Mechanical contribution of lamellar and interlamellar elastin along the mouse aorta.

The mechanical properties of aortic elastin vary regionally, but the microstructural basis for this variation is unknown. This study was designed to identify the relative contributions of lamellar and interlamellar elastin to circumferential load bearing in the mouse thoracic and abdominal aortas. Forces developed in uniaxial tests of samples of fresh and autoclaved aorta were correlated with elastin content and morphology obtained from histology and multiphoton laser scanning microscopy. Autoclaving should render much of the interlamellar elastin mechanically incompetent. In autoclaved tissue force per unit sample width correlated with lamellar elastin content (P≪0.001) but not total elastin content. In fresh tissue at low strain where elastin dominates the mechanical response, forces were higher than in the autoclaved tissue, but force did not correlate with total elastin content. Therefore although interlamellar elastin likely contributed to the stiffness in the fresh aorta, its contribution appeared not in proportion to its quantity. In both fresh and autoclaved tissue, elastin stiffness consistently decreased along the abdominal aorta, a key area for aneurysm development, and this difference could not be fully accounted for on the basis of either lamellar or total elastin content. These findings are relevant to the development of mathematical models of arterial mechanics, particularly for mouse models of arterial diseases involving elastic tissue. In microstructural based models the quantity of each mural constituent determines its contribution to the total response. This study shows elastin's mechanical response cannot necessarily be accounted for on the basis of fibre quantity, orientation, and modulus.

A simple and affordable calorespirometer for assessing the metabolic rates of fishes.

Calorimetry is the measurement of the heat liberated during energy transformations in chemical reactions. When applied to living organisms, it measures the heat released due to the energy transformations associated with metabolism under both aerobic and anaerobic conditions. This is in contrast to the often-used respirometric techniques for assessing energy turnover, which can only be used under fully aerobic conditions. Accordingly, calorimetry is considered the 'gold standard' for quantifying metabolic rate, yet despite this, it remains a seldom-used technique among comparative physiologists. The reasons for this are related to the expense and perceived difficulty of the technique. We have designed and constructed an inexpensive flow-through calorespirometer capable of detecting rates of metabolic heat loss and oxygen consumption (O2) in fish under a variety of environmental conditions over long-term experiments. The metabolic heat of the fish is detected as a voltage by a collection of Peltier units wired in series, while oxygen optodes placed on the inflowing and outflowing water lines are used for the calculation of O2. The apparatus is constructed in a differential fashion to account for ambient temperature fluctuations. This paper describes the design and construction of the calorespirometer for ~$1300 CDN. Using the goldfish (Carassius auratus auratus), we show that the calorespirometer is sensitive to changes in metabolic rate brought about by pharmacological manipulation and severe hypoxia exposures.

Cardiovascular design in fin whales: high-stiffness arteries protect against adverse pressure gradients at depth.

Fin whales have an incompliant aorta, which, we hypothesize, represents an adaptation to large, depth-induced variations in arterial transmural pressures. We hypothesize these variations arise from a limited ability of tissues to respond to rapid changes in ambient ocean pressures during a dive. We tested this hypothesis by measuring arterial mechanics experimentally and modelling arterial transmural pressures mathematically. The mechanical properties of mammalian arteries reflect the physiological loads they experience, so we examined a wide range of fin whale arteries. All arteries had abundant adventitial collagen that was usually recruited at very low stretches and inflation pressures (2-3 kPa), making arterial diameter largely independent of transmural pressure. Arteries withstood significant negative transmural pressures (-7 to -50 kPa) before collapsing. Collapse was resisted by recruitment of adventitial collagen at very low stretches. These findings are compatible with the hypothesis of depth-induced variation of arterial transmural pressure. Because transmural pressures depend on thoracic pressures, we modelled the thorax of a diving fin whale to assess the likelihood of significant variation in transmural pressures. The model predicted that deformation of the thorax body wall and diaphragm could not always equalize thoracic and ambient pressures because of asymmetrical conditions on dive descent and ascent. Redistribution of blood could partially compensate for asymmetrical conditions, but inertial and viscoelastic lag necessarily limits tissue response rates. Without pressure equilibrium, particularly when ambient pressures change rapidly, internal pressure gradients will develop and expose arteries to transient pressure fluctuations, but with minimal hemodynamic consequence due to their low compliance.

Contribution of elastin and collagen to the inflation response of the pig thoracic aorta: assessing elastin's role in mechanical homeostasis.

This study was undertaken to understand elastin's role in the mechanical homeostasis of the arterial wall. The mechanical properties of elastin vary along the aorta, and we hypothesized this maintained a uniform mechanical environment for the elastin, despite regional variation in loading. Elastin's physiological loading was determined by comparing the inflation response of intact and autoclave purified elastin aortas from the proximal and distal thoracic aorta. Elastin's stretch and stress depend on collagen recruitment. Collagen recruitment started in the proximal aorta at systolic pressures (13.3 to 14.6 kPa) and in the distal at sub-diastolic pressures (9.3 to 10.6 kPa). In the proximal aorta collagen did not contribute significantly to the stress or stiffness, indicating that elastin determined the vessel properties. In the distal aorta, the circumferential incremental modulus was 70% higher than in the proximal aorta, half of which (37%) was due to a stiffening of the elastin. Compared to the elastin tissue in the proximal aorta, the distal elastin suffered higher physiological circumferential stretch (29%, P=0.03), circumferential stress (39%, P=0.02), and circumferential stiffness (37%, P=0.006). Elastin's physiological axial stresses were also higher (67%, P=0.003). These findings do not support the hypothesis that the loading on elastin is constant along the aorta as we expected from homeostasis.

Mechanical anisotropy of inflated elastic tissue from the pig aorta.

Uniaxial and biaxial mechanical properties of purified elastic tissue from the proximal thoracic aorta were studied to understand physiological load distributions within the arterial wall. Stress-strain behaviour was non-linear in uniaxial and inflation tests. Elastic tissue was 40% stiffer in the circumferential direction compared to axial in uniaxial tests and approximately 100% stiffer in vessels at an axial stretch ratio of 1.2 or 1.3 and inflated to physiological pressure. Poisson's ratio v(thetaz) averaged 0.2 and v(ztheta) increased with circumferential stretch from approximately 0.2 to approximately 0.4. Axial stretch had little impact on circumferential behaviour. In intact (unpurified) vessels at constant length, axial forces decreased with pressure at low axial stretches but remained constant at higher stretches. Such a constant axial force is characteristic of incrementally isotropic arteries at their in vivo dimensions. In purified elastic tissue, force decreased with pressure at all axial strains, showing no trend towards isotropy. Analysis of the force-length-pressure data indicated a vessel with v(thetaz) approximately 0.2 would stretch axially 2-4% with the cardiac pulse yet maintain constant axial force. We compared the ability of 4 mathematical models to predict the pressure-circumferential stretch behaviour of tethered, purified elastic tissue. Models that assumed isotropy could not predict the stretch at zero pressure. The neo-Hookean model overestimated the non-linearity of the response and two non-linear models underestimated it. A model incorporating contributions from orthogonal fibres captured the non-linearity but not the zero-pressure response. Models incorporating anisotropy and non-linearity should better predict the mechanical behaviour of elastic tissue of the proximal thoracic aorta.

Limits to the durability of arterial elastic tissue.

To engineer a better blood vessel, we must identify which structural, mechanical, and biological features of the native vessel must be replicated to ensure long-term survival of the implant. In this study, we tested autoclave-purified elastic tissue from along the pig thoracic aorta under long-term static and cyclic loading to identify factors that affected its durability. Samples were tested in water or in sucrose, which enhances viscoelasticity. Samples failed between 50% and 80% extension, which is lower than the failure extension in shorter, quasi-static tensile tests. Cyclic loading had a small effect on the durability of samples tested in water. Samples from the distal thoracic aorta and samples pre-treated in 70% ethanol showed enhanced durability. Failure between 50% and 80% extension appears associated with structural features of the individual fibre, and indirect evidence suggests it may be due to failure of the microfibrils, not the elastin. Cross-linked elastin may be necessary but insufficient to prevent failure. Durability appears also affected by regional differences in tissue structure, possibly the three-dimensional fibre organization. These results suggest ensuring normal fibre synthesis and organization may be crucial to the design of a successful vascular implant.

Mechanical properties of elastin along the thoracic aorta in the pig.

Understanding the mechanical environment of each component within the arterial wall is fundamental for understanding vascular growth and remodelling and for engineering artificial vascular conduits. We have investigated the mechanical status of arterial elastin by measuring the circumferential mechanical properties of purified elastin as function of position along the descending thoracic aorta of the pig. The tensile circumferential secant modulus, E(sec), measured in uniaxial mechanical tests, increased 30% (P<0.001), from a value of 0.88 MPa in the proximal tissue near the aortic arch to 1.14 MPa in the distal tissue near the diaphragm, indicating the stiffness of the elastin sample increased with position. Breaking stress was 54% higher in the distal tissue compared to the proximal (P<0.001), but the breaking stretch ratio did not change. E(sec) correlated with the ratio of radius to wall thickness measured in the no load state, r(nl)/h(nl), suggesting that the rise in stiffness was linked to ring morphology. The higher stiffness and strength of the distal tissue might be explained by a higher proportion of circumferentially oriented fibres in the distal tissue, which would indicate that the elastin meshwork in the thoracic aorta may become progressively anisotropic with distance from the heart. The ratio r(nl)/(h(nl)E (sec))rose only 7%, which suggests that the in vivo circumferential strain on the elastin may be constant along the pig thoracic aorta. The positional variation in elastin's properties should be taken into account in mechanical studies on purified elastin and in mathematical models of aorta mechanics.

Tensile residual strains on the elastic lamellae along the porcine thoracic aorta.

AIMS: This study determines the residual strains on the elastic lamellae in the porcine thoracic aorta to understand the distribution of strains amongst the components of the vascular wall. METHODS: Residual strains in aortic rings were released by cutting and purifying the elastin. Strains were calculated from lamellar contour lengths based on lamellar waviness and from mechanical tests. RESULTS: On the release of residual strains, waviness decreased 2-7%, the inner lamellae shortened 2.1 +/- 0.6% and the outer lamellae shortened 7.1 +/- 0.4% (p < 0.001), indicating that all lamellar elastin was under tension in fresh aortic tissue. Lamellar shortening was 3% greater in the distal than in the proximal tissue. Mechanical tests confirmed the morphological results and showed that the residual strains in fresh tissue required both the elastic tissue and the vascular smooth muscle. Tensile residual strains averaging 1.0% remained in the uncut elastin rings. CONCLUSION: When waviness is considered, the residual strains on the individual wall components can differ from the surface residual strains based only on the ring perimeter. The residual strains on the inner elastic lamellae are tensile, not compressive. The strain distribution amongst the individual components is non-uniform and not adequately understood to determine the physiological strains in the aortic wall.

Unusual swelling of elastin.

The swelling behavior of the elastin network has been investigated by comparing the linear expansion of samples of purified elastin with the volume expansion of the network, calculated on the basis of composition. Elastin sample dimensions and sample masses were measured under three conditions in which volume changes: thermal expansion at fixed water contents, deswelling due to dehydration, and swelling to greater than normal levels due to the swelling agent, sodium dodecyl sulfate. Isotropic network swelling usually changes length in proportion to the cube root of network volume, but length was found to be directly proportional to volume, showing a greater increase in length than expected. This unusual swelling behavior is attributed to an unusual elastin structure at the subfiber level, but there is insufficient detail on elastin's molecular organization to identify a mechanism to explain how it occurs. Assuming the network swells homogeneously, we describe two models that correctly predict swelling behavior, but these models imply a significant deviation from the structure generally assumed for an elastomeric polymer network of kinetically free molecular chains. Assuming that the network swells heterogeneously removes part of the difficulty with the models, but the observed direct proportionality between length and network volume remains to be explained.

Effects of lipids on elastin's viscoelastic properties.

Sodium dodecyl sulfate (SDS) was used as a model lipid to identify the molecular basis of possible lipid-induced changes in the viscoelastic behavior of arterial elastin. The chemical composition of the elastin network and the interfibrillar space was calculated from the chemical content of the elastin sample and its swelling behavior. Viscoelastic behavior was measured in aqueous SDS and in SDS plus 1 M sucrose, a deswelling agent. Viscoelastic behavior was also measured in sucrose and potassium thiocyanate solutions to identify the effects of swelling and of changes in network composition exclusive of any direct SDS effects. The hydration of the elastin network decreased at low SDS levels and increased at higher SDS levels. The elastin was stiffer in the dehydrated network and less stiff in the hydrated network. However, once the degree of hydration exceeded that of elastin in pure water, no further decrease in stiffness was obtained despite continued increase in swelling. The stiffness of the network could be accounted for entirely by changes in network hydration. There was no evidence that SDS had any effect on elastin's conformation. We predict that arterial lipids will interact with elastin in a similar way and will have only small effects on elastin's viscoelastic behavior.

The viscoelastic basis for the tensile strength of elastin.

Purified aortic elastin displays failure behaviour characteristic of an amorphous, noncrystalizing elastomer with failure properties showing a strong dependence on viscoelastic behaviour. Tensile breaking stresses and breaking strains measured over a range of temperatures, hydration levels, and strain rates are reducible to single curves by the application of shift factors obtained from dynamic mechanical tests. The breaking stress of rubbery elastin is similar to that found in other elastomers, but glassy elastin is about an order of magnitude less strong than expected. We suggest elastin's ability to be strengthened through viscous dissipation of strain energy and crack tip blunting is limited by its fibrillar structure.

The mechanical design of spider silks: from fibroin sequence to mechanical function.

Spiders produce a variety of silks, and the cloning of genes for silk fibroins reveals a clear link between protein sequence and structure-property relationships. The fibroins produced in the spider's major ampullate (MA) gland, which forms the dragline and web frame, contain multiple repeats of motifs that include an 8-10 residue long poly-alanine block and a 24-35 residue long glycine-rich block. When fibroins are spun into fibres, the poly-alanine blocks form (&bgr;)-sheet crystals that crosslink the fibroins into a polymer network with great stiffness, strength and toughness. As illustrated by a comparison of MA silks from Araneus diadematus and Nephila clavipes, variation in fibroin sequence and properties between spider species provides the opportunity to investigate the design of these remarkable biomaterials.

Micromechanics of the equine hoof wall: optimizing crack control and material stiffness through modulation of the properties of keratin.

Small-scale components of the equine hoof wall were tested to determine their mechanical roles in the morphological hierarchy. Macroscale tensile tests conducted on samples of the inner wall tubules and intertubular material showed a sixfold difference in mean initial stiffnesses (0.47 and 0.08 GPa, respectively), indicating that the inner wall tubules stiffen the wall along its longitudinal axis. The similarity in material properties of tubule and intertubular samples from the mid-wall suggests that tubules in this region offer only minor reinforcement along the longitudinal axis. Microscale tests conducted on rows of keratin strands from the inner wall tubules and intertubular material, and on intertubular keratin strands of the mid-wall, produced estimates of the stiffnesses of the hydrated matrix (0.03 GPa) and intermediate filament (IF; 3-4 GPa) components of the nanoscale ( &agr; -keratin) composite. The results from these tests also suggest that the properties of the keratin composite vary through the wall thickness. Birefringence measurements on inner wall and mid-wall regions agree with these observations and suggest that, although the keratin IF volume fraction is locally constant, the volume fraction changes through the thickness of the wall. These findings imply that modulation of the hoof wall properties has been achieved by varying the IF volume fraction, countering the effects of specific IF alignments which serve another function and would otherwise adversely affect the modulus of a particular region.

The hydrophobicity of vertebrate elastins.

An evolutionary trend towards increasing hydrophobicity of vertebrate arterial elastins suggests that there is an adaptive advantage to higher hydrophobicity. The swelling and dynamic mechanical properties of elastins from several species were measured to test whether hydrophobicity is associated with mechanical performance. Hydrophobicity was quantified according to amino acid composition (HI), and two behaviour-based indices: the Flory-Huggins solvent interaction parameter (chi1), and a swelling index relating tissue volumes at 60 and 1 degrees C. Swelling index values correlated with chi1 and, for most species studied, with HI, suggesting that the different approaches used to quantify hydrophobicity are equally valid. Dynamic mechanical properties were measured both in a closed system, to control the effects of water content, and in an open system, to determine whether the increased swelling of hydrophobic materials at low temperatures offsets the direct stiffening effect of cold. There were no biologically significant differences in mechanical behaviour in either open or closed systems that could be attributed to hydrophobicity. Therefore, although the original function of hydrophobicity in an ancestral elastin may have been to produce molecular mobility, mechanical performance did not drive a subsequent increase in hydrophobicity. Higher hydrophobicities may have arisen to facilitate the manufacture of the elastic fibre.

Asymmetric bipedal locomotion--an adaptive response to incomplete spinal injury in the chick.

The purpose of this study was to compare the asymmetric gait induced by unilateral spinal cord injury in chicks with asymmetric gaits of other bipeds and quadrupeds. After lateral hemisection of the left thoracic spinal cord, kinetic (ground reaction forces) and kinematic (distance and timing) data were recorded as chicks moved overground unrestrained. Ground reaction forces were analyzed to obtain the mechanical energy changes throughout the stride. Kinematic measurements were obtained over a range of speeds to determine the velocity-dependent characteristics of the gait. Hemisected chicks adopted an asymmetric hopping gait in which the animals hopped from the right leg (contralateral to the lesion) onto the left (ipsilateral) leg but then fell forward onto the right leg. Mechanical energy fluctuations throughout a single stride (i.e., two steps) approximated the oscillations that occur during a single walking step of control animals. When examined over a range of velocities, asymmetries in limb timing remained constant, but distance measurements such as step length became more symmetric as speed increased. The results show that, after spinal hemisection, adaptations of the remaining neural circuitry permitted the production of a locomotor pattern that, in addition to providing effective support and propulsion, incorporated some of the energy-conserving mechanisms of the normal walk. Adjustment of this novel locomotor pattern for different velocities further demonstrates the flexibility of locomotor circuitry. Comparisons with other studies shows that this gait shares some temporal and energetic features with asymmetric gaits of several bipedal species, including humans. In particular, hemisected chicks and some hemiplegic humans adopt an asymmetric gait in which maximum energy recovery occurs during the stance of the affected limb; these similarities probably relate to common mechanical constraints imposed on bipedal forms of terrestrial locomotion.

Mechanical role of elastin-associated microfibrils in pig aortic elastic tissue.

The contribution of microfibrils to the mechanical performance of the meshwork of elastic tissue in mature pig aorta was investigated by comparing the properties of autoclaved tissue containing elastin and microfibrils with autoclaved tissue that had been treated with dithiothreitol (DTT) or hot alkali to remove the microfibrils from the elastin. The uniaxial tensile stress-strain curve of the autoclaved tissue was linear to a strain of 0.6 or 0.7 and increased nonlinearly up to the breaking strain. The nonlinearity at high strains could not be accounted for by nonGaussian behavior and was attributed to the progressive alignment of the elastic fibers with strain. Removal of the microfibrils with DTT or treatment with calcium reduced the modulus at low strains by 12% and 4% respectively and increased the modulus at high strains, suggesting that the microfibrils have the capacity to change the orientation of the elastin fibers, possibly transmitting some of the load from one elastin fiber to another. Our findings suggest two possible roles for the microfibrils in vivo: distributing the load throughout the elastic fibers of the arterial wall and direct load bearing. The modulus and the breaking stress of the rings decreased linearly with the duration of alkali treatment starting immediately. By 45 min the modulus had dropped by 30% and the breaking stress by 50%, even though the amino acid content of the extract gave little evidence of elastin hydrolysis. Alkali treatment should not be used on autoclaved pig aortic tissue to be used for mechanical testing.

Exploring the possible functions of equine hoof wall tubules.

Possible functions of equine hoof wall tubules were investigated in this study. Hydration tests were conducted on blocks of hoof wall tissue in order to test the hypothesis that hollow tubules facilitate the conduction of water vapour distally. Although water loss or gain was inhibited through the outer wall surface, the increase in surface area provided by medullary spaces was ineffective in facilitating hydration through the face with exposed tubule ends. Rather, hollow tubules appear to allow for a higher dehydration rate through their exposed ends. Analysis of medullary space indicates that the presence of these voids does not provide either a significant increase in flexural stiffness, or a decrease in thermal conductivity. These findings suggest that nonmechanical roles of hoof wall tubules are unlikely and, therefore, the hollow nature of tubules may be a reflection of manufacturing constraints in producing keratin fibres at steep angles to the coronary border.

Design complexity and fracture control in the equine hoof wall.

Morphological and mechanical studies were conducted on samples of equine hoof wall to help elucidate the relationship between form and function of this complex, hierarchically organized structure. Morphological findings indicated a dependence of tubule size, shape and helical alignment of intermediate filaments (IFs) within the lamellae on the position through the wall thickness. The plane of the intertubular IFs changed from perpendicular to the tubule axis in the inner wall to almost parallel to the tubule axis in the outer wall. Morphological data predicted the existence of three crack diversion mechanisms which might prevent cracks from reaching the sensitive, living tissues of the hoof: a mid-wall diversion mechanism of intertubular material to inhibit inward and upward crack propagation, and inner- and outer-wall diversion mechanisms that prevent inward crack propagation. Tensile and compact tension fracture tests were conducted on samples of fully hydrated equine hoof wall. Longitudinal stiffness decreased from 0.56 to 0.30 GPa proceeding inwardly, whereas ultimate (maximum) properties were constant. Fracture toughness parameters indicated that no compromise results from the declining stiffness, with J-integral values ranging from 5.5 to 7.8 kJ m-2 through the wall thickness; however, highest toughness was found in specimens with cracks initiated tangential to the wall surface (10.7 kJ m-2). Fracture paths agreed with morphological predictions and further suggested that the wall has evolved into a structure capable of both resisting and redirecting cracks initiated in numerous orientations.

Elastin dehydration through the liquid and the vapor phase: a comparison of osmotic stress models.

The swelling and viscoelastic behaviors of samples of purified arterial elastin were investigated to develop a model for studying the viscoelastic behavior of elastin. Two osmotic stress models were used: the vapor phase model (VPM), in which the stress on the elastin sample was applied through the vapor phase by equilibrating the sample over a saline solution, and the liquid phase model (LPM), in which the stress was applied through the liquid phase by equilibrating the sample in aqueous solutions of large molecular weight polymers. The elastin in the VPM showed a highly varied viscoelastic response, and was slightly stiffer and had a slightly higher damping coefficient than the elastin in the LPM at equivalent nominal relative humidities. We believe the difference in behavior of the elastin in the two models was due to geometric distortions of the elastin that occur during dehydration in the VPM. In the LPM, the spaces between the elastin fibrils are filled with water, and in the VPM these spaces collapse when the water is removed. Removal of only the interfibrillar water deswelled the tissue and increased its stiffness and damping coefficient. Viscoelastic spectra obtained at different levels of osmotic stress in the LPM were reducible to one master curve, indicating that the dominant effect of dehydration is a nonspecific reduction of molecular mobility. We conclude that the LPM is a better model than the VPM for studying the effects of dehydration on the mechanical behavior of elastin.

Swelling and viscoelastic properties of osmotically stressed elastin.

The swelling and viscoelastic properties of purified elastin were studied in aqueous solutions of superswelling agents or osmotic deswelling agents to develop models to study the behavior of elastin at frequencies not easily accessible by direct measurement. Increasing the concentration of any of the deswelling solutes (glucose, sucrose, sodium chloride, ammonium sulphate, dextran, and polyethylene glycol) increased the tensile storage and loss moduli. The viscoelastic behavior was independent of solute when compared on the basis of swelling behavior. The data collected at various solute concentrations at 37 degrees C could be reduced to one master curve, and the master curves for elastin in each of the deswelling solutes were themselves superposable. The ability to reduce the data indicates that dehydration can be used to model elastin's viscoelastic behavior at high frequencies or over short times. The viscoelastic behavior of elastin in the superswelling agents [potassium thiocyanate (KSCN), dimethyl sulfoxide (DMSO), and ethylene glycol (EG)] depended on the solute and was independent of swelling behavior. In KSCN the behavior of elastin seemed to be a continuation of the pattern established by the deswelling agents in that an increase in swelling was accompanied by a decrease in both moduli, and the viscoelastic spectra were reducible to one master curve. In high concentrations of DMSO and EG the spectra were not reducible. KSCN appears a suitable superswelling solute to model elastin's viscoelastic behavior at low frequencies or over long times.