Princeton engineers create soft architected materials with applications in robotics, medicine and sporting goods.
In the latest example of using 3D printing to drive advancements in materials, a team of engineers at Princeton University have found a way to use thermoplastic elastomers to create soft 3D printed structures with tunable stiffness. By designing a specific print path, the engineers were able to program the plastic’s physical properties so that a device can stretch and flex repeatedly in one direction while remaining rigid in another.
Emily Davidson, an assistant professor of chemical and biological engineering, said this approach to engineering soft architected materials could have many uses, such as soft robots, medical devices and prosthetics, strong lightweight helmets and custom high-performance shoe soles.
The research team used a type of block copolymer which forms stiff cylindrical structures that are 5-7 nanometers thick inside a stretchy polymer matrix. Orienting these nanoscale cylinders with 3D printing results in a material that is hard in one direction but soft and stretchy in nearly all others. Moreover, designers can orient these cylinders in different directions throughout a single object, leading to soft architectures which exhibit stiffness and stretchiness in different regions of an object.
The researchers used their knowledge of how block copolymer nanostructures form and how they respond to flow to develop a 3D printing technique that effectively induces alignment of stiff nanostructures. The researchers analyzed the way that printing rate and controlled under-extrusion could be used to control the physical properties of the printed material.
Alice Fergerson, a graduate student at Princeton and lead author on the published research, spoke about the technique and the key role played by thermal annealing.
“I think one of the coolest parts of this technique is the many roles that thermal annealing plays— it both drastically improves the properties after printing, and it allows the things we print to be reusable many times and even self-heal if the item gets damaged or broken.”
Davidson said annealing also enables self-healing properties of the material. As part of the work, the researchers can cut a flexible sample of the printed plastic and reattached it by annealing the material. The repaired material demonstrated the same characteristics as the original sample. The researchers said they observed “no significant differences” between the original and the repaired material.
Davidson said that one of the goals of the project was to create soft materials with locally tunable mechanical properties in a way that is both affordable and scalable for industry. It is possible to create similar structures with locally controlled properties using materials such as liquid crystal elastomers.
But Davidson said those materials are both expensive (upwards of $2.50 per gram) and require multi-stage processing involving carefully controlled extrusion followed by exposure to ultraviolet light. The thermoplastic elastomers used in Davidson’s lab cost about a cent per gram and can be printed with a commercial 3D printer.
As a next step, the research team expects to being exploring new 3D printable architectures that will be compatible with applications such as wearable electronics and biomedical devices.
The research is published in the journal Functional Materials.
