At Harvard University in Cambridge, Mass., scientists have reported that microorganisms can play the part of “microoxen,” being hitched to a load, directed to pull it to the desired location and unharnessed. In a proof-of-principle demonstration of the technique, which suggests applications in the guided assembly of microscopic devices, they employed 500-nm LEDs to take the reins of the unicellular green algae Chlamydomonas reinhardtii and to lead the cells to cart polystyrene beads along microfluidic channels.By exploiting a phototropism in the unicellular green algae Chlamydomonas reinhardtii, the researchers induce the microorganisms to carry polystyrene beads where they want using 500-nm LEDs. A photocleavable group in the syntheticpeptide that binds the beads to the cells enables the scientists to unhitch loads from the “microoxen” with a mercury lamp. Here, overlays of sequential images show C. reinhardtii hauling one 3-μm-diameter bead from the bottom of the frame to the top (left) and a string of two beads from the top of the frame to the bottom (right). Even under loads of five or more beads, the 10-μm-diameter algae swim at more than 100 μm/s. Courtesy of Douglas Weibel, Harvard University.Well-known to biology laboratories, C. reinhardtii is a fecund and hardy photosynthetic microorganism approximately 10 μm in diameter that moves at up to 200 μm/s using a pair of 12-μm-long flagella. It displays several tropisms, including a response to 505- and 443-nm light. The algae swim toward light at these wavelengths — unless it is too intense, in which case they swim in the opposite direction.The researchers exploited this behavior in their demonstration. They loaded the cells using a synthetic polypeptide that coated the 1- to 6-μm-diameter beads and that bound to the outer cell walls of the organisms by a noncovalent interaction. The peptide included a photocleavable group that enabled them to unhitch a bead from a cell by irradiating it with focused 365-nm radiation from an 80-W mercury lamp.By adjusting the intensities of the 500-nm LEDs placed at either end of the microfluidic channels, which were fabricated by photolithography in polydimethylsiloxane, they could induce C. reinhardtii to haul the beads back and forth along the passage — for two to three hours repeatedly in one round of the tests. The swimming speed of organisms carrying a single bead tended to be unaffected if the bead did not attach to the cell wall too near a flagellum, and the algae could carry five or more 3-μm-diameter beads simultaneously at speeds of more than 100 μm/s.The scientists note that the technique is simpler to implement than those that propose using isolated and reconstituted biological motors, such as the algae’s flagella, to manipulate microscale components.PNAS, Aug. 23, 2005, pp. 11963-11967.