Abstract
Photonic force optical coherence elastography (PF-OCE) is a new approach for volumetric characterization of microscopic mechanical properties of three-dimensional viscoelastic medium. It is based on measurements of the complex mechanical response of embedded micro-beads to harmonically modulated radiation-pressure force from a weakly-focused beam. Here, we utilize the Generalized Stokes-Einstein relation to reconstruct local complex shear modulus in polyacrylamide gels by combining PF-OCE measurements of bead mechanical responses and experimentally measured depth-resolved radiation-pressure force profile of our forcing beam. Data exclusion criteria for quantitative PF-OCE based on three noise-related parameters were identified from the analysis of measurement noise at key processing steps. Shear storage modulus measured by quantitative PF-OCE was found to be in good agreement with standard shear rheometry, whereas shear loss modulus was in agreement with previously published atomic force microscopy results. The analysis and results presented here may serve to inform practical, application-specific implementations of PF-OCE, and establish the technique as a viable tool for quantitative mechanical microscopy.
Highlights
Optical micromanipulation was first demonstrated by Ashkin in 1970, when he showed that radiation pressure from a low numerical aperture (NA) beam was able to accelerate neutral microparticles in aqueous suspension [1]
An example of the reconstructed space-domain M-mode optical coherence tomography (OCT) image shows a 1.7-μm bead located at the focal plane of the OCT beam, which was co-aligned to the focal plane of the PF beam (Fig. 2(a))
Our analysis of measurement noise culminated in data exclusion criteria, based on OCT signal-to-noise ratio (SNR), oscillation amplitude-to-noise ratio, and instability of the measured response over time, to enable quantitative Photonic force optical coherence elastography (PF-optical coherence elastography (OCE))
Summary
Optical micromanipulation was first demonstrated by Ashkin in 1970, when he showed that radiation pressure from a low numerical aperture (NA) beam was able to accelerate neutral microparticles in aqueous suspension [1]. Optical micromanipulation by low-NA radiation pressure has led to relatively fewer applications in the life sciences [12,13,14]. Using optical coherence tomography (OCT) to monitor the dynamics of accelerated beads in real-time, we recently performed the ‘OCT-version’ of Ashkin’s seminal experiment and measured depth-resolved radiation-pressure force profile from a low-NA Gaussian beam on polystyrene micro-beads in various viscous fluids [15]. The combination of radiation pressure from a lowNA beam and OCT detection of particle displacements has the potential to open up new modes of quantitative optical micromanipulation and sensing with large depth coverage
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