Anisotropic hydrogels formed by magnetically-oriented nanoclay suspensions for wound dressings.

Anisotropic hydrogels are produced, by magnetic alignment of magnetically sensitized nanoclays followed by polymerization of the hydrogel to freeze the developed oriented structure. The anisotropy in these hydrogels is quantitatively investigated using birefringence and 2D small angle X-ray scattering (SAXS) techniques. The oriented nanoclays being intrinsically birefringent provide optical anisotropy to the hydrogel and this orientation increases with the increase of the applied magnetic field strength. Moreover, 2D SAXS patterns also confirm that the nanoclays are oriented parallel to the permanent magnetic field in the hydrogel with an orientation order parameter of up to 0.67. The field-induced birefringence and 2D SAXS orientation results exhibit a linear correlation over the range of 0 to 9 tesla (T). The resultant anisotropic hydrogels exhibit substantial swelling anisotropy, making them suitable for wound dressings where the out of plane swelling is substantially higher than in-plane swelling to minimize in-plane stress damage to the wounds during healing.

Tunable Piezoresistivity from Magnetically Aligned Ni(Core)@Ag(Shell) Particles in an Elastomer Matrix.

Core-shell (Ni@Ag) particles are aligned through the thickness of a poly(dimethylsiloxane) (PDMS) film using a magnetic field in a continuous roll-to-roll process. The alignment of the particles dramatically decreases the percolation threshold for electrical conductivity through the thickness of the film by nearly an order of magnitude from 28 vol % without the field to ≈1 vol % with a 52 mT magnetic field during curing. However, the magnetic forces lead to rough surface topography for intermediate Ni@Ag loadings, but confining the Ni@Ag/PDMS composite by a glass constraint provides a smooth surface. This difference in geometry changes the morphology of the vertically aligned "chains" of Ni@Ag particles where the chains are more aggregated when the film is unconstrained. As the Ni@Ag concentration is decreased below 3.6% for the constrained film, breaks in the aligned particles evident from X-ray tomography lead to pressure sensitive resistance across that film with a large decrease in resistance above a threshold pressure. The threshold pressure is demonstrated to be controllable from ≈15 to ≈290 kPa through the loading of aligned Ni@Ag in the PDMS, but this threshold pressure decreases on cyclic loading. These magnetically aligned composites represent a facile route to mechanically responsive films that could be used in a variety of applications where cyclic loading above and below the threshold pressure is not required, such as disposable pressure sensors for ensuring reliability of products through transportation and embedded structural health monitoring for identifying critical displacements.