RESUMO
Dynamic simulations of spin-transfer and spin-orbit torques are increasingly important for a wide range of spintronic devices including magnetic random access memory, spin-torque nano-oscillators and electrical switching of antiferromagnets. Here we present a computationally efficient method for the implementation of spin-transfer and spin-orbit torques within the Landau-Lifshitz-Gilbert equation used in micromagnetic and atomistic simulations. We consolidate and simplify the varying terminology of different kinds of torques into a physical action and physical origin that clearly shows the common action of spin torques while separating their different physical origins. Our formalism introduces the spin torque as an effective magnetic field, greatly simplifying the numerical implementation and aiding the interpretation of results. The strength of the effective spin torque field unifies the action of the spin torque and subsumes the details of experimental effects such as interface resistance and spin Hall angle into a simple transferable number between numerical simulations. We present a series of numerical tests demonstrating the mechanics of generalised spin torques in a range of spintronic devices. This revised approach to modelling spin-torque effects in numerical simulations enables faster simulations and a more direct way of interpreting the results, and thus it is also suitable to be used in direct comparisons with experimental measurements or in a modelling tool that takes experimental values as input.
RESUMO
Angiogenesis is an important repairing mechanism in response to ischemia. The administration of pro-angiogenic proteins is an attractive therapeutic strategy to enhance angiogenesis after an ischemic event. Their labile structures and short circulation times in vivo are the main obstacles that reduce the bioactivity and dosage of such proteins at the target site. We report on poly(d,l-lactic-co-glycolic acid) (PLGA) nanocapsules (diameter < 200 nm) containing bioactive vascular endothelial growth factor-165 (VEGF165) in the inner core and superparamagnetic iron oxide nanoparticles (SPIONs) embedded in the polymeric shell. The system showed good encapsulation efficiencies for both VEGF165 and SPIONs and a sustained protein release over 14 days. In vitro studies confirmed protein bioactivity in the form of significantly increased proliferation in human microvascular brain endothelial cell cultures once the protein was released. Through magnetic resonance imaging (MRI) measurements we demonstrated excellent T2 contrast image properties with r2 values as high as 213 mM-1 s-1. In addition, magnetic VEGF165-loaded PLGA nanocapsules could be displaced and accumulated under an external magnetic field for guiding and retention purposes. We therefore suggest that using VEGF165-loaded magnetic PLGA nanocapsules may become a new targeted protein-delivery strategy in the development of future pro-angiogenic treatments, as for instance those directed to neurorepair after an ischemic event.
