Minuscule Robots for Targeted Drug Delivery

In the future, delivering therapeutic drugs exactly where they are needed within the body could be the task of miniature robots. Not little metal humanoid or even bio-mimicking robots; think instead of tiny bubble-like spheres.

Such robots would have a long and challenging list of requirements. For example, they would need to survive in bodily fluids, such as stomach acids, and be controllable, so they could be directed precisely to targeted sites. They also must release their medical cargo only when they reach their target, and then be absorbable by the body without causing harm.

Now, microrobots that tick all those boxes have been developed by a Caltech-led team. Using the bots, the team successfully delivered therapeutics that decreased the size of bladder tumors in mice. A paper describing the work appears in the journal Science Robotics.

"We have designed a single platform that can address all of these problems," says Wei Gao, professor of medical engineering at Caltech, Heritage Medical Research Institute Investigator, and co-corresponding author of the new paper about the bots, which the team calls bioresorbable acoustic microrobots (BAM). "Rather than putting a drug into the body and letting it diffuse everywhere, now we can guide our microrobots directly to a tumor site and release the drug in a controlled and efficient way," Gao says.

The concept of micro- or nanorobots is not new. People have been developing versions of these over the past two decades. However, thus far, their applications in living systems have been limited because it is extremely challenging to move objects with precision in complex biofluids such as blood, urine, or saliva, Gao says. The robots also have to be biocompatible and bioresorbable, meaning that they leave nothing toxic behind in the body.

The Caltech-developed microrobots are spherical microstructures made of a hydrogel called poly(ethylene glycol) diacrylate. Hydrogels are materials that start out in liquid or resin form and become solid when the network of polymers found within them becomes cross-linked, or hardens. This structure and composition enable hydrogels to retain large amounts of fluid, making many of them biocompatible. The additive manufacturing fabrication method also enables the outer sphere to carry the therapeutic cargo to a target site within the body.

To develop the hydrogel recipe and to make the microstructures, Gao turned to Caltech's Julia R. Greer, the Ruben F. and Donna Mettler Professor of Materials Science, Mechanics and Medical Engineering, the Fletcher Jones Foundation Director of the Kavli Nanoscience Institute, and co-corresponding author of the paper. Greer's group has expertise in two-photon polymerization (TPP) lithography, a technique that uses extremely fast pulses of infrared laser light to selectively cross-link photosensitive polymers according to a particular pattern in a very precise manner. The technique allows a structure to be built up layer by layer, in a way reminiscent of 3D printers, but in this case, with much greater precision and form complexity.

Greer's group managed to "write," or print out, microstructures that are roughly 30 microns in diameter—about the diameter of a human hair.

"This particular shape, this sphere, is very complicated to write," Greer says. "You have to know certain tricks of the trade to keep the spheres from collapsing on themselves. We were able to not only synthesize the resin that contains all the biofunctionalization and all the medically necessary elements, but we were able to write them in a precise spherical shape with the necessary cavity."

In their final form, the microrobots incorporate magnetic nanoparticles and the therapeutic drug within the outer structure of the spheres. The magnetic nanoparticles allow the scientists to direct the robots to a desired location using an external magnetic field. When the robots reach their target, they remain in that spot, and the drug passively diffuses out.

Gao and colleagues designed the exterior of the microstructure to be hydrophilic—that is, attracted to water—which ensures that the individual robots do not clump together as they travel through the body. However, the inner surface of the microrobot cannot be hydrophilic because it needs to trap an air bubble, and bubbles are easy to collapse or dissolve.

To construct hybrid microrobots that are both hydrophilic on their exterior and hydrophobic, or repellent to water, in their interior, the researchers devised a two-step chemical modification. First, they attached long-chain carbon molecules to the hydrogel, making the entire structure hydrophobic. Then the researchers used a technique called oxygen plasma etching to remove some of those long-chain carbon structures from the exterior, leaving the outside hydrophilic and the interior hydrophobic.

"This was one of the key innovations of this project," says Gao, who is also a Ronald and JoAnne Willens Scholar. "This asymmetric surface modification, where the inside is hydrophobic and the outside is hydrophilic, really allows us to use many robots and still trap bubbles for a prolonged period of time in biofluids, such as urine or serum."

Indeed, the team showed that the bubbles can last for as long as several days with this treatment versus the few minutes that would otherwise be possible.

The presence of trapped bubbles is also crucial for moving the robots and for keeping track of them with real-time imaging. For example, to enable propulsion, the team designed the microrobot sphere to have two cylinder-like openings—one at the top and another to one side. When the robots are exposed to an ultrasound field, the bubbles vibrate, causing the surrounding fluid to stream away from the robots through the opening, propelling the robots through the fluid. Gao's team found that the use of two openings gave the robots the ability to move not only in various viscous biofluids, but also at greater speeds than can be achieved with a single opening.

Trapped within each microstructure is an egg-like bubble that serves as an excellent ultrasound imaging contrast agent, enabling real-time monitoring of the bots in vivo. The team developed a way to track the microrobots as they move to their targets with the help of ultrasound imaging experts Mikhail Shapiro, Caltech's Max Delbruck Professor of Chemical Engineering and Medical Engineering, a Howard Hughes Medical Institute Investigator; co-corresponding author Di Wu, research scientist and director of the DeepMIC Center at Caltech; and co-corresponding author Qifa Zhou, professor of ophthalmology and biomedical engineering at USC.

The final stage of development involved testing the microrobots as a drug-delivery tool in mice with bladder tumors. The researchers found that four deliveries of therapeutics provided by the microrobots over the course of 21 days was more effective at shrinking tumors than a therapeutic not delivered by robots.

"We think this is a very promising platform for drug delivery and precision surgery," Gao says. "Looking to the future, we could evaluate using this robot as a platform to deliver different types of therapeutic payloads or agents for different conditions. And in the long term, we hope to test this in humans."

The lead authors of the paper, "Imaging-guided bioresorbable acoustic hydrogel microrobots," are Hong Han (MS '23) and Xiaotian Ma (MS '24) from Gao's lab, Weiting Deng (PhD '24), now a post-doc at UCLA who conducted this work while in Greer's lab, and Junhang Zhang from Zhou's lab at USC. Additional Caltech authors are Songsong Tang, Ernesto Criado-Hidalgo, Emil Karshalev (now at General Atomics), Jounghyun Yoo, Ming You, Ann Liu, Canran Wang (MS '23), Hao K. Shen, Payal N. Patel, Claire L. Hays, Peter J. Gunnarson (PhD '24), Lei Li (PhD '19), Yang Zhang, John O. Dabiri (PhD '05), Caltech's Centennial Professor of Aeronautics and Mechanical Engineering; and Lihong V. Wang, Caltech's Bren Professor of Medical Engineering and Electrical Engineering, and the Andrew and Peggy Cherng Medical Engineering Leadership Chair. Additional authors are On Shun Pak of Santa Clara University, Lailai Zhu of National University of Singapore, and Chen Gong of USC.

The work was supported by the Kavli Nanoscience Institute at Caltech as well as by funding from the National Science Foundation; the Heritage Medical Research Institute; the Singapore Ministry of Education Academic Research Fund; the National Institutes of Health; the Army Research Office through the Institute for Collaborative Biotechnologies; the Caltech DeepMIC Center, with support of the Caltech Beckman Institute and the Arnold and Mabel Beckman Foundation; and the David and Lucile Packard Foundation.