Hydrodynamic fabrication and characterization of a pH-responsive microscale spherical actuating element

In this paper, we report a hydrodynamic fabrication method for pH-responsive microspheres, which act as an actuating component, and we characterize their volume-changes according to variations in pH values. We fabricated the microsphere-producing apparatus by using a poly(dimethylsiloxane) (PDMS) based substrate and a pulled-glass pipette. The swelling and the shrinking motions that varied according to the size, the shape and the concentration of AA were characterized with alternating application of acidic and basic solutions. The fabricated microspheres exhibited a larger and a faster volume transition than the cylindrical shaped post, which results generally from a conventional in-situ photopolymerization method. The repeated motions resulting from the alternating change of pH value were stable and reproducible. This finding indicates that the produced microspheres can be used as an actuation component in other microdevices. To exemplify such an application, we integrated a single microsphere into a PDMS-based microfluidic valve, and the fabricated microvalve resulted in stable operations relative to pH change.