Tiny "bio-bots" inspired by sperm could swim inside the human body to deliver drugs or target cancer someday. The swimming bio-hybrid machines move by combining live heart cells with the flexible body of a synthetic polymer.
Past research has spawned a magnetic cork-screw swimmer and a flagellar swimmer made of magnetic beads and DNA molecules—both engineered creations dependent upon an outside magnetic force to move. By comparison, the new bio-bots represent the first swimming machines based on the flagellar movement of sperm that can propel themselves by harnessing the contractile power of the heart cells.
"It's the minimal amount of engineering—just a head and a wire," said Taher Saif, a professor of mechanical science and engineering at the University of Illinois, in a news release. "Then the cells come in, interact with the structure, and make it functional."
Movement of the 2-millimeter long bio-bots relies upon a small cluster of heart cells grown where the flexible tail meets the rigid head. The heart cells synchronize to beat together and create a wave motion in the tail that propels the bot forward at speeds of 5 - 10 micrometers per second.
The researchers from the University of Illinois and Arizona State University also created a two-tailed bio-bot capable of swimming even faster—81 micrometers per second. Their work is detailed in the 17 January issue of the journal Nature Communications. (The team previously created tiny "walking" robots made from 3-D printed hydrogel and rat heart cells.)
Such engineered creations still pale in comparison to their biological counterparts. For instance, a 70-micrometer long bull sperm can swim at speeds of 97 micrometers per second—139 percent of its body length per second—because it moves its entire tail. But the single-tailed and two-tailed bio-bots can only swim at 0.5 percent and 8.3 percent of their body length per second, respectively.
Still, the work marks a good first step in harnessing biological motion for a new class of bio-hybrid machines. The researchers hope to eventually use optogenetically enhanced muscle cells for light-actuated swimming, as well as a combination of neurons and muscle cells for "intelligent swimming" based on sensing.
