Download 2 - Department of Biomedical Engineering

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.
AcoustoOptic
deflector
Scanning
Mirrors
Tweezers principle
Electric Field
-
+
Confocal microscope principle
Sample
Stage
Detector
Raster
display
Scanning
Mirror
Pinhole
Laser
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 PhysicsCondensed Matter
8, 9477-9482
(1996).
Experiment Details
Ti-Sapphire
Laser
#
P
APD
BS
AOD *
OL
*
#
CCD
Camera
DM
Lock-in
Amplifier
#
SM
*
AOD
Driver
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
dx

   x  a sin( t )   L(t )
dt
x: trap position
: viscous drag
: tweezer spring constant
a: amplitude of trap oscillation
: frequency of trap oscillation L(t): Brownian forcing function
du

  u   a cos t
dt
u  x  a sin t
I (t )  1   u (t )
2
Experimental Results
0.30
80
0.20
70
60
0.15
50
40
0.10
30
20
0.05
10
0.00
0
0
1
2
3
4
5
t
6
7
8
9
nd
j2 2 Harmonic phase (deg)
90
0.25
nd
A2 2 Harmonic amplitude (a.u)
100
Signal to Noise Ratio
Relative Position Detection
0 2
c
a
SNR  2
0  c2  k BT 1/ 2 8


  
Absolute Position Detection
0c
c
a
SNR  2
0  c2  k BT 1/ 2 8


  
30
25
Confocal
-1/2
~f
20
15
SNR
 10
Valentine-3/2
~f
5
0
0
1000
2000
3000
frequency (Hz)
4000
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.
Microfluidics
New 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
Probe Bead
Flow On
Probe Bead
r
a
Oscillating
Laser Trap
a
Oscillating
Laser Trap
Experimental velocity from tweezers ( m m/sec)
Comparison of tweezer and video velocity measurement
40
35
30
25
20
15
10
5
0
0
5
10
15
20
25
30
35
40
video velocity m m /sec )
Note offset induced by Brownian motion of probe bead
Flow Measurement
17mm
Flow scale bar
mm / 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
Tayyaba Hassan and Paal Selbo, Wellman Lab MGH


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.
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.