Cancer is among the leading causes of death both in the United States and worldwide. In addition to genetic and external environmental factors, biomechanics of the cancer cell is key determinant of its activities. Cellular mechanics is known to tightly relate to its biological functions and activities, such as proliferation, migration and gene expression (Bao and Suresh, Nature materials 2, 715-725(2003); Vogel and Sheetz, Nat Rev Mol Cell Biol. 7, 265-275 (2006)). For example, when a cell needs to move forward, the contractile force will be generated within the cell body (Stossel, Science 260, 1086-1094(1993)). It is consistently found from the various biophysical measurements that cancer cells are softer than normal and benign cells and that this cellular compliance correlates with an increased metastatic potential (Cross et al., “Nanomechanical analysis of cells from cancer patients”, Nat. Nanotechnology 2, 780-783 (2007) and Guck et al.,“Optical deformability as an inherent cell marker for testing malignant transformation and metastatic competence”, Biophys J. 88, 3689-3698 (2005)). This correlation is likely from the required optimal mechanical properties of the cancer cell to efficiently migrate through a 3D matrix and/or penetrate through an endothelium during metastasis. It thus strongly indicates that cellular stiffness could be an inherent phenotyping to grade the cancer progression and metastasis.
In the past two decades, many methods have been developed to probe mechanical properties of cells, such as micropipette aspiration, optical tweezer, optical stretcher, deformability cytometry, atomic force microscopy (AFM), magnetic twisting cytometry, and micro-rheology. However, the existing methods either need physical contact (such as micropipette aspiration and AFM) to deform the cell, or can only provide an average measurement of the whole cell (such as micropipette aspiration, optical tweezer, optical stretcher and deformability cytometry), or is invasive (such as optical tweezer, magnetic twisting cytometry and micro-rheology). In addition, most of existing methods have very low throughput and thus cannot analyze a large number of cells within limited time.
Recently, a label-free flow cytometry (called Brillouin flow cytometry) was used to quantify the mechanical properties of cells when flowing through a microfluidic channel, and have successfully demonstrated its capability to directly probe the mechanical properties of the subcellular region with submicron resolution. This technique relies on the basic principle of Brillouin light scattering, which arises from the interaction of incoming light with acoustic phonons within a sample (Dil, “Brillouin scattering in condensed matter,” Rep. Prog. Phys. 45, 286-334 (1982)).
Specifically, Brillouin scattering is the phenomena of inelastic light scattering induced by acoustic phonon of a material. In order to separate the small (typically in the order of GHz) Brillouin frequency shift from elastically scattered light, high-resolution spectrometer such as a multi-pass scanning Fabry-Perot interferometer is usually used in conventional Brillouin spectroscopy (Lindsay S M, Burgess S and Shepherd I W, “Correction of Brillouin linewidths measured by multipass Fabry-Perot spectroscopy,” Appl. Opt. 16(5), 1404-1407 (1977)). Since the dynamics of acoustic phonon is directly linked to the viscoelastic properties of a material, mechanical information can be acquired by measuring the Brillouin frequency shift of the scattered light (Dil J G, “Brillouin scattering in condensed matter,” Rep. Prog. Phys. 45, 286-334 (1982)).
Specifically, the mechanical longitudinal modulus can be acquired by measuring the frequency shift of the scattered light in a non-contact, non-invasive and label-free manner. The high-frequency longitudinal modulus measured by Brillouin technique had a log-log linear relationship with common quasi-static modulus measured by conventional stress-strain test (Scarcelli G, Kim P and Yun S H, “In Vivo Measurement of Age-Related Stiffening in the Crystalline Lens by Brillouin Optical Microscopy”, Biophysical J. 101, 1539-1545 (2011), which indicates that Brillouin technique is a true quantitative metrics of mechanical properties.
Accordingly, a new technique is required to distinguish cancer cells from non-cancer cells based on their mechanical phenotyping.