Can you imagine a world where it would be possible to 3D print organs or tissues directly into your body? While this may sound like the most outlandish science fiction, researchers at Duke University and Harvard Medical School may have made it possible. They have developed a new 3D printing process using focused ultrasound and biocompatible ink that together can be used for everything from bone healing to heart valve repair. The hope is that this will lead to safer and less invasive surgical procedures.
While we have long extolled the benefits of additive manufacturing in the medical sector, including the growing field of bioprinting, there are still obstacles, especially when it comes to in-body printing. This is because many of the current processes use either extrusion or photopolymerization, neither of which are suitable for 3D printing through tissue.
With extrusion the reason it can't be done inside the body is obvious, how would you fit the apparatus into the body and if you have to cut it anyway why not do it outside? However, in-body 3D printing is also not possible with the various photopolymerization methods being developed. Why? Well, very simply, light cannot pass through the skin and organs because it scatters as it passes through them. This is also the case with volumetric printing, on which this latest 3D printing technology is based, which uses photopolymerization and transparent inks.
This new process, called deep-penetrating acoustic volumetric printing, or DVAP, can help overcome these problems. Randy King, Ph.D., program director in the Division of Applied Science & Technology at NIBIB explains: "Focused ultrasound has been used for decades to treat a wide variety of conditions, highlighting its safety and utility as a clinical tool. This potential new application, built on years of technological progress, could usher in something previously thought impossible: ultrasonic 3D printing through tissue.”
How does ultrasonic 3D printing work?
As Dr. King mentioned, focused ultrasound is not new to the medical field. Defined by the National Institute of Health as "a non-invasive therapeutic technique that directs ultrasound waves to a specific location", it is already used in the treatment of liver tumor, uterine fibroids and even Parkinson's disease. However, this is the first time it will be used specifically with medical 3D printing.
In terms of how DVAP works, it can be compared to other biomedical 3D printing processes, particularly those that use photosensitive inks and focused light, including volumetric 3D printing. In this case, however, what has been developed is ultrasound-processed ink, or sono-ink, an ink that is sensitive to ultrasound. The ink itself is made up of four separate components: a compound to absorb ultrasound waves, a microparticle that helps control viscosity, a polymer that provides structure, and a salt that absorbs heat to cause hardening. Working together, this enables the printing of biocompatible structures even through the dense, multi-layered tissues of the body.
This ink can also be adapted for different applications, such as adding bone mineral particles to treat bone loss. Additionally, it can be made more durable or degradable depending on the patient's needs. A level of flexibility that is crucial in medical applications and that works well with the researchers' self-made high-intensity confocal ultrasound printer, which has been adapted to improve both speed and resolution.
The process was developed by Y. Shrike Zhang, an associate bioengineer at Brigham and Women's Hospital and an associate professor at Harvard Medical School, and Junjie Yao, an associate professor of biomedical engineering at Duke. As for how it works, Yao expands, “DVAP relies on the sonothermal effect, which occurs when sound waves are absorbed and raise the temperature to harden our ink. Ultrasound waves can penetrate more than 100 times deeper than light while still being spatially confined, so we can reach tissues, bones and organs with high spatial precision that was not achievable with printing methods based on light.”
Furthermore, DVAP would not only enable 3D printing in the body thanks to the use of ultrasound waves, but could even be compatible with biological tissues. Zhang and Yao have already tested the process by creating tissues for a pig liver, as well as a simulated operation involving a goat heart. These experiments also showed promising results, meaning that in the future it may be possible to use a process like DVAP to replace highly invasive surgical procedures.
Yao concludes, "Because we can print through tissue, it enables many potential applications in surgery and therapy, which traditionally involve very invasive and destructive methods. This work opens up an exciting new path in the world of 3D printing, and we are excited to explore the potential of this tool together.”
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