Microfluidic Bead Sorter

MEMS (fall 2024)

Introduction:

This project aims to develop a microfluidic bead sorter designed to separate 10 μm polystyrene microspheres from a water-based mixture. The device efficiently divides the input stream into two distinct output streams: one containing both water and polystyrene beads, and the other containing only water. This type of microfluidic sorting has potential applications in fields such as biomedical research, diagnostics, and materials science, where precise particle separation is essential. The design and fabrication of the device are influenced by the constraints of the SU-8/PDMS micro-molding process, which requires careful consideration of channel dimensions and layout to ensure effective sorting performance.

Methods:

Device Fabrication

The microfluidic bead sorter is fabricated using a combination of SU-8 photolithography and PDMS casting. The fabrication process consists of the following steps:

  1. SU-8 Mold Creation:

    • Spin-Coating: SU-8 photoresist is spin-coated onto a silicon wafer to achieve a uniform layer.

    • UV Exposure: The coated wafer is exposed to UV light through a photomask, which defines the channel pattern.

    • Development: The unexposed SU-8 is removed during the development process, leaving behind the patterned mold.

  2. PDMS Casting:

    • Casting and Curing: PDMS is poured over the SU-8 mold and cured to solidify the material.

    • Mold Removal: The cured PDMS layer is carefully peeled off the mold, revealing the microfluidic channels.

  3. Device Assembly:

    • Plasma Treatment: The PDMS layer and a glass substrate undergo plasma treatment to promote adhesion.

    • Bonding: The PDMS is bonded to the glass substrate to form enclosed microfluidic channels, ensuring effective fluid containment.

Design Constraints

The design is shaped by the limitations of the SU-8/PDMS micro-molding process. Specifically:

  • Minimum Feature Size: In-plane channel features must be at least 20 μm wide, which is significantly larger than the 10 μm polystyrene beads being sorted.

  • Aspect Ratio: Channel widths must be no less than half of the channel height to maintain structural integrity and reliable molding.

These constraints determine the channel dimensions and overall layout, guiding the design to achieve efficient separation of polystyrene beads from the water-based mixture.

Results:

The microfluidic bead sorter successfully separated 10 μm polystyrene microspheres from a water-based mixture using a spiral channel design inspired by Kuntaegowdanahalli et al. The device was fabricated using SU-8 photolithography and PDMS molding, achieving an 80 μm channel height. Despite the device functioning as intended, an issue with outlet asymmetry caused fluid to favor the shorter outlet path initially, which was temporarily resolved by kinking the tubing. Experimental tests using a syringe pump revealed that bead separation depended on the flow rate. At constant and low flow rates, beads primarily exited through the outer outlet, while at higher flow rates, they shifted to the inner outlet, demonstrating the influence of Dean flow. The results confirmed that higher flow rates improve particle separation efficiency. Future work should address outlet asymmetry, refine flow control at lower rates, and explore alternative geometries and particle sizes to enhance the device’s performance and applicability.