Biomedical Engineering Reference
In-Depth Information
can also undergo mechanical bending due to a differential surface stress caused by adsorp-
tion. The signals can be detected using optical detection with diode lasers. Thus, there are
specific biosensor signals the result of which can be a shift in resonance frequency, deflection
due to mass loading, or damping effects. Microcantilevers are very sensitive and can detect
forces in the pico-Newton range and displacements in the Angstrom range. Thus, they can
also be employed to detect force, pressure, or viscosity. Specific metal-coated cantilevers are
also very sensitive to temperature changes. Microcantilevers can also operate in a liquid
medium making them practical for biosensor design.
The resonance frequency of a microcantilever rod is shown by Equation 7.1.
1
2
(7.1)
M
is the spring constant of the oscillating cantilever,
and
M
is the effective mass of the cantilever. For commercial silicon nitride cantilevers for
example, the spring constant
Where
is the resonance frequency,
can be 0.03-0.06 N/m and
M
can be (0.14-0.18) * mass of
the individual rod or beam. A second method used is mechanical displacement, and
Equation 7.2 below describes the relationship between mechanical displacement
z
and dif-
ferential surface stress
s
.
2
3(1 )
L
2
(7.2)
z
s
Yt
where
z
is the displacement,
L
and
t
are the length and thickness of the cantilever,
is
the Poisson's ratio, and
Y
is the Young's modulus.
Recently, Calleja et al. (2) have reported on the fabrication of cantilever arrays for biosen-
sors using materials other than silicon. The cantilevers were made by spin coating a photo-
sensitive polymer that had a Young's modulus much lower than that of silicon. The spring
constant, resonant frequency, and other mechanical properties of the cantilevers were inves-
tigated as a function of the dimensions and the medium. The devices were then applied for
the adsorption of ss-DNA. It was demonstrated by the authors that this cantilever biosensor
had six times the sensitivity of commercial silicon nitride cantilevers.
Nugaeva et al. (3) demonstrated the application of micromechanical cantilever arrays
for the capture and detection of fungal spores. The authors utilized either microfabricated
uncoated or gold-coated silicon cantilevers. These were functionalized with concanavalin
A (Con A), fibronectin, or immunoglobulin G. The proteins were utilized to bind to the
spores of either
Aspergillus niger
or
Saccharomyces cerevisiae
. The authors found that bind-
ing resulted in shifts in resonance frequencies of the cantilever arrays. The immunoglobu-
lin G-functionalized cantilever surfaces produced the best response, and very low levels
of spores could be detected in several hours compared to days with conventional
approaches. The proposed techniques may have potential applications as mold or odor
sensors or in medical and agricultural diagnostics, food- and water-quality monitoring.
Recently Shu et al. (4) describe an interesting mechanical motion-based biosensor using
a DNA molecular motor. The DNA motors were integrated with an array of microfabri-
cated silicon cantilevers. The forces exerted by the precise conformational changes were
probed via differential measurements using an in situ reference cantilever coated with a
nonspecific sequence of DNA. Both the direction and amplitude of motion of the can-
tilevers were a function of buffer pH and ionic strength. The authors showed that chang-
ing the pH in a controlled fashion produced compressive stress repeatedly and the process
was reversible. Thus, their device converts biochemical energy from conformational
changes into micromechanical work.
