AFM-based mechanical characterization of single nanofibres

Nanofibres are found in a broad variety of hierarchical biological systems as fundamental structural units, and nanofibrillar components are playing an increasing role in the development of advanced functional materials. Accurate determination of the mechanical properties of single nanofibres is thus of great interest, yet measurement of these properties is challenging due to the intricate specimen handling and the exceptional force and deformation resolution that is required. The atomic force microscope (AFM) has emerged as an effective, reliable tool in the investigation of nanofibrillar mechanics, with the three most popular approaches—AFM-based tensile testing, three-point deformation testing, and nanoindentation—proving preferable to conventional tensile testing in many (but not all) cases. Here, we review the capabilities and limitations of each of these methods and give a comprehensive overview of the recent advances in this field

Head motion predictability explains activity-dependent suppression of vestibular balance control

Vestibular balance control is dynamically weighted during locomotion. This might result from a selective suppression of vestibular inputs in favor of a feed-forward balance regulation based on locomotor efference copies. The feasibility of such a feed-forward mechanism should however critically depend on the predictability of head movements (HMP) during locomotion. To test this, we studied in 10 healthy subjects the differential impact of a stochastic vestibular stimulation (SVS) on body sway (center-of-pressure, COP) during standing and walking at different speeds and compared it to activity-dependent changes in HMP. SVS-COP coupling was determined by correlation analysis in frequency and time domains. HMP was quantified as the proportion of head motion variance that can be explained by the average head trajectory across the locomotor cycle. SVS-COP coupling decreased from standing to walking and further dropped with faster locomotion. Correspondingly, HMP increased with faster locomotion. Furthermore, SVS-COP coupling depended on the gait-cycle-phase with peaks corresponding to periods of least HMP. These findings support the assumption that during stereotyped human self-motion, locomotor efference copies selectively replace vestibular cues, similar to what was previously observed in animal models

Walking in orthostatic tremor modulates tremor features and is characterized by impaired gait stability

Primary orthostatic tremor (OT) is characterized by high-frequency lower-limb muscle contractions and a disabling sense of unsteadiness while standing. Patients consistently report a relief of symptoms when starting to ambulate. Here, we systematically examined and linked tremor and gait characteristics in patients with OT. Tremor and gait features were examined in nine OT patients and controls on a pressure-sensitive treadmill for one minute of walking framed by two one-minute periods of standing. Tremor characteristics were assessed by time-frequency analysis of surface EMG-recordings from four leg muscles. High-frequency tremor during standing (15.29 +/- 0.17 Hz) persisted while walking but was consistently reset to higher frequencies (16.34 +/- 0.25 Hz;p < 0.001). Tremor intensity was phase-dependently modulated, being predominantly observable during stance phases (p < 0.001). Tremor intensity scaled with the force applied during stepping (p < 0.001) and was linked to specific gait alterations, i.e., wide base walking (p = 0.019) and increased stride-to-stride fluctuations (p = 0.002). OT during walking persists but is reset to higher frequencies, indicating the involvement of supraspinal locomotor centers in the generation of OT rhythm. Tremor intensity is modulated during the gait cycle, pointing at specific pathways mediating the peripheral manifestation of OT. Finally, OT during walking is linked to gait alterations resembling a cerebellar and/or sensory ataxic gait disorder

Graphene Oxide Reduces the Hydrolytic Degradation in Polyamide-11

Graphene oxide (GO) was incorporated into polyamide-11 (PA11) via in-situ polymerization. The GO-PA11 nano-composite had elevated resistance to hydrolytic degradation. At a loading of 1 mg/g, GO to PA11, the accelerated aging equilibrium molecular weight of GO-PA11 was higher (33 and 34 kg/mol at 100 and 120 C, respectively) compared to neat PA11 (23 and 24 kg/mol at 100 and 120 C, respectively). Neat PA11 had hydrolysis rate constants (kH) of 2.8 and 12 ( 10(exp -2) day(exp -1)) when aged at 100 and 120 C, respectively, and re-polymerization rate constants (kP) of 5.0 and 23 ( 10(exp -5) day(exp -1)), respectively. The higher equilibrium molecular weight for GO-PA11 loaded at 1 mg/g was the result of a decreased kH, 1.8 and 4.5 ( 10(exp -2) day(exp -1)), and an increased kP, 10 and 17 ( 10(exp -5) day(exp -1)) compared with neat PA11 at 100 and 120 C, respectively. The decreased rate of degradation and resulting 40% increased equilibrium molecular weight of GO-PA11 was attributed to the highly asymmetric planar GO nano-sheets that inhibited the molecular mobility of water and the polymer chain. The crystallinity of the polymer matrix was similarly affected by a reduction in chain mobility during annealing due to the GO nanoparticles' chemistry and highly asymmetric nano-planar sheet structure

Cones, pringles, and grain boundary landscapes in graphene topology

A polycrystalline graphene consists of perfect domains tilted at angle {\alpha} to each other and separated by the grain boundaries (GB). These nearly one-dimensional regions consist in turn of elementary topological defects, 5-pentagons and 7-heptagons, often paired up into 5-7 dislocations. Energy G({\alpha}) of GB computed for all range 0<={\alpha}<=Pi/3, shows a slightly asymmetric behavior, reaching ~5 eV/nm in the middle, where the 5's and 7's qualitatively reorganize in transition from nearly armchair to zigzag interfaces. Analysis shows that 2-dimensional nature permits the off-plane relaxation, unavailable in 3-dimensional materials, qualitatively reducing the energy of defects on one hand while forming stable 3D-landsapes on the other. Interestingly, while the GB display small off-plane elevation, the random distributions of 5's and 7's create roughness which scales inversely with defect concentration, h ~ n^(-1/2)Comment: 9 pages, 4 figure

Nanoscale tunable reduction of graphene oxide for graphene electronics

International audienceGraphene is now recognized as the most likely carbon-based successor material for CMOS electronics. Recently, interest in graphene oxide (GO) has risen for producing large-scale flexible conductors and for its potential to open an electronic gap in graphene structures. We report on a means to tune the topographical and electrical properties of graphene-based materials with nanoscopic resolution by local thermal reduction of GO with a nano-size tip. The reduced GO nanostructures show an increase in conductivity up to four orders of magnitude as compared to pristine GO. No sign of tip wear or sample tearing was observed. Variably conductive nanoribbons with dimensions down to 12 nm have been produced in oxidized epitaxial graphene films in a single step that is clean, rapid and reliable

Mechanically Assisted Exfoliation and Functionalization of Thermally Converted Graphene Sheets

Published versio

Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons.

Published versio

Magnetic Properties of Single Transition-Metal Atom Absorbed Graphdiyne and Graphyne Sheet

The electronic and magnetic properties of single 3d transition-metal(TM) atom (V, Cr, Mn, Fe, Co, and Ni) adsorbed graphdiyne (GDY) and graphyne (GY) are systematically studied using first-principles calculations within the density functional framework. We find that the adsorption of TM atom not only efficiently modulates the electronic structures of GDY/GY system, but also introduces excellent magnetic properties, such as half-metal and spin-select half-semiconductor. Such modulation originates from the charge transfer between TM adatom and the GDY/GY sheet as well as the electron redistribution of the TM intra-atomic s, p, and d orbitals. Our results indicate that the TM adsorbed GDY/GY are excellent candidates for spintronics.Comment: 8 pages, 7 figure