a b s t r a c tAn exponential law is presented for modeling piezoelectric behavior of bone tissues. The law is established based on experimental observation and existing empirical decay function. The model is then used to investigate the relaxation behavior of pizeovoltages induced by external load. Piezovoltages between the two opposing surfaces of bovine tibia bone samples under three point bending deformation are measured using an ultra high input impedance bioamplifier. The experimental results indicate that the pizeovoltages of bone show different relaxation behaviors during loading and unloading process. It is found that the piezovoltage decay follows a stretched exponential law when the load increases from zero to its maximum value, while it follows a typical relaxation exponential law when the load is kept its maximum value. The stretching-exponential behavior is independent of loading amplitude and rate. One possible reason for causing the stretched exponential behavior may be due to the triple helices structure of collagen fibrils distributed randomly in bone, which can experience relatively large deformation under external loads. The deformation process may include self-deformation and relative slipping between the molecule chains. The relative slipping movements may change the dielectric constants and resistances of bone, which can lead to multiple relaxation time behaviors during the deformation process of bone.

Bone is a complex composite material with hierarchical structures and anisotropic mechanical properties. Bone also processes electromechanical properties, such as piezoelectricity and streaming potentials, which termed as stress generated potentials. Furthermore, the electrostrictive effect and flexoelectric effect can also affect electromechanical properties of the bone. In the present work, time responses of bending deflections of bone cantilever in an external electric field are measured experimentally to investigate bone's electromechanical behavior. It is found that, when subjected to a square waveform electric field, a bone cantilever specimen begins to bend and its deflection increases gradually to a peak value. Then, the deflection begins to decrease gradually during the period of constant voltage. To analyze the reasons of the bending response of bone, additional experiments were performed. Experimental results obtained show the following two features. The first one is that the electric polarization, induced in bone by an electric field, is due to the Maxwell-Wagner polarization mechanism that the polarization rate is relatively slow, which leads to the electric field force acted on a bone specimen increase gradually and then its bending deflections increase gradually. The second one is that the flexoelectric polarization effect that resists the electric force to decrease and then leads to the bending deflection of a bone cantilever decrease gradually. It is concluded that the first aspect refers to the organic collagens decreasing the electric polarization rate of the bone, and the second one to the inorganic component influencing the bone's polarization intensity.

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Space charge injection and accumulation is attracting much attention in the field of dielectric insulation especially for electronic devices, power equipment and so on. This paper proposes using the inhibition effect of graphene for the injection and accumulation of space charge in low-density polyethylene (LDPE). Scanning electron microscope (SEM) and transmission electron microscopy (TEM) images were employed to observe the dispersion of graphene with a two-dimensional structure in LDPE. The time-dependent space charge dynamic behaviors of graphene/LDPE nanocomposites with the filler content of 0, 0.003, 0.005, 0.007 and 0.01 wt % were characterized by the pulsed electro-acoustic (PEA) test at 40, 60 and 80 °C, and the charge mobility was evaluated by its depolarization processes. The experimental results show that for the undoped LDPE film, large amounts of space charges were injected from the electrodes into samples, especially at 60 and 80 °C. The graphene/LDPE nanocomposites with a filler content of 0.005 wt % could markedly suppress the space charge injection and accumulation even at 80 °C, which is attributed to the large quantities of graphene-polymer in interface regions. These interface regions introduced numbers of deep trap sites within the forbidden band of nanocomposites, which can reduce the de-trapping rate of charges and suppress the space charge accumulation in the polymer bulks. The graphene/LDPE nanocomposites are suggested for dielectric applications, intending the inhibition of space charge injection and accumulation.

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Due to its excellent electrical and thermal performance, as well as satisfying the needs for developing the environmentally friendly and recyclable cable insulation material, polypropylene has caused widespread concern. Nanodoping can effectively improve the electrical, thermal and mechanical properties of polypropylene nanocomposites, which provides a new method to solve the problems in its application in HVDC cable insulation. This chapter introduces research achievements on polypropylene and polypropylene/inorganic nanocomposites, which states the effects of nanodoping on the electrical properties, such as space charge behaviors, electrical tree aging, breakdown strength, etc. thermal conductivity and mechanical properties of the polypropylene and its multi-blends. The aging mechanism under different conditions is also discussed. The analysis shows that the surface treatment of nanoparticles can reduce the aggregation of nanoparticles and strengthen the interface effect, thus improving the comprehensive properties of polypropylene nanocomposites. This chapter also summarized the feasibility and future development of the polypropylene and its nanocomposites application in the insulation of HVDC cables.

When load is applied to bone, it deforms and causes fluid pressure to build up in bone. The pressure gradient between different portions of the bone microcnannels drives fluid flow through them. This kind of bone fluid flow can induces the streaming potentials which are considered to play a role for bone remodeling. Aimed to determine the impact of ribbed rough inner surfaces of the microchannels on the streaming potentials, streaming potentials were measured as bone fluid flowed through the mcrospaces of thin cylinder bone samples under different pressure loading rates. The results show that the streaming potentials decrease with the increase of the pressure loading rates. A digital simulation calculation was performed and the results demonstrated that there were turbulent flows near the inner wall surfaces, which making the streaming potentials smaller in bone microchannels.bone, streaming potentials, pressure rising rate, turbulent flow Citation:

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