Page 183 - Computational Modeling in Biomedical Engineering and Medical Physics

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2019). The MNPs are then injected into the bloodstream at a suitable location,
directed to the tumor site using an external magnetic field produced by a permanent
magnet (PM), and directed to the capillary bed of the diseased tissue (Papisov et al.,
1987; Papisov and Torchilin, 1987; Alexiou et al., 2000, 2006a,b). Here they could in
turn be activated by an enzyme, pH, or trough temperature (Papisov et al., 1987).
However this procedure also has its challenges, largely due to the dynamics of the
MNPs within the blood flow, which is not well understood hence mastered at the
moment (Kaminski et al., 2003; Babincova and Babinec, 2009; Price et al., 2017). In
the attempt to alleviate this difficulty, monitoring and control techniques have been
suggested to ensure the MD proper localization (Bai et al., 2016; Chuzawa et al.,
2013; Shapiro, 2009).
Despites the efforts the magnetic field “design” (spectrum and intensity) is still a
challenge (Gleich et al., 2007; Morega et al., 2013a,b,c; Manea et al., 2014; Shapiro
et al., 2014; Sharma et al., 2015; S˘ andoiu, 2019). To produce the field that ought to
guide and immobilize particles, MDT utilizes, usually, widely available, inexpensive
and powerful PMs. In vivo and in vitro studies have demonstrated the potency of
PMs to immobilize MNPs provided they are placed close enough to the target.
Several studies were conducted with the aim of optimizing their size, shape, and ori-
entation for MNPs control (Dobre and Morega, 2010; S˘ andoiu, 2019; Liu et al.,
2019; Liu, 2019). Moreover, Halbach arrays (Riegler et al., 2011; Barnsley et al.,
2016), magnetic field concentrator ferromagnetic wires (Iacob and Chiriac, 2004), or
transdermal ferromagnetic implants (Avilés et al., 2005; Cregg et al., 2010; Cregg
et al., 2012) were alternatively proposed.
Although significant progress was made using PMs to control MNPs, they have sev-
eral disadvantages. The strength of the magnetic field that they produce decreases fast
(power law) with the distance to the source. Consequently it has a rather narrow region
of influence on the circulatory system, and it cannot act efficiently upon the MD that is
injected deeper into the body. Moreover, the magnetic field created by a PM is fixed
and static and can only be changed by the physical displacement of the magnet.
To overcome these limitations, electromagnets have been proposed and experi-
mental, and numerical studies have shown that electromagnetic MDT is feasible (Xu
et al., 2005; Morega et al., 2013a,b,c). Several works were published on electromagnet
systems specially built for MDT that can produce extremely intense, high gradient
magnetic fields, confined in rather narrow regions (Hoskins et al. 1990; Gleich et al.,
2007; Li et al., 2018). Their magnetic field may be controlled to guide, to some
extent, the MNPs by conveniently adjusting the current. Nevertheless, controlling
MNPs flow in micrometric capillaries is unrealistic by cause of the rather high rate of
the blood flow (Furlani and Furlani, 2007; Shaw, 2020).
The implant-assisted MDT combines the advantages of PMs with the guiding prop-
erty of electromagnets, and it involves a stent, wire or cylinder that are invasively