Densimeter-on-chip (DoC): measuring a single-cell mass density by sedimentation in microchannel flows†

Abstract

Intrinsic biophysical and morphological features are essential for the label-free identification of different cell types. Indeed, apart from object size, density could represent a key parameter for single-cell analysis. However, the measurement of such a parameter is challenging. Therefore, we present a straightforward and versatile microfluidic chip. The densimeter-on-chip (DoC) measures single-cell mass densities thanks to a hydrodynamically induced sedimentation process inside the microchannel. In detail, in-flow buoyant components become more relevant than viscoelastic alignment forces, leading to precise in-flow sedimentation. DoC is based on precise three-dimensional cell alignment, followed by an abrupt change in cross-section to induce calibrated sedimentation. Based on the balance of acting forces and tracking the in-flow cell trajectory, we have developed a self-written mathematical model to precisely measure the single-cell densities of multiple cell types of any shape. Both cell velocity and fall length define the resulting cell density. The working range of object diameters for which density can be estimated is 0.75–22.5 μm. As result, the minimum measured density is 998 kg m⁻³ and a sensitivity of 0.001 can be obtained. Great agreement between the computational and the literature findings about red blood cells (∼1159 ± 29.5 kg m⁻³), lymphocytes (∼1073 ± 49 kg m⁻³) and neutrophils (∼1093 ± 27 kg m⁻³) is obtained without chip modification. Indeed, the computational error between the mean density values is ∼1%. Thereby, DoC as an easy-to-use and reproducible solution for label-free single-cell density measurement, provides a universal approach for characterizing a wide range of cell types, independently of their size and shape.