Silicone is an excellent material for biomedical applications due to its inherent biocompatibility. The ability to 3D print implantable medical devices would be highly desirable because it would allow for: quick turnaround, local production in a medical facility, precise renderings of highly complex objects, and the ability to customize each device to fit each individual patient. However, 3D-printing with silicone has proven difficult.
According to Chris O’Bryan, a Mechanical & Aerospace Engineering doctoral student at the University of Florida, “the problem with printing silicone elastomers as that before they actually turn into a hard elastomeric structure, they’re actually a fluid and they have no structural rigidity. This leads to instabilities in the printing process, the material starts sagging, the structures lack definition, and the level of precision that can be achieved is very limited.”
O ‘Bryan is the lead author of a paper published in the journal Science Advances describing a new process for successfully 3D-printing highly precise detailed objects from silicone.
Previous attempts to print silicone focused on rapidly vulcanizing tiny droplets of silicone liquid as they were laid down with UV light. According to O’Bryan’s advisor, M&AE Professor Tom Angelini, this type of approach has limitations as to the precision that can be achieved.
|Using an organic microgel bed, researchers were able to 3D print various silicone structures. (Image source: Science Advances /|
What they did instead was to utilize an organic microgel bed to print into. In O’Bryan’s words, the microgels are “very soft gels that swell in organic solvent, that behave like a solid, but can be easily be yielded and fluidized as the print nozzle moves through it,” re-solidifying afterward.
“As we print this liquid silicone elastomer, it is supported in a liquid, which allows us to print much finer structures. Then, after we print it, we go back and cure it. It allows you to print very fine, complex structures out of nothing but liquid, and they will hold their shape.”
This is a major departure and a major advantage over other approaches because it removes the time factor. In approaches that treat the silicone more like a thermoplastic resin getting it to support itself by quickly vulcanizing it, there will always be some distortion until the liquid material is cured.
In this approach, the support is provided by soft microscale particles suspended in the gel, that “smoothly transition between fluid and solid states.”
O’ Bryan made the analogy of the ball pit in playgrounds that children play in. They support the child’s weight by packing together. In the same way, the microgel particles trap support the liquid silicon in precisely the shape it is applied, until it is cured and removed.
The key, according to Angelini, was in finding the right level of yield stress in the microgel that would support the silicone adequately, while flowing and rearranging itself around the injection needle as the silicone enters it. The main driver “is the amount of force that the needle applies as it traverses, which is set by the yield stress of the material.” The print mechanism they used was fairly generic; the innovation lies in the microgel that receives and supports the silicone.
Angelini’s lab developed an aqueous version of the gel in 2015, to be used for bioprinting with living cells, which, like this new organic version, works as “the granular gel fluidizes at the point of injection and then rapidly solidifies, trapping injected material in place.”
Both of these developments are spinoffs of Angelini’s core mission, which is to develop materials and processes in support of bioprinting.
When they turned to look into 3D-printing silicon for implantable devices, they realized it wouldn’t work with the aqueous gel, because, essentially, oil and water don’t mix. The interfacial tension between the aqueous gel and the silicone oil was high enough to overcome the yield stress, thereby rearranging the particles in the gel and destroying the object being printed. So, they set out to developed a new organic material to support the silicone oil and came up with this microgel.
The result is a new 3D printing method that promises vastly superior biomedical devices, such as tubes with valves for draining bodily fluids, or small joint replacements, or cranio-facial reconstruction, or any of the other many applications for which silicone has been used. Only now, they can be produced in a manner that allows for customized, on-demand, low-cost parts to be made, and in some cases, highly intricate parts that cannot be made any other way.