Spectroscopic photonic force optical coherence elastography

Abstract

We demonstrate spectroscopic photonic force optical coherence elastography (PF-OCE). Oscillations of microparticles embedded in viscoelastic hydrogels were induced by harmonically modulated optical radiation pressure and measured by phase-sensitive spectral-domain optical coherence tomography. PF-OCE can detect microparticle displacements with pico-to nano-meter sensitivity and millimeter-scale volumetric coverage. With spectroscopic PF-OCE, we quantified viscoelasticity over a broad frequency range from 1 Hz to 7 kHz, revealing rich microstructural dynamics of polymer networks across multiple microrheological regimes. Reconstructed frequency-dependent loss moduli of polyacrylamide hydrogels were observed to follow a general power scaling law G ″~ω0.75 , consistent with that of semiflexible polymer networks. Spectroscopic

Light-sheet photonic force optical coherence elastography for high-throughput quantitative 3D micromechanical imaging

“…The measurement of G′ by LS-pfOCE is in good agreement with that of shear rheometry at the same modulation frequency of 20 Hz. However, LS-pfOCE measurement of the loss modulus, G″ , differs from that of shear rheometry due to distinct viscous responses (e.g., viscous drag of fluid flow through the porous polymer network) at the micro-versus macroscale, which is in agreement with our prior PF-OCE work based on a Gaussian-beam excitation 37,38 .…”

“…More sophisticated light-sheet generation optics can be introduced to generate laterally uniform light-sheet PF excitation with extended depth of focus, such as those previously developed for light-sheet fluorescence microscopy 45–47 . In addition, the volumetric throughput of LS-pfOCE can be further improved by increasing the mechanical response amplitude-to-noise ratio 37,38 (such as by increasing the power of the PF beam or reducing the OCT system noise), which would allow reliable measurements of bead mechanical response to be made with a lower number of temporal frames per slow-axis position 37,38 . One way to reduce noise in the current LS-pfOCE system is by replacing the motorized actuator with a high-fidelity stepper motor.…”

“…The acquisition scheme of LS-pfOCE can be flexibly tailored to best fit the experimental needs of specific mechanobiological applications—by adjusting the number of temporal (BM-mode) frames acquired at each slow-axis position. This navigates the trade-off between acquisition time and the maximum stiffness that can be reliably quantified (i.e., the displacement sensitivity supported by a given number of temporal frames 37,38 ). Meanwhile, LS-pfOCE may also be implemented with other high-speed OCT imaging approaches such as swept-source and parallel line-field OCT, in order to further improve its imaging speed 49 .…”

“…36 ). The pulse-train excitation, with the duty cycle determined by the “dwell time” of the excitation beam on each bead, is: 1) inefficient due to over an order of magnitude reduction in time-averaged force exerted on each probe bead compared to continuous excitation 36 , and 2) does not readily support microrheological quantification of viscoelasticity 37,38 due to the presence of higher harmonics in the comb-like excitation profile.…”

Light-sheet photonic force optical coherence elastography for high-throughput quantitative 3D micromechanical imaging

“…36 ). The pulse-train excitation, with the duty cycle determined by the "dwell time" of the excitation beam on each bead, is: 1) inefficient due to over an order of magnitude reduction in time-averaged force exerted on each probe bead compared to continuous excitation 36 , and 2) does not readily support microrheological quantification of viscoelasticity 37,38 due to the presence of higher harmonics in the comb-like excitation profile. Thus, a more efficient localized mechanical excitation scheme is needed in order to realize practical volumetric quantitative micromechanical imaging with PF-OCE.…”