Episode 11: Synthetic hydrogel combines stiffness and self-healing properties

In this podcast episode, MRS Bulletin’s Laura Leay interviews Hang Zhang from Aalto University in Finland about his group’s creation of a composite material that is both stiff and self-healing. The composite involves a hydrogel where the long polymer chains are confined between nanosheets of synthetic hectorite. This material mimics skin that is both stiff and self-healing. Applications may be forthcoming in self-healing soft robots or artificial tissues that can self-heal like synthetic skin. This work was published in a recent issue of Nature Materials. 

Transcript

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on hot topics in materials research. My name is Laura Leay. Natural materials such as skin have an impressive ability to be stiff while also being able to heal themselves. In contrast, many synthetic materials can be stiff or self-healing but don’t usually have both attributes. New research has led to the creation of a composite material that, like skin, is both stiff and self-healing. The composite involves a hydrogel where the long polymer chains are confined between nanosheets of synthetic hectorite. The nanosheets are fully delaminated and are oriented parallel to each other by introducing a mild shear flow. The spacing between the sheets is tuned by adjusting the concentration, as Dr. Hang Zhang from Aalto University in Finland, explains.

HANG ZHANG: We are using a very special type of clay nanosheet. They are synthetic ones and very unique. It was only produced by Professor Josef Breu’s group. They are very unique in the sense that you can really make monolayer dispersions of them, so they are really like 1 nm thick nanosheets dispersed in water. And that was not possible with other previous nanosheet systems.

LAURA LEAY: The nanosheets can achieve a very uniform, parallel orientation. They are much longer than the spacing between them, which hinders rotation and leads to a nematic liquid crystal where the planes are not well-defined. This structure serves as a scaffold around which the hydrogel is polymerized creating densely packed, entangled polymer chains. The resultant stiff material has a modulus of 50 MPa, similar to skin, and tensile strength up to 4.2 MPa. 

HANG ZHANG: We were indeed surprised that the stiffness got enhanced by so many times. That something we did not expect in the beginning. We thought, ok, there may be some mechanical reinforcement by adding the clay but actually it goes beyond our expectations. That what was very surprising and we were, of course, very excited to discover that. And we were also very excited to discover that the material is capable of self-healing despite the very high stiffness.

LAURA LEAY: When self-healing takes place where the nematic structure is end-to-end the ultimate tensile strength recovers by 33%, and when self-healing involves a plane parallel to the nanosheets, recovery is close to 100%. They also have good adhesion properties.

HANG ZHANG: They adhere really well on a substrate and that’s precisely because they are delaminated, they are like monolayer. If they are not, if they are multi-layer in the gel, then when they undergo some force, then those layers might tear apart which breaks the material. But because they are already monolayer dispersed, that doesn’t happen, and that actually makes the gel even stronger so that the adhesion is also very good in this system.

LAURA LEAY: Combining the nanosheets with the polymer is straightforward but the stiffness of the material led to a challenge when it came to testing the mechanical properties.

HANG ZHANG: Because they are so stiff it’s difficult to measure the mechanical properties actually, because they slip out of the grip of the tensile tester. So we really had to find proper ways of fixing them to achieve reproducible, repeatable tensile test measurement results.

LAURA LEAY: Other substances or materials can be added to the composite to provide novel functionalities. Glycerol was added to produce an organohydrogel while soaking in an iron chloride solution led to an iron-3 coordinated hydrogel. Although these could have different applications, Dr. Zhang points out that there is more work to do.

HANG ZHANG: In terms of potential one can think of, for example, self-healing soft robots or artificial tissues that can self-heal like synthetic skin. So those are kind of like the potential – where it can be applied – but of course there are still technical challenges we have to overcome to make those applications real. People got really interested in, ok you have something that is skin-like, so to speak, and it can still self-heal. So people are very much interested – we got many contacts – but of course we are still in the fundamental research stage so it’s not yet applicable to the market. But actually we really very much look forward to the opportunity to later on – let’s say – translate this technique or technology into real products in the future.

LAURA LEAY: This work was published in a recent issue of Nature Materials. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on X, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.