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06 Aug 2008
A low-cost and high-resolution microscope has for the first time been engineered to fit onto a single chip.
Scientists in Switzerland and the US have built the first on-chip microscope that they claim will provide clinicians with a rugged and high-resolution instrument that can be carried around in a pocket. The system disposes of bulky lenses in favour of a CMOS sensor combined with a microfluidic channel for a highly compact design (Proceedings of the National Academy of Sciences 105 10670).
"Our device combines the ability of microfluidics to easily transport cells in suspension, with the ability of optics to perform sensitive detection," Changhuei Yang, assistant professor at the California Institute of Technology, US, told optics.org. "So far, we have demonstrated a resolution of 800 nm, but we believe that this could be further improved to around 50 nm."
According to Yang, the simple design means that tens or even hundreds of these microscopes can be built on a single chip and operate in parallel to speed up the imaging process at a cost per chip of around $10. This portable and cheap device is particularly appealing for third-world applications where it could be used in the field to analyze blood samples for malaria or check water supplies for pathogens.
Since the lenses used in current microscopes are difficult and expensive to miniaturize, the Caltech team came up with a different approach.
The device is based on optofluidic microscopy (OFM) and uses a microfluidic flow to deliver specimens across an array of micrometer-size apertures etched onto a metal-coated CMOS sensor.
"By covering the sensor grid with a thin metal layer and etching small apertures onto the layer at the centre of each pixel, the sensor pixel will be sensitive only to light transmitted through the aperture," explained Yang. "This means that the resolution is determined only by the aperture size and not by the pixel size of the CMOS sensor."
In the setup, a voltage of 25 V is applied across the inlet and outlet of a microfluidic channel measuring 2.4 mm in length, 40 µm in width and 13 µm in height. The electric field draws the specimen across the aperture array in a controlled manner. The array consists of 120 holes of diameter 0.5 µm and separation of 10.4 µm fabricated on a 2D CMOS imaging sensor. The CMOS sensor comprises a grid lattice of 1280 × 1024 square pixels with a pixel size of 5.2 µm.
The grid is tilted at a small angle to create a diagonal line with respect to the flow direction causing the images to overlap slightly. All of the images are then pieced together to create a precise two-dimensional picture of the object with a resolution of 800 nm.
Yang is now working on translating the technology into a commercially available product and is optimistic that the resolution of the device could be improved. "In principle, we can push the resolution down past the diffraction limit to around 50 nm as the resolution is limited only by the size of the holes that we can punch," he concluded.
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