Imagine getting surgery without ever being cut open. Researchers at Duke University and Harvard Medical School have successfully demonstrated a proof of concept in new research published Thursday in Science using a 3D printer that uses ultrasound to print biomaterials inside an organ.
Growing up, Junjie Yao, a bioengineer at Duke University and one of the primary investigators of the study, had heard stories about scientists coming up with great ideas over coffee or while chatting in the break room, but he never thought that would happen to him. About three years ago, Yao, and his co-primary investigator Yu Shrike Zhang, who have been friends and collaborators since their grad school days, were talking casually at an industry conference about big problems in their field. This included how to break the limit of bioprinting technologies used to create things like engineered tissue, flexible electronics, or medical devices.
But they kept running into the same problem: Most of these 3D printing technologies used light to convert the ink into a solid structure and they needed to be printed outside or on a surface. And in the case of manufacturing something meant to be minimally invasive, the researchers thought this was impossible.
If surgeons want to use a 3D printed tissue, not only does light not penetrate solid objects, but they would need to do surgery to implant the printed structure. “That’s really the limit of current bioprinting technology,” said Yao. So, the researchers came up with the idea to make a printer that uses ultrasound waves which travel deeper through opaque materials. And they also had to create an ink that can be hardened using sound.
“So we are developing this ultrasound printer, which uses ultrasound waves to convert the sono-ink developed by Shirke’s lab into three-dimensional structures actually inside the tissue without the need to print it out, then implant it into the tissue,” Yao told STAT. This emerging technology, called deep-penetrating acoustic volumetric printing, opens the door to many potential applications in medicine.
Yao, whose background is in ultrasonic imaging, worked with his team to build a 3D printer that has a focused ultrasound transducer, which converts electrical energy into sound waves. The waves can then be manipulated remotely to travel through the tissue to create any structure, layer by layer, including a honeycomb, a tube, or even an irregular structure like a patch.
Zhang, a biomaterials scientist, associate bioengineer at Brigham and Women’s Hospital, and associate professor at Harvard Medical School, created an “ink cocktail,” which includes a variety of chemicals or a “concoction of polymers, particles and chemical initiators” that respond to sound waves. The ink’s components varied if it was meant to be bone-like or as flexible as heart tissue. For the proof of concept, the researchers injected the ink into several different pig organs to see if it would work inside actual tissue.
The excess ink that remains inactivated by the ultrasound will still remain fluid, Yao told STAT. The remaining liquid could either be cleared out by the tissue itself or it could be sucked out using a catheter or a syringe.
The researchers were able to print a bone-shaped structure through 10 millimeters of pig skin and muscle to simulate bone reconstruction. They were also able to show a way to treat atrial fibrillation, by printing a patch on the left atrial appendage of an ex vivo heart, or one that is outside the body. The ink could also be loaded up with a chemotherapy drug, which was printed through a 14-millimeter-thick pig liver.
“I think what was pretty innovative was the use of ultrasound to do this kind of gelling of materials so deep into the [organ] with a non-invasive technology like ultrasound, which really has minimal side effects,” said Adam Feinberg, a professor at Carnegie Mellon University with expertise in biomedical engineering and material science. Feinberg, who did not participate in the study, described the research as a “unique” application of technology.
For now the ink does not include tissue and that might be enough. Many tissues in the human body have an intrinsic regenerative capacity and maybe the tissues just need a scaffold to push them in the right direction. “Bone, muscle, fat tissue — these are pretty good examples of parts of the body where probably the right cells are already in the body, we just need to create the right environment,” said Feinberg, who did similar research using a collagen scaffold that needed to be implanted. The technology could one day be used to print other materials, like medical devices and other forms of drug delivery, said Anthony Atala, director of the Wake Forest Institute for Regenerative Medicine in North Carolina.
As promising as this research may sound, though, the researchers encountered many problems over the course of this project.
One of the key challenges, according to Yao, was achieving “a balance between efficient printing and tissue safety.” Everyone is familiar with the photothermal effect or when you get a sunburn. Yao explained that the ink molecules can convert the ultrasound wave into heat, which is called the sonothermal effect and can lead to a “soundburn,” damaging tissue in the process if the temperature goes above 70 degrees Celsius. “So, how much temperature rise is tolerable, is safe, is compatible for the patient? That’s something we have to really pay attention to because we do want the printing process to be as efficient as possible,” Yao said.
Xuanhe Zhao, a professor of mechanical, civil and environmental engineering at the Massachusetts Institute of Technology, agreed. “While the biocompatibility and printing resolution of the method requires further improvement, there is great potential for future applications,” Zhao said by email. He was not part of the study.
Another concern raised is that since the sono-ink must be injected at high concentrations inside of the body, it may cause toxicity. Feinberg added that once this technology is tested in vivo, or in the body, there might be some regulatory hurdles with the biomaterials being used and how it could be regulated by the Food and Drug Administration.
“If you’re repurposing materials already used in vivo, that’s usually a much lower bar than if you’re introducing new materials that have never been in vivo in this kind of indication,” Feinberg said. In the future, he added that he expects 3D printed tissue scaffolds (printed outside the body) to be in clinical trials within the next five years. In June 2022, the regenerative medicine company 3DBio Therapeutics started Phase 1/2a clinical trials using an implantable living tissue scaffold to reconstruct the ears of people with congenital ear deformity.
Yao, however, thinks that his vision for the future is not that far off either. He imagines that in a clinical setting a patient would either be lying on a table or sitting in a chair comfortably. There will be a robotic arm holding an ultrasound transducer and performing its printing in a precise pattern that’s already been preloaded to the computer. The robotic arm will be controlled remotely with the help of artificial intelligence for precise printing and the ink will be delivered by a catheter or a small syringe.
“Why this is so interesting is because we don’t have to open the patient anymore, so the surgeon’s role is much reduced,” Yao said, adding in addition to reducing the surgeon’s workload, patients will also have improved treatment outcomes, and will be less likely to suffer from complications like inflammation, infection, and long recovery times.
“This is really outstanding technology. It really has great potential for the future,” said Atala. “It’s like manufacturing devices and therapies inside the body with remote control.”
Yao believes that this research is the highlight of his career. When people enjoy their work and having conversations about it, that can generate the spark for many good ideas from different people, Yao explained, “so I really say we should go to the coffee break more often and meet friends for coffee breaks more often to have better ideas.”