Researchers from the Kaneko Higashimori Lab at Osaka University and the Arai Lab at Nagoya University have observed an interesting phenomenon when studying red blood cells in microfluidic channels. Instead of flowing along the channel in a smooth motion as expected, some cells bounce back and forth between the channel walls in a pinball-like motion at much slower speed. In addition to these ‘cell pinballs’, there are also cells that move at a similar reduced speed, but don’t hit the channel walls.
This altered behaviour could have detrimental effects on microfluidic devices, caused by non-uniform movement of the cells in the channels. In order to prevent these potential problems, the authors have investigated the cause of this behaviour. They noted that cell pinballs only occur when the saline medium is hypotonic, as this causes the cells to inflate due to intake of water. By attaching microbeads to the cells and using a high speed camera, the motion of the cells were studied in more detail. The pinball cells rotated clockwise as they moved to the left of the channel and anticlockwise as they moved to the right of the channel (relative to the direction of the flow).
This observation, combined with the knowledge that the cells were inflated in the hypotonic solution, led the authors to believe that the pinball-motion was occurring due to both the shape of the red blood cell and contact with the channel walls. 3D images obtained using confocal microscopy showed that the upper and lower surfaces of the cells were flattened, confirming that the cells were in contact with the walls.
By studying the different possible deformations of the inflated red blood cells when subjected to flow, the authors found that the contact line (between the cell and wall) and the centre line of the cell were not the same. This explains both types of unexpected cell motion – if the contact line is downstream of the centre line, the cell is unstable to rotational motion and this causes it to move at an angle to the flow, leading to the pinball cells, whereas if the contact line is upstream of the centre line the cell is stable to rotational motion and no displacement occurs, leading to the slow moving non-pinball cells.
From these studies, the authors were able to propose mechanisms that successfully explained the two types of altered red blood cell behaviour in hypotonic solutions, and hopefully in the future this should allow microfluidic systems to be used which will avoid this pinball-motion occurring.
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Cell pinball: phenomenon and mechanism of inertia-like cell motion in a microfluidic channel
Ryo Murakami, Chia-Hung Dylan Tsai, Makoto Kaneko, Shinya Sakuma and Fumihito Arai
Lab Chip, 2015, 15, 3307-3313
DOI: 10.1039/ c5lc00535c
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