Mutian Hua, Shuwang Wu, Ximin He
University of California, Los Angeles
A hydrogel can be viewed as a unique combination of liquid and solid, the liquid side properties endow it with generally good biocompatibility and high porosity for fast diffusion, yet they are shape fixable as a solid material, by applying knowledge of molecular and structural engineering, they can also be made to bear moderate external loads. These unique combinations of physical and chemical properties make them promising materials for a number of applications.
However, fragileness and low durability are major examples of the obstacles that hindered broad application of hydrogels in real life scenarios. It is a long dealt and studied topic over the past decade, and a number of methods have been developed to tackle this issue, such as double-networking, mechanical stretching, etc. Ximin He’s group recently developed a class of strong and tough hydrogels by tailoring hydrogel structure across several hierarchical length-scales via modulation of polymer aggregation. The series of hydrogels were prepared by combining freeze-casting and salting out in sequential steps, which could realize simultaneously high strength, toughness, stretchability and fatigue resistance (Nature), and also tunability of those properties over broad ranges (Advanced Materials).
Standing on the shoulder of Giants
- Double-network tough hydrogel
Two chemically crosslinked interpenetrating networks1 or one chemically crosslinked and another physically crosslinked interpenetrating networks2.
- Ice templating hydrogel
Hydrogels with micro-aligned channels created by growth of columnar ice crystals3.
- Mechanical Stretching / Training tough hydrogel
Anisotropic hydrogels with polymer alignments created by stretching, accompanied by chain crystallization induced by mechanical training or drying4-6.
- Low hysteresis hydrogel
Composite hydrogels showing both high toughness, fatigue resistance and low hysteresis7,8.
Motivation and Design
Natural materials offer unique combination of attractive properties that many synthetic materials could not achieve. For example, Wood is light and strong; nacres are hard and resilient; muscles and tendons are soft and tough. These combination of normally contradicting mechanical properties, are often attributed to their hierarchical structures across multiple length scales, in which the basic building blocks are tightly joint together by physical or chemical bonds, and those bundles or clusters are joint together in a more macroscopic architecture. Current bottom-up (e.g. self-assembly) and top-down (e.g. freeze-casting) fabrication methods often have difficulty in effectively tuning material structure across broad length-scales, therefore, it becomes inevitable to combine those methods together to fabricate hierarchically structured hydrogels.
1. Introducing tough hydrogels via freezing-assisted salting out (2021 Nature)
Fig. 1 Tough Hydrogels via Freezing-assisted Salting Out
- A freezing assisted salting out method was introduced to fabricate hydrogel with hierarchical structures, involving both bottom-up and top-down processes (Fig. 1a).
- Directional freezing created the micrometer and up length scale structures (Fig. 1b, c). Directional freezing concentrated PVA to form the aligned pore walls and increased the local concentration of PVA to higher values than the nominal concentration. Freezing served as an indispensable step as the concentration and closer packing of polymer during freezing prepared the polymer chains for subsequent strong aggregation and crystallization induced by salting out.
- Salting out strongly induced the aggregation and crystallization of PVA by phase separation to form the nanofibrils (Fig. 1f). Under the influence of kosmotropic ions, the pre-concentrated PVA chains strongly self-coalesced and phase-separated from the original homogeneous phase, which in turn formed the mesh-like nanofibril network on the surface of the micrometer-scale aligned pore walls (Fig. 1d, e).
- The produced hydrogel with nominal 5% PVA concentration showed an ultra-high ultimate stress of 11.5 MPa and ultimate strain of 2900% (Fig. 1g).
- The HA-5PVA hydrogel showed a remarkable gradual failure mode featuring stepwise fracture and pull-out of fibers typical for highly anisotropic materials like tendons (Fig. 1g).
- Even with pre-existing cracks, the hydrogel showed a remarkable crack-blunting ability, and the initial crack did not advance into the material at high strains, indicating flaw-insensitivity.
- The HA-PVA hydrogels showed high ultimate stress and strain that well surpassed the values seen in many reported tough hydrogels. The HA-PVA hydrogels demonstrated excellent toughness of 175 ± 9 MJ m−3 to 210 ± 13 MJ m−3 in the absence of flaws, as the direct result of their combination of high strength and high ductility (Fig. 1i-k). At a water content of over 70% in these hydrogels, these toughness values are well above those of water-free polymers such as polydimethylsiloxane (PDMS), Kevlar and synthetic rubber, even surpassing the toughness of natural tendon1 and spider silk.
2. Highly Tunable Mechanical Properties and Microstructure (2021 Adv. Mater.)
Fig. 2 Broad tunability of mechanical properties.
- Depending on the ion species, there are three kinds of possible interactions between the ions, the polymer chains, and the hydration water of polymer. First, some anions can polarize the hydration water molecules, which destabilizes the hydrogen bonds between the polymer and its hydration water molecules. Second, some ions can interfere with the hydrophobic hydration of the macromolecules by increasing the surface tension of the cavity surrounding the backbone. Third, other anions can bind directly and thus add extra charges to the PVA chains, which increase the solubility of the polymer (Fig. 2a, b).
- Ions such as SO42− and CO32− exhibit the first and second effects and could lead to the salting-out of polymers, thereby resulting in the collapse of polymer chains and forming small pores (Fig. 2c). Hydrogen bonds formed between the hydroxyl groups, which resulted in aggregation/ crystallization of the polymer chains.
- Other ions like NO3− and I− exhibit the third interaction and lead to the salting-in of polymers, thereby resulting in the dissolution of polymer chains and forming large pores. The hydrogen bonds were dissociated, and the solubility increased when the frozen samples were melted in solutions of these other ions (Fig. 2d).
- By comparing the mechanical properties of 5 wt% PVA in different anions, a typical Hofmeister series emerged following the sequence SO42- > CO32- > Ac- > Cl- > NO3- > I-, with Na+ as the constant counterion. The cation sequence based on critical PVA gelation concentration followed K+ > Na+ ≈ Cs+ > Li+ ≈ Ca2+ ≈ Mg2+.
- The specific ion effect is usually concentration sensitive. Here, Na2SO4 was used as an example to study the influence of concentrations. As concentration of Na2SO4 increased from 0.5 M to saturated (~1.8 M at room temperature), the ultimate stress and maximum strain of the resulted hydrogel increased significantly from 1.0 MPa to 15.0 MPa and from 1500% to 2100%, respectively (Fig. 2e).
- Via changing the ions or concentrations, the modulus of PVA hydrogels can be easily tuned within a broad range, from near 24 kPa to 2500 kPa, which covered all the moduli of soft tissues in the human body (Fig. 2f).
3. Versatile application in 3D Printing (2020 ACS Appl. Mater. Interfaces)
Fig. 3 3D printed tough hydrogel architectures and forceful actuators
- By chemical modification of PVA, the polymer is enabled the UV polymerization capability. By blending the modified PVA with functional monomers such as NIPAM, stimuli responsive tough hydrogels could be fabricated.
- Fig. 3a showed three types of printed lattice, respectively with Kelvin cell, Simple Cubic and Octet truss unit cells. . When using the lattice design with octet truss cells, the lattice could tolerate high external loading and deformation and still recover (Fig. 3b).
- A thermally activated bilayer tough hydrogel gripper was 3D printed (Figure 3c). Upon heating, the gripper arm bent toward the object and locked onto the object, which enabled the subsequent lifting of the object out of the water bath. The tough gel showed 20X higher actuation force than conventional pNIPAM hydrogel grippers of the same size.
- Remote actuation could be realized by coating the hydrogel with poly(pyrrole) and gained remote actuatability using an IR laser (50 mW). The illuminated part on the hydrogel actuators were locally heated quickly to induce bending, while the unilluminated parts remained cool and static. By controlling the illumination, good spatial control of actuation could be realized in the hydrogel actuator.
Future and beyond
· Tough hydrogel coating
The PVA solution could easily infiltrate into or coat upon various structures, after freezing and salting out in appropriate salt solution, the structure would gain a tough hydrogel coating for reinforcement or protection.
· Application in Soft Robotics
Blending PVA and stimuli responsive polymers yield hydrogels with increased strength and toughness, which leads to improved actuation force compared to conventional hydrogels.
· Application in Tissue / Organ Replacement
With 3D printing, the hydrogel could be fabricated into biomimetic geometries, the printed structure could then be toughened to desired level by choosing appropriate salt and concentration to achieve the compatible mechanical strength. PVA is bio-compatible and serves as good scaffold for cell seeding.
1. More understanding on the hierarchical structure- property relationship is needed. For example, how many levels are need in the structure to have a prominent effect?
2. More understanding on role of feature at specific length scale is needed. For example, is smaller structures more effective at toughening than larger structures?
3. More understanding of polymer-ion interaction mechanism is needed to explain Hofmeister effect. Currently, all trends are empirical and we do notice differences in effectiveness for different polymers.
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