Photonic force optical coherence elastography for three-dimensional mechanical microscopy

Nichaluk Leartprapun,Jeffrey A Mulligan,Steven G Adie,Rishyashring R Iyer,Gavrielle R Untracht

doi:10.1038/s41467-018-04357-8

Nichaluk Leartprapun, Jeffrey A Mulligan + Show 3 more

Open Access

https://doi.org/10.1038/s41467-018-04357-8

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Abstract

Optical tweezers are an invaluable tool for non-contact trapping and micro-manipulation, but their ability to facilitate high-throughput volumetric microrheology of biological samples for mechanobiology research is limited by the precise alignment associated with the excitation and detection of individual bead oscillations. In contrast, radiation pressure from a low-numerical aperture optical beam can apply transversely localized force over an extended depth range. Here we present photonic force optical coherence elastography (PF-OCE), leveraging phase-sensitive interferometric detection to track sub-nanometer oscillations of beads, embedded in viscoelastic hydrogels, induced by modulated radiation pressure. Since the displacements caused by ultra-low radiation-pressure force are typically obscured by absorption-mediated thermal effects, mechanical responses of the beads were isolated after independent measurement and decoupling of the photothermal response of the hydrogels. Volumetric imaging of bead mechanical responses in hydrogels with different agarose concentrations by PF-OCE was consistent with bulk mechanical characterization of the hydrogels by shear rheometry.

Highlights

Optical tweezers are an invaluable tool for non-contact trapping and micro-manipulation, but their ability to facilitate high-throughput volumetric microrheology of biological samples for mechanobiology research is limited by the precise alignment associated with the excitation and detection of individual bead oscillations
In order to address the unmet need for volumetric, time-lapse mechanical microscopy of engineered systems in mechanobiology research, we revisited Ashkin’s original idea of using radiation pressure from a low-numerical aperture (NA) beam11— as a potential mechanism to apply localized mechanical excitation to micro-beads embedded in aqueous biological media
We present photonic force optical coherence elastography39–41 (OCE) (PF-OCE) as a technique for 3D mechanical microscopy, leveraging the interferometric displacement sensitivity of OCT to detect picometerto-nanometer bead oscillations induced by modulated radiation pressure from a low-NA beam

Summary

Introduction

Optical tweezers are an invaluable tool for non-contact trapping and micro-manipulation, but their ability to facilitate high-throughput volumetric microrheology of biological samples for mechanobiology research is limited by the precise alignment associated with the excitation and detection of individual bead oscillations. Developed by Ashkin et al.[4], optical tweezers (OTs) have enabled the manipulation of biological systems at the molecular-to-cellular scale This has led to many seminal studies, including measurement of the elastic properties of bacterial flagella[5], direct observation of the movement and forces generated by molecular motors[6,7], the study of mechanotransduction pathways in living cells[8], and measurement of the mechanical properties and biophysical interactions of DNA9,10. In order to address the unmet need for volumetric, time-lapse mechanical microscopy of engineered systems in mechanobiology research, we revisited Ashkin’s original idea of using radiation pressure from a low-NA beam11— as a potential mechanism to apply localized mechanical excitation to micro-beads embedded in aqueous biological media. OCT has been used previously to monitor the radiation-pressure-induced trajectories of beads in liquid media[44]; but this method has yet to be applied to solid viscoelastic materials

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