Measuring Fluid Properties on a Microscopic Scale Using Optically Trapped Microprobes

Mark Cronin-Golomb

Biomedical Engineering

Tufts University

With the help of:

Boaz Nemet Yossef Shabtai Lisa Goel

at Tufts University Tayyaba Hasan Paal Selbo

at Wellman Laboratories of Photomedicine, MGH

Scanning Probe Microfluidic Analysis

Viscosity is an important indicator of biopolymer concentration. Flow analysis is important in development of microfluidic devices. Method: Confocal phase sensitive detection of optical tweezer beam

reflected from a trapped probe bead set in sinusoidal oscillation by the tweezer beam enables micrometer scale spatially resolved viscosity measurements at 10kHz data acquisition rates.

Acousto-Optic deflector

Scanning Mirrors

Tweezers principle

Electric Field

+-

Confocal microscope principle

Laser

Detector

Sample Stage

Scanning Mirror

Raster display

Pinhole

Beam Expander

Prior methods to measure viscoelasticity

Video microscopy of magnetically induced fluctuations Schmidt F.G., Ziemann,F. & Sackmann,E. Eur.

Biophys. J. 24, 348 (1996). Positional and temporal statistics of trapped

bead trap strength and viscosity A. Pralle, E.L. Florin, E.H.K. Stelzer & J.K.H. Horber, Appl. Phys. A-Mat. Sci. & Proc. 66, S71 (1998).

Viscosity measurement using position sensing detector

M.T. Valentine, L.E. Dewalt & H.D. OuYang, “Forces on a colloidal particle in a polymer solution: A study using optical tweezers.” Journal of Physics-Condensed Matter 8, 9477-9482 (1996).

Experiment Details

Ti - Sapphire Laser

Lock - in Amplifier

CCD Camera

OL

DM

P APD

*

*

AOD Driver

BS

*

AOD

#

#

#

SM

At large oscillation amplitudes the potential well splits

•As the tweezer beam is moved back and forth, the probe bead lags behind.

•The bead is bright when the tweezer beam illuminates it.

•The confocal signal is highest when the tweezer beam is centered on the probe bead.

Theoretical Background

sin( ) ( )dx

x a t L tdt

x: trap position : viscous drag

: tweezer spring constant a: amplitude of trap oscillation

: frequency of trap oscillation L(t): Brownian forcing function

2

cos

sin

( ) 1 ( )

duu a t

dtu x a t

I t u t

Experimental Results

0 1 2 3 4 5 6 7 8 90.00

0.05

0.10

0.15

0.20

0.25

0.30

A2

2n

d Har

mo

nic

am

pli

tud

e (a

.u)

0

10

20

30

40

50

60

70

80

90

100

2

nd H

arm

on

ic p

has

e (d

eg

)

Signal to Noise Ratio

Absolute Position Detection

Relative Position Detection

01/ 22 2

0 8c c

c B

aSNR

k T

20

1/ 22 20 8

c

c B

aSNR

k T

0 1000 2000 3000 4000 0

5

10

15

20

25

30

SNR

~ f -3/2

~ f -1/2

Confocal

Valentine

frequency (Hz)

Viscosity Image

Viscosity distribution around A. pullulans imaged by raster scanning an optically trapped probe bead.

This blastospore has a halo of the polysaccharide pullulan around it. Note the viscosity gradient.

Flow field measurement

An optically trapped microsphere is used as a probe for two-dimensional velocity field imaging using scanning optics.

A fluid viscosity map may be obtained simultaneously. Calibration is based on a single length measurement only. Applications are anticipated in the design of microfluidic

devices.

MicrofluidicsNew microfluidic devices are being constantly developed. Their fluid dynamics need to be understood.

After A. D. Stroock, S. K. W. Dertinger, A. Ajdari, I. Mezi, H. A. Stone, G. M. Whitesides, Science 295, 647 (2002)

Flow Off

Oscillating Laser Trap

Probe Bead Probe Bead

a

r

Flow On

Probe BeadProbe Bead

aOscillating Laser Trap

0 5 10 15 20 25 30 35 400

5

10

15

20

25

30

35

40

Ex

per

imen

tal

vel

oci

ty f

rom

tw

eeze

rs (m

/sec

)

video velocity m/sec)

Note offset induced by Brownian motion of probe bead

Comparison of tweezer and video velocity measurement

Flow Measurement

17m

Flow scale bar m / sec

Force Measurement

Flow measurement is one example of force measurement.

We can use tweezers to apply forces to probe beads and measure their response.

Bead stuck on pullulan around blastospore:

Use To Study Effects Of Photodynamic Therapy On Adhesive Properties Of Cancer Cells Photodynamic Therapy (PDT) is frequently

extremely effective in controlling the primary malignancy, but have also been associated with an increase in distant metastasis.

PDT, used as clinical cancer therapy worldwide, is a method in which photosensitizers (PS) are administered to tumor cells and are activated by light at the appropriate wavelength, where a combination of light, oxygen, and PS are toxic to tumors.

Tayyaba Hassan and Paal Selbo, Wellman Lab MGH

E-Cadherin/Catenin Complex Overview

Van Aken, E. et al., Virchows Arch., 2001

Conclusions

Our scanning confocal tweezers microscope can measure velocity and viscosity simultaneously.

Viscosity can be measured rapidly with microspheres on microscopic scale.

Absolute measurements are obtained in real time for the flow velocity with minimal calibration.

Results from the measurements of the flow shear in z suggest that this technique has the potential of mapping the full 3-D distribution of fluid flow and viscosity.