The nucleus is the largest and stiffest organelle of eukaryotic cells, and as such, its mechanical properties are tightly related to various cell functions. Many efforts have been devoted to characterize the mechanical properties of nucleus, but the current techniques generally need physical contact of the cell and staining of the nucleus and thus cannot acquire the mechanical information directly. Brillouin microscope is an integration of a confocal microscope and a Brillouin spectrometer, which measures the spectral shift due to the spontaneous Brillouin light scattering, and from that the longitudinal modulus of the sample can be quantified. In this work, by combining the standard Brillouin microscope with the microfluidic technique, we developed a Brillouin flow cytometry that can quantify the mechanical properties of the intact cellular nucleus in a non-contact and label-free manner. As cell flows through a microfluidic channel, its mechanical property at different regions will be sampled by a sub-micron beam spot of the Brillouin microscope. The mechanical information of the nucleus from the cell population can then be identified and extracted via data post-processing, which is further confirmed by co-registering Brillouin data with fluorescence data from the same cell. Currently, the overall throughput of this technique is about 200 cells per hour, mainly relies on the acquisition speed of the spectrometer, which could be readily improved with available technology. We verified the capability of this all-optical technique by distinguishing the stiffness changes of the nucleus that are relevant to physiological and pathological phenomena.
Brillouin spectroscopy is able to measure material’s mechanical properties by analyzing the optical spectrum of acoustically-induced light scattering within a sample. In the past decade, the development of high-resolution Brillouin spectrometers based on virtually-imaged phased array (VIPA) has greatly increased the spectral detection efficiency thus enabling mechanical characterization of biological tissue and biomaterials. Further improvements in spectrometer performances have enabled in vivo measurements at safe power levels and 2D/3D imaging of biological cells. However, it remains a slow technique compared to other imaging modalities, because only one point of the sample can be measured by the traditional backward-scattering configuration at a time. In this work, we demonstrate a parallel detection configuration with 90-degree geometry where the Brillouin shift of hundreds of points in a line can be measured simultaneously. In a 1.1mm-by-1.5mm samples, this novel configuration effectively shortens the acquisition time of 2D Brillouin imaging from hours to ~30 seconds with spatial resolution of ~3um, thus making it a powerful technology for label-free mechanical characterization of tissue and biomaterials.