Using Optical Tweezers to Manipulate Cells

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Using Optical Tweezers to Manipulate Cells

• Monish Shah
• Feb. 25, 1999

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Outline of Presentation

• What are Optical Tweezers?
• History of optical tweezers
• How do optical tweezers work?
• How safe are optical tweezers?
• Interesting experiments with tweezers
• Optical Tweezers in my research

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What are Optical Tweezers?

• A low power, continuous wave laser that is
  focused through a high N.A. objective can trap
  particles of diameter ?10 ?m.
• Can move the trapped particle by moving the laser
  or stage, hence the laser acts as a tweezer by
  picking up and moving an individual particle.

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History of Optical Tweezers

• Most of the early work in this field was done by
  Arthur Ashkin of Bell Labs.
• 1978 two opposing laser beams were used to trap
  and cool atoms 1.
• 1986 a single laser focused through a microscope
  was used to trap polystyrene balls with diameters
  10 ?m to 25 nm 2.
• 1987 bacteria and protozoa were trapped, first
  with a 514.5 nm Ar laser, followed by a 1064 nm
  NdYAG laser 3, 4.

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How do Optical Tweezers work?

• Two regimes of operation
• Rayleigh regime (diameter of particle ltlt ?)
• Mie regime (diameter of particle gtgt ?)
• Ray optics used for simple explanation in Mie
  regime 2

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Ray Optics

• Two main forces
• Scattering force
• Gradient force
• Scattering force caused by reflection of incident
  beam
• Gradient force caused by the deflection
  (transmission) of incident beam
• Gradient force dominates scattering force

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More details 5

• R,T are the Fresnel coefficients for reflection
  and transmission. ? and r are the angles of
  reflection and transmission.
• Need to integrate over all angles to get total
  force

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Interesting Notes 5

• Maximum trapping force occurs along the z axis,
  when the laser is focused on the furthest edge of
  the particle -- gradient and scattering forces
  are in the same direction.
• For a TEM00 mode laser, with n
  nparticle/nsolution 1.2, the maximum force
  0.490nsolutionincident power/speed of light
• equilibrium point is offset from the center of
  the particle by 0.06radius towards the beam

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Physiological Effects of Tweezers

• First experiments using a 514.5 nm Ar laser
  killed bacteria at power levels of 100 mW 3.
• When a 1064 nm NdYAG laser was used, there was
  no noticeable damage. In fact, cells that were
  trapped reproduced, and the offspring remained in
  the trap 4.
• Liu et. al used the membrane probe Laurdan to
  monitor cellular temperature changes during
  trapping 6.
• For motile cells (ex. human sperm), ?Temp 0.93
  ?C/100 mW
• For immotile cells (ex. CHO), ?Temp 1.1 ?C/100
  mW
• In addition, Liu et al. found that continuous
  wave trapping had no effect on intracellular pH
  or DNA.

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Interesting experiments

• Optical Tweezers have been used to
• manipulate organelles 7
• measure forces associated with transport and
  adhesion8
• study the swimming forces of sperm 9
• study kinesin motors 10
• stretch DNA molecules to their full length 11

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My use of Tweezers

• In our laboratory, we are trying to create
  patterned living neural networks
• Ideally, cell soma should reside on the nodes,
  and axons and dendrites should connect along
  pathways. An electrode would be placed
  underneath the soma for recording and stimulation

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Cont

• However, since cells are flooded onto the
  surface, the node compliance is low.
• Therefore, optical tweezers are used to try to
  move the cells onto the nodes

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Current Status and Problems

• Cells can be trapped and manipulated easily
• However, current setup is not optimal for
  transfer of cells from a reservoir to a pattern
• Also, the B104 cells tend to stick rather
  quickly. Once stuck, they cannot be moved by the
  tweezer
• Working distance at 100x is quite small
• Gravity!

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Conclusion

• Optical tweezers are a useful tool for the
  manipulation of cells
• easy to use
• physiologically safe
• can do a variety of experiments with them

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References

• 1 Ashkin A., Trapping of Atoms by Resonance
  Radiation Pressure, Physical Review Letters 40,
  pp 729-732 (1978).
• 2 Ashkin, A. Dziedzic, J. M. Bjorkholm, J.
  E. and Chu, S., "Observation of a Single-Beam
  Gradient Force Optical Trap for Dielectrical
  Particles", Optics Letters 11, pp 288-290 (1986).
• 3 Ashkin, A. Dziedzic, J.M., "Optical Trapping
  and Manipulation of Viruses and Bacteria",
  Science 235, (4795) pp 1517-20 (1987).
• 4 Ashkin, A. Dziedzic, J. M. and Yamane, T. ,
  "Optical Trapping and Manipulation of Single
  Cells using Infrared Laser Beams", Nature 330,
  (24-31 Dec.) pp 769-771 (1987).
• 5Ashkin A., Forces of a Single-beam Gradient
  Laser Trap on a Dielectric Sphere in the Ray
  Optics Regime, Biophysical Journal 61, pp
  569-582 (1992).
• 6Liu, Y. Sonek, G. J. Berns, M. W. Tromberg,
  B. J., Physiological Monitoring of Optically
  Trapped Cells Assessing the Effects of
  Confinement by 1064-nm Laser Tweezers Using
  Microfluorometry, Biophysical Journal 71, pp
  2158-2167 (1996).
• 7 Aufderheide, K.J. Du, Q. and Fry, E. S.,
  "Directed Positioning of Micronuclei in
  Paramecium tetraurelia with Laser Tweezers
  Absence of Detectable Damage after Manipulation",
  Journal of Eukaryotic Microbiology 40, pp 793-796
  (1993).

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• 8 Finer, J. T. Simmons, R. M. and Spudich, J.
  A., "Single Mysoin Molecular Mechanics
  Piconewtons Forces and Nanometer Steps, Nature
  368, pp 113-119 (1994).
• 9 Tadir, Y. Wright, O. Vafa, O. Ord, T.
  Asch, R.H. and Berns, M.W., "Micromanipulation
  of Sperm by a Laser Generated Optical Trap",
  Fertility and Sterility 52, pp 870-873 (1989).
• 10 Kuo, S. C. and Sheetz, M. P., "Force of
  Single Kinesin Molecules Measured with Optical
  Tweezers, Science 260, pp 232-234 (1993).
• 11 Chu S., Laser manipulation of atoms and
  particles, Science 253, pp.861-6 (1991).