Yew Yan Seng,Elijah
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bieyyse@nus.edu.sg
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Publication Vectorial approach to studying second harmonic generation in collagen using linearly and radially polarized beams(2006) Yew, E.Y.S.; Sheppard, C.J.R.; BIOENGINEERINGThe study of second harmonic generation (SHG) has been examined using a vectorial approach for both linearly and radially polarized beams. This approach is necessary for situations when the beam is tightly focused such as in a microscope. Using the vectorial approach, the result of including the y and z components of the electric field is that previously ignored 'cross-component' terms are now found to have an influence on the SHG polarization and the radiation patterns obtained. Since SHG is dependent on the susceptibility tensor, the inclusion of these 'cross-component' terms can help to identify structural changes in biological materials simply by studying the changes in the tensor via the SHG polarization. In particular, we calculate the second harmonic polarization induced in collagen for both linearly and radially polarized beams.Publication Speckle image analysis of cortical blood flow and perfusion using temporally derived contrasts(2004) Tan, Y.K.; Liu, W.Z.; Yew, Y.S.; Ong, S.H.; Paul, J.S.; ELECTRICAL & COMPUTER ENGINEERING; CTR FOR REM IMAGING,SENSING & PROCESSINGContrast values estimated from temporal statistics is used for Laser Speckle Contrast Analysis (LASCA) of cortical blood flow and perfusion. Using temporal statistics, we are able to reliably estimate the blood flow and perfusion by processing only a selected number of pixels from the raw speckle images. For the number of frames (N > 6) for estimating the temporal contrast, the Root Mean Square (RMS) of the inverse decorrelation time (τ c) is found to be relatively constant with a variance of ±3.52%. For a given window size used for estimation of τ c, it is observed that the RMS of 1/τ c values exhibit a low variance of ± 4.62% as compared to spatially derived contrasts (SDC) with/without averaging. For a given reduction in binning size, the image processing using the temporally derived contrasts (TDC) is found to be 1.41 times faster as compared to obtained using SDC. ©2004 IEEE.Publication Polarization conversion in confocal microscopy with radially polarized illumination(2009-07-15) Tang, W.T.; Yew, E.Y.S.; Sheppard, C.J.R.; BIOENGINEERINGThe effects of using radially polarized illumination in a confocal microscope are discussed, and the introduction of a polarization mode converter into the detection optics of the microscope is proposed. We find that with such a configuration, bright-field imaging can be performed without losing the resolution advantage of radially polarized illumination. The detection efficiency can be increased by three times without having to increase the pinhole radius and sacrificing the confocality of the system. Furthermore, the merits of such a setup are also discussed in relation to surface plasmon microscopy and single-molecule orientation studies, where the doughnut point spread function can be engineered into a single-lobed point spread function. © 2009 Optical Society of America.Publication Imaging the development of an ischemic core following photochemically induced cortical infarction in rats using Laser Speckle Contrast Analysis (LASCA)(2006-01-01) Paul, J.S.; Luft, A.R.; Yew, E.; Sheu, F.-S.; BIOENGINEERING; BIOLOGICAL SCIENCESLaser Speckle Contrast Analysis (LASCA) has been used to image the development of an ischemic core following photochemically induced infarction in rats up to 1 h post-lesion. Using LASCA, we have been able to image a central ischemic core which had little or no perfusion surrounded by a penumbral region with reduced perfusion. In addition, we have shown the existence of a surrounding region of hyperemic tissue. A potential feature of this imaging approach is its capability to track cerebral blood flow (CBF) changes in the region within and outside the ischemic core besides demonstrating the real-time progression of the ischemic core into the penumbral region. We have demonstrated the continuous disruption of CBF to the ischemic core that eventually affected the blood supply to the surrounding regions. The penumbral flow is shown to exhibit a sudden increase post-ischemic induction followed by a slow decline to the final baseline level. Interestingly, we observed an interaction (P < 0.03) between penumbral flow peak and the time effects of increase in pixel area from the infarct region to the surrounding penumbral region. Using a paired-sample t test, we observed that the mean pixel area was larger for the infarct region than for the penumbral region (P < 0.004) during the time interval between the induction of ischemia and the time point of peak flow in the penumbral region. Crown Copyright © 2005 Published by Elsevier Inc. All rights reserved.Publication Electro-spinning of pure collagen nano-fibres - Just an expensive way to make gelatin?(2008-05) Zeugolis, D.I.; Khew, S.T.; Yew, E.S.Y.; Ekaputra, A.K.; Tong, Y.W.; Yung, L.-Y.L.; Hutmacher, D.W.; Sheppard, C.; Raghunath, M.; BIOENGINEERING; CHEMICAL & BIOMOLECULAR ENGINEERINGScaffolds manufactured from biological materials promise better clinical functionality, providing that characteristic features are preserved. Collagen, a prominent biopolymer, is used extensively for tissue engineering applications, because its signature biological and physico-chemical properties are retained in in vitro preparations. We show here for the first time that the very properties that have established collagen as the leading natural biomaterial are lost when it is electro-spun into nano-fibres out of fluoroalcohols such as 1,1,1,3,3,3-hexafluoro-2-propanol or 2,2,2-trifluoroethanol. We further identify the use of fluoroalcohols as the major culprit in the process. The resultant nano-scaffolds lack the unique ultra-structural axial periodicity that confirms quarter-staggered supramolecular assemblies and the capacity to generate second harmonic signals, representing the typical crystalline triple-helical structure. They were also characterised by low denaturation temperatures, similar to those obtained from gelatin preparations (p > 0.05). Likewise, circular dichroism spectra revealed extensive denaturation of the electro-spun collagen. Using pepsin digestion in combination with quantitative SDS-PAGE, we corroborate great losses of up to 99% of triple-helical collagen. In conclusion, electro-spinning of collagen out of fluoroalcohols effectively denatures this biopolymer, and thus appears to defeat its purpose, namely to create biomimetic scaffolds emulating the collagen structure and function of the extracellular matrix. © 2008 Elsevier Ltd. All rights reserved.Publication Fractional Gouy phase(2008-06-15) Yew, E.Y.S.; Sheppard, C.J.R.; BIOENGINEERINGIn general, the total Gouy phase shift has the form n π, where n need not be an integer. As a result of the Fourier transforming property of a lens, the Gouy phase is found to be related to the types of discontinuities at the upper or lower range of the pupil function Q(c) resulting from the asymptotic order of the Fourier transform. The sign of the Gouy phase is also related to the slope of the pupil function. The oscillations of the Gouy phase shift arise from the strength of the nondominant discontinuity. © 2008 Optical Society of America.Publication Effects of axial field components on second harmonic generation microscopy(2006-02) Yew, E.Y.S.; Sheppard, C.J.R.; BIOENGINEERINGThe effects of the axial field components of a focused beam under high NA on the second harmonic generation (SHG) in collagen was examined using a vectorial approach. We find that with high NA, the cross-component terms that are most likely to have an effect on SHG will be ExEx, ExEy, ExEz and EzE z as a result of tight focusing. By considering the tensor and the presence of the other electric field components the possibility of different polarization states of the generated second harmonic as a result of the nonlinear susceptibility tensor making it possible to generate radially polarized modes with linearly polarized beams. © 2006 Optical Society of America.Publication Effects of pupil functions on tightly focused radially polarized beams in microscopy(2008) Yew, E.Y.S.; Sheppard, C.J.R.; BIOENGINEERINGWe examine the effects of tightly focusing a radially polarized beam with uniform, Gaussian or Bessel-Gauss pupil functions. FWHM is smallest for a uniform amplitude profile while the Bessel-Gauss beam results in the largest FWHM. © 2007 Optical Society of America.Publication Performance parameters for focusing of radial polarization(2008-03-01) Sheppard, C.J.R.; Yew, E.Y.S.; BIOENGINEERINGPerformance parameters are presented for high-aperture radially polarized focusing systems. These can be used for comparing the focusing performance of different optical systems, including the effect of pupil filters. © 2008 Optical Society of America.Publication Tight focusing of radially polarized Gaussian and Bessel-Gauss beams(2007-12-01) Yew, E.Y.S.; Sheppard, C.J.R.; BIOENGINEERINGWe examine the effects of tightly focusing a radially polarized beam with uniform, Gaussian, or Bessel-Gauss pupil functions. The resulting FWHM is smallest for the case of a uniform amplitude profile, while the Bessel-Gauss beam results in the largest FWHM. The uniform amplitude profile also results in an axial field component that increases fastest with increasing NA. The ratio of the axial component to the transverse component is also the greatest for the uniform pupil function. On the other hand, the Bessel-Gauss beam benefits the most from the use of an annulus. © 2007 Optical Society of America.