Freezing Time in 3-D

Dennis P. Doherty, Prior Scientific Inc.

In microscopy, researchers strive for better resolution and faster data processing. Piezoelectric positioning devices, which have been used in some form for years, can help them meet these needs.

When using a piezoelectric Z-axis positioning device, an electric field is applied to a specifically engineered crystalline structure. The structure changes size, providing a force that is converted into motion. Piezo devices by nature can “push” well but have some issues “pulling.” To get around this problem, a preloaded mechanism is used within the positioning device to oppose the drive direction. This mechanism can use gravity, a spring or a material with similar properties to work as a counterforce against the push direction to accurately control the Z-axis position.

Specifically designed for researchers using deconvolution and 3-D imaging, this nanopositioning piezoelectric Z-axis stage features 100-μm travel, 1-nm repeatability and closed-loop control using a subangstrom-resolution piezoresistive sensor.

Some systems can achieve near perfection with this symbiotic push-pull relationship, although others experience a bit of “ringing” where there is some bounce and settle time between moves. Design of the piezoelectric device is critical; it must be engineered with different sample types in mind to limit or eliminate ringing and to provide smooth, accurate movement.

For example, to maximize speed and efficiency, a stage for use with microtiter plates would be different from a stage used for 35-mm petri dishes or single slides because travel requirements and sample weights are different. Engineering harmony in movement between the piezo forces and the preloaded counterforces, plus incorporating the various sample weights and keeping the end result of motion perpendicular to the optical axis are key. Also, the more travel required, the thicker the stage becomes, which can be an issue, as the correct height position of the sample must be within the focal range of the optical system.

Piezoelectric Z-positioning technology can achieve nanometer-scale movement resolutions, repeatabilities and accuracies. However, because microscopes commonly use flexure-motion stages, or similar preloaded devices, maximum loads are usually small and travel ranges limited to hundreds of microns.

Regulating speed

The speed at which movement takes place also determines how well piezo technology works. For example, positioning at slow speeds can be a bit jumpy because the motion can be seen as a tic of sorts, depending on how it is controlled. Samples that move quickly, however, have linear velocities that exceed standard stepper- or servo-style Z drives.

A piezo Z-axis positioner can offer fast movement, but it is only part of the system. The way it is controlled is critical. Positioners that use a data-acquisition card, or some other device with similar direct input/output capability, are extremely fast because they directly control output voltage. Those that use serial or USB control often experience lower speeds because there is a lag in communication times.

Sample-manipulation in microscopy typically uses piezo technology because its speed can keep up with the speeds of other devices, such as cameras and PCs, which have improved so much. Another advantage is that, as opposed to objective movers, sample movers don’t alter the optical path of the microscope, and any objective can be used.

Piezo-based devices are becoming more popular in some common biological applications. Simply by offering more speed of movement, they can increase the speed of high-throughput devices. Saving hundreds or more milliseconds per scan when one is scanning thousands of areas of interest each day can add up to hours of throughput productivity at the end of the week. Autofocus routines that could take a few seconds can be tweaked to run in a matter of several hundred milliseconds.

Also, a Z-axis stepper can be useful for taking images from a series of planes to construct 3-D models of a sample. For this application, piezo-based positioners have a real advantage over standard Z-axis stepper or servo setups because they can move extremely fast. Using a piezoelectric Z-stage, a high-speed camera and a computer with extensive processing capabilities, one can acquire data in close to real time as a rapid succession of Z planes are imaged.

Once these images are stored and compiled, time can be made to nearly stand still in 3-D. The ability to view the beginning or end segment — or the entire duration — of a high-speed biological reaction, or to pinpoint a particular sample of morphology, can provide remarkable insight and data that was previously unattainable. Missing one second of processing time can mean missing a critical piece of data.

As PCs and cameras get faster and processing becomes easier, rapid Z-axis positioning via piezo drives will be useful in an increasing number of applications.

Meet the author

Dennis P. Doherty is sales manager for Prior Scientific Inc. of Rockland, Mass.; e-mail: ddoherty@prior.com.