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When an antibody (Ab) is immobilized on its surface, a carbon nanotube (CNT) becomes a biosensor that detects the corresponding antigen (Ag) because Ag-Ab complexes formed on the CNT surface moderate the current flow through it. We synthesized a biological ink containing CNTs that are twice functionalized, first with magnetic nanoparticles and thereafter with the anti-c-Myc monoclonal Ab. The ink is pipetted and dynamically self-organized by an external magnetic field into a dense electrically conducting sensor strip that measures the decrease in current when a sample containing c-Myc Ag is deposited on it. Prototypes are rapidly fabricated materials that cost less than 20 cents (Canadian) for each sensor. With larger current decreases due to real-time specific Ag-Ab binding for higher c-Myc concentrations, the biosensor distinguishes between picomolar c-Myc concentrations within a minute, offering proof of concept of a simple, rapid, economical, and sensitive method to detect specific molecules recognizable by Abs.

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In the context of emerging methods to control particle organization in particle-matrix composite materials, we explore, using finite element analysis, how to modulate the material bulk mechanical stiffness. Compared to a composite containing randomly distributed particles, material stiffness is enhanced 100-fold when filler particles are aligned into linear chains lying parallel to the loading direction. In contrast, chains aligned perpendicular to that direction produce negligible stiffness change. These outcomes reveal how zigzag chains, which provide intermediate results, can modulate stiffness. The stiffness decreases gradually with increasing zigzag angle θ over a range spanning 2 orders of magnitude.

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Microfluidics has advanced magnetic blood fractionation by making integrated miniature devices possible. A ferromagnetic microstructure array that is integrated with a microfluidic channel rearranges an applied magnetic field to create a high gradient magnetic field (HGMF). By leveraging the differential magnetic susceptibilities of cell types contained in a host medium, such as paramagnetic red blood cells (RBCs) and diamagnetic white blood cells (WBCs), the resulting HGMF can be used to continuously separate them without attaching additional labels, such as magnetic beads, to them. We describe the effect of these ferromagnetic microstructure geometries have on the blood separation efficacy by numerically simulating the influence of microstructure height and pitch on the HGMF characteristics and resulting RBC separation. Visualizations of RBC trajectories provide insight into how arrays can be optimized to best separate these cells from a host fluid. Periodic microstructures are shown to moderate the applied field due to magnetic interference between the adjacent teeth of an array. Since continuous microstructures do not similarly weaken the resultant HGMF, they facilitate significantly higher RBC separation. Nevertheless, periodic arrays are more appropriate for relatively deep microchannels since, unlike continuous microstructures, their separation effectiveness is independent of depth. The results are relevant to the design of microfluidic devices that leverage HGMFs to fractionate blood by separating RBCs and WBCs.

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We present a rapid and controllable method to create microscale heterogeneities in the 3D stiffness of a soft material by printing patterns with a ferrofluid ink. An ink droplet moved through a liquid polydimethylsiloxane (PDMS) volume using an externally applied magnetic field sheds clusters of magnetic nanoparticles (MNPs) in its wake. By varying the field spatiotemporally, a well-defined three-dimensional curvilinear feature is printed that contains MNP clusters. Subsequent cross-linking of the PDMS preserves the feature in place after the magnetic field is removed. Since the ferrofluid ink interferes with the cross-linking of PDMS, a 3D print containing ink density variations leads to corresponding spatial deviations in the elastic modulus of the matrix. The modulus is mapped in the experiments with atomic force microscopy. This rapid method to print 3D heterogeneities in soft matter promises the ability to mimic mechanical variations that occur in natural biomaterials.

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We report the serendipitous discovery of a rapid and inexpensive method to attach nanoscale magnetic chaperones to carbon nanotubes (CNTs). Nickel nanoparticles (NiNPs) become entangled in CNTs after both are dispersed in kerosene by sonication and form conjugates. An externally applied magnetic field manipulates the resulting CNTs-NiNP ink without NiNP separation, allowing us to print an embedded circuit in an elastomeric matrix and fabricate a strain gage and an oil sensor. The new method to print a circuit in a soft material using an NiNP-CNT ink is more rapid and inexpensive than the complex physical and chemical means typically used to magnetize CNTs.

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Using whole blood, we demonstrate the first realization of a novel macroscale, contactless, label-free method to print in situ three-dimensional (3D) cell assemblies of different morphologies and sizes. This novel bioprinting method does not use nozzles that can contaminate the cell suspension, or to which cells can adhere. Instead, we utilize the intrinsic diamagnetic properties of whole blood cells to magnetically manipulate them in situ in a nontoxic paramagnetic medium, creating (a) rectangular bar, (b) three-pointed star, and (c) spheroids of varying sizes. We envision the technique to be transferable to other cell lines, with potential applications in tissue engineering and drug screening.

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Magnetic nanoparticles (MNPs) in a liquid dispersion can be organized through controlled self-assembly by applying an external magnetic field that regulates inter-particle interactions. Thus, micro- and nanostructures of desired morphology and superlattice geometry that show emergent magnetic properties can be fabricated. We describe how superferromagnetism, which is a specific type of emergence, can be produced. Here, superparamagnetic nanoparticles that show no individual residual magnetization are organized into structures with substantial residual magnetization that behave as miniature permanent magnets. We investigate the emergence of superferromagnetism in an idealized system consisting of two MNPs, by considering the influence that interparticle magnetostatic interactions have on the dynamics of the magnetic moments. We use this model to illustrate the design principles for self-assembly in terms of the choice of material and MNP particle size. We simulate the dynamics of the interacting magnetic moments by applying the stochastic Landau-Lifshitz-Gilbert equation to verify our principles. The findings enable a method to pattern material magnetization with submicron resolution, a useful feature that has potential applications for magnetic recording and microfluidic particle traps. The analysis also yields useful empirical generalizations that could facilitate other theoretical developments.

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We report a novel method to pattern the stiffness of an elastomeric nanocomposite by selectively impeding the cross-linking reactions at desired locations while curing. This is accomplished by using a magnetic field to enforce a desired concentration distribution of colloidal magnetite nanoparticles (MNPs) in the liquid precursor of polydimethysiloxane (PDMS) elastomer. MNPs impede the cross-linking of PDMS; when they are dispersed in liquid PDMS, the cured elastomer exhibits lower stiffness in portions containing a higher nanoparticle concentration. Consequently, a desired stiffness pattern is produced by selecting the required magnetic field distribution a priori. Up to 200% variation in the reduced modulus is observed over a 2 mm length, and gradients of up to 12.6 MPa·mm-1 are obtained. This is a significant improvement over conventional nanocomposite systems where only small unidirectional variations can be achieved by varying nanoparticle concentration. The method has promising prospects in additive manufacturing; it can be integrated with existing systems thereby adding the capability to produce microscale heterogeneities in mechanical properties.

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A method to produce and pattern magnetic microstructure in a soft-polymer matrix is demonstrated. An externally applied magnetic field is used to influence the dynamics of magnetophoretic transport and dipolar self-assembly of magnetic nanoparticle clusters in the liquid precursor of poly-dimethylsiloxane (PDMS). Magnetic nanoparticles agglomerate by an interplay of van der Waals forces and dipolar interactions to form anisotropic clusters. These clusters are concentrated on a substrate by magnetophoresis, wherein they self-organize by dipolar interactions to form microscopic filaments. The polymer is cured in the presence of the magnetic field to preserve the microstructure shape. The externally applied magnetic field and its gradient are the two main control variables of interest when considering magnetic control during nanoparticle self-assembly. Their influence on microstructure geometry is investigated through correlations with the height of a characteristic self-assembled filament, fraction of the substrate area covered by the microstructure and its shape anisotropy. These relations enable a priori design.