Reducing time to market for medical ultrasonic sensors & transducers

by Amir R. Mirza, Ph. D

22 May 2023

14:00

Today ultrasonic technology has become pervasive in the medical devices market from surgical instruments to nebulisers to hospital and home dialysis machines.

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Ultrasonics is used for both diagnostic applications such as ultrasound imaging as well as therapeutics, including HIFU (high intensity focused ultrasound) which is reported to reach $486 million in market size by 2027.

The largest single market for ultrasonic technology is ultrasound imaging which had a market size of $7.9 billion in 2021 and is projected to grow to $14.5 billion by 2030. Another key growth market is ultrasonic electrosurgical instruments which had revenues of $3.86 billion in 2021.

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Ultrasonic technology is widely used in haemodialysis and peritoneal dialysis. In the USA, according to the CDC one in seven adults is affected by chronic kidney disease. Home dialysis is also becoming a key growth area, as according to WHO the world's population aged over 60 will reach 2 billion by 2050 from 900 million in 2015.

Ultrasonic technology is particularly powerful for fluid management in medical equipment, such as the detection of air bubbles in lines, liquid flow and level sensing. For example, when measuring blood flow in instruments there is no contact between the ultrasonic flow sensor and patient blood itself. Ultrasonics is often the preferred solution in medical applications over optical, mechanical or a capacitive sensing technology.

Importance of numerical computer modelling in ultrasonics

A key element of reducing product development times for ultrasonic sensors and transducers is the use of the latest numerical computer modelling software. Ultrasonic devices are based on piezoelectric materials, that is a ceramic material which can generate an electric field in response to a mechanical stress and vice-versa.

Piezoelectric components are typically operated in resonant mode as this is the frequency that electrical energy is most efficiently converted into mechanical energy. However, when exciting piezoelectric plates and discs into resonance a number of different resonant modes can be created in the mechanical system and therefore computer modelling is essential to optimise the desired resonance mode and discriminate against other resonances.

A multi-physics approach to numerical modelling needs to be taken as for example an ultrasonic transducer assembly will consist of piezoceramic discs as well as metal or plastic components. Therefore, the computer modelling must take into account the different domains involved - electrical signals, piezoelectric effects (conversion of electrical signals into mechanical stress) and the thermo-mechanical behaviour of the entire mechanical system. This multi-physics solution then needs to provide electronic designers the information they need such as primary resonance frequency, impedance, capacitance, Q-factor and so on in order to design the appropriate control electronics.

The performance of piezoceramic components is governed by a wide variety of fundamental material properties, such as mechanical elastic constants, dielectric properties and coupling coefficients (electrical to mechanical coupling) as well as dimensional control of ceramic discs and plates. All of these parameters will be dependent on the piezoceramic manufacturing process itself. The value of any computer-numerical modelling will depend on the repeatability of these material and mechanical properties and their tolerances. Computer models that are not calibrated against manufacturing performance will only increase product development lead times as a trial-and-error approach would have to be used rather than accurate modelling and first-time success.

Control of piezoceramic materials and manufacturing is key

Piezoceramic materials are complex mixtures of powders, binders and additives that are fine tuned to give the desired performance in terms of component piezoelectric sensitivity, capacitance, dielectric losses and power handling. Lead zirconate titanate (PZT) is the most commonly used piezoceramic for electronic applications. The global piezoelectric materials market was valued at $1.51 billion in the year 2021.

PZT materials offer good sensitivity and temperature performance as well as being mechnically robust for a wide variety of electronic applications for markets including medical, automotive, industrial and defence.

The actual manufacturing process to create PZT plates, cylinders and discs consists of many steps. These include grinding, sintering, shaping and machining, firing at elevated temperatures (above 1000°C), metallisation to form electrodes and high voltage polarisation to enable piezoelectric behaviour. The purity and control of raw materials is critical as well as the mixing of materials to enable the correct final ceramic material homogeneity and morphology. Tight dimensional control, including substrate parallelism, plays a crucial role in obtaining the highest performance from piezoceramic components.

If proper control is not maintained over these many manufacturing steps or the incoming raw materials, then a significant variation in piezoceramic performance will be observed. The resultant effect would be increased manufacturing costs due to lower process yields.

This piezoceramic variability would also impact final transducer assembly yields and the ability to rely on computing modelling to design new products with first-time success. Therefore, in order to reduce product development times and to maintain high yields once a product goes into mass production, both at the piece part and final assembly stages, it is key to control the piezoceramic manufacturing process itself. Ideally, an ultrasonic sensor or transducer supplier to the medical market should have complete control of the product process from modelling/design to piezoceramic manufacture to final product assembly and test.

Achieving consistent product performance and reducing time to market

It is seen that ultrasonic sensors and transducers are used in a wide range of commercial markets. However, in medical applications where patients depend on the consistent performance of medical sensors, equipment and instruments, product reliability is vitally important.

Figure 1 shows an example of the manufacturing tolerance for resonant frequency of an ultrasonic transducer designed by CeramTec for a medical device application. It is seen here that the resonant frequency is well controlled with very little variation. The same device is shown in Figure 2, but this time the impedance (Ohms) of the transducer in plotted and again shows this parameter is also well centred. This level of consistency in piezoceramic materials and manufacturing enables ultrasonic design engineers to develop medical devices with predictable performance and high yield at final assembly and test.

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The above predictability ultimately reduces overall development cost and time and lowers manufacturing costs when the design is released for mass production. Figure 3 (above) shows what can happen in the absence of consistent piezoceramic material performance, that is a continuous cycle can develop where a computer modelling based on certain assumptions about piezoceramic material properties are made which result in a specific design. Later on, and this could be several months later the piezoceramic discs or plates are manufactured, assembled and tested.

However, the performance of the final product is not as expected as the modelling predicted. Therefore, the design is modified to match the actual piezo material performance. The cycle is then repeated and several months later, data is collected on the revised design and a new batch of piezo material.

Ultimately, this approach could lead to an inefficient transducer design with wider specifications than desired or over-designed, higher power and more expensive control electronics that can compensate with a transducer design with fluctuating performance.

Worst case, these lessons may be learnt after the medical device goes into production. Therefore, it is vital to understand the ultrasonic component supplier's full capabilities from design through to final mass production.

In summary

Ultrasonics is a powerful technology for medical devices with unique benefits in critical patient applications. In order to design new, innovative ultrasonic sensors and transducers the use of advanced multi-physics numerical computer modelling is advised. These modelling tools depend heavily on repeatable fundamental piezoceramic material properties to avoid designing by a trial-and-error approach. Piezoceramic material and mechanical properties can only be controlled through tight quality control of incoming raw materials as well as a the many process steps involved in going from powders to finished machined ceramic. A lack of control in the design process or in piezoceramic manufacturing will ultimately lead to increased product development times and costs as well as in ultimate unit costs when the product is released for volume production.

CeramTec will be exhibiting at Med-Tech Innovation Expo on 7th-8th June at the NEC, Birmingham on Stand B10. To register for FREE, visit www.med-techexpo.com

by Amir R. Mirza, Ph. D

22 May 2023

14:00