Ultrasound News: A New Direction
By Rebekah Moan
Radiology Today
Vol. 26 No. 6 P. 30

Personalized medicine is the way of the future in health care, and 3D bioprinting allows for just that. The technique has the potential to create functional, living tissues and organs for transplantation, which can reduce the need for organ donors, and 3D bioprinting can improve drug development as well as testing. The problem? Implantation.

“Our lab works on implantable devices, and 3D printing is very attractive and suitable for prototyping and creating a personalized construct,” says biomedical engineer and Caltech professor Wei Gao, PhD. “But the question is how to do it without surgery?” Surgery is invasive and expensive, but in vivo printing enables direct fabrication of bioconstructs at defect sites within the body. This eliminates the need for traditional implantation and facilitates rapid, on-site tissue repair as well as drug delivery.

One method that enables in vivo 3D bioprinting is ultrasound—it penetrates deep tissue and is noninvasive. Not only that, but real-time imaging means precise targeting and control while fabricating biomaterials. More specifically, focused ultrasound can target energy delivery and facilitate processes such as acoustic cavitation- induced radical polymerization of materials, including polydimethylsiloxane, or sonothermally induced polymerization of poly(ethylene glycol) diacrylate.

Sound Printing at a Glance
Gao and colleagues developed an imaging- guided deep tissue in vivo sound printing (DISP) platform that uses low-temperature-sensitive liposomes (LTSLs) as carriers for cross-linking agents to enable precise and controlled in situ fabrication of biomaterials within deep tissues (Science, May 8, 2025). An issue with in vivo printing is “versatile bioink formulations capable of accommodating diverse biomaterials for wide-ranging applicability across various medical scenarios while ensuring high biocompatibility and minimal toxicity from residual prepolymers,” the authors write.

The liposomes they used solved that problem. They enhance biocompatibility by encapsulating cross-linking agents. These agents prevent premature interaction with surrounding tissues and are only released on-demand via focused ultrasound.

DISP uses ultrasound-responsive bioinks (US-inks) specifically designed for precise and controlled in situ fabrication. These inks are composed of biopolymers, cross-linking agent-encapsulated LTSLs, and gas vesicles that act as ultrasound imaging contrast agents. The bioinks are delivered to the target sites through injection or catheters and are located using an ultrasound imaging setup integrated into a 3D-printing platform. The system USink and continuously monitors the printing process.

The LTSLs remain stable at 37°C and rapidly release encapsulated materials at ~41.7°C, which occurs upon focused ultrasound exposure. The temperature increases locally, and that induces a phase transition in the LTSL lipid bilayer from a solid to a liquid state, creating nanopores in the bilayer structure.

“Grain boundary defects in the solid phase were found to be crucial for the formation of stable nanopores in the lipid bilayers,” the authors write. “To enhance lipid mobility and facilitate pore formation, pore-forming lysolipids and a small percentage of PEGylated lipids were incorporated into the lipid bilayer. During heating, these defects expand rapidly, enabling controlled payload release while minimizing premature leakage at physiological temperatures, ensuring precise and reliable crosslinking for sound printing.”

Continued Testing
The ultrasound transducer is controlled by an automatic positioning system and scans the US-ink following a predefined G-code. Due to localized heating from the focused ultrasound, the crosslinking agent from the LTSLs is released, and there’s immediate in situ cross-linking of the US-ink.

The method prints in high resolution (~150 μm) and very quickly (up to 40 mm s−1). Gao and colleagues successfully printed a wide range of functional biomaterials, including conductive, drug-loaded, cell-laden, and bioadhesive hydrogels. Hydrogels can seal wounds, act as a drug or cell carrier, or regenerate tissue. As a proof of concept, the researchers demonstrated in vivo printing within the bladders and muscles of live animals. Postprocedure analyses confirmed the high biocompatibility of both the prepolymers and printed hydrogels.

In a mouse with bladder cancer, drug-loaded ultrasound gels were printed near the tumor site. Ultrasound imaging monitored the printing process while the animal was under anesthesia. The catheter’s position was monitored using B-mode ultrasound imaging to ensure successful instillation before injecting the ultrasound ink into the bladder. With focused ultrasound, the ink was released, and a clear ultrasound gel was observed from the abdomen of the animal, confirming successful in vivo printing.

However, the researchers wanted to test DISP on a larger animal and used a rabbit. They successfully printed on the exposed abdominal muscle, deep within the adductor muscle, and beneath the biceps femoris muscle. Doing so means DISP can target deeper tissue layers for applications such as tissue replacement.

“The technology is very promising, but it has its limitations,” Gao says. “For example, we demonstrated 3D printing in a relatively stable environment but not in a dynamic and changing one like a beating heart or lungs.” Combining AI with imaging could support printing in those dynamic environments. For instance, machine learning-based adaptive transducer positioning could enhance precision in complex scenarios such as cardiac printing.

“The technology sounds promising, but we’re in the early stages,” he says. “We demonstrated a proof of concept, but we’re five to 10 years from bringing DISP to a clinical study in humans. But this printing technology is a new direction for precision medicine because it directly delivers therapy into the body without surgery or hospitalization.”