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Laser Tweezers and
Laser ScissorsBrian Hull
Physics 461
March 21st, 2004
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Laser Tweezers and Scissors
Beginnings Laser scissors came first
Donald E. Rounds and Michael W. Berns wrote apaper in Scientific American, february 1970 titled
Cell Surgery by Laser.
Laser tweezers were first demonstrated by
Arthur Ashkin (mid 1980s)
They performed optical trapping inside a cellwithout causing any damage to the cell wall or
membrane
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Early Laser Tweezers
Experiments (Ashkin) A continuous-wave (cw), low-power (
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Early Laser Tweezers
Experiments Steven Chu from Stanford University showed
that lasers could also manipulate molecules
They demonstrated this by placing
polystyrene beads at the ends of DNA andpulling on the beads to stretch the DNA
Blockof Princeton and Sheetzof Duke have
used laser scissors to study kinesin motors(the structures that drive a flagellum or sperm
tail)
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Early Laser Scissors
Experiments Berns and his colleagues focused on defining
the laser parameters and determining whichorganelles could be manipulated (~.25 m)
Later found that laser scissors could be usedto study organelles of the nucleus of a cell,
including chromosomes, and the mitotic
spindle Also began to study structures such as
mitochondria, microfilaments & microtubules
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Physics of Laser Tweezers A small relatively transparent (to a certain
laser frequency) object refracts incidentbeams, bending the light
This refraction transfers momentum from the light
to the target object
The momentum causes a Lilliputian force (pN)
that pulls the object toward the focal volume ofthe beam
By moving the focal volume of the beam one can
cause the object to move as well
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Physics of Laser Tweezers Force Measurements:
Force is linearly dependent on the laser power
F = QP/c
Where Q is a trapping efficiency parameter dependent upon
the optical properties of the trapped object
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Laser Tweezers Diagram
Cell
Trapping Beam
Fgrad
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More about Laser Scissors Important to achieve precision and selectivity
Precision refers to targeting Due to technology (high level of optical elements in
microscopes), precision is relatively straightforward
Selectivity refers to alteration of the target leavingthe surroundings unaffected
Laser is a Gaussian distribution of energy Energy at the focal spot is characterized by a bell-
shape curve
Only the peak of the curve may have sufficient energy
to alter a target so the effective spot can be much lessthan the diameter of the measured focal spot
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More about Laser Scissors Unknowns about interactions between laser
light and biological targets Makes selectivity very difficult to guarantee
without empirically noting what works
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Differences between Laser
Tweezers and Laser Scissors Obvious
Cutting versus managing and manipulating
Scissors are short pulsed and high irradiance
(energy reaching the surface in a given time)beams while tweezers are continuous and of
low-irradiance
Target MUST be transparent to tweezers inorder not to pass significant energy
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Side effects and Difficulties of
Laser Tweezers and Scissors Defining exact laser effects is difficult
Submicron interractions
Energy controls in small volumes
Heat is a significant issue with Tweezers The cw lasers can cause a 1.15 to 1.45 degree
Celsius rise per 100 milliwatts of laser power
If the dissipation is inefficient the laser canproduce ten times that power
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More Side effects and
Difficulties Can cause microplasma to form
A cloud of ionized gas that stresses the cell andcan cause it to rupture
Occurs when the laser is focused on or above the
glass slide (of a microscope) Single photon absorption may heat a target
and can initiate chemical reactions resulting
in free-radicals Photoablation
High energy photons (ultraviolet) can breakmolecular bonds, tearing molecules apart
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Applications Chromosome alteration
Scissors produce changes in chromosomes andcan inactivate a selected chromosome
Can cut and paste chromosomes during meiosis
Leaves the chromosomes apparently unaffected
Study mitotic spindle
Discovered that movement of fragments outside thespindle was easy but fragments in the spindle could
not be moved
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Applications cont Optoporation
Cutting of micron sized holes in a cell
The cell closes the hole in a fraction of a second with
no apparent permanent damage
Within that time, molecules can be inserted into a cell
Suitable for genetic manipulation of plants
especially (cell walls make manipulation difficult)
Exploit microplasma formation
A glass capillary tube collects the cell contents
Applicable in single-cell analytical chemistry
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Applications cont UC Irvine: study immune system T and B-
cells T cells have a polarized appearance, a kind of
front and back
When B-cells were trapped and placed at the
back of the T-cells, nothing occurred but when
placed at the front, the immune system cascade
began
(increasing Ca+ ions in T-cells and promoting their
proliferation)
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Applications cont UC Irvine: Tweezers can grasp highly motile
cells including male sperm cells Study the swimming force of sperm
Trapped sperm cells and analyzed their escape
abilities
Discover zigzag pattern swimmers create a greater
force than the straight pattern swimmers
Note however that exposure to tweezers can
hamper motility of object or cell
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Applications cont Merging of two distinct cells to create a third
cell with the genetics of both the previous two Manipulation of gametes
assisted hatching Scissors thin or remove a small area of the
protective cover of an egg cell
Fertilization by trapping and moving sperm cells toa pore in an egg created by laser scissors
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Cutting Edge technology
(pardon the pun) National Institues of Health Laser Microbeam
and Medical Program Biotechnology Resource Built a microscope workstation with two laser
tweezers and one laser scissors beam into a
confocal laser fluorescence microscope
Lasers are tunable to any wavelength
Researchers can observe fluorescing cells during and
after use of laser tweezers and laser scissors
Useful in studying DNA sequencing
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References Liang, Hong et al. Directed Movement of Chromosome Arms and
Fragments in Mitotic Newt Lung Cells Using Optical Scissors and
Optical Tweezers. Beckman Laser Institute and Medical Clinic,UC Irvine
Berns, Michael. Laser Scissors and Tweezers. ScientificAmerican. Apr98 V. 278, Issue 4
Konig, Karsten. Laser tweezers and multiphoton microscopes inlife sciences. Robert Feulgen Prize Lecture. Springer-Berlag2000. pub. On-line 19 July 2000
Fuhr, Gunter R. Living cells in opto-electrical cages. Departmentof Biology, Humboldt-University of Berlin, Invalidenstr. 42.Elsevier Science B.V. 2000
www.intracel.co.uk/cellrmws.htm
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Questions?