Contactless micro-elastography of single cells using oscillating microbubbles as shear wave sources

TL;DR

This work introduces a fully contactless micro-elastography approach that uses acoustically oscillating gas bubbles as localized shear-wave sources to probe single cells. By exciting bubbles near megakaryocytes at $f=30.8\ \text{kHz}$ and leveraging subharmonic components at $15.4\ \text{kHz}$, the method generates propagating shear waves inside cells and enables rapid (less than $1\ \text{ms}$), high-resolution velocity mapping with no mechanical contact. Across multiple bubble configurations, a $15\ \text{kHz}$ content in the cell displacement is shown to be necessary for detectable wave propagation, while spherical or nonspherical bubble oscillations can be employed depending on practical considerations; the technique demonstrates measurements in cells as small as $10\ \mu\text{m}$ and yields velocity maps ranging from $0.1$ to $1.3\ \text{m/s}$, indicating robust intracellular viscoelastic contrasts. The approach offers real-time, label-free monitoring of dynamic cellular processes and opens the path toward elastographic cytometry without perturbing cell viability.

Abstract

The mechanical properties of cells play key roles in their physiology, function, physiological and pathological transformations. Micro-elastography has recently emerged as a promising tool to estimate cellular viscoelastic properties within a millisecond, without the need for mechanical modeling. Here, we report a fully contactless approach to single-cell micro-elastography, using acoustically oscillating gas microbubbles positioned near individual cells (20~\textmu m diameter megakaryocytes) as localized shear wave sources. Using this approach, we successfully performed micro-elastography on cells up to five times smaller than those studied in previous works, establishing the smallest single-cell elastography measurements to date. Spherical or non-spherical bubble oscillations generated 15~kHz elastic waves, which we detected using a high-speed camera coupled to a standard bright-field microscope. Noise correlation elastography enabled the measurement of average and local shear-wave velocities within single cells. Our results demonstrate that this method is robust and reproducible across multiple cells from the same cell line, paving the way for real-time, label-free mechanical monitoring of single cells during fast biological processes.

Figures (4)

• Figure 1: (A) Experimental setup consisting of a water tank filled with cell culture medium placed under an optical microscope with a $\times$60 lens connected to a high-frame-rate camera. A bubble is formed using two electrolysis electrodes, and it is excited using an ultrasonic transducer plunged into the liquid. A cell is placed very close from the bubble. (B) Example of an image recorded by the camera, showing a bubble of 43 of diameter and a cell of 28 placed 7 apart.
• Figure 2: (A) Snapshots of the large field-of-view and low frame rate video showing the bubble and the cell in the same image. (B) Results of the bubble dynamics analysis, showing the temporal dynamics of the spherical (red points) and nonspherical (green points) oscillations. Exemplary snapshot of (C) the narrow field-of-view and higher frame rate video showing only the cell, and (D) of the X-component of the displacement field in the cell. (E) X-component of the displacement field at point * plotted as a function of time and (F) its spectrum.
• Figure 3: Shear wave velocity measurement inside the cell. (A) Spectrum of the displacement field. Average spatio-temporal correlation matrices at (B) 30, (C) 45 and (D) 15. (E) Snapshots of the displacement field filtered at 15. (F) Two correlation focal spots calculated at different points marked by *, and their corresponding lateral profiles. (G) Final wave velocity map estimated inside the cell.
• Figure 4: Other configurations that have been tested, varying the oscillation type, the bubble radius and the excitation frequency. For each configuration, the average spatio-temporal correlation matrices (A and C) and the wave velocity maps (B and D) are displayed. (E) Graphics summarizing all the experiments that have been made in the context of this paper: the 4 bubble oscillation conditions are represented with different symbols, and plotted as a function of the 15 kHz peak amplitude measured in the cell displacements spectrum and the measured wave speed in the cell. When a wave speed could not be measured, the marker is left white and put on the 0 m/s axis.