Soft Matter Physics Division - Biophysics at the University of Leipzig University of Leipzig
IntroductionOptical Stretcher & Rotator

Optical Stretcher
Schematic of an optical stretcher: The Cells are in suspension in a flow chamber. They can be trapped by two opposing laserbeams of low intensity, emanating from optical fibers. Raising the intensity of the laserlight increases the forces acting on the cell surface, leading to measurable deformation. (Animation by J. Guck et al. [7])
The optical stretcher is a novel laser tool to micro-manipulate single biological cells and probe their viscoelastic properties in suspension [1-3]. In the stretcher, an individual cell is trapped between two divergent, opposing laser beams. A stress is exerted on the cell where the light hits the surface causing an elongation of the cell body along the laser beam axis (stress-strain elasticity experiment).
Since reflection (<0.5%) and absorption (<0.01%) of the laser beam are negligible at the chosen wavelength (780 nm), the laser light is almost completely transmitted through the cell. The stretching forces arise from a momentum transfer of light to the surface. The momentum of the laser beam increases inside the cell because the cell has a higher refractive index than the surrounding medium. By conservation of momentum, the beam gives an impulse to the cell, resulting in the stretching force on the cell surface where the light enters and leaves.
The amount of stretching, or optical deformability, depends on the force on the surface (which we can be controlled by adjusting the laser power) as well as the physical properties of the cell, such as size and refraction index.
The stretching forces (pN to nN) exerted by the optical stretcher can be up to 100 times higher than the holding forces for optical tweezers without causing any radiation damage, since the two laser beams are not focused and the optical stretcher does not rely on gradient forces.
Combined with a microfluidic flow chamber, the processing of large numbers of cells is possible, which is a big advantage over conventional techniques for measuring cell elasticity. (Those techniques are often limited due to laborious sample preparation).

We have derived an analytical model to calculate the stress profile on the cell's surface. This model has been verified by stretching objects with well-defined elasticity.
The response of the cells to the applied stress profile can be well described by a viscoelastic three-parameter model, and reveals a lot of information about the material properties of the cell.

Comparison between the deformation of a red blood cell observed in the optical stretcher and the deformations expected from membrane theory (white lines) showing an excellent agreement. The peak stresses sigma_0 calculated using ray optics were used for the membrane theory calculations. (Figure taken from [2].)

Application of the Optical Stretcher for Cancer Diagnosis

Using an optical stretcher, the cell elasticity can be accurately measured. This material property of cells can be used to differentiate between different cell types or between normal and unhealthy cells.

Since changes in the cytoskeleton are characteristic in the pathology of cancer, we investigated whether single malignant cells and precancerous cells can be detected by elasticity measurements with the optical stretcher. Model cell lines were used to explore to what extent cell elasticity is a good parameter to detect cancer cells. We compared fibroblasts to clonal populations of these cells malignantly transformed by H-ras, SV40, or v-rel and normal neutrophils to leukemia cell lines. It turned out that malignant cells generelly are easier to stretch and show a lower elastic strength [7, 10, 13].

The ultimate goal of these cell elasticity studies with the optical stretcher is to distinguish dysplastic cells from early cancer cells and to monitor the progress of cancer from preinvasive to invasive. We also investigate whether we are able to separate pluripotent stem cells from blood stem cells in umbilical cord blood based on cell elasticity. This could open new routes in stem cell research avoiding the ethical problems with stem cells from embryos. It is planned to have a clinical device for the early diagnostics of cancer on the market within the next 5 years.

Optical Cell Rotator
Schematic of an optical rotator, which is is mounted on an inverted microscope. The suspended cells are trapped and rotated by two opposing laserbeams of low intensity, emanating from optical fibers. (Figure by Anatol Fritsch, 2009.)
One inherent problem of optical tomography, for example when using an confocal laser scanning microscope, is its resolution in axial direction. The lateral resolution exceeds the axial at least by a factor of 2 to 3. Rotation of the object of interest along a horizontal axis can resolve this issue and the higher lateral resolution for every rotation angle could be used to image the desired cell-detail and eventually recalculate an isoresolution 3D image.

The optical cell rotator is a modified divergent dual-beam laser trap for holding and controlled rotation of suspended cells [14]. Cell align with their long axis inside this optical trap when using two gaussian beam profiles like in the optical stretcher. With the optical rotator, an asymmetric beam is introduced on one side of the trap. Since cells are naturally inhomogeneous, they align somehow to this beam profile. When now the asymmetric beam profile is rotated, a rotation of the trapped cell itself is induced. Since the optical rotator is fully decoupled from imaging optics, it could be a beneficial tool for tomographic microscopy.

A cluster of several MCF-7 cells is rotated by 360 degrees. This videos already allow a good impression of the 3D topology of the cells. (Figure by Anatol Fritsch & Tobias Kießling, 2009.)

J. Guck, R. Ananthakrishnan, T. J. Moon, C. C. Cunningham, J. Käs: Optical Deformability of Soft Biological Dielectrics, Phys. Rev. Lett. 84(23):5451-5454 (2000)
J. Guck, R. Ananthakrishnan, H. Mahmood, T. J. Moon, C. C. Cunningham, J. Käs: The Optical Stretcher: A Novel Laser Tool to Micromanipulate Cells, Biophys. J. 81(2):767-784 (2001)
J. Guck, R. Ananthakrishnan, C. C. Cunningham, J. Käs: Stretching biological cells with light, J. Phys.: Condens. Matter 14(19):4843-4856 (2002)
B. Lincoln, H. M. Erickson, S. Schinkinger, F. Wottawah, D. Mitchell, S. Ulvick, C. Bilby, J. Guck: Deformability-based flow cytometry, Cytometry Part A 59A(2):203-209 (2004)
F. Wottawah, S. Schinkinger, B. Lincoln, S. Ebert, K. Müller, F. Sauer, K. Travis, J. Guck: Characterizing single suspended cells by optorheology, Acta Biomaterialia 1(3):263-271 (2005)
R. Ananthakrishnan, J. Guck, F. Wottawah, S. Schinkinger, B. Lincoln, M. Romeyke, J. Käs: Modelling the structural response of an eukaryotic cell in the optical stretcher, Current Science 88(9):1434-1440 (2005)
J. Guck, S. Schinkinger, B. Lincoln, F. Wottawah, S. Ebert, M. Romeyke, D. Lenz, H. M. Erickson, R. Ananthakrishnan, D. Mitchell, J. Käs, S. Ulvick, C. Bilby: Optical Deformability as an Inherent Cell Marker for Testing Malignant Transformation and Metastatic Competence, Biophys. J. 88(5):3689-3698 (2005)
F. Wottawah, S. Schinkinger, B. Lincoln, R. Ananthakrishnan, M. Romeyke, J. Guck, J. Käs: Optical Rheology of Biological Cells, Phys. Rev. Lett. 94(9):98103 (2005)
R. Ananthakrishnan, J. Guck, F. Wottawah, S. Schinkinger, B. Lincoln, M. Romeyke, T. Moon, J. Käs: Quantifying the contribution of actin networks to the elastic strength of fibroblasts, Journal of Theoretical Biology 242(2):502-516 (2006) 
M. Martin, K. Mueller, F. Wottawah, S. Schinkinger, B. Lincoln, M. Romeyke, J. A. Käs: Feeling with light for cancer, Proceedings of SPIE 6080:126-135 (2006)
B. Lincoln, S. Schinkinger, K. Travis, F. Wottawah, S. Ebert, F. Sauer, J. Guck: Reconfigurable microfluidic integration of a dual-beam laser trap with biomedical applications, Biomedical Microdevices 9(5):703-710 (2007)
S. Ebert, K. Travis, B. Lincoln, J. Guck: Fluorescence ratio thermometry in a microfluidic dual-beam laser trap, Optics Express 15(23):15493-15499 (2007)
T. W. Remmerbach, F. Wottawah, J. Dietrich, B. Lincoln, C. Wittekind, J. Guck: Oral Cancer Diagnosis by Mechanical Phenotyping, Cancer Research 69(5):1728-1732 (2009)
M. K. Kreysing, T. Kießling, A. Fritsch, C. Dietrich, J. R. Guck, J. A. Käs: The optical cell rotator, Optics Express 16(21):16984-16992 (2008)

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