Fabrication of MnxFe1-xO colloidal solid solution as a dual magnetic-resonance-contrast agent

Coupling metal organic frameworks with molybdenum disulfide nanoflakes for targeted cancer theranostics

The superior properties of metal organic frameworks (MOF) can provide great opportunities for merging functional nanoparticles to construct smart and versatile cancer theranostic agents. In this study, on the basis of non-mesoporous nanoparticles (molybdenum disulfide, MoS2), the structure of the MOF shell layer with an adjustable structure can be constructed through the natural coordination interaction between polydopamine (PDA) and iron ion, and the tumor cell target ligand was modified on the surface of the nanocomposite after loading the anticancer drug doxorubicin hydrochloride (DOX) to form a multifunctional cancer theranostics nanoplatform (DOX@MoS2-PMA). Benefiting from the excellent properties of MoS2 and MOF, the favorable photothermal properties and pH/near-infrared (NIR) laser-triggered DOX release behavior of composite nanoparticles were demonstrated. Its well-defined nanostructure, adequate colloidal stability, and satisfactory biocompatibility were further evidenced. Furthermore, the selective tumor cell targeting ability of DOX@MoS2-PMA can improve the cellular uptake efficacy and the photothermal-chemotherapy combination therapy can significantly enhance the killing effect on cancer cells both in vitro and in vivo. In addition, fluorescence imaging results show that nanoparticles can efficiently accumulate inside tumors. The photoacoustic (PA) and magnetic resonance (MR) imaging capabilities derived from different components of nanoparticles can perform better imaging effects. To the best of our knowledge, this is the first attempt to merge the performance of MoS2 with MOF for PA/MR dual-modality imaging-guided photothermal-chemotherapy combination therapy. Our work presented herein proves that MOF can be combined with non-mesoporous nanoparticles and exhibits excellent performance, thus opening a new avenue for endowing non-mesoporous nanoparticles with an efficient drug loading capacity and practical applications of MOFs in nanomedicine.

Tumor microenvironment responsive hollow mesoporous Co9S8@MnO2-ICG/DOX intelligent nanoplatform for synergistically enhanced tumor multimodal therapy

The development of multifunctional nanoplatform with combination of tumor microenvironment (TME)-responsive dual T₁/T₂ magnetic resonance (MR) imaging and synergistically self-enhanced photothermal/photodynamic/chemo-therapy is of significant importance for tumor theranostic, which still remains a great challenge. Herein, a novel hollow mesoporous double-shell Co₉S₈@MnO₂ nanoplatform loaded with photodynamic agent of indocyanine green molecules (ICG) and chemotherapy drug of doxorubicin (DOX) was designed for TME responsive dual T₁/T₂ enhanced MR imaging and synergistically enhanced anti-tumor therapy. The designed nanoplatform with MnO₂ shell can act as a TME-responsive oxygen self-supplied producer to alleviate tumor hypoxia and simultaneously improve photodynamic therapy (PDT) efficiency. Moreover, the TME-induced MnO₂ dissolving and near-infrared (NIR) triggered photothermal nature from Co₉S₈ shell can further promote the tumor-targeted DOX release, leading to the synergistically improved anti-tumor efficacy. And the simultaneous enhancement in dual T₁/T₂ MR signal was achieved for highly specific tumor diagnosis. The in vivo and in vitro results confirmed that the designed TME-triggered nanoplatform with synergistic combination therapy presented good biocompatibility, and superior inhibition of tumor growth than monotherapy. This study provides the opportunities of designing intelligent TME-activated nanoplatform for highly specific tumor MR imaging and collaborative self-enhanced tumor therapy.

Size-selected Fe3O4-Au hybrid nanoparticles for improved magnetism-based theranostics

Size-selected Fe₃O₄-Au hybrid nanoparticles with diameters of 6-44 nm (Fe₃O₄) and 3-11 nm (Au) were prepared by high temperature, wet chemical synthesis. High-quality Fe₃O₄ nanocrystals with bulk-like magnetic behavior were obtained as confirmed by the presence of the Verwey transition. The 25 nm diameter Fe₃O₄-Au hybrid nanomaterial sample (in aqueous and agarose phantom systems) showed the best characteristics for application as contrast agents in magnetic resonance imaging and for local heating using magnetic particle hyperthermia. Due to the octahedral shape and the large saturation magnetization of the magnetite particles, we obtained an extraordinarily high r ₂-relaxivity of 495 mM^-1·s^-1 along with a specific loss power of 617 W·g_Fe ^-1 and 327 W·g_Fe ^-1 for hyperthermia in aqueous and agarose systems, respectively. The functional in vitro hyperthermia test for the 4T1 mouse breast cancer cell line demonstrated 80% and 100% cell death for immediate exposure and after precultivation of the cells for 6 h with 25 nm Fe₃O₄-Au hybrid nanomaterials, respectively. This confirms that the improved magnetic properties of the bifunctional particles present a next step in magnetic-particle-based theranostics.

T1-T2 molecular magnetic resonance imaging of renal carcinoma cells based on nano-contrast agents

BACKGROUND

The development of T₁-T₂ dual contrast agent (CA) favors the visualization of the lesion in a more accurate and reliable manner by magnetic resonance imaging (MRI). The relaxivity and the interference between T₁ and T₂ CA are the main concerns for their design.

METHODS

In this work, we constructed an Fe₃O₄@mSiO₂/PDDA/BSA-Gd₂O₃ nanocomplex where BSA-Gd₂O₃ NPs and Fe₃O₄ NPs were chosen as T₁ and T₂ MRI CAs and a 20 nm mesoporous silica (mSiO₂) nanoshell was introduced to reduce the interference between them. We performed transmis sion electron microscopy, X-ray powder diffraction, UV-vis absorption spectra, and Fourier transform infrared absorption (FTIR) spectra to characterize the prepared nanocom-plex and MRI scanning to evaluate their MRI behaviors. Furthermore, 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and hematologic and biochemical analyses were introduced to evaluate their in vitro and in vivo toxicity. Finally, the specific MRI of 786-0 cells with Fe₃O₄@mSiO₂/PDDA/BSA-Gd₂O₃-AS1411 nanoprobe in vitro was realized. In vivo biodistribution of Fe₃O₄@mSiO₂/PDDA/BSA-Gd₂O₃ nanocomplex in the mouse was determined by the quantification of the Gd element by inductively coupled plasma-mass spectrometry.

RESULTS

The prepared Fe₃O₄@mSiO₂/PDDA/BSA-Gd₂O₃ nanocomplex possessed high longitudinal (r₁=11.47 mM s^-1 Gd) and transverse (r₂=195.1 mM s^-1 Fe) relaxivities, enabling its use as a T₁-T₂ dual contrast agent for MRI. MTT testing and hematologic and biochemical analysis indicated the good biocompatibility of Fe₃O₄@mSiO₂/PDDA/BSA-Gd₂O₃ nanocomplex in vitro and in vivo. After further conjugation with AS1411 aptamer, they could target tumor cells successfully by T₁ and T₂ MRI in vitro. The possible metabolic pathway of the tail vein-injected Fe₃O₄@mSiO₂/PDDA/BSA-Gd₂O₃ nanocomplex in mouse was mainly via kidney.

CONCLUSION

A T₁-T₂ dual-mode contrast agent, Fe₃O₄@mSiO₂/PDDA/BSA-Gd₂O₃ nano-complex, was developed and its good performance for tumor cell targeting in vitro and kidney contrast-enhanced MRI in mice indicated its promising potential as an effective T₁-T₂ dual-mode contrast agent for in vivo MRI with self-confirmation.

Magnetite-Gold nanohybrids as ideal all-in-one platforms for theranostics

High-quality, 25 nm octahedral-shaped Fe3O4 magnetite nanocrystals are epitaxially grown on 9 nm Au seed nanoparticles using a modified wet-chemical synthesis. These Fe3O4-Au Janus nanoparticles exhibit bulk-like magnetic properties. Due to their high magnetization and octahedral shape, the hybrids show superior in vitro and in vivo T2 relaxivity for magnetic resonance imaging as compared to other types of Fe3O4-Au hybrids and commercial contrast agents. The nanoparticles provide two functional surfaces for theranostic applications. For the first time, Fe3O4-Au hybrids are conjugated with two fluorescent dyes or the combination of drug and dye allowing the simultaneous tracking of the nanoparticle vehicle and the drug cargo in vitro and in vivo. The delivery to tumors and payload release are demonstrated in real time by intravital microscopy. Replacing the dyes by cell-specific molecules and drugs makes the Fe3O4-Au hybrids a unique all-in-one platform for theranostics.

MnO2-Functionalized Co-P Nanocomposite: A New Theranostic Agent for pH-Triggered T1/T2 Dual-Modality Magnetic Resonance Imaging-Guided Chemo-photothermal Synergistic Therapy

Construction of stimuli-responsive theranostic nanoagents that can increase the accuracy of imaging diagnosis and boost the therapeutic efficacy has been demonstrated for a promising approach for diagnosis and treatment of cancer. Herein, we constructed a novel theranostic agent with Co-P nanocomposites as core, mesoporous silica as shell, and manganese dioxide (MnO₂) nanosheets as gatekeeper, which have been employed for pH-activatable T₁/T₂ dual-modality magnetic resonance imaging (MRI)-guided chemotherapeutical and photothermal combination anticancer therapy in vitro and in vivo. Co-P core-enabled theranostic platform could be applied for both photothermal therapy and T₂-weighted MRI in the normal circulation owing to its strong  near-infrared absorbance and intrinsic magnetic properties. In the acidic environment of tumors, MnO₂ cap could be dissolved into Mn²⁺ ions to not only realize pH-responsive on-demand drug release but also activate T₁-weighted MRI contrast enhancement. Such T₁/T₂ dual-mode MR imaging provides further comprehensive details and accurate information for tumor diagnosis, and the on-demand chemo-photothermal synergetic therapy greatly improved the therapeutic effectiveness and effectively mitigated side effects. These findings demonstrate that Co-P@mSiO₂@DOX-MnO₂ are promising as pH-responsive theranostic agents for tumor diagnosis and treatment, and stimulate interest in exploration of novel stimuli-responsive theranostic nanoagents which posssess good potential for clinical application in the future.

Synthesis of Ferromagnetic Fe0.6 Mn0.4 O Nanoflowers as a New Class of Magnetic Theranostic Platform for In Vivo T1 -T2 Dual-Mode Magnetic Resonance Imaging and Magnetic Hyperthermia Therapy

Uniform wüstite Fe0.6 Mn0.4 O nanoflowers have been successfully developed as an innovative theranostic agent with T1 -T2 dual-mode magnetic resonance imaging (MRI), for diagnostic applications and therapeutic interventions via magnetic hyperthermia. Unlike their antiferromagnetic bulk counterpart, the obtained Fe0.6 Mn0.4 O nanoflowers show unique room-temperature ferromagnetic behavior, probably due to the presence of an exchange coupling effect. Combined with the flower-like morphology, ferromagnetic Fe0.6 Mn0.4 O nanoflowers are demonstrated to possess dual-modal MRI sensitivity, with longitudinal relaxivity r1 and transverse relaxivity r2 as high as 4.9 and 61.2 mm(-1) s(-1) [Fe]+[Mn], respectively. Further in vivo MRI carried out on the mouse orthotopic glioma model revealed gliomas are clearly delineated in both T1 - and T2 -weighted MR images, after administration of the Fe0.6 Mn0.4 O nanoflowers. In addition, the Fe0.6 Mn0.4 O nanoflowers also exhibit excellent magnetic induction heating effects. Both in vitro and in vivo magnetic hyperthermia experimentation has demonstrated that magnetic hyperthermia by using the innovative Fe0.6 Mn0.4 O nanoflowers can induce MCF-7 breast cancer cell apoptosis and a complete tumor regression without appreciable side effects. The results have demonstrated that the innovative Fe0.6 Mn0.4 O nanoflowers can be a new magnetic theranostic platform for in vivo T1 -T2 dual-mode MRI and magnetic thermotherapy, thereby achieving a one-stop diagnosis cum effective therapeutic modality in cancer management.

Fluorescent Heterodoped Nanotetrapods as Synergistically Enhancing Positive and Negative Magnetic Resonance Imaging Contrast Agents

In this work, we report Mn-Fe heterodoped ZnSe tetrapod nanocrystals (NCs) synthesized to synergistically enhance contrast in both T1- and T2-weighted magnetic resonance imaging (MRI). The proposed NCs were prepared using a customized heteroarchitecture such that the manganese (Mn) is confined in the core and iron (Fe) in the branches of the tetrapods. The elemental composition and profile of these NCs were studied using X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, and inductively coupled plasma mass spectroscopy. Photoluminescence quantum yield of these heterodoped NCs in water is ∼30%. Magnetic measurements reveal the simultaneous presence of superparamagnetic and paramagnetic behavior in these NCs because of the coexistence of Mn(2+) and Fe(2+) dopants. Their potential as simultaneous positive and negative MRI contrast agents was demonstrated by relaxivity measurements and in vivo MRI. From the in vivo studies, we also found that these NCs (with a hydrodynamic diameter of 20 nm) are excreted from the body within 24 h after the injection. Therefore, these heterodoped tetrapods NCs, while being fluorescent and safe, hold great future as a synergistically enhancing dual-modal MRI contrast agent.

Synthesis and controllable oxidation of monodisperse cobalt-doped wüstite nanoparticles and their core-shell stability and exchange-bias stabilization

Cobalt-doped wüstite (CWT), Co0.33Fe0.67O, nanoparticles were prepared via the thermal decomposition of CoFe2-oleate complexes in organic solvents. A controllable oxidation process was then performed to obtain Co0.33Fe0.67O/CoFe2O4 core-shell structures with different core-to-shell volume ratios and exchange bias properties. The oxidized core-shell samples with a ∼4 nm CoFe2O4 shell showed good resistance to oxygen transmission. Thus, it is inferred that the cobalt ferrite shell provides a better oxidation barrier performance than magnetite in the un-doped case. The hysteresis loops of the oxidized 19 nm samples exhibited a high exchange bias field (H(E)), an enhanced coercivity field (H(C)), and a pronounced vertical shift, thus indicating the presence of a strong exchange bias coupling effect. More importantly, the onset temperature of H(E) was found to be higher than 200 K, which suggests that cobalt doping increases the Néel temperature (T(N)) of the CWT core. In general, the results show that the homogeneous dispersion of Co in iron precursors improves the stability of the final CWT nanoparticles. Moreover, the CoFe2O4 shells formed following oxidation increase the oxidation resistance of the CWT cores and enhance their anisotropy energy.

Highly monodisperse low-magnetization magnetite nanocubes as simultaneous T(1)-T(2) MRI contrast agents

We report the first study of highly monodisperse and crystalline iron oxide nanocubes with sub-nm controlled size distribution (9.7 ± 0.5 nm in size) that achieve simultaneous contrast enhancement in both T1- and T2-weighted magnetic resonance imaging (MRI). Here, we confirmed the magnetite structure of iron oxide nanocubes by X-ray diffraction (XRD), selected area electron diffraction (SAED) pattern, optical absorption and Fourier transformed infrared (FT-IR) spectra. These magnetite nanocubes exhibit superparamagnetic and paramagnetic behavior simultaneously by virtue of their finely controlled shape and size. The magnetic measurements reveal that the magnetic moment values are favorably much lower because of the small size and cubic shape of the nanoparticles, which results in an enhanced spin canting effect. As a proof-of-concept demonstration, we showed their potential as dual contrast agents for both T1- and T2-weighted MRI via phantom studies, in vivo imaging and relaxivity measurements. Therefore, these low-magnetization magnetite nanocubes, while being non-toxic and bio-compatible, hold great promise as excellent dual-mode T1 and T2 contrast agents for MRI.

Design of FeBi nanoparticles for imaging applications

A variety of imaging technologies are now routinely used in the medical field, their use being continuously enlarged through the development of contrast agents. Recently nanoparticles (NPs) proved efficient to improve imaging in vivo by increasing contrast and targeting capabilities. The current trend is now focused on the development of dual contrast agents combining two or more functionalities on the same NP. Motivated by this new challenge we developed FeBi NPs as new nanomaterials with potential application as a contrast agent for MRI and CT imaging. In addition to the well-known use of iron in the development MRI contrast agents, we chose Bi as a CT imaging agent rather than the more documented gold, because it possesses a larger X-ray attenuation coefficient and is much less expensive. Two sets of NPs, with sizes around 150 nm and 14 nm, were synthesized using organometallic approaches. In both cases, the NPs are spherical, and contain distinct domains of Fe and Bi, with the surface being enriched with Fe, and a hydrophobic coating. This coating differs from one sample to the other: the surfaces of the 150 nm large NPs are coated by amine ligands, while those of the 14 nm large NPs are coated by a mixture of an amine and its hydrochloride salt. Exchange of the surface ligands to afford water soluble NPs has been attempted. We show that only the larger NPs could be functionalized with water soluble ligands, which is in agreement with the lability of their initial surface coating. Colloidal aqueous solutions of FeBi NPs with glycoPEG ligands have been obtained.

Developing Fe3O4 nanoparticles into an efficient multimodality imaging and therapeutic probe

A rapid ligand-exchange method was developed to transfer high quality hydrophobic magnetite nanocrystals into water-soluble NPs by using protocatechuic acid as a ligand via homogenous reaction. After ligand exchange, the magnetite nanocrystals not only exhibited outstanding stability in water, but also maintained high crystallinity and saturation magnetization. Cell viability experiments demonstrated good biocompatibility of the NPs. For 12 nm magnetite nanoparticles (NPs), the small hydrodynamic size of 14 nm enabled a high T1 relaxivity of 17.8 mM(-1) s(-1) while high saturation magnetization of 77.8 emu g(-1) enabled the NPs to exhibit a high T2 relaxivity of 220 mM(-1) s(-1) in MRI phantom experiments. In vivo MR imaging experiments further confirmed that the NPs were eminent T1 and T2 contrast agents. Moreover, the high quality NPs can be used as excellent magnetic heating agents under an alternating magnetic field. With all those features, including multimodality imaging and magnetic hyperthermia, the NPs can be used as single compound multifunctional agents for various biomedical applications, especially for cancer diagnosis and therapy.

Manganese-loaded dual-mesoporous silica spheres for efficient T1- and T2-weighted dual mode magnetic resonance imaging

A novel class of manganese-based dual-mode contrast agents (DMCAs) based on the core-shell structured manganese-loaded dual-mesoporous silica spheres (Mn-DMSSs) for simultaneous T1- and T2-weighted magnetic resonance imaging (MRI) has been successfully reported. The in vitro MR tests demonstrate that the Mn-based DMCAs display an excellent simultaneous T1-weighted and T2-weighted MR imaging effect with a noticeably high T1 relaxivity (r1) of 10.1 mM(-1) s(-1) and a moderately high T2 relaxivity (r2) of 169.7 mM(-1) s(-1). The Mn-based DMCAs exhibit negligible cytotoxicity with >80% cell viability at a concentration of up to 200 μg/mL in human liver carcinoma (HepG2) and mouse macrophage (RAW264.7) cells after 24 h. Confocal laser scanning microscopy (CLSM) results show that the Mn-DMSSs were internalized via endocytosis and located in the cytoplasm but not in the nucleus. The in vivo experiment shows that the signals of rat liver increased by 29% under T1-weighted imaging mode and decreased by 28% under T2-weighted imaging mode in 5 min postinjection of Mn-DMSSs, which reveal that the novel Mn-loaded DMSSs can be used as both positive (T1-weighted) and negative (T2-weighted) MR contrast agents in further biomedical applications.

Manganese-enhanced MRI of rat brain based on slow cerebral delivery of manganese(II) with silica-encapsulated Mn x Fe(1-x) O nanoparticles

In this work, we report a monodisperse bifunctional nanoparticle system, MIO@SiO2 -RITC, as an MRI contrast agent [core, manganese iron oxide (MIO); shell, amorphous silica conjugated with rhodamine B isothiocyanate (RITC)]. It was prepared by thermal decomposition and modified microemulsion methods. The nanoparticles with varying iron to manganese ratios displayed different saturated magnetizations and relaxivities. In vivo MRI of rats injected intravenously with MIO@SiO2-RITC nanoparticles exhibited enhancement of the T1 contrast in brain tissue, in particular a time-delayed enhancement in the hippocampus, pituitary gland, striatum and cerebellum. This is attributable to the gradual degradation of MIO@SiO2-RITC nanoparticles in the liver, resulting in the slow release of manganese(II) [Mn(II)] into the blood pool and, subsequently, accumulation in the brain tissue. Thus, T1-weighted contrast enhancement was clearly detected in the anatomic structure of the brain as time progressed. In addition, T2*-weighted images of the liver showed a gradual darkening effect. Here, we demonstrate the concept of the slow release of Mn(II) for neuroimaging. This new nanoparticle-based manganese contrast agent allows one simple intravenous injection (rather than multiple infusions) of Mn(II) precursor, and results in delineation of the detailed anatomic neuroarchitecture in MRI; hence, this provides the advantage of the long-term study of neural function.

A synergistically enhanced T(1) -T(2) dual-modal contrast agent

Monodisperse Gd(2) O(3) -embedded iron oxide (GdIO) nanoparticles can simultaneously enhance the local magnetic field intensities of each other under an external magnetic field and result in synergistic enhancement of T(1) and T(2) effects. GdIO nanoparticles have the unique property to be both T(1) and T(2) contrast agents and can potentially lead to higher accuracy in cancer diagnosis, particularly liver tumors.

A thermolysis approach to simultaneously achieve crystal phase- and shape-control of ternary M-Fe-O metal oxide nanoparticles

Significant studies have achieved beautiful control in particle size, while the shape- and phase-control synthesis of nanoparticles remains an open challenge. In this study, we have developed a generalized methodology to selectively prepare either NaCl-type (reduced form) or spinel-type ferrite (oxidized form) M-Fe-O (M = Mn, Co) crystallites with high reproducibility. A two-step heating process was able to control formation of two types of crystal phase, either a thermodynamic spinel-type under air or a kinetic-control of NaCl-type (rock salt structure) under Ar in a cubic morphology. On the other hand, the three-step heating procedure in air obtained the spinel-type with a thermodynamic equilibrium octahedral shape exclusively. Either using metal acetates (M(ac)(2)) or metal acetylacetonates (M(acac)(2)) as the starting precursors (M = Mn, Co) can be introduced to prepare NaCl-type (reduced form) or spinel-type ferrite (oxidized form) crystallites with identical experimental parameters, including precursor concentration, reaction temperature, reaction time, and heating rate. The oleic acid molecule, reaction temperature, and heating rate employed in the synthesis were carefully examined and found acting as determined roles behind the reaction processes. Apart from the previous literature reports as shape-directed and/or stabilizing agents, the oleic acid molecule played an additional phase-tuning role.

Amphiphilic polymer-coated hybrid nanoparticles as CT/MRI dual contrast agents

We describe hybrid nanoparticles, composed of iron oxide and gold nanoparticles, as potential dual contrast agents for both computed tomography (CT) and magnetic resonance imaging (MRI). The hybrid nanoparticles are synthesized by thermal decomposition of mixtures of Fe-oleate and Au-oleylamine complexes. Using a nano-emulsion method, the nanoparticles are coated with amphiphilic poly(DMA-r-mPEGMA-r-MA) to impart water-dispersity and antibiofouling properties. An in vitro phantom study shows that the hybrid nanoparticles have high CT attenuation, because of the constituent gold nanoparticles, and afford a good MR signal, attributable to the contained iron oxide nanoparticles. Intravenous injection of the hybrid nanoparticles into hepatoma-bearing mice results in high contrast between the hepatoma and normal hepatic parenchyma in both CT and MRI. These results suggest that the hybrid nanoparticles may be useful as CT/MRI dual contrast agents for in vivo hepatoma imaging.

Nanoparticles as magnetic resonance imaging contrast agents for vascular and cardiac diseases

Advances in nanoparticle contrast agents for molecular imaging have made magnetic resonance imaging a promising modality for noninvasive visualization and assessment of vascular and cardiac disease processes. This review provides a description of the various nanoparticles exploited for imaging cardiovascular targets. Nanoparticle probes detecting inflammation, apoptosis, extracellular matrix, and angiogenesis may provide tools for assessing the risk of progressive vascular dysfunction and heart failure. The utility of nanoparticles as multimodal probes and/or theranostic agents has also been investigated. Although clinical application of these nanoparticles is largely unexplored, the potential for enhancing disease diagnosis and treatment is considerable.

Fabrication of Mn-ferrite nanoparticles from MnO colloids

The reaction mechanism for conversion of MnO nanoparticles to Mn-ferrite nanoparticles was studied, which involved sequential consumption of MnO and the growth of ferrite. The method could be applied to other ferrite nanoparticles including cobalt ferrite.