Chemically-tunable synthesis of MPECsWe synthesized acrylate-based MPECs using a facile, single-step fabrication method via Liquid Crystal Display (LCD)-stereolithography. We first prepared a homogeneous photoresin solution that consisted of acrylic acid (AA) monomers and sodium acrylate (SA) co-monomer buffer that serve as chain builders, combined with metal ion species as dynamic crosslinkers. As illustrated in Fig. 1a,  PAA was formed by photopolymerization, initiated by addition of ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate (TPO-L) photoinitiator and tartrazine UV blocker (yellow-dye). To produce environmentally-stable, high-longevity materials, glycerol and water were used as co-solvents to suppress gel dehydration. For fair comparison, most of synthesis parameters, except the metal valency and system pH, were kept consistent between the gels. The total monomer concentration was kept constant at 8.8 M to ensure consistent polymerization. Concurrently, the maximum number of potential carboxylate–metal cation bonds was conserved at 2 mol% of monomer concentration, irrespective of the number of available binding sites on the polymer chains (see details in Methods and Supplementary Tables 1 and 2). The 3D-printed gels, after post processing, maintained their mass at 85% – 90% of the as-printed state for more than 90 days (Supplementary Fig. 2). Themogravimentric analysis (TGA) confirms that all equilibrated gels, irrespective of the metal ions and the system pH chosen, contain  ~14% ± 2.14% water by weight. This uniformity underscores the consistency in solvent content across all samples (Supplementary Discussions 6). X-ray fluorescence mapping confirmed that metal crosslinkers were homogeneously distributed throughout the sample. This methodology is generic and can be used to produce a broad range of material with different compositions through selection of an aqueous anionic monomer and various metal salts in the initial resin. Figure 1b demonstrates several multivalent metal species used to synthesize MPECs into 3D structures with μm to mm resolution and complex geometries.Fig. 1: Fabrication of Metallo-polyelectrolytes with different metal ions.a Single-pot synthesis of MPEC using stereolithography. b Optical images of synthesized photo resins (without tartrazine to show coordination color) and 3D-printed lattices (with tartrazine) with different metal ions. Scale bar 15 mm. c Magnified images of 3D-printed octet lattices, showing multiple unit cells. Scale bar 2 mm. d Binding energy vs. metal-carboxylate separation of multivalent metal ions printable with the developed fabrication method. Dashed curves represent theoretical metal cation-PAA dipole interactions. The inset highlights how the different charges of metal cations lead to distinct coordination numbers around the metal center. e pH versus charge sparsity of polyanion chains, with varying cation valency of fixed concentration (2 mol%). Strong association of multivalent metal cations pushes the equilibrium forwards below the Henderson–Hasselbach theory in the range of pH of interest.To screen for candidate metal nitrate salts we used quantum Density Functional Theory (DFT) to calculate the binding energy between various metal cations and carboxylate (acetate) (see Methods and Fig. 1d). The binding energy scales linearly with the cation valency, highlighting control over both the coordination number of complexed polyanion sites19,20,21 and the binding energy. We observe that metal ions with available d-orbitals, such as Ni2+ and Cr3+, typically deviate from the expected scaling behavior due to the formation of overlapping π-orbitals (see Supplementary Discussions 1). We focus on three representative hard-ions for each valency, Na+, Ca2+, and Al3+, to isolate the effects of valency without the concomitant effects of d-orbital interactions. Discussions relating to the metal identity can be found in Supplementary Discussion 2. Owing to the low binding energy of Na+ relative to protonation, pure PAA gels and Na+-MPEC gels are used as a system control, depending on the pH range explored. The weak electrostatic interactions of Na+ ions allow the monovalent gel to serve as a reference system where entanglement is the major contributor to the material response. For comparison, non-dynamic PAA-gels covalently crosslinked with N,N’-Methylenebisacrylamide (MBAA) were also fabricated to demonstrate the contribution of the dynamic bonds. For the remainder of this article, each crosslinker is represented by a consistent color except where otherwise specified: blue for Na+, green for Ca2+, yellow for Al3+, and gray for the MBAA.Another key parameter is the pH of the resin, which was controlled using nitric acid and sodium hydroxide, complementary to the sodium acrylate buffer and nitrate salts. During printing, each  ~5 um-thick layer is saturated in the resin with exposure time of 30 s, allowing for equilibration with the solution. To minimize pH-dependent chain conformation effects induced from anion repulsion and the changes in hydrogen bonding22, we maintained the pH in our experiments below the pKa of polyacrylic acid (~4.5). Fourier Transform Infra-red Spectroscopy (FTIR) confirmed that the change of hydrogen bonding was minimal within the range of pH explored (Supplementary Discussions 4). The high ionic strength of the gel23,24, at pH far from the pKa of the system, allows us to correlate the pH with charge sparsity of polymerized polyanion following the Henderson–Hasselbach (HH) equation, 〈ℓ〉 = [COOH]:[COO−]. The strong association of multivalent metal salts with the charged sites on the polymers shifts the deprotonation equilibrium forwards25,26, reducing the charge sparsity of the polyanion relative to the HH expectation. We account for this effect using a modified HH equation, as shown in Fig. 1e (see Supplementary Discussions 3 for derivation). Under the experimental conditions, the modified HH equation demonstrates the availability of sufficient number of binding sites on the polymer backbone to complex with the available metal ions ([COO−] ≫ n[Mn+]). Figure 1e conveys the three pH regimes experimentally explored in this work: (1) low (1.5 < pH < 2.5), (2) intermediate (2.5 < pH  < 3.0), and (3) high (3.0 < pH < 3.5). Using pH measurements of the photoresin as a proxy, these values correspond to the range of polyanion charge sparsity 〈ℓ〉, of ~100:1 –10:1. (see Supplementary Table 3).The presented fabrication platform lends itself to modulating the two key levers, metal valency and pH, to understand the extent of molecular control on the material properties. Another lever, albeit less impactful and more challenging to control, is the solvent content, which, for the sake of conciseness, is examined in greater detail in Supplementary Discussions 7.Structural bonding of metal–carboxylatesIn Fig. 2a, representative FTIR spectra are shown for MPEC samples with Na+, Ca2+, and Al3+. These spectra indicate that metal ions associate with the polymer, forming stoichiometrically charge balanced complexes. The polyacid nature of MPEC gels gives spectral character for carboxyl (R-COOH) and carboxylate (R-COO−) functional groups, with the local modes of R-COOH/COO− groups identified in Fig. 2a, at characteristic carboxyl absorption at  ~1760 cm−1 (monomer) /  ~1700 cm−1 (dimer) and alcohol C-OH stretch at  ~1240 cm−1. The carboxylate anion exhibits clear bond-and-a-half character with asymmetric and symmetric stretches at  ~1545 cm−1 and  ~1410 cm−1, respectively. Pure PAA gels exhibited the expected carboxyl stretches only, which indicates that the polymer is completely protonated (see Supplementary Discussions 4). The presence of symmetric and asymmetric modes of the carboxylate anion at pH ≲ 2.5 indicates complexation of metal cations in MPECs. The structural mode of complexation of each metal species was revealed by the separation of the R-COO− stretches, using sodium polyacrylate salt at pH = 13 as a spectral reference27,28. Taken together with the hard ionic nature of the selected metal cations, these results support a bidentate chelation of each metal cation, with direct coordination of the metal species by the polymer. Maintaining charge neutrality requires correspondence between the number of coordinating carboxylates and the metal cation valency, respectively. Our interpretation of the local coordination environment is further supported by DFT simulations of pure acetate anions (CH3COO−) bonding to each dynamic crosslinker and their associated IR signature, irrespective of the presence of solvent (see Supplementary Discussions 4).Fig. 2: Material-level properties of the additively manufactured MPECs crosslinked with Na+, Ca2+, and Al3+ in all plots.a FTIR spectra exhibit signatures of both carboxyl and carboxylate functional groups, with the later being associated with bidentate chelation of Na+, Ca2+, and Al3+. b Storage modulus with overall DSC thermogram and c Loss factor of MPECs at different pH (curves are vertically shifted for visual aid). Inset: Optical images of water pocket formed within gel. Scale bar 2 mm. Different linestyles on the pH scale in inset represents pH regime of the characterized gels. d Experimental stress–strain data of gels crosslinked with each metal crosslinks versus with MBAA under uniaxial tensile loading. Inset: Optical images of Ca2+ stretchability. Scale bar 10 mm. e MD simulation snapshots (top) and stress–strain data (bottom) for different valency (left: divalent, right: trivalent) and charges parsity (solid: high sparsity, dashed: low sparsity) under uniaxial tensile loading. f Experimental stress—strain data as a function of pH, which shows degradation in mechanical performance in increasingly alkaline environments. g Optical snapshots obtained at strains of 0%, 85%, and 225% during quasi-static fracture experiments on MPEC samples subjected to pure shear loading. Scale bar 20 mm. h Box plots depicting fracture energy and critical strain for crack initiation calculated based on stress–strain data (inset) during fracture experiments for charge-sparse MPEC (left panel) and charge-dense MPEC (right panel).Thermal characterizationDifferential thermal analysis was conducted at a low-ramp rate of \(\sim 5\frac{{\!\,}^{\circ} {{{\rm{C}}}}}{\min}\) from −30 °C to 200 °C to identify phases and phase transitions of the gels at ambient conditions. The Differential Scanning Calorimetry (DSC) thermogram (Fig. 2b, bottom) reveals the existence of a water solvation shell that participates in two processes: solvating the polymer and contributing to the metallo–polyelectrolyte complexation. The MPEC gels show a distinct and repeatable non-crystallization exotherm at 140 °C < T < 170 °C prior to an evaporative endotherm, caused by desolvation and evaporation of water in a manner consistent with the literature for the dehydration of related oxalate compounds within a similar temperature range. This evaporative thermal signature, with associated nucleation of vapor pockets, persists even when other co-solvents are removed. It is observable in dimethylformamide free MPECs synthesized by use of a water soluble photo-initiator, lithium phenyl (2,4,6-trimethylbenzoyl) phosphinate (Supplementary Discussions 7.1.2). Optical micrographs in the insets show the presence of water vapor pockets within the gels, with no other observable phase transitions present. Thermogravimetric analysis of MPECs confirms that the extent of loosely bound (≈15%) and solvated (≈5%) water remains in all equilibrated gels, which allows for segmental polymer motion and ion exchange (Supplementary Discussions 6).Dynamic Mechanical Analysis (DMA) demonstrates visually no dependence of the thermal-phase transitions on metal valency: glassy regime below 0 °C followed by 102–103 fold decrease in the storage modulus (\({E}^{{\prime} }\)) around 5–50 °C, indicative of the glassy-to-rubbery transition (Fig. 2b, top). The rubbery phase initiates above 50–70 °C, characterized by enhanced polymer chain mobility at higher temperatures. The thermal signatures from DSC and DMA indicate the absence of gel melting until the onset of water evaporation at 170 °C. Water evaporation embrittles the material, increasing \({E}^{{\prime} }\) by a factor of 20–30 and leading to unstable measurements of the loss factor (tan(δ)) at temperatures higher than 170 °C (Fig. 2b, c). This signifies the importance of the solvent phase in mediating the polymer motions in MPEC gels. Figure 2c shows that the peak of tan(δ) occurs at 15–25 °C for all samples, with a broad transition range in 0–50 °C, characteristic of glass transition Tg in radical acrylate chemistries. In contrast to the similar phase transitions among different metal species, increasing pH from 1.5 to 3.5 induces a noticeable shift in Tg and the emergence of a secondary transition at 70–100 °C, (as seen in Fig. 2c). This Tg shift with a wider distribution implies a significant effect of pH on polymer morphology and uniformity.Mechanical characterizationWe conducted uniaxial tensile experiments on dog bone-shaped specimens at a constant strain rate, \(\dot{\epsilon }\, \approx\) 0.03 s−1 at 25 °C. In Fig. 2d, we show the stress–strain data for MPEC gels infused with different metal ions within the low pH regime. This figure demonstrates a valency-dependent effect: gels with higher valency metal ions have greater stiffness and strength, and a lower stretchability. The Young’s modulus, estimated as the slope of the initial linear regime up to 10% strain, increases from 0.92 MPa for Na+-MPEC gel to 1.30 MPa for Ca+-MPEC gel to 2.82 MPa for Al+-MPEC gel. The stresses and strains at rupture are 2.68 MPa and 1750% for the Na+-MPEC gel, 3.12 MPa and 1600% for the Ca2+-MPEC gel, and 3.56 MPa and 870% for the Al3+-MPEC gel. These results reveal that despite the maximum number of potential carboxylate–metal bonds being kept constant for all gels, local functionality induces a noticeable difference in mechanical response. The covalently-crosslinked gel, shown in Fig. 2d, has a similar elastic modulus to the mono- and divalent gels, most probably due to similar molecular network connectivity, which governs the material properties at low strains29. The permanent nature of covalent bonds leads to stiffening of the gel at larger strains and results in limited stretchability of 550%. This implies that the dynamic ion crosslinkers can dissociate at the time scale of the applied deformation, allowing stretched chain relaxation and a local slip-and-stick motion of bonds under stress (Supplementary Fig. 4).In Fig. 2e (solid lines), the mechanical response of the experimentally-equivalent polyelectrolyte gel networks in the low pH regime obtained from coarse-grained Molecular-Dynamics (MD) simulations is shown. These simulations qualitatively reproduce the trends of metal ion valency on gel stiffness. The evolution of the polymer network at different strains (Fig. 2e, top) reveals that polymer chains in the divalent gels are able to re-orient along the loading direction during uniaxial deformation; in the trivalent gels, the polymer chains are more-closely packed and tend to form large percolating structures through the dynamic crosslinks (clusters) that inhibit re-orientation. This behavior is supported by the observation that trivalent gels maintain a higher fraction of inter-crosslinked metal ions, finter than that of divalent gels under high strains (Supplementary Fig. 5).The MD simulations predict that reducing the charge sparsity on the polyanion (increasing pH) leads to a reduction in finter relative to the high sparsity system, with a corresponding reduction in gel stiffness (dashed lines in Fig. 2e, bottom). This is consistent with experimental results, shown in Fig. 2f. Gels with charge-dense polyanions exhibit a 2 × reduction in modulus and tensile strength and a concomitant decrease of tensile strain from  >1600% to 1000% for Na+- and Ca2+-MPEC samples. The stiffness and stretchability of Al3+-MPEC gels are reduced by a lesser amount.To probe the effects of the molecular-level controls on the polymer network topology, we conducted pure shear fracture experiments of MPECs. Using a notched thin gel sheets in MTS load frame (MTS Systems Co., Eden Prairie, MN), we followed the methodology first developed by Thomas, Rivlin, and Lake30,31 and more recently adapted to probe fracture of tough hydrogels32 (Details in Methods and Supplementary Fig. 6). During the experiment, a thin MPEC sample with dimensions (W × H0 × th) of 40 × 10 × 1.5 mm3 and an initial notch (a0) of 16 mm (a0/W = 0.4) was stretched uniaxially at a strain rate of \(\dot{\epsilon }\, \approx\) 0.02 s−1, to induce crack propagation from the notch until complete sample rupture (Fig. 2g). We used Digital Image Correlation (DIC) to capture the exact event of crack initiation, defined as critical strain (εc), which allowed for calculating the fracture energy, Γ = \({H}_{0}\int_{0}^{{\varepsilon }_{c}}\sigma d\lambda\).In Fig. 2h, we show the Γ and εc for MPEC samples cross-linked with each metal ion and demonstrate that the fracture energy increases with stiffness. In a range of pH of 1.5–2.5 (left panel), the Al3+-MPEC gels achieve fracture energies of 7.32 ± 1.04 kJ/m2,  ≈50% greater than that of the Ca2+ MPEC, whose fracture energy is 4.66 ± 0.98 kJ/m2 and  ≈120% higher than that of the Na+-MPEC gel, 3.27 ± 0.94 kJ/m2. We observed a similar trend for higher pH (right panel). Stiffer gels that correspond to MPECs with highest valency (Al3+-MPEC gels) are also the toughest. Additionally, the critical strain for crack propagation correlates with fracture energy. As the functionality of the dynamic crosslinker increases, gels become stiffer and tougher, exhibiting greater resistance to crack initiation. We find all MPEC gels are 2–10 times tougher than their covalently crosslinked counterparts.
