bio sphere
Microfluidic field-effect device for heparin Heparin and other glycosaminoglycans can be directly measured in minute quantities by the silicon field effect.
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cal opportunity more for the low-mothe AT-III surfaces measured fondaeparin, a complex, negatively lecular-weight heparins,” says Manalis. parinux and unfractionated heparin. charged molecule, is widely used Ganesh Venkataraman of Momenta Mark Meyerhoff at the University of as an anticoagulant in hospitals. To Pharmaceuticals says that the device Michigan explains that the AT-III funcdate, only indirect assays have been may be applicable to other heparinavailable to measure it. Now, Scott related molecules. For instance, Manalis and colleagues at MassaSensor chondroitin sulfate is thought to be chusetts Institute of Technology useful for studies of inflammation have developed a microfluidic siliand neuronal regeneration. But if con field-effect device that directly Electrode researchers want to analyze its bemeasures heparin or its low-molechavior in humans, they will need a ular-weight analogs, enoxaparin and way to quantify blood concentrafondaparinux, at a detection limit 50 µm tions of the molecule. “For simple, of 0.001 units/mL (Proc. Natl. Sensor small molecules, I can develop a Acad. Sci. U.S.A. 2006, 103, HPLC-based assay. But when it’s a 13,374–13,379). Control sensor channel complex mixture [like chondroitin The field effect is a change in Active sensor channel sulfate], I need this kind of novel silicon’s conductivity when highly technology to get a measure of how charged species get close and alter Optical micrograph shows an array of glass microflu2 much of the [molecule] is still in the material’s surface potential— idic channels, each containing two 50 50-µm silicon circulation,” he says. the magnitude of change can be surfaces and a gold signal electrode. Differential meaA clinical version of the device directly related to the amount of surements involve two channels, one with active surthat can deal with whole-blood charge. “The concept of the device faces and the other with surfaces passivated with samples is not quite at hand. Alis not novel,” Manalis explains. bovine serum albumin. (Adapted with permission. though the fabrication of the de“It’s been around for decades. In Copyright 2006 National Academy of Sciences, U.S.A.) vices was reproducible, the funcfact, it’s even been used in the past tionalization of surfaces wasn’t, and the to detect heparin.” But previous meationalization is important because the device always had to be precalibrated. surements were indirect and difficult enzyme is selective for a biologically acSample handling also was a problem: to perform. tive pentasaccharide domain in heparin serum-based measurements of heparin Manalis and colleagues created their and fondaparinux. “The five-unit bindworked well, but measurements in plasdevice so that it could directly measure ing site is critical because there are a lot ma were thwarted by fouling of the silismall amounts of heparin and its anaof companies trying to make drugs” con surface. Manalis recognizes that the logs solely on the basis of their large based on it, he says. surfaces need to be better engineered to negative charge. The device was comWell-established assays are used to be more reproducible and robust. posed of arrays of glass microfluidic measure heparin in blood; it must be The investigators are most excited channels with two 50 50-µm2 eleccarefully monitored in patients to preabout the next application—studying invent unchecked bleeding. In contrast, trolyte-insulator silicon surfaces embedconcentrations of enoxaparin and fonda- teractions between glycans on cell surded in each channel. parinux in blood can’t be measured by a faces and protein receptors. Current asThe investigators functionalized the says require micrograms of samples and simple, on-site assay. The lack of an silicon surfaces with a heparin antidote, are labor-intensive. However, with the assay usually isn’t critical because the protamine, or an enzyme, antithrombin field-effect device, the researchers can drugs generally don’t need to be meaIII (AT-III), and demonstrated that the create an array of surfaces functionalized sured in the majority of patients. But surfaces could detect clinically relevant with different proteins. Cells can be concentrations of heparin and its analogs. for the minority of patients in whom flowed through the array, and a fingerSuccessive layers of AT-III or protamine the drugs need to be measured, doctors print can be obtained of the cell-surface are forced to send samples to a centralcould be deposited onto the surfaces; glycans. Manalis says, “It would be ized clinical lab and wait a couple of this permitted multiple measurements quantitative, and you could use very days for the results. The silicon field-efwithout significant signal degradation. small volumes.” a fect device could be useful as an on-site The protamine-functionalized surfaces —Rajendrani Mukhopadhyay assay for these drugs. “We see the cliniwere selective for enoxaparin, whereas © 2006 AMERICAN CHEMICAL SOCIETY
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