An optofluidic tweeze-and-drag cell stretcher in a microfluidic channel

The mechanical properties of biological cells are utilized as an inherent, label-free biomarker to indicate physiological and pathological changes of cells. Although various optical and microfluidic techniques have been developed for cell mechanical characterization, there is still a strong demand for non-contact and continuous methods. Here, by combining optical and microfluidic techniques in a single desktop platform, we demonstrate an optofluidic cell stretcher based on a tweeze-and-drag mechanism using a periodically chopped, tightly focused laser beam as an optical tweezer to trap a cell temporarily and a flow-induced drag force to stretch the cell in a microfluidic channel transverse to the tweezer. Our method leverages the advantages of non-contact optical forces and a microfluidic flow for both cell stretching and continuous cell delivery. We demonstrate the stretcher for mechanical characterization of rabbit red blood cells (RBCs), with a throughput of similar to 1 cell per s at a flow rate of 2.5 mu l h(-1) at a continuous-wave laser power of similar to 25 mW at a wavelength of 1064 nm (chopped at 2 Hz). We estimate the spring constant of RBCs to be similar to 14.9 mu N m(-1). Using the stretcher, we distinguish healthy RBCs and RBCs treated with glutaraldehyde at concentrations of 5 x 10(-4)% to 2.5 x 10(-3)%, with a strain-to-concentration sensitivity of similar to -1529. By increasing the optical power to similar to 45 mW, we demonstrate cell-stretching under a higher flow rate of 4 mu l h(-1), with a higher throughput of similar to 1.5 cells per s and a higher sensitivity of similar to -2457. Our technique shows promise for applications in the fields of healthcare monitoring and biomechanical studies.