Lensless Fluorescent Microscope

FlatScope captures three-dimensional data that passes through a mask and to a fingernail-sized camera chip. It sends that data to a computer that processes it back into an image. The camera may be used as an implantable endoscope, a large-area imager or a flexible microscope. (Source: J. Fitlow)

Lenses are no longer necessary for some micro­scopes, according to Rice Univer­sity engineers deve­loping FlatScope, a thin fluore­scent micro­scope whose abilities promise to surpass those of old-school devices. Rice engineers Ashok Veerara­ghavan, Jacob Robinson, Richard Baraniuk and colleagues describe a wide-field micro­scope thinner than a credit card, small enough to sit on a fingertip and capable of micro­meter reso­lution over a volume of several cubic milli­meters. Flat­Scope eli­minates the tradeoff that hinders tradi­tional micro­scopes in which arrays of lenses can either gather less light from a large field of view or gather more light from a smaller field.

The team began deve­loping the device as part of a federal ini­tiative by the Defense Advanced Research Projects Agency as an implan­table, high-resolution neural interface. But the device’s potential is much greater. The researchers claim FlatScope, an advance on the labs’ earlier FlatCam, could be used as an implan­table endo­scope, a large-area imager or a flexible micro­scope. “We think of this as amping up FlatCam so it can solve even bigger problems,” Baraniuk said.

Tradi­tional fluorescent micro­scopes are essential tools in biology. They pick up fluorescent signals from particles inserted into cells and tissues that are illuminated with specific wave­lengths of light. The technique allows scientists to probe and track biolo­gical agents with nanometer-scale reso­lution. But like all tradi­tional micro­scopes, tele­scopes and cameras, their reso­lution depends on the size of their lenses, which can be large and heavy and limit their use in bio­logical appli­cations.

The team takes a different approach. It uses the same charge-coupled device (CCD) chips found in all electronic cameras to capture incoming light, but the compa­risons stop there. Like the FlatCam project that inspired it, FlatScope’s field of view equals the size of the CCD sensor, which can be as large or as small as required. It’s flat because it replaces the array of lenses in a tradi­tional micro­scope with a custom ampli­tude mask. This mask, which resembles a bar code, sits directly in front of the CCD. Light that comes through the mask and hits the sensor becomes data that a computer program inter­prets to produce images.

The algorithm can focus on any part of the three-dimen­sional data the scope captures and produce images of objects smaller than a micron anywhere in the field. That resolution is what makes the device a micro­scope, Robinson said. “A camera in your mobile phone or DSLR typically gets on the order of 100-micron reso­lution,” he said. “When you take a macro photo, the resolution is about 20 to 50 microns. I think of a micro­scope as something that allows you to image things on the micron scale,” he said. “That means things that are smaller than the diameter of a human hair, like cells, parts of cells or the fine structure of fibers.”

Achieving that resolution required modi­fications to the FlatCam mask to further cut the amount of light that reaches the sensor as well as a rewrite of their software, Robinson said. “It wasn’t as trivial as simply applying the FlatCam algo­rithm to the same tech­niques we used to image things that are far away,” he said. The mask is akin to the aperture in a lensed camera that focuses light onto the sensor, but it’s only a few hundred micro­meters from the sensor and allows only a fraction of the available light to get through, limiting the amount of data to simplify proces­sing.

“In the case of a mega­pixel camera, that compu­tational problem requires a matrix of a million times a million elements,” Robinson said. “It’s an incre­dibly big matrix. But because we break it down through this pattern of rows and columns, our matrix is just 1 million elements.” That cuts the data for each snapshot from six tera­bytes to a more practical 21 mega­bytes, which trans­lates to short processing times. From early versions of FlatCam that required an hour or more to process an image, FlatScope captures 30 frames of 3-D data per second.

Veerara­ghavan said the burgeoning internet of things may provide many appli­cations for flat cameras and micro­scopes. That in turn would drive down costs. “One of the big advan­tages of this tech­nology compared with tradi­tional cameras is that because we don’t need lenses, we don’t need postfabri­cation assembly,” he said. “We can imagine this rolling off a fabri­cation line.” But their primary targets are medical uses, from implan­table scopes for the clinic to palm-sized micro­scopes for the battle­field. “To be able to carry a micro­scope in your pocket is a neat tech­nology, Veerara­ghavan said.

The researchers noted that while their current work is focused on fluore­scent appli­cations, FlatScope could also be used for bright-field, dark-field and reflected-light micro­scopy. They suggested an array of FlatScopes on a flexible background could be used to match the contours of a target. (Source: Rice U.)

Reference: J. K. Adams et al.: Single-frame 3D fluorescence microscopy with ultraminiature lensless FlatScope, Sci. Adv. 3, e1701548 (2018); DOI: 10.1126/sciadv.1701548

Link: Nanophotonic Computational Imaging and Sensing Laboratory, Rice University, Houston, USA

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