Novel ‘ink’ for light-based 3D printing enables color-changing, conducting polymer structures

A new type of “ink” makes it possible to 3D print electrochemically switchable, conducting polymers using a light-based process. Researchers from the universities of Heidelberg and Stuttgart have succeeded in making so-called redox polymers useful for additive manufacturing with digital light processing.

The complex two- and three-dimensional structures created in this way can be manipulated electrochemically to change color. This opens up new perspectives for manufacturing 3D-printed optoelectronic devices.

The research work was conducted within the Research Training Group “Mixed Ionic-Electronic Transport: From Fundamentals to Applications,” which is supported by both universities. The study is published in the journal Advanced Functional Materials.

Digital light processing (DLP) is a light-based 3D-printing process in which a light-sensitive “ink” is built, layer by layer, into a three-dimensional object through selective radiation of UV light. Compared to other additive manufacturing processes, DLP enables fast manufacturing of complex structures.

“Although the technology has already been successfully used in dentistry, for example, until now DLP printing of conducting polymers for applications in optoelectronics has been challenging,” explains Prof. Dr. Eva Blasco.

The researcher and her team at the Institute for Molecular Systems Engineering and Advanced Materials of Heidelberg University are investigating unique functional materials for 3D printing. The project was carried out in close cooperation with Prof. Dr. Sabine Ludwigs and her group at the Institute of Polymer Chemistry at the University of Stuttgart, who are experts in conducting polymers and electrochemical switching.

Developing and testing the new redox ink

The two research teams developed a new methacrylate-based ink that carries redox-active carbazole groups. These redox units enable such materials to donate or accept electrons in their polymer chains, which makes them electrically conducting and able to change color depending on their oxidation or reduction state. In their current work, the researchers were able to use this photoconducting ink formulation to fabricate structures that can be manipulated electrochemically even after printing, their properties remaining switchable.

“This research was made possible by a close, interdisciplinary cooperation in our labs in Heidelberg and Stuttgart,” emphasize Christian Delavier and Svenja Bechtold, both of whom are working on their dissertations within the Research Training Group.

Color-changing structures and future applications

Using this carbazole-containing ink formulation, two-dimensional pixel arrays and checkerboard patterns as well as a multi-layered three-dimensional pyramid were additively manufactured directly. Originally almost transparent, these complex structures first took on a light green color through electrochemical stimulation and then turned dark green and ultimately practically black.

“This process is completely reversible and can be controlled down to the pixel level depending on the structure. Control in the third dimension, i.e., with respect to the architectures’ height, is especially exciting,” adds Prof. Dr. Ludwigs.

According to Prof. Blasco and Prof. Ludwigs, the combination of high-resolution, light-based 3D printing with redox polymers opens up new possibilities for the additive manufacturing of pixel displays or actuators for soft robotic applications in which the volume can be electrochemically switched.