Digital Design: Engineers used AutoCAD to
design the reaction chamber for the CD4 monitor prototype. Medical
personnel pump a few drops of blood through an orange inlet tube into the
microchip. A metal holder with a screw cap allows for assembly and
disassembly of the unit.
Newton, MA—Two boulders block the road to fighting AIDS in poor countries: the high cost of drugs and figuring out who needs the medicine. Thanks to the work of activists and the emergence of generic drug companies, drug costs have plummeted over the last two years. And now a simple-to-use CD4 T-cell monitor may provide a portable, plausible way to identify when to begin treatment.
Working on shoestring funds, William Rodriguez, an AIDS researcher at Massachusetts General Hospital and instructor at Harvard Medical School, and John McDevitt, a chemist at the University of Texas in Austin, have created a prototype for a CD4 monitor that eventually will fit into a shirt pocket.
CD4 cells are the lymphocytes, or white blood cells, that orchestrate the body's response to invading micro-organisms such as viruses. Often called the generals of the immune system, their levels show when a person with AIDS needs to begin therapy. They also gauge how the patient is responding to medicines.
In developing countries, there are some six million people with AIDS in need of life-sustaining antiretroviral drug therapy. Only 230,000 of those have access, according to World Health Organization estimates. But even as patients struggle to deal with the "reduced" cost of drugs, adequate measuring of CD4 counts has been limited to countries that can afford it.
Flow cytometers, the gold standard for measuring CD4 levels, cost an average of $100,000. Beyond the cost is another concern: Electricity needed to run the machines doesn't exist in many remote corners of the world. And keeping the temperamental giants in operation is an additional expense. The size of a refrigerator, they break down regularly and maintenance contracts cost another $15,000 a year. Operating them is the job of trained lab assistants, who need to prepare and centrifuge blood samples.
In West Africa, where the population is 150 million and AIDS has reached epidemic proportions, there are only four flow cytometers, and these are located at health clinics far from most villages. "What they need is something that is portable, idiot-proof, and electricity-independent," says Rodriguez. "It's as desperately needed as the drugs are."
The pocket-sized CD4 monitor may be the answer. It does not require blood to be pre-processed, and everything that is normally done in the lab takes place on a tiny microchip that costs about $5.
Lab-on-a-chip technology has been around for several years. The same principles used to develop miniature electronic circuits have shrunk the size of a chemistry lab to a device the size of a dime. The chip looks more like a microscope slide, with hair-sized pathways and tiny reservoirs carved into it using standard microfabrication techniques.
The beauty of the CD4 monitor is that it requires little skill to operate. A health worker with 15 minutes of training can administer the test, according to Rodriguez. The nut of the prototype CD4 monitor is a stainless steel reaction chamber designed in AutoCAD. It contains the microchip and has an inlet and outlet tube with valves.
A few drops of blood, placed in the inlet tube, are pumped into the microchip using a microprofusion system. A tiny polycarbonate microfiltration membrane filters out the red blood cells and captures lymphocytes on the top of the chip, where the experiment takes place. As the T-cells move through the channels on the chip, they bind with antibodies that are coupled to light-emitting fluorophores. The process is similar to that used by flow cytometers.
A high-intensity mercury pressure lamp excites the fluorophores so that each T-cell emits a different color. CD4 cells light up yellow and CD8 cells light up red. A charged couple device, similar to a digital camera, positioned at the end of a microscopic lens, snaps a picture of the cells. The image resembles a field of colored stars. An automated algorithm reads and analyzes the pixels in the image and gives quick results on the ratio of CD4 to CD8 cells. The whole operation takes place in about 10 minutes.
"It's really not that technically sophisticated," admits Rodriguez. "All we're doing is miniaturizing everything."
Future prototypes will use built-in lasers and photodiodes, fabricated out of silicon chips that would easily fit into a pencil eraser, to analyze the fluorescent glow given off by the sample. Also in the works are plans for a molded plastic chip to replace the glass one. Plastic chips cost only pennies to produce and may become part of a disposable cartridge that will contain the bio waste.
Securing funding for the project was surprisingly difficult, says Rodriguez. Although many groups were excited about the idea, getting them to pitch in was another matter. McDevitt was able to obtain funds from the military to support the chip's use in Anthrax testing. And the National Institute of Health later gave $300,000 to show the technology could be applied to HIV testing.
Rodriguez and McDevitt had hoped for larger scale support. "Bill and I always laugh at how much we've done for this much money," says McDevitt in retrospect. "But that's how it is sometimes. You have to start out modestly."
It wasn't until after Rodriguez spoke about the prototype CD4 monitor in February at the 10th Conference on Retroviruses and Opportunistic Infections in Boston that the phone started to ring, and foundations like Bill & Melinda Gates, Doris Duke Charitable, and Rockefeller showed an interest.
With additional funding in the pipeline, the team's goal is to continue working through different prototypes and various manufacturability issues. "The prototype we have now has everything housed in separate components," says Rodriguez. "We need to put the lamp, the lens, the microfluidics, and the microchip chamber together into one small box in a way that makes sense for mass production."
It may be several years before the device is approved by the Food and Drug Administration (FDA) for use in the United States, but its potential impact in the treatment of AIDS in third world countries could be monumental.