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3-D Printed Polymer Devices Change Color When Stretched

Materials Science: Three-dimensional printing simplifies the process of incorporating functional polymers into sensors

by Melissae Fellet
December 18, 2014

Photos of mechanosensitive polymer that changes color when stretched.
Credit: ACS Appl. Mater. Interfaces
A polymer device contains strips of a mechanosensitive spiropyran-containing polycaprolactone (left). The strips turn purple when the device is stretched (right).

Researchers have used a three-dimensional printer to create polymer structures that change color when stretched (ACS Appl. Mater. Interfaces 2014, DOI: 10.1021/am506745m). The 3-D printing approach can be used to build easy-to-read mechanical force sensors that would be difficult, if not impossible, to make with standard methods, the team says.

Structure of spiropyran.
Credit: ACS Appl. Mater. Interfaces
Spiropyran units in a polymer (left) isomerize into a purple merocyanine (right) when the material is stretched.

Functional polymers change shape or composition in response to stimuli such as light, heat, and mechanical force, and hold promise as sensors or in drug delivery devices. But it’s challenging to incorporate such materials into devices using standard manufacturing techniques. Methods that involve light or heat, for example, could trigger the material’s functional response prematurely.

So researchers often use molds when working with functional polymers. This tactic, however, can limit the shape and complexity of the final structure. To build devices with unique capabilities, Andrew J. Boydston of the University of Washington looked to 3-D printing as a powerful and versatile way to shape functional polymers.

Boydston and his colleagues wanted to use a commercial 3-D printer to create force sensors that change color when stretched. Such devices might be used to measure the structural load on a particular material or track the amount of times the material experiences a particular force, he says.

The researchers first developed a mechanosensitive polymer that could withstand being extruded by a commercial 3-D printer without changing color or being damaged by heat. They synthesized polycaprolactone polymers containing 50% by weight of a spiropyran. Applying mechanical force causes the spiropyran to isomerize to a purple merocyanine.

The researchers then used a 3-D printer to create a dog-bone-shaped device made mainly of commercial polycaprolactone, but with an inner strip of the spiropyran-containing polymer. When they pulled on one end of the device, it stretched and permanently deformed, and the sensor strip inside turned purple.

To test the polymer’s ability to act as a sensor, the researchers printed another device that could record the maximum amount of force applied to the material. They embedded four squares of spiropyran polymer inside commercial polycaprolactone. The number of squares that ended up changing color depended on how hard the researchers pulled on the device. By noting the length of the device and the amount of force applied, the researchers could calibrate the applied force to the observed color change. When they stretched other devices, they could quickly estimate the amount of applied force just by counting the number of purple squares.

A device with squares of one polymer embedded inside another material would be difficult to prepare with molds, Boydston says. The 3-D printer creates these devices quickly and reproducibly, he adds.

Charles E. Diesendruck of Technion—Israel Institute of Technology says this work will make life easier for researchers building devices with mechanically sensitive polymers. The flexibility of 3-D printers, he says, will allow them to experiment with different types of functional polymers and device shapes.



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