University of Washington scientists have created a process for printing out molecules that can respond to their surroundings.

As a test, they created a bone-shaped plastic tab that turns purple under stretching, offering an easy way to record the force on an object.

“At the UW, this is a marriage that’s been waiting to happen – 3-D printing from the engineering side, and functional materials from the chemistry side,” said Andrew J. Boydston, a UW assistant professor of chemistry. He is corresponding author on a recent  open-access paper in the American Chemical Society’s Applied Materials and Interfaces journal.

To create the test material, they fed polycaprolactone into one print head of a 3D printer (similar the flexible filament used in conventional 3D printing). For the other print head, they used a plastic that is 99.5 percent identical, but designed to have occasional insertions of a molecule, spiropyran, that changes color when it is stretched.

The result was a printed piece of white plastic with barely visible stripes that turn purple under force. It acts as an inexpensive, mechanical sensor with no electronic parts. The whole device took about 15 minutes to print from materials that cost less than a dollar.

Low-cost instant customized sensors

Such a sensor might be used to record force or strain on a building or other structure. The researchers also want to create a version that records the speed of the force, or impact, which could allow for a football helmet that changes color when hit with sufficient force.

Different instructions can program the machine to print the plastics in any configuration — with the color-changing part in stripes in the middle, completely encased in the other plastic, or in any other desired shape. Varying how the plastic is made could yield molecules that respond in different ways. For example, a material could record load history.

The research was funded by the University of Washington.