ABSTRACT
The original iron oxide nanoparticle-based contrast agents are fabricated by the coprecipitation of iron oxides from iron salts and consist of a crystalline iron oxide core ranging 5-10 nm in diameter and a dextran coating ranging approximately 50-150 nm in diameter [8-10]. It was discovered that these particles are both passively and preferentially taken up by the liver and spleen, and the first imaging efforts were aimed at providing contrast enhancement of these tissues [6, 7]. These materials were soon joined in clinical use by silica-coated particles made by a similar process [10, 11]. In the late 1990s, improved coating methods allowed for the industrial scale production of ultrasmall iron oxide nanoparticles, which, due to their smaller size, could be passively transported into the lymph system [12, 13]. Most of the clinically available iron oxide contrast agents share these characteristics: facile industrial fabrication by the coprecipitation method, dextran-based or silica-based surface coating, passive uptake by certain tissues due to their size, and a relatively wide size distribution.In addition to these well-characterized materials, academia has produced many novel iron oxide-based nanostructures with potential as diagnostic and/or therapeutic tools. Many particle formulations have been developed with different physical, chemical, and biological properties [14]. Furthermore, active targeting mechanisms have been well established in the academic sector for over a decade and advanced materials employing both diagnostic and therapeutic techniques (theranostic nanomedicine) are
quickly approaching clinical feasibility [15]. These new materials must exhibit large magnetizations, narrow size distributions, colloidal stability, and biocompatibility that matches or exceeds that of existing materials to be clinically efficacious [8, 9, 16, 17]. In order to improve these nanomaterials for clinical translation, the quality of these parameters and others must be continually improved. The primary means for doing so are through improving the control of nanoparticle fabrication and/or by altering the surface presented by the particles to cells and tissues. In this review, we will examine the physical and biological properties of iron oxide-based nanomaterials for clinical uses and explore more novel materials poised for clinical translation. We will present multiple fabrication techniques and conclude with some basic recommendations aimed at facilitating the clinical translation of new magnetic nanomaterials. 18.2 Properties of Iron Oxide NanoparticlesAlthough the metal component of magnetic nanoparticles is not intentionally presented to biological structures, the iron oxide core contributes greatly to the diagnostic or therapeutic efficacy of these materials and is responsible for their magnetic behavior. One of the most important parameters of these particles is particle diameter. The size of the iron oxide core will largely determine the magnetic properties of the particle and, therefore, will control the contrast produced in MR images. Core size also controls, in part, functional characteristics pertinent to other uses such as heat production in magnetically induced hyperthermia. The biocompatibility of these nanomaterials is also of the utmost importance for clinical translation. Particle surface coatings must be carefully chosen to preserve and enhance the biocompatibility of the metallic oxide core. 18.2.1 Geometric PropertiesMost clinically available and investigational iron oxide nanomaterials consist of magnetite, Fe3O4, maghemite, γ-Fe2O3, or some combination of the two. Hematite, α-Fe2O3, has been investigated as well, but has not received the same degree of attention due
to smaller magnetization values and more arduous fabrication routes [18]. Due to its inverse spinal crystalline structure with no vacancies, magnetite generally has stronger magnetization than magnetite given an identical applied magnetic field. This behavior is observed because magnetite has a defect spinal structure with repeating iron atom vacancies making it essentially an iron-deficient form of magnetite [19]. However, magnetite is very sensitive to oxidation during fabrication. The result is that most particle formulations are either partly or entirely maghemite.The diameter of an iron oxide nanoparticle greatly affects multiple magnetic parameters and the nanometer scale of these particles offers unique magnetic, thermal, and biological properties [20]. The saturation magnetization, spontaneous or resting magnetization, and the magnetic susceptibility are all controlled by particle size [21]. This results in a highly non-linear influence of size on particle magnetic behavior, though smaller particles tend to have smaller saturation magnetizations [22, 23]. This behavior is further complicated by the presence of a magnetically inert layer at the surface of the particle caused by a disordered crystalline structure, a result of the fabrication method [21, 23, 24]. The crystalline structure of the entire particle also plays a role in the magnetic properties, with highly crystalline particles exhibiting stronger magnetizations, making them more efficacious for clinical uses [8]. All of these effects are represented by a single phenomenon describing the enhanced magnetic properties of nanoscale iron oxide nanoparticles: superparamagnetism. 18.2.2 Superparamagnetism
Superparamagnetism is a size-dependent magnetic phenomenon that occurs in nanoscale crystals of many ferro-, ferri-, and antiferromagnetic materials. First predicted in 1930, superpara-magnetism is a state of enhanced magnetic properties in a particle due to the presence of a single magnetic domain [1, 25]. In a bulk sample of these materials, the magnetic moments of multiple do-mains act unidirectionally to produce a stable spontaneous mag-netization in ferromagnetic and ferromagnetic materials, or cancel out to produce a state of no magnetism as in antiferromagnetic materials. However, when the particle size is sufficiently small, the iron oxide crystal contains a single magnetic domain. Due to direc-
tionally randomizing thermal fluctuations, bulk solutions of these particles have no magnetization at room temperature in the absence of an externally applied magnetic field [26]. This behavior is similar to paramagnetic behavior and follows many of the same physical laws as paramagnetism; however, single domain particles exhibit stronger magnetization compared to multi-domain para-magnetic materials, giving rise to the term superparamagnetism or superparamagnetic iron oxides (SPIOs) [25].Particle size plays an important role in the magnetic properties of SPIOs: above a particular diameter, an iron oxide particle forms multiple domains. For example, an iron oxide crystal 150 nm in diameter exhibits magnetization, susceptibility, and coercivity that are predictable using the bulk properties and relationships of that material [23]. As particle size decreases to approximately 30 nm for iron oxides, the crystal becomes single domain, loses its coercive behavior, deviates from bulk property characteristics and displays enhanced magnetization [5]. It is the enhanced magnetization and magnetic susceptibility that makes SPIOs effective as MRI contrast agents. 18.2.2.1 Superparamagnetic mechanisms for MRI contrast
One of the most well known applications for SPIOs is for enhancing contrast in MRI. SPIOs are a strong enhancers of transverse relaxation, resulting in dark negative contrast when they are employed for T2-and T 2 * -weighted MRI [27]. They are also capable of producing positive contrast when employed at lower concentrations and smaller sizes in T1-weighted MRI [28]. Negative contrast T2 agents also undergo relaxation on the order of 1-100 ms, which is favorable for moving systems like the bowel, and act to reduce ghost artifacts [5]. 18.2.3 Biocompatibility
One of the major advantages of SPIO contrast agents is that they are highly biocompatible. The gadolinium used in other contrast agents is toxic and must be delivered with a chelating agent to increase biocompatibility [29]. The high biocompatibility of SPIO agents are due to two factors: the natural metabolism of iron in vivo and the enhanced biocompatibility of the surface coating
[30]. A coating layer is necessary for clinical SPIO nanomaterials in order to prevent aggregation and impart hydrophilicity and colloidal stability in biological fluids [8]. The oldest clinically available SPIOs are coated in dextran and are cleared rapidly from circulation [31]. This is due primarily to the total size of the particles. In most cases, the ultimate fate of clinically available SPIOs is dependent on the total particle size derived from the iron oxide core and the thickness of the surface coat. The most common method of clearance for most of these particles, which are approximately 10-200 nm, is uptake by the cells of the reticuloendothelial system (RES). The majority of particles in this size range are endocytosed by Kupffer cells in the liver or into endothelial cells or RES cells of the spleen. The particles are then metabolized via the lysosomal pathway 1-2 days after internalization, and the iron is incorporated into hemoglobin over 28-40 days [31, 32]. Larger particles are quickly opsonized by plasma proteins and are cleared by phagocytic cells in the spleen [16]. Advanced fabrication methods have allowed for the repeatable production of ultrasmall superparamagnetic iron oxides (USPIOs) [13, 33]. These smaller particles, often smaller than 10 nm in diameter, are capable of evading RES clearance due to their size and can enter the lymph system, penetrate tissue, and are cleared by the renal system [10, 34]. The clinically available SPIO and USPIO materials, as well as newer investigational materials, have repeatedly been shown to be highly biocompatible. 18.3 Fabrication
The fabrication mechanism chosen for SPIOs and USPIOs plays a critical role in their efficacy as diagnostic and therapeutic agents. Particle magnetization and susceptibility depend largely on particle size, population size distribution, crystallinity, and surface coat [16]. These parameters are established during fabrication and different synthetic routes, though they produce particles of similar sizes, may produce dissimilar materials due to differences in size distribution, crystallinity, or other parameters [35, 36]. The fabrication route chosen to design novel materials for clinical applications is non-trivial and each method has advantages and disadvantages.
