R OBOTS COME in all shapes and sizes. Some are humanoid. Others resemble animals. Many are just a jumble of arms slaving away on a production line. But one thing all robots have in common is that they are mechanical, not biological devices. They are built from materials like metal and plastic, and stuffed with electronics. No more, though—for a group of researchers in America have worked out how to use unmodified biological cells to create new sorts of organisms that might do a variety of jobs, and might even be made to reproduce themselves.

There are several ways to tinker with living organisms. Selective breeding and, more recently, genetic engineering permit the production of novel plants and animals for agriculture and horticulture, and as pets. Souped-up bugs for industrial processes can also be made in these ways. Researchers are working, too, on growing isolated animal organs for testing drugs and eventually, perhaps, for transplant surgery.

What Joshua Bongard of the University of Vermont and Michael Levin of Tufts University in Massachusetts have come up with is different. As they report in the Proceedings of the National Academy of Sciences, they and their colleagues have designed organic robots from their cellular components, and then set about realising those designs by joining together specific types of stem cells taken from a well-studied species of African frog, Xenopus laevis. The result (pictured) is close to matching the biological definition of an organism, in that it is capable of behaving autonomously and contains cell types that are specialised to perform different roles.

Though only a millimetre or so across, the artificial organisms Dr Bongard and Dr Levin have invented, which they call xenobots, can move and perform simple tasks, such as pushing pellets along in a dish. That may not sound much, but the process could, they reckon, be scaled up and made to do useful things. Bots derived from a person’s own cells might, for instance, be injected into the bloodstream to remove plaque from artery walls or to identify cancer. More generally, swarms of them could be built to seek out and digest toxic waste in the environment, including microscopic bits of plastic in the sea.

To design their bots Dr Bongard and Dr Levin employed a computer program called an evolutionary algorithm. This worked by creating virtual representations of thousands of arrangements of cells that might achieve a particular task. It then tested those arrangements, using what is known about the biophysics of Xenopus cells, for suitability to perform the task in question, picked the most promising versions to form the basis for thousands more cellular arrangements, and then repeated the process until something properly fit for purpose emerged. That done, it was merely a matter of building the pattern which the algorithm had arrived at out of actual Xenopus cells, using microsurgical techniques to shape groups of cells in the way the pattern dictated.

The demonstration bots Dr Bongard and Dr Levin have made use two types of stem cell. Some are so-called pluripotent cells taken from early-stage embryos. These embryonic cells retain wide powers to turn into other cell types. The others are cardiac progenitor cells, a more specialised type of stem cell already destined to generate heart muscle.

Placed in a dish, bots made in this way were able to propel themselves along the dish surface via contractions of the heart-muscle cells within them. Besides pushing single pellets, groups of bots put into a dish together were able to work collectively, moving around in circles and gathering the pellets into neat piles.

Exactly how that happens is not yet clear. “It is possible”, says Dr Bongard, “that the cells are signalling to one another in a way we’re not aware of.” That possibility, and many other questions, will be the subject of further research. The team are also trying to work out how cells can be motivated to build complex, functioning bodies. Such knowledge, says Dr Levin, would be immensely useful in regenerative medicine, which seeks to repair organs and build body parts for transplant.

Go forth and multiply?

For xenobots to have a practical future, though, someone will have to find a less fiddly way of making them. At present, it takes a microsurgeon hours to handcraft each individual bot, peering down a microscope and using tiny tweezers to do so. One way the process might be automated is by employing three-dimensional printing to build up the necessary layers of cells.

The new organisms could also do with upgrading in certain ways. At present, for example, they have short lives—a couple of weeks at most. This is because they do not have any apparatus for feeding themselves. In one sense that is a good thing, for it soothes fears about safety. If a bot should escape it would expire at the end of its allotted time and, being made simply of frog cells, would be biodegradable and non-toxic. But because longer-lived bots would be more useful, the researchers are looking at ways to extend their creations’ lives.

A more controversial suggestion is to equip xenobots with reproductive systems—perhaps as simple as allowing them to divide themselves in two, in the way that flatworms can. This would help any application that required a swarm of the critters. It might also, though, raise concerns about escapees establishing themselves in the wild. All this, says Dr Bongard, means it will be necessary to work with policymakers to decide how the production of future life forms, as useful as they might be, might be regulated. ■