Thermal stabilization of collagen fibers by calcification

The diagenetic fate of collagen as revealed by analytical pyrolysis of fossil fish scales from deep time

The mechanism of protein degradation has remained a topic of debate (specifically concerning their preservation in deep time), which has recently been invigorated due to multiple published reports of preservation ranging from Miocene to the Triassic that potentially challenge the convention that protein preservation beyond the Cenozoic is extremely uncommon or is expected to be absent altogether, and thus have attracted skepticism. In this paper, we analyze fossil fish scales from the Cretaceous, Jurassic, and Triassic using comprehensive pyrolysis gas chromatography coupled with time-of-flight mass spectrometry and compare the pyrolytic products so obtained with a well-preserved fish scale from Late Pliocene, in an attempt to better understand the effects of diagenesis on protein degradation at the molecular level through deep time. We find that the Pliocene fish scale displays a large number of N-bearing pyrolytic products, including abundant substituted cyclic 2,5-diketopiperazines (2,5-DKPs) which are diagnostic products of peptide and amino acid pyrolysis. We identify N-bearing compounds in the Mesozoic fish scales-however, among the 2,5-DKPs that were identified in the Pliocene scale, only diketodipyrrole (or cyclo (Pyr-Pyr)) is present in the Mesozoic scales. We discuss the implications of N-bearing pyrolytic products with emphasis on 2,5-DKPs in geological samples and conclude that the discrepancy in abundance and variety of N-bearing products between Pliocene and Mesozoic scales indicates that the protein component in the latter has been extensively diagenetically altered, while a suite of DKPs such as in the former would imply stronger evidence to indicate preservation of protein. We conclude that analytical pyrolysis is an effective tool for detecting preservation of intact proteins, as well as for providing insights into their degradation mechanisms, and can potentially be utilized to assign proteinaceous origin to a fossil sample of unknown affinity.

A substrate protection approach to applying the calcium ion for improving the proteolysis resistance of the collagen

Enzymatic dehairing, as a crucial part of cleaner leather processing, has reached processive advancement with potentially replacing the traditional hair removal due to increasing pressure from environmental demand. However, this cleaner technology based on proteases has a problem that the hide grain (collagen-rich structure) is susceptible to be hydrolyzed, decreasing the quality of finished leather. From the perspective of improving the stability of collagen fibers and their resistance to proteolysis, a method for protecting the hide grain during the enzymatic dehairing process was developed. The results showed that calcium ions had a swelling effect on collagen fibers under near-neutral conditions (pH 6.0-10.0), decreasing the thermal stability of collagen and the proteolysis resistance of collagen significantly. The alkaline environment (pH 10.0-12.0) will promote the dissociation of carboxyl groups in hide collagen, promoting the combination of calcium ions and carboxyl groups. This strategy can change the surface charge of collagen fibers and strengthen the connection between collagen fibers, thus improving protease resistance and the thermal stability of collagen. However, collagen fibers could swell violently once the alkalinity of the solution environment was extreme. Despite the above situation, calcium ion was still conducive to maintain the structural stability of collagen fibers. At pH 10.0-12.0, pretreating animal hide with a solution containing calcium ions can improve the protease resistance of hide grain, making the hide grain well-protected. This method provided an effective way to establish a safer enzymatic unhairing technology based on substrate protection. KEY POINTS: • A collagen protection method for hair removal of animal hide was developed. • This method applied calcium ions to collagen at alkaline conditions (pH 10.0-12.0). • Pretreatment results of calcium ions at different pH values on animal hide were compared.

Heat impact during laser ablation extraction of mineralised tissue micropillars

The underlying constraint of ultrashort pulsed laser ablation in both the clinical and micromachining setting is the uncertainty regarding the impact on the composition of material surrounding the ablated region. A heat model representing the laser-tissue interaction was implemented into a finite element suite to assess the cumulative temperature response of bone during ultrashort pulsed laser ablation. As an example, we focus on the extraction of mineralised collagen fibre micropillars. Laser induced heating can cause denaturation of the collagen, resulting in ultrastructural loss which could affect mechanical testing results. Laser parameters were taken from a used micropillar extraction protocol. The laser scanning pattern consisted of 4085 pulses, with a final radial pass being 22 [Formula: see text] away from the micropillar. The micropillar temperature was elevated to 70.58 [Formula: see text], remaining 79.42 [Formula: see text] lower than that of which we interpret as an onset for denaturation. We verified the results by means of Raman microscopy and Energy Dispersive X-ray Microanalysis and found the laser-material interaction had no effect on the collagen molecules or mineral nanocrystals that constitute the micropillars. We, thus, show that ultrashort pulsed laser ablation is a safe and viable tool to fabricate bone specimens for mechanical testing at the micro- and nanoscale and we provide a computational model to efficiently assess this.

Peptide mass mapping in bioapatites isolated from animal bones

Bioapatite ceramics produced from biogenic sources provide highly attractive materials for the preparation of artificial replacements since such materials are not only more easily accepted by living organisms, but bioapatite isolated from biowaste such as xenogeneous bones also provides a low-cost material. Nevertheless, the presence of organic compounds in the bioapatite may lead to a deterioration in its quality and may trigger an undesirable immune response. Therefore, procedures which ensure the elimination of organic compounds through bioapatite isolation are being subjected to intense investigation and the presence of remaining organic impurities is being determined through the application of various methods. Since current conclusions concerning the conditions suitable for the elimination of organic compounds remain ambiguous, we used the mass spectrometry-based proteomic approach in order to determine the presence of proteins or peptides in bioapatite samples treated under the most frequently employed conditions, i.e., the alkaline hydrothermal process and calcination at 500 °C. Since we also investigated the presence of proteins or peptides in treated bioapatite particles of differing sizes, we discovered that both calcination and the size of the bioapatite particles constitute the main factors influencing the presence of proteins or peptides in bioapatite. In fact, while intact proteins were detected even in calcinated bioapatite consisting of particles >250 µm, no proteins were detected in the same material consisting of particles <40 µm. Therefore, we recommend the use of powdered bioapatite for the preparation of artificial replacements since it is more effectively purified than apatite in the form of blocks. In addition, we observed that while alkaline hydrothermal treatment leads to the non-specific cleavage of proteins, it does not ensure the full degradation thereof.

Cytoskeletal and extracellular matrix proteins resist the burning of bones

Due the proteins from bone remains are highly resistant to pass of time and environmental conditions, they could tell us about the events that probably happened in the past. In the forensic and physical anthropology context, burnt bone remains are one of the most common pieces of recovered evidence and, generally, they are associated with funerary practices, criminal scenes or massive catastrophic events. In the present study, bone pieces of pigs were calcined at different calcination temperatures, and proteins were searched using biochemical, immunochemical and ultrastructure visualization under these experimentally conditions. For this purpose, it was successfully developed a non-demineralizing protein extraction method from burnt bone remains and the use of specific antibodies permitted the identification of different extracellular matrix and intracellular proteins. While collagen proteins type I and IV were identified and detected under middle and high calcination temperatures (300°C and 600°C); cytoskeletal proteins as actin, tubulin and, the microtubule associated protein Tau, were found under calcination process, even up high calcination temperatures. Under ultrastructural analysis, fibrous materials with a classical disposition of collagens were observed even at high calcination temperatures of the burnt bone remains. The protein identification and characterization in burnt bones as performed in present studies, is clearly demonstrating that using specific strategies for protein characterizations it is possible to found protein biomarkers in burnt bone remains and this strategy could be useful for forensic and anthropological purposes.

Effects of the aspect ratio of multi-walled carbon nanotubes on the structure and properties of regenerated collagen fibers

Collagen is a natural one-dimensional nanomaterial. Multi-walled carbon nanotubes (MWNTs) have been previously shown to interact with biomolecules and to have promising applications in reinforced biopolymers for tissue engineering and regenerative medicine. In this work, collagen/MWNT composite fibers are prepared using dry-jet wet-spinning technology. Three types of MWNTs with aspect ratios of 40, 150, and 4000 are used to investigate the effects of the MWNT aspect ratio on the properties of the composite fibers. There results show that there are strong molecular interactions between the MWNTs and collagen molecules. The mechanical properties and thermal stability of the composite fibers are significantly improved compared to those of the collagen fibers. The diameter and aspect ratio of the MWNTs are the main factors affecting the self-assembled structure of the collagen molecules, the alignment of the microfibrils, and the mechanical and thermal performance of the composite fibers.

Heparinization of a biomimetic bone matrix: integration of heparin during matrix synthesis versus adsorptive post surface modification

This study intended to evaluate a contemporary concept of scaffolding in bone tissue engineering in order to mimic functions of the extracellular matrix. The investigated approach considered the effect of the glycosaminoglycan heparin on structural and biological properties of a synthetic biomimetic bone graft material consisting of mineralized collagen. Two strategies for heparin functionalization were explored in order to receive a three-component bone substitute material. Heparin was either incorporated during matrix synthesis by mixing with collagen prior to simultaneous fibril reassembly and mineralization (in situ) or added to the matrix after fabrication (a posteriori). Both methods resulted in an incorporation of comparable amounts of heparin, though its distribution in the matrix varied as indicated by TOF-SIMS analyses, and a similar modulation of their protein binding properties. Differential scanning calorimetry revealed that the thermal stability and thereby the degree of crosslinking of the heparinized matrices was increased. However, in contrast to the a posteriori modification, the in situ integration of heparin led to considerable changes of morphology and composition of the matrix: a more open network of collagen fibers yielding a more porous surface and a reduced mineral content were observed. Cell culture experiments with human mesenchymal stem cells (hMSC) revealed a strong influence of the mode of heparin functionalization on cellular processes, as demonstrated for proliferation and osteogenic differentiation of hMSC. Our results indicate that not only heparin per se but also the way of its incorporation into a collagenous matrix determines the cell response. In conclusion, the a posteriori modification was beneficial to support adhesion, proliferation and differentiation of hMSC.

Partial removal of pore and loosely bound water by low-energy drying decreases cortical bone toughness in young and old donors

With an ability to quantify matrix-bound and pore water in bone, (1)H nuclear magnetic resonance (NMR) relaxometry can potentially be implemented in clinical imaging to assess the fracture resistance of bone in a way that is independent of current X-ray techniques, which assess bone mineral density as a correlate of bone strength. Working towards that goal, we quantified the effect of partial dehydration in air on the mechanical and NMR properties of human cortical bone in order to understand whether NMR is sensitive to water-bone interactions at low energy and whether such interactions contribute to the age-related difference in the toughness of bone. Cadaveric femurs were collected from male and female donors falling into two age groups: 21-60 years of age (young) and 74-99 years of age (old). After extracting two samples from the medial cortex of the mid-shaft, tensile tests were conducted on Wet specimens and paired, Partially Dry (PtlD) specimens (prepared by low-energy drying in air to remove ∼3% of original mass before testing). Prior analysis by micro-computed tomography found that there were no differences in intra-cortical porosity between the Wet and PtlD specimens nor did an age-related difference in porosity exist. PtlD specimens from young and old donors had significantly less toughness than Wet specimens, primarily due to a dehydration-related decrease in post-yield strain. The low-energy drying protocol did not affect the modulus and yield strength of bone. Subsequent dehydration of the PtlD specimens in a vacuum oven at 62°C and then 103°C, with quantification of water loss at each temperature, revealed an age-related shift from more loosely bound water to more tightly bound water. NMR detected a change in both bound and pore water pools with low-energy air-drying, and both pools were effectively removed when bone was oven-dried at 62°C, irrespective of donor age. Although not strictly significant due to variability in the drying and testing conditions, the absolute difference in toughness between Wet and PtlD tended to be greater for the younger donors that had higher bone toughness and more bound water for the wet condition than did the older donors. With sensitivity to low-energy bone-water interactions, NMR, which underpins magnetic resonance imaging, has potential to assess fracture resistance of bone as it relates to bone toughness.

Effect of "in vitro" induced glycation on thermostability of bone tissue

The aim of the study was to test the hypothesis that glycation would influence thermal stability of bone tissue collagen. Bone samples were incubated in buffer or in ribose solution. Then, half of the ribosylated and half of the control samples were completely demineralized in formic acid. Differential scanning calorimetry was performed for temperatures from 40 °C to 220 °C in nitrogen atmosphere on intact (mineralized) and demineralized bone samples, partially dehydrated at room temperature. Samples were thermally active in temperatures from 110 °C to 210 °C. Few endotherms of a complex nature were found in demineralized and intact bone. Thermodynamics of collagen conformations was affected by glycation, especially in demineralized bone where a significant increase of denaturation temperature (by 3-4 °C) and enthalpy drop (above 20%) were stated after glycation.

Protein and mineral characterisation of rendered meat and bone meal

We report the characterisation of meat and bone meal (MBM) standards (Set B-EFPRA) derived from cattle, sheep, pig and chicken, each rendered at four different temperatures (133, 137, 141 and 145 °C). The standards, prepared for an EU programme STRATFEED (to develop new methodologies for the detection and quantification of illegal addition of mammalian tissues in feeding stuffs), have been widely circulated and used to assess a range of methods for identification of the species composition of MBM. The overall state of mineral alteration and protein preservation as a function of temperature was monitored using small angle X-ray diffraction (SAXS), amino acid composition and racemization analyses. Progressive increases in protein damage and mineral alteration in chicken and cattle standards was observed. In the case of sheep and pig, there was greater damage to the proteins and alteration of the minerals at the lowest treatment temperature (133 °C), suggesting that the thermal treatments must have been compromised in some way. This problem has probably impacted upon the numerous studies which tested methods against these heat treatments. We use protein mass spectrometric methods to explore if thermostable proteins could be used to identify rendered MBM. In more thermally altered samples, so-called 'thermostable' proteins such as osteocalcin which has been proposed as a ideal target to speciate MBM were no longer detectable, but the structural protein type I collagen could be used to differentiate all four species, even in the most thermally altered samples.

FTIR spectroscopy: A new diagnostic tool to aid DNA analysis from heated bone

Deoxyribonucleic acid (DNA) extracted from skeletal tissue can be invaluable in genetic profiling applications, as it is often the only source available. Like all forensic samples, skeletal tissue may have been exposed to a variety of environmental insults, including heat. This study has focussed upon characterising changes in the material properties of bone that has been compromised by controlled heat treatments. These changes were then examined in relation to the subsequent success or failure of nuclear DNA (nDNA) amplification, using a range of differently sized amplicons, relevant to alternate profiling strategies. The results presented demonstrate that the ability to amplify nDNA correlates well with particular changes in mineral and organic content of bone. As such, we propose the application of a 'diagnostic triage tool' that can be performed quickly and at low cost on individual bone samples, in order to determine whether nDNA analysis is likely to be a viable option.

A novel collagen/hydroxyapatite/poly(lactide-co-ε-caprolactone) biodegradable and bioactive 3D porous scaffold for bone regeneration

The goal of this study was to design a nontoxic scaffold with both composition and microstructure suitable for bone engineering using collagen (Coll), hydroxyapatite (HA), and poly(lactide-co-ε-caprolactone) (PLCL). Mineralized type I Coll was produced by direct nucleation of HA particles inside self-assembled Coll fibers to obtain a Coll/HA complex, which was then added to dissolved PLCL (70:30) in 1,4-dioxane. A 3D porous Coll/HA/PLCL scaffold was subsequently produced through freeze-drying/lyophilization and salt-leaching procedures. The resulting Coll/HA/PLCL scaffold displayed a high uniform porosity and highly interconnected pores. X-ray photoelectron spectrometer and Fourier transform infrared analyses revealed the presence of both collagen and HA particles on the surface of the Coll/HA/PLCL scaffold. Proliferation assay, microscopic observations, and gene analysis with quantitative RT-PCR showed that osteoblast cells were able to attach, proliferate, and maintain an osteoblastlike phenotype when cultured on the Coll/HA/PLCL scaffold. In summary, we produced a nontoxic scaffold that contains natural polymers (Coll and HA) and synthetic polymer (PLCL). Through its chemical composition and porous morphology, this scaffold may be useful for osteoblast growth, differentiation, and bone tissue formation.

The critical barrier to progress in dentine bonding with the etch-and-rinse technique

OBJECTIVES

The lack of durability in resin-dentine bonds led to the use of chlorhexidine as MMP-inhibitor to prevent the degradation of hybrid layers. Biomimetic remineralisation is a concept-proven approach in preventing the degradation of resin-dentine bonds. The purpose of this study is to examine the integrity of aged resin-dentine interfaces created with a nanofiller-containing etch-and-rinse adhesive after the application of these two approaches.

METHODS

The more established MMP-inhibition approach was examined using a parallel in vivo and in vitro ageing design to facilitate comparison with the biomimetic remineralisation approach using an in vitro ageing design. Specimens bonded without chlorhexidine exhibited extensive degradation of the hybrid layer after 12 months of in vivo ageing.

RESULTS

Dissolution of nanofillers could be seen within a water-rich zone within the adhesive layer. Although specimens bonded with chlorhexidine exhibited intact hybrid layers, water-rich regions remained in those hybrid layers and degradation of nanofillers occurred within the adhesive layer. Specimens subjected to in vitro biomimetic remineralisation followed by in vitro ageing demonstrated intrafibrillar collagen remineralisation within hybrid layers and deposition of mineral nanocrystals in nanovoids within the adhesive.

CONCLUSIONS

The impact was realized by understanding the lack of an inherent mechanism to remove water from resin-dentine interfaces as the critical barrier to progress in bonding with the etch-and-rinse technique. The experimental biomimetic remineralisation strategy offers a creative solution for incorporating a progressive hydration mechanism to achieve this goal, which warrants its translation into a clinically applicable technique.

Thermal analysis reveals differential effects of various crosslinkers on bovine annulus fibrosis

Treatment of a pathological spinal disc in vivo by injection of protein crosslinking reagents to restore the disc's mechanical properties is a new approach to the treatment of degenerative disc disease. In this study, the thermal stability of the collagen in disc annulus was measured by differential scanning calorimetry following treatment with six different crosslinking agents. The crosslinkers used were; L-threose (LT), genipin (GP), methylglyoxal (MG), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC), glutaraldehyde (GA), and proanthrocyanidin (PA). Untreated tissue displayed a prominent peak at about 66-68°C. Comparison of endothermal patterns of untreated and crosslinker-treated disc annulus tissue samples showed that a new peak appeared at a higher temperature following treatment. The temperature of the new peak qualitatively depended on the crosslinker in the following order GA > MG > GP > PA = EDC > LT, suggesting that the enhanced thermal stability of collagen in the annulus tissue was related to the nature of the crosslinker. Also, the enthalpic ratios of the lower temperature (noncrosslinked) peaks in the treated and untreated tissue, and of the higher and lower temperature peaks in the treated tissue, both indicated that the various agents crosslinked the tissue with different efficiencies. Our data suggest that the ability of GP to penetrate into the disc and form long- and short-range crosslinks may make it the most suitable candidate for clinical development. In addition, binary combinations of long- and short-range crosslinkers, such as PA with LT, may also provide synergistic effects due to their substantially different physicochemical properties.

Imperfect hybrid layers created by an aggressive one-step self-etch adhesive in primary dentin are amendable to biomimetic remineralization in vitro

Degradation of hybrid layers created in primary dentin occurs as early as 6 months in vivo. Biomimetic remineralization utilizes "bottom-up" nanotechnology principles for interfibrillar and intrafibrillar remineralization of collagen matrices. This study examined whether imperfect hybrid layers created in primary dentin can be remineralized. Coronal dentin surfaces were prepared from extracted primary molars and bonded using Adper Prompt L-Pop and a composite. One-millimeter-thick specimen slabs of the resin-dentin interface were immersed in a Portland cement-based remineralization medium that contained two biomimetic analogs to mimic the sequestration and templating functions of dentin noncollagenous proteins. Specimens were retrieved after 1-6 months. Confocal laser scanning microscopy was used for evaluating the permeability of hybrid layers to Rhodamine B. Transmission electron microscopy was used to examine the status of remineralization within hybrid layers. Remineralization at different locations of the hybrid layers corresponded with quenching of fluorescence within similar locations of those hybrid layers. Remineralization was predominantly intrafibrillar in nature as interfibrillar spaces were filled with adhesive resin. Biomimetic remineralization of imperfect hybrid layers in primary human dentin is a potential means for preserving bond integrity. The success of the current proof-of-concept, laterally diffusing remineralization protocol warrants development of a clinically applicable biomimetic remineralization delivery system.

Is amino acid racemization a useful tool for screening for ancient DNA in bone?

Many rare and valuable ancient specimens now carry the scars of ancient DNA research, as questions of population genetics and phylogeography require larger sample sets. This fuels the demand for reliable techniques to screen for DNA preservation prior to destructive sampling. Only one such technique has been widely adopted: the extent of aspartic acid racemization (AAR). The kinetics of AAR are believed to be similar to the rate of DNA depurination and therefore a good measure of the likelihood of DNA survival. Moreover, AAR analysis is only minimally destructive. We report the first comprehensive test of AAR using 91 bone and teeth samples from temperate and high-latitude sites that were analysed for DNA. While the AAR range of all specimens was low (0.02–0.17), no correlation was found between the extent of AAR and DNA amplification success. Additional heating experiments and surveys of the literature indicated that d/l Asx is low in bones until almost all the collagen is lost. This is because aspartic acid is retained in the bone within the constrained environment of the collagen triple helix, where it cannot racemize for steric reasons. Only if the helix denatures to soluble gelatin can Asx racemize readily, but this soluble gelatine is readily lost in most burial environments. We conclude that Asx d/l is not a useful screening technique for ancient DNA from bone.

Heat resistance of allograft tissue

Lyophilized allograft musculoskeletal tissue is generally intended to be stored at "ambient" or "room" temperature, and usually is kept in climate controlled indoor storage areas. However, there is a question of what temperature extremes tissue may see, especially during transportation, in that these extremes may exceed even the limits of "ambient" conditions. Temperatures may become quite hot, but only for a few hours and only during daytime. Damage from high temperatures, if it occurs, is expected to be evident by damage to the collagen component of bone, soft tissue, and demineralized bone, as well as to the growth factors contained in demineralized bone. If damage is significant, then temperature monitoring requirements for lyophilized allograft tissue might be necessary. To answer this question, a literature review was carried out to look at the short-term temperature resistance of collagen and demineralized bone. Both collagen and the growth factors in demineralized bone show remarkable short term tolerance to elevated temperatures in the dry state, and it was concluded that temperature excursions of 50 degrees C or less, lasting for a few days or less, would not cause any significant deterioration. This means that temperature monitoring also should not be required.

Efeito do pH na calcificação in vitro de pele porcina

A engenharia de tecidos tem sido utilizada como alternativa na reconstrução de tecidos duros e moles. Este estudo teve como objetivo a calcificação "in vitro" de pele porcina visando à obtenção de um material para regeneração de tecido duro. As matrizes de pele porcina foram calcificadas em cela dupla termostatizada a 37 °C em pH 7,4 e pH 9,0 e caracterizadas por microscopia eletrônica de varredura (MEV), termogravimetria (TGA), espectroscopia no infravermelho (FTIR), calorimetria exploratória diferencial (DSC) e difração de raios X. Os resultados obtidos por DSC mostraram que as amostras calcificadas têm um pequeno aumento nos valores de temperatura de desnaturação em relação à amostra não calcificada, enquanto as curvas termogravimétricas mostraram uma porcentagem maior de material inorgânico para o pH 7,4 em comparação com as amostras obtidas em pH 9,0. A formação de sais de fosfato de cálcio nas fibras de colágeno foi confirmada por difração de raios X (DRX), espectroscopia no infravermelho (FTIR) e microscopia eletrônica de varredura (MEV).

Effects of polar solvents and adhesive resin on the denaturation temperatures of demineralised dentine matrices

OBJECTIVES

To measure the denaturation temperature (Td) of demineralised dentine matrix as a function of infiltration with water vs. polar solvents vs. adhesive resins.

METHODS

Small discs of normal dentine were completely demineralised in 0.5M EDTA. Dried demineralised specimens were placed in water, methanol, ethanol, acetone, eta-butanol or HEMA. Additional specimens were infiltrated with Prime&Bond NT and polymerised. All specimens sealed in high-pressure pans and scanned using differential scanning calorimetry (DSC).

RESULTS

Demineralised dentine saturated with water showed a Td of 65.6 degrees C that increased with saturation by methanol, ethanol, acetone, eta-butanol or HEMA to 148.5 degrees C. These increases in Td were inversely related to the molar concentration of the solvents and to their Hoy's solubility parameter for hydrogen bonding (delta h, p<0.01), as well as directly related to the cube root of their molecular weights (p<0.001). The presence of adhesive resins also increased the Td of demineralised matrices to even higher values depending if the resin bonded dentine was measured after 24h of water storage (166.8 degrees C) or dry (172.7 degrees C) storage.

CONCLUSIONS

Solvents and monomers with low delta(h) values (i.e., 100% HEMA) increase the Td of demineralised dentine above that produced by solvents with higher delta h values such as methanol and water.

Denaturation temperatures of dentin matrices. I. Effect of demineralization and dehydration

The denaturation temperature (T(d)) of dentin collagen in mineralized versus demineralized teeth was examined as a function of dentin age and the extent of dehydration. Using differential scanning calorimetry, T(d) of mineralized dentin was shown to be between 160 degrees C to 186 degrees C, depending on whether it was from young or old dentin that was hydrated or dehydrated, respectively. Demineralized dentin exhibited a T(d) of 65.6 degrees C that increased with dehydration to 176 degrees C. The presence of apatite crystallites or interpeptide bonding increased the T(d) of demineralized matrices. Interpeptide hydrogen bonding seems to stabilize collagen to thermal challenge. Water breaks interpeptide hydrogen bonds making collagen more susceptible to thermal denaturation. Rises in intracanal temperature are unlikely to cause extensive denaturation of mineralized root dentin walls. However, hydrated or partially dehydrated root canal walls that have been partially demineralized with chelating agents or mild acids may be susceptible to thermal denaturation.

Thermal stabilization of collagen molecules in bone tissue

Differential thermal calorimetry (DSC) analysis of partially dehydrated bovine bone, demineralized bone and bovine tendon collagen was performed up to 300 degrees C to determine factors influencing stability of mineralized collagen in bone tissue. Two endothermal regions were recognized. The first, attributed to denaturation of collagen triple helix, was hydration dependent and had a peak at 155-165 degrees C in bone, 118-137 degrees C in tendon and 131-136 degrees C in demineralized bone. The second region extended from 245 to 290 degrees C in bone and from 200 to 280 degrees C in tendon and was connected with melting and decomposition of collagen. Differences in thermodynamic parameters between cortical and trabecular bone tissue were stated. Activation energy of collagen unfolding in native bone tissue increased with dehydration of the bone. From the results of the present study we conclude that dehydrated bone collagen is thermally very stable both in native and in demineralized bone. Presence of mineral additionally stabilizes bone tissue.

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.

Collagen and bone viscoelasticity: a dynamic mechanical analysis

The purpose of this study was to explore the effects of changes in Type I collagen on the viscoelasticity of bone. Bone coupons were heated at either 100 or 200 degrees C to induce the thermal denaturation of Type I collagen. Half of these specimens were rehydrated after heat treatment; the other half were tested in a dry condition. The degree of denatured collagen (DC%) was analyzed by a selective digestion technique with the use of alpha-chymotrypsin. Isothermal (37 degrees C) and variable temperature tests (scans from 35 to 200 degrees C) were performed with the use of a dynamic mechanical analyzer to evaluate changes in bone viscoelastic properties as a function of collagen damage, specifically, changes in the loss factor (tan delta) and storage modulus (E') were assessed. Significant collagen denaturation occurred only when bone was heated at 200 degrees C irrespective of the hydration condition. Also, DC% did not show a significant effect on tan delta. However, higher values of tan delta were observed in wet samples compared to dry specimens. The temperature-scan tests revealed that the hydration condition, but not DC%, significantly affected the behavior of tan delta. However, E' was not strongly influenced either by DC% or by water content. These results suggest that at a constant frequency the denaturation of collagen triple-helical molecules may have few effects on the viscoelasticity of bone, but moisture may play a prominent role in determining this property.

The role of collagen in determining bone mechanical properties

The hypothesis of this study was that collagen denaturation would lead to a significant decrease in the toughness of bone, but has little effect on the stiffness of bone. Using a heating model, effects of collagen denaturation on the biomechanical properties of human cadaveric bone were examined. Prior to testing, bone specimens were heat treated at varied temperatures (37-200 degrees C) to induce different degrees of collagen denaturation. Collagen denaturation and mechanical properties of bone were determined using a selective digestion technique and three-point bending tests, respectively. The densities and weight fractions of the mineral and organic phases in bone also were determined. A repeated measures analysis of variance showed that heating had a significant effect on the biomechanical integrity of bone, corresponding to the degree of collagen denaturation. The results of this study indicate that the toughness and strength of bone decreases significantly with increasing collagen denaturation, whereas the elastic modulus of bone is almost constant irrespective of collagen denaturation. These results suggest that the collagen network plays an important role in the toughness of bone, but has little effect on the stiffness of bone, thereby supporting the hypothesis of this study.

The use of dynamic mechanical analysis to assess the viscoelastic properties of human cortical bone

The purpose of this study was to examine the use of a dynamic mechanical analyzer (DMA) system to study the viscoelastic nature of bone. Cortical bone specimens from human femora were tested isothermally for 150 min at 37 degrees C and the loss factor (tan delta) and storage modulus (E') were measured. To explore the effects of test conditions on tan delta and E', different levels of applied stress, two specimen sizes, and two hydration conditions (wet and vacuum-dried) were evaluated. Finally, nonisothermal tests were performed, wherein specimens were heated up to 70 degrees C at different heating rates: 1 degrees C/min, 3 degrees C/min, and 5 degrees C/min. The results indicated that a threshold level of minimum applied stress was required to obtain repeatable and relatively constant values of tan delta. Specimen size did not significantly affect tan delta although it influenced E'. Moisture content had a significant effect on tan delta; vacuum-dried specimens exhibited a lower tan delta compared to wet specimens. Lastly, heating rates influenced tan delta values with lower rates producing more consistent results. The study demonstrated that DMA can be used as an effective tool to test bone.

Thermal stability of bone collagen as an indicator of bone turnover in gonadectomized and multiparous rats

Previous findings indicate that the thermal stability of bone collagen is related to age. In this study, collagen from rat bone with reported different turnover rates was investigated. Cortical and trabecular bone from femur were obtained from intact, ovariectomized, orchidectomized and multiparous breeder rats. Thermal stabilities of fibrillar collagen in decalcified bone matrix and molecular collagen obtained by pepsin treatment were measured as shrinkage (Ts) and 'melting' temperature (Tm), respectively. Both Ts and Tm of cortical collagen from intact female rats decreased in parallel with age as previously found in male rats indicating that Ts and Tm measurements are interchangeable techniques in characterizing the thermal stability of bone collagen. Tm of trabecular collagen from intact rats decreased with age, however, with a decay only one-third of that for cortical collagen. The different rates possibly reflect different ages of collagen due to remodeling activity present in trabecular and minimal in cortical bone. Compared with control rats the Tm of trabecular collagen from gonadectomized and multiparous rats with a reported increased trabecular turnover rate was elevated, whereas only minor variations in Tm of cortical collagen were found. In conclusion, the thermal stability of bone collagen decreases with the age of the collagen. Increased bone turnover implies elevated thermal stability of bone collagen. Thus, thermal stability of bone collagen appears to be an indicator of bone turnover.