Shape-Shifting Substance Stays Soft

Shape-memory polymers have long intrigued materials scientists because they can be bent or twisted into a new configuration, which they’ll hold until some stimulus—such as heat—reverts them to their original shape. But the same material properties that help these polymers hold a new shape also tend to make them fairly inflexible, limiting their use in biomedical applications. Now, using a simple technique, researchers have made a soft, rubbery shape-memory elastomer by combining two different polymers (ACS Macro Lett. 2015, DOI: 10.1021/acsmacrolett.5b00106).

Patrick T. Mather of Syracuse University and his group made the shape-memory composite by combining polymers with distinct melting and glass transition temperatures. While one remains soft and rubbery at a given temperature, the other crystallizes to stiffen the composite enough to hold a new form. Finding the right ratio of polymers allows the composite to hold its shape without becoming too rigid.

To make their material, the researchers chose a commercially available thermoplastic polyurethane that stays soft at room temperature. The other polymer, poly(ε-caprolactone), holds the material’s shape until heated above the polymer’s melting temperature of 56 °C. The researchers electrospun separate solutions of the two polymers onto a metal cylinder, forming a composite fiber mat. They then compressed and heated the mat into a dense film, and made that into a dog-bone shape. By heating the object to 80 °C, twisting it, and letting it cool, the researchers reshaped the polymer.

Different combinations of commercially available polymers could lead to a range of shape-memory composites with various degrees of stiffness or hardness and different transition temperatures, Mather says. Such materials are flexible enough to make custom medical probes, such as catheters, shaped to fit a particular patient or organ, he says. And Mather says the materials should be easily adaptable to three-dimensional printing, which is gaining in popularity for rapid prototyping of many objects.