Researchers in South Korea have recently developed a novel 3D printing method that allows soft actuators made from a single material to stretch or contract when exposed to heat.
The project was carried out by researchers at Pusan National University (PNU) and collaborators at the Oak Ridge National Laboratory (ORNL) in Tennessee, US. They created a 3D-printable liquid crystal elastomer (LCE) whose molecular alignment can be controlled during printing.
By changing the printing speed or temperature, the team was able to determine how the material responds to heat. As per the team, the material can be used in soft robots, artificial muscles, wearables, and other shape-changing devices.
“Our work provides the first demonstration of switching molecular alignment between two orthogonal directions using a single 3D-printable smectic LCE ink, simply by tuning the printing speed and temperature,” Suk-kyun Ahn, PhD, a professor at the university, said.
Movement control
Soft actuators are materials or devices that can bend, stretch, contract, or change their shape in response to an external trigger. Liquid crystal elastomers are useful for these applications, because their molecular structure can change in response to stimuli such as heat.
However, conventional extrusion-based 3D printing typically aligns the molecules inside each printed filament in one direction. As a result, each filament is mostly limited to one type of movement. To overcome this challenge, the team used a smectic LCE ink that can switch its molecular alignment during printing.
Changing these printing conditions caused the molecules to align in one of two perpendicular directions. It determined whether the finished material elongated or contracted when heated.
Importantly, both behaviors were reached with the same printable material rather than requiring different materials for different movements. “Unlike conventional liquid crystal elastomer printing, which fixes molecular alignment along the print direction, the new approach exploits the unique behavior of smectic liquid crystal inks,” the researchers reported.
Shape-shifting structures
To better understand the process, the research team combined direct ink writing with several experimental and computational methods. They included rheological measurements to study how the ink flows, wide-angle X-ray scattering to examine its molecular structure, and molecular dynamics simulations.
The researchers then utilized the material to print both 2D and 3D structures with programmed movements. These included lattices, curved structures, and surfaces capable of changing their topography. The printed materials also kept performing reliably through repeated heating and cooling cycles.
The ability to program opposite movements into a single material could simplify the creation of soft machines with more complex behaviors. “Potential real-life applications include soft robotic actuators and artificial muscles, reconfigurable surfaces for haptic displays, and adaptive textures that regulate aerodynamic drag,” Ahn added.
According to the team, the approach could also advance 4D printing, in which 3D-printed objects are designed to change their shape or properties following fabrication.
Ahn emphasized that over the next five to 10 years, this work could allow 3D-printed objects to do more than hold a fixed shape. “Instead, they could actively change shape and carry out specific functions,” he concluded in a press release.
The study has been published in the journal Nature Communications.