Leapfrogging Microfluidics?

“Microfluidics” is one of the hottest buzzwords in biotechnology and diagnostics research these days, with good reason: these lab-on-a-chip devices are about the coolest technology to come along since monoclonal antibodies. The designs vary widely, but the basic principle is to take traditional lab assays and miniaturize them onto silicone or plastic chips, often using manufacturing techniques developed by the semiconductor industry. I’ve blogged about these nifty devices [once][1] or twice (okay, maybe three times) before.

While I’ve found the technology fascinating to watch, it’s remained a bit of a laboratory curiosity. Everyone seems to agree that the “killer app” for microfluidics will be field-portable devices that that will let minimally-trained people diagnose diseases or detect specific compounds in the environment, especially in poor countries. That’s because chip-based labs can incorporate all of their equipment and reagents onto a disposable device no larger than a credit card. In principle, a technician could place a drop of fluid, such as blood, onto one end of the chip, and micrometer-size channels would siphon it around, mixing it and moving it to different chambers to perform assays that would normally require a fully-equipped lab. The volumes are so small that the reactions tend to occur very quickly, often shortening multi-hour tests to a few minutes. But that’s where the good news ends.

Because the reactions produce subtle chemical changes inside minuscule containers, detecting the result usually requires sophisticated analytical equipment, at which point we’re right back to building a full-size laboratory. No matter how cheap or portable the chips get, the assay readout has generally remained huge and pricey.

Until now. Two recent papers highlight what I think is a new stage in the development of microfluidics, where researchers are finally addressing the readout problem. In one effort, scientists at Columbia University report on a microfluidic clinical testing system that incorporates a whole slew of new ideas. More importantly, it actually seems to work in the field. As senior investigator Samuel Sia says in an accompanying press release:

“We have engineered a disposable credit card-sized device that can produce blood-based diagnostic results in minutes,” said Sia. “The idea is to make a large class of diagnostic tests accessible to patients in any setting in the world, rather than forcing them to go to a clinic to draw blood and then wait days for their results.”

Sia’s lab at Columbia Engineering has developed the mChip devices in collaboration with Claros Diagnostics Inc., a venture capital-backed startup that Sia co-founded in 2004. The microchip inside the device is formed through injection molding and holds miniature forms of test tubes and chemicals; the cost of the chip is about $1 and the entire instrument about $100.

The injection-molding process is a departure from most microfluidic construction methods. Instead of engraving the device onto a silicon chip, the researchers made a mold and cast duplicates in a mass-production system. Injection molding gives us plastic cups and soda bottles, so it’s clearly a mature technology that can be scaled way, way up. That’s what drives the per-chip cost down so low.

The investigators used this cheap chip to build a miniaturized ELISA, or enzyme-linked immunosorbent assay, platform. If you’ve ever been tested for any infectious disease, you’ve probably had an ELISA; it’s one of the most common and important assays in clinical diagnosis. To perform it, one needs to incubate a sample with a reagent that will bind some analyte – let’s say an antigen that will bind antibodies against HIV in a patient’s blood. Once the analyte binds, a series of washes and secondary reagents clears up background reactions and causes some kind of easily-detected chemical change. It typically takes a skilled lab technician a few hours to perform an ELISA, and it requires careful attention to detail through the various washing and incubation steps.

On the new chips, a simple channel meanders through the plastic. The binding reagent is stuck to one section of the channel, and Sia and his colleauges feed the blood sample, wash solutions, and other reagents through the tube sequentially. To separate the reagents, they simply added tiny bubbles between them, like you might see in a very small straw that’s reached the bottom of the glass. A common medical syringe provides the vacuum force to draw the whole train of reagents through the system.

Completing this tour de force of clever ideas, the team used a nanoparticle-based detection system that deposits visible quantities of silver in the channel if there’s been a reaction. A cheap absorbance meter quantifies the amount of silver, and determines whether a test is positive or negative. The researchers walk through the system’s advantages in a nicely-produced video interview Nature Medicine released to accompany the piece:

As you’ll see in the video, the researchers also put their system to the ultimate test, hauling it to Rwanda and testing actual patient samples in an underfunded, overworked clinic. The results were impressive: the new assay is about as accurate as traditional ELISA tests for detecting HIV and syphilis, but much faster and cheaper.

This makes me wonder whether we’re about to see another example of leapfrogging in poor countries. The most popular (and really only) current example of this phenomenon is cellular phones. There are virtually no landline connections in most poor countries, but nearly everyone has a phone. By missing the first telecommunication revolution, these countries have “leapfrogged” to the second, gaining all of the advantages of instant communication without going through the intermediate stages of rural electrification, Ma Bell, party lines, and rotary dials. If microfluidic devices can bring modern medical tests to the bedside in Rwanda, will we see them and other poor countries catapulting into 21st century medicine without having to establish 20th (or even 19th) century medical infrastructure first? Maybe.

What makes me optimistic about this is that Sia and his colleagues aren’t the only ones working on this problem. Indeed, around the time their paper came out, a less-noticed but equally interesting bit of work came out in the journal Analytical Chemistry. In that paper, Aydogan Ozcan and his colleagues at UCLA and elsewhere describe a system for performing flow cytometry on a microfluidic device. Flow cytometry, or cell sorting, is a sort of ELISA on speed. Rather than incubate the bulk sample with the reagent, cell sorters separate individual cells into a stream of droplets, like one would get by shaking a running garden hose. The droplets pass through a detector that measures specific parameters of the cell, such as its light diffraction characteristics or whether it bound a fluorescently labeled antibody. Researchers can then quantify exactly how many cells of each type were in a sample.

It’s a tremendously powerful technique for immunological research, and can also be used to perform a variety of blood-counting assays, but it requires even more skill and money than an ELISA. Research-grade cell sorters are massive machines that usually occupy a small room of their own and employ a dedicated technician.

Ozcan’s team decided to use a cell phone instead. With about $5 worth of parts, they cobbled together an adapter that connects an inexpensive microfluidic cell sorter to the camera on a Sony-Ericsson phone. As they explain in an accompanying press release:

The microfluidic assembly is placed just above a separate, inexpensive lens that is put in contact with the cell phone’s existing camera unit. This way, the entire cross-section of the microfluidic device can be mapped onto the phone’s CMOS sensor-chip. The sample fluid is delivered continuously through a disposable microfluidic channel via a syringe pump.

The device is illuminated from the side by the LEDs using a simple butt-coupling technique. The excitation light is then guided within the cross-section of the device, uniformly exciting the specimens in the imaging fluid. The optofluidic pumping scheme also allows for the use of an inexpensive plastic absorption filter to create the dark-field background needed for fluorescent imaging. In addition, video post-processing and contour-detection and tracking algorithms are used to count and label the cells or particles passing through the microfluidic chip.

While they haven’t taken it into a poor country’s clinics yet, the investigators did put the system through its paces in the lab. So far, they’ve demonstrated that it can measure white blood cell density as a cell sorter, and also operate as a mid-power fluorescent microscope. The former capability could provide tests for leukemia and AIDS progression, while the latter could be useful for a variety of analyses, including detecting pathogens in drinking water.

It’s going to take more than a couple of new testing systems to fix the health problems of poor countries, but papers like these – and I suspect others will follow shortly – show at least part of the solution. Perhaps these technologies will even make their way back to the developed world, as we seem to have some of our own issues with medical costs these days.

References:

ResearchBlogging.org

Chin, C., Laksanasopin, T., Cheung, Y., Steinmiller, D., Linder, V., Parsa, H., Wang, J., Moore, H., Rouse, R., Umviligihozo, G., Karita, E., Mwambarangwe, L., Braunstein, S., van de Wijgert, J., Sahabo, R., Justman, J., El-Sadr, W., & Sia, S. (2011). Microfluidics-based diagnostics of infectious diseases in the developing world Nature Medicine DOI: 10.1038/nm.2408

Seo, S., Isikman, S., Sencan, I., Mudanyali, O., Su, T., Bishara, W., Erlinger, A., & Ozcan, A. (2010). High-Throughput Lens-Free Blood Analysis on a Chip Analytical Chemistry, 82 (11), 4621-4627 DOI: 10.1021/ac1007915

[1]:
http://alandove.com/content/2010/02/sewing-a-lab-chip/

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