Specific Absorption Rate Determination of Magnetic Nanoparticles Through Hyperthermia Measurements in Non-Adiabatic Conditions

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    both samples, solutions containing 10 mg/ml of particles in deio-

    nised water have been prepared. Room temperature hysteresis

    loops of as-received dried particles have been measured with a

    LakeShore 7410 VSM: the results are reported in Fig. 1.

    Sample A is constituted by larger particles, with a lower surface

    to volume ratio with respect to sample P; therefore, as expected,

    their saturation magnetisation is higher, and almost equal to the

    saturation of bulk magnetite. Sample P, instead, is characterised by

    smaller particles, with a higher surface to volume ratio leading to a

    reduced saturation, that display a superparamagnetic behaviour,

    with zero coercive  eld and a much slower approach to saturation.

    These two samples are representative of two different classes of 

    particles often studied for MPH applications.

    3. Hyperthermia setup: calibration and modelling 

    The hyperthermia setup used in our experiments consists in a

    water-cooled copper coil made of 4 turns, connected to a rf gen-

    erator through a matching network, and able to generate elec-

    tromagnetic   elds with intensity up to 100 mT at a frequency of 

    100 kHz. Inside the coil, a PTFE sample holder hosts an Eppendorf 

    test tube containing 1 ml of water into which known concentra-

    tions of magnetic nanoparticles are dispersed. At equilibrium and

    with no rf   eld applied, the sample remains at a constant tem-

    perature   T a¼23.5   °C thanks to the water-cooling of the coil. In

    order to measure the temperature of the water solution, a type-T

    (wires diameter 0.127 mm) thermocouple is available, that doesnot allow the measurement of the sample temperature during the

    rf irradiation process. However, it can be inserted in the test tube

    immediately after the rf  eld is switched off, in order to measure

    the time-dependent temperature decay of the sample towards  T a.

    As we will show, if a suitable calibration of the experimental setup

    is available, an accurate determination of the specic absorption

    rate (SAR), and consequently of intrinsic loss power (ILP), can be

    obtained by just knowing the two temperatures   T a   and   T M , the

    latter being the equilibrium temperature reached by the nano-

    particles solution at steady state during irradiation.

    An accurate determination of SAR would require a hy-

    perthermia setup operating in ideally adiabatic conditions [10,18],

    i.e. a calorimeter would be the optimum choice for this kind of 

    measurements. However, such complex setups are often not

    available and SAR is therefore determined by measuring the initial

    slope of the sample temperature vs. time when the rf   eld isswitched on [11]. This procedure leads to approximate results that

    may be affected by how the temperature vs. time curve is analysed

    and by the accuracy of the adiabatic hypothesis in the actual ex-

    perimental setup, and requires suitable corrections for compen-

    sating the heat exchange with the surrounding environment

    [12,13].

    To avoid these problems, we opted to keep simple the hy-

    perthermia experiment, while taking into account the heat ex-

    change of the sample with the surrounding environment in the

    physical model that describes its thermodynamics. In this way,

    after a suitable calibration procedure, the SAR can be accurately

    measured even in non-adiabatic conditions.

    The hyperthermia system can be modelled as described in

    Fig. 2. A heat source  S   (e.g. the nanoparticles inside the solution)provides energy Q s in,   to the system (e.g. through the power lossesof the magnetic particles excited by the rf   eld), and exchanges

    heat with the water   W ,   Q s out ,   and   Q w in,   representing the sameamount of heat, respectively,  owing out of the  S  subsystem and

    into the  W  one. Finally, the water exchanges heat   Q w out ,   with theenvironment (at temperature  T a). At each instant of time, the heat

    exchange equations for the  S  and  W  subsystems are the following:

    δ    δ δ = +

    ( )

    dt 

    dt 

    dt    1as   s in s out  , ,

    δ    δ δ = +

    ( )

    dt 

    dt 

    dt    1bw   w in w out  , ,

    where each term is taken with its proper sign (positive for heatentering the subsystem, negative for heat released by the

    subsystem).

    The   rst term  δ Q 

    dt 

    s in, is given by the power   P   that is provided

    through the source. The two terms   δ Q dt 

    s out , and  δ Q 

    dt 

    w in, are of opposite

    sign and given by the following expression:

    δ δ 

    τ = − = − [ ( ) − ( )]

    ( )

    dt 

    dt 

    c mT t T t  

    2s out w in   s s

    ss w

    , ,

    where c s is the specic heat of the source,  ms  is its mass,   τ s is the

    time constant of the heat exchange process between the source

    and the water,   T s   is the source temperature and   T w   is the tem-

    perature of the water. Both quantities  T s  and  T w  depend on time,

    but in typical hyperthermia experiments involving magnetic

    Fig. 1.   Room temperature hysteresis loops of sample A (blue curve) and sample P

    (red curve). Inset: magnication at low   elds to better show the coercivity. (For

    interpretation of the references to colour in this   gure caption, the reader is re-

    ferred to the web version of this paper.)

    Fig. 2.   Thermodynamic scheme of the hyperthermia system.  S  is the source sub-

    system (i.e. the magnetic nanoparticles).  W  is the water subsystem in which the

    nanoparticles are dispersed.  P  is the exciting power.  Q  terms represent the heat

    exchanged by the two subsystems with each other and with the environment.

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    particles only T w is accessible.

    Finally, the term   δ Q dt 

    w out , needs to be evaluated. To this purpose, a

    rst calibration procedure has been carried on, consisting in put-

    ting in the test tube 1 ml of water that has been pre-heated on a

    hot plate. The test tube is in its sample holder and the cooling

    water is let circulate in the rf coil, even though no rf  eld is gen-

    erated. The thermocouple logs the time dependence of the tem-

    perature of water as it gradually recovers to the temperature  T a of 

    the surrounding environment.  Fig. 3 reports the results of one of the several tests that have been performed on water that has been

    pre-heated at different temperatures. An exponential decay of the

    temperature over time is expected. However, the experimental

    data could not be   tted with a single exponential function,

    whereas two were required to reproduce the data. The time con-

    stants of the two exponential functions turned out to be fairly

    reproducible within the whole set of measurements; an average

    value has then been calculated, giving rise to the   nal results:

    τ    =  137.5 sw1   and   τ    = 498.1 sw2   . The presence of two time constants

    indicates that the dissipation of heat towards the external en-

    vironment is constituted by two mechanisms: the   rst possibly

    involves heat transfer through the test tube walls and sample

    holder, and the second through the top surface of the liquid in the

    test tube.

    Therefore, it is necessary to express   δ Q dt 

    w out , as the sum of two

    terms in the form:

    δ 

    τ τ = − [ ( ) − ] − [ ( ) − ]

    ( )

    dt 

    c mT t T 

      c mT t T 

    3aw out    w w

    ww a

    w w

    ww a

    ,

    1 2

    τ τ 

    τ τ = −

      ( + )[ ( ) − ]

    ( )

    c mT t T 

    3bw w w w

    w ww a

    1 2

    1 2

    where   c w   and   mw   are the specic heat and mass of the water,

    respectively.   τ w1  and   τ w2   are the time constants of the two heat

    transfer mechanisms to the surrounding environment.

    Therefore, Eq. (1) can be written in the following way:

    δ 

    τ = − [ ( ) − ( )]

    ( )

    dt   P 

      c mT t T t  

    4as s s

    ss w

    δ 

    τ 

    τ τ 

    τ τ = [ ( ) − ( )] −

      ( + )[ ( ) − ]

    ( )

    dt 

    c mT t T t  

      c mT t T 

    4bw s s

    ss w

    w w w w

    w ww a

    1 2

    1 2

    The term P  appearing in Eq. (4a) is the power that is released to

    the system by the source. In a typical hyperthermia experiment,

    this term is the power generated by the nanoparticles that aresubmitted to the rf  eld (this is not the power emitted by the rf 

    generator driving the coil, that is usually much higher). From the

    rst principle of thermodynamics, Δ = Δ − ΔU Q L, with U  being the

    internal energy of the system, and   Q   and   L   the heat and work

    exchanged by the system with the outside, respectively. If we as-

    sume that in the studied temperature range the water does not

    appreciably change its volume, then  Δ =L   0. The internal energy of 

    both subsystems   S   and   W   can then be expressed in terms of 

    temperature through the expressions   =Δ

    Δ  c m

    T    s w s w, ,s w

    s w

    ,

    ,. As a con-

    sequence, the thermodynamic behaviour of the hyperthermia

    setup can be modelled with the following set of coupled differ-

    ential equations:

    τ − [ ( ) − ( )] =

      ( )

    ( )P 

      c m

    T t T t c m

      dT t 

    dt    5as s

    ss w s s

    s

    τ 

    τ τ 

    τ τ [ ( ) − ( )] −

      ( + )[ ( ) − ]

    =  ( )

    ( )

    c mT t T t  

      c mT t T 

    c m  dT t 

    dt    5b

    s s

    ss w

    w w w w

    w ww a

    w ww

    1 2

    1 2

    In order to solve the system (5) for the two unknowns  T s(t ) and

    T w(t ), values for   c s,   ms   and   τ s   should be provided. Instead,   c w   is

    known (4187 J kg1 K1), as well as mw, since the amount of water

    put in the test tube can be measured. However, the specic heat,

    mass and time constant of the source are not easily determined for

    a typical hyperthermia sample consisting on magnetic nano-

    particles. Therefore, we performed a second calibration step where

    the source term is replaced by a known resistor having a small sizeand a value of 10.2 Ω. When a known current of constant intensityis let pass through the resistor, it produces a known power   =P RI 2

    that is used to heat the water. This is also the power term that

    enters Eq. (4a) and following. To simplify the system and ease the

    calibration process, the resistor has been immersed in a solution of 

    water and ice, that is therefore at temperature   ≈ °T    0 Cice   ; this

    temperature will not change when the resistor heats up due to the

    current that   ows through it, leading to a much simplied ther-

    modynamic description of the system. The thermocouple has been

    placed in direct contact with the resistor, in order to measure its

    temperature as it heats up when the current  ows through it. In

    this way, the system   (5) reduces to a single equation with the

    temperature of the source (resistor) being the only unknown:

    τ 

    ( )= − [ ( ) − ]

    ( )c m   dT t 

    dt   P    c m T t T 

    6s s

    s s s

    ss ice

    where the quantities with suf x   s  now refer to the resistor that

    acts as heat source in this particular case. Eq.  (6) can be rearranged

    as:

    τ τ 

    ( )= − ( ) + +

    ( )

    ⎣⎢

    ⎦⎥

    dT t 

    dt   T t 

      P 

    c m

    T 1

    7

    s

    ss

    s s

    ice

    s

    that is in the form   ′   = − y ay b   that, for the initial condition

    ( ) = y y0 0, has the analytical solution   = + [ − ] y y eb

    a

    b

    aat 

    0   . Therefore,

    Eq. (6)  can be solved as:

    τ ( ) = + [ − ]

    ( )

    τ −T t T   P 

    c m

      e1

    8

    s ices

    s s

    t /  s

    0 500 1000 1500 200020

    25

    30

    35

    40

    45

    time (s)

    experimental data

    Fig. 3.   Symbols: time dependence of the temperature of 1 ml of pre-heated watercooling down in the hyperthermia setup. Red line:   tting with the sum of two

    exponential functions. (For interpretation of the references to colour in this  gure

    caption, the reader is referred to the web version of this paper.)

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    for the initial condition   ( ) =T T 0s ice. The experimental results of the

    time dependence of the temperature of the resistor immersed in

    water and ice for a value of  P ¼0.41 W are reported in Fig. 4, that is

    representative of the several measurements that have been per-

    formed with different heating powers up to E1 W (higher powers

    would have led to an excessive temperature increase of the water,

    well beyond the range that is interesting for MPH applications).

    The data reported in   Fig. 4, as well as all the other available

    data, have been  tted with Eq. (8)  leaving   τ s and the product  c ms sas free parameters. The obtained values have been averaged and

    the  nal results have been obtained:   τ   =  89.0 ss   and   =c m   7.0 J/Ks s   .

    We will assume that these values describe the properties of the

    resistor when it exchanges heat with water, both in the conditions

    exploited for calibration purposes and in an actual hyperthermia

    experiment represented in Fig. 2  and modelled by Eq.  (5), when

    the water will effectively heat up.

    It is therefore now possible to validate the thermodynamic

    model by performing a measurement of the temperature of the

    water in the test tube in its sample holder with the cooling water

    circulating in the rf coil but without the rf  eld, as the heat source

    will be the power provided by the calibration resistor. In practice,

    an experiment in the same conditions as a normal hyperthermia

    measurement is performed, with the difference that the heatsource is not the magnetic nanoparticles excited by the rf  eld, but

    the calibration resistor that heats the water by means of the Joule

    effect with a known power. Since the rf  eld is off, in this case it is

    possible to use the thermocouple to measure   T w(t ) during both

    heating and cooling. The results, for heating powers of 0.41 W and

    0.10 W (taken as examples) applied for 7200 s and 1800 s, re-

    spectively, are reported in Fig. 5 (black symbols).

    Eq. (5) is solved numerically using the values for   c ms s,   τ s,   τ w1and   τ w2  obtained in the two calibration steps, and with a volume

    of 0.7 ml (mw¼0.7 g) of water. The solutions, calculated for the

    same heating powers and heating times adopted in the experi-

    ment, are plotted in Fig. 5 as the green lines. The agreement with

    the experimental results is excellent. It is important to point out

    that no free parameter has been adjusted to obtain this

    agreement: all the quantities involved in Eq.  (5) have been either

    measured during the two calibration processes (time constants of 

    the heat transfer process from the test tube to the environment,

    heat capacity of the calibration resistor, time constant of the heat

    transfer process from the calibration resistor to water, power), or

    are otherwise known (volume of water, its specic heat).

    As a result, we can therefore consider the system of  (5) as an

    adequate thermodynamic representation of our hyperthermia

    setup. We can then exploit them to determine the SAR of an un-

    known source (magnetic nanoparticles). If the system is brought to

    thermal equilibrium during the heating process, the energy en-

    tering the system through the rf   eld exciting the nanoparticles

    and the energy released to the environment are equal and oppo-

    site in sign; as a consequence, both   ( )dT t dt s and   ( )dT t 

    dt w are equal to

    zero. Therefore, from Eq.  (5):

    τ τ 

    τ τ =

      ( + )[ − ]

    ( )P 

      c mT T 

    9w w w w

    w wM a

    1 2

    1 2

    where   T M   is the maximum (equilibrium) temperature reached

    during the heating process. With this approach, it is not necessary

    to measure the temperature of the water during the heating pro-

    cess and to correct its time derivative for the residual heat ex-

    change with the surrounding environment. Instead, it is suf cient

    to know T M , that can be measured in stationary conditions with an

    optical thermometer or immediately after the rf  eld is switched

    off with just a thermocouple, to determine   P , as all the other

    quantities entering Eq. (9)  are now known and the model Eq.  (9)

    belongs to has been validated. With   P   obtained in this way, by

    knowing the mass of particles used in the experiment, their spe-

    cic absorption rate is obtained accurately for the given rf    eld

    intensity and frequency.

    4. SAR measurements on magnetic nanoparticles

    Magnetic hyperthermia has been measured on both samples A

    0 100 200 300 400 500 6000

    2

    4

    6

    8

    10

    t (s)

    experimental data

    thermodynamic model

    s> = 89.0 s

    = 7 J / K

    <

    Fig. 4.  Symbols: time dependence of the temperature of the calibration resistor

    immersed in iced water when an electrical current ows through it giving a heating

    power of 0.41 W. Blue line:  tting with Eq. (8)  with   τ s and the product  c ms s   left as

    free parameters. (For interpretation of the references to colour in this  gure cap-

    tion, the reader is referred to the web version of this paper.)

    0 2 4 6 8 1020

    25

    30

    35

    40

    t (103 s)

    experimental data

    thermodynamic model

    P = 0.41 W

    P = 0.10 W

    Fig. 5.   Symbols: time dependence of the temperature of the water heated by

    means of the calibration resistor with a power of 0.41 W. and 0.10 W. Green lines:

    numerical solutions of Eq. (5) with the values obtained in the course of the two

    calibration processes, for the same heating powers. (For interpretation of the re-

    ferences to colour in this  gure caption, the reader is referred to the web version of 

    this paper.)

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    and P under three different values of rf  eld (30, 60, and 90 mT)

    applied for 1800 s. According to our preliminary investigations,

    carried on by performing several measurements at different

    heating times, 1800 s are suf cient to reach the equilibrium stable

    state. The thermocouple is inserted in the test tube immediately

    after switching off the rf   eld, therefore the   rst measured tem-

    perature is assumed to be equal to  T M . The samples are then left

    cool down to T a. Fig. 6 shows the three temperature vs. time curvesfor both samples A and P; for convenience, the abscissa is relative

    to the switching on of the rf  eld; as only the cooling part of the

    curve is acquired, the horizontal axis in Fig. 6 starts at 1800 s.

    The equilibrium temperatures  T M  for the three applied rf  eld

    intensities are shown in the inset: as expected, the system heats at

    higher temperatures when higher rf   elds are applied. With the

    measured values of  T M  for both samples A and P it is then possible

    to calculate SAR using Eq. (9):

    =( )

    SAR  P 

    m   10 particles

    where m particles  is the mass of the particles dispersed in the solu-

    tion. The results are reported in Fig. 7a. As expected, SAR increases

    with increasing   eld value; for rf   elds below 30 mT the power

    losses of the studied particles become negligible.

    However, the intrinsic loss power (ILP) has been proposed as a

    quantity more suitable for comparing hyperthermia results ob-

    tained on different families of particles, since the power losses are

    normalised not only to the sample mass, but also to  eld intensity

    and frequency [19]:

    ν=

    ( )ILP 

      SAR

    H    112

    where  ν  is the frequency of the rf exciting eld H . In Fig. 7b values of 

    intrinsic loss power are reported for both samples A and P. A cleardistinction between the two samples can be observed: sample A,

    which is constituted by larger particles that display static hysteresis

    (see Fig. 1), has a non-monotonous dependence of ILP with the rf 

    eld, indicating non-linear magnetisation dynamics giving rise to

    power losses that depend on   eld intensity. Conversely, sample P,

    that is constituted by smaller particles having a superparamagnetic

    behaviour, is characterised by an almost constant ILP as a function

    of  H , having a lower value with respect to sample A.

    5. Conclusions

    A thermodynamic model and a calibration procedure have been

    proposed for a magnetic hyperthermia setup that does not need to

    Fig. 6.   Time dependence of the temperature of 1 ml of water containing 10 mg of 

    particles of samples A (top) and P (bottom), submitted to three values of rf  eld for

    1800 s. The origin of the abscissa is at the instant of switching on of the rf   eld.

    Insets: dependence of the equilibrium temperature  T M  as a function of the rf  eld

    intensity.

    Fig. 7.   Specic absorption rate (SAR, panel (a)) and intrinsic loss power (ILP, panel

    (b)) as a function of rf exciting  eld of samples A and P.

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    operate in adiabatic conditions. Accurate determination of the

    specic absorption rate is possible without measuring the initial

    slope of the temperature vs. time curve; instead, by measuring the

    equilibrium temperature at the stationary state, SAR can be ob-

    tained. The thermodynamic model is validated on the actual ex-

    perimental setup through a proper calibration procedure, and the

    method is  nally applied on a set of samples consisting in water

    solutions of magnetic nanoparticles allowing the experimental

    determination of SAR and ILP.

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