Dielectric studies of proton transport in air-dried fully calcified and decalcified bone

The bioelectrical properties of bone tissue

Understanding the bioelectrical properties of bone tissue is key to developing new treatment strategies for bone diseases and injuries, as well as improving the design and fabrication of scaffold implants for bone tissue engineering. The bioelectrical properties of bone tissue can be attributed to the interaction of its various cell lineages (osteocyte, osteoblast and osteoclast) with the surrounding extracellular matrix, in the presence of various biomechanical stimuli arising from routine physical activities; and is best described as a combination and overlap of dielectric, piezoelectric, pyroelectric and ferroelectric properties, together with streaming potential and electro-osmosis. There is close interdependence and interaction of the various electroactive and electrosensitive components of bone tissue, including cell membrane potential, voltage-gated ion channels, intracellular signaling pathways, and cell surface receptors, together with various matrix components such as collagen, hydroxyapatite, proteoglycans and glycosaminoglycans. It is the remarkably complex web of interactive cross-talk between the organic and non-organic components of bone that define its electrophysiological properties, which in turn exerts a profound influence on its metabolism, homeostasis and regeneration in health and disease. This has spurred increasing interest in application of electroactive scaffolds in bone tissue engineering, to recapitulate the natural electrophysiological microenvironment of healthy bone tissue to facilitate bone defect repair.

Electroactive Biomaterials for Facilitating Bone Defect Repair under Pathological Conditions

Bone degeneration associated with various diseases is increasing due to rapid aging, sedentary lifestyles, and unhealthy diets. Living bone tissue has bioelectric properties critical to bone remodeling, and bone degeneration under various pathological conditions results in significant changes to these bioelectric properties. There is growing interest in utilizing biomimetic electroactive biomaterials that recapitulate the natural electrophysiological microenvironment of healthy bone tissue to promote bone repair. This review first summarizes the etiology of degenerative bone conditions associated with various diseases such as type II diabetes, osteoporosis, periodontitis, osteoarthritis, rheumatoid arthritis, osteomyelitis, and metastatic osteolysis. Next, the diverse array of natural and synthetic electroactive biomaterials with therapeutic potential are discussed. Putative mechanistic pathways by which electroactive biomaterials can mitigate bone degeneration are critically examined, including the enhancement of osteogenesis and angiogenesis, suppression of inflammation and osteoclastogenesis, as well as their anti-bacterial effects. Finally, the limited research on utilization of electroactive biomaterials in the treatment of bone degeneration associated with the aforementioned diseases are examined. Previous studies have mostly focused on using electroactive biomaterials to treat bone traumatic injuries. It is hoped that this review will encourage more research efforts on the use of electroactive biomaterials for treating degenerative bone conditions.

A review on computer modeling of bone piezoelectricity and its application to bone adaptation and regeneration

Bone is a hierarchical, multiphasic and anisotropic structure which in addition possess piezoelectric properties. The generation of piezoelectricity in bone is a complex process which has been shown to play a key role both in bone adaptation and regeneration. In order to understand the complex biological, mechanical and electrical interactions that take place during these processes, several computer models have been developed and used to test hypothesis on potential mechanisms behind experimental observations. This paper aims to review the available literature on computer modeling of bone piezoelectricity and its application to bone adaptation and healing. We first provide a brief overview of the fundamentals of piezoelectricity and bone piezoelectric effects. We then review how these properties have been used in computational models of bone adaptation and electromechanical behaviour of bone. In addition, in the last section, we summarize current limitations and potential directions for future work.

The application of nanogenerators and piezoelectricity in osteogenesis

Bone is a complex organ possessing both physicomechanical and bioelectrochemical properties. In the view of Wolff's Law, bone can respond to mechanical loading and is subsequently reinforced in the areas of stress. Piezoelectricity is one of several mechanical responses of the bone matrix that allows osteocytes, osteoblasts, osteoclasts, and osteoprogenitors to react to changes in their environment. The present review details how osteocytes convert external mechanical stimuli into internal bioelectrical signals and the induction of intercellular cytokines from the standpoint of piezoelectricity. In addition, this review introduces piezoelectric and triboelectric materials used as self-powered electrical generators to promote osteogenic proliferation and differentiation due to their electromechanical properties, which could promote the development of promising applications in tissue engineering and bone regeneration.

Novel gigahertz frequency dielectric relaxations in chitosan films

Molecular relaxations of chitosan films have been investigated in the wide frequency range of 0.1 to 3 × 10(9) Hz from -10 °C to 110 °C using dielectric spectroscopy. For the first time, two high-frequency relaxation processes (in the range 10(8) to 3 × 10(9) Hz) are reported in addition to the low frequency relaxations α and β. These two relaxation processes are related to the vibrations of OH and NH2/NH3(+), respectively. The high-frequency relaxations exhibit Arrhenius-type dependencies in the temperature range 10 °C to 54 °C with negative activation energy; this observation is traceable to hydrogen bonding reorientation. At temperatures above the glass transition temperature (54 °C), the activation energy changes from negative to positive values due to breaking of hydrogen bonding and water loss. Upon cooling in a sealed environment, the activation energies of two relaxation processes are nearly zero. FTIR and XRD analyses reveal associated structural changes upon heating and cooling. These two new high-frequency relaxation processes can be attributed to the interaction of bound water with OH and NH2/NH3(+), respectively. A plausible scenario for these high-frequency relaxations is discussed in light of impedance spectroscopy, TGA, FTIR and XRD measurements.

Relevance of collagen piezoelectricity to "Wolff's Law": a critical review

According to "Wolff's Law", bone is deposited and reinforced at areas of greatest stress. From a clinical perspective, this "law" is supported by the strong association between bone density and physical activity. From a mechanistic standpoint, however, the law presents a challenge to scientists seeking to understand how osteocytes and osteoblasts sense the mechanical load. In the 1960s, collagen piezoelectricity was invoked as a potential mechanism by which osteocytes could detect areas of greater stress but piezoelectricity diminished in importance as more compelling mechanisms, such as streaming potential, were identified. In addition, accumulating evidence for the role of fluid-related shear stress in osteocyte's mechanosensory function has made piezoelectricity seemingly more obsolete in bone physiology. This review critically evaluates the role of collagen piezoelectricity (if any) in Wolff's Law--specifically, the evidence regarding its involvement in strain-generated potentials, existing alternate mechanisms, the present understanding of bone mechanosensation, and whether piezoelectricity serves an influential role within the context of this newly proposed mechanism. In addition to reviewing the literature, this review generates several hypotheses and proposes future research to fully address the relevance of piezoelectricity in bone physiology.

Interrelationships between electrical properties and microstructure of human trabecular bone

Microstructural changes, such as reduction of trabecular thickness and number, are characteristic signs of osteoporosis leading to diminished bone strength. Electrical and dielectric parameters might provide diagnostically valuable information on trabecular bone microstructure not extractable from bone mineral density measurements. In this study, structural properties of human trabecular bone samples (n=26) harvested from the distal femur and proximal tibia were investigated using the computed microtomography (microCT) technique. Quantitative parameters, e.g. structural model index (SMI) or trabecular bone volume fraction (BV/TV), were calculated. In addition, the samples were examined electrically over a wide frequency range (50 Hz-5 MHz) using a two-electrode impedance spectroscopy set-up. Relative permittivity, loss factor, conductivity, phase angle, specific impedance and dissipation factor were determined. Significant linear correlations were obtained between the dissipation factor and BV/TV or SMI (|r| 0.70, p<0.01, n=26). Principal component analyses, conducted on electrical and structural parameters, revealed that the high frequency principal component of the dissipation factor was significantly related to SMI (r=0.72, p<0.01, n=26). The linear combination of high and low frequency relative permittivity predicted 73% of the variation in BV/TV. To conclude, electrical and dielectric parameters of trabecular bone, especially relative permittivity and dissipation factor, were significantly and specifically related to a trabecular microstructure as characterized with microCT. The data gathered in this study constitute a useful basis for theoretical and experimental work towards the development of impedance spectroscopy techniques for detection of bone quality in vitro or in special cases of open surgery.

Dielectric properties of a protein–water system in selected animal tissues

Dielectric spectroscopy has been applied to study aspects of the organization of water in selected animal tissues (tendon, bone and horn). The measurements of the relative permittivity epsilon' and the dielectric loss epsilon'' were carried over the frequency range of 10-100 kHz and at temperatures from 22 to 240 degrees C. The water content was 10% for bone and horn, and 22% for tendon by mass at room temperature at a relative humidity of 70%. The temperature dependencies of epsilon' and epsilon'' reveal distinctively the temperature ranges corresponding to the release of water in temperatures up to about 200 degrees C for all tissues and the melting of the crystalline structure only for tendon and horn, above this temperature. The frequency dependencies of epsilon' and epsilon'' show a remarkable dispersion in the low-frequency at selected temperatures up to 200 degrees C for all tissues due to the release of the loosely and strongly bound water. The results were discussed in terms of the interfacial (Maxwell-Wagner) polarization and polarization mechanism involving hopping charge carriers interacting with the bound water molecules. The information on the effect of temperature, water content and frequency of the electromagnetic field on the dielectric behaviour of the tissues studied is of importance in the design and construction of medical diagnostic or therapeutic instruments based on the use of electric signals.

Changes in thermal and electrical properties of bone as a result of 1 MGy-dose γ-irradiation

Determination of temperature dependencies of electric conductivity and thermal properties by differential scanning calorimetry (DSC) allow to analyse the processes of charge and heat transport in the bone being a complex collagen-hydroxyapatite (HAP)-water system. Modification of the bone structure by high doses of gamma-radiation changes the electrical and thermal properties of the bone. Electrical conductivity (sigma) of the bone decreases with consecutive heating runs. The decrease in sigma observed for irradiated samples was explained by the scission of the main chain of collagen macromolecule. Irradiation decreased the hydration level in the bone, its denaturation temperature and increased both enthalpy and entropy of the denaturation process.

The effect of gamma-irradiation on the temperature dependence of D.C. electrical conductivity of dry bone

The effect of gamma-irradiation with doses from 10 to 500 kGy on the electrical conductivity (g) of dry bone was studied. Temperature measurement of electrical conductivity were made from 393 to 533 K. The dependence obtained indicates the increase in g with temperature. An increase in irradiation dose resulted in a decreased g value for each dose up to temperature 462 K. Temperature 462 K was interpreted as the temperature of collagen melting point in dry bone. Above 462 K, g values were dose independent. A dose of 500 kGy shifted the melting point to lower temperature. In addition, the activation energy for the charge conduction process was calculated. Obtained values for electrical conductivity and activation energy were typical for dielectrics and indicated degradation of the organic component of bone.

Dielectric relaxation of air-dried horn keratin

Dielectric measurements as a function of temperature and frequency are reported for horn keratin. The measurements of dielectric constant epsilon' and loss factor epsilon", in keratin, were made in an electric field. This was done in the frequency range 10(1)-10(5) Hz and at temperatures from 22 to 220 degrees C. The samples contained 8% water by mass at room temperature at a relative humidity of 40%. A remarkable dispersion observed in the range of lower frequencies was attributed to interfacial polarization. The temperature dependences of the dielectric constants of horn keratin revealed the occurrence of two main molecular processes. The first process corresponded to the removal of water in the temperature range 22-170 degrees C. The activation energy associated with the release of loosely and strongly bound water, was about 35 and 7 kcal/mol, respectively. The second process, above 170 degrees C, was related to the denaturation of the alpha-helical phase in keratin.