We present a detailed study of static and dynamic magnetic behavior of Fe 3 O 4 nanoparticles with average particle sizes ͗d͘ ranging from 5 to 150 nm. Bulk-like properties such as saturation magnetization, hyperfine parameters, coercive field, and Verwey transition are observed in 150 nm particles. For decreasing particle size, the Verwey temperature, T V , shifts down to ϳ20 K for ͗d͘ϭ50 nm and is no longer observable for smaller particles. The smallest particles (͗d͘ϭ5 nm) display superparamagnetic behavior at room temperature, with transition to a blocked state at T B ϳ45 K, which depends on the applied field. The existence of surface spin disorder can be inferred from the decrease of saturation magnetization M S at low temperatures, as the average particle size is reduced. This disordered surface did not show effects of exchange coupling to the particle core, as observed from hysteresis loops after field cooling in a 7 T magnetic field. For particles with ͗d͘ϭ5 nm, dynamic ac susceptibility measurements show a thermally activated Arrhenius-Néel dependence of the blocking temperature with applied frequency. The interparticle interactions are found to influence the energy barriers yielding an enhancement of the estimated magnetic anisotropy. From the calculus of the magnetic anisotropy, it is inferred that there is no structural transition from cubic to triclinic symmetry for ͗d͘ϭ5 nm, in agreement with the absence of the Verwey transition. A value K 1 ϭ4.68ϫ10 5 erg/cm 3 is obtained for the magnetocrystalline anisotropy constant of the cubic phase.

Nanocomposite membranes based on thermosensitive, poly(N-isopropylacrylamide)-based nanogels and magnetite nanoparticles have been designed to achieve “on-demand” drug delivery upon the application of an oscillating magnetic field. On-off release of sodium fluorescein over multiple magnetic cycles has been successfully demonstrated using prototype membrane-based devices. The total drug dose delivered was directly proportional to the duration of the “on” pulse. The membranes were non-cytotoxic, biocompatible, and retained their switchable flux properties after 45 days of subcutaneous implantation.

Drug delivery devices based on nanocomposite membranes containing thermoresponsive nanogels and superparamagnetic nanoparticles have been demonstrated to provide reversible, on-off drug release upon application (and removal) of an oscillating magnetic field. We show that the dose of drug delivered across the membrane can be tuned by engineering the phase transition temperature of the nanogel, the loading density of nanogels in the membrane, and the membrane thickness, allowing for on-state delivery of model drugs over at least two orders of magnitude (0.1-10 µg/ hr). The zero-order kinetics of drug release across the membranes permit drug doses from a specific device to be tuned according to the duration of the magnetic field. Drugs over a broad range of molecular weights (500-40,000 Da) can be delivered by the same membrane device. Membrane-to-membrane and cycle-to-cycle reproducibility is demonstrated, suggesting the general utility of these membranes for drug delivery. KeywordsIron oxide; nanogel; nanoparticles; triggered drug release; on-demand; superparamagnetism Sustained drug release technology has been applied in a wide variety of medical fields1. Many devices are passive, exhibiting release kinetics that are either constant or decreasing over time. However, drug delivery devices that can be repeatedly switched on and off would be optimal for effective treatment of conditions such as diabetes, chronic pain, or cancer. 2 * To whom correspondence should be addressed. Daniel.Kohane@childrens.harvard.edu. † These authors contributed equally to this report. To this end, environmentally responsive ("smart") materials have been developed that can respond to stimuli that are either internal to the patient (e.g. body temperature) or external (e.g. a remotely-applied magnetic field). Temperature-sensitive drug delivery devices have been developed based on the thermoreversible polymer poly(N-isopropylacrylamide) (PNIPAm) 3 , which has been incorporated into implantable hydrogels4 -9, microparticles10, nanoparticles [11][12][13][14] , and surface-grafted polymers [15][16][17][18][19][20][21][22][23][24][25][26][27] . Examples of magnetically-activated materials include superparamagnetic nanoparticles, which absorb power when placed in an oscillating magnetic field and transfer heat to the surrounding medium. These nanoparticles have been used to achieve drug release from polymer scaffolds 28 , sheets 29 , liposomes 30 , microspheres 31,32 , microcapsules 33 , and nanospheres [34][35][36] , typically by mechanical disruption of the drug-biomaterial matrix. However, the quantity of drug contained by most of these "smart" carriers is relatively small, and drug release is often characterized either by a single burst event or inconsistent dosing as a function of triggering cycle. Supporting InformationTo achieve both triggered drug release and consistent dosing, we previously reported composite membranes containing both temperature-sensitive polymer nanoparticles (nanogel) and magnetically activated superparamagn...

Nanoparticles engineered for biomedical applications are meant to be in contact with protein-rich physiological fluids. These proteins are usually adsorbed onto the nanoparticle's surface, forming a swaddling layer that has been described as a 'protein corona', the nature of which is expected to influence not only the physicochemical properties of the particles but also the internalization into a given cell type. We have investigated the process of protein adsorption onto different magnetic nanoparticles (MNPs) when immersed in cell culture medium, and how these changes affect the cellular uptake. The role of the MNPs surface charge has been assessed by synthesizing two colloids with the same hydrodynamic size and opposite surface charge: magnetite (Fe3O4) cores of 25-30 nm were in situ functionalized with (a) positive polyethyleneimine (PEI-MNPs) and (b) negative poly(acrylic acid) (PAA-MNPs). After few minutes of incubation in cell culture medium the wrapping of the MNPs by protein adsorption resulted in a 5-fold increase of the hydrodynamic size. After 24 h of incubation large MNP-protein aggregates with hydrodynamic sizes of ≈1500 nm (PAA-MNPs) and ≈3000 nm (PEI-MNPs) were observed, each one containing an estimated number of magnetic cores between 450 and 1000. These results are consistent with the formation of large protein-MNPs aggregate units having a 'plum pudding' structure of MNPs embedded into a protein network that results in a negative surface charge, irrespective of the MNP-core charge. In spite of the similar negative ζ-potential for both MNPs within cell culture, we demonstrated that PEI-MNPs are incorporated in much larger amounts than the PAA-MNPs units. Quantitative analysis showed that SH-SY5Y cells can incorporate 100% of the added PEI-MNPs up to ≈100 pg/cell, whereas for PAA-MNPs the uptake was less than 50%. The final cellular distribution showed also notable differences regarding partial attachment to the cell membrane. These results highlight the need to characterize the final properties of MNPs after protein adsorption in biological media, and demonstrate the impact of these properties on the internalization mechanisms in neural cells.