A team of researchers from the California Institute of Technology (Caltech) has developed a method for 3D printing polymers at specific locations deep within living tissues using focused ultrasound. This technique, known as Deep Tissue In Vivo Sound Printing (DISP), enables the solidification of injectable bioinks directly inside internal organs. DISP has been used to print polymer capsules and glue-like materials to seal internal wounds, demonstrating potential for a wide range of medical applications.
The study, published in Science, was co-led by Wei Gao, Professor of Medical Engineering at Caltech and Investigator at the Heritage Medical Research Institute, and Elham Davoodi, Assistant Professor of Mechanical Engineering at the University of Utah. The research was a multi-institutional collaboration involving contributors also from the Terasaki Institute for Biomedical Innovation, the University of Southern California (USC), the University of California, Los Angeles (UCLA), and Rice University. It received funding from the National Institutes of Health (NIH), the American Cancer Society, the Heritage Medical Research Institute, and the Challenge Initiative at UCLA.
The Origin of DISP and How it Works
To develop deep tissue in vivo printing, Gao and his team turned to ultrasound, a technique commonly used in medicine for its ability to penetrate deeply into tissues. The challenge they faced was controlling polymer solidification, or crosslinking, precisely at the right locations and times within the body.
To address this, they combined ultrasound with temperature-sensitive liposomes. These small, spherical structures, typically used for drug delivery, were loaded with a crosslinking agent and incorporated into a polymer solution containing the monomers for the polymer they aimed to print. The composite bioink was then injected directly into the body.
The liposomes used in this method are designed to respond to slight changes in temperature. By applying focused ultrasound, the researchers raise the temperature in a targeted area by around 5°C. This small increase is enough to cause the liposomes to release their stored crosslinking agents, triggering localized polymerization exactly where it is needed.
To monitor the printing process in real time, the team used gas vesicles derived from bacteria—microscopic, air-filled protein structures that are highly visible in ultrasound imaging. These vesicles act as contrast agents that respond to chemical changes during polymerization. As monomers crosslink and form a solid gel, the vesicles shift in contrast on ultrasound, providing a clear signal of when and where polymerization occurs inside the body.
Testing DISP and Future Potential
In their initial experiments, the team used DISP to print drug-loaded polymers containing doxorubicin (a chemotherapy drug) near bladder tumors in mice. They found that the drug treatment resulted in significantly more tumor cell death over several days compared to traditional drug injections. “We have already shown in a small animal that we can print drug-loaded hydrogels for tumor treatment,” Gao said. “Our next stage is to try to print in a larger animal model, and hopefully, in the near future, we can evaluate this in humans.”
The researchers also see a future for machine learning to further refine the technique. “In the future, with the help of AI, we would like to be able to autonomously trigger high-precision printing within a moving organ such as a beating heart,” Gao said.
Advances in Ultrasound-Based 3D Printing Technologies
In 2022, researchers from Concordia University developed a technique called Direct Sound Printing (DSP), which uses ultrasound waves to fabricate complex and precise objects. This technology works by creating sonochemical reactions in minuscule cavities, enabling the production of pre-designed geometries that were previously unattainable with conventional techniques. As Muthukumaran Packirisamy, a Professor in Concordia’s Department of Mechanical, Industrial and Aerospace Engineering, explained, “Ultrasonic frequencies are already being used in destructive procedures like laser ablation of tissues and tumors. We wanted to use them to create something.”
In 2021, a team of researchers from the University of Bath and the University of Bristol introduced a new 3D bioprinting method known as Sonolithography. This technique harnesses acoustic energy and uses computer-controlled ultrasound waves to precisely deposit particles and droplets into predetermined patterns on a substrate. The ability to control aerosol sprays in a non-contact manner makes it highly suitable for a range of applications, including biofabrication, tissue engineering, complex drug delivery, and even wound healing.
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Featured image shows Drug delivery for tumor treatment. Panels A–L simulate drug distribution following direct injection. Panels A–D show random deposition of drug-loaded hydrogel on the bladder surface, while panels E–H and I–L illustrate precise printing of the hydrogel directly onto the tumor site. Photo via Caltech.
