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A Laurin Publication
Photonic Solutions for Biotechnology and Medicine
Stage Positioning Provides
High-Speed Precision for 3-D Images
by Dr. William O’Brien, Mad City Labs Inc.
T
o acquire three-dimensional information from biological structures requires
moving the focus of the microscope vertically along the sample’s Z-axis. Gaining this information quickly and accurately is
important for time-resolved or
video-rate imaging or if the sample may photobleach or be damaged by a long exposure to excitation light. High-speed motion
and nanometer precision in the
Z-axis are necessary for these
types of studies.
Although various options are
Three-dimensional images like these of a pollen grain (a), cells labeled with GFP (b) and
available for rapid imaging of
mouse intestine require Z-positioning. Accomplishing this with a positioning stage can be
the X- and Y-planes, Z-axis data
faster than doing so by moving the objective. Courtesy of Molecular Expressions.
collection has typically involved
vertically moving the objective
ple. An objective lens can be brought up
lens, and it can take as long as 20 ms per
A common stage for this type of posito the sample through the bottom of the
step to achieve nanometer precision. For
tioning would include a guidance mechaperture, while the top surface of the samapplications where this is too time-conanism, an actuator and a position senple remains accessible to other probes.
suming, Z-axis information can be obsor. For example, a stage could use flexThe low profile is necessary for experitained by moving the sample with a
ure hinges for guidance. Flexure hinges
ments where it is important to keep
nanopositioning stage.
are unique in that they contain no conchanges in the optical path length to a
In general, moving the objective lens is
tacting parts that move relative to each
minimum. An example might be an exdifficult because of its relatively large
other. The stage moves when strain is
periment involving phase-sensitivity meamass and the limited amount of useful
placed on the hinge, and its movement
surement.
space available on most microscopes.
is 100 percent reversible, in most cases.
Although stages are available with reDealing with these limitations places sesponse times approaching 1 ms, end users
vere design constraints on the objective
Stage options
usually give up some range of motion in
lens translator. For instance, designers
In contrast, bearing-guided stages have
exchange for the higher speed. Typical
usually want to make a fast-moving commoving contact positions between the
range of motion, for instance, might be
ponent very stiff to maintain precision.
bearings and a hardened surface. The
25 µm. Stages can be built with a range
However, with microscopes, the objectranslation of the stage is not reversible
as great as 500 µm, but they do have a
tive lens is usually on a nose cone, and
at the nanometer level. Piezoelectric aco
slower response time.
space constraints may limit the amount
tuators, which change length under an
of stiffness that can be added to a transapplied voltage, supply the driving force,
lator design.
have high load capacities, fast response
References
Using a positioning stage to move only
time and subnanometer resolution. 1
1. B. Andersen (2001). Piezoelectric comthe sample allows the use of stiffer, faster
Feedback control is accomplished with
ponents in the telecom industry. Global
translators to boost positioning speed.
an integrated piezoresistive position senphotonics applications and technolMicroscopes also tend to have more
sor, which is usually necessary to provide
ogy. Business Briefings Ltd.
space available for a stage. Z-positionthe nanometer-level resolution required
2. W. O’Brien (July 2003). Piezoresistive
ing requires only one stage per microof higher-precision nanopositioning.2
sensors facilitate high-speed nanoposcope no matter what objective is used,
This type of stage exhibits a fast response
sitioning. PHOTONICS SPECTRA, pp.
but moving the objective lenses typically
time, often in the range of 3 ms for a step,
90-92.
requires a positioner for each one used.
with subnanometer resolution.
Thus, a system using Z-positioning costs
The typical Z-axis device also includes
Meet the author
less and is simpler in design; for instance,
a large aperture (2.6 3 2.6 in.) with a
William O’Brien is a member of the R&D
there are fewer cables linked to the conlow profile (0.8 in.). The large aperture is
group at Mad City Labs Inc. in Madison, Wis.;
e-mail: [email protected].
troller.
useful for accessing both sides of the samReprinted from the June 2004 issue of Biophotonics International © Laurin Publishing Co. Inc.