Physicists in India have shown that red blood cells can transfer the angular momentum in a circularly polarized laser beam into rotational motion. The "motor" developed by Deepak Mathur and colleagues at the Tata Institute of Fundamental Research (TIFR) in Mumbai could find use in a variety of applications, including biosensors and cellular micromachines (J A Dharmadhikari et al. 2004 Appl. Phys. Lett. 85 6048).
Several groups have already shown that optical forces can be used to make micron-sized objects rotate, but these objects all had to be made with complicated microfabrication techniques. The advantage of the Tata approach is that it exploits naturally occurring materials.
“Close collaboration between biologists and physicists made it possible for us to overcome the need to microfabricate specially designed shapes for the ‘rotor’,” Mathur told PhysicsWeb. “The use of biological matter, in the form of red blood cells, allowed nature to do all the hard work for us as far as fabrication was concerned.”
Red blood cells are normally disk-shaped but the forces generated by the laser beams in an optical trap deform the disks into cylinders (see figure 1). These cylinders align themselves so that they are edge-on with respect to the direction of the incident laser beam, and then begin to rotate by following the polarization of the beam.
Mathur and co-workers found that the red blood cells — which came from humans and from mice — could rotate at up 42 revolutions per minute without being damaged, and that larger cells rotated faster than smaller ones. The speed of rotation could also be increased by increasing the laser intensity, although the cells were destroyed when the laser power exceeded about 100 milliwatts (see figure 2).
The group is now repeating its experiments with cells from different species. The elasticity of the cell membrane is central to the process because it governs how the cells deform in the laser beam.
“The torques generated in our ‘motor’ are enormous,” says Mathur,” but the key question is: can we use such a single-cell motor to perform tasks on the micron level? We await the answer with bated breath!”
