Princeton engineers tap TPEs for ‘stretchable, flexible, recyclable’ 3D printing materials

Princeton, New Jersey – Researchers at Princeton University have developed “an easily scalable” 3D printing technique to manufacture soft parts with programmed stretchiness and flexibility, while being “recyclable and inexpensive.”

The team, led by Emily Davidson, used thermoplastic elastomers (TPEs) to create soft 3D printed structures with tuneable stiffness, reported the US university 12 Dec.

The research team used a type of block copolymer which forms stiff cylindrical structures that are 5-7 nanometres thick (for comparison, human hair measures about 90,000 nanometers) inside a stretchy polymer matrix.

They then used 3D printing to “orient” the nanoscale cylinders in different directions, leading to soft architectures which exhibit stiffness and stretchiness in different regions of an object.

“The elastomer we are using forms nanostructures that we are able to control,” Davidson explained.

This allows designers a great degree of control over finished products, with “tailored properties in different directions,” she added.

To start the process, researchers chose a TPE that could be heated and processed as a polymer melt, but which solidifies into an elastic material when it cools.

Traditional homopolymers are long chains of one repeating molecule, whereas block copolymers are made of different homopolymers connected to each other.

According to the Princeton report, the different regions of a block copolymer chain are like “oil and water” which separate instead of mixing.

The researchers used this property to produce material with stiff cylinders within a stretchy matrix.

Using their knowledge of how block copolymer nanostructures form, the researchers analysed the way that printing rate and controlled under-extrusion could be used to control the physical properties of the printed material.

A key part of the technique is “the many roles that thermal annealing plays,” said lead author of the paper Alice Fergerson, a graduate student at Princeton.

“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,” she added.

The goal of the project, according to Davidson, was to create soft materials with locally tuneable mechanical properties in a both “affordable and scalable” manner.

While it is possible to create similar structures using materials such as liquid crystal elastomers, such materials are both expensive and require multi-stage processing, involving carefully controlled extrusion followed by exposure to ultraviolet light.

The thermoplastic elastomers used in Davidson’s lab, however, cost “about a cent per gramme” and can be printed with a commercial 3D printer.

The researchers have shown their technique’s ability to incorporate functional additives into the thermoplastic elastomer without reducing the ability to control material properties.

In one example, they added an organic molecule that makes the material glow red after exposure to ultraviolet light.