Continuous Mechanical Indexing of Single-Cell Spheroids Using a Robot-Integrated Microfluidic Chip

In single-cell/spheroid analyses, flow cytometry plays a key role in high-throughput measurements. Among the considerable numbers of indexes available for a target evaluation, mechanical characteristics such as a Young's modulus have been focused upon as new indexes related to physiological condition. However, a continuous mechanical indexing system has yet to be achieved due to the difficulty in flow control, which brings about a fluctuation of the force sensor probe. In this letter, we propose an automated mechanical indexing system of spheroids. By utilizing a combination of two syringe pumps, we succeeded in positioning the target spheroids without an undesirable pinching. We conducted experiments on the continuous mechanical indexing of single-cell spheroids using 26 mesenchymal stem cell spheroids. The evaluation results of positioning showed that the error in the position control was 14.1 mu m with a standard deviation of 87.7 mu m, which corresponded to a measurement error of the Young's modulus of under 2%. In addition, we succeeded in achieving a measurement throughput of 3.14 spheroids per minute, which is approximately 2 times faster than a manual operation. Finally, we demonstrated the on-chip sorting of single spheroids based on the mechanical index to show the adaptability of the proposed method toward the flow cytometry.