Pathogen Detection Using Magnetoelastic Biosentinels - Biomedical Engineering Systems and Technologies - page 52

[21]. Motion of the biosentinels is the result of forces ( F ) and torques ( T ) that are expe-
rienced by the biosentinel that arise in the presence of an external magnetic field ( H) .
The forces and torques are given in terms of vector quantities by [22]:

(4)

(5)

where M is the magnetization of the biosentinel, µ 0 is the permeability of free space, and
v is the material volume. In general, M and H vary over the body of the biosentinel. At
the micrometer dimensional scales of the biosentinels, the external magnetic field can be
assumed to be constant with little error. However, for magnetically soft materials, such
as the amorphous magnetoelastic materials used for the biosentinels, the magnetization
is a nonlinear function of the external magnetic field and is dependent on the body
shape. Only ellipsoids may possess a uniform magnetization throughout the body. For
other shapes, geometric effects give rise to demagnetizing fields along different direc-
tions (shape anisotropy), causing some directions to be more easily magnetized than
others. The modeling of the magnetic fields has typically been achieved with finite
element analysis which is unsuitable for real-time control. Recently, analytical models
employing equivalent ellipsoids to determine the body magnetization [23] have been
published to aid with the magnetic propulsion and navigation of microrobots [24, 25].
Models such as these enable the real-time actuation and control of magnetic microsen-
sors and robots for targeted medical therapeutic and diagnostic applications.

2.3 Fabrication of ME Resonators

Magnetoelastic resonators may be formed by bulk cutting from commercially available
ribbons [26-28], electrochemical deposition [29-31], electroless plating [32], and phys-
ical vapor deposition [33, 34]. The ME resonators described here are fabricated using
standard microelectronic fabrication techniques of photolithography and physical vapor
deposition (sputtering). A resonator fabricated by this technique is shown in Figure 6
and the fabrication process is shown schematically in Figure 7. A full description of the
process is given in [33]. Magnetoelastic resonators are fabricated on a patterned wafer
by co-depositing iron and boron at controlled rates under vacuum. The resonators are
coated with gold to protect against oxidation and provide a biocompatible surface to
immobilize the biorecognition layer. Fabrication of the sensor platform begins by coat-
ing a plain silicon test wafer with a layer of chromium followed by gold, each to a
thickness of 30−40 nm. Next, a spin-coated layer of photoresist is deposited on the gold
surface, at least twice as thick as the desired ME resonator thickness. The photoresist is
then UV exposed using a positive mask that patterns the shape and dimensions of the
ME resonators. The wafer is then developed, rinsed, dried, and inspected for pattern
integrity and thickness.
To begin forming the resonators, a gold layer is first deposited onto the patterned wafer
to a thickness of 30-40 nm. Iron (dc) and boron (rf) are then co-sputtered to form the
magnetostrictive alloy. The thickness of the alloy film depends on process conditions, and
is generally limited by the thickness of the photoresist layer. Highly magnetostrictive films