable tissue compatibility [7,9]. The three-dimensional polymer network of a hydrogel can provide ionic conduction pathways, enabling electrical signal output without the incorporation of electronic conductors [10,11]. Moreover, the mechanical properties of hydrogels are comparable in magnitude to those of biological soft tissues, a characteristic that endows them with good skin conformability and wearing comfort [10].

Nevertheless, existing conductive hydrogels still face several common challenges in strain-sensing applications. First, there is often an inherent trade-off between conductivity and mechanical strength: increasing the ionic concentration or introducing conductive fillers may improve signal output but can simultaneously compromise the structural integrity of the network or increase stiffness [6]. Second, under prolonged cyclic loading, networks held together solely by conventional physical or single covalent cross-links are prone to irreversible damage, leading to signal attenuation or baseline drift [12]. Third, once fracture occurs, most hydrogel systems cannot restore the conductive pathways, and sensing functionality is consequently lost [13,14]. Furthermore, in practical usage scenarios involving significant variations in temperature and humidity, hydrogels also face stability challenges such as dehydration-induced shrinkage and low-temperature freezing [15]. These issues indicate that conventional cross-linking strategies alone are insufficient to meet the multi-property synergy required for strain sensing, highlighting the need for new design approaches.

1.3. The Unique Role of Metal-Ion Coordination

Metal–ligand coordination interactions have been widely introduced into hydrogel network design in recent years, offering a promising pathway to address the aforementioned challenges [16,17]. Unlike irreversible covalent cross-links, metal-coordination bonds possess a dynamic and reversible character: their bond energy lies between that of covalent bonds and typical hydrogen bonds, allowing them to rupture under mechanical loading to dissipate energy. Upon load removal, these bonds reform, thereby imparting self-healing and mechanical recovery capabilities to the network [18,19]. More importantly, both the strength and the dynamic exchange rate of metal-coordination bonds can be systematically tuned by varying the metal-ion species, the ligand structure, and the coordination stoichiometry, providing a molecular-level means of purposefully tailoring the mechanical response behavior of the network [17,20].

In the context of strain sensing, metal-ion-coordinated hydrogels present three noteworthy advantages. First, metal ions within the network can serve simultaneously as dynamic cross-linking centers that participate in network construction and as free ions that contribute to charge transport [21]; consequently, coordination interactions may affect both the mechanical structure and the conductive pathways, giving rise to a coupled structure–signal effect. This dual allocation arises because, at any given metal-salt loading, only a fraction of the metal ions are consumed by coordination with ligand groups on the polymer chains; the remainder exist as solvated free ions in the aqueous phase. The partition ratio is governed by the coordination equilibrium constant, which depends on the metal-ion species, ligand type, and stoichiometry, thereby providing a molecular-level handle for adjusting the balance between network stiffness and ionic conductivity [22,23]. Second, dynamic coordination bonds can act as sacrificial bonds that rupture preferentially under stress, thereby preventing irreversible damage to the backbone or permanent network and improving toughness and cyclic stability [20,24]. At the molecular level, the intermediate bond energy of metal–ligand coordination bonds—typically 50–200 kJ mol⁻¹, situated between that of covalent C–C bonds (~350 kJ mol⁻¹) and hydrogen bonds (~10–40 kJ mol⁻¹)—ensures that they rupture before the covalent backbone is compromised [17,18]. The energy absorbed during bond scission is dissipated as heat, manifesting macroscopically as hysteresis in cyclic tensile tests. Upon unloading, the thermodynamic driving force for metal–ligand re-association—originating from the negative free energy of complex formation—enables bond reformation on time scales ranging from seconds to hours, depending on the ligand-exchange kinetics of the specific ion–ligand pair [25]. Third, the re-association of coordination bonds at fractured interfaces can facilitate structural reconstruction and the recovery of conductive pathways, providing a molecular basis for self-healing sensor functionality [21,26]. Because ionic conduction in these hydrogels relies on the continuity of the hydrated polymer network as the medium for ion migration, the re-establishment of coordination cross-links at a fractured interface simultaneously reconstructs both the load-bearing scaffold and the ion-transport pathways. This intrinsic coupling between structural and electrical recovery distinguishes coordination-based self-healing from external healing agents or thermal remolding approaches, which may restore mechanical integrity without necessarily ensuring conductive-pathway continuity [26,27]. Taken together, the synergistic contributions of metal-ion coordination to conductivity regulation, mechanical toughening, and self-healing make it an important design strategy for strain-sensing hydrogels.

1.4. Scope and Organization of This Review

Several recent reviews have addressed related topics from various perspectives. For example, Tang et al. [28] and Cao et al. [6] provided broad overviews of conductive hydrogels for flexible strain sensors, covering a wide range of conductive mechanisms (ionic, electronic, and hybrid) without focusing on a specific crosslinking strategy. Zhu et al. [7] discussed toughening and conductive network design as two parallel pathways toward high-performance hydrogel sensors, while Li et al. [15] examined hydrogel sensor performance under harsh environmental conditions. Zhang et al. [29] reviewed flexible and wearable strain sensors based on conductive hydrogels, offering a broad survey that encompassed both electronic and ionic conduction strategies across diverse hydrogel platforms but did not focus on the specific role of metal-ion coordination in governing sensing-relevant properties. From the standpoint of coordination chemistry, Khare et al. [17] systematically reviewed how transition-metal coordinate bonds govern the mechanical properties of bioinspired macromolecules, but without extending the discussion to electrical sensing behavior. These reviews collectively provide valuable context for understanding conductive hydrogel sensors; however, they are organized primarily by conductive mechanism, sensor architecture, or application scenario, few of them adopt metal-ion coordination as the central organizing principle to systematically examine how the choice of metal-ion species and ligand chemistry governs the interplay among network formation, conductivity, mechanical behavior, self-healing, and sensing output. As a result, the field still lacks a more unified comparative framework that connects coordination-level design parameters to device-level sensing performance across different metal-ion systems. The scope and organization of this review are illustrated schematically in Figure 1.

2. Representative Metal-Ion Coordination Systems

2.1. Fe³⁺-Based Coordination Systems

Fe³⁺ is one of the most extensively investigated and mechanistically informative metal ions in coordination-crosslinked hydrogel systems for strain sensing. Its prominence in the literature stems not necessarily from universal superiority in sensing performance, but from a particularly versatile coordination chemistry: Fe³⁺ readily forms dynamic bonds with catechol, carboxyl, and hydroxyl groups [16,17], while its redox activity (Fe³⁺/Fe²⁺) can additionally be exploited to trigger free-radical polymerization through Fenton-type reactions, in which Fe²⁺ generated by in situ reduction activates peroxides (e.g., H₂O₂ or persulfate) to produce hydroxyl or sulfate radicals that initiate vinyl monomer polymerization. This redox-mediated pathway enables rapid gelation under ambient conditions—often within seconds to minutes—without the need for external initiators, UV irradiation, or elevated temperatures, thereby substantially simplifying the preparation procedure and broadening the processing window [30,31,32,33,34]. In strain-sensing contexts, Fe³⁺ can simultaneously serve as a dynamic crosslink center and an ionic charge carrier, making Fe³⁺-based hydrogels a useful reference case for illustrating how a single ion may couple gelation, network mechanics, ionic transport, and sensing behavior. The representative Fe³⁺-based systems reported to date can be grouped into three design strategies—single-network catalytic systems, functional-separation architectures, and dual-network partitioning—each of which negotiates this multi-role coupling in a distinct manner.

2.1.1. Single-Network Catalytic Systems

The simplest and most widely adopted strategy employs Fe³⁺ as a combined catalyst, crosslinker, and charge carrier within a single polymer network. Three representative systems illustrate the capabilities and inherent trade-offs of this approach. Jia et al. developed a dual self-catalytic system composed of dopamine (DA) and Fe³⁺ that triggers acrylic acid polymerization within 5 s at room temperature, or even at temperatures as low as 6 °C, without any external energy input [30]. The underlying mechanism involves a stable redox couple between DA and Fe³⁺, which activates ammonium persulfate to generate sulfate and hydroxyl radicals. The resulting DA-Fe-PAA hydrogel exhibited a fracture strain of 2600%, a fracture energy of 2145 J m⁻², and an ionic conductivity up to 38 S m⁻¹—a value notably higher than those reported for most ionic conductive hydrogels, which the authors attributed to the abundant free Fe³⁺ ions serving as charge carriers within the network. After 6 h of self-healing, the stretchability and conductivity recovered to approximately 90% and 98% of their original values, respectively. This material was additionally validated as a bioelectrode for electrocardiogram (ECG) and electromyography (EMG) signal acquisition [30].

Liu et al. introduced an alternative autocatalytic system based on alkali lignin (AL) and Fe³⁺ to prepare PAA hydrogels via a Fenton-type mechanism [32]. The phenolic hydroxyl and methoxy groups on AL reduce Fe³⁺ to Fe²⁺ in situ, which subsequently activates H₂O₂ to generate hydroxyl radicals, enabling gelation in as little as 1 min without toxic initiators or crosslinkers. The mechanical properties showed a characteristic non-monotonic dependence on Fe³⁺ content: tensile strength and toughness peaked at 0.6 wt% Fe³⁺ (38 kPa and ~1 MJ m⁻³, respectively), beyond which excessive crosslinking led to embrittlement. The ionic conductivity increased monotonically with Fe³⁺ content, reaching 1.61 S m⁻¹. The hydrogel-based sensor exhibited GF values of 1.68 (0–120% strain), 2.98 (120–200%), and 4.38 (200–300%), with no appreciable signal degradation after 1000 cycles. Furthermore, the sensor was integrated with an Arduino microcontroller and a Bluetooth module for wireless remote motion monitoring [32].

Wang et al. extended the single-network catalytic concept by additionally addressing environmental adaptability. They employed a tannic acid (TA)–Fe³⁺ dual catalytic system combined with a glycerol/water binary solvent to achieve rapid gelation of BCW-TA/PAA/Fe³⁺ hydrogels within 4 s at ambient temperature [34]. The incorporation of glycerol endowed the hydrogel with anti-freezing capability, enabling stable operation from −20 to 60 °C. The material attained a tensile strength of 203 kPa, a fracture strain of 1950%, a self-healing efficiency of 91%, and a GF of 5.2 in the 1200–1900% strain range with stable signals after 2000 loading cycles [34]. Notably, this is the only Fe³⁺-based system in the present survey that explicitly integrates environmental adaptability at the gelation stage; the preceding systems [30,32] were designed and tested exclusively under ambient laboratory conditions.

A comparison across these three single-network systems reveals both the strengths and limitations of the “one ion, multiple roles” design philosophy. On the one hand, all three achieve rapid room-temperature gelation through Fe³⁺-mediated redox catalysis, and the free Fe³⁺ ions not consumed by coordination crosslinking directly serve as charge carriers—yielding ionic conductivities that range from 1.61 S m⁻¹ [32] to 38 S m⁻¹ [30], depending primarily on the total Fe³⁺ loading and the fraction immobilized at coordination nodes. On the other hand, this same coupling implies an inherent trade-off: any attempt to increase the crosslink density for better mechanical properties simultaneously reduces the population of freely mobile Fe³⁺ ions available for conduction, as evidenced by the non-monotonic conductivity–strength relationship reported by Liu et al. [32]. The 23-fold conductivity difference between [30] and [32] further illustrates that performance variations among single-network systems are driven not only by Fe³⁺ concentration but also by the ligand chemistry (catechol versus phenolic hydroxyl) that governs the coordination equilibrium and thus the free-ion fraction.

2.1.2. Functional-Separation Strategy: Coordination Bonds and Conductive Fillers in Parallel

A conceptually distinct approach to alleviating the conductivity–crosslinking coupling is to assign coordination bonds and conductive fillers to separate functional roles. Kondaveeti et al. reported a multifunctional hydrogel (designated PPGP) composed of dopamine-functionalized pectin (PT-DA), poly(acrylic acid) (PAA), polydopamine-coated reduced graphene oxide (rGO-PDA), and Fe³⁺ as an ionic crosslinker [31]. In this system, the reversible coordination between Fe³⁺ and the carboxyl groups of PAA as well as the catechol groups of PT-DA formed a dynamic crosslinked network responsible for mechanical integrity and self-healing, while the dispersed rGO-PDA provided separate electronic conduction pathways. At an optimal rGO-PDA loading of 0.05 wt%, this system reached a fracture strength of 133 kPa and an elongation of approximately 1590%. In terms of sensing performance, the gauge factor (GF) reached 14.6 in the 650–1000% strain range, with a response time of 169 ms and stable signal output over 150 cycles. Notably, its skin adhesion strength of 85 kPa enabled conformal attachment to human skin for the direct detection of eyebrow movement, eye blinking, carotid pulse, and joint bending [31].

A quantitative comparison with the single-network DA-Fe-PAA system [30] highlights the trade-offs inherent in this functional-separation approach. The pure ionic conduction design [30] yields a substantially higher bulk conductivity (38 S m⁻¹ versus a lower value for PPGP, which relies on a dilute rGO-PDA filler phase for electronic transport), because the free Fe³⁺ ions are the sole charge carriers and their concentration is not diluted by a secondary filler phase. However, the PPGP system achieves a markedly higher GF (14.6 versus values of 1.68–4.38 reported for other single-network systems [32]), likely because the percolation-sensitive rGO-PDA electronic network amplifies resistance changes under deformation in a manner that is absent in purely ionic conductors. This contrast illustrates a general design principle: introducing electronic conductive fillers can enhance strain sensitivity at the expense of baseline conductivity, whereas relying solely on free metal ions maximizes conductivity but may limit the achievable gauge factor.

2.1.3. Dual-Network Partitioning: Fe³⁺ in Functionally Distinct Sub-Networks

A third strategy partitions Fe³⁺ into two interpenetrating networks so that it fulfills different roles in each. Sun et al. designed a synergistic dual-network hydrogel consisting of an Fe³⁺-coordinated poly(acrylamide-co-acrylic acid) network and a polyaniline (PANI) conductive network [33]. By controlling the degree of protonation, the partitioning of Fe³⁺ between its roles as a coordination crosslinker and as an oxidizing agent for aniline polymerization could be precisely adjusted, yielding tunable ultimate tensile stresses (0.071–0.305 MPa) and fracture strains (145–880%). The resulting sensor maintained a stable GF of approximately 0.48 over the 0–400% working range with reliable output over 500 loading cycles, suggesting suitability for large-deformation monitoring scenarios [33].

It is notable that the GF of this dual-network system (0.48) is the lowest among all Fe³⁺-based systems surveyed here. This reduction likely reflects the buffering effect of two interpenetrating networks: when coordination crosslinks rupture under strain, the PANI network sustains parallel electronic conduction pathways, limiting the net change in overall resistance. In effect, the dual-network architecture trades peak sensitivity for a wider and more stable working range—a trade-off that favors applications requiring reliable signal output across large deformations rather than maximum sensitivity to small strains. The tunability of mechanical properties over a wide range (0.071–0.305 MPa) further distinguishes this design from the single-network systems, which offer limited post-synthesis adjustability.

2.1.4. Summary and Cross-Strategy Comparison

Taken together, Fe³⁺-based coordination systems are characterized by their multifunctional yet coupled nature: Fe³⁺ simultaneously participates in dynamic crosslinking and energy dissipation, contributes to ionic conductivity, and can trigger polymerization reactions. However, this multiplicity of roles also implies complexity in parameter optimization. Excessive Fe³⁺ content leads to over-crosslinking and brittleness [30,32], while insufficient content compromises network stability. The three design strategies described above negotiate this coupling in different ways: single-network systems [30,32,34] maximize simplicity and ionic conductivity but accept the conductivity–crosslinking trade-off; functional separation [31] enhances sensitivity through electronic fillers at the cost of baseline conductivity; and dual-network partitioning [33] broadens the working range and enables tunability at the expense of peak GF.

The reported GF values across these systems span a wide range (from 0.48 to 14.6), and this variation can be interpreted in terms of at least three factors, rather than being attributed solely to differences in testing conditions. First, GF is strain-interval-dependent: systems that report GF in high-strain regimes (e.g., 650–1000% for [31]) tend to exhibit higher values because the resistance change accelerates at large deformations where pore collapse and chain alignment amplify ion-transport disruption. Second, the conduction mechanism matters: hybrid ionic/electronic systems [31] exhibit larger resistance changes than purely ionic conductors [30,32,33,34], because filler disconnection at the percolation threshold introduces an abrupt resistance jump. Third, the degree of Fe³⁺ role-coupling affects sensitivity: in the dual-network system [33], where PANI provides a parallel conduction pathway, the GF is inherently buffered, whereas in single-network systems the entire resistance response originates from a single ionic channel. These considerations underscore the importance of reporting GF together with the associated strain interval and conduction mechanism, a point that complicates direct cross-system ranking and that is further discussed in Section 5.

From a design perspective, the most pressing unresolved question in Fe³⁺-based systems is not how to further improve any single performance metric, but how to decouple the multiple roles of Fe³⁺—as crosslinker, catalyst, and charge carrier—so that each can be optimized independently. The dual-network strategy [33] and the functional-filler separation approach [31] represent two initial attempts in this direction, but neither has yet achieved full orthogonal control over mechanical, conductive, and catalytic functions within a single formulation. More broadly, Fe³⁺ should be viewed not as the benchmark against which all other ions must be judged, but as a mechanistically rich reference case whose multifunctionality makes the coupling problem especially visible. Its current prominence in the literature may reflect, at least in part, the historically earlier development and broader functional repertoire of this ion, rather than a clearly demonstrated superiority over other metal-ion systems for strain sensing. The following sections therefore shift the emphasis from Fe³⁺-specific design strategies to ion-dependent design windows, highlighting where Ca²⁺, Zn²⁺, Al³⁺, Cu²⁺, Ti⁴⁺, and Zr⁴⁺ may offer simpler, faster, more stable, or more application-specific solutions.

2.2. Ca²⁺/Zn²⁺-Based Coordination Systems

Among the divalent metal ions employed in coordination-crosslinked hydrogels, Ca²⁺ and Zn²⁺ are the two most extensively investigated species. Both ions can participate in hydrogel network formation through coordination with carboxylate (–COO–), hydroxyl (–OH), and amino (–NH₂) groups on polymer chains; however, they differ considerably in coordination strength, ligand-exchange kinetics, and functional roles within the network. Ca²⁺ coordination is generally characterized by moderate ionic crosslinking and is widely used to construct reversible sacrificial networks while simultaneously providing baseline ionic conductivity. Zn²⁺, by contrast, exhibits a stronger coordination tendency with nitrogen- and oxygen-containing ligands, enabling more versatile structural regulation and enhanced self-healing capability.

2.2.1. Ca²⁺-Based Coordination Systems

The most representative application of Ca²⁺ coordination in hydrogel strain sensors is the construction of double-network (DN) architectures based on sodium alginate (SA). Ca²⁺ ions coordinate with the α-L-guluronic acid (G) blocks of SA chains to form the well-known “egg-box” structure, creating physical crosslinking junctions that serve as a sacrificial energy-dissipating network alongside a covalently crosslinked polyacrylamide (PAM) backbone [35]. Sun et al. demonstrated this principle in PAM/calcium alginate DN hydrogels that sustained tensile strains exceeding 1700% [35]. Cyclic loading–unloading experiments confirmed that the Ca²⁺–carboxylate coordination bonds dissipate energy during large deformation while the PAM network bridges cracks and maintains structural integrity. The sensing mechanism relies on stretching-induced changes in cross-sectional area and ionic transport path length for the free Ca²⁺ and Cl⁻ ions, yielding detectable resistance variations over a 0–1700% strain range with a response time of approximately 800 ms and stable output over 200 cycles. Wang et al. extended this Ca²⁺–alginate coordination system to 3D-printed agar/calcium alginate/PAM composites, in which Ca²⁺ crosslinking increased the tensile strength from approximately 143 to 489 kPa and yielded a gauge factor (GF) of 3.83 within a linear sensing range of 0–150% [36]. Chen et al. extended Ca²⁺ coordination from alginate carboxylate groups to the sulfate ester (–OSO₃⁻) groups of κ-carrageenan, with co-added K⁺ synergistically stabilizing the double-helix network [37]. The resulting PPK-Ca²⁺/K⁺ hydrogel achieved a tensile strength of 1190 kPa, an ionic conductivity of 0.726 S m⁻¹, and segmented GF values up to 3.692 (250–400% strain) with a response time of 152 ms, while the thermoreversible sol–gel transition of κ-carrageenan further enabled full recyclability through melt-remolding. These three studies share a common design logic: Ca²⁺ serves simultaneously as a sacrificial crosslinker for mechanical toughening and as a source of free ions for ionic conduction.

Despite these advantages—mild reaction conditions, simple preparation, and dual crosslinking–conduction functionality—Ca²⁺-based systems have notable limitations. The relatively low bond energy of Ca²⁺–carboxylate coordination may restrict the reversibility of network reconstruction under high-strain or prolonged cycling conditions. Furthermore, the GF values of Ca²⁺-based ionic hydrogel sensors are generally lower than those of hydrogels containing electronic conductive fillers (typically <10) [28], which may limit their applicability in scenarios requiring the detection of subtle deformations.

2.2.2. Zn²⁺-Based Coordination Systems

Compared with Ca²⁺, Zn²⁺ exhibits a stronger coordination tendency and more diverse coordination modes—forming bonds with carboxylate, imidazole, amino, and phenolic hydroxyl groups—which enables the construction of multi-crosslinked network architectures that combine mechanical robustness with rapid self-healing. Dong et al. exploited this versatility to construct a gelatin/Zn²⁺-cooperated triple-crosslinked hydrogel (PMAGZ) comprising covalent bonds, hydrogen bonds, and Zn²⁺–COO⁻ coordination bonds [38]. At an optimal gelatin content of 3 g, the hydrogel achieved a tensile strength of 1.08 MPa and an elongation of 782%, with segmented GF values of 1.98, 3.12, and 5.09 over the 0–400% strain range, a response time of 195 ms, and stable output over 1000 cycles at 20% strain. The mechanical stability and signal reproducibility of this system further enabled 99.8% accuracy in handwritten letter recognition using a gradient boosting algorithm [38]. Wang et al. similarly leveraged the reversible nature of Zn²⁺–COO⁻ bonds in a PAA/silk fibroin/Ti₃C₂T_x MXene composite (PZS-MXene), where the dynamic Zn²⁺ coordination network conferred injectability and self-healing within 30 s, while MXene nanosheets provided conductive pathways [39]. This system reached an elongation at break of 1750% and, configured as a capacitive strain sensor, delivered segmented GF values up to 1.78 with a response time of 0.2 s [39]. These two systems illustrate that the stronger and more kinetically labile Zn²⁺ coordination bonds, relative to Ca²⁺, can support both rapid dynamic network reconstruction and reliable sensing output. At a more fundamental level, Tunn et al. showed that varying the Zn²⁺ concentration in histidine-coordinated star-PEG hydrogels could tune the storage modulus approximately fourfold and the relaxation time over nearly three orders of magnitude, underscoring the breadth of ligand environments available to Zn²⁺ beyond carboxylate coordination [40]. This versatility extends to phenolic hydroxyl ligands as well: Su et al. developed a lignin sulfonate (LS)–Zn²⁺ self-catalytic system in which Zn²⁺–catechol coordination establishes a reversible redox equilibrium that initiates acrylamide polymerization within 5 min at room temperature [41]. With high-concentration ZnCl₂ as an anti-freezing agent, the hydrogel retained an ionic conductivity of 0.367 S m⁻¹ and a tensile strain of 1750% at −40 °C, enabling strain and temperature dual-mode sensing over a wide temperature range [41]. Collectively, these studies demonstrate that the versatile coordination chemistry of Zn²⁺—spanning carboxylate, histidine, and catechol ligands—provides a broad design palette for tailoring both the mechanical and sensing characteristics of conductive hydrogels.

2.2.3. Summary and Remarks

Ca²⁺ and Zn²⁺ coordination systems each possess distinct advantages for strain-sensing hydrogels. Ca²⁺ systems benefit from simple preparation and low cost, and are well-suited for SA-based DN sacrificial-bond architectures that provide effective energy dissipation under large deformation; however, the limited bond energy of Ca²⁺–carboxylate coordination restricts dynamic network reconstruction and rapid self-healing. Zn²⁺, owing to its ability to form coordination bonds of varying strength with multiple ligand types, generally offers broader tunability in network stability [40], self-healing rate, and synergistic mechanical–electrical recovery. Nevertheless, the strong coordination tendency of Zn²⁺ may, under certain conditions, excessively restrict chain-segment mobility and free-ion migration, thereby adversely affecting ionic conductivity and signal sensitivity. The selection between Ca²⁺ and Zn²⁺ thus depends on the specific balance among toughness, self-healing capability, conductivity, and sensitivity required by the target application. The practical choice between Ca²⁺ and Zn²⁺ may ultimately depend less on maximizing any single metric than on matching the coordination exchange kinetics to the target strain regime: Ca²⁺, with its rapid and weak coordination exchange, may be better suited for large-deformation scenarios where fast energy recovery between cycles is important, whereas Zn²⁺, with its stronger and more diverse coordination modes, may be more suitable for moderate-strain applications where network stability and self-healing fidelity take precedence [18,20]. From a design standpoint, the value of Ca²⁺ and Zn²⁺ systems lies less in pursuing record-setting sensitivity than in offering simpler and potentially more application-matched coordination regimes: Ca²⁺ favors low-cost sacrificial-network construction with rapid energy recovery, whereas Zn²⁺ provides a broader window for balancing self-healing, network stability, and moderate-range sensing performance.

2.3. Other Multivalent Metal Ion Coordination Systems

In addition to the widely studied Fe³⁺, Ca²⁺, and Zn²⁺ systems, several other multivalent metal ions—including Al³⁺, Cu²⁺, Ti⁴⁺, and Zr⁴⁺—have been progressively introduced into conductive hydrogel strain sensors. This section summarizes representative studies on these systems, with emphasis on how the choice of metal ion influences the structure–property–sensing relationship.

Al³⁺ has been one of the earlier multivalent ions systematically explored as a coordination crosslinker, owing to its optical transparency, low cytotoxicity, and relatively high charge density. Pang et al. constructed a ternary PAA-CMC-Al³⁺ composite hydrogel in which Al³⁺ simultaneously coordinates with the carboxyl groups on both CMC and PAA chains, synergizing with the intercomponent hydrogen-bond network to form a hierarchical energy-dissipating crosslinked structure [42]. This composite delivered a tensile strength of ~249 kPa, a toughness of ~1.57 MJ m⁻³, a gauge factor (GF) ranging from 0.22 to 6.7 over a 0–2066% strain window, and a self-healing efficiency of ~96.3% within 60 min. Zhang et al. further introduced sulfated polysaccharide from Enteromorpha prolifera (SPE) into the PAA-Al³⁺ system [43]; the –COO− and –OSO₃⁻ groups on SPE chains established multiple coordination bonds with Al³⁺, yielding an elongation at break exceeding 4000%, an ionic conductivity of 1.34 S m⁻¹, a GF of 6.76 in the 300–700% strain regime, and rapid self-healing within 60 s. Both systems were demonstrated for real-time human motion monitoring, including joint bending, pulse detection, and phonation recognition.

Compared with Fe³⁺ and Zn²⁺ systems, one notable advantage of Al³⁺ coordination is its optical transparency and efficient crosslinking with carboxylate-type ligands; however, the resulting mechanical and sensing performance still depends strongly on the ligand type and auxiliary network design rather than on ion concentration alone.

The introduction of Cu²⁺ typically involves an additional functional mechanism—the formation of dynamic redox coordination couples with polyphenolic or catechol-type ligands, which simultaneously accelerate hydrogel polymerization and impart interfacial adhesion. Sun et al. established a self-catalytic Ls–Cu²⁺ redox system to trigger ultrafast free-radical polymerization of PHEAA in a glycerol/water binary solvent within 30 s at room temperature [44]. The resulting organohydrogel exhibited UV-blocking capacity, anti-freezing tolerance down to −50 °C, and ionic conductivity of 0.0248–0.21 S m⁻¹, enabling real-time epidermal sensing of finger bending and pronunciation. Zong et al. further developed a Cu²⁺–tannic acid autocatalytic system combining TA@CNF and glycerol to fabricate CuTCG conductive hydrogels [45]. This system achieved a tensile strength of ~156.4 kPa, an elongation of ~1624.8%, a conductivity of 0.0436 S m⁻¹, and a maximum GF of ~4.35 within 0–400% strain, with a response time of ~120 ms and maintained flexibility and repeatable adhesion from −20 to 60 °C. Wu et al. employed Cu²⁺–alanine coordination bonds to construct a 3D-printed hybrid network hydrogel in which in situ generated Cu²⁺ crosslinks a PDA network while graphene sheets provide a parallel conductive filler network [46]. The dynamic coordination network contributed to stress sensitivity, and the reversible water loss and reabsorption behavior of the hydrogel additionally enabled airflow sensing for breath monitoring.

Unlike Al³⁺ which serves primarily as a crosslinking center, the distinctive contribution of Cu²⁺ lies in its redox activity—it drives rapid room-temperature gelation and sustains durable interfacial adhesion through the reversible catechol–quinone equilibrium, though its direct contribution to strain-sensing sensitivity warrants further evaluation.

Ti⁴⁺, possessing vacant d orbitals for strong coordination bond formation, has been more recently explored. Wang et al. exploited tris-catechol coordination between dopamine and Ti⁴⁺ to construct a CMC/DA@Ti conductive hydrogel, using a titanium ion precursor to circumvent Ti⁴⁺ hydrolysis [47]. The resulting CMC/DA@Ti gel reached a conductivity of 2.564 S m⁻¹ (~4.9× that of the Ti⁴⁺-free control) and a self-healing efficiency of ~93.66% within 3 h, with fast sensing/recovery behavior demonstrated during wrist bending, finger flexion, and letter pronunciation tests. The reported Ti⁴⁺ system suggests that the strong coordination afforded by this ion may contribute to enhanced conductivity; however, studies on this ion in strain-sensing contexts remain scarce.

Zr⁴⁺, with its colorless nature and multiple binding sites afforded by its aqueous tetrameric structure, has attracted growing attention. Fang et al. combined Zr⁴⁺ with SBMA, hydrophobic micelles, and calcium lignosulfonate to build a multiple-dynamic-network hydrogel with a tensile strength of ~111 kPa and 100% self-healing efficiency within 6 h [48]. A GF of 14.0 was recorded in the 400–800% strain regime, enabling smart-glove gesture recognition. Zhang et al. introduced Zr⁴⁺ into a polyacrylamide/gelatin system, yielding the PMG2Z2 sensor that sustained 924% strain, exhibited a GF of 5.744 (150–300% strain), and completed 800 stretch–release cycles with stable signal output for Morse code transmission [49]. The high coordination number of Zr⁴⁺ facilitates simultaneously achieving high crosslink density and multiple dynamic energy dissipation within a single-ion system, though the partitioning between its structural and ion-transport roles remains to be clarified. Ren et al. exploited coordination between Zr⁴⁺ and the dicarboxylic acid groups of poly(N-acryloyl aspartic acid) to construct an anti-freezing hydrogel (PAASP-Zr-LiCl) that exhibited an ionic conductivity of 8.45 S m⁻¹, a GF of 3.21, and a crystallization temperature below −80 °C, enabling continuous sensing at −30 °C [50]. Li et al. combined Zr⁴⁺ coordination with carboxylated cellulose nanofibers to form a double-network hydrogel that achieved a GF of 1.241 with a detection limit of 0.1% strain and a response time of 63 ms, while additionally serving as a vehicle for controlled transdermal drug delivery [51].

Taken together with the Fe³⁺, Ca²⁺, and Zn²⁺ systems discussed in Section 2.1 and Section 2.2, several cross-cutting observations emerge. First, the available studies suggest a broad—though not strictly monotonic—tendency linking the coordination strength of the metal ion to the dominant functional role it plays in the hydrogel network [17]. Weakly coordinating ions such as Ca²⁺ serve primarily as sacrificial crosslinkers and free-ion charge carriers, yielding mechanically tough yet rapidly recoverable networks with moderate sensing sensitivity. Strongly coordinating ions such as Fe³⁺, Ti⁴⁺, and Zr⁴⁺ tend to form more stable crosslink junctions that enhance toughness and self-healing efficiency, but their strong binding can simultaneously immobilize a larger fraction of ions at structural nodes, potentially reducing ionic conductivity. Intermediate cases such as Zn²⁺ and Al³⁺ offer a broader window for balancing these competing demands through ligand selection and stoichiometric control. Cu²⁺ occupies a distinctive position owing to its additional redox activity, which enables rapid gelation and interfacial adhesion but introduces parameters (e.g., catechol oxidation state) that are largely absent in the other systems. Second, although controlled comparisons of different metal ions within the same polymer–ligand platform have been performed for viscoelastic properties [18] and for conductivity and self-healing [52], to our knowledge, no such study has yet been reported in the specific context of strain-sensing performance, making it difficult to disentangle intrinsic coordination-chemistry contributions from confounding material-design factors. The reported performance differences therefore reflect not only the intrinsic coordination chemistry of each ion but also the confounding effects of different polymer backbones, co-crosslinking mechanisms, and testing protocols. Establishing such controlled comparisons represents an important direction for future work in this field. Other transition metal ions such as Ni²⁺ and Mn²⁺ have also been sporadically explored but are not expanded upon here.

For clarity, representative metal-ion-coordinated conductive hydrogel systems for strain sensors are summarized in

Table 1. Representative metal-ion-coordinated conductive hydrogel systems for strain sensors.

Metal Ion	Polymer Matrix/Key Components	Main Coordinating Ligand Group(s)	Key Feature	Ref.
Fe3+	dopamine (DA); poly(acrylic acid) (PAA)	catechol (DA)	dynamic Fe3+–catechol/carboxyl coordination	[30]
Fe3+	dopamine-functionalized pectin; PAA; polydopamine-coated reduced graphene oxide	carboxyl; catechol	hybrid ionic/electronic network	[31]
Fe3+	poly(acrylamide-co-acrylic acid); polyaniline (PANI)	carboxyl	synergistic dual-network sensor	[33]
Ca2+	polyacrylamide (PAM); sodium alginate/calcium alginate	guluronic acid blocks in alginate	sacrificial egg-box double network	[35]
Zn2+	gelatin; poly(acrylamide-co-acrylic acid)	carboxyl (–COO−); amino/amide groups (gelatin)	triple-crosslinked network	[38]
Al3+	poly(acrylic acid) (PAA); sulfated polysaccharide from Enteromorpha prolifera	carboxyl (–COO−); sulfate (–OSO3−)	multi-coordination stretchable network	[43]
Cu2+	sodium lignosulfonate (Ls); poly(N-hydroxyethyl acrylamide) (PHEAA); glycerol/water	catechol-like phenolic hydroxyl groups in lignin	ultrafast organohydrogel formation	[44]
Ti4+	carboxymethyl cellulose (CMC); dopamine	catechol (dopamine)	tris-catechol coordination network	[47]
Zr4+	sulfobetaine methacrylate (SBMA); stearyl methacrylate (SMA); calcium lignosulfonate (LS); hydrophobic micelles	sulfonate (–SO3−) groups	multiple dynamic networks	[48]
Zr4+	polyacrylamide (PAM); gelatin	carboxyl/amino groups (gelatin); amide groups (PAM)	resilient large-strain sensor network	[49]

Abbreviations: DA, dopamine; PAA, poly(acrylic acid); PANI, polyaniline; PAM, polyacrylamide; PHEAA, poly(N-hydroxyethyl acrylamide); CMC, carboxymethyl cellulose; SBMA, sulfobetaine methacrylate; SMA, stearyl methacrylate; Ls, sodium lignosulfonate; LS, calcium lignosulfonate.

, with emphasis on polymer matrices, major coordinating ligand groups, and characteristic coordination/network features. Note that Table 1 lists selected systems to illustrate the principal coordination strategies for each metal ion; additional systems discussed in the text (e.g., refs [37,41,46,50,51]) are omitted for brevity.

As shown in Table 1, different metal ions are typically associated with distinct ligand preferences and network-construction strategies, which in turn influence the structure and functional characteristics of the resulting hydrogels.

These differences in coordination chemistry and network architecture provide the basis for the variations in conductivity, mechanical robustness, self-healing capability, and environmental adaptability discussed in the following section.

3. Key Properties for Strain Sensing

3.1. Conductivity

In hydrogel-based strain sensors, signal transduction relies predominantly on ionic conduction: mobile ions—typically dissolved metal cations and their counter-anions—serve as charge carriers that migrate directionally through the hydrated polymer network under an applied electric field [28,53]. When the hydrogel is stretched or compressed, the resulting changes in geometry and internal pore structure alter the ion transport pathways, causing detectable variations in resistance or capacitance [53]. Ionic conductivity therefore constitutes the material basis for signal output and directly affects the signal-to-noise ratio, sensitivity, and detection limit of the device.

In metal-coordination hydrogel systems, the introduction of metal ions influences conductivity in multiple ways. Most directly, the dissolution of metal salts (e.g., FeCl₃, ZnCl₂, AlCl₃, CaCl₂) releases free ions that serve as charge carriers, thereby increasing the ionic conductivity of the system [52,54,55]. Quantitative evidence from several systems illustrates this effect: Zhao et al. observed that increasing the ZnCl₂ content from 0 to 0.10 wt% in a poly(vinyl alcohol) (PVA)/gelatin system raised the conductivity from 0.038 to 0.067 S m⁻¹ [54]; Chen et al. found that Al³⁺-crosslinked alginate/polyacrylamide hydrogels exhibited the highest conductivity (6.45 × 10⁻² S m⁻¹, approximately 323% above the uncrosslinked control), possibly owing to the smaller ionic radius and higher charge density of Al³⁺ [52]; and Zhang et al. achieved 0.85 S m⁻¹ (∼2.8× that of pure PAM hydrogels) through in situ Ca²⁺ release from pre-seeded CaCO₃ microparticles [55]. These results collectively indicate that metal ions contribute to conductivity not only through coordination-based network regulation but also, and sometimes more directly, through their role as free charge carriers.

Importantly, not all metal ions in coordination hydrogels exist in a freely mobile state. A substantial fraction is immobilized at coordination crosslinking sites, serving as structural nodes rather than participating in ion transport [33,56]. At the molecular level, ions immobilized at coordination nodes are confined within the ligand coordination sphere and lack the translational freedom required for long-range charge transport. Moreover, increasing the crosslink density restricts the segmental motion of the polymer chains between adjacent junctions, reduces the effective mesh size, and narrows the water-filled channels through which free ions migrate, collectively diminishing both ionic mobility and the volume fraction available for ion transport [22,23,57]. The coexistence of structural and free ions and their strain-dependent redistribution are illustrated schematically in Figure 2. This dual-role allocation between “conductive ions” and “structural ions” gives rise to a fundamental trade-off: while increasing the coordination crosslinking density generally strengthens the mechanical network, excessive crosslinking may restrict polymer chain mobility, reduce pore connectivity, and compress the available space for free-ion migration, potentially decreasing conductivity [53]. Fu et al. provided a representative example in their gelatin/DATNFC/Fe³⁺ (GDIH) system, where ionic conductivity increased from 0.31 × 10⁻² to 2.27 × 10⁻² S m⁻¹ as the Fe³⁺ concentration rose from 0.04 to 0.10 mol L⁻¹ [56]. Within this range, the increase in free ions and coordination crosslinking density appeared to act synergistically; however, at still higher concentrations, network densification induced by excessive crosslinking may counteract the benefit of additional free ions, a tendency also reported in other multivalent ion systems [52]. A comparable effect has been observed in Ca²⁺ systems: Sun et al. reported that ionic conductivity in SF/PAM hydrogels decreased with increasing polyacrylamide content because the denser polymer network restricted ion mobility, even though the total Ca²⁺ loading remained constant [58]. Yang et al. similarly found that increasing the dialdehyde cellulose content from 0 to 0.4 wt% in their CaTPD system reduced the conductivity from 4.47 to 3.90 S m⁻¹, attributable to the coordination between aldehyde groups and Ca²⁺ that narrowed ionic transport channels [59].

To address this trade-off, several strategies have been explored. Sun et al. designed a PANI-P(AAm-co-AA)@Fe³⁺ hydrogel employing a synergistic architecture in which Fe³⁺ played distinct roles in two networks—acting as a crosslinker in the polymer backbone and as an oxidant for polyaniline formation—thereby partially decoupling structural and conductive functions and achieving a stable conductivity of 2.03–3.67 S m⁻¹ [33]. More broadly, adjusting salt concentration, incorporating zwitterionic monomers, or engineering porous architectures have been noted as effective approaches to enhance ionic conductivity without excessively compromising network integrity [28,53].

In summary, optimizing conductivity in metal-coordination hydrogel strain sensors is not simply a matter of maximizing ion concentration. The coordination crosslinking density, the proportion of free versus structurally bound ions, the pore architecture, and the polymer chain mobility are interrelated parameters that collectively determine the effective ionic conductivity. Achieving a balance between conductivity enhancement and network structural stability remains essential for reliable electrical signal output. An important methodological caveat is that the conductivity values compiled from different studies in this section are not directly commensurable, because they were obtained using different measurement techniques under different conditions. Among the systems discussed above, Sun et al. [33] employed a four-probe resistance meter, Kondaveeti et al. [31] used a two-probe method, and Pang et al. [42] derived conductivity from current–voltage curves measured with an electrochemical workstation in a twin-electrode configuration. These three approaches differ in a fundamental respect: the two-probe method includes the contact resistance between the electrode and the hydrogel surface in the measured value, which can systematically underestimate conductivity; the four-probe method eliminates contact resistance but, in systems containing electronically conductive fillers such as polyaniline [33] or reduced graphene oxide [31], reports a composite conductivity that does not distinguish ionic from electronic contributions; and electrochemical impedance spectroscopy (EIS) can, in principle, resolve these contributions through frequency-dependent analysis but is sensitive to electrode geometry and electrolyte conditions [60]. As Daso et al. have recently emphasized, such methodological inconsistencies preclude direct quantitative comparisons between independently characterized hydrogel systems and represent a significant barrier to systematic benchmarking [60]. Readers should therefore interpret the absolute conductivity values cited in this section as indicative of the order of magnitude achievable within each coordination system, rather than as a basis for precise cross-system ranking. The development of standardized conductivity measurement protocols for hydrogel materials—analogous to the sensing-metric standardization discussed in Section 4.3 and Section 5.4—would be a valuable contribution to the field. The mechanical dimensions of this challenge—how coordination bonds enhance toughness and stretchability—are examined in the following section.

3.2. Mechanical Robustness and Stretchability

Reliable strain sensing requires that the hydrogel substrate endure repeated and often large-amplitude deformation without structural failure. In wearable scenarios, sensors attached to joints or skin are subjected to cyclic bending, stretching, and compression, and the cumulative mechanical loading can reach thousands of cycles per day [13,14,29]. Premature fracture, plastic deformation, or progressive softening of the hydrogel matrix leads directly to signal distortion or complete device failure. Moreover, practical deployment often occurs under non-ideal conditions—temperature fluctuations, partial dehydration, and uneven contact—that further accelerate mechanical degradation [15]. Therefore, mechanical robustness and stretchability are not secondary attributes but essential prerequisites for any hydrogel strain sensor intended for real-world use.

A fundamental challenge in hydrogel mechanics is the trade-off between strength and extensibility [13,14]. Chemically cross-linked hydrogels tend to be stiff but brittle, whereas lightly cross-linked or physically entangled networks are extensible but mechanically weak. The introduction of dynamic metal–ligand coordination bonds offers a way to negotiate this trade-off. Because coordination bonds possess intermediate binding energies that lie between permanent covalent bonds and weak noncovalent interactions, they can serve as sacrificial cross-links: breaking preferentially under stress to dissipate energy while protecting the primary network backbone from irreversible rupture [61]. Once the external load is removed, the reversible nature of these bonds allows them to reform, restoring the network connectivity. This energy dissipation mechanism is analogous to the role of sacrificial bonds in biological materials such as bone and cartilage, and it has emerged as a general strategy for the design of tough, stretchable hydrogels [62]. The superior effectiveness of coordination bonds as sacrificial cross-links, relative to hydrogen bonds or hydrophobic associations, can be attributed to two factors. First, their higher bond dissociation energy (typically 50–200 kJ mol⁻¹ versus 10–40 kJ mol⁻¹ for hydrogen bonds [17]) means that each bond-breaking event absorbs substantially more energy before the primary covalent network is compromised. Second, the kinetics of coordination bond reformation are governed by the ligand-exchange rate of the metal aquo-ion, which can be tuned over several orders of magnitude by varying the metal-ion species [18,20]—for example, from ~10⁷ s⁻¹ for labile Ca²⁺ and Zn²⁺ to ~10² s⁻¹ for more inert Fe³⁺. This tunability allows the material designer to match the network recovery time scale to the application’s duty cycle, a level of control that is generally not achievable with hydrogen-bonding systems. The underlying molecular mechanism is illustrated schematically in Figure 3.

A number of representative systems illustrate how coordination-mediated cross-linking can substantially enhance tensile strength and toughness. Zhou et al. reported a physically cross-linked double-network hydrogel in which the first network was formed by helical κ-carrageenan chains and the second network was reinforced by Pluronic F127 diacrylate micelles together with Fe³⁺–COO− tridentate coordination. The synergy among these three noncovalent cross-linking mechanisms yielded a tensile strength of 2.7 MPa, a fracture strain of 1400%, and a toughness of 9.82 MJ·m⁻³, placing this material among the toughest ion-conductive hydrogels reported for sensing applications [63]. At the mechanistic level, the Fe³⁺ coordination cross-links served as a high-energy sacrificial network that dissipated substantially more energy upon stretching than hydrogen bonds or hydrophobic associations alone. Das Mahapatra et al. further demonstrated the magnitude of this toughening effect in a macro-cross-linked poly(acrylic acid-co-acrylamide) system: introducing Ca²⁺ coordination bonds enabled the hydrogel to be stretched to 108 times its original length with a maximum toughness of 177 MJ·m⁻³ [64]. This notable performance was attributed to the cooperative action of electrostatic interactions, micellar elasticity, and metal–ligand coordination, suggesting that the toughening contribution of coordination bonds is maximized when they operate in concert with complementary energy dissipation mechanisms.

Beyond maximizing absolute strength or toughness, a practically important feature of coordination cross-linking is its tunability. Sun et al. developed a synergistic dual-network hydrogel, PANI-P(AAm-co-AA)@Fe³⁺, in which the mechanical properties could be regulated over a wide range by controlling the coordination cross-linking density through the degree of protonation. The ultimate tensile stress was tunable from 0.071 to 0.305 MPa and the fracture strain from 145% to 880%, enabling the same material platform to be adapted to different application scenarios—stiffer formulations for joint monitoring and softer ones for pulse or throat detection [33]. In a different approach, Li et al. combined dynamic imine bonds, Al³⁺–carboxyl coordination, and hydrogen bonds within a single network, achieving an elongation at break of 3168% and a toughness of 0.79 MJ·m⁻³, while simultaneously maintaining a conductivity of 2.33 S·m⁻¹ and a gauge factor of 4.12 at 1600% strain [65]. These results indicate that ultra-high stretchability and effective signal output are not mutually exclusive, provided that the coordination network is designed to sustain both ionic transport and structural integrity over the full deformation range.

It should be emphasized that the value of coordination cross-linking is not limited to the pursuit of extreme stretchability. In many cases, the primary contribution is to convert an otherwise brittle system into a more damage-tolerant material with adequate working range. Liu et al. designed an MPASP-PAA/Fe³⁺ hydrogel with a physical–chemical dual cross-linking structure, achieving a tensile strength of 208.75 kPa and a breaking strain of 623.1% [66]. Although these values are moderate compared with the above systems, the result underscores a practical point: Fe³⁺ cross-links converted an otherwise rigid and fragile chemically cross-linked hydrogel into a material with sufficient toughness and flexibility for sensing use.

For wearable applications, mechanical robustness must be sustained not only in a single tensile test but over prolonged cyclic deformation. In this regard, coordination-based hydrogels have shown encouraging durability. The κ-CG/P(AAm-co-AAc)-Fe³⁺ hydrogel reported by Zhou et al. exhibited recoverable hysteresis loops under cyclic loading at 400% strain and maintained its elastic response upon resting at room temperature, indicating that the triple noncovalent cross-links could partially reform between successive cycles [63]. Feng et al. demonstrated that metallogel-based fibrous sensors (MOG@TPE) with Zn²⁺ coordination could sustain more than 3000 loading–unloading cycles with only a 13% decrease in maximum stress [67]. The rapidly reversible sol–gel transition of the metallogel core ensured that conductivity remained stable throughout the entire cycling process. Even for milder coordination systems, such as the Ca²⁺-cross-linked sodium alginate hydrogel fibers reported by Tong et al., the tensile strength (1.55 MPa) and sensing signals remained satisfactorily stable over 330 cycles at 10% strain [68]. These examples collectively suggest that mechanical robustness under cyclic loading is not exclusive to strongly coordinating metal ions such as Fe³⁺. Rather, it can be achieved across a range of coordination strengths when the network architecture is properly designed.

In summary, metal-ion coordination offers a useful strategy for enhancing the mechanical properties of conductive hydrogels for strain sensing. By serving as sacrificial cross-links that dissipate energy during deformation and reform upon unloading, coordination bonds enable a favorable combination of high toughness, large stretchability, and sustained structural integrity under cyclic loading. The tunability of coordination strength and density further allows researchers to tailor the mechanical behavior to specific application requirements. A caveat in interpreting the mechanical data compiled above is that tensile testing conditions—including strain rate, specimen geometry, and the definition of toughness (integration limits of the stress–strain curve)—vary considerably among studies, making direct numerical comparisons of strength, stretchability, and toughness values across different systems inherently approximate. Nevertheless, mechanical robustness alone does not guarantee reliable sensing. Maintaining structural and electrical integrity over extended periods and under varying environmental conditions requires additional material-design strategies, which are examined in Section 3.3.

3.3. Self-Healing, Interfacial Adhesion, and Environmental Stability

The preceding section examined how dynamic metal–ligand bonds dissipate energy and reconstruct during mechanical cycling. However, the practical viability of a hydrogel strain sensor depends not only on its response to cyclic deformation but also on its ability to maintain functional integrity under realistic service conditions over extended periods. Three environmental factors are particularly detrimental to hydrogel-based devices: (i) poor interfacial adhesion, which causes the sensor to slip or detach from the target surface during use; (ii) freezing of the entrapped water at sub-zero temperatures, which stiffens the network and suppresses ionic conduction; and (iii) gradual dehydration under ambient exposure, which leads to volume shrinkage, modulus increase, and baseline drift in the electrical signal [15,69]. Because these degradation pathways are environmental rather than mechanical in origin, they cannot be addressed by the sacrificial-bond strategies discussed in Section 3.2 and require complementary material-design approaches. This section first examines how the dynamic reversibility of coordination bonds enables self-healing and sensing recovery, and then reviews how metal-coordination chemistry has been leveraged—often through the same ligands and ions that serve structural roles—to confer adhesion, freezing resistance, and water retention.

3.3.1. Self-Healing and Sensing Recovery

For strain sensors constructed from metal-ion-coordinated hydrogels, the practical value of self-healing lies not merely in structural repair but in the concurrent recovery of conductive pathways and, consequently, sensing functionality. The underlying mechanism is rooted in the dynamic and reversible nature of metal–ligand coordination bonds: upon fracture, the broken coordination cross-links at the damaged interface can re-associate when the severed surfaces are brought back into contact, progressively reconstructing both the polymer network and the ion-transport channels embedded within it [21,26]. Because ionic conduction in these hydrogels depends on the continuity of the hydrated network, the recovery of mechanical integrity and electrical conductivity are inherently coupled processes. The macroscopic manifestations of these two properties—large stretchability with elastic recovery and autonomous self-healing at fractured interfaces—are illustrated schematically in Figure 4.

Data reported across the systems surveyed in Section 2 illustrate the breadth of self-healing performance enabled by different coordination chemistries. Fe³⁺-based systems generally exhibit high healing efficiencies under ambient conditions—for example, the DA-Fe-PAA hydrogel recovered approximately 90% of its stretchability and 98% of its conductivity after 6 h of autonomous healing [30], while the BCW-TA/PAA/Fe³⁺ system achieved 91% mechanical recovery [34]. Zn²⁺-coordinated networks can offer faster kinetics; the PZS-MXene hydrogel regained injectability and structural continuity within 30 s, attributable to the rapid ligand-exchange dynamics of Zn²⁺–COO– bonds [39]. Among the other multivalent ions, the Zr⁴⁺-based P(SBMA-SMA)/LS system reached 100% self-healing efficiency within 6 h [48], and the Al³⁺-crosslinked PAA-CMC hydrogel attained approximately 96% recovery within 60 min [42]. These results indicate that metal-ion coordination can support effective autonomous self-healing across a range of time scales, from seconds to hours, depending on the coordination strength and ligand-exchange kinetics of the chosen ion–ligand pair. As discussed in Section 3.2, the water-exchange rate constant of the metal ion sets an upper bound on the rate of ligand substitution and, consequently, on the healing kinetics [18]; labile ions such as Ca²⁺ and Zn²⁺ heal within seconds to minutes, whereas more substitution-inert ions such as Fe³⁺ require hours but may yield thermodynamically more stable reconstituted junctions [70,71]. It should be noted, however, that most reported healing efficiencies are measured under quiescent, ambient laboratory conditions; whether comparable recovery can be achieved under simultaneous mechanical loading and variable humidity in real wearable use remains to be validated.

3.3.2. Interfacial Adhesion

Wearable strain sensors are typically mounted directly on the skin, and any slippage between the sensor and the epidermis introduces mechanical coupling errors that distort the measured signal [69]. Conventional hydrogels often require external adhesive tapes or bandages for fixation, which compromise comfort and long-term wearability [27]. In metal-coordination hydrogels, intrinsic adhesion can arise from the same catechol- or polyphenol-containing ligands that participate in coordination cross-linking [72]. The unoxidized catechol groups on these ligands interact with substrate surfaces through a combination of hydrogen bonding, π–π stacking, and covalent or metal-coordination coupling, while the coordination network itself provides the bulk cohesive strength needed to prevent cohesive failure during peeling [31].

Kondaveeti et al. developed a PT-DA/PAA/rGO-PDA hydrogel in which Fe³⁺ served as both the ionic cross-linker and the coordination partner for catechol groups on dopamine-functionalized pectin [31]. Skin adhesion reached approximately 85 kPa and was sustained over more than 300 consecutive attach–detach cycles without measurable degradation. Wang et al. reported comparable durability in a PDA@Fe³⁺-mediated cellulose composite system, where the adhesion to porcine skin remained at roughly 75 kPa after 10 peel-off cycles [73]. In a tannic acid–Fe³⁺ catalytically polymerized hydrogel (PATG-B-Fe), Wang et al. measured adhesion strengths of approximately 39.8 kPa on aluminum and 16.6 kPa on porcine skin, both retaining serviceable levels after five repeated cycles [34]. A common feature across these systems is that the adhesive interface is regenerated through the same reversible, noncovalent interactions that underpin the bulk network’s dynamic character, enabling repeated adhesion without a dedicated adhesive layer. One recognized limitation is the susceptibility of catechol groups to irreversible oxidation into quinone species under aerobic or alkaline conditions, which progressively reduces the density of available adhesion sites [31]. Strategies to retard this oxidation, such as incorporating antioxidant co-monomers or operating under mildly acidic conditions, have been proposed but remain to be validated under prolonged wearable use.

3.3.3. Freezing Resistance

Hydrogels typically contain 70–95% water by weight, the majority of which exists as free water that crystallizes readily below 0 °C [15]. Ice formation disrupts ionic transport pathways, increases the elastic modulus by orders of magnitude, and can permanently damage the network microstructure through expansion-induced cracking. For sensors intended for outdoor or cold-chain applications, freezing resistance is therefore highly desirable.

Within metal-coordination hydrogel systems, two anti-freeze mechanisms are commonly exploited, often in combination. The first relies on the hydration shells around dissolved metal ions, which compete with ice-lattice hydrogen bonding and thereby depress the freezing point [74]. Ghosh et al. showed that in Ca²⁺–dicarboxylate cross-linked poly(acrylamide-co-maleic acid) hydrogels, progressively increasing the Ca²⁺ content shifted the differential scanning calorimetry (DSC) endothermic peak from approximately −15 °C to −24 °C [21]. After 24 h at −15 °C, the Ca²⁺-containing hydrogel remained flexible and stretchable to roughly five times its original length, whereas the metal-free control was rigid and brittle. The second strategy introduces polyol co-solvents—most commonly glycerol—into the coordination network. Chen et al. constructed a stilbazolium-modified poly(vinyl alcohol) (PVA-SbQ)/SA double-network hydrogel cross-linked by Fe³⁺ and swollen in a glycerol/water binary solvent [75]. The synergistic action of Fe³⁺ hydration (disrupting inter-water hydrogen bonds) and glycerol (replacing free water with non-crystallizable solvent) lowered the exothermic peak to −42.3 °C. Crucially, the hydrogel retained an ionic conductivity of 0.32 S m⁻¹ at −30 °C and could detect finger, elbow, and knee bending as well as speech vibrations under sub-zero conditions [75]. A practical caveat is that glycerol competes with polymer chains for Fe³⁺ coordination sites, and excessive glycerol content was found to reduce the coordination cross-link density and consequently the tensile strength [75]. Optimizing the salt–polyol–polymer composition to balance freezing resistance against mechanical and electrical performance thus remains an important design consideration.

3.3.4. Resistance to Dehydration

Under ambient conditions, the evaporation of free water from the hydrogel surface causes progressive volume shrinkage, an increase in elastic modulus, and a decline in ionic conductivity, all of which manifest as baseline drift and sensitivity changes in the sensor output over time [15,69]. Unlike the mechanical damage discussed in Section 3.2, dehydration is a slow, continuous process that is not reversed by the dynamic reassociation of coordination bonds. Metal-coordination networks nevertheless contribute to water retention through two mechanisms: the dense cross-linked structure physically retards water diffusion toward the surface, and the hygroscopic metal salts within the network lower the internal vapor pressure.

Ghosh et al. quantified the former effect by comparing Ca²⁺-cross-linked and metal-free poly(acrylamide-co-maleic acid) hydrogels stored at 30 °C and 75% relative humidity [21]. After 30 days, the Ca²⁺-containing sample had lost only approximately 16% of its initial water content, compared with roughly 55% for the control—a difference attributed to both the tighter network pore structure created by Ca²⁺ coordination and the water-absorbing capacity of the residual CaCl2 salt. When polyol co-solvents are additionally incorporated, water retention is further improved. Chen et al. reported that introducing glycerol into the PVA-SbQ/SA/FeCl₃ system raised the 96 h water retention rate from approximately 74% to 89% at 25 °C and 75% RH [75]. Wang et al. achieved over 90% water retention after 30 days in a TA–Fe³⁺ glycerol/water system, with the gauge factor and tensile strain remaining essentially unchanged from their initial values [34]. These results are encouraging, yet a caveat applies: nearly all published water-retention data have been obtained under constant temperature and humidity in the laboratory. Real wearable environments involve fluctuating temperature, intermittent perspiration, and mechanical perturbation. The long-term stability of these systems under such conditions has not yet been rigorously validated.

In summary, self-healing capability, interfacial adhesion, freezing resistance, and dehydration resistance collectively address the functional degradation pathways that are distinct from—and complementary to—the cyclic-loading durability discussed in Section 3.2. Metal-coordination chemistry contributes to all four aspects: dynamic coordination bonds enable autonomous structural and electrical recovery, polyphenol–metal bonds provide repeatable skin adhesion, metal-ion hydration suppresses ice crystallization, and dense coordination networks combined with hygroscopic co-solvents retard water loss. A recurring theme, however, is that each of these auxiliary enhancements introduces trade-offs—catechol oxidation limits adhesion longevity, glycerol reduces coordination density, and high salt loading may alter ionic transport kinetics. Achieving comprehensive environmental adaptability without compromising the core sensing metrics of conductivity, sensitivity, and mechanical resilience remains an open challenge that calls for more systematic, multi-variable optimization.

4. Typical Strain-Sensing Applications

4.1. Large-Strain Motion Monitoring

Monitoring large-amplitude joint movements—such as bending of the fingers, wrists, elbows, and knees—is arguably the most straightforward yet demanding application scenario for hydrogel-based strain sensors [13,76]. The host material must accommodate strains that routinely exceed several hundred percent, sustain thousands of loading–unloading cycles without catastrophic failure, and maintain continuous electrical output throughout. Earlier studies on non-hydrogel flexible sensors, such as elastomer-based systems, had already suggested that dynamic metal–ligand coordination can enhance mechanical resilience and support strain-responsive signal output [77]. This design principle has subsequently been translated into hydrogel-based strain sensors, which rely on a fundamentally different sensing mechanism from electronic-conductor-based sensors. In electronic systems, resistance changes arise primarily from the disconnection or reconnection of percolating conductive filler networks [78]. In ionic hydrogels, by contrast, the dominant mechanism involves geometry-dependent changes in ion transport: as the hydrogel is stretched, the cross-sectional area decreases and the ion migration path length increases, both of which raise the measured resistance in accordance with R = ρL/A, where ρ is the resistivity, L the length, and A the cross-sectional area [78]. For metal-coordination hydrogels, an additional contribution arises from strain-induced disruption of coordination cross-links, which alters the local network mesh size and shifts the partition between free and structurally bound ions, thereby modulating the effective ionic conductivity in a manner that is coupled to the mechanical deformation of the coordination network. This coupled electromechanical response—rather than simple geometric scaling alone—underpins the typically nonlinear gauge factor profiles observed in metal-coordination hydrogel sensors and is the mechanistic basis for the application demonstrations discussed below. Figure 5 illustrates the working principle of a representative wearable hydrogel strain sensor for knee motion monitoring.

This principle has been validated in a growing number of hydrogel systems. Liang et al. [79] developed a dual ionically cross-linked hydrogel based on poly(acrylamide-co-acrylic acid)–Fe³⁺ and chitosan–SO₄²⁻ networks, achieving a tensile strain capacity of approximately 1225%, a toughness of 32.1 MJ m⁻³, and a gauge factor of 6.0 at 700% strain. In this architecture, the strong Fe³⁺–carboxylate network maintained structural integrity during stretching, while the weaker chitosan–sulfate network acted as a sacrificial component to dissipate energy and supply free ions for conduction. The sensor reliably tracked knuckle motion with fast recovery between cycles. This work illustrates a recurring design motif in large-strain applications: pairing a mechanically robust coordination network with a more labile secondary network so that neither extreme deformability nor load-bearing capacity is sacrificed.

Beyond Fe³⁺-based systems, Ca²⁺–dicarboxylate coordination has also proven effective for large-strain monitoring. Ghosh et al. [21] incorporated Ca²⁺ into a poly(acrylamide-co-maleic acid) matrix, obtaining a hydrogel that could stretch to 15–16 times its original length with a fracture energy comparable to that of cartilage (~1500 J m⁻²). The reversible Ca²⁺–carboxylate cross-links served as sacrificial bonds, and the hydrogel recovered approximately 94% of its dissipated energy after only 10 min of rest. When assembled as a strain sensor, the material monitored finger, elbow, knee, and wrist bending with gauge factors increasing from 1.9 (0–50% strain) to 4.05 (at 300% strain), and it further demonstrated Morse-code communication through finger gestures—an illustrative example of how adequate large-strain reliability enables more sophisticated functional demonstrations. The use of Ca²⁺ rather than Fe³⁺ also underscores the point that the coordination chemistry need not rely on transition metals with high ligand-field stabilization; the weaker and more rapidly exchanging Ca²⁺–carboxylate bonds can offer a favorable balance between deformability and self-recovery that suits large-motion scenarios.

A further notable feature of metal-coordination hydrogels is that a single coordination platform can often be tuned to cover a broad deformation window. Sun et al. [33] reported an Fe³⁺-coordinated poly(acrylamide-co-acrylic acid)/polyaniline dual-network hydrogel whose mechanical stiffness could be adjusted over a wide range by controlling the degree of protonation. The optimized formulation exhibited a stable gauge factor of 0.48 across 0–400% strain with excellent durability over 300 cycles, making it suitable for detecting large joint movements such as finger and wrist bending. Liu et al. [80] employed a Tara tannin–Fe³⁺ dynamic redox system to prepare a dual-network hydrogel sensor within minutes at room temperature. The resulting device captured walking and finger flexion with clear, reproducible signal waveforms at strains up to 125%. These two studies share a common implication: once the coordination network is appropriately balanced between structural rigidity and dynamic exchange, the sensor can accommodate large deformations without requiring a separate design for each target joint.

At the device-integration level, recent work has begun to address the practical requirements—uniform network structure, interfacial adhesion, and long-term cycling stability—that bridge the gap between laboratory coupons and wearable prototypes. Guan et al. [81] introduced a masking strategy that promoted uniform permeation of Fe³⁺ complexes into (PAM-co-PAA)/PVA hydrogels, nearly doubling the fracture stress (~1.55 MPa) and toughness (~2.14 MJ m⁻³) relative to conventional soaking samples. The improved mechanical homogeneity enabled the fabrication of smart gloves capable of distinguishing different hand gestures and smart insoles that estimated foot-strike frequencies during walking, jogging, and running—a step toward real-world motion-classification applications. Similarly, Zeng et al. [82] used a controlled ion-penetration approach to construct an Fe³⁺/alginate–polyacrylamide dual-network hydrogel exhibiting 1800% elongation and strong adhesion (61.8 kPa on glass). Sensing performance remained stable over 100 stretch–release cycles at 200% strain. Luo et al. developed a double-network P(AM-co-VEImBr)/CMC-Na/Fe³⁺ hydrogel in which Fe³⁺ coordinates with the carboxyl groups of CMC-Na to form ionic crosslinks, yielding a tensile strain of 1425% and segmented GF values of 12.27 (0–100%), 6.25 (100–300%), and 5.94 (300–500%) [83]. Notably, this system exhibited its highest sensitivity in the low-strain regime, an inverse pattern compared with most systems surveyed here, which may be advantageous for detecting subtle deformations; the authors further demonstrated Morse code transmission and a 3 × 3 pressure-sensing array for spatial force mapping. The emphasis on adhesion in that study is noteworthy, because inadequate skin–sensor coupling is a common but often underreported source of signal noise in large-motion monitoring.

Beyond Fe³⁺ and Ca²⁺, several other metal-ion systems have also demonstrated large-strain monitoring capabilities. Zhang et al. [43] developed a PAA-Al³⁺/SPE hydrogel reinforced by sulfated polysaccharide from Enteromorpha prolifera, achieving an elongation exceeding 4000% and a GF of 6.76 in the 300–700% strain range, with real-time monitoring of joint bending and breathing demonstrated; however, the long-term cycling stability of this ultra-stretchable system was not explicitly quantified, and the contribution of Al³⁺ to signal sensitivity—as distinct from its role as a crosslinker—remains unclear. Zhao et al. achieved markedly higher performance in an Al³⁺ system by incorporating a polydopamine/sodium caseinate crosslinked network into a P(AM-co-AA)/Al³⁺ matrix, obtaining an ultra-high stretchability of 3700%, a conductivity of 27.0 S m⁻¹, and segmented GF values of 2.7 (0–1000%), 6.5 (1000–2000%), and 23.7 (2000–2500%) with stable output over 300 cycles [84]. The GF of 23.7 is notably higher than those of most systems surveyed in this review, although it is measured in a narrow high-strain window where geometric amplification effects are pronounced. In the Zr⁴⁺ system, Fang et al. [48] constructed a P(SBMA-SMA)/LS/Zr⁴⁺ hydrogel with a GF of 14.0 in the 400–800% strain regime and integrated it into smart gloves for hand-gesture recognition; the high GF was attributed to the multiple dynamic networks afforded by Zr⁴⁺’s high coordination number, yet the reliance on hydrophobic micelles may complicate scalable fabrication. Zhang et al. [49] introduced Zr⁴⁺ into a polyacrylamide/gelatin system, yielding a sensor that sustained 924% strain with a GF of 5.744 (150–300% strain) and demonstrated Morse code transmission through finger gestures over 800 cycles; a limitation noted was the relatively slow self-healing kinetics (hours), which may restrict continuous-use scenarios requiring rapid recovery. These examples collectively demonstrate that the large-strain monitoring capability of metal-coordination hydrogels is not confined to Fe³⁺-based systems; Al³⁺ and Zr⁴⁺ each offer competitive or superior performance in specific metrics, broadening the design space available for wearable motion sensing.

Taken together, these examples highlight two points that are particularly relevant to large-strain wearable sensing. First, survivability under repeated large deformation is often more critical than an extremely high gauge factor; a sensor that fractures or exhibits progressive baseline drift after moderate cycling provides limited practical value regardless of its peak sensitivity. Metal-coordination hydrogels address this requirement through reversible energy dissipation and dynamic network reconstruction. Second, most reported systems achieve only moderate gauge factors (typically 0.5–6) in their large-strain regimes, yet they reliably distinguish different joint angles and motion types—suggesting that for large-motion wearable monitoring, signal consistency and mechanical durability, rather than raw sensitivity, are the true performance-limiting factors. From the standpoint of ion selection, no single metal ion uniformly dominates all large-strain sensing requirements. Fe³⁺ offers the richest set of demonstrated design strategies, but this comes with complex multi-role coupling; Ca²⁺ favors simplicity and rapid energy recovery for high-frequency cyclic motions; Zn²⁺ provides a balanced dynamic-network platform in the moderate-strain regime; and Al³⁺ and Zr⁴⁺ demonstrate that high stretchability or relatively high GF can also be achieved outside the Fe³⁺ framework. Accordingly, the practical design question is less which ion is “best” in general than which coordination regime is most appropriate for the target deformation mode, operating environment, and sensing priority.

4.2. Subtle Physiological Signal Detection

Beyond large-scale joint movements, the detection of subtle physiological signals represents another important application direction for metal-ion-coordinated conductive hydrogel strain sensors. Signals such as arterial pulse, respiration, swallowing, vocal cord vibration, and facial expression involve only minor tissue deformations, typically on the order of a few percent strain or less [29]. Reliable capture of these weak and transient signals places stringent demands on the sensing materials, including high sensitivity at low strain, a low detection limit, fast response and recovery times, and a stable baseline with minimal signal drift [11,29].

Several metal-ion-coordinated conductive hydrogel systems have shown the ability to detect vibration- and deformation-related signals originating from the throat and face. Liu et al. reported an MPASP-PAA/Fe³⁺ composite conductive hydrogel in which Fe³⁺ ions served as physical crosslinking agents to coordinate with carboxyl groups, simultaneously improving toughness and ionic conductivity. When attached to a volunteer’s throat, the assembled strain sensor distinguished the pronunciation of different English phrases and detected the swallowing motion during water intake through reproducible changes in relative resistance [66]. Pang et al. developed a PNA/PVP/TA/Fe³⁺ dual-network hydrogel, where Fe³⁺–catechol coordination between tannic acid and Fe³⁺ strengthened the network and contributed to ionic conduction. The sensor showed stable signal responses when applied to throat-related deformation monitoring, including swallowing actions [85]. Wang et al. prepared a P(AA-SBMA)/CMC/Fe-TA@SWNT/PPy hydrogel in which Fe³⁺–tannic acid coordination compounds were coated onto carbon nanotubes to establish conductive pathways and dynamic crosslinks. The resulting sensor detected facial expressions such as smiling and chewing, breathing patterns, and vocal cord vibrations corresponding to different spoken phrases [86]. These examples suggest that Fe³⁺-coordinated hydrogels can offer adequate sensitivity for detecting weak mechanical deformations in the throat and facial regions.

The detection of arterial pulse waveforms has also attracted attention because of its relevance to cardiovascular status assessment. Wang et al. constructed a PVA/PAA/Fe³⁺-GaIn conductive hydrogel, where Fe³⁺ coordination with carboxyl groups enhanced the mechanical integrity and ionic transport of the hydrogel matrix. When applied as a surface electrode for pulse wave and electrocardiogram signal collection, the material yielded waveform features comparable to those recorded with conventional Ag/AgCl electrodes [87]. Sun et al. reported a PANI-P(AAm-co-AA)@Fe³⁺ hydrogel in which Fe³⁺ simultaneously acted as a network reinforcer through coordination with carboxyl groups and as an oxidizing agent for in situ polyaniline formation. The sensor detected pulse signals with relative resistance changes on the order of 1%, indicating that the Fe³⁺-mediated dual-network design may support the capture of subtle cardiovascular signals [33]. In a related study, Chen et al. demonstrated that a Ca²⁺-containing RSF/AgNW/Ca(II) ionotronic skin—relying on Ca²⁺ ions for ionic conductivity rather than strain-sensitive coordination crosslinks—could resolve the percussion wave, tidal wave, and diastolic wave within a single arterial pulse cycle [88]. Although this system differs from typical metal-ion-coordinated hydrogel strain sensors, it illustrates the broader role of metal ions in enabling physiological signal detection.

Although the examples discussed above are predominantly Fe³⁺-based, emerging evidence from other metal-ion systems suggests that subtle physiological signal detection is not exclusive to Fe³⁺ coordination chemistry. Zong et al. [45] fabricated a Cu²⁺–tannic acid autocatalytically polymerized CuTCG hydrogel that detected joint bending, swallowing motions, and spoken words with a GF of up to 4.35 in the 0–400% strain range, and notably maintained flexibility and repeatable adhesion from −20 to 60 °C; however, the relatively low ionic conductivity (0.0436 S m⁻¹) may limit the signal-to-noise ratio for the weakest physiological signals. Wang et al. [47] exploited tris-catechol Ti⁴⁺ coordination to construct a CMC/DA@Ti hydrogel with a conductivity of 2.564 S m⁻¹ that demonstrated fast sensing and recovery during wrist bending, finger flexion, and letter pronunciation tests; the high conductivity is advantageous for low-amplitude signal capture, though the scarcity of Ti⁴⁺ studies limits the generalizability of these findings. Fang et al. [48] reported that their Zr⁴⁺-reinforced zwitterionic hydrogel could detect not only large joint movements but also pulse waveforms and vocal cord vibrations, benefiting from the multiple dynamic networks that maintain structural continuity under subtle deformations; a caveat is that the self-healed sensor’s sensitivity to weak signals has not been independently verified, and the complex multi-component formulation may pose reproducibility challenges. These results indicate that the scope of metal-ion-coordinated hydrogel sensors for physiological signal detection extends beyond Fe³⁺ to include Cu²⁺, Ti⁴⁺, and Zr⁴⁺, although the number of validated systems remains limited and systematic comparisons across ion types are lacking.

The studies discussed above indicate that metal-ion-coordinated conductive hydrogels show growing potential for wearable monitoring of weak physiological deformations across a range of metal-ion systems. Fe³⁺ remains the most extensively validated ion for this application, but Cu²⁺, Ti⁴⁺, and Zr⁴⁺ have each demonstrated promising initial results in detecting swallowing, vocalization, and pulse signals. A common enabling factor is that metal-ion coordination stabilizes the dynamic network under small deformations, maintaining the continuity of ion-transport pathways and thereby preserving signal fidelity at low strain amplitudes.

4.3. Comparative Analysis of Sensing Performance

The cross-system comparisons discussed in this section are based on the representative systems compiled in

Table 2. Representative quantitative sensing performance of metal-ion-coordinated conductive hydrogels for strain sensors.

Metal Ion	Representative System	Reported Sensing Interval (%)	Gauge Factor (GF)	Other Reported Sensor Merits	Representative Application	Ref.
Fe3+	PPGP hydrogel	0–1000	14.6 (650–1000%)	169 ms; 150 cycles	Eyebrow/eye blinking, carotid pulse, joint bending	[31]
Fe3+	AL/PAA/Fe3+ hydrogel	0–300	1.68 (0–120%), 2.98 (120–200%), 4.38 (200–300%)	1000 cycles	Wireless motion monitoring	[32]
Fe3+	PANI-P(AAm-co-AA)@Fe3+ hydrogel	0–400	0.48	500 cycles	Finger/wrist bending; pulse detection	[33]
Fe3+	BCW-TA/PAA/Fe3+ hydrogel	0–1900	5.2 (1200–1900%)	91% self-healing; 2000 cycles	Flexible wearable sensing in harsh environments	[34]
Fe3+	P(AAm-co-AA)-Fe3+/CS- SO42− hydrogel	0–700	6.0 (700%)	Fast recovery; 100 cycles	Knuckle motion; speaking; swallowing	[79]
Ca2+	3D-printed agar/calcium alginate/PAM hydrogel	0–150	3.83	Rapid and stable response	Finger bending	[36]
Ca2+	PAAm-co-maleic acid/Ca2+ hydrogel	0–300	1.9 (0–50%), 4.05 (300%)	~98% conductivity recovery in 1 min;	Finger, elbow, knee, wrist bending; Morse code	[21]
Zn2+	PMAGZ hydrogel	0–400	1.98 (0–100%), 3.12 (100–250%), 5.09 (250–400%)	195 ms; 1000 cycles	Multimodal sensing; handwriting recognition	[38]
Zn2+	PZS-MXene hydrogel	0–200	1.39 (0–75%), 1.78 (75–150%), 0.52 (150–200%)	Self-healing within 30 s; 0.2 s; 100 cycles	Wearable capacitive strain sensing	[39]
Al3+	PAA-CMC-Al3+ composite hydrogel	0–2066	0.22 (0–200%), 6.7 (1600–2066%)	~96.3% self-healing within 60 min; 1000 cycles	Joint bending; pulse; phonation	[42]
Al3+	PAA-Al3+/SPE hydrogel	0–700	2.10 (25–100%), 3.23 (100–300%), 6.76 (300–700%)	Rapid self-healing within 60 s	Joint bending, breathing, phonation	[43]
Cu2+	CuTCG conductive hydrogel	0–400	1.77–4.35 (0–400%)	~120 ms; flexible/adhesive from −20 to 60 °C	Joint bending; swallowing; phonation	[45]
Zr4+	P(SBMA-SMA)/LS/Zr4+ hydrogel	0–800	3.4 (0–200%), 7.6 (200–400%), 14.0 (400–800%)	100% self-healing within 6 h	Joint bending; pulse; gesture recognition	[48]
Zr4+	PMG2Z2 hydrogel sensor	0–300	1.395 (0–50%), 2.637 (50–150%), 5.744 (150–300%)	800 cycles; fracture strain 924%	Motion monitoring; phonation; Morse code	[49]

Abbreviations: GF, gauge factor. Where multiple GF values are reported, the corresponding strain intervals are indicated in parentheses. Ref. [39] reports capacitive gauge factor; all other entries report resistive gauge factor.

and visualized in Figure 6, which were selected because they report relatively complete and comparable sensing datasets, particularly with respect to the working strain interval and gauge factor. Additional application cases discussed elsewhere in Section 4 are used to broaden the qualitative discussion of wearable sensing scenarios, but were not incorporated into this comparative framework when the reported metrics were incomplete or not directly commensurable. The most prominent trend is an apparent trade-off between the maximum gauge factor and the sensing interval breadth: Among the Table 2 entries, the highest GF values—such as the PPGP hydrogel (GF = 14.6 at 650–1000% strain [31]) and the Zr⁴⁺-reinforced hydrogel (GF = 14.0 at 400–800% strain [48])—achieve these figures within relatively narrow, high-strain windows, whereas systems designed for broad working ranges (e.g., 0–1900% for [34], 0–1000% for [31]) exhibit lower GF values at the low-strain end. This trade-off is not a design flaw but a physical consequence of the ionic sensing mechanism: at low strains, geometric changes in cross-section and length are modest, producing small resistance variations; at high strains, additional contributions from coordination bond disruption and pore-structure collapse amplify the resistance response, resulting in the nonlinear GF profiles observed across most systems.

A cross-comparison by metal-ion species further reveals that the choice of coordination ion influences the accessible sensing parameter space. Fe³⁺-based systems span the widest range of both GF (0.48–14.6) and sensing intervals (0–1900%), reflecting the large number of Fe³⁺ studies and the diversity of network architectures explored, rather than an inherent superiority of Fe³⁺ coordination for sensing. Ca²⁺ systems occupy a more compact region with moderate GF values (1.9–4.05) and intermediate strain ranges (0–300%), consistent with the weaker and more rapidly exchanging Ca²⁺–carboxylate bonds that favor energy recovery over high sensitivity. Zn²⁺ systems cluster in the 0–400% strain range with GF values of 1.98–5.09, offering a balance between sensitivity and self-healing speed. Notably, the two Zr⁴⁺-based systems [48,49] occupy a favorable region in the GF–strain landscape, combining high sensitivity (GF up to 14.0) with appreciable working ranges (up to 800%), suggesting that Zr⁴⁺ coordination merits more systematic investigation. Al³⁺ and Cu²⁺ systems each contribute only two data points to the current dataset, making generalization premature; however, the ultra-high stretchability of the Al³⁺ system [42] (0–2066%) and the environmental tolerance of the Cu²⁺ system [45] (−20 to 60 °C) highlight unique capabilities that could complement the more widely studied ions.

It must be acknowledged that the quantitative comparisons drawn from Table 2 are subject to significant methodological caveats. First, GF values reported at different strain intervals are not directly commensurable; a GF of 14.6 measured in the 650–1000% range [31] and a GF of 14.0 measured in the 400–800% range [48] may not reflect equivalent intrinsic sensitivity because the baseline resistance and the deformation mode differ at these strain levels. Second, testing conditions—including strain rate, specimen geometry, electrode configuration, and ambient humidity—vary widely among studies and are frequently under-reported, further limiting cross-study comparability. Third, several performance metrics that are critical for practical wearable deployment—such as the minimum detection limit, signal-to-noise ratio, response time under realistic multi-axial deformation, and long-term baseline stability beyond a few hundred cycles—are absent from most reports. These gaps underscore the need for the community to develop standardized testing and reporting protocols for hydrogel-based strain sensors, a point that is further elaborated in the outlook section.

5. Challenges and Current Limitations

The preceding sections have demonstrated that metal-ion-coordinated conductive hydrogels offer a versatile materials platform for strain sensing, with representative systems spanning seven metal-ion species and encompassing a broad range of sensing metrics. Nevertheless, several fundamental and practical challenges must be addressed before these materials can transition from laboratory demonstrations to reliable wearable devices. This section identifies five key areas in which the current state of the field falls short of practical requirements.

5.1. The Conductivity–Mechanics Coupling Bottleneck

As established in Section 2.1 and Section 3.1, metal ions in coordination hydrogels serve a dual role as structural crosslink nodes and as mobile charge carriers, and the partition between these two populations is governed by the coordination equilibrium [21,53]. This dual-role allocation gives rise to an intrinsic trade-off: strengthening the mechanical network requires higher crosslink density, which immobilizes more ions and reduces conductivity, while maximizing conductivity demands a large free-ion fraction that weakens the network [32,56]. Three strategies have been explored to alleviate this coupling—dual-network partitioning [33], functional-filler separation [31], and porous-architecture engineering [53]—yet each introduces secondary trade-offs. The dual-network approach [33] stabilizes conductivity but buffers the gauge factor, as discussed in Section 2.1.3; functional-filler separation [31] enhances sensitivity but lowers the baseline ionic conductivity; and porous architectures risk accelerating dehydration by increasing the surface-area-to-volume ratio [15]. Crucially, to our knowledge, no study has yet demonstrated the ability to tune ionic conductivity and mechanical stiffness independently within a single metal-coordination hydrogel, indicating that the fundamental coupling remains unresolved and represents one of the central design bottlenecks for this class of materials. This bottleneck is compounded by the disproportionate concentration of research effort on Fe³⁺-based systems (Section 2.1), which limits the diversity of coordination chemistries and network architectures available for exploring alternative decoupling strategies.

5.2. Insufficient Long-Term Operational Stability

Most studies reviewed in this work report cycling stability over a few hundred to at most 2000 loading–unloading cycles under controlled laboratory conditions [34]. For perspective, a finger joint undergoes an estimated 5000–10,000 flexion cycles per day [29], and a practical wearable sensor would be expected to operate continuously for weeks to months—implying a requirement on the order of 10⁵–10⁶ cycles, one to three orders of magnitude beyond the longest tests reported here. Under such prolonged service, at least three degradation mechanisms can compromise performance. (i) Mechanical fatigue: although dynamic coordination bonds can reform after rupture, the reformation is rarely quantitative, and progressive accumulation of unrecovered damage—manifesting as creep, stress relaxation, and hysteresis growth—can lead to gradual baseline drift. Recent reviews have highlighted fatigue resistance as a prerequisite that remains inadequately characterized for most conductive hydrogel systems [89]. (ii) Ion redistribution: under prolonged or asymmetric mechanical loading, concentration gradients of free metal ions may develop within the hydrogel, causing spatially inhomogeneous conductivity and unpredictable changes in the baseline resistance. This phenomenon has been noted qualitatively [15,90] but has not been systematically quantified for any metal-ion system reviewed here. (iii) Dehydration and aging: despite the encouraging water-retention data reported under constant laboratory humidity (Section 3.3.4), the combined effects of intermittent perspiration, temperature cycling, and UV exposure on long-term network integrity and ionic transport remain largely unexplored. Two studies have begun to address specific aspects of these gaps. Xiao et al. reported that a Zn²⁺/Fe³⁺ dual-metal-coordinated polyampholyte hydrogel maintained an anti-dehydration lifespan exceeding 300 days under ambient open-air storage [91], although the sensing performance was characterized only at a single time point rather than tracked continuously over the storage period. Li et al. incorporated the zwitterionic osmolyte betaine into a Zr⁴⁺-crosslinked P(AA-co-AM) hydrogel, depressing the freezing point to −50.81 °C while maintaining conductivity at −20 °C [92], though performance under repeated freeze–thaw cycling was not quantified. These gaps collectively indicate that the field has yet to establish whether the dynamic coordination bond mechanism that confers short-term self-healing can also guarantee long-term signal reliability under real-world operating conditions.

5.3. Biosafety and Skin-Contact Compatibility

Biosafety and skin-contact compatibility have been largely overlooked in the current literature on metal-ion-coordinated hydrogel strain sensors. The majority of the systems reviewed in this work employ transition-metal salts (FeCl₃, ZnCl₂, CuCl₂, AlCl₃) at concentrations sufficient for crosslinking and conductivity. While certain ions—such as Ca²⁺ and Zn²⁺—are generally regarded as biologically benign at moderate concentrations, others—particularly Fe³⁺ at high loadings and Cu²⁺ with its redox activity—may pose risks of local cytotoxicity, oxidative stress, or skin sensitization upon prolonged or repeated exposure [90,93]. A further concern is that the polyphenol-based ligands (dopamine, tannic acid) widely used for adhesion and coordination may undergo irreversible oxidation on the skin surface, generating quinone species with known irritant potential [31]. Among the systems surveyed in Section 2, Section 3 and Section 4, only a small number have reported any biocompatibility assessment. Wang et al. [34] evaluated their BCW-TA/PAA/Fe³⁺ hydrogel using L929 fibroblast cells and observed 93% cell viability after 48 h of incubation, along with antibacterial activity against S. aureus and E. coli. Wang et al. [36] similarly conducted U87-MG cell viability tests on their 3D-printed Ca²⁺–alginate/PAM hydrogels and confirmed low cytotoxicity after removal of unreacted monomers. Pang et al. [42] selected Al³⁺ specifically for its reported low cytotoxicity, though no independent cell-viability assay was presented. Chen et al. evaluated their Al³⁺-crosslinked chitosan/tannic acid/PAA dual-network hydrogel in an L929 cell viability assay and further tested its antibacterial efficacy and wound healing promotion in a mouse skin infection model [94]. Although this represents one of the few in vivo assessments in the field, standard dermal sensitization testing per ISO 10993 was not performed. These scattered results are encouraging but far from sufficient: none of the studies conducted skin-irritation or sensitization testing in accordance with international standards such as ISO 10993 [93], and no study has quantified the rate or cumulative extent of metal-ion release under simulated wear conditions (e.g., perspiration, mechanical abrasion, elevated temperature). For metal-ion-coordinated hydrogels to advance toward commercial wearable products, systematic evaluation of metal-ion release kinetics, percutaneous absorption, and dermal cytotoxicity under realistic long-term exposure will be indispensable [95].

5.4. Lack of Standardized Evaluation Protocols

As discussed in detail in Section 4.3, the reported gauge factor values across different systems are not directly commensurable because they are measured at different strain intervals, under different strain rates, and with different electrode configurations. Beyond the GF comparability issue, additional metrics that are critical for practical deployment—including the minimum detection limit, the signal-to-noise ratio under realistic multi-axial loading, and the long-term baseline stability over days to weeks—are absent from the majority of published reports [28,29]. The field would benefit from a community-wide effort to define minimum reporting requirements for hydrogel strain sensors, analogous to the standardized figures of merit used in photovoltaics or battery research. At a minimum, such a framework should require reporting of GF with explicit strain-interval boundaries, the testing strain rate, the electrode configuration, the ambient temperature and humidity, the number of pre-conditioning cycles applied before data acquisition, and the duration over which baseline stability was monitored.

5.5. Device Integration and Manufacturing Scalability

Beyond the material-level challenges discussed above, the transition from a laboratory hydrogel coupon to a functional wearable device introduces additional engineering barriers that have received limited attention. Signal conditioning—including amplification, filtering, and analog-to-digital conversion—will generally require compact, skin-mountable electronics, yet the interface impedance between ionic hydrogels and metallic electrodes can introduce artifacts that are rarely characterized in the current literature [3,8,95]. Encapsulation strategies that prevent dehydration while maintaining skin breathability remain an open problem: hermetic sealing preserves hydration but blocks perspiration, whereas breathable encapsulants accelerate water loss. Furthermore, nearly all studies reviewed here rely on manual casting, mold-filling, or soaking procedures that are inherently difficult to scale. Recent work by Zheng et al. demonstrated a post-printing ion-exchange approach in which a UV-cured NAGA/carboxymethyl chitosan/methyl cellulose hydrogel was sequentially immersed in FeCl₃ and LiCl solutions, yielding a 3D-printed reticulated sensor with a GF of 2.57 and stable output over 100 cycles at 60% strain [96]. This strategy separates the printing and coordination-crosslinking steps, circumventing the spontaneous gelation-kinetics challenge noted above, although the additional soaking process may complicate throughput in continuous production workflows. While 3D printing [36,97] and controlled ion-penetration [82] have been explored as potentially scalable approaches, their throughput, batch-to-batch reproducibility, and compatibility with continuous manufacturing workflows (e.g., roll-to-roll coating or screen printing) have not been validated, leaving manufacturing scalability as an unresolved barrier to practical deployment.

6. Conclusions and Outlook

This review has provided a systematic examination of metal-ion-coordinated conductive hydrogels for strain sensing, organized around the central principle that the choice of metal-ion species and ligand chemistry governs the interplay among network formation, ionic conductivity, mechanical robustness, self-healing capability, and sensing performance. Seven representative metal-ion systems (Fe³⁺, Ca²⁺, Zn²⁺, Al³⁺, Cu²⁺, Ti⁴⁺, and Zr⁴⁺) have been surveyed, and—taking the most extensively studied Fe³⁺ as a case study—three distinct design strategies (single-network catalytic systems, functional-filler separation, and dual-network partitioning) have been identified, each negotiating the inherent multi-role coupling of the metal ion in a different manner. The key sensing-relevant properties—conductivity, stretchability, self-healing, interfacial adhesion, and environmental stability—have been discussed in relation to the underlying coordination chemistry, with emphasis on the molecular-level mechanisms that link coordination bond dynamics to macroscopic sensor behavior. A comparative analysis of the sensing performance across 14 representative systems (Table 2 and Figure 6) suggests that the gauge factor and sensing range are influenced not only by the metal-ion species but also by the conduction mechanism and the strain interval over which GF is measured, underscoring the need for standardized reporting. The challenges identified in Section 5—including the conductivity–mechanics coupling bottleneck, insufficient long-term stability, biosafety concerns, the lack of standardized evaluation protocols, and device-integration barriers—collectively highlight a current gap between laboratory-demonstrated material performance and the requirements of practical wearable deployment.

Looking forward, several research directions may contribute to narrowing this gap. First, the development of controlled comparative studies that systematically vary the metal-ion species within a common polymer–ligand platform—while holding all other variables constant—would be highly valuable for isolating the intrinsic contribution of coordination chemistry to sensing performance. Such studies could employ a modular hydrogel platform (e.g., a single PAA or PAM backbone with standardized ligand density) and characterize a panel of metal ions under identical testing conditions, thereby generating the quantitative structure–property–sensing relationships that are currently lacking.

Second, advancing the scalability of hydrogel sensor fabrication is essential for practical translation. Current preparation methods—predominantly manual casting and ion-soaking—are inherently low-throughput and yield variable film thickness and ion distribution. Emerging additive manufacturing techniques, including direct ink writing (DIW) and digital light processing (DLP) [36,97], offer the potential for geometrically precise and reproducible hydrogel patterning; however, the spontaneous and often uncontrollable gelation kinetics of metal-coordination systems—where polymerization initiates immediately upon mixing of the metal salt with the monomer precursor [30,34]—pose challenges for maintaining a processable ink window during printing, and the scalability to continuous production workflows (e.g., roll-to-roll coating or screen printing) remains to be demonstrated. Process–property relationships linking printing parameters to sensing performance represent a particularly promising area for future investigation.

Third, bridging the gap from material coupon to integrated wearable device requires concerted attention to encapsulation, electrode interfacing, and signal conditioning. Encapsulation strategies must balance dehydration prevention with skin breathability—a trade-off that may be addressed through semi-permeable coatings or integrated moisture-management layers. The impedance mismatch between ionic hydrogels and metallic readout electrodes deserves systematic characterization, as it directly affects the signal-to-noise ratio and detection limit of the device [95]. Integration with miniaturized wireless communication modules, as demonstrated in a preliminary fashion by Liu et al. [32], will be necessary for untethered, real-time motion monitoring.

Fourth, establishing community-wide standardized testing and reporting protocols could substantially accelerate progress. Building on the analysis in Section 4.3 and Section 5.4, we recommend that future studies report, at a minimum, the gauge factor with explicit strain-interval boundaries and strain rate, the electrode configuration, the ambient temperature and humidity, the number of pre-conditioning cycles, the baseline stability duration, and the minimum detection limit. Adoption of such a framework would enable meaningful meta-analyses across material systems and facilitate the identification of optimal ion–ligand combinations for specific application scenarios.

Fifth, the integration of machine learning algorithms with hydrogel sensor arrays opens opportunities for multimodal sensing and intelligent motion recognition. Preliminary demonstrations—such as handwritten letter recognition with 99.8% accuracy using a Zn²⁺-coordinated hydrogel and a gradient boosting classifier [38], and hand-gesture classification using Fe³⁺-based smart gloves [81]—illustrate the potential of data-driven approaches to extract high-level information from the analog resistance signals of hydrogel sensors. Ren et al. recently fabricated a Janus Fe³⁺-coordinated hydrogel sensor array and coupled it with a Bi-LSTM deep learning model to achieve continuous dynamic gesture recognition, providing a concrete demonstration of this approach within a metal-coordination hydrogel platform [98]. Future work could extend these approaches to multi-sensor arrays capable of simultaneously capturing strain, temperature, and humidity signals, enabling context-aware health monitoring that distinguishes, for example, exercise-induced motion from pathological tremor.

With continued interdisciplinary efforts spanning coordination chemistry, polymer science, manufacturing engineering, and bioelectronics, metal-ion-coordinated conductive hydrogels have the potential to evolve from a promising laboratory materials platform toward practical wearable strain-sensing applications, provided that the fundamental and engineering challenges identified in Section 5 are systematically addressed.