A team of Princeton University engineers has created an easily scalable 3D printing technique to manufacture soft plastic. This new plastic has qualities that we do not typically see combined in commercially available materials; they are programmed with flexibility and stretchiness, and are also inexpensive and recyclable.
Alice S. Fergerson, Benjamin H. Gorse, Shawn M. Maguire, Emily C. Ostermann, and Emily C. Davidson, all from Princeton, wrote about their research and achievements in the peer-reviewed academic journal Advanced Functional Materials (citation below).
Team leader, Emily Davidson, an Assistant Professor of Chemical and Biological Engineering, wrote that they used a class of thermoplastic elastomers, a widely available class of polymers, to create a soft 3D-printed structure with tunable stiffness.
Amazing Properties of the New Plastic
Engineers have developed a way to control the way the 3D printer moves while creating the new plastic, allowing them to program specific physical characteristics into the material.
For example, the plastic can be designed to bend and stretch repeatedly in one direction while staying firm and inflexible in another.
Prof. Davidson explained that this method of designing soft, customizable materials could have a variety of applications, including soft robotics, medical devices, prosthetics, lightweight and durable helmets, and personalized high-performance shoe soles.
Structures Just a Few Nanometers Thick
The material’s exceptional performance comes from its intricate internal structure at the microscopic level.
The scientists used a kind of block copolymer which forms incredibly thin cylindrical structures inside a stretchy polymer matrix. These structures are only 5-7 nanometers thick. Human hair, as a comparison, is approximately 90,000 nanometers thick, or between 12,857 and 18,000 times thicker.
They used 3D printing to orient these mega-tiny cylinders, which leads to a 3D-printed material that is hard in one direction only, while in nearly all other directions, it is soft and stretchy.
A designer can orient these cylinders in various directions throughout one single object, allowing for the creation of objects with diverse and customizable material properties.
Prof. Davidson said:
“The elastomer we are using forms nanostructures that we are able to control. This allows designers a great degree of control over finished products. We can create materials that have tailored properties in different directions.”
Developing the Process
The first step was to choose the right polymer. The scientists chose a thermoplastic elastomer, that is, a block copolymer they could heat and process as a polymer melt, but which, when cooled, solidified into an elastic material.
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Homopolymers
Polymers consist of tiny building blocks, like a chain made of links. In simple polymers (homopolymers), the chain is made up of the same type of building block repeated over and over.
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Copolymers
In more complex polymers (block copolymers), the chain is made by connecting two or more different types of building blocks together.
These different regions of a block copolymer separate instead of mixing—like oil and water. The scientists exploited this property to make a new plastic with stiff cylinders within a stretchy matrix.
They used their understanding of how these tiny building blocks form and behave in fluid to create a 3D printing method that aligns these stiff nanostructures.
The team studied how the speed of printing and precise control of material flow (called under-extrusion) could influence and fine-tune the strength and flexibility of the printed material.
Thermal Annealing
Lead author, Alice Fergerson, a fifth-year PhD student in Chemical and Biological Engineering, said the following about the process and the importance of thermal annealing, a controlled heating and cooling cycle that manipulates material properties.
“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.”
The Project’s Goals
One of the researchers’ goals was to create soft materials with locally tunable mechanical properties. These new plastics had to be both affordable and scalable for industry.
We can create similar structures with locally controlled properties using liquid crystal elastomers and some other materials. However, Davidson said those materials are expensive (upwards of $2.50 per gram). Additionally, their production involves complex, multi-step processes that include carefully controlled extrusion and exposure to ultraviolet light. In extrusion, a machine forces material through a mold to create a desired shape.
In contrast, the thermoplastic elastomers used in Prof. Davidson’s research are far more affordable, costing just about one cent per gram.
These materials also offer the advantage of being compatible with standard 3D printers, making the manufacturing process of the new plastics faster, simpler, and more accessible.
Expanding the Possibilities of New Plastics
Prof. Davidson and colleagues have demonstrated their technique can incorporate additives into the thermoplastic elastomer while maintaining precise control over its material properties.
For example, an organic molecule that Prof. Lynn Loo’s group created was added. After exposing it to ultraviolet light, this molecule makes plastic grow red.
They also showed how the printer can produce complex and multilayered structures. Their device printed text that spelled out the word PRINCETON and created a very small plastic vase.
Annealing plays a key role in their process by improving the arrangement of the internal nanostructures. Prof. Davidson said that annealing also allows the material to self-repair.
As part of their work, the research team is able to cut a flexible piece of the printed new plastic and bond it back together by annealing the material.
The characteristics of the repaired material were the same as their original sample. The scientists observed “no significant differences.”
The research team’s next goal is to explore new 3D printing designs that could work well for technologies like electronic wearables and biomedical devices.
Video – 3D Printer Creating New Plastic
Citation
Fergerson, A. S., Gorse, B. H., Maguire, S. M., Ostermann, E. C., & Davidson, E. C. (2024). Reprocessable and Mechanically Tailored Soft Architectures Through 3D Printing of Elastomeric Block Copolymers. Advanced Functional Materials, 34(48), 2411812. https://doi.org/10.1002/adfm.202411812