Scientists have developed a new 3D printing technique that can create soft materials programmed to either stretch or shrink when heated.
The advance could eventually lead to more capable soft robots, artificial muscles, wearable devices and medical tools that change shape inside the body.
The research was led by Professor Suk-kyun Ahn of Pusan National University in South Korea, working with scientists from Oak Ridge National Laboratory in the United States.
The study was published in Nature Communications.
Researchers have long been interested in soft materials that respond to temperature. One promising type is called a liquid crystal elastomer, or LCE.
These materials combine the flexibility of rubber with an internal molecular structure that can reorganize in response to heat. This allows them to move without traditional motors or mechanical parts.
However, 3D printing these materials has faced an important limitation.
In a common technique called extrusion-based 3D printing, soft material is pushed through a small nozzle to create individual lines, or filaments. This process tends to align molecules in a single direction. As a result, each printed filament normally has only one type of heat-driven movement.
The researchers have now found a way around this problem using a special material known as a smectic liquid crystal elastomer.
Their new ink allows the direction of its molecules to be changed during printing. Simply adjusting the printing speed or temperature can make the molecules line up in one of two directions that are perpendicular to each other.
That difference determines what the finished material does when heated. A filament printed under one set of conditions can contract, while the same ink printed under different conditions can elongate.
This means a single material and printing system can produce different types of movement without changing the chemical ingredients.
To understand why this happens, the researchers combined 3D printing experiments with measurements of how the ink flows, X-ray techniques that reveal molecular structures and computer simulations of molecular behavior.
They then demonstrated the technology by printing both two-dimensional and three-dimensional objects. These included lattice structures, curved shapes and surfaces capable of changing their textures. Importantly, the materials continued to perform reliably through repeated cycles of heating and cooling.
The ability to program stretching and shrinking into different parts of the same printed structure could make soft machines simpler and more versatile.
Possible applications include artificial muscles and actuators for soft robots, surfaces that change texture for touch-based displays, and adaptive surfaces that alter airflow and aerodynamic drag. The technology could also be useful for wearable electronics and minimally invasive medical devices that change shape when exposed to heat.
The research is still at an early stage. The experiments used one specific liquid crystal elastomer formulation under laboratory conditions, so further work will be needed before the technology can be manufactured on a large scale.
Over the next five to 10 years, however, the researchers believe the approach could help transform 3D printing into true “4D printing,” in which printed objects do more than keep their original shape. Instead, they could actively transform, move and perform useful tasks in response to their surroundings.
