Ms. Pei-Jung Wu Profile

Pei-Jung Wu

at Imperial College London

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Proceedings Article | 13 March 2019 Presentation

Brillouin spectroscopy is insensitive to stiffness after correcting for the influence of water content in hydrogels (Conference Presentation)

Darryl Overby, Pei-Jung Wu, Irina Kabakova, Jeffrey Ruberti, Joseph Sherwood, Iain Dunlop, Carl Paterson, Peter Török

Proceedings Volume 10880, 1088012 (2019) https://doi.org/10.1117/12.2507841

KEYWORDS: Spectroscopy, Tissues, Oxides, Optical spectroscopy, Elastography, Microscopy

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Brillouin spectroscopy provides a non-invasive, label-free method to evaluate the mechanical properties of biological materials. Prior studies have shown that the longitudinal elastic modulus, M, measured by Brillouin spectroscopy correlates with the Young’s modulus, E, of cells and tissues. However, M and E for hydrated materials are both influenced by water content. Using hydrogels as a simple model for hydrated biological materials, we designed experiments to separate the effects of E and water content on M. Polyethylene oxide (PEO) hydrogels were prepared with an average molecular weight of 1, 4 or 8 MDa and water content of 92, 95 or 98.5% (v/v). Polyacrylamide (PA) hydrogels were prepared with 10% acrylamide and 0.03-0.30% N-methylene-bis-acrylamide (w/v). E was measured by rheometry for PEO hydrogels and by unconfined compression for PA hydrogels. M was measured using a custom-built Brillouin spectrometer. For PEO hydrogels, E increased with molecular weight, whilst M was unaffected by molecular weight and decreased with increasing water content. For PA hydrogels, both M and E decreased over time due to swelling, but no single relationship could describe how M changed in terms of E. Regardless of swelling, all values of collapsed onto a single relationship that depended only on water content. After correcting for water content, measurements from Brillouin spectroscopy no longer correlate with Young’s modulus for hydrated materials. This work cautions against the straightforward application of Brillouin spectroscopy for optical elastography, but suggests that Brillouin spectroscopy and microscopy may be useful for investigating mechanisms involving changes in local water content.

Proceedings Article | 24 April 2017 Presentation

Mechanical characterisation of hydrogels using Brillouin microscopy, ultrasound and unconfined compression tests (Conference Presentation)

Pei-Jung Wu, Irina Kabakova, ChengZe Song, Carl Paterson, Darryl Overby, Peter Török

Proceedings Volume 10067, 100670L (2017) https://doi.org/10.1117/12.2252813

KEYWORDS: Mechanics, Microscopy, Ultrasonography, Acoustics, Tissues, Light scattering, Scattering, Photons, Phonons, Tissue optics

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Mechanical characterisation of biomaterials provides the basis for investigating disease-related changes in the biomechanical properties of living tissues and cells. Brillouin microscopy offers a non-invasive and label-free method to measure material properties. Briefly, Brillouin scattering involves energy exchange between photons and acoustic phonons, resulting in an optical frequency shift of the scattered light. This shift is proportional to the speed of sound in the material, and consequently to the longitudinal elastic modulus (M). However, it is unclear how Brillouin measurements, which characterize the mechanical response at GHz frequencies, relate to mechanical properties measured at much lower frequencies (~1 Hz) relevant to physiological conditions. Furthermore, as most biomaterials are hydrated, it remains unclear how the relative incompressibility of water influences the acoustic wave speed so as to affect Brillouin measurements of hydrated biomaterials. In this study, we aim to establish the relationship between Brillouin frequency shift, acoustic wave speed and quasi-static elastic modulus of hydrogels of varying stiffness. Hydrogels are homogeneous and isotropic materials that mimic the poroelastic nature of biological tissues. Each measurement probes the mechanics of hydrogels in a significantly different frequency range: GHz for Brillouin imaging, MHz for ultrasound and Hz for unconfined compression tests. The acoustic wave speed falls into range from 1490 to 1533 m/s in both MHz (ultrasound) and GHz (Brillouin) frequency ranges. The quasi-static modulus correlates positively with Brillouin frequency shift, increasing from 6 to 54 kPa. All the results indicate the measurements obtained by Brillouin microscopy are capable of representing the material properties of hydrogels in quasi-static condition.

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