Magnetohipertermia em nanopartículas core-shell
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2016-05-04
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Universidade Federal de Goiás
Resumo
The phenomenon of heat dissipation by magnetic materials interacting with an alternating magnetic field,
known as magnetic hyperthermia, is an emergent and promising therapy for many diseases, mainly cancer.
The scientific community has endeavored to identify the properties that lead to maximum efficiency
dissipation of magnetic nanoparticles. However, the diameter in which this efficiency reaches maximum is
sometimes bigger than 10 nm, presenting several incompatibilities with biomedical aplications. On the
other hand, small nanoparticles (< 8 nm}) do not suffer from the same disadvantages. On the contrary, they
benefit from a biodistribution convenient for cancer treatment, affinity for the lymphatic system, further
penetration of tumor tissue and renal clearance. However, the use of small nanostructures as heat centers
never received much attention, in part because the model most used to describe the magnetic hyperthermia
phenomenon, the linear response theory (LRT), provides a very small dissipation in these systems. Recently,
experimental results have questioned this inefficiency and evidences that it is possible to produce a
biological response (including cell death) without necessarily measuring a temperature variation opened up
new possibilities for small nanostructures. This research, therefore, proposes a change in magnetic
nanostructure tailoring strategy for biomedical applications of hyperthermia: to make more efficient
dissipation in small nanoparticles. Therefore, it is necessary to rebuild the theoretical framework of
hyperthermia, making the description of these small systems more accurate. This thesis deals with the
development of modeling tools to enable a distinction between the most superficial and internal region of
the nanoparticle, recognizing that many of the properties at the nanoscale has its origin in surface effects
and the surface-to-volume ratio. A model for the description of core-shell system magnetization was
developed, based on the Heisenberg Hamiltonian and a mean field theory in which different parameters may
be assigned to each region. The combination of this model with the LRT has given rise to a new description
of hyperthermia phenomenon in which the importance of surface effects and can be explicitly considered,
making also possible the description of heterogeneous systems. The model was compared with original
(homogeneous nanoparticles) and literature (heterogeneous nanoparticles) experimental data, with good
qualitative agreement with the results. In an attempt to verify the influence of effects of nonlinearity in these
systems, a non-linear response theory was developed from the generalization of the LRT, and applied to
core-shell systems. The fundamental role of these theoretical tools is to point the direction in which the
nanomaterials tailoring should advance to make viable the proposed hyperthermia with small
nanostructures. The models proposed here suggest that a higher dissipation efficiency in small systems is
obtained with a combination of materials which lead to the reduction ratio of shell-to-core damping factors,
increasing of the exchange constant in the interface and maximizing the shell-to-core anisotropy constants,
indicating that better results should be found in Soft@Hard systems.
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CARRIAO, M. S. Magnetohipertermia em nanopartículas core-shell. 2016. 125 f. Tese (Doutorado em Fisica) - Universidade Federal de Goiás, Goiânia, 2016.