Label-free on-chip selective extraction of cell aggregate-laden microcapsules from oil into aqueous solution with optical sensor and dielectrophoresis.

Microfluidic encapsulation of cells or tissues in biocompatible solid-like hydrogels has wide biomedical applications. However, the microfluidically encapsulated cells/tissues are usually suspended in oil and need to be extracted into aqueous solution for further culture or use. Current extracting techniques are either non-selective for the cell/tissue-laden hydrogel microcapsules or rely on fluorescence labeling of the cells/tissues that may be undesired for their further culture or use. Here we developed a micro-electro-mechanical system to achieve label-free on-chip selective extraction of cell aggregate-laden hydrogel microcapsules from oil into aqueous solution. The system includes a microfluidic device, an optical sensor, and a dielectrophoretic (DEP) actuator. The microfluidic device is for encapsulating cell aggregates in hydrogel microcapsules using the flow-focusing function. The optical sensor is to detect the cell aggregates, based on the difference of the optical properties between the cell aggregates and surrounding solution before their encapsulation in hydrogel microcapsules. This strategy is used because the difference in optical property between the cell aggregate-laden hydrogel microcapsules and empty microcapsules is too small to tell them apart by commonly used optical sensor. The DEP actuator, which is controlled by the sensor, is for selectively extracting the targeted hydrogel microcapsules by DEP force. The results indicate this system can achieve selective extraction of cell aggregate-laden hydrogel microcapsules with ~100% efficiency without compromising the cell viability, and can improve the purity of the cell aggregate-laden microcapsules by more than 75 times compared with non-selective extraction.