Silicone-based components are an important structural ingredient in innumerable technologies and consumer goods — from electronic devices and automobiles to aircraft and medical devices. High-quality silicone printing is a specialized technological feat that currently depends on a few, very restrictive commercially available systems, using costly proprietary silicones to manufacture structures that are typically not very soft, nullifying one of the primary benefits of the material.
But Thomas Angelini, Ph.D., associate professor in the Department of Mechanical and Aerospace Engineering (MAE), and Senthilkumar Duraivel, a graduate from the Department of Materials Science and Engineering (MSE) working out of Angelini’s Soft Matter Lab, have collaborated on an approach to 3D print soft silicone structures like miniscule vascular bodies by turning the conventional process on its head. Their paper for this breakthrough was published in the journal Science on March 23.
“There’s not a really good way to print soft silicones, and the few that do are using their own specially formulated system for rapid prototyping,” Dr. Angelini explained. “If you want to take your favorite formulation to make a silicone object, you just can’t do it. None of these printers can just take an off-the-shelf silicone and print a structure out of it. Our approach utilizes unmodified silicone, not a one-off, which is an important distinction. While there may be a need for some fine-tuning, there’s no limitation on the formulations that can be employed.”
Flipping the Script on Inks
Present technology in 3D silicone printing is focused on developing novel inks. These new inks are often formulated to hold their own shape as they are printed without any support medium (the support medium is another soft material that serves as the host in which the ink is suspended during printing). The problem with this standard is manifold: the inks are very expensive; thousands of existing silicone formulations cannot be printed using other techniques — they require specialized inks; and the specialized inks are formulated with additives that are not desirable in other applications.
By contrast, Dr. Angelini and his team devised a method that allows the use of almost any silicone formulation whatsoever, including the thousands already in use, without modification.
The secret to the Angelini Lab approach to creating these soft silicone structures is in crafting a type of support material that is chemically similar to the printed inks, eliminating the disruptive role of interfacial tension, the same force that drives oil droplets to become circular in water, which constitutes the greatest threat to the integrity of the silicone structures.
“This simple approach eliminates an entire category of disruptive physical forces that deform or destroy materials like silicone as they’re being printed,” Dr. Angelini said. “The secret is — if you’re printing silicone, use a silicone-based support.”
Others have tried developing support materials for 3D printing silicones, like an early version with a watery-based support material. Those produced imperfect outcomes due to a very high interfacial tension between the silicone ink and water.
“In 2017, we improved on this approach by making a hydrocarbon-based support material, reducing the interfacial tension by about a factor of 3,” Dr. Angelini said. “Then we developed the AMULIT (Additive Manufacturing at Ultra-Low Interfacial Tension) concept, which reduced that by a factor of roughly 500 relative to the original silicone/water approach. We have now reached a threshold where interfacial tension simply doesn’t matter. This allows us to print the world-record finest stable features with silicone, down to 8 micrometers in diameter.”
The muse in the drug store
Inspiration strikes in the least likely of places, and for this novel method derived by Dr. Angelini, that place was the local drug store while on an errand for baby supplies.
“We had obviously been thinking about the ideal properties of a support material to pair with the silicone ink,” Dr. Angelini said. “One day I was in the baby-products aisle at Target and saw this oily gel. I knew that it would be a great support material for printing silicone. This experience inspired the 2017 work on hydrocarbon materials. A few years later, it dawned on me — I said ‘Let’s go all the way. Let’s just make the surrounding environment out of silicone oil.’”
The conventional wisdom being bucked by the UF team was that those two elements were too similar and would mix with each other. But their investigation of the body of scientific literature over the last 10 years in the study of liquid/liquid phase separation indicated that the ink and the supporting medium could remain distinct despite their similarity to each other. And more interestingly, it reduced the interfacial tension to a factor so undetectable as to make it a non-issue as a manufacturing concern.
Duraivel, who also partnered with Dr. Angelini for their 2017 silicone printing test with a hydrocarbon-based alternative, investigated various microgels and emulsions to find one that would respond well with silicone. “I saw that silicone-oil-based emulsions have been used in shampoos and conditioners since the 1970s, and they have widely been studied,” he said. “I created a formulation of an inverse emulsion, which is basically a bunch of tiny water drops packed together, permeated by silicone oil. If you immerse something into it, it will remain trapped in place. The oil is a lot like mayonnaise, but I optimized its appearance so it’s crystal-clear so we can more easily monitor its performance.”
Heart valves and heart vessels, silicone 3D printed
With the capacity to reliably manufacture extremely small structures with a high degree of precision, the UF team made the logical leap into the biomedical realm, printing heart valves and “microfluidic devices” like blood vessels. The need for small, robust, pliable silicone blood vessels for cardiac and neurological applications has the potential to create a cottage industry for next-generation patient simulation on hyper-realistic vascular structures, and, aspirationally, for possible use as prosthetics replacing the vessels themselves farther down the road. This particular biomedical application of the AMULIT concept was readily apparent to Dr. Angelini and his researchers.
“When we started looking at where we knew for sure this 3D printing of soft silicone was needed, it was in these biomedical applications, specifically neurovascular models for neurosurgeons to gain valuable experience and practice,” Dr. Angelini said. “And for the purposes of medical prostheses, we have at least demonstrated an ability to make super-thin objects that are strong, pliable and resistant to fatigue and failure.”