Ultrasonic piezo transducers can be used in a wide variety of applications, including medical devices. Because there is no one-size-fits-all solution, transducer manufacturers need to provide flexibility in materials and mechanical design.
Ultrasound has been used in nature for navigation above and under water. Bats and dolphins are but two species that have perfected its transmission, reception, and processing over millions of years (see Figure 1).
Frequencies above 20 kHz — above the hearing range of the human ear — are labeled as ultrasound. What works so well in nature has also found many applications in industrial and medical engineering and scientific research. Examples include distance measurement and object recognition, filling level or flow rate measurements, welding or bonding, high-resolution materials testing, and medical diagnosis and therapy. Electroceramics — materials that change their shape when an electric field is applied and generate electricity when a force is applied — work great for these applications. Usually this class of piezoelectric ceramics is used because they can be manufactured cost-effectively in many shapes perfectly matched to the application requirements.
Piezoelectric materials produce an electrical charge when a force is applied (direct piezo effect), and they change their dimensions when an electrical field is applied (inverse piezo effect) as shown in Figures 2 and 3. In other words, they convert mechanical power into electrical power, and vice versa, which is why they are often referred to as transducers. Piezo transducers are based on solid-state effects and are basically free of wear and tear. While the direct piezo effect can be used for sensor applications, the inverse piezo effect lends itself particularly for fast and precise motion generation.
Frequencies and Amplitudes
When an AC voltage is applied, piezo elements begin to oscillate. The extremely fast response in the microsecond range and below allows for ultrasound generation with frequencies of up to 20 MHz.
Ultrasonic applications can be classified in sensor applications and power ultrasound where the energy densities are higher and frequencies range from 20 to 800 kHz. Power ultrasound can be used in medical applications, for example, to crush kidney stones and for removing dental plaque, and in industrial processes such as cleaning and welding or bonding.
In addition to the material selection, different geometric variants and resonant frequencies can be realized to optimize the transducer for the respective application. While most common piezo transducers are based on PZT formulations, lead-free PIC700 material is now also available, and is suitable for ultrasonic transducers in the megahertz range.
Components such as disc- or plate-shaped transducers in thickness vibration mode, piezoceramic rings, piezo tubes, and shear elements are available as semi-finished products. OEMs often require custom geometries with integration in the final product. This includes the contacting of the elements according to OEM specifications, as well as mounting in provided components, gluing, or potting of the ultrasonic transducers.
Flow Measurement, Filling Level Measurement
The range of applications for piezoceramic components is diverse. The direct, as well as the inverse piezo effect, is used in ultrasonic measurement of propagation times. A typical application for propagation time measurement is the measurement of filling levels. Unlike the biological solutions where sender and receiver are separate, superfast muscles in the bat’s larynx create the sound while highly sensitive ears detect the reflection. Similarly, a piezo transducer can work as both a transmitter and a receiver, almost at the same time. The ultrasonic pulse it transmits is reflected by the filling medium. The propagation time required is a measure of the distance traveled in the empty part of the container.
Flow measurement is based on the propagation time difference during alternate transmission and receiving of ultrasonic pulses in and against the flow direction (see Figure 2). Here, two piezo transducers operating as both transmitter and receiver are arranged diagonally to the direction of flow in an acoustic path. With the Doppler effect, the phase and frequency shift of the ultrasonic waves, which are scattered and reflected by particles of liquid, are evaluated. The frequency shift between the reflected wavefront emitted and received by the same piezo transducer is proportional to the flow speed. Many other tasks can be effectively solved in a similar way, such as object recognition or high-resolution material tests.
Flexible Piezo Transducers and Patch Transducers
Piezo patch transducers can be designed to provide multiple functions in a very compact and flexible package. By embedding the ceramic patch into a laminated flexible polymer material, the whole transducer itself becomes bendable.
Applications include active vibration damping by combining a piezo sensor with a closed-loop controller and an actuator for active vibration cancellation, structural health monitoring, and fast switching.
Direct and Inverse Piezo Effect
The direct piezoelectric effect (see Figure 3) occurs when a force on the piezo ceramic substrate or a deflection or deformation generates an electric field, and thus a voltage/current that can be amplified and is proportional to strain or pressure. If the contact between the piezo transducer and the structure is good, this effect works from DC to high frequencies.
The inverse piezoelectric effect (see Figure 4) comes about when an electric field is applied to a poled piezo material. It results in a deformation that can be used to create a displacement or force. If the transducer is glued to a structure, it can be used to actively control the structure, damp vibrations, etc.
Using direct and inverse piezo effect (see Figure 5) in a structural health monitoring system, for example, one piezo transducer is electrically excited to induce vibration which can be detected by a multitude of piezo transducers operated in sensor mode. By evaluating the excitation signal and the various senor output signals, conclusions on the state of the structural health can be drawn.
Medical Transducers and Nanoliter Dosing
Piezo elements and transducers have also become indispensable in medical engineering. In addition to propagation time measurements such as in air bubble detection, typical applications are in pumping and dosing.
The dosing amounts range from the microliter and nanoliter range to the picoliter range. In these applications, piezo-based precision systems excel due to their minute dimensions, low energy consumption, and low costs. The same also applies to devices for aerosol generation, in which a piezo element excites a diaphragm to ultrasonic vibrations. The frequency is approximately 35 kHz. Due to the resulting pressure changes at the diaphragm, the fluid is pressed through the holes in the diaphragm and thereby aerosolized.
Ultrasonic transducers are also becoming mainstream on surgical instruments for minimally invasive surgery — they can replace traditional scalpels, giving the surgeon more control while reducing injuries to nerves and vessels. Some ultrasonic tips can simultaneously cut and cauterize tissue, resulting in a faster recovery. Dental hygienists use ultrasonic scalers to remove plaque and dental tartar, and urologists rely on high-power piezo sound transducers for shock wave lithotripsy, crushing kidney stones into small particles.
Ultrasonic piezo transducers can be used in a wide variety of applications, including many different types of medical devices. In addition to the material selection, different geometric variants and resonant frequencies can be realized to optimize the transducer for the respective application. Because there is no one-size-fits-all solution, the transducer manufacturer needs to provide flexibility in materials, mechanical design, and the willingness to work with OEMs from markets as diverse as medical engineering and nanobiotechnology. The ability to control all critical manufacturing steps reduces the design cycles and time to market.