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MEETING NEWS Rajendrani Mukhopadhyay reports from the Biomedical
Engineering Society 2005 Annual Fall
Meeting—Baltimore, Md. Obtaining trustworthy data
452 A
250-nm volume of aqueous solution that lies above the surface and creates shear stress in the fluid. The shear stress shakes off molecules weakly bound to the sensing element or to the surface. “We took a device very commonly used (a)
(b)
Antigen Antibody Protein G
Antigen Antibody Protein G Parylene
Parylene Gold Sensing area
Quartz
Gold Sensing area
Quartz
(a) Nonspecifically adsorbed proteins are removed from the device’s sensing region by resonating the quartz crystal to provide (b) more accurate measurements.
as a gravimetric biosensor, and we just [increased] the amplitude [to generate] a shear stress in the fluid,” explains Meyer. So, when an immunoassay is run in the device, only an antigen that is tightly bound to the antibody remains in the sensing region long enough to be detected. Preliminary data suggest that the sensing molecules attached to the gold squares aren’t adversely affected by the shear stress in the fluid. “People will certainly ask, ‘With the shear stress, are you doing any damage to the proteins?’ Our initial experiments have demonstrated that the proteins maintain activity even at high-amplitude drives,” says Meyer. “As this progresses, we have to [do more] rigorous characterization to ensure that we’re not doing any damage to any of the proteins.” The investigators used atomic force
A N A LY T I C A L C H E M I S T R Y / D E C E M B E R 1 , 2 0 0 5
COURTESY OF GRANT MEYER
An assay serves no purpose if the data it churns out are unreliable. That’s why Grant Meyer, a graduate student in Harold Craighead’s group at Cornell University, and Darren Branch at the Sandia National Laboratories in Albuquerque, N.M., are developing a microfluidic device that can eliminate false-positive and -negative signals in bioassays. “You want the most sensitive device possible, but you have to make absolutely sure that the information you’re getting is valid,” states Meyer. Two common sources of complications in measurements are surface fouling and nonspecific binding of extraneous molecules to the sensing region. Surface fouling occurs when molecules in the sample or reagents adsorb and clutter the walls and sensing regions of a device. Nonspecific binding arises when miscellaneous molecules bind to the sensing entity. For example, antibodies, the sensing element in immunoassays, are often selected because they have high affinities for their corresponding antigens. However, antibodies can also weakly interact with other molecules. These low-affinity, nonspecific interactions can lead to false signals in the data. The device Meyer and colleagues are developing is based on a quartz crystal that usually is used in mass sensing. In their instrument, gold is evaporated onto the quartz and then coated with parylene, a polymer that resembles sticky tape. By etching away defined squares of parylene, the investigators selectively expose areas of gold. The gold can then be modified by adding the molecule of interest to produce a bioassay. Meyer and colleagues tackle the problems of nonspecific binding and surface fouling by applying an ac bias to the quartz crystal. The crystal resonates at 5 MHz by a shearing motion. The motion of the crystal couples into the first
microscopy and fluorescence intensity measurements to confirm that the motion of the quartz crystal removed nonspecifically bound molecules and surface foulants. Meyer says he also determined that the removal process was relatively rapid by introducing albumin into the device. Albumin, an abundant protein in serum, often binds nonspecifically and fouls surfaces; this fouling can be a particular problem if researchers are attempting to detect a rare biomarker in serum. Meyer reports that the new system removed nonspecifically bound albumin in
