“Layers of color-changing plastic can be tuned by hosting organic or metallic ‘guests’, enabling easy-to-read measurements of heat and friction
A smart polymer developed at Keio University responds to stimuli by absorbing different wavelengths of light, allowing minor changes to objects to be visualized that might escape other techniques1−6. For example, the polymer can indicate how hard a pen presses on an object through vivid color transformations. This technology, which can also be used to display three-dimensional temperature changes, may be used to sense mechanical stress in material degradation, and for biomedical applications such as diagnosing pressure ulcers.
Inspired by dirt
The clay minerals found in soil naturally exemplify the versatility of layered materials. Consisting of stacked sheets of alumina and silica, clays can swell and hold water by absorbing molecules between these sheets through a process known as intercalation. People learned as early as two millennia ago that the intercalation of different ‘guest’ particles into sheets could create rudimentary dyes, and applications based on this principle continue to this day.
One modern layered material being investigated by Yuya Oaki and his colleagues at Keio University’s Department of Chemistry is a conductive plastic known as polydiacetylene. The conjugated electrons that flow through polydiacetylene make it sensitive to light — polymerization is usually indicated by a color change. Because the particular color that emerges depends on the length and twisting of the polymer backbone, researchers have explored ways to exploit these compounds as chemical and biomolecular sensors.
Oaki realized, however, that producing polydiacetylene sensors required a structure that was flexible enough to change in response to an external force. “Layered materials such as liquid crystals or lipids have soft structures that give them dynamic properties,” he explains. “To get similar sensitivity and controllability, we used intercalation to tune the response to stimulus.”
Many layers, many uses
In the team’s approach, a substrate, such as filter paper, is dipped into a solution containing long, floppy hydrocarbon chains with rigid diacetylene cores, and then air dried. The resulting thin film self-organizes into highly aligned layers, similar to a liquid crystal. The sample is subsequently immersed in a solution of metallic ions, which intercalate between the organic sheets.
Experiments revealed that different metal ions enlarged the distances and altered the structures of the organic layers in distinct ways, creating a tunable output of possible colors. When one of Oaki’s students tested for heat sensitivity, the results showed the intercalated material responded to rises or dips in temperature by changing color within seconds — a surprise, Oaki recalls, because most polydiacetylene hues are irreversible.
The team also discovered that mechanical force, such as grinding in a mortar, could initiate color switching. To refine this effect, they exchanged the metallic guests for more flexible ones based on organic amines. The new system enabled innovative detection of friction — for example, it could readily indicate the number of times a pencil rubbed a particular spot.
“It’s not easy measuring and visualizing applied friction forces, but our color-change method can even indicate accumulated amounts,” says Oaki, who was recently awarded the Young Scientists’ Prize of the Commendation for Science and Technology by the Japanese government for his research. “Next, we hope to enhance the sensitivity by using an informatics approach and also to realize materials that are based on this polydiacetylene material, and are responsive to multiple stimuli.””