19 Apr 2005
Shape-memory polymers that are activated by UV light make their debut.
Polymers that can be frozen into a variety of shapes by illuminating them with UV light have been fabricated by a US-German team (Nature 434 879). The shape-memory polymers, which can fixed into tubes, arches and spirals, could have a wide range of applications in medicine. Potential examples include light activated stents for inflating blood vessels or staples for sealing wounds.
“Now instead of using heat, we can induce the shape-memory effect in polymers with light,” said Andreas Lendlein, from the GKSS Research Center in Tetlow, Germany and an author of the paper. “We are currently developing medical and industrial applications using their photosensitivity.”
According to Lendlein and his co-workers at RWTH Aachen and the Massachusetts Institute of Technology, the new shapes are stable for long periods of time, even when heated up to 50 °C. They also recover to their original shape at ambient temperatures when exposed to UV light of a shorter wavelength.
The key to the polymer’s fascinating properties are photosensitive groups that are grafted onto a polymer network. On illumination with UV light the photosensitive groups crosslink and bind to one another, causing the material to hold its new shape. Subsequent irradiation at a shorter UV wavelength cuts the crosslinks and the material returns to its original shape.
To date, the team has stretched a polymer film by 20% and then frozen it in its new state by irradiating both sides evenly with 260 nm light for 60 minutes. Illuminating it with light below 260 nm for the same period of to time caused it to return to its original length.
What’s more, the researchers have created 3D shapes by irradiating just one side of a stretched polymer strip. When the external tension is released, the illuminated side wants to keep its new shape while the dark side wants to return to its original shape. The imbalance of the forces causes the strip to deform into an arch or a spiral. Once again irradiation with short wavelength UV light reverses the effect.