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October 29, 2013

Super-Thin Membranes Shrink Lab-on-a-Chip's Power Supply

  • Despite its promise, the lab-on-a-chip has been missing an essential component: a miniature pump—or, rather, a miniature pump with a suitably diminutive power supply. Typical electroosmotic pumps (EOPs), which impel fluids through porous media in the presence of an electric field, are readily miniaturized, but they require bulky power sources, defeating the whole concept of portable microfluidics.

    Now researchers have developed a silicon-based platform that integrates low-voltage, on-chip electroosmotic pumps into portable microfluidic devices. The researchers’ secret? They used a super-thin silicon membrane to drastically shrink the power source. This approach may lead to diagnostic devices the size of a credit card, realizing the lab-on-a-chip concept.

    The researchers, based at the University of Rochester, published their latest findings October 28 in the online version of the Proceedings of the National Academy of Sciences. The researchers’ paper, entitled “High-performance, low-voltage electroosmotic pumps with molecularly thin silicon nanomembranes,” describes how the membranes presented small resistances and permitted high electric fields across their tiny spans.

    The researchers were led by James L. McGrath, Ph.D., associate professor of biomedical engineering, director of the Nanomembrane Research Group, and cofounder of SiMPore, a firm that develops and commercializes membrane technology. In their paper, McGrath and his coauthors write: “We have developed EOPs fabricated from 15 nm-thick porous nanocrystalline silicon (pnc-Si) membranes. Ultrathin pnc-Si membranes enable high electroosmotic flow per unit voltage.”

    In the prototype EOP described by the authors, the use of pnc-Simembranes and Ag/AgCl electrodes was shown to pump microliter per minute-range flow through a 0.5 mm-diameter capillary tubing with as low as 250 mV of applied voltage. “Up until now, electroosmotic pumps have had to operate at a very high voltage—about 10 kilovolts,” said Dr. McGrath. “Our device works in the range of one-quarter of a volt, which means it can be integrated into devices and powered with small batteries.”

    Besides achieving high flow rates, the authors realized their techniques offered them a high degree of control. “Pore distributions of pnc-Si membranes can be imaged via TEM,” write the authors, “and we show using Rice-Whitehead theory that flow rates can be predicted for a given membrane.” In addition, the authors note that they could modify surfaces through oxidation and silanization techniques to change the zeta potential of the material, and thereby adjust electroosmotic flow rates.

    Reflecting on the significance of his team’s results, Dr. McGrath said, “Up until now, not everything associated with miniature pumps was miniaturized. Our device opens the door for a tremendous number of applications.”

    According to the McGrath lab’s website, the ultrathin membranes are also transparent and fully biocompatible, so that cells of different types can be grown on either side of the membrane to remain separated by a molecularly thin, porous layer. By developing the membrane material as a cell culture substrate, the lab intends to help biomedical scientists and developmental biologists address long-standing questions about short-distance cell-cell communication.

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