Researchers in Taiwan have developed a stretchable, self-healing gel that changes color when pulled or heated. It combines strength and built-in sensors in a single material that could have interesting applications in wearable devices and soft robotics.
In short, the new material can be thought of as a smart, rubbery material that tells you when it’s stressed out by literally changing color. This breakthrough is important because most soft or stretchable materials either stretch well but break easily, or stay tough but don’t heal themselves or sense stress.
This new gel, however, manages to combine strength, healing, and sensing capabilities in one material, which is a rare feat. The secret sauce behind the breakthrough is a clever manipulation of its molecular design.
The researchers used mechanically interlocked molecules called rotaxanes, which are ring-shaped molecules that slide along a “rod.” These are linked together in daisy chains with two rotaxanes linked together, which can expand/contract more like a spring.
Self-healing, strong, and dual-sensing
Using these, the team also attached a special fluorescent unit called DPAC to these molecules. When free to move, DPAC glows orange, but when restricted (like when stretched or bent), DPAC glows blue.
So, when you pull the gel, the rings slide and restrict DPAC’s movement, making the gel visibly shift from orange to blue under UV light. The interlocked molecules were chemically bonded into a polyurethane gel reinforced with cellulose nanocrystals (tiny, strong fibers).
The cellulose helps the gel self-heal by forming reversible hydrogen bonds across the network. Because the sliding molecules are built into the gel (not just mixed in), their motion directly couples with the gel’s stretching.
When tested, the team found that the new material is extremely stretchy, able to safely handle ~4600% strain (like stretching 1 cm of gel to 46 cm without breaking). It is also extremely tough, exhibiting a toughness of 142 MJ/m³, which is approximately 2.6 times tougher than the same gel without these molecules.
Since the material also changes color under strain (shifts from orange to blue), you can map the stress distribution of the gel simply by observing the coloration. It also benefits from dual sensing as it also changes color with heat (orange at higher temps, blue when cool/strained).
But one of the main benefits of the material is its self-healing properties. This means it can damage heals at room temperature in hours, or faster with mild heating.
Interesting potential applications
This kind of material could prove very useful for wearable devices that monitor stress and strain in real-time. The gel could also have some interesting applications in soft robotics, where parts need to be both strong and responsive.
It could also, theoretically, be used to make artificial skin or biomedical implants that can sense and self-repair. The gel could also open the door for damage-tolerant electronics that don’t fail suddenly but show visible signs of strain.
In short, this is a smart gel where tiny sliding molecules act like both shock absorbers and stress indicators. Stretch it, and it heals itself while lighting up with a color change that tells you how much it’s being strained or heated.
The study is available in the journal Advanced Functional Materials.