Phononic crystals are resonant structures with great potential to be implemented in applications as liquid sensors. The use of the symmetry reduction technique allows introducing relevant transmission features inside bandgaps by creating defect modes in a periodic regular structure. These features can be used as measures to quantify changes in the speed of sound of liquid samples that could be related to the concentration of analytes or the presence of pathogens among other interesting applications. In order to be able to implement this new technology in more challenging applications, such as biomedical applications, it is necessary to have a very precise and accurate measurement. Changes in temperature greatly affect the speed of sound of the liquid samples, causing errors in the measurements. This article presents a phononic crystal sensor that, by introducing additional defect modes, can carry out differential measurements as a temperature compensation mechanism. Theoretical studies using the transmission line model and analytes at various temperatures show that the proposed temperature compensation mechanism enhances the performance of the sensor in a significant way. This temperature compensation strategy could also be implemented in crystals with different topologies.

Phononic crystals are periodic composite structures with specific resonant features that are gaining popularity in the field as liquid sensors. The introduction of a structural defect in an otherwise periodic regular arrangement can generate a resonant mode, also called defect mode, inside the characteristic band gaps of phononic crystals. The morphology, as well as the frequency in which these defect modes appear, can give useful information on the composition and properties of an analyte. Currently, only gain and frequency measurements are performed using phononic crystal sensors. Other measurements like the transient response have been implemented in resonant sensors such as quartz microbalances showing great results and proving to be a great complimentary measure to the gain and frequency measurements. In the present paper, a study of the feasibility of using the transient response as a measure to acquire additional information about the analyte is presented. Theoretical studies using the transmission line model were realized to show the impact of variations in the concentration of an analyte, in this case, lithium carbonate solutions, in the transient time of the system. Experimental realizations were also performed showing that the proposed measurement scheme presents significant changes in the resulting data, indicating the potential use of this measure in phononic crystal sensors. This proposed measure could be implemented as a stand-alone measure or as a compliment to current sensing modalities.

Phononic crystals (PnC) consist of two or more materials arranged in a spatially periodic manner. Introduction of a defect into the crystal lattice is a technique to create resonant modes. The idea behind the application of a PnC as sensor element is the dependence of the characteristic features of those modes on speed of sound. Currently the sensor designs are based on common 2D phononic crystal geometries. A liquid-filled defect in the crystal lattice creates a resonant mode used for the determination of sound velocity of the confined analyte. This (complex) value allows the determination of thermodynamic values which are linked to molecular properties of (bio)chemical samples. The objective of our current research is two-fold: a new geometry considering disposable elements containing the analyte while keeping the high sensitivity of the acoustic sensor signal and suppressing influences of unavoidable inaccuracies when replacing the analyte container.

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.

We report on first steps towards a phononic crystal sensor for biomedical applications. Phononic crystal sensors are a new concept following the route of photonic crystal sensors. Basically, the material of interest constitutes one component of the phononic crystal. Here a liquid analyte is confined in a disposable element which acts as defect in a phononic crystal. In an application as biomedical sensor the concentration of a compound in liquid must be related to speed of sound of the liquid in the cavity. A change in the concentration e.g. caused by the appearance of a toxic compound or as a result of a biochemical reaction, results in measurable changes in the transmission coefficient, specifically a shift of a resonance induced transmission peak. Three additional key design challenges for a medical sensor are the strong restriction coming from limitations to approved materials for the analyte container, geometric dimensions in the mm-or sub-mm-range common in hospital or point-of-care environment and acoustic coupling between sensor platform and the analyte container.