Jim McMahon for Physik Instrumente (PI) L.P.
A key impetus for medical and bioresearch companies to engage during product development cycles is the opportunity to capitalize on advances in technology for the manufacture of better-operating, lower-cost and more-efficient devices. A recent improvement in high-speed laser scanning, for example, has facilitated the release of Harvard Medical School’s latest optical imaging technique, optical frequency-domain imaging (OFDI), which provides unprecedented ultradetailed 3-D visualization of the coronary arteries. OFDI operates at several magnitudes of improvement over its predecessor, optical coherence tomography, which itself was enabled by advances in laser scanning 15 years prior.
Just as refinements in laser scanning technology have had widespread applications, no less influential are recent advances in motor technology, specifically piezoelectric motors and actuators. Medical device manufacturers increasingly are choosing to use piezoelectric motors and actuators in preference to conventional electromagnetic motors because they exhibit substantial inherent advantages.
PZT disks also are featured in eFlow electronic nebulizers made by Pari Pharma GmbH. The atomizer heads of the eFlow Rapid Electronic Nebulizer series employ annular ultrasonic piezo transducers. Courtesy of Pari Pharma GmbH.
Piezoelectric devices are used successfully in a widening range of medical applications, including ultrasonic emitters, nanoliter and microliter pumps, surgical devices, MRI-compatible robotics, microdose dispensers, materials handling, drug delivery, 3-D scanners, and laser beam steering devices for ophthalmology, dermatology and cosmetology.
Actuators and motors
A piezoelectric actuator, or piezoactuator, is a type of solid-state actuator based on the ability of a piezoelectric material to change shape when an electric field is applied to it. A piezoelectric ceramic element produces mechanical energy in response to electrical signals and, conversely, can produce electrical signals in response to a mechanical stimulus.
Shown here are custom ceramic-encapsulated piezo stacks with apertures.
The use of piezoelectric materials dates back to 1881, when Pierre and Jacques Curie observed that quartz crystals generate an electric field when stressed along a primary axis. The term piezoelectric derives from the Greek word piezein, meaning to squeeze or press.
Piezoelectric ceramics consist of ferroelectric materials and quartz. High-purity PZT, or plumbum zirconate titanate, powders are processed, pressed to shape, fired, fitted with electrodes and polarized. Polarization is achieved using high electric fields to align material domains along a primary axis. Piezoactuators in their basic form provide very small displacement, but they can generate huge forces. The minute size of the displacement is the basis for the high-precision motion they can deliver.
PZT disks are used in micropumps made by thinXXS Microtechnology AG. Custom piezoelectric disk actuators precisely dose liquids and gases in these devices. Courtesy of thinXXS Microtechnology AG.
For long travel ranges, a clever arrangement of multiple actuators, and the operation of a single piezoelement at its resonance frequency, have proved to be viable concepts. These types of devices are called piezomotors.
The latest designs of piezomotors have a number of advantages over electromagnetic motors when being considered for use in medical equipment and devices. Two types of piezomotors in particular have considerable attributes making them ideal for medical applications: ultrasonic linear piezomotors (also called resonant motors) and piezo stepper motors. Both versions can basically provide unlimited travel, yet are very different in design, specifications and performance.
In ultrasonic piezomotors, the piezoelectric ceramic material produces high-frequency acoustic vibrations on a nanometer scale to create linear or rotary motion. For large travel ranges, especially when high speeds also are required, ultrasonic linear drives are used. With resolutions as good as 50 nm, they become a better alternative to electromagnetic motor-spindle combinations. Ultrasonic drives are substantially smaller than conventional electromagnetic motors, and the drivetrain elements needed to convert rotary to linear motion are not required.
Ultrasonic piezoelectric linear motors employ a rectangular monolithic piezoceramic plate (the stator), segmented on one side by two electrodes. Depending on the desired direction of motion, one of the electrodes of the piezoceramic plate is excited to produce high-frequency eigenmode oscillations (one of the normal vibrational modes of an oscillating system) of tens to hundreds of kilohertz. An alumina friction tip (pusher) attached to the plate moves along an inclined linear path at the eigenmode frequency. Through its contact with the friction bar, it provides microimpulses that drive the moving part of the mechanics (slider and turntable) forward or backward. With each oscillatory cycle, the mechanics execute a step of a few nanometers. The macroscopic result is smooth motion with a virtually unlimited travel range.
The miniature linear piezomotor slide with onboard driver can reach velocities of 200 mm/s.
New ultrasonic resonant motors, such as the PILine model developed by Physik Instrumente (PI) – which has played a pioneering role in advancing development of piezodevices for medical applications – are characterized by speeds of up to 500 mm/s and a very compact and simple design. Such motors can produce accelerations to 10 g. They are also very stiff, a prerequisite for their fast step-and-settle times (approximately a few milliseconds), and provide resolution to 0.05 µm.
Piezo stepper linear motors usually consist of several individual piezoactuators and generate motion through a succession of coordinated clamp/unclamp and expand/contract cycles. Each extension cycle provides only a few microns of movement but, running at hundreds to thousands of hertz, achieves continuous motion. Even though the steps are incremental – in the nanometer to micrometer range – they can move along at about 10 mm/s, taking thousands of steps per second.
Illustrated here is the operating principle of a PiezoWalk piezo stepper motor.
Piezo stepper motors, such as PiezoWalk, also developed by PI, can achieve much higher forces of up to 700 N and picometer resolution compared with ultrasonic piezomotors. Resolution of 50 pm has been demonstrated. The motor performs extremely high-precision positioning over long travel ranges, and when the position has been reached, performs highly dynamic motions for tracking, scanning or suppressing vibrations. As with the ultrasonic piezomotors, these motions can be conducted in the presence of strong magnetic fields or at very low temperatures.
Improving performance 11 ways
Medical devices can be made smaller, more precise, lighter and easier to control by employing piezoelectric motors.
Piezoelectric motors are better suited for miniaturization than electromagnetic motors, yet they provide greater force for their size. The efficiency of electromagnetic motors decreases as their dimensions are reduced, with more of the electrical power converted to heat, whereas a piezomotor’s efficiency stays virtually constant. The stored energy density of a piezomotor is 10 times greater than that of an electromagnetic motor of the same volume and weight. The most advanced piezomotors are configured into extremely compact, high-speed micropositioning stages that are smaller than a matchbox; the smallest piezomotor-driven stages are used in autofocus devices for cell phone cameras. Because piezomotors provide a higher force per motor size, equipment and instrumentation (including medical devices) can be reduced in size while maintaining or increasing performance.
The direct-drive principle of the piezomotor eliminates the need for a supplementary transmission or gear train found in conventional electromagnetic motors. This avoids the usual backlash effects that limit accurate tracing, which creates a critical reduction in positioning accuracy in electromagnetic servomotors. The mechanical coupling elements otherwise required to convert the rotary motion of classical motors to linear motion are unnecessary. The intrinsic steady-state autolocking capability of piezoelectric motors does away with the servo ditherinherent in electromagnetic motors. Piezo-motors can be designed to hold their positions to nanometer accuracy, even when powered down.
Compared to other technologies, piezodevices can react in a matter of microseconds. Acceleration rates of more than 10,000 g (response times of 0.01 ms) are obtainable.
For many medical and biotechnology applications, piezomotors are ideal because they neither create nor are influenced by electromagnetic interference, eliminating the need for magnetic shielding. This feature is particularly important for motors used within strong magnetic fields, such as with MRI equipment. Small piezomotors are used for MRI-monitored microsurgery, and large piezomotors for rotating patients and equipment. Magnetic fields and metal components in conventional electronic motors make it impossible for motorized medical devices to function within MRI equipment.
Because the piezo motion depends on crystalline effects and involves no rotating parts such as gears or bearings, piezomotors are maintenance-free and do not require lubrication. They can be sterilized at high temperatures, which is a significant advantage in medical applications.
Static operation of piezodevices, even when holding heavy loads for long periods, consumes virtually no power. Also, since the efficiency of piezoelectric motors is not reduced by miniaturization, they are effective in the power range lower than 30 W. This makes piezomotors attractive for use in battery-operated, portable and wearable medical devices because they can extend the life of a battery by as much as 10 times.
When at rest, piezomotors generate no heat. Because they eliminate servo dither, piezomotors also eliminate the undesirable accompanying heat generation. In addition, they are, in principle, vacuum-compatible, a requirement for many applications in the medical industry, and are operable at cryogenic temperatures, down to near 0 K, making them suitable for operation in laboratory storage facilities and in cryogenic research.
Lastly, piezodevices are nonflammable and therefore safer in the event of an overload or short circuit at the output terminal – a considerable advantage for portable and wearable medical devices – and they can be used to harvest energy. For example, they can use a person’s motion to power small medical devices such as pacemakers or monitors.
Switching to piezodevices
In optical coherence tomography, piezoelectric motors are used to impart rapid periodic motion to the unit’s reference mirror and imaging optics. To enable creation of 2- and 3-D images from optical interference patterns, optical fibers must be moved both axially and laterally during a scan. Piezomotors have proved to provide more precise movements, resulting in improved image resolution over conventional electromagnetic motors.
The advantages of piezodevices have inspired their use in a number of medical and biotechnology applications, including point-of-care and medical test equipment, transdermal drug delivery, monitoring of endoscopic devices, microsurgery and noninvasive surgery tools, confocal microscopy for ophthalmological purposes and orthodontic 3-D cone beam imaging.
Electromagnetic devices dominate the drive mechanisms in medical equipment designs today. However, increasing accuracy requirements in the micron and nanometer ranges, along with an inclination to miniaturization, dynamics streamlining and interference immunity, are pushing the physical limitations of electromagnetic drive systems. Piezomotors are proving to be a viable alternative.
Meet the author
Jim McMahon is the senior writer at Zebra Communications in Simi Valley, Calif., where he writes about instrumentation technology. He can be reached at firstname.lastname@example.org. For more information on Physik Instrumente (PI) L.P.’s devices, contact Stefan Vorndran, the company’s director of corporate product marketing and communications; email@example.com.