laser tweezers and laser scissors

8/2/2019 Laser Tweezers and Laser Scissors

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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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