Revolutionary reversible 4D printing

Reversible 4D printing technology could make these 'smart curtains' a reality without the use of any sensors or electrical devices, and instead rely on the changing levels of heat during the different times of the day to change its shape.

4D printing essentially refers to the ability of 3D printed objects to change its shape over time caused by either heat or water while the reversibility aspect of it allows it to revert to its original shape. However, to have it change back to its original shape usually requires the manual stretching or pulling of the object, which can be laborious and time consuming.

In recent years, there have been successful breakthroughs in the study of reversible 4D printing, where the object gets back its original shape without any human intervention. This usually involved the use of hydrogel as a stimulus to achieve reversible 4D printing.

As hydrogel lacks mechanical strength, it became a limitation when used for load-bearing applications. At the same time, other research work that utilised various layers of material as an alternative to hydrogel, only made the procedure to enable reversible actuation more tedious.

To address these challenges, researchers from the Singapore University of Technology and Design collaborated with Nanyang Technological University to revolutionise 4D printing by making it reversible, without the need for hydrogel nor human interference. This paper has been published in the Engineering journal.

This research work utilised only two materials, VeroWhitePlus and TangoBlackPlus, which were more readily available and compatible for printing in a 3D polyjet printer compared to using a hydrogel. The researchers also proved in their paper that the materials were able to retain considerable mechanical strength during and after actuation.

The process consisted of the swelling of elastomer with ethanol to replace the function of hydrogel swelling to induce stress on the transition material. When heated, the transition material changes its shape to a second shape. After the ethanol is being dried out of the elastomer, heating the transition material again will then allow it to revert to its original shape, as the elastomer will pull the transition material back due to elastic energy stored in it after drying.

The elastomer plays a dual function in this whole process. It is used to both to induce stress in the programming stage and to store elastic energy in the material during the recovery stage.

This process of reversible 4D printing has also proven to be more precise when the material reverts to its original shape compared to manually stretching or inducing stress on it. While it is still in its infancy, this breakthrough development provides a wide variety of applications in the future when more mechanisms and more materials become available for printing.

"While reversible 4D printing in itself is a great advancement, being able to use a more robust material while ensuring a more precise reversal during shape change is revolutionary as it allows us to produce complex structures that cannot easily be achieved through conventional fabrication. By relying on environmental conditions instead of electricity, it makes it a game changer across various industries, completely changing the way we design, create, package and ship products," said Professor Chua Chee Kai, lead researcher and Head of Engineering Product Development in SUTD.