Min Jun Kim
Imagine a microscopic machine that could swim through a person’s blood vessels on its way to delivering medicine to a cancerous tumor. Or one that unclogs an artery to prevent a heart attack, or even performs delicate vision-saving surgery from inside the eye. These feats aren’t possible yet. But researchers are designing miniature robots, called microswimmers, that may soon do all of these things. And more.
Imagine a robot and Star Wars’ beloved R2-D2 may come to mind, or his pal C-3PO. The robots scientists want to deploy in the human body are much, much smaller. They’re typically less than 1 millimeter (0.04 inch) in size. At their largest, they might be the size of a few grains of table salt. But they can be much smaller — so small that they can be seen only with a microscope.
The human body is made up mostly of wet stuff. Blood, spinal fluid and other liquids make up about 60 to 65 percent of the volume of the human body. So to move through this environment with ease, robots must swim. But finding the right materials and designs to send robots swimming through even the tiniest blood vessels has proven tough. But not impossible. Indeed, scientists have been inching toward this vision throughout the past decade.
The payoff should be worth the wait, says David Cappilleri. He’s a microroboticist at Purdue University in West Lafayette, Ind. “Microrobots can go places that larger robots can’t.” What’s more, he argues, “they can handle tools with finer precision.”
Humans don’t have much experience moving things around micro-environments of the body, explains Bradley Nelson. He’s a roboticist at the Swiss Federal Institute of Technology in Zurich. But some other organisms do this well. So Nelson and other robot designers are looking to them for inspiration.
Copying nature’s microswimmers
Before scientists could create robots to swim inside the body, they first had to solve the problem of scale. The physics of swimming changes as an object gets smaller and smaller, Nelson observes. That’s because on a microscopic scale, liquids become much more viscous. (Viscosity is the thickness of a liquid.)
“If you were to shrink down to the size of a [microrobot] and jump into a pool of water,” says Nelson, "the water wouldn’t feel like water anymore. It would feel like thick honey."
That’s why he and other scientists have copied some of nature’s tiniest swimmers: single-celled lifeforms that infect people. These master swimmers can navigate swiftly through our bodies.
For design tips, Nelson focused on a type of microbe known as a protozoan. He picked the species Trypanosoma brucei (Try-PAN-oh-SO-mah BRU-see-eye). In people, this parasite causes African sleeping sickness. Affected people often can’t sleep at night but are overcome with drowsiness by day.
T. brucei swims through the bloodstream by swishing a whiplike tail. This tail is called a flagellum (Fla-JEL-um).
As it moves, T. brucei also changes its shape. This helps it get from one part of the body to another. The protozoan can go from short and stumpy to slender and needle-like. That lets it penetrate blood vessel walls and invade the spinal cord.
Nelson liked the idea of building a microrobot with this talent for shape-shifting. Such a robot could do jobs in different parts of the body, depending on its shape. These tasks might include unclogging arteries or delivering medicines, Nelson explains. Or maybe the robot could make microscopic repairs in the blood vessels of the eye.
He used a gel-like material to build a soft microrobot. By altering the gel’s temperature, he can make his robot short and stumpy or long and needle-like.
Nelson described this new design last year in the journal Nature Communications.
Putting tiny robots to work inside the body
Nelson’s soft-bodied microswimmer isn’t the only nano-medic that researchers are building. Some at Drexel University in Philadelphia, Penn., created a different type to bring cancer-killing medicine to tumors.
One challenge to designing robots that will go inside the body is powering them. These robots are too small to carry an engine or battery pack, notes Min Jun Kim, who led the Drexel team.
Kim is a mechanical engineer. (He now works at Southern Methodist University in Dallas, Texas.) Mechanical engineers use rules of motion, energy and force to design and build machinery. Kim’s team used magnets to solve the power problem. Unlike Nelson’s soft microswimmer, Kim’s design is made entirely of metal.
He looked to bacteria for inspiration. And he found it in Borrelia burgdorferi (Bor-REL-ee-ah Burg-DOR-fur-eye). This spiral-shaped germ causes an illness known as Lyme disease. People fall ill after being bitten by an infected tick. The bacterium uses a corkscrew-like motion to swim and bore through tissue.
Kim mimicked its corkscrew shape by joining together strands of magnetic beads. Those beads are so small they can barely be seen without a microscope.
He used magnets to propel his microrobot through artificial blood vessels in a petri dish. By increasing or decreasing the strength of the magnets, Kim can make the robot swim faster or slower. Changing the direction of the magnetic field lets him steer the robot left or right. He can join several chains together to make strands as long as 20 to 30 beads. Or he can break them apart into strands as short as three beads.
Kim says the surfaces of these beads could be coated with medicines. To treat cancer, a microswimmer that’s a longer chain could bore into a thick tumor. Once inside, it could break apart into smaller chains to spread its cancer-killing medicine around, he says.
Kim’s team shared its findings last year in the journal Scientific Reports.
Some microrobots can already swim in petri dishes. But there would likely be big hurdles before they could be released in people, notes Zhang Li. He studies nanotechnology — the design and harnessing of extremely small devices — at the Chinese University of Hong Kong. Scientists need to know, for instance, how safe the itty-bitty devices are. They also need to know how to remove those tiny swimmers once their work is done, Li notes.
For now, researchers plan to test their microscopic devices in animals such as mice. Those tests can also help scientists make sure that the materials in their robots aren’t toxic.
Scientists also can use animals to practice inserting, tracking and removing microswimmers before they put them in people. Working with robots in a real body — whether a human or a mouse — is a lot different than doing it in a petri dish under a microscope, Li explains. In a petri dish, there’s no skin, blood or other tissues to block the view.
Despite these challenges, scientists find this “an exciting time” Li observes. “There’s so much left to learn.”
artery Part of the body’s circulation system, these tubes carry blood from the heart to all parts of the body.
bacterium (plural bacteria) A single-celled organism. These dwell nearly everywhere on Earth, from the bottom of the sea to inside animals.
cancer Any of more than 100 different diseases, each characterized by the rapid, uncontrolled growth of abnormal cells. The development and growth of cancers, also known as malignancies, can lead to tumors, pain and death.
engine A machine designed to convert energy into useful mechanical motion. Sometimes an engine is called a motor.
engineer A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.
environment The sum of all of the things that exist around some organism or the process and the condition those things create for that organism or process. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature, humidity and placement of components in some electronics system or product.
federal Of or related to a country’s national government (not to any state or local government within that nation). For instance, the National Science Foundation and National Institutes of Health are both agencies of the federal government of the United States.
flagellum (plural: flagella) A long, tapering appendage that some cells use for locomotion.
force Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
gel A gooey or viscous material that can flow like a thick liquid.
heart attack Permanent damage to the heart muscle that occurs when one or more regions of it become starved of oxygen, usually due to a temporary blockage in blood flow.
magnetic field An area of influence created by certain materials, called magnets, or by the movement of electric charges.
mechanical Having to do with the devices that move, including tools, engines and other machines (even, potentially, living machines); or something caused by the physical movement of another thing.
micro A prefix for fractional units of measurement, here referring to millionths in the international metric system.
microscope An instrument used to view objects, like bacteria, or the single cells of plants or animals, that are too small to be visible to the unaided eye. Objects this small are referred to as microscopic.
microswimmer A microscopic robot that can swim through the human body. Engineers are still developing these types of robots.
nanotechnology Science, technology and engineering that deals with things and phenomena at the scale of 100 billionths of a meter or less.
navigate To find one’s way through a landscape using visual cues, sensory information (like scents), magnetic information (like an internal compass) or other techniques.
organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.
petri dish A shallow, circular dish used to grow bacteria or other microorganisms.
physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion.
protozoan (plural: protozoa) Any of many types of single-celled organisms, most of them too small to be seen with the unaided eye. They include amoebas, parameciums and many others. Some can cause disease in humans and other animals.
robot A machine that can sense its environment, process information and respond with specific actions. Some robots can act without any human input, while others are guided by a human.
spinal cord A cylindrical bundle of nerve fibers and associated tissue. It is enclosed in the spine and connects nearly all parts of the body to the brain, with which it forms the central nervous system.
tick A small eight-legged blood-sucking arthropod, related to spiders and mites. Although they look like bugs, these are not insects. They attach themselves to the skin of their host and feed on their blood. But in the process, they may spread any germs that could have been present in the blood of an earlier host.
tissue Any of the distinct types of material, comprised of cells, which make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms.
toxic Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.
tumor A mass of cells characterized by atypical and often uncontrolled growth. Benign tumors will not spread; they just grow and cause problems if they press against or tighten around healthy tissue. Malignant tumors will ultimately shed cells that can seed the body with new tumors. Malignant tumors are also known as cancers.
viscosity (adj. viscous) The measure of a fluid’s resistance to stress. Viscosity corresponds to the idea of how “thick” a liquid is. Honey is very viscous, for instance, while water has relatively low viscosity.
JOURNAL: U.K. Cheang et al. Versatile microrobotics using simple modular subunits. Scientific Reports. Vol. 6, July 28, 2016. doi: 10.1038/srep30472.
JOURNAL: H.W. Huang et al. Soft micromachines with programmable motility and morphology. Nature Communications. Vol. 7, July 22, 2016. doi: 10.1038/ncomms12263.