Saturday, March 19, 2022

These tiny robots could work inside your body

[This is a transcript of the video embedded below. Some of the explanations may not make sense without the animations in the video.]

When I grew up, one of my favorite movies was “Innerspace”, in which a man is miniaturized and, by accident, ends up inside somebody else’s body. We’re not going to shrink people to make them fit into blood vessels any time soon. But injecting tiny remote controlled robots into the human body isn’t all that far-fetched. What tiny robots are scientists working on? How far along is the technology? And, aside from leaving Rohin unemployed, what could they be good for? That’s what we’ll talk about today.

First things first, “tiny robots” isn’t a technical term. Scientists like to be more precise and talk about microbots or nanobots, for robots of the size of micrometers or nanometers. Just for reference, the width of a human hair is about a tenth of a millimeter. A micrometer is one hundred times smaller than a hair width. And a nanometer is one hundred thousand times smaller. So, yeah, that’s really really tiny. But some “tiny” robots are up to a few millimeters in size. I guess we should call those millibots. And then are xenobots. We’ll talk about those later.

You may think the problem with tiny robots is that they’re tiny. But actually that’s not the problem. Modern technology has been extremely successful at miniaturization, and I’m not talking about cellphones. Take a look at this image of a few gears next to a dust mite. That’s what I am talking about. We already have the technology to custom-build tiny things.

No, the problem with tiny robots is a different one. It’s that, regardless of whether the prefix is nano, micro, or xeno, at such small scales, different laws of physics become relevant. You can’t just take a human sized robot and scale it down, that makes no sense.

For tiny robots, forces like friction and surface tension become vastly more important than they are for us. That’s why insects can move in ways that humans can’t, like walking on water, or walking upside-down on the ceiling, or like, flying. Tiny robots can indeed fly entirely without wings. They just float on air like dust grains. Tiny robots need different ways of moving around, depending on their size and the medium they’re supposed to work in, or on.

There is another reason why tiny robots are more than just small versions of big robots, it’s that you need them in large numbers and they must be able to coordinate their tasks. Imagine for example you want to deliver drugs to cancer cells with robots of a size comparable to that of a cell. Well, then you need a lot of those robots just because there’s lots of cells in a tumor. A tumor of one cubic centimeter is typically made of some hundred million cells. That means, the production of these robots must be easy, cheap and fast.

But enough talk about the problems, let’s look at some robots that engineers have built.

Tiny robots often rely on flexible materials that can change shape. Here is an example of a little robot that’s a few millimeters long. It was developed by a group of researchers from the Max Planck Institute in Stuttgart, Germany. They published their results in Nature magazine in 2018.

This tiny robot is basically an elastic piece of silicone with some magnetic material added. Because of the magnetic material, one can use a magnetic field to bend and move it, in quite a variety of ways for which the group has taken inspiration from worms, caterpillars and jellyfish. The magnetic field they used has a strength of typically 10 milli Tesla. That’s strong, but not super strong, about several thousand times less than what you need for an MRI.

A few millimeters are still pretty large if you want to move around the human body. Here is another recent example of a robot that’s less than a tenth of a millimeter. It was developed by researchers at Cornell and Pennsylvania University. This tiny robot basically consists of a body that has a solar cell and four legs. If one illuminates the solar cell with laser pulses, then the legs move.

These robots can easily be mass produced using the same techniques that are currently used to mass produce microchips. The team estimates that for one US dollar, you could produce about a thousand of such robots, each equipped with a clock, sensors, and a programmable controller. To give you a sense of scale, these robots are so small that they can be sucked up injected with a syringe needle, which I’m sure will delight conspiracy theorists and anti-vaxers. The researchers hope that in the future their robots can operate with sunlight as power source.

That looks neat. Except, well, there isn’t a lot of sunlight in the human body. Indeed, how to get microbots in the desired place to do their job is maybe the biggest challenge at the moment. There are basically two ways to get it done. Either you use some kind of external control. That could be using magnetic or electric fields or ultrasound or lasers. Or you use some kind of self-propulsion mechanism.

An example of a self-propelled robot comes from two Japanese researchers who published a paper in 2019. Their tiny robot is basically a little tube that has an anode and a cathode, and those act on organic molecules which you would also find inside the human body. As a result, the robot moves forwards because momentum is conserved. It’s all physics!

Another method to move around a tiny robot is by pushing it with bacteria that themselves can be controlled with magnetic fields. Yes, there are bacteria that respond to magnetic fields, the species is called Magnetobacterium. This idea was put forward in 2006 by a group from the NanoRobotics Laboratory in Quebec, Canada. The magnetic field they used is half a Gauss, which is about an order of magnitude smaller than the field they used to bend the flexible millibot. Again that’s strong but not super strong. But this idea of bacteria-aided propulsion hasn’t been much further explored.

The major motivation for tiny robots is that one day they can be used to perform tasks in blood vessels or inside human tissue, to make surgeries less invasive or to entire avoid them. They might also be used to deliver drugs to a specific target to make a treatment more effective and protect the surrounding tissue. Besides this, they could passively collect data about their environment with very exact time and location markers. But these aren’t the only things tiny robots could be good for.

A team of researcher from the Czech Republic has for example developed a microbot that can capture and destroy microplastics. This robot is star-shaped and only about 4 micrometers in size, so that’s smaller than all the previous ones we have looked at. It is also powered by sunlight. In a paper published a few weeks ago the researchers show how their tiny robot gets stuck onto microplastic bits as soon as it touches them. The robot then accelerates the degradation of plastic.

Their test result numbers are not very impressive: In lab experiments, the robots reduced the weight of the plastics by only 3% in one week. Then again, this is just a proof of concept. Maybe one day we could release trillions of these robots at sea or wherever you want to clean and let them do the work for you.

Except, well, you might just swap microplastic pollution for microbot pollution. This is why Michael Sailor from the University of California San Diego has proposed to drill nanometer sized holes into tiny robots to make then less durable. They would then easily degrade within days or months, depending on the conditions, and decay into nontoxic silicon compounds.

Some researchers are thinking about robots differently. They combine biological materials, like parts of cells, with synthetic materials. These hybrid robots aren’t just promising because they allow researchers to use propulsion mechanisms that evolution has developed, but also because they can remedy another problem. It’s that robots in the human body may be attacked by the immune system. Hybrid robots which resemble cells or parts of cells can greatly alleviate this issue.

An example of a tiny hybrid robot that can swim through blood was developed by researchers at the University of California San Diego in 2018. Their goal was to use the robot to remove harmful bacteria and the toxins which the bacteria produce. Their robots have a size of about one micrometer, are powered by ultrasound, and can travel up to 35 micrometers per second.

They are made of gold nanowires coated with membranes from red blood cells and platelets. The gold nanowire responds to ultrasound, which allows the researchers to control where the robots go. The coating maintains much of the function of those cells, and therefore can absorb and neutralize toxins produced by bacteria.

The researchers have tested their tiny robots on blood samples contaminated with bacteria. After five minutes, the blood samples had three times less bacteria and toxins than untreated samples. Again this isn’t technology that will become common use any time soon, but it’s a promising proof of principle.

Finally, let’s talk about the xenobots. Xenobots are robots the size of a few tenths of a millimeter made from frog embryo cells. These cells are called Xenopus laevis, hence the name xenobots. This is a fairly new idea; it’s only been around since 2020.

The way it’s done is by using two types of cells: skin cells to create a barrier and heart muscle cells, which provide movement when they contract. Depending on how they are combined, the xenobot can perform different functions. This could be cleaning a medium of a certain substance, like microplastics or certain chemicals, or delivering drugs to a specific location, or clearing an obstructed artery.

A few weeks ago, a new paper appeared in the Proceedings of the National Academy of Sciences in which the authors presented for the first time xenobots capable of self-replication. The xenobots replicate by collecting cells and assembling them to new xenobots. Not really how we are used to self-replication from biology, but self-replication nevertheless.

But the production of these xenobots currently requires a lot of craftsmanship. It’s done by hand with a lot of cutting and twisting under the microscope, like an especially tiny form of microsurgery. This is clearly totally impractical for mass production, so until a cheap and easy technology has been developed to create xenobots, they’re not going to be of much use.

As you can see, tiny robots are a super-active research area at the moment, and the potential of this new technology is amazing. We will certainly come back with updates in the future, so don’t forget to subscribe.

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