Bacteria are simple organisms with a remarkable capacity for survival by adapting to different environments, which is a result of their long evolutionary history. Taking into consideration these adapting mechanisms, this work now investigates the effect of electrically active microenvironments on bacteria and on how this stimulation may trigger bacteria growth inhibition or proliferation. Electrical microenvironments are generated via stimulation of a piezoelectric polymer with a mechanical cue, thus developing an electrical response and a variation on the surface charge of the polymeric material. Specifically, Grampositive Staphylococcus epidermidis and Gram-negative Escherichia coli were grown overnight under static and dynamic conditions on piezoelectric poly(vinylidene) fluoride (PVDF) films to further study bacteria behavior under: (i) the effect of the material surface charge in static conditions, (ii) the mechanical effect, and (iii) the piezoelectric effect, the last two performed under dynamic conditions. Bacteria viability in planktonic and biofilm forms was measured, and the microorganism morphology was characterized. Whereas E. coli responds little to any of the stimuli application, S. epidermidis growth can be regulated through the material surface charge and by the applied frequency. Positively charged PVDF induces bacterial growth inhibition in planktonic and adhered cells in static conditions, whereas antifouling properties are obtained when a mechanical or piezoelectric effect at 4 Hz stimuli is applied. By increasing the stimuli to 40 Hz, however, the adhesion of bacteria is promoted. In conclusion, the behavior of certain bacteria species is tailored through the application of piezoelectric materials, which provide sufficient mechanoelectrical stimuli for growth or inhibition of bacteria, allowing for the design of suitable anti-and promicrobial strategies. Such strategies are only found in studies related to mammalian cells, whereas in bacterial cells this type of stimuli are still unknown. Thus, this work provides one of the first insights on the effect of piezoelectric stimuli on bacterial cells.

Magnetic nanowires, conceived as individual building blocks for spintronic devices, constitute a well-suited model to design and study magnetization reversal processes, or to tackle fundamental questions, such as the presence of topologically protected magnetization textures under particular conditions. Recently, a skyrmion-tube mediated magnetization reversal process was theoretically reported in diameter modulated cylindrical nanowires. In these nanowires, a vortex nucleates at the end of the segments with larger diameter and propagates, resulting in a first switching of the nanowire core magnetization at small fields. In this work, we show experimental evidence of the so-called Bloch skyrmion-tubes, using advanced Magnetic Force Microscopy modes to image the magnetization reversal process of FeCoCu diameter modulated nanowires. By monitoring the magnetic state of the nanowire during applied field sweeping, a detected drop of magnetic signal at a given critical field unveils the presence of a skyrmion-tube, due to mutually compensating stray field components. That evidences the presence of a skyrmion-tube as an intermediate stage during the magnetization reversal, whose presence is related to the geometrical dimensions of the cylindrical segments.

The anomalous Nernst effect (ANE) is a thermomagnetic phenomenon with potential applications in thermal energy harvesting. While many recent works studied the approaches to increase the ANE coefficient of materials, relatively little effort was devoted to increasing the power supplied by the effect. Here, we demonstrate a nanofabricated device with record power density generated by the ANE. To accomplish this, we fabricate micrometer-sized devices in which the thermal gradient is 3 orders of magnitude higher than conventional macroscopic devices. In addition, we use Co/Pt multilayers, a system characterized by a high ANE thermopower (∼1 μV/K), low electrical resistivity, and perpendicular magnetic anisotropy. These innovations allow us to obtain power densities of around 13 ± 2 W/cm 3 . We believe that this design may find uses in harvesting wasted energy, e.g., in electronic devices.

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