1. Measuring Fluid Properties on a Microscopic Scale Using Optically Trapped Microprobes Mark Cronin-Golomb Biomedical Engineering Tufts University

2. With the help of: Boaz Nemet Yossef Shabtai Lisa Goel at Tufts University Tayyaba Hasan Paal Selbo at Wellman Laboratories of Photomedicine, MGH

3. 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.

4. Tweezers principle Electric Field + -

5. Confocal microscope principle

6. 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).

7. 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).

8. Experiment Details

9. At large oscillation amplitudes the potential well splits

10. 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.

11. Theoretical Background x: trap position  : viscous drag  : tweezer spring constant a: amplitude of trap oscillation  : frequency of trap oscillation L(t): Brownian forcing function

12. Experimental Results 0123456789 0.00 0.05 0.10 0.15 0.20 0.25 0.30 A 2 2 nd Harmonic amplitude (a.u)  0 10 20 30 40 50 60 70 80 90 100   2 nd Harmonic phase (deg)

13. Signal to Noise Ratio Absolute Position Detection Relative Position Detection

15. 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.

16. 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.

17. 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)

18. Flow Off Oscillating Laser Trap Probe Bead a r Flow On Probe Bead a Oscillating Laser Trap

19. Note offset induced by Brownian motion of probe bead Comparison of tweezer and video velocity measurement

20. Flow Measurement 17  m Flow scale bar  m / sec

21. 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:

22. 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

23. E-Cadherin/Catenin Complex Overview Van Aken, E. et al., Virchows Arch., 2001

24. 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.