2.5-Dimensional Parylene C micropore array with a large area and a high porosity for high-throughput particle and cell separation

Large-area micropore arrays with a high porosity are in high demand because of their promising potential in liquid biopsy with a large volume of clinical sample. However, a micropore array with a large area and a high porosity faces a serious mechanical strength challenge. The filtration membrane may undergo large deformation at a high filtration throughput, which will decrease its size separation accuracy. In this work, a keyhole-free Parylene molding process has been developed to prepare a large (>20 mm × 20 mm) filtration membrane containing a 2.5-dimensional (2.5D) micropore array with an ultra-high porosity (up to 91.37% with designed pore diameter/space of 100 μm/4 μm). The notation 2.5D indicates that the large area and the relatively small thickness (approximately 10 μm) of the fabricated membranes represent 2D properties, while the large thickness-to-width ratio (10 μm/ < 4 μm) of the spaces between the adjacent pores corresponds to a local 3D feature. The large area and high porosity of the micropore array achieved filtration with a throughput up to 180 mL/min (PBS solution) simply driven by gravity. Meanwhile, the high mechanical strength, benefiting from the 2.5D structure of the micropore array, ensured a negligible pore size variation during the high-throughput filtration, thereby enabling high size resolution separation, which was proven by single-layer and multi-layer filtrations for particle separation. Furthermore, as a preliminary demonstration, the prepared 2.5-dimensional Parylene C micropore array was implemented as an efficient filter for rare cancer cell separation from a large volume, approximately 10 cells in 10 mL PBS and undiluted urine, with high recovery rates of 87 ± 13% and 56 ± 13%, respectively. Polymer films that detect cancer cells in blood or urine samples using size-based filtrations can benefit from a strategy for improving the strength and throughput rate of micropore arrays. Yaoping Liu from Peking University in China and colleagues first fabricated a silicon wafer of close-packed micropillars, and then coated this template with the polymer film Parylene C. The team used reactive ion etching to carve out a hexagonal array of microscale openings in the polymer with unique dimensions—smaller-than normal pore separations for high flow rates, and thickness more than twice the width of the sidewall to give exceptional mechanical stability. Experiments proved the new array structure could selectively trap particles ranging from microbeads to cancer cells with throughputs hundreds of times faster than conventional filtering devices.