We report on first steps towards a phononic crystal sensor for biomedical applications. Phononic crystals and metamaterials allow for unprecedented control of sound propagation. The classical ultrasonic sensors, acoustic microsensors and MEMS resonator sensors face severe limitations when applying them to small volume liquid analytes. Phononic crystal sensors are a new concept following the route of photonic crystal sensors. Basically, the material of interest, here a liquid analyte confined in a cavity of a phononic crystal having a solid matrix constitutes one component of the phononic crystal. In an application as chemical sensor the value of interest, let’s say the concentration of a toxic compound in liquid, is related to acoustic properties of the liquid in the cavity. A change in the concentration causes measurable changes in the properties of the phononic crystal. Transmission or reflection coefficients are appropriate parameters for measurement. Specifically, a resonance induced well separated transmission peak within the band gap is the most favorable feature. The sensor scheme therefore relies on the determination of the frequency of maximum transmission as measure of concentration.
Promising applications like biomedical sensors, point-of-care diagnostics or fast screening introduce further engineering challenges, specifically when considering a disposable element containing the analyte. The three key challenges are the strong restriction coming from limitations to approved materials for the analyte container, geometric dimensions in the mm-range common in hospital or point-of-care environment and acoustic coupling between sensor platform and analyte container.
Q-Factor Enhancement of Thin-Film Piezoelectric-on-Silicon MEMS Resonator by Phononic Crystal-Reflector Composite Structure