Pathogen Detection Using Magnetoelastic Biosentinels - Biomedical Engineering Systems and Technologies - page 50

2.1 The Resonator Platform

Acoustic wave (AW) resonators as biosensing platforms have been widely investi-
gated [2, 3] with quartz crystal microbalances [4-6], microcantilevers [7, 8], and
magnetoelastic resonators [9-12] among the platforms. Acoustic resonators are mass
sensitive devices where a change in the mass load on the sensor surface causes a
change in the sensor's resonant frequency. Acoustic resonators are characterized by
two important parameters: 1) the sensitivity ( S m ) which represents the resonant fre-
quency shift per unit mass load; and 2) the resonance performance ( Q factor), which
is defined as the ratio of the energy stored in the resonant structure to the total energy
losses per oscillation cycle. A high Q factor means lower attenuation and a sharper
resonant peak and thus better resolution in determining the resonant frequency. In a
viscous environment, all AW resonators suffer from the damping effects of the media
on the vibratory response of the platform. Viscous damping causes a decrease in f , S m
and Q [13]. This is why some AW resonators work well in vacuum or air but not in
liquids. The Q value of ME resonators was found to exhibit a much higher Q value
(>1000 in air & ~100 in water) than microcantilevers of the same length [14]. This
advantage originates from the longitudinal mode of vibration for the freestanding ME
resonator's design compared to the transverse vibration mode of cantilevers [15]. The
minimum detectable mass for an acoustic sensor platform depends on the ability to
resolve resonant frequency shifts as a result of the mass loading.
Magnetoelasticity is the bidirectional coupling of magnetic and elastic fields within
a material [16, 17]. With the application of an external magnetic field, elastic distor-
tions arise from the rearrangement of magnetic domains (magnetostrictive behavior)
causing the material to change shape. This deformation induces stresses which, con-
versely, cause the magnetization of the material to change (Villari effect). In the pres-
ence of an oscillating external magnetic field ( H ), the ME resonator will align with
the field along its easy axis and will resonate by elastically deforming in response
inducing an opposing magnetic field that may be measured with an induction pickup
coil. When the frequency of H matches the natural resonant frequency of the plat-
form, the platform will resonate with further enhanced motion. In air, the fundamental
resonant frequency f 0 of the longitudinal vibratory motion of a freestanding thin rib-
bon of magnetoelastic material (with length L ) is [18]:

1

1

(1)

2

where E is Young's modulus of elasticity, ρ is the density, and σ is Poisson's ratio.
For biological detection, the surface of the ME resonator is coated with a biorecog-
nition element such as an antibody or phage to form a biosentinel. This biorecognition
element is designed to specifically bind the target of interest. When the ME biosenti-
nel comes into contact with the target pathogens, the biorecognition element will cap-
ture/bind the pathogens, creating an additional mass load on the biosentinel resulting
in a decrease in the resonant frequency. Therefore, the presence and concentration of
any target pathogens can be identified by monitoring the resonant frequency shifts of
the biosentinel. For the uniform addition of a small mass ∆m to the original mass m 0
(such that ∆m << m 0 ), the resulting mass loaded resonant frequency is [13]: