Molecularly Engineered Metal Coordination Interactions for Strong, Resilient, and Fast Recovery Hydrogels

Tissues that support loads, such as muscles and cartilage, generally show high elasticity, toughness, and rapid recovery rates. However, combining such mechanical properties in the laboratory to build synthetic biomaterials is a fundamental challenge. In a new study now published in Scientific advancesWenxu Sun and a research team in physics, mechanical engineering and smart devices in China developed a strong, strong and fast-recovery hydrogel. The team designed the material using crosslinkers with dynamic cooperative interactions. They engineered a histidine-rich decapeptide (10-amino acid chain) containing two tandem zinc (Zn) binding motifs (consecutive) to facilitate thermodynamic stability, stronger binding force, and faster construction binding rate , compared to motifs of single-binding proteins or isolated ligand proteins Hybrid network hydrogels designed with the zinc peptide complex exhibited high stability, toughness, and rapid recovery within seconds. The research team hopes the scaffolds will effectively manage loaded tissue engineering applications and function as building blocks for soft robotics. The new results provide a general route to adjust the mechanical and dynamic properties of hydrogels at the molecular level.

When we walk, our muscles, cartilage, and tendons are subject to substantial mechanical loads, but biological tissues can quickly recover to function reliably during many mechanical cycles. Bioengineers have explored soft hydrogels with muscle-like mechanical properties such as biomechanical actuators, synthetic cartilage, artificial muscle, ionic skin, and soft robotics. Much effort has been devoted to improving the mechanical strength and toughness of hydrogels by introducing special energy dissipation mechanisms. Rapid recovery is also a unique trait for soft, load-bearing tissues apart from mechanical strength and toughness, but synthetic hydrogels still lack a mechanism for rapid recovery. For example, traditional dual-network (DN) or hybrid-network (HN) hydrogels with short polymer chains as sacrificial networks generally cannot be recovered early, often taking minutes to days.

The strength of a hydrogel depends on the life of its crosslinkers, where slow binding / dissociation kinetics leads to strong hydrogels, while rapid exchange rates produce soft ones. To obtain high strength and toughness, crosslinkers must be slow, but to achieve rapid recovery, crosslinkers must be dynamic with high rates of association and dissociation. To overcome this contradiction, materials bearing natural loads have used cooperative weak interactions. In this work, Sun et al. Similar engineered hybrid network (HN) hydrogels with a peptide-metal complex specifically designed as the physical crosslinker. The team formed efficient metal binding sites on a peptide sequence to design hydrogels with the required characteristics.

Mesh size, sol / gel fractions and the actual percentage of peptides that are incorporated into the hydrogel network. (A-C) SEM images of the HN-PH1 gel (A), the HN-PH3 gel (B) and the HN-PH3 gel (C) before adding Zn2 + ions. (D-F) Mesh size distributions of the HNPH1 (D) gel, the HN-PH3 (E) gel and the HN-PH6 (F) gel estimated from SEM images using ImageJ software. (G) Average mesh size of HN-PHn gels in the absence of Zn2 + ions. (H) Sol / gel fractions of different HN-PHn gels before adding zinc. (I) The percentage of peptides that are incorporated into the hydrogel network. Initial peptide concentrations were 0.3 M, 0.10 M, and 0.05 M for PH1, PH3, and PH6, respectively. The percentage of the peptides incorporated into the hydrogels was similar, as estimated by subtracting the fraction of eluted peptides from the total amount used. Error bars indicate the mean ± S.D. NS: p> 0.05. Credit: Science Advances, doi: 10.1126 / sciadv.aaz9531

The team first designed three short histidine-rich peptides (HR peptides) as ligands to bind with zinc ions (Zn²⁺) and build HN hydrogels. They denoted the peptide sequences as PH_onePH₃ and PH_{6 6} based on the number of histidines bound. Sun et al. He synthesized the peptides using solid phase peptide synthesis and purified it with high performance liquid chromatography. They observed the formation of Zn²⁺ Histidine coordination complexes by ultraviolet (UV) and Raman spectroscopy. The specifically designed peptide sequence allowed synergistic and cooperative Zn²⁺ binding affinity, compared to peptides with random histidine residues in their sequences. Scientists studied the molecular mechanism of cooperative binding of zinc ions to PH_{6 6} using circular dichroism, results suggest conformal changes of the first HP coordination site_{6 6} be critical to cooperative union and showed how structural changes favored additional Zn²⁺ Union.

Sun et al. used advanced techniques such as atomic force microscopy (AFM) based on single molecule force spectroscopy (SMF) to measure the mechanical stability of the peptide-Zn HR²⁺ complexes, i.e. hydrogel crosslinkers at the molecular level. Average breaking forces were much higher for PH_{6 6} compared to other types of hydrogels, confirming the hardness of the hydrogel. The results showed that the mechanical stability of the metal-ligand complexes could be improved considerably depending on the binding sites.

The team explored whether changes in the intrinsic properties of the crosslinkers could alter the hydrogel’s macroscopic mechanical properties by preparing a series of hybrid network (HN) hydrogels. They used the HR-Zn peptide²⁺ as sacrifice crosslinkers and covalent bonds as permanent crosslinkers in the constructs and named the resulting hydrogels as HN-PH_one, HN-PH₃and HN-PH_{6 6}, based on the peptide sequence used. The network structures were similar in the three hydrogels, but the HN-PH_{6 6} The gel was more compressible compared to the others, while working effectively in stressful mechanical environments. Interestingly, scientists could even twist the HN-PH_{6 6} Spiral hydrogel and compress the material with a sharp blade without causing permanent damage.

The team performed mechanical tensile tests on the gels and correlated the results at the mass level with those at the molecular level, to show markedly higher break stress, Young’s modulus, and resistance for HN-PH._{6 6} Sun gels et al. he then examined the material’s recovery property as a function of charge and discharge cycles and found HN-PH_{6 6} gels to almost completely recover their macroscopic mechanical properties in minutes. However, if they cut the HN-PH_{6 6} gels in pieces, the hydrogel could not heal itself as covalent crosslinkers do not reform after fracture. To understand the experimental results, the research team also performed theoretical analyzes and proposed the cooperative union of zinc in PH_{6 6} being an important factor, among other factors, in forming strong and resistant hydrogels with fast recovery rates.

Structure and properties of HN-PHn HN hydrogels crosslinked by peptide-Zn2 + coordination complexes. (A) Schematic illustration of the network structure of HN-PHn hydrogels. The network comprises covalent bonds as primary crosslinkers and ligand-metal interactions as secondary crosslinkers. (B) Optical images of the hydrogels HN-PH1 (above), HN-PH3 (medium) and HN-PH6 (below) under a compression-relaxation cycle. The HN-PH1 and HN-PH3 gels fractured, while the HN-PH6 gel almost completely recovered. (C) Optical images of the HN-PH6 gel under an extreme compression condition (compressed at> 70% tension for 100 times at 1.6 Hz). (D) Optical images of the HN-PH6 gel under extreme stress conditions (stretched at> 150% stress for 100 times at 1.6 Hz). (E) Optical image of the spirally twisted HN-PH6 gel. (F) Optical images of HN-PH6 gel compressed with a sharp, relaxed blade. No detectable cut was observed in the gel. PAM, polyacrylamide. Photo credits: Wenxu Sun, Nanjing University. Credit: Science Advances, doi: 10.1126 / sciadv.aaz9531

In this way, Wenxu Sun and colleagues developed a new hydrogel material, bioinspired by histidine residues found in natural, load-bearing materials. The combination of such outstanding mechanical properties in the laboratory has remained a challenge due to the inability to effectively take advantage of the unique metal ion binding properties that are encoded in natural proteins. In this work, Sun et al. Used bioinspired zn²⁺binding peptide as crosslinkers to form the desired hydrogels at the molecular level, highlighting the importance of cooperative metal coordination during material synthesis. They are intended to examine additional mechanical characteristics, such as adhesion to other fabrics, before making practical applications in tissue engineering.

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