"We can take point-of-care to the next level," says Changhuei Yang, assistant professor of bioengineering and electrical engineering and a co-inventor of the mini-microscope. "The microscope design is really simple because we wanted it to have widespread use. You don’t even need a light source. You could use sunlight."Coupled with computer software, the optofluidic microscope (OFM) could become the heart of an in-home monitoring test to track immune cells in HIV-AIDS patients. When treating HIV-AIDS, doctors watch for changes in CD4 cells, the immune cells impaired by HIV. If the CD4 cell count drops below 200, patients are at risk for new infections. In-home monitoring would serve as an early alert for important changes in the patient’s condition and would reduce the number of patient office visits. Progressive tracking of CD4 cell counts would allow doctors to adjust therapies sooner to keep patients in optimal condition.
When integrated into a cell phone, the OFM also could have a big impact in remote locations around the globe where lack of electricity and unreliable equipment make diagnosing diseases problematic. "This is an enabling technology," says Charles DiMarzio, associate professor of electrical and computer engineering at Northeastern University (NEU) and a pioneer in biomedical imaging techniques. "In places where you don’t have access to pathology labs, this device could be used to transmit information to pathologists around the world."
"Look Ma, No Lenses"
It took just five years for Yang and former Caltech colleague Demetri Psaltis, now dean of engineering at the Ecole Polytechnique Federale de Lausanne, to turn the miniature microscope into reality. In that time, they had to completely rethink optical microscopy. Conventional optical microscopes rely on a series of lenses and a light source to image a sample. Miniature lenses for a mini-microscope are expensive and difficult to make.
Breaking with tradition, Yang and Psaltis considered how to make a lensless microscope. They didn’t need to go far for an answer. The concept for the OFM was staring them in the eye – literally. Floaters, the specks that pass across your eyes and are especially visible when you look at a blue sky, are seen because light causes them to cast shadows directly onto the retina (the image-forming layer of the eye). No lenses are needed to create the floater images.
Turning the concept of direct imaging into reality was assisted by two factors: (1) the field of microfluidics, in which tiny tubes and tunnels guide and measure various liquids, had matured, and (2) the price of image sensor chips had plummeted. "It was the right time for this because microfluidics is now a relatively easy technology to master, and sensors are about $10 to $20 a chip," says Yang. Microfluidics allows researchers to examine specimens, such as blood cells, in their native fluids and eliminates the need to stain samples. Low-cost image sensor chips help keep the cost of the OFM low (less than $50) and provide high image resolution.
To build the OFM, Yang began with an image sensor like those used in digital cameras. Sitting right on top of the sensor is a base layer made of metal and dotted with a diagonal line of holes from end to end. A transparent polymer protects the base layer. Above the base layer is a microfluidic chip that contains a channel through which specimens can flow. Specimens are injected into the microscope chamber through a syringe.
White light, of intensity comparable to sunlight, illuminates the sample from above as it passes through the microscope chamber. As the sample floats over the diagonal line, it interrupts light transmission through the holes. The time-varying transmission signal changes are picked up by the sensor and transmitted directly to a computer. A simple program then creates an image of the sample based on the information.
One drawback of the initial OFM design was its inability to image nearly transparent cell components, called organelles, within the cell. Microscopes with five-figure price tags use a technique called phase imaging to image these tiny parts. To address this challenge and enable the tiny microscope to perform phase imaging, Yang and his group are replacing single holes on the base layer with clusters of holes and tweaking the software so that it assists with focusing.
The team demonstrated the power of the OFM for biological research in a recent experiment in which they imaged a round worm, Caenorhabditis elegans, as well as algae, pollen spores, and plastic microspheres. The OFM images were comparable to those of a conventional microscope.
Mass Producing the Mini
To reach a broad user group, Yang had to find a fast, high-yield way to manufacture the OFM. Rather than have graduate students handcraft the microscopes & ndash; a two-day process – the lab has partnered with a semiconductor manufacturer to mass produce the device. Yang anticipates shipments will be ready for evaluation by clinicians and biologists during the second half of this year.
In the future, an array of laboratory applications, from white blood cell counts to cancer cell counting and drug evaluation, may get a boost from the OFM. By connecting hundreds of OFMs in compact packages, it may be possible to process 10,000 or more cells at a time. Within the next decade Yang hopes to create a device that could be implanted in the body to track various blood components. This project will require development of biocompatible materials as well as long-lasting power sources.
"A little microscope like this connected to a lot of computing power opens up a lot of possibilities," says NEU’s DiMarzio. "This won’t replace all microscopes, but it will find a wonderful niche."
This work is funded in part by the National Institute of Biomedical Imaging and Bioengineering.
Cui X, Lee LM, Heng X, Zhong W, Sternberg PW, Psaltis D, Yang C. Lensless high-resolution on-chip optofluidic microscopes for Caenorhabditis elegans and cell imaging. PNAS 2008 Aug 5; 105:10670–5.