What happens when biologists and computer experts join forces? They cross-breed frog cells with micromachines. The result seems spooky and is called Xenobots. Such tiny hybrids were presented last year by scientists from Tufts University and the University of Vermont (UVM) in the United States. Xenobots can haul payloads (such as pharmaceuticals) to defined destinations and even exhibited collective behavior in the presence of a swarm of other Xenobots.

And now the story becomes even eerier. The same team has recently created life forms that self-assemble a body from single cells. In other words, the Xenobot 3.0 creates itself, elevating the level of spookiness to the second power. This new generation of Xenobots also “lives” longer and moves faster than its predecessors, even navigating different environments. Since "Xeno3" also has a type of memory it’s even able to learn while retaining the ability to work together in groups and to heal itself if damaged.  

Let’s delve into the “genesis” of Xenobots that’s somewhat reminiscent of a microbiological variety of the classic “Frankenstein” creature. For the 1.0 version of the millimeter-sized bio machines, the scientists constructed a locomotor system from frog skin tissue and cardiac cells. For the next generation, the biologists at Tufts University took stem cells from embryos of the African frog Xenopus laevis (hence the name “Xenobots”) and allowed them to self-assemble and grow into spheroids (spherical micro forms of life). After a few days, some of the cells differentiated to produce cilia – tiny hair-like projections that move back and forth and literally give the spheroids “legs.”

The third generation is now able to collect stem cells and form replicants of itself from them.

Turbo evolution

But why are Xenobots machines? Senior scientist Douglas Blackiston provides some answers: “In a way, the Xenobots are constructed much like a traditional robot. Only we use cells and tissues rather than artificial components to build the shape and create predictable behavior.”

Michael Levin, Distinguished Professor of Biology at Tufts University and corresponding author of the study, adds that “In a frog embryo, cells cooperate to create a tadpole. Here, removed from that context, we see that cells can re-purpose their genetically encoded hardware, like cilia, for new functions such as locomotion. It is amazing that cells can spontaneously take on new roles and create new body plans and behaviors without long periods of evolutionary selection for those features.”

But how do Xenobots know what they’re supposed to do? A supercomputer at UVM assists them in this context. “We know the task, but it’s not at all obvious – for people – what a successful design should look like. That’s where the supercomputer comes in and searches over the space of all possible Xenobot swarms to find the swarm that does the job best,” says UVM robotics expert Josh Bongard. By means of this type of clustering using an evolutionary algorithm that simulates hundreds of thousands of random environmental conditions the bots are supposed to be optimized for more complex behaviors – or programmed for a kind of high-speed evolution, in a manner of speaking. Bongard continues, “We want Xenobots to do useful work. Right now, we’re giving them simple tasks, but, ultimately, we’re aiming for a new kind of living tool that could, for example, clean up microplastics in the ocean or contaminants in soil.” 

Initial experiments in that direction have produced some promising results: Specialized Xenobots work together in a swarm to sweep through a petri dish and gather large piles of iron oxide particles. They can also cover large flat surfaces, or travel through narrow capillary tubes.

Going forward

But the scientists are pursuing even greater aims. “When we bring in more capabilities to the bots, we can use the computer simulations to design them with more complex behaviors and the ability to carry out more elaborate tasks,” says Bongard. “We could potentially design them not only to report conditions in their environment but also to modify and repair conditions in their environment.”

Step one: The scientists at Tufts provided the Xenobot 2.0 with a read/write capability by injecting it with a messenger RNA to record one bit of information. When a Xenobot prepared in this way is exposed to a specific kind of light it will permanently change its color, enabling a “travel experience” to be recorded. Step two: An extended molecular memory could detect and record the presence of radioactive contamination, chemical pollutants, drugs or a disease condition, for example. Step three: Further engineering is intended to enable the bots to release compounds or change behavior upon sensation of stimuli.