THE TIME DEPENDENCE OF THE CHANGES OF ULTRASONIC WAVE ATTENUATION IN BIOCOMPATIBLE MAGNETIC FLUIDS

Jozepczak A., Skumiel A.

Institute of acoustics, Adam Mickewicz University, Umultowska 85, 61- 614, Poznan, Poland.

Aqueous magnetic fluid have been used for diagnostics and therapy in clinical applications and in biotechnology. Every application needs its own tailored magnetic core / shell composition [1]. In order to be used for biomedical purposes, magnetic particles must be pro-coated with substances that ensure their stability, biodegradability and non-toxicity in a physiological medium. After pre-coating, biological effectors can be absorbed at the particle surface to produce biocompatible materials with specific biomedical applications [2]. Biocompatible magnetic fluids may be used as a delivery system for anticancer agents in locoregional tumour therapy [3 – 5] and for magnetic fluid hyperthermia [5 – 8].

The use of biocompatible magnetic fluids in living organisms (in vivo) needs a thorough recognition of their properties. This paper presents acoustic properties of biocompatible magnetic fluids containing magnetic particles coated with one or two biological layers and subjected to an external magnetic field. The time dependence of ultrasonic wave absorption coefficient is obtained for the magnetic fluid in an external magnetic field and without it. The results are expected to bring information on the effect of the second layer of the surfactant on the stability of the ferrofluid structure subjected to an external magnetic field.

Two magnetic fluids based on water were used in this work. In the first one MF the magnetite particles Fe₃O₄ were coated with a single layer of a surfactant being oleat sodium, while in the second one MF dextran the magnetite particles were coated with two surfactant layers: oleat sodium and dextran.

The ultrasonic wave absorption coefficient was measured as a function of time in the following conditions: when the magnetic field increased in 1 minute to a certain value at different sweep rates (80 kA/m at a sweep rate dH / dt = 1.3 kA / ms, 160 kA / m at a sweep rate dH / dt = 2.7 kA / ms, 340 kA / m at a sweep rate dH / dt = 5,7 kA / ms), when a constant value of the field was maintained for 30 minutes and then when the field decreased in 1 minute at the same sweep rate as that of the increase and after the field duisappearance (60 minutes). The area between the broken lines corresponds to the range of time in which the fluid was subjected to a maximum value magnetic field.

In medical applications a ferrofluid is subjected to a magnetic field for a certain time. It is important to know its behavior under the effect of the magnetic field and after its decay. Figure 1a and 1b present the ultrasonic wave absorption coefficient changes in the ferrofluid subjected to a magnetic field, for the MF and MF dextran fluids, respectively. For the ferrofluid MF with a single surfactant layer the changes in Da changes were small, but later the ultrasonic wave absorption coefficient significantly increased. This phenomenon was explained by progressing aggregation of magnetic particles in clusters [9] – it is know that the fluid structure needs some time to reach a new state of equilibrium [10]. With increasing magnetic field intensity, the viscosity of the ferrofluid increases because the applications of a magnetic field induced formation of particle clusters [11]. According to Stokes-Kirchoff equation with increasing the viscosity increase the absorption coefficient of ultrasonic wave. The maximal in Da are a result of an additional resonance absorption of the ultrasonic wave by the spherical clusters formed in the fluid [12]. After the magnetic field decay, the wave absorption coefficient drastically decrease but not to the initial value. The absorption coefficient shows a hysteresis as has been reported earlier [13].

After total decay of the magnetic field the absorption coefficient if the ultrasound wave does not remain constant but it slightly increases with time. Most probably the process of aggregation induced by the magnetic field still proceeds as a result of the particles-particles interactions. The greatest changes in Da were observed after the application of the strongest field leading to the greatest increase in the absorption.

The application of the second biological coating brings about further changes in the ferromagnetic fluid properties. Figure 1b clearly shows that the changes in the absorption coefficient produced by the same magnetic field in MF dextran are a few times smaller than those in MF. An addition of the second layer of the surfactant modifies the particles surface properties, and the attractive Van der Waals forces are reduced, which prevents aggregation of particles even in a strong magnetic field. Interestingly, the maximum increase in absorption is independent of the field applied, it is just quicker reached in a high field. The magnetic field of 80 kA / m caused the most stable changes in Da oin time, a decay of the field has not led to an abrupt decrease in the absorption coefficient. The decay of the magnetic fields of 160 kA / m and 340 kA/ m has resulted in a jump-wise decrease in absorption whici then remained unchanged in time.

In conclusion, the biocompatible magnetic fluid containing nanoparticles covered with two layers of surfactants is characterized by a high stability of its acoustic parameters in time. Results of determination of the absorption coefficient od ultrasound waves in a ferrofluid subjected to a magnetic field have shown that the field has little effect of the dextran dferrofluid structure. The finding is very promising from the point of view of future biomedical applications of such magnetic fluids.

Figure 1. The changes in the ultrasonic wave absorption coefficient

as a function of time in the ferrofluids: a) MF and b) MF DEXTRAN.

The authors gratefully acknowledge Professor M. Timko from Slovak Academy of Science (Kosice) for sample of ferrofluids.

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