Biomedical Engineering Reference
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Fig. 9.7 New NMR coils for higher sensitivity .( a ) A solenoidal coil is embedded along with a
microfluidic channel. The entire bore of the coil is available to samples, maximizing the filling
factor (
1). ( b ) The new coil, due to its high filling factor, offered >350 % enhancement in SNR
compared to a similar coil wrapped around a tubing. ( c )TheR 2 changes can be amplified by
concentrating MNP-labeled samples inside the microcoil. Based on the coil design in ( a ), we
incorporated a filter that captures target objects. The photo shows a prototype device containing
100-nm pore filter (Adopted from [ 28 ]and[ 25 ]. Copyright 2009 National Academy of Sciences,
USA and John Wiley and Sons, Inc.)
samples by producing more homogeneous radio-frequency magnetic fields and had
smaller electrical resistance (e.g., 0.3 vs. 3:2 at 20 MHz).
We have further modified the embedded coil to improve the overall SNR and to
streamline the assay procedure. In the new probe design, a cell-capturing membrane
filter was incorporated at the outlet side of the coil (Fig. 9.7 c). The filter serves
two essential functions. First, it size selectively captures cells and concentrates
them inside the NMR detection coil, which leads to more pronounced R 2 changes
(Eq. 9.2 ). The filter also provides a way to detect a small number of targets from
large sample volumes. Second, the filter enables on-chip separation of cells from
unbound MNPs, therefore obviating the need for separate off-chip purification steps
(e.g., centrifugation). Together with the highly magnetic Fe@ferrite MNPs, this new
probe has been applied to diagnose tuberculosis [ 25 ]. Using the attenuated bacillus
Calmette-Guerin (BCG) as a surrogate for Mycobacterium tuberculosis , we detected
as few as 20 colony-forming units (CFUs) in sputum (1 mL). Importantly, the entire
detection procedure was performed in a single-chip format, minimizing sample loss
and making the assay simple and fast (in less than 30 min).
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