Optical Tweezers


Infrared lasers focused with high numerical aperture into a microscope are used to exert and measure forces in the piconewton range. Simple light pressure and the more complex ‘gradient forces’ are exploited. Unlike any other micromanipulation tool, optical tweezers can, for example, be used to work in the interior of unopened living cells to support cell fusion, to stimulate specificity in the interaction of immune cells and to simulate heart infarction. Here, a variant of optical tweezers, erythrocyte‐mediated force application is particularly helpful. Other applications are in human in vitro fertilisation, where hundreds of children owe their existence to optical tweezers, and blood pressure studies. Studies on infection of a single cell by a virus or bacterium and on the organisation, of one of the fundamental processes of life – the generation of motion (by the centrosome) – complement the wide field of applications of optical tweezers in the life sciences.

Key Concepts:

  • Light is focused with very high aperture into a microscope so that light pressure and gradient forces can be used to hold, manipulate and measure forces of microscopic objects.

  • Optical tweezers can be added to most standard microscopes with only little effort.

  • Infrared light is used, which has a large penetration depth in biological tissue. Thus, one can work in the interior of unopened objects.

  • With red blood cells as handle, forces of optical tweezers can be fully exploited (erythrocyte‐mediated force application, EMFA).

  • Optical tweezers can be used to establish contact between microsopic objects as well as to move particles and organelles in the interior of closed objects.

  • The force transduction is gentle and thus also suitable for fragile targets.

Keywords: lasers; microscopes; immune cells; cell fusion; piconewton forces; heart beating; blood pressure; cell infection; centrosome

Figure 1.

Example of optical tweezers combined with a laser microbeam. The two lasers are coupled into the microscope through the fluorescence illumination path. The pulsed titanium–sapphire or nitrogen laser serves as a microbeam for cutting, welding or drilling a micrometre‐sized hole; the continuous infrared lasers serve as optical tweezers for handling objects. NdYAG, neodymium yttrium–aluminium–garnet.

Figure 2.

Two cells from the immune system of the mouse just fusing with one another after being treated with a short series of laser pulses. The optical tweezers serve to push the two cells towards each other.

Figure 3.

Moving subcellular structures in the interior of unopened living algae cells. The infrared neodymium yttrium–aluminium–garnet (NdYAG) laser is invisible; its position is indicated by a circular disc added to the image by image processing. (a)–(d) are snapshots of a movie on organelle elongation at different times.

Figure 4.

Induction of a calcium wave in a layer of fibroblasts. An erythrocyte (centre, a) is pushed with optical tweezers onto fibroblasts. The erythrocyte solely acts as a handle that has no biological function. This process starts the liberation of calcium in the fibroblast (b), which spreads over the whole cell (c), and finally (d), after 2–3 s, couples over to neighbouring cells and spreads over a wide area in the tissue. The remarkable aspect of this experiment, which works even better with excitable heart muscle cells, is that it indeed does work with nonexcitable fibroblasts. Copyright © Karl Otto Greulich.

Figure 5.

(HUVECs) become stiffer with repeated application of pressure. The first calcium ion burst (peak) is obtained with a laser power of 70 mW. Later peaks need higher laser powers (arrows) to achieve the same burst size, for example, 360 mW in the ninth cycle at 650 s. The peak for the tenth cycle is somewhat higher, but 100 mW more than in the ninth cycle are needed. The overall fluorescence intensity decrease is due to photobleaching. Copyright © Karl Otto Greulich.



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Further Reading

Berns MW and Greulich KO (2007) Laser manipulation of cells and tissues. In: Methods in Cell Biology, vol. 82. San Diego, CA: Academic Press.

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Greulich, Karl Otto(Dec 2010) Optical Tweezers. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0003038.pub2]