3796 wileyonlinelibrary.com nanorods as the fi llers are believed to be the best candidates for materials working in extreme environment and hence they are on demand in aerospace and space exploration applications. [39][40][41][42][43] In composite fi lms needed for inductor and antennae applications, the alignment of magnetic particles is required to increase the high frequency permeability and to lower the hysteresis losses. [44][45][46][47] In applications to manufacturing of high density recording fi lms and discs, spin alignment is required to carry the recorded information when spins in magnetic nanorods are prone to align parallel to the nanorod axis. [ 37,[48][49][50] In optical applications, magnetic liquid crystals are attractive candidates for making reconfi gurable magnetooptical devices with the fast time response measured in milliseconds. [11][12][13][14][15][16][17][18][51][52][53] In all these applications, one needs to control the alignment of magnetic nanorods in liquid medium. Therefore, this problem is actively discussed in the literature; however, the general strategy for making macroscopic samples with ordered nanorods has not been developed yet and it remains the main challenge of materials engineering [32][33][34][35][39][40][41][42]54 ] Processing of multifunctional coatings and thin fi lms require many steps and hence one needs to control the fi lm properties at each step.Rheological properties of the fi lms at each stage of their processing play the most important role in nanorod ordering and keeping nanorods in place. Characterization of thin coating fi lms during composite manufacturing remains challenging. Different experimental methods have been proposed and developed for in situ characterization of rheological properties of thin fi lms and coatings. [ 20,[25][26][27][55][56][57][58][59][60] It appears that unique features of rotation of ferromagnetic nanorods can be used for characterization of very thin fi lms when other methods fall short. Recently introduced magnetic rotational spectroscopy (MRS) with nanoparticles and nanorods allows one to probe fl uid rheology in very thin fi lms and nanoliter droplets. [ 12,20,22,25,60,61 ] MRS takes advantage of a distinguishable behavior of rotating magnetic tracers as the frequency of applied rotating fi eld changes. Unlike many methods based on the analysis of small oscillations, which are diffi cult to interpret when the fi lm is very thin, MRS with magnetic nanorods enjoys analysis of full revolutions of magnetic tracers. [ 20,22,24,25,60,62,63 ] Understanding of the characteristic features of rotation of a single nanorod in complex fl uids and alignment of an assembly

Magnetic nanoparticles (MNPs), primarily iron oxide nanoparticles, have been incorporated into cellular spheroids to allow for magnetic manipulation into desired shapes, patterns and 3-D tissue constructs using magnetic forces. However, the direct and long-term interaction of iron oxide nanoparticles with cells and biological systems can induce adverse effects on cell viability, phenotype and function, and remain a critical concern. Here we report the preparation of biological magnetic cellular spheroids containing magnetoferritin, a biological MNP, capable of serving as a biological alternative to iron oxide magnetic cellular spheroids as tissue engineered building blocks. Magnetoferritin NPs were incorporated into 3-D cellular spheroids with no adverse effects on cell viability up to 1 week. Additionally, cellular spheroids containing magnetoferritin NPs were magnetically patterned and fused into a tissue ring to demonstrate its potential for tissue engineering applications. These results present a biological approach that can serve as an alternative to the commonly used iron oxide magnetic cellular spheroids, which often require complex surface modifications of iron oxide NPs to reduce the adverse effects on cells.

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In this paper, we provide a complete analytical description of the alignment kinetics of magnetic nanorods in magnetic field. Nickel nanorods were formed by template electrochemical deposition in alumina membranes from a dispersion in a water-glycerol mixture. To ensure uniformity of the dispersion, the surface of the nickel nanorods was covered with polyvinylpyrrolidone (PVP). A 40-70 nm coating prevented aggregation of the nanorods. These modifications allowed us to control alignment of the nanorods in a magnetic field and test the proposed theory. An orientational distribution function of nanorods was introduced. We demonstrated that the 0.04% volume fraction of nanorods in the glycerol-water mixture behaves as a system of non-interacting particles. However, the kinetics of alignment of a nanorod assembly does not follow the predictions of the single-nanorod theory. The distribution function theory explains the kinetics of alignment of a nanorod assembly and shows the significance of the initial distribution of nanorods in the film. It can be used to develop an experimental protocol for controlled ordering of magnetic nanorods in thin films.

4738www.MaterialsViews.com wileyonlinelibrary.com the remote control of the particle selfassembly and the generated structures and generation of anisotropic nanostructured materials. [ 1 ] The ability of magnetic fl uids to form chains of magnetic particles in a magnetic fi eld [ 2 ] is being used to form anisotropic composites when alignment of the magnetic nanoparticles in a monomer solution or a polymer melt is followed by polymerization, crosslinking, glass or crystallization transitions. [ 1c , 3 ] Optical, mechanical, electrical, and thermal properties of nanocomposites can be tuned by changing the materials anisotropy with a magnetic fi eld during the composite formation. [ 4 ] The magnetic nanocomposites (as any other fi ber loaded composites) [ 3e ] are reinforced by magnetic chains. [ 3b , 5 ] The synthesized nanocomposites show mechanical, [ 6 ] thermal, [ 5b,c ] and optical anisotropy [ 3a , 7 ] and can be used to create patterned microstructures with controllable periodicity, [ 8 ] materials with magnetically controlled anisotropic porosity, [ 9 ] magnetically controlled MEMS devices for drug delivery, [ 10 ] tunable diffraction gratings, [ 11 ] magnetic actuators for microfl uidics, [ 12 ] dynamic absorbers, [ 13 ] and electromagnetic shielding composites. [ 14 ] Thermal properties [ 5b,c ] and conductivity [ 15 ] of materials can be controlled by changing the directions of the applied magnetic fi eld during the composite formation. Fabrication of complex composite materials with combination of different physical properties calls for the development of new tools and methodologies. It is desirable to control the alignment and interactions of the building blocks constituting the composite material; especially important is to control the interactions between the blocks by switching them from attractive to repulsive and vice versa. Making the building blocks magnetic and applying magnetic fi eld, one provides the versatile possibilities to direct the assembly and control the interactions between the blocks.To date, there have been several successful strategies for formation of stable chains/nanowires from nanoparticles as the building blocks of these 1D structures. [ 16 ] These 1D structures can be made through the template-directed synthesis inside the channels in solids or other mesostructures self-assembled from block copolymers, cylindrical polymer brushes, 1D biological A method for the generation of remotely reconfi gurable anisotropic coatings is developed. To form these coatings, locking magnetic nanoparticles (LMNPs) made of a superparamagnetic core and a two-component polymer shell are employed. Two different polymers form phase-separated coaxial shells. The outer shell provides repulsive interactions between the LMNPs while the inner shell exerts attractive forces between the particles. Applying a non-uniform magnetic fi eld, one gathers the particles together, pushing them to come in contact when the internal shells could effectively hold the particles together. When the magnetic fi ...

Precursors derived from the hydrolysis of organic or inorganic salts have been widely used to produce ceramic coatings for a broad variety of applications. When applying the liquid precursors to the substrates, it is extremely challenging to control the film uniformity and homogeneity. The rate of solvent evaporation at different locations is different, causing the viscosity variation and flows in the film. There is very limited knowledge about the viscosity change in evaporating ceramic precursors. Therefore, it is crucial to understand the effect of evaporation on viscosity variation in thin films and droplets. We use magnetic rotational spectroscopy to study the time dependence of viscosity in mullite precursors. A correlation between the viscosity change and evaporation kinetics is revealed. This correlation was used to relate the change of viscosity to the concentration of mullite. A master curve relating viscosity to the mullite concentration was constructed and used to propose a possible scenario of the viscosity increase during solvent evaporation.

Magnetic nanorods offer many opportunities in manufacturing of multifunctional composites with unprecedented magnetic and mechanical properties. [1][2][3][4][5][6][7] Coating fi lms with ordered magnetic nanorods are especially attractive for different hightech applications. These fi lms provide additional functionality, for example, enabling magnetic recording or magneto-optic features. [ 4,[8][9][10] However, the strategy for nanorod alignment in the macroscopic samples has not been developed so far. In the processing of magnetic fi lms, one important step lacking understanding of the behavior of nanorods is related to the complexity of rheological behavior of solidifying fi lm during the sol-gel processing. Many liquids used for sol-gel processing rapidly react on the environmental conditions by changing their rheological properties. [ 11,12 ] The time-dependent viscosity of many practically important carriers is typically described by the following equation η ( t ) = η 0 exp( t / τ 0 ), where η 0 is the initial viscosity of the carrier, t is the time, and τ 0 is the characteristic time of polymerization. [ 11,12 ] Recent experiments showed that the spinning behavior of a single nanorod in a fi lm with this type of viscosity variation is drastically different from the spinning behavior of nanorods in fl uids with constant viscosity. [ 13 ] However, alignment of an assembly of magnetic nanorods in a solidifying fi lm has not been discussed in the literature.In this paper, we study the behavior of an orientational distribution function of nanorods in the fi lm when its viscosity exponentially increases with time. Nanorods are assumed to have a large diameter to length ratio and are confi ned within the fi lm, keeping their longer axes parallel to the fi lm plane. The concentration of nanorods is assumed to be small, so that the dipole-dipole interactions between nanorods are insignifi cant. We also assume that the torque caused by the external fi eld is strong and that only viscous friction forces can balance this torque; the thermal fl uctuations are insignifi cant. These conditions are of practical interest in nanocomposite manufacturing; recent experiments on Newtonian fi lms confi rmed the validity of these assumptions. This paper sets up the basic theory needed for interpretation of experiments on non-Newtonian fi lms. [ 14 ] Assuming that in the fi rst moment of time the nanorods are randomly distributed in the fi lm, we show that the kinetics of their alignment cannot be explained using the theory for a single nanorod. The analysis of the time dependence of orientational distribution function reveals different regimes of nanorod alignment. We focus mainly on the specifi cation of a window of materials parameters, where the nanorods can be completely aligned along the fi eld during a specifi c time period. . Rotation of a Single Nanorod in a Solidifying FilmConsider a single nanorod suspended in fi lm. At time t = 0, one applies an external uniform fi eld B acting in the fi lm plane. In thin fi lms, the na...