# An SV-GMR Needle Sensor-Based Estimation of Volume Density of Magnetic Fluid inside Human Body

Authors: C. P. Gooneratne; S. C. Mukhopadhyay; S. Yamada
Journal: Journal of Sensors
 (2008)

---

## Abstract

A spin-valve giant magneto-resistive (SV-GMR) sensor of needle-type configuration is reported to estimate the volume density of magnetic fluid inside human body. The magnetic fluid is usually injected into human body to kill cancerous cell using hyperthermia-based treatment. To control the heat treatment, a good knowledge of temperature is very much essential. The SV-GMR-based needle-type sensor is used to measure the magnetic flux density of the magnetic fluid inside the human body from which the temperature is estimated. The needle-type sensor provides a semi-invasive approach of temperature determination.


---

## Body

## 1. Introduction

In recent times, due to the development of several novel sensors and sensing technologies, there
is a tremendous progress in biomedical sensing. Modern biomedical sensors
developed with advanced microfabrication and signal processing techniques are
becoming inexpensive, accurate, and reliable [1]. To develop sensors to analyze
cells at a single-cell level, it is very demanding to have high-performance
sensing and instrumentation techniques [2, 3]. A hyperthermia cancer treatment based on induction
heating is achieved by injecting magnetic fluid with magnetic nanoparticles into
the tumor. The potential benefit of
using magnetic nanoparticles is the use of localized magnetic field gradients to attract the particles to a chosen site, to
hold them there until the therapy is complete, and then to remove them
[4]. To destroy the tumor, a time-varying magnetic field is applied to the
nanoparticles for heating up the tumor. The applied heat depends on the amount
of nanoparticles, magnetic flux density, exciting supply frequency, and the
distance from the exciting coil to the location of tumor inside the body. To
confirm that the tumor can be destroyed when exposed to heating, it is required to measure
the temperature of the magnetic liquid inside the body. The control of temperature is a paramount important task in
achieving success using this treatment method. But it is difficult to fabricate temperature
sensor of very small size with sufficient accuracy to measure the temperature.
So an indirect approach has been taken to measure the temperature in the tumor
area inside the human body. This approach is based on the measurement of
magnetic flux density of the magnetic liquid which is injected for treatment.
In order to calculate temperature in an indirect way, knowledge
about the density of nanoparticles is important before and during the
treatment. This paper reports the development of a needle-type nanosensor based on the spin-valve
giant magneto-resistive (SV-GMR) technique to measure the magnetic flux density
inside the body via pricking the needle inside the body. The modeling and
experimental results of flux-density measurement have been reported here. From
the information of flux density, the temperature rise can be estimated to permit
the delivery of controlled heating to precisely defined locations in controlled hyperthermia cancer
treatment. The sensor needs to be used in a uniform magnetic field region which
is provided by Helmholtz coils.The complete paper is divided into six sections. After the introduction that covers the first
section, sensor has been discussed in Section2. The hyperthermia treatment
and use of the sensor have been described in Section 3. The Helmholtz coil,
analytical results, and experimental results are provided in Sections 4 and 5. The
experimental results are reported in Section 6. Finally the conclusion is
summarized in Section 7.


## 2. SV-GMR Needle-Type Sensor

The
SV-GMR sensor as shown in Figure1 is used to estimate volume as well as weight
density of the magnetic fluid inside human body. The needle-type construction
paves for easier application for measurement inside the human body. The SV-GMR
element with sensing area of 75μm×40μm is on the tip of the needle. Sensing
direction is parallel to the needle. The volume/weight density of the magnetic
fluid can be estimated by measuring and comparing the applied magnetic flux
density (outside body) and the flux density in the magnetic fluid. A constant
current of 0.5 mA is applied to the SV-GMR sensor. Figure 2 shows the AC
characteristics of the sensor and the sensitivity is approximately
10μV/μT.SV-GMR needle sensor.
(a)(b)Figure 2
AC characteristics of the SV-GMR
sensor.


## 3. SV-GMR Sensor and Hyperthermia Treatment

The block diagram representation of
hyperthermia-based cancer treatment is shown in Figure3. Hyperthermia
treatment is a rapidly evolving cancer cure that has many advantages compared
to other common methods of treatment. These include no side effects or pain for the patient, which is a major problem in treatments such as chemotherapy. A low/minimally invasive treatment method such as the one reported here takes less time for treatment compared to chemotherapy and radiotherapy.
Even though the cancer cells do not die completely, they could quite possibly be more
susceptible to treatments such as ionizing radiation or chemotherapy, allowing such
therapy to be given in small doses.Figure 3
Hyperthermia treatment as a cancer therapy.Hyperthermia is based on the principle that
cancer cells are more sensitive to heat than normal healthy cells. Magnetic
fluid with magnetic nanoparticles as shown in Figure4 is inserted into the
affected area.Model showing magnetic nanoparticles distributed  uniformly in magnetic fluid.
(a)
Uniform distribution of magnetic nanoparticles(b)
Close-up view of  a single magnetic nanoparticleWhen a time-varying external magnetic field is applied on
the magnetic particle, magnetic particles absorb and generate heat due to hysteresis
loss, and temperature is thus increased. It is essential to correctly control
the temperature in hyperthermia treatment. The magnetic fluid density in the
body is an important parameter to estimate the heat that needs to be given.
This serves as a critical aspect in the success of the treatment. However when
injected into the affected area, the magnetic fluid spreads inside the cancer
cells and as a result, the volume as well as weight density of magnetic fluid
decreases. As a consequence, it is vital to estimate the density of magnetic
fluid in the body. The magnetic fluid weight density is the ratio of weight of
magnetic nanoparticles in 1 mL volume to weight of magnetic nanoparticles and
water in 1 mL volume.The essential parameters of magnetic fluid are(i)
wf: weight rate of magnetic bead in grams per cubic centimeter (g/cc),(ii)
Dw: weight density of the magnetic fluid in
milligrams of ferrite per cubic centimeter (mgFe/cc),(iii)
Dv: magnetic fluid volume density measured as a
percentage.Given thatDv and
Dw are small, then the relationship between them can be simplified to
(1)Dv=Dwγf,where γf=4.58 (W-35 sample, Taiho Co., Japan) and is the specific gravity
of magnetic bead.Table1 provides the volume and weight
densities for different combination of nanoparticles and water. From Table 1, it
can be seen that the Dv percentage change between the lower weight
densities is low. For example, the Dv percentage change between Dw of 0.00479 and  0.00574 is 0.021%. So it is essential that the percentage change
in uniform magnetic field region be less than this value. An error percentage
of ≤0.01% was decided for 0.05 m in the positive and negative axial direction
(z-axis) between the coils, and the same was in the radial direction.Table 1
Magnetic fluid parameters.
Ratiowf (g/cc)Dw (mgFe/cc)Dv (%)1:1200.004810.004790.1051:1000.005760.005740.1261:700.008200.008140.1791:500.011410.011310.2491:400.014200.014040.311:270.020790.020450.4541:220.025300.024810.5521:180.030630.029920.6691:130.041570.040260.9081:100.052910.050811.1551:80.064670.061561.4121:70.072750.068841.5881:60.083140.078071.8151:50.097000.090162.1181:40.116400.106692.541


## 4. Helmholtz Coil and Experimental Setup

Helmholtz  coils are used in a variety of
applications, primarily due to its ability to produce a relatively uniform
field configuration, ease of construction and flexibility. The accuracy of the
relative magnetic field produced depends on how precise the Helmholtz coils are
constructed and how accurate the current through them is maintained [4–9]. A helmholtz coil system consists of two coils, either circular or
square, of equal radius and equal number of turns along an axis through the
centre of the coils, separated by a distance equal to the radius of the coils.
The total field produced is the sum of the two coils. The helmholtz coil has
many applications in a variety of areas such as bioelectromagnetic/biomedical,
diagnostic studies on electron beams, calibration of magnetic instruments and
probes and in the study of the magnetic properties of materials.Figure5 shows the setup used for
measurement of volume density of magnetic fluid; the whole system is inside a
2-square Helmholtz coil. The theoretical distribution of magnetic flux is shown
in Figure 6.Figure 5
A 2-square Helmholtz coil system.Figure 6
Model of magnetic flux distribution produced
by Helmholtz coils and the flux distribution inside an embedded cavity.The Helmholtz coil system is used to
produce a magnetic flux density ofB0. The flux density will
concentrate on the magnetic fluid-filled cavity. The needle-type sensor is
placed at the center of the cavity, hence the center of the common axis of the
coil pairs. The magnetic flux in the cavity can be assumed as B1. B1 changes with the content density of the magnetic fluid. The theoretical
equation estimating the magnetic fluid volume and thus the weight density from
the difference between the applied flux and the flux in the cavity is [10](2)B1−B0B0≅4(1−N)Dv.The
change between the applied magnetic flux and the flux in the container is
directly proportional to the magnetic fluid volume density. The change is
usually of the order of 10%–12%.The above equation is based on the assumption that the 2-square Helmholtz coils
produce a uniform magnetic field distribution. But in practice, the magnetic
field distribution is not very uniform as shown in Figure7. So a new coil
system has been explored and developed.Magnetic field distribution inside a 2-square
Helmholtz coil system.
(a)
Analytical calculation(b)
Measured value


## 5. Development of a New Helmholtz Coil System

A Helmholtz coil system has been designed
and constructed to obtain a uniform magnetic field distribution with an
estimated error≤0.01% for 0.05 m in the z and r axes. After a lot of
discussions to meet the specifications and experimental setup, the ideal system
was decided as a planar coil system consisting of three coil pairs. The
intended coil is shown in Figure 8. The three-coil system is used to produce a
uniform magnetic flux distribution inside the region of interest.Three-coil pair system Helmholtz coil.
(a)
Diagram(b)
Fabricated systemThe magnetic flux distribution as well as
the comparison (error in Figures9 and 10) of the magnetic flux density at any
point with respect to the center has been  calculated analytically. Figures 9 and 10 show the surface plot and contour plot of analytical results for Helmholtz
tri-coil system. It is seen that the error margin with the region of interest
is negligible.Figure 9
3D surface plot of error in thez and ρ axes.Figure 10
Contour plot of error in theρ and z axes.The magnetic flux density distribution
inside Helmholtz tri-coil system has been measured using a gaussmeter (MTI-HM315). An
area of400mm2 in the uniform region was tested for a Z-direction
range from −20 mm to +20 mm. However it must be noted that the display of the
gaussmeter was accurate only to 0.01% and the error in the system was
approximately 1%. Figures 11 to 13 show measured error margin of contour plot
at different positions.Figure 11
Contour plot atZ=0mm.Figure 12
Contour plot atZ=+20mm.Figure 13
Contour plot atZ=−20mm.


## 6. Experiments and Experimental Results

Helmholtz tri-coil system has been used for
providing magnetic flux. Magnetic fluid is injected into a tray with embedded
cylindrical cells as shown in Figure5. The SV-GMR needle sensor tip is set-up
in the center of the Helmholtz coil. The tray is moved to get the needle in the
middle of the magnetic fluid-filled cylindrical shell. The function generator
provides the AC excitation current with the Helmholtz coil to produce a magnetic
flux density of 100 μT. The frequency of operation is set at 100 Hz and 200 Hz,
respectively. Constant current of 0.5 mA is applied to the SV-GMR sensor. The
output of the GMR sensor is obtained by the NF Electronic Instruments 5610B
two-phase lock in the amplifier and shown in Figure 14.Figure 14
Change in magnetic flux density
for varying fluid weight density.The experiments above were performed with
the tip of the needle sensor placed at the middle of the magnetic fluid.
Experiments were then performed to see the effect of penetration depth
(y-direction) on the output. The model is shown in Figure 15.Figure 15
The model showing the effect of penetration depth.Figure15 above shows the needle sensor at
the start (0 mm). This is when the needle sensor initially comes into contact
with the magnetic fluid. The fluid is filled with up to 1 cm of the cylinder. The
center is then 5 mm from the start. Between the start and the middle, four points
are tested in 1mm steps, that is, at 1 mm, 2 mm, and until 5 mm. After the middle, a further
two steps are tested. Four concentrations from the 15 samples are tested (two
extremes and two from the middle). Figures
16 and  17 show the variation of the change of magnetic flux density as a
function of penetration depth of the needle sensor for 100 Hz and 200 Hz, respectively. It is seen that the signal increases with the increase in
penetration depth in the beginning and attains a maximum value and then
it decreases slightly and reaches a steady state value. The value for 200 Hz is
slightly less than 100 Hz. The reason of the increase of change linearly in the
beginning is that first the sensor creates an indentation while it touches the
magnetic liquid and then the sensor gets more magnetic material with the
increase in the depth of the needle. When the sensor is just inside the magnetic
fluid, there is an effect of crowd of
magnetic flux at the upper layer of the magnetic fluid. With the increase in
depth, the edge effect is negligible.Figure 16
Magnetic field change for penetration depth at 100 Hz.Figure 17
Magnetic field change for penetration depth at 200 Hz.In this study, the volume (or weight)
density of the magnetic fluid has been estimated by the needle-type GMR sensor.
In practical situation, it is essential to know the actual temperature. In order
to obtain the temperature accurately, it is very important to know the volume
density of the magnetic fluid. So by using the needle-type GMR sensor, the
volume density has been estimated on trial.  While researches into the heating effect
on cells have been carried out for a long time, it is only recently that safe
methods have been found to increase the temperature in treatment or in vivo
trials. Numerous trials have been carried out on the relationship between
temperature and dextran magnetite and reported in literature with very
encouraging results [14–18]. Furthermore, there is a widespread belief that hyperthermia has the
potential to be an effective treatment, validated by the fact of the number of
clinics that carry out such treatment currently. However the side effect of
possible damage to healthy cells means that an effective and accurate method is
needed to estimate the magnetic fluid volume density. The SV-GMR sensor can be
utilized in such a way to estimate the magnetic fluid volume density in vivo paving
the way for less/no side effects for patients, as well as the widespread
possibility of being a stand-alone cancer therapy rather than an adjunct to
other cancer treatments, which seems to be the popular practice at the moment.


## 7. Conclusion

A needle-type spin-valve giant magneto-resistive(SV-GMR) sensor has been developed for measurement of flux density
inside human body. It is important to estimate the volume density of magnetite
inside the body for the hyperthermia cancer treatment based on induction
heating. A new Helmholtz three-pair coil system has been developed for
providing uniform flux distribution in the region of interest. The relationship
between the difference of magnetic flux density and magnetic fluid volume
density contained in an embedded cavity has been obtained and it is linear. It
can therefore be concluded that the volume density of magnetite injected into the
body may be estimated by the proposed technique.



---

*Source: 890293-2007-12-10.xml*