Quartz tuning forks with hydrogel patterned by dynamic mask lithography for humidity sensing

This communication reports a new humidity sensing platform based on a quartz tuning fork (QTF) with hydrogel patterned. A commercial QTF is placed into a polydimethylsiloxane (PDMS) mold to make a flat plane between two prongs and this is called a QTF-PDMS mold assembly. Then, a PDMS cover with 10-mu m deep recess is placed on top of the QTF-PDMS mold assembly. Then, hydrogel prepolymer, polyethyleneglycol-diacrylate (PEG-DA, Mw 575), mixed with a photoinitiator is introduced between the QTF-PDMS mold assembly and the PDMS cover and cured as a specific shape by digital micromirror device (DMD)-based dynamic mask lithography. Each QTF with a patterned PEG-DA structure is characterized and used for humidity sensing experiments. The humidity sensing resolution is also investigated by time-domain frequency measurement and Allan deviation analysis.