For more than half a century, the action potential (AP) has been considered a purely electrical phenomenon. However, experimental observations of membrane deformations occurring during APs have revealed that this process also involves mechanical features. This discovery has recently fuelled a controversy on the real nature of APs: whether they are mechanical or electrical. In order to examine some of the modern hypotheses regarding APs, we propose here a coupled mechanoelectrophysiological membrane finite-element model for neuronal axons. The axon is modeled as an axisymmetric thin-wall cylindrical tube. The electrophysiology of the membrane is modeled using the classic Hodgkin-Huxley (H-H) equations for the Nodes of Ranvier or unmyelinated axons and the cable theory for the internodal regions, whereas the axonal mechanics is modeled by means of viscoelasticity theory. Membrane potential changes induce a strain gradient field via reverse flexoelectricity, whereas mechanical pulses result in an electrical self-polarization field following the direct flexoelectric effect, in turn influencing the membrane potential. Moreover, membrane deformation also alters the values of membrane capacitance and resistance in the H-H equation. These three effects serve as the fundamental coupling mechanisms between the APs and mechanical pulses in the model. A series of numerical studies was systematically conducted to investigate the consequences of interaction between the APs and mechanical waves on both myelinated and unmyelinated axons. Simulation results illustrate that the AP is always accompanied by an in-phase propagating membrane displacement of ≈1 nm, whereas mechanical pulses with enough magnitude can also trigger APs. The model demonstrates that mechanical vibrations, such as the ones arising from ultrasound stimulations, can either annihilate or enhance axonal electrophysiology depending on their respective directionality and frequency. It also shows that frequency of pulse repetition can also enhance signal propagation independently of the amplitude of the signal. This result not only reconciles the mechanical and electrical natures of the APs but also provides an explanation for the experimentally observed mechanoelectrophysiological phenomena in axons, especially in the context of ultrasound neuromodulation.
Investigation of mechanical impact behavior of short carbon-fiber-reinforced PEEK composites, Composite Structures (2015), doi: http://dx.Abstract. This paper describes the results of an experimental and numerical investigation of the impact behavior of short carbon fiber reinforced polyether-ether-ketone (SCFR PEEK) composites. The biocompatibility of PEEK and its short fiber composites, their rapid processing by injection molding and suitability for modern imaging have supported technological advances in prosthetic implants used in orthopedic medicine. Surgical implants, including hip and cranial implants, can experience clinically significant impact loading during medical installation and useful life. While the incorporation of short fibers in a thermoplastic matrix can produce significant improvements in stiffness and strength, it can also cause a marked reduction in ductility, making study of their energy absorption capability essential. In this work, the mechanical impact behavior of PEEK composites reinforced with polyacrylonitrile (PAN) short carbon fibers 30 % in weight is compared with unfilled PEEK. The perforation tests conducted covered an impact kinetic energy range from 21 J to 131 J, equivalent to the range observed in a fall, the leading cause of hip fractures. Energy absorption capability, damage extension and failure mechanism have been quantified and reported. A numerical modeling that includes homogenization of elastic material and 1 anisotropic damage is presented and validated with experimental data. At all impact energies, SCFR PEEK composites showed a brittle failure and their absorption energy capability decreases drastically in comparison with unfilled PEEK.
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