We present a novel optofluidic device for real-time sorting on the basis of cell mechanical properties, measured by optical stretching. The whole mechanism, based on optical forces, does not hamper the viability of the tested cells, which can be used for further analysis. The device effectiveness is demonstrated by extracting a sample population enriched with highly metastatic cells from a heterogeneous cell mixture.

This paper presents a comprehensive review of the development of the optical stretcher, a powerful optofluidic device for single cell mechanical study by using optical force induced cell stretching. The different techniques and the different materials for the fabrication of the optical stretcher are first summarized. A short description of the optical-stretching mechanism is then given, highlighting the optical force calculation and the cell optical deformability characterization. Subsequently, the implementations of the optical stretcher in various cell-mechanics studies are shown on different types of cells. Afterwards, two new advancements on optical stretcher applications are also introduced: the active cell sorting based on cell mechanical characterization and the temperature effect on cell stretching measurement from laser-induced heating. Two examples of new functionalities developed with the optical stretcher are also included. Finally, the current major limitation and the future development possibilities are discussed.

Moreover, by comparing results of cell stretching measurements, we demonstrate that acoustic waves do not alter the optical deformability of the cells and that the acoustic prefocusing results in a considerable enhancement of throughput in optical stretching experiments.

We realized an integrated microfluidic chip that allows measuring both optical deformability and acoustic compressibility on single cells, by optical stretching and acoustophoresis experiments respectively. Additionally, we propose a measurement protocol that allows evaluating the experimental apparatus parameters before performing the cell-characterization experiments, including a non-destructive method to characterize the optical force distribution inside the microchannel. The chip was used to study important cell-mechanics parameters in two human breast cancer cell lines, MCF7 and MDA-MB231. Results indicate that MDA-MB231 has both higher acoustic compressibility and higher optical deformability than MCF7, but statistical analysis shows that optical deformability and acoustic compressibility are not correlated parameters. This result suggests the possibility to use them to analyze the response of different cellular structures. We also demonstrate that it is possible to perform both measurements on a single cell, and that the order of the two experiments does not affect the retrieved values.

The viscosity of gel-forming fluids is notoriously complex and its study can benefit from new model systems that enable a detailed control of the network features. Here we use a novel and simple microfluidic-based active microrheology approach to study the transition from Newtonian to non-Newtonian behavior in a DNA hydrogel whose structure, connectivity, density of bonds, bond energy and kinetics are strongly temperature dependent and well known. In a temperature range of 15 °C, the system reversibly and continuously transforms from a Newtonian dispersion of low-valence nanocolloids into a strongly shear-thinning fluid, passing through a set of intermediate states where it behaves as a power-law fluid. We demonstrate that the knowledge of network topology and bond free energy enables to quantitatively predict the observed behavior using established rheology models.

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Cellular mechanical properties constitute good markers to characterize tumor cells, to study cell population heterogeneity and to highlight the effect of drug treatments. In this work, we describe the fabrication and validation of an integrated optofluidic chip capable of analyzing cellular deformability on the basis of the pressure gradient needed to push a cell through a narrow constriction. We demonstrate the ability of the chip to discriminate between tumorigenic and metastatic breast cancer cells (MCF7 and MDA-MB231) and between human melanoma cells with different metastatic potential (A375P and A375MC2). Moreover, we show that this chip allows highlighting the effect of drugs interfering with microtubule organization (paclitaxel, combretastatin A-4 and nocodazole) on cancer cells, which leads to changes in the pressure-gradient required to push cells through the constriction. Our single-cell microfluidic device for mechanical evaluation is compact and easy to use, allowing for an extensive use in different laboratory environments.

In the present work, an integrated optofluidic chip for fluid viscosity measurements in the range from 1 mPa·s to 100 mPa·s is proposed. The device allows the use of small sample volumes (<1 µL) and the measurement of viscosity as a function of temperature. Thanks to the precise control of the force exerted on dielectric spheres by optical beams, the viscosity of fluids is assessed by comparing the experimentally observed movement of dielectric beads produced by the optical forces with that expected by numerical calculations. The chip and the developed technique are validated by analyzing several fluids, such as Milli-Q water, ethanol and water–glycerol mixtures. The results show a good agreement between the experimental values and those reported in the literature. The extremely reduced volume of the sample required and the high flexibility of this technique make it a good candidate for measuring a wide range of viscosity values as well as for the analysis of nonlinear viscosity in complex fluids.