BIOBOTS to Treat Brain Injury?

Tiny living robots that travel your system and repair wounds? This idea sounds like science fiction, but it might be closer to reality than you think…  

If you like sci-fi films, I bet you've seen at least one show where a fleet of some sort of tiny robots or nanoscale devices were administered to someone with severe injuries or illness to travel their body and heal them internally. Such ideas aren't limited to the theater, but are actually being explored as a part of real biotechnology research. In an actual scientific study,  researchers were experimenting with so-called biological robots, or “biobots”. 

The idea of biological robots sounds unbelievable, but biological machines have long been in existence. If you think about it, really all biological organisms - bacteria, plants, cells, and even the human body - could be considered machines. Yet, rather than containing nuts and bolts, biological machines contain DNA, proteins, and other special biological molecules that enable them to perform different functions. An exciting prospect would be if we could harness the abilities of different biologics to function in new ways, like heal wounds throughout the body, repair brain damage, kill cancer cells, or deliver medications to specific organs. Such goals lie behind biobot research.

So, what is a biobot? 

A biobot is made up of some sort of living material like bacteria or other cells, which are somehow held together in a collective that performs a specific function. A big question though is how to organize and connect cells to form a working multicellular form. Multicellular organisms are exceedingly complex. For example, consider all of the cells that make up just your hand. Not only is your hand composed of skin cells, but many different types of cells. Further, exact connections between all of these cells are required for your hand to function: networks of blood vessels, bone, capillaries, cells of the immune system, and connections with your brain. Piecing such an advanced organization together cell-by-cell would be a monumental task. But what if collectives of cells could self-assemble so we don’t have to piece them together artificially? The scientists who wrote the paper discussed here created what they call “anthrobots”, so-called because they are made out of human cells. The authors describe these as “fully biological self-constructing mobile living structures”, which are created out of a type of human lung cells. 

The researchers here grew a type of human airway cell to form their anthrobots. But why would they choose cells of the airway? Well, any good biological robot should be able to move around on its own. One way that some cells can move is by possessing cilia: projections on the outside of the cell sort of like fins that enable cells to move and swim. Cells in human airways have these special structures. Of course, cells in the lung don't swim, but in stationary cells these cilia take on a different role. Cilia pointing outwards into the lung and airspace can help push debris and mucus out of the airways. So what happens if we take human lung cells out of the airway? In the lab, the researchers formed clusters of these cells called spheroids. The researchers’ idea was that the cilia these cells naturally possess would enable them to move and swim on their own. 

But there was one challenge. The human lung isn't just a dense ball of cells. There's an open space for the air surrounded by cells: basically a hollow ball of cells. This hollow space in an organ is called the lumen. So, if we zoom in, the cells actually have a direction. They're sitting on some surface, usually other cells or some sort of tissue, and facing upward into an open space. This upward direction into the open cavity is called the apical side. And the bottom is called basal. You might hear the fancy sounding term apicobasal polarity, which essentially just means what direction the cell is facing. Cells have a phenomenal capability to organize into structures on their own. So, when the researchers created spheroids out of these airway cells, the cells organized themselves into a hollow ball with all of the cilia facing inwards and none on the outside. This was a problem because in order for the biobots to be able to swim, they would need to have the cilia on the outside. 

A huge part of what causes cells to organize into different structures and change their shape and behaviors is external conditions. These factors include not only chemicals and molecules surrounding the cells, but also physical surroundings, like the types of materials in which the cells are grown. The scientists here tweaked the growth conditions of the spheroids until they found conditions that resulted in the cells organizing into what they call “apical out” structures, or inside-out structures with the cilia on the outside. One of the main changes they made was varying the type of substance in which they were growing the cells. Specifically, the researchers transitioned the spheroids from growing in a thick gel-like substance called Matrigel to growing directly in liquid. They observed that these spheroids with cilia on the outside could move and swim on their own. Using this process, they found they could grow mobile cell structures in about a week.

These so-called anthrobots could swim in different different directions, straight ahead or in circular patterns, or wriggle around in place. But the authors were curious how these anthrobots would behave around other cells. They set up something called a scratch assay, which is a way of mimicking injury to a sheet of cells growing in a dish. Cells in a petri dish grow by dividing. These form small groups of cells called colonies that grow larger and larger. Eventually, cells will cover the entire surface of the dish, forming a continuous sheet of cells. To perform the scratch assay, this layer of cells is literally scratched with a razor blade or scalpel. Then, it can be studied how the cells grow back together to heal the scratch. Here, the authors performed scratch assays using neuronal cells. They wanted to see how the anthrobots behaved in a scenario that was completely different from an airway, the environment that the biobots originally came from. Also, healing hard-to-repair organs like brain tissue would be a fantastic potential application for the anthrobots. 

The researchers placed the anthrobots on the neuronal scratch and then watched what happened. The bots appeared to move along and within the scratch. But the researchers wanted to see in greater detail how the anthrobots affected the injured neuronal cells near the scratch sitel. Inspired by collective behavior in nature, how groups or swarms can accomplish tasks that individuals alone cannot, the researchers clustered the anthrobots into what they call “superbot” assemblies. They then put these clusters of multiple bots into the scratch and examined what happened. 

These superbot clusters were larger, and could span the width of the scratch, forming a sort of bridge. Surprisingly, after a few days they observed regrowth of the neuronal cells directly underneath the “superbot bridges”. They did not see any regrowth at other locations along the scratch, so it probably wasn't just a coincidence. Even more striking was that the regrown tissue was just as thick as the uninjured cell layer. But was this really a unique phenomenon of the super-anthro-bots, or could any material form a bridge to aid neuronal cell repair? To answer this question, the scientists tried putting agarose, a jelly-like substance that cells can grow in, at different sites along the scratch. However, no neuronal tissue re-grew. So, it seems that \these biobot collectives could somehow enable the regrowth of neurons. 

But what happens to the anthrobots after the neurons heal? The authors report that the lifespan of the bots is around 4-6 weeks, and then they just disintegrate. 

So, when can we expect to inject anthrobots into people to heal brain injury? Well, these experiments were fascinating, but also very early in the research process. For instance, the scratch assay is certainly relevant to study how cells behave, but extremely simplified compared to what happens in the body. Neurons growing on plastic are subjected to very different surroundings than neurons growing in the brain. In the body, there are multiple cell types including immune cells that are involved in injury processes. Also, exactly how the anthrobots enabled the neuron regrowth in the dish isn't fully understood. Possibly, they could secrete some sort of substance that promoted neuronal growth, but it wasn't yet explored in this study. For example, perhaps there are other cell types that the anthrobots could be made from that would work even better.

Another outstanding question is how exactly anthrobots theoretically should be introduced into the body. It’s easy enough to put them into a scratched layer of cells in a petri dish, but in the body there are many places that the bots might potentially travel. More research is needed to figure out how to target them to specific sites within a scratch or injury within the body. And, it must be determined if they could cause any undesirable side effects or could harm other non-injured organs.

What do you think? With further research, might biobots have potential as an actual medical treatment someday? 

Reference for the paper discussed in this article:

"Motile Living Biobots Self-Construct from Adult Human Somatic Progenitor Seed Cells" Gumuskaya, G. et al. Advanced Science. 2023.