Novel encapsulation of a medical ultrasound probe

Abstract

Ultrasound imaging plays an important role in the diagnosis of cardiovascular diseases. A trans-esophageal echocardiography (TEE) probe uses ultrasound waves to image the heart in real time in three dimensions (3D) from inside the esophagus of the patient. The current design of the TEE scan head in this study requires manual assembly of several prefabricated parts to provide key functions; heat spreading, electromagnetic interference (EMI) shielding, electrical isolation, and biocompatibility. The aim of this study was to develop new encapsulation concepts (based on materials, designs, or processing methods) to reduce the number of parts. This could be achieved by a twolayer encapsulation (e.g. metallized encapsulation, or encapsulation with two polymer composites). Among the requirements for this device, thermal management is important for the safety, performance, lifetime, and reliability. Therefore, this project focused on the thermal performance of new encapsulation concepts. The maximum surface temperature of the scan head (in contact with patient) must be below 43 °C to avoid thermal damage to human tissue. A two-layer encapsulation based on a metallized polymer structure was suggested for simplifying the assembly, while accommodating the key functions listed above. Experimental results showed that such an encapsulation could provide adequate heat transfer at low power levels. Thermal simulations were in good agreement with the experiments, when two different thermal contact conductance coefficients were used for the boundary condition in the finite element model, one for the heat source surface and one for the remainder of the scan head surface (the polymer encapsulation). The verified simulation model was used to identify alternative materials for the outer layer of the encapsulation, to improve the heat transfer at higher power levels. The preferred outer material is biocompatible and electrically insulating, and has high thermal conductivity. Thermally conductive polymer composites were therefore selected as the second main research topic. Composites with hBN particles (thermally conductive and electrically insulating) were studied. Composite specimens were fabricated by injection moulding, casting and 3D printing (powder bed fusion). The maximum practical hBN loading was 65 wt% for injection moulding with TPU as matrix, 55 wt% for casting with epoxy and 40 wt% for powder bed fusion with TPU. The injection moulded composite with 65 wt% hBN obtained the highest thermal conductivity in this study (2.1 W/mK), which was nearly 10 times higher than that of pure TPU. For a given hBN loading, casting resulted in a higher thermal conductivity than the other two methods. Injection moulding induced a preferred orientation of the platelets, with the platelet normal along the thickness direction of the specimens. The orientation varied through the thickness of the moulded specimens, and it increased with increasing hBN loading. Casting resulted in a low preferred orientation, and powder bed fusion resulted in an almost random orientation. The platelet orientation in injection moulded specimens (characterized by X-ray diffraction) agreed qualitatively with numerical simulations of the injection moulding process. The measured thermal conductivities were compared with four models (the effective medium approximation (EMA) model, the Ordóñez- Miranda model, the Sun model, and the Lewis-Nielsen model) to understand how the conductivity was affected by filler loading and orientation. Selecting the filler loading in the composite is a compromise; increasing the loading increases the thermal conductivity, but reduces the mechanical ductility and increases the melt viscosity. The heat transfer simulations also demonstrated the applicability of hBN/polymer composites in the two-layer encapsulation of the TEE scan head. These composites could be used in a metallized encapsulation, or in an encapsulation consisting of two layers (an outer layer of hBN/polymer and an inner layer of a thermally and electrically conductive polymer composite).