When precision is needed, piezoelectric actuation is the default choice. Similarly, piezoelectric actuation represents the gold-standard when high force or high dynamics is required. Piezo actuators have long been used in semiconductor manufacturing and testing applications for their extreme speed and precision. Now, medical device manufacturers and microscope manufacturers are warming to their benefits.

Piezo stack actuators traditionally offer limited travel ranges of less than 0.1 mm, but effective flexure lever-amplification approaches, continually improved over many years of development, today provide travels up to 2mm. The combination of very high speed (sub-millisecond response times), and extreme positional resolution of these flexure mechanisms make them ideal for use in precision dosing mechanisms, such as those employed for drug delivery. The sketch below shows the basic design of a piezo flexure valve mechanism.

In addition to their use in piezo-flexure-actuated, valve-based dosing mechanisms, piezo stack actuators are the basis for a valveless technology employed in microdispensers. As nanoliter dosing is becoming more important in the manufacture of microarrays and lab-on-a-chip systems, nanoliter generators must be able to generate finer and finer droplets while taking into account the viscosity and surface tension of the media and the dosing speed. Misting, satellite formation on impact, or dripping must be reliably prevented.

PipeJet, a newly developed direct displacement technology by Biofluidix (biofluidix.com), uses low-voltage high-force piezo stacks for high-speed direct actuation. This precision-dispenser is employed in clinical diagnostics in lateral flow assays, which are test strips requiring a specific dosage of fluids in the volume range from a few nanoliters to several microliters per second.

The design is based on an elastic polymer tube fluid line with a well-defined internal diameter; there is no contact between the piezo material and the pump medium. In order to provide sufficient force for the precise dosing of difficult media, the PipeJet system employs a PI Ceramic multilayer PICMA piezo stack to precisely compress the polymer tube via a piston by controlling the amplitude of the piezo stack. The force generated is 100 times higher than previous methods based on ring-shaped piezo actuators.

Faster switching

Another application of piezo flexure drives can be found in the field of electrophysiology. Electrophysiological studies require fast motions of a double-bore (theta-glass) pipette across cell membranes. With a cell held in a patch clamp and monitored electrically, one bore of the pipette flows a buffer solution, while the other carries a drug or other chemical. By quickly sweeping the pipette so either bore suffuses the cell, one can correlate the cell’s electrophysiological response to the chemical environment. Piezo flexure mechanisms enable fast switching, as they do not produce electromagnetic noise and offer good repeatability, high reliability, and fast speeds that enable switching in milliseconds. In fact, one of the fastest mechanisms on the market, Siskiyou’s (siskiyou.com) MXPZT-300A, is based on a P-601 piezo flexure actuator.

Improved MRI

For medical and biotechnology applications, piezoelectric motors can overcome problems of traditional motors since they neither create electromagnetic interference, nor are influenced by it, eliminating the need for magnetic shielding. This feature is particularly important for motors used within strong magnetic fields, such as those seen in MRI equipment, where small piezo motors are used for MRI-monitored microsurgery. Magnetic fields and metal components make it practically impossible for conventional motors to function within MRI equipment while their non-magnetic piezoelectric counterparts encounter and cause no such disruptions.

MRI systems need to be tuned to the correct frequency to provide optimum results. Not only does the image quality of a properly tuned MRI scanner improve, the imaging speed is also positively affected by an optimized system, so both patients and service providers benefit. Tuning can be done manually, a slow process, or with the help of automated motion control components. Typically, in an automated tuning process a number of variable capacitors are adjusted by means of different types of motors. The latest generation of PiezoWalk linear motors is non-magnetic and unaffected by the high magnetic fields in MRI scanners, which is a break-through advantage in itself. Other benefits include easy controllability by a computer and a much smaller size compared to hydraulic or pneumatic motors, in addition to higher precision.

Fast focusing

Similarly, piezo actuation’s high speed and resolution makes it the ideal technology for especially responsive autofocus implementations such as those used in life-science microscopy and drug discovery research.

A recent trend has been towards longer and longer scan travels—even beyond the several-hundred-micron travels of which conventional flexure-amplified piezo mechanisms are capable. Long travel in the millimeter range is especially important for microscopy applications with large penetration depth, such as two-photon microscopy (TPM). This specific type of microscopy is valuable for non-invasive skin cancer diagnostics as well as to study transdermal drug delivery.

Ultrasonic piezo applications

Piezoceramic elements can be excited to very high frequency vibrations above 100 kHz and beyond the range of human hearing in the ultrasonic frequency spectrum. Operated in this domain, piezo ceramics can be used as transmitters and receivers to detect irregularities (e.g. air bubbles) in the delivery of fluids to patients, or for the generation of aerosols, a principle related to ultrasonic humidifiers.

Efficient aerosol generation is an important technique in the treatment of respiratory diseases often involving medications being administered directly with atomizers. One method of atomization is to generate very fine droplets with the aid of ultrasonic vibrations. Specially designed piezo disks excite a stainless steel diaphragm with thousands of precision holes to execute oscillations beyond 100 kilohertz. This results in particularly homogenous aerosols, allowing the medications to be dosed accurately and administered in a more targeted way while cutting the time required to atomize medications by up to 50% compared to conventional systems.

Bubble detection and flow measurement

Dialysis or blood transfusions are vital and highly sensitive processes. Air bubbles or impurities can have catastrophic effects on the patient and need to be detected quickly and reliably. Piezo ultrasound transmitters/detectors, which work without direct contact with the medium and require no maintenance, allow these compact transducers to measure flow velocity and detect air bubbles. They transmit and receive ultrasonic waves, similar to the sonar used in submarines, and because they are based on sound waves are not affected by the color or opacity of the medium. The sensors are mounted on the outside of flexible tubes and operate under sterile conditions with no danger of interfering with the flow rate or contamination.

In all of the above applications piezoceramic drive technology was chosen for one or more of its unique characteristics: compactness, millisecond responsiveness, nanometer precision, non-magnetic behavior, low power consumption and sterility. While electromagnetic devices still dominate today’s medical equipment designs and will never completely be replaced, the advantages of piezoceramics are challenging the physical limitations of the electromagnetic drive principle. Design engineers can now choose from a plethora of piezoelectric devices to efficiently solve a variety of motion problems that traditional technology either cannot satisfy at all or can do so only at an uneconomical expense.

Why piezo?

Piezoelectric actuation is the foundation for a multitude of mission-critical applications, from atomic force microscopy or single molecule manipulation within cells using optical tweezers, to high-resolution bio-imaging, genomic sequencing or aerosol generation, and drug delivery.

Medical design equipment and many related life-science disciplines have multifaceted requirements for motion drives, spanning a spectrum from high-precision positioning systems to reliable, low-cost, and compact drive components with reduced power consumption. Since progress goes hand in hand with continuous miniaturization and demand for higher speed and throughput, piezoceramic drives can satisfy many of these requirements. The piezoceramic drive technology is mature and reliable, having long been put to use in the semiconductor, laser machining, and automotive industries in a variety of 24/7 applications.

Originally developed for nanometer-precision motion, piezoelectric motor principles are proving highly adaptable to new configurations and modalities. Recently developed mechanisms have overcome the former travel limitations familiar from classical nanopositioning mechanisms.

A significant improvement over conventional electromagnetic drives, the latest piezo motors and actuators are more compact and deliver higher force and faster response. They also generate less heat, are maintenance-free and require no lubricants, a great advantage for sterile environments. Piezoceramic components can be sterilized at high temperatures and are fundamentally non-magnetic and non-magnetizable, making them compatible with the high magnetic fields in MRI scanners.