Cell-sized robots can store and steer to deliver drugs.

A scanning electron microscope image shows a cell-size robotic swimmer that can be powered and steered by ultrasound waves.

A scanning electron microscope image shows a cell-size robotic swimmer that can be powered and steered by ultrasound waves.

The microscopic robots are accurately steered using ultrasound and can carry cargo.

Micro/nanorobotics has excellent potential for medical applications – mainly involving targeted drug delivery, surgical operation, and disease diagnosis. Unlike traditional drug delivery, which relies on blood circulation to reach the target, microbots can move on a microscopic scale. Meaning they can travel directly to the site of interest—making it possible to deliver drugs to hard-to-reach areas using a more targeted approach.

For this purpose, the minuscule robots can use external or internal power sources for propulsion. External sources can include magnetic fields, light energy, acoustic fields, and ultrasound. In comparison, internal power sources emanate from the robot itself, comprising chemical reactions or appendages built for locomotion. However, these parts or reactions are sometimes difficult to control, with biocompatibility still an issue in the field. And even though extensive research on nanodrugs exists, only a few enter clinical trials, and even less are approved.

Now, a study from researchers at Cornell University engineers cell-sized robots powered and steered by ultrasound waves. Despite their tiny size, the team states these micro-robotic swimmers – controlled via two air-filled compartments– are expected to be a new tool for targeted drug delivery. The study is published in the journal Lab on a Chip.

Accurate drug delivery has always been challenging to achieve in the medical arena. Therefore, the ideal method requires the therapeutic dose to be transported directly to the salient tissues or cells to produce a marked effect, but standard pharmaceuticals cannot achieve this.

In contrast, studies show that nanomedicines prolong the controlled release of drugs while increasing permeability and decreasing toxicity. The medical community is now working towards a drug delivery system that can deliver therapeutic payloads accurately, including autonomous propulsion, controlled navigation, tissue penetration, payloads towing, and releasing. However, these remain enormous challenges for current nanorobotic drug delivery systems.

The current study uses 3D printing to build micro-robotic swimmers the size of animal cells that are propelled and steered accurately using ultrasound. A pair of cavities etched into the back of the microbot enables the ultrasound to control its direction. And once the resin-based robot is submerged in solution because this material is hydrophobic, a tiny air bubble is automatically trapped in each hole. At which point the ultrasound causes the air bubbles to oscillate within their holes, forming whirlpools – creating a force known as streaming flow – that propel the swimmer forward. These streaming flows then drive the nanobot in the preferred orientation.

The lab explains that other engineers have previously built “single bubble” swimmers. But, they’re the first to develop a model that uses two bubbles, each with a different diameter opening in their respective cavity. By varying the ultrasonic frequency, the researchers can either excite each bubble separately – or tune them together – thereby controlling the swimmer’s direction.

The lab cautions that challenges lay ahead regarding the safety of these nanomachines, as they’re currently body incompatible. A fact that will have to change to enable the robots to work within a swarm while navigating the host’s blood vessels.

The team explains why this swarming behavior is vital for their drug delivery system. “For drug delivery, you could have a group of micro-robotic swimmers, and if one failed during the journey, that’s not a problem. That’s how nature survives,” Wu said. “In a way, it’s a more robust system. Smaller does not mean weaker. A group of them is undefeatable. I feel like these nature-inspired tools typically are more sustainable because nature has proved it works.”

In the future, the researchers state their robotic swimmers could be used to advance targeted drug delivery and remote microsurgery.

Source: Cornell Chronicle

Image courtesy of Cornell University

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