Imagine that doctors can precisely print capsules containing cells for tissue repair where needed, within the beating heart. Caltech scientists have taken an important step towards this ultimate goal by developing a 3D-printing method that can be used to print polymers in specific areas deep inside living animals. This technique uses sound to localize and was used in the past to create polymer capsules to deliver drugs selectively as well as to produce glue-like polymers for sealing internal wounds.
Scientists have previously used infrared lights to trigger polymerization. This is the joining of basic units or monomers of polymers inside living animals. Wei Gao is a professor of medical technology at Caltech, and an investigator with the Heritage Medical Research Institute. Gao, his co-workers and the Science journal report on their in vivo 3-D printing technique “Our new technique reaches the deep tissue and can print a variety of materials for a broad range of applications, all while maintaining excellent biocompatibility.”
The paper describes how to print bioelectric hydrogels. These are polymers embedded with conductive material for internal monitoring vital signs such as electrocardiograms. Elham Davoodi is the lead author of this study. He’s an assistant professor of Mechanical Engineering at the University of Utah who did the research while working as a postdoctoral fellow at Caltech. The Origin of a Novel Idea.
Gao, his co-workers, and their professors wanted to find a solution to deep tissue printing in vivo. They turned to ultrasound as a tool that was widely used for biomedical deep tissue penetration. They needed to find a method of triggering crosslinking or monomer binding at a particular location, and only when they wanted it.
They came up with a novel approach: Combine ultrasound with low-temperature-sensitive liposomes. Liposomes are spherical vesicles that have protective fat layers. They’re often used to deliver drugs. The scientists in the study embedded the liposomes into a solution of polymers containing monomers for the polymer that they wanted to print. They also added an imaging contrast agent to reveal the time at which the crosslinking occurred. Finally, they included the drug they were hoping to deliver. Cells and other components, like carbon nanotubes and silver, can also be added. After injecting the composite bioink directly into the human body, it was able to trigger printing. Raising the temperature just a touch to trigger printing
Liposomes are sensitive to low temperatures, so by using focused ultrasonic waves to increase the temperature in a targeted area by approximately 5 degrees Celsius the scientists could trigger the release and start the printing process of polymers. Gao. “Where the agents are released, that’s where localized polymerization or printing will happen.”
As an imaging contrast agent, the team is using gas vesicles that are derived from bacteria. These vesicles are air-filled protein capsules that show up well in ultrasound images and respond to the chemical changes which occur when the monomer liquid solution crosses links to form a network of gel. When the polymerization occurs, the vesicles change their contrast and can be detected using ultrasound imaging. This allows scientists to identify exactly where the crosslinking took place. They are then able to customise the pattern printed on live animals.
According to the team, the technique is called deep tissue in vivo Sound Printing (DISP). The team found that when they used the DISP to print polymers containing doxorubicin – a chemotherapeutic – near a tumorous bladder in mice, there was a significant increase in tumor cell death over a period of several days compared with animals who received doxorubicin directly through injection.
“We have already shown in a small animal that we can print drug-loaded hydrogels for tumor treatment,” Gao says. “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.”
According to the team, machine learning could also enhance the DISP platform’s ability precisely locate and use focused ultrasound. “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 says. Funding for the work came from the National Institutes of Health (NIH), the American Cancer Society (ACS), the Heritage Medical Research Institute and UCLA’s Challenge Initiative (19659014). Fluorescence microscopy was performed at the Advanced Light Microscopy/Spectroscopy Laboratory and Leica Center of Excellence at the California NanoSystems Institute at UCLA.
