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MEDICAL
HARVARD MEDICAL SCHOOL & MASSACHUSETTS GENERAL HOSPITAL, BOSTON, MA
Microscopic Magnetic Field Simulations with COMSOL Multiphysics
Micro-magnetic fields hold the promise of new medical treatments. Researchers turn to modeling
for designing a MEMS field generator device.
BY GIORGIO BONMASSAR, HARVARD MEDICAL SCHOOL AND A.A. MARTINOS CENTER, MASSACHUSETTS GENERAL HOSPITAL
Electrical stimulation is currently used
as a therapeutic option for treating a
wide range of human diseases including
cardiovascular, sensory and neurological
disorders. Despite the remarkable success
of electrical stimulation, there are significant
limitations to its application; these
limitations include incompatibility with
magnetic resonance imaging (MRI) and the
association with tissue inflammation. Here,
we present an alternative method that uses
micro-magnetic fields instead of direct electrical
currents to activate excitable tissue.
MEMS Microinductor
COMSOL Multiphysics allowed us to
estimate microscopic magnetic fields generated
by a custom made micro-coil. The
field estimation was performed using the
AC/DC Module, which gives the ability to
solve Maxwell equation for the description
of electromagnetic fields with the magnetic
quasi static approximation. This approximation
holds when considering low
frequencies and ignoring the contribution
of the displacement
currents. We
studied the MEMS
microinductor (Fig.
1) designed to create
a microscopic
magnetic field using
a 3D model of
electromagnetic
quasistatic approximation.
An inductor is
the ideal magnetic
field generator, and
it stores the magnetic
field energy
generated by the
supplied electric
current. The 3D
model of the square
inductor was solved
for the electric and
Fig. 1. Image of a μMS coil. The typical overall
dimensions of the coils are 350 μm × 350 μm.
magnetic potentials using the Lagrange
Quadratic vector with constant input current.
The geometry (Fig. 2) consisted of a
680×680×400 µm block, which contained
three different objects: a seven-turn coil
composed of gold traces (red), surrounded
by air and the tissue substrate (blue). The
assumption made was that no displacement
currents were present. However, a
capacitive component may still have been
Fig. 2. Geometrical model of the 7 turns spiral square
inductor (red) on top of the neuronal tissue (blue)
and connected to a current generator. This model of
the proposed coils was used in the Finite Element 3D
studies to estimate the induced currents in the tissue
(i.e., 50μm).
present in our stimulation. Future studies
should address this important point.
Micro-Magnetic Fields Simulation
In setting up the model, all set the
steps were performed in COMSOL Multiphysics,
including: (1) drawing, (2) defining
the boundary conditions, (3) meshing,
(4) solving and (5) post-processing.
The drawing procedure was facilitated
by the existence of a similar example in
the COMSOL Model Library named “Integrated
Square-Shaped Spiral Inductor”.
This model examines the case of a
MEMS square inductor that is used for
LC bandpass filters but with fewer turns
than in our case. As in the example, the
boundary conditions allowed for the definition
of the excitation with the use of a
port. The mesh was a Delaunay set with
a maximum element size of 2.5 10 -5 m on
the coil domain and with the adaptive refine
meshing option, generating a mesh
consisting of 7,782 tetrahedral base mesh
elements, and 61,685 degrees of freedom.
Fig. 3. Distribution of the magnetic field (arrows) generated
around the μMS coil as predicted by the 3D FEM simulations.
The size of the arrow is directly proportional to the magnitude of
the magnetic field (A/m) in each (x, y, z) point of space uniformly
sampled in 7×7×7 locations. The colormap represents the electric
potential (V).
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