Additive manufacturing (AM) has revolutionized many industries and holds the promise to affect many more in the not too distant future. While people are most familiar with the 3D printers that function much like inkjet printers, another type of AM offers advantages using a different approach: Building objects with light one layer at a time.
One such technology is digital light processing (DLP). Widely used in both industrial and dental applications, DLP works by converting a liquid resin into a solid part using light, essentially pulling solid objects out of a shallow pool of resin one layer at a time.
A major challenge to using this 3D printing method, however, is that the resins need to have a low viscosity, almost like water, to function properly at high resolution. Plenty of polymers that would otherwise be useful in DLP printing are solids or too viscous, requiring solvents to dilute them to an appropriate consistency.
But adding these solvents also causes significant drawbacks, like poor dimensional accuracy after printing due to part shrinkage (up to 30%) coupled with residual stress that occurs as the solvent evaporates.
In a paper published August 30 online in Angewandte Chemie International Edition, researchers at Duke University have invented a new solvent free polymer for DLP printing. Besides eliminating the shrinkage problem, the lack of solvent also results in improved mechanical properties of the part while maintaining the ability to degrade in the body.
“I wanted to create an inherently thin, low-viscosity material for DLP to use for degradable medical devices,” said Maddiy Segal, a MEMS PhD candidate working in the laboratory of Matthew Becker, the Hugo L. Blomquist Distinguished Professor of Chemistry at Duke. “It took a lot of attempts, but eventually I was able to identify optimal monomers and a synthetic technique to create a solvent free polymer that can be used in a DLP printer without any dilution.”
Being one of the first solvent free resins that can be used in DLP printing, Segal was interested in testing the properties of parts made with it. She was excited to discover that the test parts did not shrink or distort at all and in general, they were also stronger and more durable than those made with solvents. According to her findings, this is one of the first empirical demonstrations of increased mechanical properties from eliminating solvent use in DLP 3D printing of degradable polymers.
To create her new polymer, Segal analyzed the structure and properties of existing resins developed by the Becker Lab and others and modified the monomers and chain length in a step-by-step empirical approach to achieve the desired low-viscosity polymers. She essentially used a “guess and check” approach, adjusting the polymer’s monomers or “recipes” until she found a combination that worked.
The process isn’t entirely dissimilar from cooking a meal. It involves mixing specific combinations of ingredients, heating them, and then testing the results until achieving the desired outcome. In total, Segal experimented with about 60 different combinations before finally making the product she had been hoping for.
“Besides making a material that didn’t shrink and was stronger, I also wanted it to be useful for medical applications,” Segal said. “I’m trying to make prototype devices that are both biocompatible and degradable. Eliminating toxic solvents from the process will help me do that.”
Segal’s ultimate goal with this work is to apply this technique to biodegradable medical implants. Some materials used to make temporary medical implants today are not degradable and require multiple surgeries to not only implant them, but also remove them. Through her research, Segal aims to develop implants that can be degraded through the body’s natural processes.
Devices fabricated from this material could be implanted and designed to degrade naturally over time, eliminating the need for additional surgeries to remove the device. It could also potentially be used as a bone adhesive to hold fractures together temporarily or in soft robotics applications, where a soft, degradable material is needed.
“This kind of material is what makes this particular application the primary goal of my work,” Segal said. “And in reality, this technique could be used for any sort of implant that you would want to degrade after some time and not stay there forever.”
This research is supported by the National Institutes of Health (1R01HL159954-01). A provisional patent application covering the technology has been filed by Duke University (Application #63544353).
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