Exploratory Investigation of Ultrasound Interrogated Passive Sensors based on an Acoustic Metamaterial

Wireless technologies have significantly influenced the development of new sensing concepts for continuous health monitoring and disease detection. Compared to wired solutions, wireless sensors overcome limitations such as risk of infection and discomfort caused by tethered connections. Wireless systems are classified as passive (powerless) and active (powered). Active solutions face significant challenges both in terms of power management and fabrication, requiring the integration of complex electronic components and circuits. Furthermore, active devices may require power source replacement, which can lead to health risks and complications for the patient. In contrast, passive devices present reduced complications thanks to their powerless nature and simpler architecture. Wireless devices typically rely on electromagnetic coupling interrogation for powering or data transmission. Electromagnetic waves, while being a common approach for sensing interrogation, are limited by overheating risks associated with high energy absorption and scattering in tissue. Moreover, the development of customized receivers for these sensing applications requires significant design investment. Due to their mechanical nature, acoustic waves achieve comparable penetration depths to electromagnetic waves at lower power levels. Furthermore, standardized clinical equipment such as echographs can be utilized for interrogation in the MHz regime (ultrasound). Acoustic sensors based on ultrasound interrogation have already been explored, but they are mostly limited by the spatial resolution of commercially-available transducers, often insufficient to measure variations of biomedical parameters of interest (e.g. pressure, temperature). This thesis presents a new approach to perform intracorporeal sensing, investigating the advantages of frequency resolution and ultrasound interrogation. In particular, this is achieved exploiting the high resonant states generated by an acoustic metamaterial. Metamaterials (from Greek: μϵτα, "beyond" conventional matter), engineered structures by design, exhibit properties beyond those of conventional materials. While fundamental research on metamaterials spans more than three decades, their application to ultrasound for the development of new and innovative medical devices is still in the early stages. The acoustic metamaterial presented in this thesis consists of three-dimensional silicon micropillars (radius: 35 μm), arranged in a honeycomb lattice, and embedded in a polymeric matrix (PDMS, polyimethylsiloxane), which also acts as an encapsulation material. This design combines the high amplitude of the resonance mode of the metamaterial with the temperature and pressure sensitivity introduced by the presence of the PDMS matrix. The objectives of this thesis are focused on the demonstration of (1) temperature and (2) pressure sensing capabilities, within ranges of interest for medical applications. Specifically, temperature sensitivity is evaluated in the 36◦ ÷ 41◦C range and pressure sensitivity in the 0 ÷ 200 mbar range. Temperature applications are relevant for the detection of infections in failing implants. Pressure sensing requirements were selected in collaboration with our clinical partners from the Deutsches Herzzentrum der Charité, DHZC, Charité Hospital. In particular, pressure specifications were defined for bi-atrial pressure monitoring, as a key indicator of relevant cardiovascular conditions, such as heart failure. The metamaterial (henceforth, PDMS-Meta) was initially characterized in water. Its response was compared to a bilayer of silicon and PDMS (henceforth, Bilayer), and to the metamaterial without the PDMS matrix (henceforth, Si-Meta). These two structures were utilized to elucidate the physical mechanism behind the temperature sensitivity. The temperature resolution achieved by the PDMS-Meta is below 0.1 K (0.03 K), with a temperature sensitivity of −2.9 · 10−3 K-1. The physical origin of temperature sensitivity was investigated by experiments and Finite Element Method (FEM) simulations and explained as a temperature-dependency of the bulk modulus of the PDMS. Temperature characterization was repeated in presence of tissue mimicking materials, TMMs—imaging samples with acoustic properties comparable to those of human muscle—and with animal tissue (pork loin) to assess the effect of scattering and attenuation on the temperature performance. Temperature sensitivity was comparable in the three media (−3.4 · 10−3 K-1, −3 · 10−3 K-1, −3.5 · 10−3 K-1, in water and in presence of the TMM and tissue, respectively), although the temperature resolution degraded (0.02 K, 0.12 K, 0.18 K, in water, TMM and tissue). The achieved temperature resolution in presence of tissue is comparable to the resolution of infrared cameras utilized in medical thermometry (0.1 K). The measurement location was observed to strongly influence the temperature results in presence of highly inhomogeneous media as an effect of the multiple interferences introduced by the tissue. Finally, pressure characterization of the Bilayer, PDMS-Meta and Si-Meta was performed in water with a bulge-test setup. Pressure sensitivity was significantly higher in the PDMS-Meta and the Bilayer, although opposite in sign (−4.3 · 10−6 mbar-1 and 11 · 10−6 mbar-1, respectively) in comparison to the Si-Meta (−0.5 · 10−6 mbar-1). Pressure resolution was comparable in the PDMS-Meta and Bilayer (11.6 mbar vs 18.3 mbar, respectively), but significantly lower in the Si-Meta (224.8 mbar). The origin of pressure sensitivity was investigated by FEM simulations. In the Bilayer, the primary mechanism was identified as a geometrical effect in the PDMS layer. For the PDMS-metamaterial, the pressure sensitivity is potentially attributed to a strain-dependent variation in the speed of sound within the PDMS. The achieved pressure resolution enables the detection of pressure changes equivalent to systolic and diastolic pressure values (13 mbar vs 187 mbar) in ideal conditions (water). As a next step, alternative metamaterial designs could be investigated to exploit anisotropy with respect to the direction of interrogation of the ultrasound source. This approach could be particularly useful in multi-modal sensing to decouple each contribution. Furthermore, the current spatial dependency of the results should be addressed, for instance with a multichannel setup with enhanced time averaging modality, to enable the interrogation of the sensor at multiple locations with improved signal resolution. The presented sensor opens the way to a new class of zero-power devices based on acoustic metamaterials and ultrasound interrogation to perform remote sensing with limited integration and, potentially, exposure complications for the patient.