Terminator 2: Judgement Day
Terminator 2: Judgment DayTriStar Pictures

A self-healing, self-adaptive and reversible material has been created by scientists in the US. The self-adaptive composite – called SAC – can heal itself over and over again and change its shape then return to its original form, a bit like T-1000 from Terminator 2.

Researchers from Rice University in Texas announced the material in American Chemical Society journal ACS Applied Materials and Interfaces. Unlike other self-healing materials, which hold healing liquids in solid shells that leak out when the outer surface is damaged, the team wanted to create a material that was more flexible and could change itself to adapt to external stimulation.

The material is created by mixing two polymers and a solvent that evaporates when heated. This leaves behind a porous mass of gooey blobs. Pei Dong, a postdoctoral researcher who co-led the study, said: "We wanted to introduce more flexibility. We wanted a biomimetic material that could change itself, or its inner structure, to adapt to external stimulation and thought introducing more liquid would be a way. But we wanted the liquid to be stable instead of flowing everywhere."

self healing material
Rice postdoctoral researcher Pei Dong holds a sample of SAC, a new form of self-adapting composite. The material has the ability to heal itself and to regain its original shape after extraordinary compression.Jeff Fitlow

In the material, tiny spheres of polyvinylidene fluoride (PVDF) hold in most of the liquid. Viscous polydimethylsiloxane (PDMS) coats the whole surface. The spheres are very resilient, deforming easily and when they crack, it quickly and repeatedly heals itself. After it is compressed, it returns to its original shape like a sponge.

The liquid content enhances their ability to absorb strain and return to its original state, while keeping the PDMS keeps the whole sphere together. Jun Lou, co-leader of the study, said: "The sample doesn't give you the impression that it contains any liquid. That's very different from a gel. This is not really squishy; it's more like a sugar cube that you can compress quite a lot. The nice thing is that it recovers."

Another study leader, Pulickel Ajayan, added: "Gels have lots of liquid encapsulated in solids, but they're too much on the very soft side. We wanted something that was mechanically robust as well. What we ended up with is probably an extreme gel in which the liquid phase is only 50% or so."

He added that making the SAC is simple and the whole process can be fine-tuned to regulate the products mechanical behaviour. The authors say the SAC could be used as a biocompatible material for tissue engineering or as a deface-tolerant structural component in engineering, but at present the sample sizes are very limited. Dong said: "Right now, we're making it in a 150-milliliter beaker, but it can be scaled up. We have a design for that."

Concluding, the authors said: "Compared to existing self-healing and self-stiffening materials, SAC offers distinct advantages in the ease of fabrication, high achievable storage modulus, and reversibility. Such materials could provide a new class of adaptive materials system with multi-functionality, tunability, and scale-up potentials."