Recent advances in piezoelectric motors and actuators are impacting medical applications. These include ultrasonic emitters, artificial fertilization, medical nano-microliter pumps, micromonitoring, surgery devices, MRI compatible robots, microdose dispensing, cell penetration and cell imaging in cytopathology, medical material handling such as pick-n-place systems, drug delivery devices, 3D scanning and for laser beam steering in ophthalmology, dermatology and cosmetology.
The piezo difference
A piezoelectric actuator (piezo actuator) is a type of solid state actuator based upon the change in shape of a piezoelectric material when an electric field is applied. It uses a piezoelectric ceramic element to produce mechanical energy in response to electrical signals, and conversely, is capable of producing electrical signals in response to mechanical stimulus.
The use of piezoelectric materials dates back to 1881 when Pierre and Jacques Curie observed that quartz crystals generated an electric field when stressed along a primary axis. The term piezoelectric derives from the Greek word ‘piezein’, meaning to squeeze or press, relating to the electricity that results from pressure applied to a quartz crystal.
Piezoelectric ceramics consist of ferroelectric materials and quartz. High-purity PZT (plumbum, zirconate, titanate) powders are processed, pressed to shape, fired, electroded and polarized. Polarization is achieved using high electric fields to align material domains along a primary axis. Piezoelectric actuators in their basic form provide very small displacement, but can generate huge forces. The minute size of the displacement is the basis for the high precision motion they can deliver.
For long travel ranges, a clever arrangement of multiple actuators, or the operation of a single piezoelement at its resonance frequency have proven to be viable concepts. These types of piezo motion devices are called piezo motors.
The latest designs of piezo motors have advantages for use in medical equipment and devices. Two types of piezo motors, in particular, have considerable attributes for medical applications. They are: ultrasonic piezo linear motors (also called resonant motors), and piezo stepper motors. Both versions can provide virtually unlimited travel (movement), yet they are very different in their design, specifications, and performance.
In ultrasonic piezoelectric motors, the piezoelectric ceramic material produces highfrequency (inaudible to the human ear) acoustic vibrations on a nanometer scale to create a linear or rotary motion. For large travel ranges, especially when high speeds are also required, ultrasonic linear drives are used. With resolutions as good as 50 nm they become a better alternative to electromagnetic motor-spindle combinations. The ultrasonic drives are substantially smaller than conventional EM motors, and the drive train 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 micro-impulses and drives the moving part of the mechanics (slider and turntable) forward or backwards. With each oscillatory cycle, the mechanics executes a step of a few nanometers. The macroscopic result is smooth motion with a virtually unlimited travel range.
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New ultrasonic resonant motors, such as the PILine by Auburn, MAbased Physik Instrumente, are characterized by very high speeds to 500 mm/s, in 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 (on the order of a few milliseconds) and provide resolution to 0.05 μm.
Piezo stepper linear motors usually consist of several individual piezo actuators 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 speeds in the 10mm per second range, taking thousands of steps per second.
Piezo stepper motors, such as the PiezoWalk, also by Physik Instrumente, can achieve much higher forces of up to 700 N (155 lbs) and picometer (one trillionth of a meter) range resolution compared to ultrasonic piezo motors. Resolution of 50 picometers has been demonstrated. The motor is capable of performing extremely highprecision positioning over long travel ranges, and when the position has been reached then performing highly dynamic motions for tracking, scanning, or active vibration suppression. Like the ultrasonic piezo motors, these motions can be conducted in the presence of strong magnetic fields or at very low temperatures.
Basic piezo technology for motion control applications
These are the most common types of piezo actuators and piezo motors:
• “Simple” piezo actuator expands proportionally to voltage. The motion is basically proportional to the drive voltage. Sub-groups include:
– Stacked actuator – most common type. High force, fast response, short travel.
– Shear actuator – lateral motion, very fast, XY systems available. High force, high frequency possible, travel typically limited to 20 μm.
– Tube actuator – mostly for micro-dispensing applications and AFM scanners.
– Bender actuator – long travel (deflection) to several mm possible, but low force and low frequency.
• Flexure-guided, piezo actuator – frictionless flexures and motion amplifiers provide longer travel, and straight motion, which is basically proportional to the drive voltage. No wear and tear. Integrated multiaxis systems are available. Motion range up to 2 mm and above.
• Ultrasonic friction motors – based on high frequency oscillation of a piezo plate (stator). Unlimited motion, high speed, fast response (10 to 10s of millisecs). Oscillation is transferred to a slide or rotor via friction. Due to friction, resolution is limited to typically 50 nm.
• Piezo stepping motors – basically unlimited motion range. Based on accumulation of small controllable steps. Picometer resolution dither mode (direct piezo actuation). Compact and high force to 155 lbs (for off the shelf units). Fast response (less than 1 millisec). High stiffness.
• UItrasonic transducers – plate or disk-driven with a high frequency at resonance. Used as sensors or transmitters, and in nebulizers.
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Medical devices can be made smaller, more precise, lighter, and easier to control by employing piezoelectric motors, which are well-suited for miniaturization At the same volume and weight, the stored energy density of a piezo motor is 10 times greater than that of an electromagnetic motor. The most advanced versions of piezo motors are configured into extremely compact, high-speed micro-positioning stages that are smaller than a matchbox. The smallest piezo motor-driven stages are currently used in autofocus devices for cell phone cameras.
Other performance characteristics:
• Faster acceleration. Piezo devices can react in a matter of microseconds. Acceleration rates of more than 10,000 g (response times of 0.01 milliseconds) can be obtained.
• No magnetic fields. For medical and biotechnology applications, piezoelectric motors are ideal as they do not create electromagnetic interference, nor are they influenced by it, eliminating the need for magnetic shielding. This feature is particularly important for motors used within strong magnetic fields, such as with MRI equipment, where small piezo motors are used for MRImonitored microsurgery and large piezo motors 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.
• No maintenance or lubrication, aseptic enabled. Because the piezo motion depends on crystalline effects and involves no rotating parts like gears or bearings, they are maintenance free and do not require any lubrication. They can be sterilized at high temperatures, a significant advantage in medical applications.
• Reduced power consumption. Static operation, even 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 30W. This makes piezo motors attractive for use in battery-operated, portable and wearable medical devices because they can extend the life of a battery as much as 10 times longer.
• No heat generation. When at rest, piezo motors generate no heat. Piezoelectric motors also eliminate servo dither and the accompanying heat generation, an undesirable feature of electromagnetic motors.
• Vacuum-compatible. Piezo motors are in principle vacuum-compatible, a requirement for many applications in the medical industry.
• Operable at cryogenic temperatures. Piezoelectric motors continue to operate even at temperatures close to zero kelvin, making them suitable for operation in extremely cold environments, such as in medical laboratory storage facilities and in cryogenic research.
• Nonflammable. Piezo motors 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.
• Power generation. Piezo devices can be used to harvest energy. For example, using a person’s motion to power small medical or electrical devices such as pacemakers or health monitors.
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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 two- and three-dimensional images from optical interference patterns, optical fibers must be moved both axially and laterally during the scan. Piezo motors have proven to provide more precise movements resulting in improved image resolution over conventional electromagnetic motors.
Point-of-care and medical test equipment engage piezo technology. Where extremely fine tuned positioning and measuring equipment is required, piezo motors meet the need, which can create motion with extreme precision, from inches to nanometers.
Piezoelectric actuators are beginning to be used for transdermal drug delivery, such as with a needle-free insulin injection system. Piezoelectric devices are also being used in the monitoring of endoscope-gastroscope devices.
Biomedical micro-tools, such as tweezers, scissors and drills, have been adapted to a micro-robot base powered by piezo motors. Piezo motors are becoming more prevalent in micro-surgery and non-invasive surgery tools.
3D Cone Beam Imaging, used in orthodontics and for treating sleep apnea patients to obtain an exact model of the oral cavity for fitting oral appliances, employs the use of piezoelectric actuators.
Confocal microscopy used in ophthalmology for quality assurance of implants uses piezoelectric motors. Precise motion of the optics is required to adjust the focal plane and for surface scanning. Piezoelectric positioning systems are integrated directly into the optics.
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 toward miniaturization, dynamics streamlining and interference immunity are pushing the physical limitations of electromagnetic drive systems. Piezoelectric motors are proving to be a viable alternative, finding their way into a growing number of medical device applications.