Breakthroughs: Microrobots + More

Controlling microscopic processes is inherently challenging. The everyday tools we use to manipulate matter on the macroscale can’t simply be shrunk down to the size of a cell, and even if they could, the physical forces they rely on work differently when their targets are measured in nanometers. But while it’s no easy feat, attaining this type of control would pay enormous dividends: from transporting drugs to tumors for precise therapies, to making functional materials out of microscopic building blocks, to enabling new kinds of displays and sensors.

One approach for controlling these types of processes is via microrobots.

We typically think of robots as computerized machines like those on assembly lines or in warehouses, programmed to move cargo and to build complex structures like automobiles and cellphones. However, programming a device smaller than a microchip presents another kind of challenge. Robots on this scale need to be designed in a completely different way than their bigger counterparts.

Since they’re too small for their own onboard computers, microrobots’ movements are controlled by an external magnetic force. And to manipulate equally small cargo, they need to take advantage of the different physical and chemical laws that rule the microscale.

At those sizes, every object is greatly influenced by the molecules surrounding it. Whether they are surrounded by gas, like the ambient atmosphere, or dispersed in a liquid, micro robots must be designed to exploit this influence, a concept known as “physical intelligence.”

Kathleen Stebe, Richer & Elizabeth Goodwin Professor in Chemical and Biomolecular Engineering and Mechanical Engineering and Applied Mechanics, Tianyi Yao and Qi Xing Zhang, graduate students in her lab, and collaborators at the University of Ljubljana, Slovenia, are establishing some of the fundamentals of physically intelligent microrobots, publishing a pair of groundbreaking papers earlier this year.

Their studies investigated magnetic micro robots swimming through nematic liquid crystals ( NLC s), the fluid that makes up each pixel in a liquid crystal display. Colloidal systems like these, where one material is dispersed through another, can give even the simplest microrobots a way of doing work; rather than arm-like mechanical grippers you would find on an assembly line, they can extend “force fields” of elastic energy by altering the arrangement of molecules around them. “Nematic liquid crystals exist as a special phase, a structured fluid that is neither liquid nor solid,” says Stebe. “NLCs consist of elongated molecules that self-align in a configuration that requires the least amount of energy. Think of shaking a pan of rice; the grains all align. When you disturb the nematic alignment by introducing microrobots or colloidal cargo, you get really interesting dynamics that you don’t see in water, for example.”

In one study, Stebe and her colleagues deployed X-shaped microrobots, each roughly a twentieth of a millimeter across, and showed how they could pick up and move spherical particles in complicated ways. Though their four stumpy arms can’t grab this cargo on their own, their specially designed shape disrupts the surrounding NLCs in such a way that the particles are attracted to the microrobot’s edges.

The team also demonstrated more advanced maneuvers, flicking particles away with a twist, or using the geometry of stationary structures in the environment to deposit them in specific locations.

The team’s other study answered even more fundamental questions about the microrobots’ swimming and gripping abilities. There, they used a robot with an even simpler shape — a flat disk — and a simpler form of magnetic control. By rotating the magnetic field back and forth, they saw that one side of the disk would lift, creating a disruption in the liquid crystal pattern under- neath it. This created an energy gradient that the microrobot could follow, propelling itself forward without needing to be dragged along by the external magnetic field.

Stebe and her team are now able to imagine real-world applications for this technology in the optical device industry as well as in many other fields.

“Together with dedicated colleagues and graduate students, we have been working hard on this technology and are excited to see years of work come to fruition,” she says. “We are now standing on the edge of real applications and ready to explore.”

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Microrobots in Your Mouth

Applications for tiny robots are diverse, requiring the design of each robotic system to be as unique as the problem it aims to solve. At the Center for Innovation and Precision Dentistry (CiPD), directors Stebe and Hyun (Michel) Koo, Professor in Penn Dental Medicine, look to microrobots to help clean, sample and diagnose plaque buildup and infections in the mouth. Together with Edward Steager, senior research investigator at Penn Engineering, they designed physically intelligent, magnetically controlled iron oxide nanoparticles that can move through the complex anatomy of the root canal or act as a swarm to brush and floss your teeth.

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But before a patient needs a root canal procedure, microrobots can aid them in everyday dental hygiene.

The same nanoparticles can form a swarm in the shape of bristles, like a toothbrush, or elongated strings, like floss, to mechanically scrub and sweep plaque from teeth. The particles also chemically remove plaque through a catalytic reaction triggering a production of antimicrobials, providing multiple forms of attack on the biofilm in the patient’s mouth.

Text by Melissa Pappas.

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