Therapeutic Applications

Magnetic polymer particles must have the appropriate properties for use as a carrier in various in vivo therapeutic applications. The polymer must be biodegradable if the microspheres are to remain inside the organism [55]. The physi-cochemical and colloidal characteristics of the magnetic carrier and the properties of the continuous phase must be considered. The intensity and distance from the magnetic field applied must be taken into account to check the magnetic particle distribution and displacement before degradation. They must have a biocompatible surface that does not cause an immune system reaction and that generates an efficacious and specific targeting action. Finally, the diameter of the particles is also a crucial parameter. Risks of phagocytosis are limited with particles of diameter greater than 12 pm [26]. On the other hand, small particles (less than 1 pm) can penetrate the capillaries. These act more specifically but do not resist phagocytes.

FIG. 7 Schematic illustration of hydrophilic magnetic thermally sensitive latex particles via temperature flocculation process.

The total quantity of medicine introduced into the organism is sometimes limited by toxic effects beyond a certain concentration [26]. By injecting the active ingredient in a polymer capsule or matrix, it is possible to control the diffusion and release of the desired product in the organism. This also avoids repetitive administration of the medicine. The targeting is then well controlled by applying a magnetic field that improves the localization of the chemical product and prevents the particles from being captured by the reticuloendothe-liosis system [58]. Because the product is locally concentrated, the total dose administered can be reduced, with a corresponding reduction of toxicity problems. For example, the total dose of doxorubicin, injected freely, is 100 times higher than that introduced with a magnetic carrier for an equivalent concentration at the treatment site.

The active agent is released outside the particles as a function of mechanisms that depend on the properties of the polymer. Adriamycin and doxorubicin in lyophilized albumin magnetic microspheres, conserved at 4°C, diffuse spontaneously at 37°C, in an aqueous solution of 1 g/L NaCl at a rate that depends on the cross-linking of the matrix during synthesis. In the case of certain so-called smart materials, variation of the medium (i.e., pH, temperature, ionic strength) leads to modification of the polymer's properties [59]. For example, certain thermally sensitive polymer-based particles are hydrophilic and expand below their volume phase transition temperature, i.e., poly(NIPAM) particles below 32°C. On the other hand, the particles shrunk above the transition temperature, thus releasing the encapsulated active ingredient, as illustrated in Fig. 8.

FIG. 8 Release of the active ingredient by controlling incubation temperature. Tvpt is the volume phase transition temperature, the TVPT in the case of poly (W-isopropylacryl-amide) microgel particles is nearly 35°C. (1) Below the TVPT the particles are swollen by the active agent, and (2) above the TVPT the active agent is released.

FIG. 8 Release of the active ingredient by controlling incubation temperature. Tvpt is the volume phase transition temperature, the TVPT in the case of poly (W-isopropylacryl-amide) microgel particles is nearly 35°C. (1) Below the TVPT the particles are swollen by the active agent, and (2) above the TVPT the active agent is released.

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