Monolithic optofluidic constriction chip for cellular squeezing studies

Cellular mechanical properties can be exploited as an inherent cell marker for different pathologies and also as a label-free criterion to define the health state of a cell. Indeed, many studies revealed a clear correlation between cellular transformations and changes in its mechanical properties [1]. On the other hand, the great advance of lab-on-chip technology further facilitates the relevant research of this field. Among various fabrication technologies, 3D femtosecond laser micromachining appears extremely promising, because it allows the integration, in a millimetre-scale device, of cells' selection, transportation, probing and collection functionalities [2]. In the present abstract, we report a monolithic optofluidic microchip, realized using this technique, to study cellular migration ability in a passive way. The micro-structure of the realized chip, largely similar to an optical cell sorter [3], is shown in Fig.1a, where it can be seen that with respect to standard sorting chips, a constriction is embedded inside one output branch. The measurement principle is simple: at the two inlets, on the left-end of Fig 1a, we input a cell suspension and a buffer fluid, top and bottom input respectively. By balancing the pressures of the two channels, a stable laminar flow can be obtained in the common central channel and if no sorting is performed all the cells are output from the top-right branch. When a cell is to be tested, light is sent through an optical waveguide facing the central channel, so that the cell is pushed into the "lower half" of the channel, and the pure buffer flux brings it to the constriction branch. After the selected cell blocks the constriction, a slow pressure ramp is applied, thanks to high-precision micropumps, from the input part until cell passes. The pressure values required to push cells through the constriction is defined as "passing pressure" and stored. In order to evaluate the possibility to use the "passing pressure" parameter to analyze cell mechanical properties, we performed different experiments on two pairs of cellular lines: tumorigenic (MCF7) and metastatic (MDA-MB231) human breast cancer cells and metastatic (A375P) and highly metastatic (A375MC2) human melanoma cells. The obtained results, shown in Fig.1 b) and c), highlight that a statistically significant difference between the passing pressures of the considered populations is present. Our results demonstrate that this constriction chip allows distinguishing cancer cells on the basis of their metastatic potential, which is positively correlated to the pressure required by cells to squeeze through the constriction. Additionally, we investigated also the impact on cells of drugs able to affect microtubules organization and we observed significant changes in the passing-pressure distributions, thus suggesting that the proposed chip can even be applied for the analysis of drug treatments on single cells.