Ultrasound imaging plays an increasing role in the diagnosis of cardiovascular diseases and the guidance of interventional cardiac procedures. Trans-esophageal echocardiography (TEE) probe, uses ultrasound to image the heart in real time from inside the esophagus. Such an ultrasound probe calls for small packaging designs while maintaining high image quality and patient safety. The assembly of the state-of-the-art TEE probe design from GE Vingmed Ultrasound is demanding. Two major challenges in the process are the time-consuming manual handling of several prefabricated parts and the limited possibility for rework/repair of high-value modules assembled by adhesives, factors that significantly contributions to the probe manufacturing cost. In the MMIMI project, advanced materials and techniques have been investigated to achieve more compact packaging designs and more cost-effective manufacturing of the TEE probes.
Reworkable anisotropic conductive adhesives
Anisotropic conductive adhesives (ACA) provides both electrical conduction and mechanical strength for assembled modules. ACA is used for assembling an electro acoustic module to a signal cable module in the TEE probes. Since the cost of these modules are high, it is a huge advantage if ACA bonds are reworkable if any failure appears. To be able to separate and reuse functional module(s), either during production or when repairing returned products, is of high interest in regards to cost saving. Reworkable ACA with comparable performance as conventional ACA are developed in this work. Adhesives comprising a blend of a thermosetting epoxy and a thermoplastic polymer have shown potential to ensure good electrical and mechanical integrity whilst still allowing reworkability for ACA assemblies.
Thermomechanical properties of the thermoplastics play an important role in tuning the mechanical properties as well as the reworkability of ACA, even with a concentration as low as 30 wt% thermoplastic. Glass transition temperature (Tg) of the thermoplastics should be in-between the target operation temperature and the rework temperature. Results from blends of an epoxy and a high-Tg thermoplastic for silicon-based samples show high mechanical strength at temperatures relevant for production/storage and operation of the TEE probes, proper reworkability, and sufficient electrical conduction, as compared to commercially available ACA. On the other hand, the use of a low-Tg thermoplastic in blends with a suitable epoxy enables the reworkability of ACA at a lower temperature while maintaining acceptable adhesion strength to flexible printed circuits. These reworkable ACAs have potential to be used in the assembly of ultrasound probes but also for other applications.
Advanced encapsulation for TEE probes
Simplification of the TEE probe packaging design, by means of fewer parts and less manual processes, can lead to a more efficient manufacturing process. Our studies shows that a double-layer encapsulation based on metallized polymer structure is promising in terms of simplifying the assembly, heat management, electrical isolation and electromagnetic interference shielding. Since thermal management plays an important role with respect to patient safety, the thermal performance of a double-layer encapsulation has been evaluated in both experiments and numerical simulations. Experiments in a relevant setup for a TEE probe, show that a metallized polymer encapsulation, fabricated by 3D printing and electroplating, can provide adequate thermal dissipation. Simulations are in good agreement with the experiments. The verified model suggests alternative materials for improving the thermal performance of the metallized encapsulation. Hence, a biocompatible and electrically insulating polymer-based composite material with high thermal conductivity is studied by experiments. Composite samples, containing a polymer matrix (TPU or epoxy) and a highly thermal conductive ceramic filler BN, were fabricated by 3D printing, injection moulding, and casting. Injection moulded composite containing 35 % TPU and 65 % BN have shown the highest thermal conductivity and that composites can provide better thermal performance for the TEE probe. A metallized encapsulation fabricated by injection moulding of (TPU-BN) composite and electroplating will be a useful approach for simplifying the TEE probe assembly.
This project represents an innovation at national and international level by developing new materials and processes for more compact, cost-effective TEE probes manufactured by GEVU. The planned innovation involves:
- New processes that allow for more compact design in interventional TEE ultrasound probes allowing for improving the image quality in relation to the size. This is important when the product must enter the body and fit with the human physiology. This will allow both improvements in image quality and the development of smaller probes with the same image quality as todays adult probes.
- The processes being investigated are means of preparing single components or processes that solve requirements that previously necessitated several different parts each requiring a certain amount of space. Such components could be the outer housing that needs to be robust, seal against fluid, transfer heat efficiently, protect the internal electronics and shield against electromagnetic interference in the operating room.
- The project will also focus on increasing the number of reworkable processes. This will significantly reduce the amount of scrap that is created if some process or component fails or is damaged. This is important as these components are very delicate and the process is very demanding as more and more imaging power is being packed into the same very small space needed to safely use the probe inside the body.
These projects will together help keep the Norwegian assembly facility to establish methods that are more competitive than other plants for the TEE probes.