Temperature effect on cell mechanics by optofluidic microchips

Cell mechanics plays a very important role in various cellular functions and in many disease-related mutations, especially for carcinogenesis [1], and has been extensively studied in the last years thanks to the rapid development of lab-on-chip technologies. In particular, it was proved that cell's mechanical properties can be used as an intrinsic and reliable marker of cells' conditions [2,3]. Cells' cytoskeleton has a complex and dynamic structure which constantly and continuously changes according to cell status and also to external environment stimuli and temperature. Here we show how the cell temperature deeply impact the cytoskeleton mechanical properties using two optofluidic microchips: the first is a traditional optical stretcher, exploiting optical forces to induce cell deformation; while, the second one (shown in Fig.1) is substantially constituted by an optical singlecell sorter with an additional constriction embedded along the output branch, and is used to test cell's capability to pass through a small constriction in a passive way [4]. To explore the temperature impact on cell mechanics, we equipped the two systems with an additional temperature control system, which allows keeping the chip, the cells, and the solutions at a controlled temperature. The properties of investigated cell line (highly metastatic human melanoma cells - A375MC2) were studied at four different temperatures: 5°C, 15°C, 25°C and 35°C. The results obtained by the optical stretcher chip clearly demonstrate that cell optical deformability, measured as the optical forces-induced change of cell ellipticity, is strongly temperature-dependent, as it increases by a factor of ?3 when the temperature is increased from 15°C to 35°C. Additionally, the results obtained by the constriction-chip show (quite surprisingly) that at higher temperature the cells require a higher pressure to be applied at the chip input in order to pass through the constriction. After a preliminary analysis we speculate that this behaviour is due to the fact that softer cells are strongly deformed when they reach the constriction and exhibit larger "interaction area" with the surrounding constriction-surface leading to higher friction and adhesion to the glass walls, thus requiring a higher pressure to pass the obstacle. The combination of the two results demonstrate the effectiveness of the two integrated chips in analyzing changes of cells' cytoskeleton and also highlight the strong relevance of temperature parameter, which should be well controlled in order to perform accurate comparisons between the mechanical properties of different cells samples