## 4 Analysis
The curve illustrates that the streaming potential is approximately proportional to the length of the high charge density zone. The characteristic of MWp is that the electric charges induced by an electric field accumulate gradually on the inner pore surfaces and the interfaces between the different constituents, such as collagens and hydroxyapatite in bone. The multicomponent materials and hierarchical microporous structure of bone may lead to the rate of MWp being much less than those of the general ionic displacement and dipole orientation polarizations. The polarization time of MWp ranges from \(10^{-1}\) s to several hours [36; 37], while ionic displacement and dipole orientation polarizations last less than milliseconds [38]. The electrical conductivities of bone collagens and hydroxyapatite crystals are in different orders of magnitude [39] and the hierarchical structure of bone are the conditions of MWp [36; 37]. On the one hand, the electric charges on the bone solid cause electric double layers in bone microcanals; on the other hand, the charges form nonuniform electric fields inside bone. When bone fluid flows through the microcanals, the electric charges carried by the fluid can accumulate at the canal's turns, branches and sharp tapers where the electric field densities are relatively strong and the accumulations occur gradually like the MWp in bone. Figure 5:9 (**A**) Streaming potentials versus the ratio of \(l_{1}\)/\(L\). (**B**) Normalized streaming potentials versus the radius of a capillaryBecause the microcanals in bone are curved and branched, the distribution of the electric charges on the inner surface of the microcanals or solid surfaces are not uniform, which can be explained by the mechanism of Maxwell–Wagner polarization in bone. Maxwell–Wagner polarization (MWp), also known as interfacial polarization or space charge polarization, occurs in a porous composite dielectric composed of segregated constituents with different dielectric permittivity and electrical conductivity [35]. From the above analysis, a conclusion can be reached that the linear variations of the measured streaming potentials under constant fluid pressure are caused by the local accumulation of electric charges carried by the fluid, and Equation (18) can describe the action of the charges. The streaming potential pulse amplitudes plotted in Figure 3 represent the typical streaming potential features in bone, and they are consistent with those in literature [16; 21; 28]. Of course, the electric charges' accumulation and distribution are complex, for positive and negative charges can accumulate at different locations in bone. The accumulations depend on the bone's microstructure, components and electric charge distributions in bone, which may possibly include the piezoelectric charges induced by the stresses in bone. Further, the fluid flow states in the microcanals are also a significant factor to affect the charge accumulation. Another property of bone may also be related to the streaming potentials. That is the spontaneous polarization of bone [40; 41], which explains that bone can be polarized without any external action. The spontaneous polarization causes a nonuniform distribution of electric charge in bone. In the micropore's inner surfaces contacting the fluid, some areas have positive electric charge density and others have negative charge density, and the two types of areas are not necessarily equal to each other. The difference of the two densities may be associated with the slopes of streaming potentials. If the positive density can increase the slopes, the negative density can decrease the slopes, and can even change the direction of the slopes. This may explain the negative slopes of specimen one and specimen four, for which the pressure dependences of the slopes are uncertain. Figure 2E shows intuitively that the slopes of some curves (red) corresponding to 150 kPa are even less than those (black) corresponding to 100 kPa, though the slopes may increase with the increase of the pressures statistically.