Optofluidic microscope enables lensless imaging of microorganisms
Gwynne D. Koch
In biological research or clinical applications, optical imaging
typically is performed using large microscope systems with expensive optics. Now
researchers at California Institute of Technology in Pasadena have developed a device
that integrates optics and microfluidics on the same platform and offers resolution
comparable to that of conventional microscopes.
The instrument, called an optofluidic microscope,
does not require bulky optical elements for imaging. It consists of an illumination
source, a CCD sensor and a slanted linear array of apertures etched onto an opaque
metal film that is bonded to a microfluidic chip.
The scientists fabricated the aperture
array and microfluidic chip in two steps: First, using reactive ion etching, they
created a series of 600-nm-diameter holes spaced 5 μm apart on a layer of aluminum.
They then constructed a microfluidic structure using
photolithography techniques and transferred it onto a polydimethylsiloxane elastomer.
To image with the device, they flowed a sample
through the 30 x 15-μm microfluidic channel until it passed over the aperture
array, which was uniformly illuminated from above by a white LED source. Light transmitted
through the apertures was detected by a CCD sensor from Princeton Instruments. Each
hole scanned a line across the object as it was conveyed through the channel. The
composite of the line scans from all of the apertures generated a transmission image
of the object.
A transmission microscope image of an optofluidic microscope — a lensless imaging
device that integrates optics and microfluidics — shows the pattern of apertures
that are used to capture line scans of a sample as it flows through a microfluidic
To evaluate the performance of their
prototype, they flowed Caenorhabditis elegans through the channel and used
an inverted microscope from Olympus to relay the transmitted light through the aperture
array to the CCD; however, the inverted microscope
played no direct role in imaging.
They demonstrated that the device has an imaging
throughput rate of about 40 worms per minute and a resolution limit of ~490
nm. To achieve this resolution, the sample must be as close as possible to the aperture
A prototype device with 600-nm-diameter apertures captured images of C. elegans with comparable
resolution (bottom) to images acquired using a conventional microscope with a 403
Changhuei Yang, an assistant professor
at the institute and associate director of the Defense Advanced Research Projects
Agency’s center for optofluidic integration (which provided support for the
project), said that resolution limits as low as 100 nm are possible with smaller
aperture diameters. In comparison, conventional microscopes provide resolutions
from about 0.2 to 1 μm.
Optofluidic devices offer several advantages
over conventional microscopes for bioscience and clinical applications: They are
smaller, are less expensive, can be easily reconfigured to handle a variety
of samples and potentially can be used to perform high-resolution, high-throughput
Their simplicity and compactness facilitate
fabrication of multiple units on a single microfluidic chip. Using such devices
in parallel, biologists could increase imaging throughput of microorganisms for
phenotype characterization. The optofluidic microscopes also could be used to directly
image individual cells in suspension, enabling more accurate type-sorting in clinical
blood sample analyses.
The metal film can be deposited directly
onto a CCD or CMOS sensor to create compact, portable imaging devices. Direct imaging
of cells achieved with a portable version may aid definitive diagnoses of parasitic
infections such as malaria, which is currently diagnosed by examining blood samples
for infected cells using a 100x immersion objective on a conventional microscope.
The main limitation of the optofluidic microscope is that the sample
effectively must be static during the imaging process. According to Yang, there
are two ways to overcome this limitation.
“For nonmotile objects, such as blood cells,
we simply need to control the flow well enough so that the cells
don’t tumble or roll. Electro-osmotic-driven flow should help because it generates
a flat flow front.” In addition, if the imaging time is sufficiently short,
motile objects will not have time to change shape or orientation significantly.
In the experiment, flow was actuated by gravitational
pull — tilting the chip caused the C. elegans to slide down the microfluidic
channel. The researchers are investigating other mechanisms to induce flow, including
pressure, electro-osmosis and dielectrophoresis. They also are working on incorporating
all components on a single platform, reducing the aperture diameter to improve resolution
and developing versions for fluorescence and differential interference contrast
Lab on a Chip, online edition, Aug. 4, 2006, doi: 10.1039/b604676b.
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