Acoustic Materials for Wearable Ultrasound patches

Abstract

With today’s aging population and the prevalence of chronic illnesses, healthcare systems worldwide are struggling to accommodate the increasing number of patients. This projected growth and rising demand underscore the importance of advancing wearable devices. In the realm of diagnostic healthcare, the pursuit of non-invasive and safe techniques is paramount. Until now, body patches have predominantly focused on monitoring surface-level body parameters such as temperature, humidity, pH, oxygen saturation, and electric potentials. The introduction of ultrasound patches extends the realm of possibilities, enabling a deeper exploration of physiological processes within the body. Additionally, ultrasound serves as a non-invasive diagnostic technique. To facilitate medical ultrasound imaging, an acoustic interface is indispensable for the unimpeded transmission of waves through the skin and tissue. This interface must maintain proper hydration and adhere to the skin to ensure a conforming acoustic connection. Initially, this MSc. thesis research aimed to conduct lifetime performance tests on promising ultrasound acoustic interface materials. These tests were conducted by placing wearable ultrasound patches with the acoustic interface materials in place on an ultrasound phantom. With the ultrasound transducer, ultrasound images were made over a fixed period. This experiment was done to see to which extent the image quality would degrade over time for the different interface materials. The available ultrasound phantoms did not meet the requirements of skin-mimicking properties, on which the lifetime of the acoustic materials would be tested. Consequently, this research opted to simulate specific skin conditions: temperature, moisture, and acoustic properties like human tissue. Different iterations were made and evaluated during the development of the final ultrasound phantom model. In this thesis, five different models were evaluated, and eventually, the final model was presented: A three-layer model. The phantom model consists of a gel wax filled with scattering objects, visible with ultrasound, at specific depths inside the filling. To mimic the water loss rate of the skin, a hydrating layer of agar was placed on top of the gel wax filling. A PET foil was deployed with a specific number of holes to let water through from the agar layer to regulate the amount of water evaporation over time. To mimic skin temperature, the phantom model was placed in an oven at T = 34 °C. With this final model, the lifetime experiments were conducted with six potential interface materials: AquaFlex (solid hydrogel), HydroAid (solid hydrogel), Ecoflex (silicone), Axelgaard (ECG solid hydrogel), HH5023 (ECG solid hydrogel), and HH5450 (ECG solid hydrogel). The duration of this experiment was eight days, after which the agar layer started to degrade and shrink. The filling and scattering objects of the phantom model are reusable, while the hydration layer has to be replaced or disposed of after five days. Further, acoustic properties, like materials-specific attenuation coefficient, of potential acoustic interface materials and materials used in the phantom were measured using a though-transmission setup. Also, a validation assessment of skin compatibility of the potential interface materials for a long duration of time was conducted. This was done with the consultation of experts in medical devices, medical professionals and literature. With these three different subjects of this MSc. the attenuation coefficient of six different acoustic interface materials are characterized and validated to be compatible with human skin for longer periods. The phantom model developed satisfies the requirements set and, most importantly, mimics skin temperature and water loss rate. One round lifetime (eight days) performance experiments of acoustic interface materials using the phantom model. It is difficult to conclude to which extend the image quality degraded over time for the different interface materials due to the agar layer dehydration after eight days and that the experiment was only conducted once. For future recommendations, it is suggested that the lifetime experiments be repeated using the phantom model for these six different interface materials. It could also be an option to renew the hydration layer every seven days to prevent the agar layer from dehydrating over the acceptable limit if the experiment requires longer periods.