Mechanical measurement of the human cerebellum under compressive loading

The cerebellum is responsible for controlling the posture and walking stability of the body. The cerebellum can subject to the traumatic injuries following by complicated clinical problems, i.e., the cerebellar pathologies. Application of the computational models can be helpful to figure out the injury mechanisms of the cerebellum, however, there is a lack of knowledge on the mechanical properties of the cerebellum under compressive loading. Therefore, this study aimed to perform an experimental study to measure the mechanical properties of 17 male individuals’ cerebellum under the series of compressive loadings. The resulted stress-strain data of the cerebellum revealed the elastic modulus and maximum/failure stress of 13.48?±?2.65 (Mean?±?SD) and 19.65?±?3.89?kPa, respectively. The findings of this study have implications not only for understanding the mechanical properties of the human cerebellum tissue under the compressive loading, but also for providing a raw data for the doctors and biomechanical experts as the mechanical threshold of the cerebellum as well as computational modelling of the traumatic brain injuries.     

Dissection properties and mechanical strength of tissue components in human carotid bifurcations

Carotid artery dissections can be triggered by several factors. The underlying biomechanical phenomena and properties are unclear. This study investigates the dissection properties of 62 human carotid bifurcations using two experimental methods: direct tension and peeling tests. Direct tension tests study the mechanical strength of the tissue components in radial direction, while peeling tests quantify the fracture energy required to propagate a dissection in a tissue. Results show that the interface between the healthy adventitia and media has the highest radial failure stress (132 ± 20 kPa, mean ± SD, n = 25), whereas the lowest value occurs between the diseased intima and media (104 ± 24 kPa, n = 18). The radial tissue strength at the bifurcation is the highest compared with locations that are away from the central region of the bifurcation. Force/width values required to separate the individual layers and to dissect the media in the circumferential direction are always lower than related values in the axial direction, suggesting anisotropic dissection properties. Dissection energies per reference area generated during the peeling tests are also lower for strips in the circumferential direction than for axial strips, and they vary significantly with the location, as shown for the media. Histological investigations demonstrate that interfacial ruptures mainly occur in the media in both types of tests and are 2-5 elastic lamellae away from the external and internal elastic laminae. A remarkably “rougher” dissection surface is generated during axial peeling tests when compared with tests performed in the circumferential direction.     

不同病理学等级关节炎软骨的三相力学特性

目的基于超声膨胀观测技术与三相理论提取关节软骨的轴向弹性模量,探索不同等级关节炎软骨的三相力学特性。方法对不同阶段的兔子关节炎软骨样本进行病理学分级,基于高频瞬态超声测量技术获取软骨因自由膨胀引起的组织应变量,结合组织的固定电荷密度和水体积分数,运用三相模型估计软骨的轴向弹性模量,并进行相关性分析。结果正常软骨与关节炎软骨的轴向弹性模量存在着明显差异(P<0.05),不同等级软骨样本的轴向弹性模量存在明显差异(P<0.05)。正常软骨组织的平均弹性模量为(15.87±6.30)MPa,随着关节炎的发生与关节炎程度的加重,软骨的弹性模量逐渐减小,1、2、3级软骨样本的轴向弹性模量分别为(11.33±5.21)、(9.15±5.68)、(6.05±4.99)MPa。结论不同病理学关节炎等级软骨样本的三相力学特性有明显差异,该研究为应用软骨组织的力学属性定量评判关节炎的严重程度提供了新的思路。     

Elastic modulus and collagen organization of the rabbit cornea: Epithelium to endothelium

The rabbit is commonly used to evaluate new corneal prosthetics and study corneal wound healing. Knowledge of the stiffness of the rabbit cornea would better inform the design and fabrication of keratoprosthetics and substrates with relevant mechanical properties for in vitro investigations of corneal cellular behavior. This study determined the elastic modulus of the rabbit corneal epithelium, anterior basement membrane (ABM), anterior and posterior stroma, Descemet’s membrane (DM) and endothelium using atomic force microscopy (AFM). In addition, three-dimensional collagen fiber organization of the rabbit cornea was determined using nonlinear optical high-resolution macroscopy. The elastic modulus as determined by AFM for each corneal layer was: epithelium, 0.57 ± 0.29 kPa (mean ± SD); ABM, 4.5 ± 1.2 kPa, anterior stroma, 1.1 ± 0.6 kPa; posterior stroma, 0.38 ± 0.22 kPa; DM, 11.7 ± 7.4 kPa; and endothelium, 4.1 ± 1.7 kPa. The biophysical properties, including the elastic modulus, are unique for each layer of the rabbit cornea and are dramatically softer in comparison to the corresponding regions of the human cornea. Collagen fiber organization is also dramatically different between the two species, with markedly less intertwining observed in the rabbit vs. human cornea. Given that the substratum stiffness considerably alters the corneal cell behavior, keratoprosthetics that incorporate mechanical properties simulating the native human cornea may not elicit optimal cellular performance in rabbit corneas that have dramatically different elastic moduli. These data should allow for the design of substrates that better mimic the biomechanical properties of the corneal cellular environment.     

The orthotropic elastic properties of fibrolamellar bone tissue in juvenile white‐tailed deer femora

Fibrolamellar bone is a transient primary bone tissue found in fast‐growing juvenile mammals, several species of birds and large dinosaurs. Despite the fact that this bone tissue is prevalent in many species, the vast majority of bone structural and mechanical studies are focused on human osteonal bone tissue. Previous research revealed the orthotropic structure of fibrolamellar bone, but only a handful of experiments investigated its elastic properties, mostly in the axial direction. Here we have performed for the first time an extensive biomechanical study to determine the elastic properties of fibrolamellar bone in all three orthogonal directions. We have tested 30 fibrolamellar bone cubes (2 × 2 × 2 mm) from the femora of five juvenile white‐tailed deer (Odocoileus virginianus) in compression. Each bone cube was compressed iteratively, within its elastic region, in the axial, transverse and radial directions, and bone stiffness (Young's modulus) was recorded. Next, the cubes were kept for 7 days at 4 °C and then compressed again to test whether bone stiffness had significantly deteriorated. Our results demonstrated that bone tissue in the deer femora has an orthotropic elastic behavior where the highest stiffness was in the axial direction followed by the transverse and the radial directions (21.6 ± 3.3, 17.6 ± 3.0 and 14.9 ± 1.9 Gpa, respectively). Our results also revealed a slight non‐significant decrease in bone stiffness after 7 days. Finally, our sample size allowed us to establish that population variance was much bigger in the axial direction than the radial direction, potentially reflecting bone adaptation to the large diversity in loading activity between individuals in the loading direction (axial) compared with the normal (radial) direction. This study confirms that the mechanically well‐studied human transverse‐isotropic osteonal bone is just one possible functional adaptation of bone tissue and that other vertebrate species use an orthotropic bone tissue structure which is more suitable for their mechanical requirements.     

Novel collagen scaffolds prepared by using unnatural D-amino acids assisted EDC/NHS crosslinking

This work discusses the preparation and characterization of novel collagen scaffolds by using unnatural D-amino acids (Coll-D-AAs)-assisted 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC)/N-hydroxyl succinimide(NHS)-initiated crosslinking. The mechanical strength, hydrothermal and structural stability, resistance to biodegradation and the biocompatibility of Coll-D-AAs matrices were investigated. The results from Thermo mechanical analysis, Differential scanning calorimetric analysis and Thermo gravimetric analysis of the Coll-D-AAs matrices indicate a significant increase in the tensile strength (TS, 180?±?3), % elongation (% E, 80?±?9), elastic modulus (E, 170?±?4) denaturation temperature (T _d, 108?±?4) and a significant decrease in decomposition rate (T _g, 64?±?6). Scanning electron microscopic and Atomic force microscopic analyses revealed a well-ordered with properly oriented and well-aligned structure of the Coll-D-AAs matrices. FT-IR results suggest that the incorporation of D-AAs favours the molecular stability of collagen matrix. The D-AAs stabilizing the collagen matrices against degradation by collagenase would have been brought about by protecting the active sites in collagen. The Coll-D-AAs matrices have good biocompatibility when compared with native collagen matrix. Molecular docking studies also indicate better understanding of bonding pattern of collagen with D-AAs. These Coll-D-AAs matrices have been produced in high mechanical strength, thermally and biologically stable, and highly biocompatible forms that can be further manipulated into the functional matrix suitable in designing scaffolds for tissue engineering and regenerative medical applications.     

The preparation of PLL–GRGDS modified PTSG copolymer scaffolds and their effects on manufacturing artificial salivary gland

We prepared two-dimentional (2D) and three-dimentional (3D) scaffolds with biodegradable poly(butylene terephthalate)-co-poly(butylene succinate)-b-poly(ethylene glycol) (i.e. PTSG), mainly for the purpose of investigating its cytocompatibility and mechanical property as artificial salivary gland material. The surface of 2D scaffold (i.e. PTSG film) was modified by O₂ plasma treatment and the following coating of Gly–Arg–Gly–Asp–Ser (GRGDS) decorated poly(L-lysine) (i.e. PLL–GRGDS). The obtained film was named PLL–GRGDS/PTSG (O). Its surface properties were characterized using contact angles, surface energies, X-ray photoelectron spectroscopy and Fourier transform infrared; and cytocompatibility tests in vitro including morphology, attachment and proliferation of human salivary gland (HSG) epithelial cells were further performed on PTSG films. Meanwhile, 3D scaffold with the shape of porous tube was constructed using hydrogel-rapid prototyping and the performance of 3D scaffold including mechanical property, pore structure, degradation and water uptake was also evaluated. Results revealed that PLL–GRGDS/PTSG (O) possessed the high surface free energy (63.89?mJ/m²) and could immobilize a great amount of PLL–GRGDS, which attributed to the formation of some polar oxygen-containing groups such as carboxyl and carbonyl ones in the process of O₂ plasma treatment. Cell tests in vitro suggested the efficiency of surface modification in enhancing the cytocompatibility of PTSG. Furthermore, the manufacturing scaffold was proved to possess excellent pore structures (porosity 88.9%, connectivity 97.5% and average pore size 35.4?μm) and good mechanical properties (E-modulus 88.4?±?4.1?kPa, yield stress 45.7?±?2.3?kPa, yield strain 56?±?2%, fracture stress 52.2?±?3.5?kPa and fracture strain 63?±?3%). After four weeks hydrolysis reaction, the degradation of the scaffold reached 8% and equilibrium water uptake declined from 51 to 45%. The decline of water uptake was probably caused by the decrease of the hydrophilic units in PTSG copolymer during degradation. These results satisfied the demands for constructing the artificial salivary gland scaffold.     

Zonal Uniformity in Mechanical Properties of the Chondrocyte Pericellular Matrix: Micropipette Aspiration of Canine Chondrons Isolated by Cartilage Homogenization

The pericellular matrix (PCM) is a region of tissue that surrounds chondrocytes in articular cartilage and together with the enclosed cells is termed the chondron. Previous studies suggest that the mechanical properties of the PCM, relative to those of the chondrocyte and the extracellular matrix (ECM), may significantly influence the stress–strain, physicochemical, and fluid-flow environments of the cell. The aim of this study was to measure the biomechanical properties of the PCM of mechanically isolated chondrons and to test the hypothesis that the Young's modulus of the PCM varies with zone of origin in articular cartilage (surface vs. middle/deep). Chondrons were extracted from articular cartilage of the canine knee using mechanical homogenization, and the elastic properties of the PCM were determined using micropipette aspiration in combination with theoretical models of the chondron as an elastic incompressible half-space, an elastic compressible bilayer, or an elastic compressible shell. The Young's modulus of the PCM was significantly higher than that reported for isolated chondrocytes but over an order of magnitude lower than that of the cartilage ECM. No significant differences were observed in the Young's modulus of the PCM between surface zone (24.0 ± 8.9 kPa) and middle/deep zone cartilage (23.2 ± 7.1 kPa). In combination with previous theoretical biomechanical models of the chondron, these findings suggest that the PCM significantly influences the mechanical environment of the chondrocyte in articular cartilage and therefore may play a role in modulating cellular responses to micromechanical factors.     

Measuring anisotropic muscle stiffness properties using elastography

Physiological and pathological changes to the anisotropic mechanical properties of skeletal muscle are still largely unknown, with only a few studies quantifying changes in vivo. This study used the noninvasive MR elastography (MRE) technique, in combination with diffusion tensor imaging (DTI), to measure shear modulus anisotropy in the human skeletal muscle in the lower leg. Shear modulus measurements parallel and perpendicular to the fibre direction were made in 10 healthy subjects in the medial gastrocnemius, soleus and tibialis anterior muscles. The results showed significant differences in the medial gastrocnemius (μ_‖ =0.86 ± 0.15 kPa; μ_⊥ = 0.66 ± 0.19 kPa, P < 0.001), soleus (μ_‖ = 0.83 ± 0.22 kPa; μ_⊥ = 0.65 ± 0.13 kPa, P < 0.001) and the tibialis anterior (μ_‖ = 0.78 ± 0.24 kPa; μ_⊥ = 0.66 ± 0.16 kPa, P = 0.03) muscles, where the shear modulus measured in the direction parallel is greater than that measured in the direction perpendicular to the muscle fibres. No significant differences were measured across muscle groups. This study provides the first direct estimates of the anisotropic shear modulus in the triceps surae muscle group, and shows that the technique may be useful for the probing of mechanical anisotropy changes caused by disease, aging and injury. Copyright © 2013 John Wiley & Sons, Ltd.     

Experimental and computational analysis of soft tissue stiffness in forearm using a manual indentation device

A hand held stiffness meter can be used to measure indentation stiffness of human soft tissues, sensitively altered, e.g., by pathological tissue swelling. Under indentation load, the relative contribution of each soft tissue component (i.e., skin, adipose tissue and muscle) to the biomechanical response is not known. In the present study, we evaluated the biomechanical role of different soft tissues in relaxed, physically stressed and oedemic human forearm. Soft tissue stiffness of the forearms of nine healthy human subjects was measured under four different test protocols: (1) forearm at rest, (2) forearm under isometric flexor loading, (3) forearm under isometric extensor loading, and (4) forearm under venous occlusion. In (2) and (3) the loading forces were monitored using a dynamometer, and in (4) the soft tissue swelling was induced by venous occlusion using a pressure cuff. At the site of indentation, thickness of different tissue layers (skin, adipose tissue and muscle) was measured using B-mode ultrasound imaging. Layered, hyperelastic finite element (FE) model of the indentation measurement was created and the model response was matched with that of the stiffness meter to determine the elastic modulus for each tissue in the model. Optimized values of the elastic modulus for skin and adipose tissue at rest were 210 kPa and 1.9 kPa, respectively. Further, significance of the variations in stiffness of different tissues on the indentation response was tested. Experimentally, indentation stiffness of the forearm increased during isometric extensor and flexor loads as well as under venous occlusion by 53, 91 and 15%, respectively. The FE model could reproduce the experimental responses primarily by the increased modulus of skin; 112% (446 kPa), 210% (651 kPa) and 21% (254 kPa) under flexor and extensor loading as well as during venous occlusion, respectively. The indentation response was 9–16 times more sensitive to changes in the mechanical properties of skin than those of adipose tissue and muscle. In conclusion, the present stiffness meter may be used to quantify in vivo mechanical properties of soft tissues in the forearm, sensitively modulated by soft tissue swelling and muscle loading.     

Subcutaneous Tissue Mechanical Behavior is Linear and Viscoelastic Under Uniaxial Tension

Subcutaneous tissue is part of a bodywide network of “loose” connective tissue including interstitial connective tissues separating muscles and surrounding all nerves and blood vessels. Despite its ubiquitous presence in the body and its potential importance in a variety of therapies utilizing mechanical stretch, as well as normal movement and exercise, very little is known about loose connective tissue's biomechanical behavior. This study aimed to determine elastic and viscoelastic mechanical properties of ex-vivo rat subcutaneous tissue in uniaxial tension with incremental stress relaxation experiments. The elastic response of the tissue was linear, with instantaneous and equilibrium tensile moduli of 4.77 kPa and 2.75 kPa, respectively. Using a 5 parameter Maxwell solid model, material parameters μ₁ = 0.95 ± 0.24 Ns/m and μ₂ = 8.49 ± 2.42 Ns/m defined coefficients of viscosity related to time constants τ_1M = 3.83 ± 0.15 sec and τ_2M = 30.15 ± 3.16 sec, respectively. Using a continuous relaxation function, parameters C = 0.25 ± 0.12, τ_1C = 1.86 ± 0.34 sec, and τ_2C = 110.40 ± 25.59 sec defined the magnitude and frequency limits of the relaxation spectrum. This study provides baseline information for the stress-strain behaviors of subcutaneous connective tissue. Our results underscore the differences in mechanical behaviors between loose and high-load bearing connective tissues and suggest that loose connective tissues may function to transmit mechanical signals to and from the abundant fibroblasts, immune, vascular, and neural cells present within these tissues.     

Topographical variations of the strain-dependent zonal properties of tibial articular cartilage by microscopic MRI

The topographical variations of the zonal properties of canine articular cartilage over the medial tibia were evaluated as the function of external loading by microscopic magnetic resonance imaging (µMRI). T2 and T1 relaxation maps and GAG (glycosaminoglycan) images from a total of 70 specimens were obtained with and without the mechanical loading at 17.6?µm depth resolution. In addition, mechanical modulus and water content were measured from the tissue. For the bulk without loading, the means of T2 at magic angle (43.6?±?8.1?ms), absolute thickness (907.6?±?187.9?µm) and water content (63.3?±?9.3%) on the meniscus-covered area were significantly lower than the means of T2 at magic angle (51.1?±?8.5?ms), absolute thickness (1251.6?±?218.4?µm) and water content (73.2?±?5.6%) on the meniscus-uncovered area. However GAG (86.0?±?15.3?mg/ml) on the covered area was significantly higher than GAG (70.0?±?8.8?mg/ml) on the uncovered area. Complex relationships were found in the tissue properties as the function of external loading. The tissue parameters in the superficial zone changed more profoundly than the same properties in the radial zone. The tissue parameters in the meniscus-covered areas changed differently when comparing with the same parameters in the uncovered areas. This project confirms that the load-induced changes in the molecular distribution and structure of cartilage are both depth-dependent and topographically distributed. Such detailed knowledge of the tibial layer could improve the early detection of the subtle softening of the cartilage that will eventually lead to the clinical diseases such as osteoarthritis.     

A human pericardium biopolymeric scaffold for autologous heart valve tissue engineering: cellular and extracellular matrix structure and biomechanical properties in comparison with a normal aortic heart valve

The objective of our study was to compare the cellular and extracellular matrix (ECM) structure and the biomechanical properties of human pericardium (HP) with the normal human aortic heart valve (NAV). HP tissues (from 12 patients) and NAV samples (from 5 patients) were harvested during heart surgery. The main cells in HP were pericardial interstitial cells, which are fibroblast-like cells of mesenchymal origin similar to the valvular interstitial cells in NAV tissue. The ECM of HP had a statistically significantly (p < 0.001) higher collagen I content, a lower collagen III and elastin content, and a similar glycosaminoglycans (GAGs) content, in comparison with the NAV, as measured by ECM integrated density. However, the relative thickness of the main load-bearing structures of the two tissues, the dense part of fibrous HP (49 ± 2%) and the lamina fibrosa of NAV (47 ± 4%), was similar. In both tissues, the secant elastic modulus (Es) was significantly lower in the transversal direction (p < 0.05) than in the longitudinal direction. This proved that both tissues were anisotropic. No statistically significant differences in UTS (ultimate tensile strength) values and in calculated bending stiffness values in the longitudinal or transversal direction were found between HP and NAV. Our study confirms that HP has an advantageous ECM biopolymeric structure and has the biomechanical properties required for a tissue from which an autologous heart valve replacement may be constructed.     

Magnetic resonance elastography of skeletal muscle deep tissue injury

The current state‐of‐the‐art diagnosis method for deep tissue injury in muscle, a subcategory of pressure ulcers, is palpation. It is recognized that deep tissue injury is frequently preceded by altered biomechanical properties. A quantitative understanding of the changes in biomechanical properties preceding and during deep tissue injury development is therefore highly desired. In this paper we quantified the spatial–temporal changes in mechanical properties upon damage development and recovery in a rat model of deep tissue injury. Deep tissue injury was induced in nine rats by two hours of sustained deformation of the tibialis anterior muscle. Magnetic resonance elastography (MRE), T₂‐weighted, and T₂‐mapping measurements were performed before, directly after indentation, and at several timepoints during a 14‐day follow‐up. The results revealed a local hotspot of elevated shear modulus (from 3.30 ± 0.14 kPa before to 4.22 ± 0.90 kPa after) near the center of deformation at Day 0, whereas the T₂ was elevated in a larger area. During recovery there was a clear difference in the time course of the shear modulus and T₂. Whereas T₂ showed a gradual normalization towards baseline, the shear modulus dropped below baseline from Day 3 up to Day 10 (from 3.29 ± 0.07 kPa before to 2.68 ± 0.23 kPa at Day 10, P < 0.001), followed by a normalization at Day 14. In conclusion, we found an initial increase in shear modulus directly after two hours of damage‐inducing deformation, which was followed by decreased shear modulus from Day 3 up to Day 10, and subsequent normalization. The lower shear modulus originates from the moderate to severe degeneration of the muscle. MRE stiffness values were affected in a smaller area as compared with T₂. Since T₂ elevation is related to edema, distributing along the muscle fibers proximally and distally from the injury, we suggest that MRE is more specific than T₂ for localization of the actual damaged area.     

Biomechanical and Microstructural Properties of Common Carotid Arteries from Fibulin-5 Null Mice

Alteration in the mechanical properties of arteries occurs with aging and disease, and arterial stiffening is a key risk factor for subsequent cardiovascular events. Arterial stiffening is associated with the loss of functional elastic fibers and increased collagen content in the wall of large arteries. Arterial mechanical properties are controlled largely by the turnover and reorganization of key structural proteins and cells, a process termed growth and remodeling. Fibulin-5 (fbln5) is a microfibrillar protein that binds tropoelastin, interacts with integrins, and localizes to elastin fibers; tropoelastin and microfibrillar proteins constitute functional elastic fibers. We performed biaxial mechanical testing and confocal imaging of common carotid arteries (CCAs) from fibulin-5 null mice (fbln5 ^−/−) and littermate controls (fbln5 ^+/+) to characterize the mechanical behavior and microstructural content of these arteries; mechanical testing data were fit to a four-fiber family constitutive model. We found that CCAs from fbln5 ^−/− mice exhibited lower in vivo axial stretch and lower in vivo stresses while maintaining a similar compliance over physiological pressures compared to littermate controls. Specifically, for fbln5 ^−/− the axial stretch λ = 1.41 ± 0.07, the circumferential stress σ _θ  = 101 ± 32 kPa, and the axial stress σ _z  = 74 ± 28 kPa; for fbln5 ^+/+ λ = 1.64 ± 0.03, σ _θ  = 194 ± 38 kPa, and σ _z  = 159 ± 29 kPa. Structurally, CCAs from fbln5 ^−/− mice lack distinct functional elastic fibers defined by the lamellar structure of alternating layers of smooth muscle cells and elastin sheets. These data suggest that structural differences in fbln5 ^−/− arteries correlate with significant differences in mechanical properties. Despite these significant differences fbln5 ^−/− CCAs exhibited nearly normal levels of cyclic strain over the cardiac cycle.     

Effects of simulated injury on the anteroinferior glenohumeral capsule

Glenohumeral dislocation results in permanent deformation (nonrecoverable strain) of the glenohumeral capsule which leads to increased range of motion and recurrent instability. Minimal research has examined the effects of injury on the biomechanical properties of the capsule which may contribute to poor patient outcome following repair procedures. The objective of this study was to determine the effect of simulated injury on the stiffness and material properties of the AB-IGHL during tensile deformation. Using a combined experimental and computational methodology, the stiffness and material properties of six AB-IGHL samples during tensile elongation were determined before and after simulated injury. The AB-IGHL was subjected to 12.7?±?3.2?% maximum principal strain which resulted in 2.5?±?0.9?% nonrecoverable strain. The linear region stiffness and modulus of stress?Cstretch curves between the normal (52.4?±?30.0?N/mm, 39.1?±?26.6?MPa) and injured (64.7?±?21.3?N/mm, 73.5?±?53.8?MPa) AB-IGHL increased significantly (p?=?0.03, p?=?0.04). These increases suggest that changes in the tissue microstructure exist following simulated injury. The injured tissue could contain more aligned collagen fibers and may not be able to support a normal range of joint motion. Collagen fiber kinematics during simulated injury will be examined in the future.     

Application of dynamic computed tomography for measurements of local aortic elastic modulus

A novel computed tomographic (CT) technique used for the instantaneous measurement of the dynamic elastic modulus of intact excised porcine aortic vessels subjected to physiological pressure waveforms is described. This system was comprised of a high resolution X-ray image intensifier based computed tomographic system with limiting spatial resolution of 3.2mm⁻¹ (for a 40mm field of view) and a computer-controlled flow simulator. Utilising cardiac gating and computer control, a time-resolved sequence of 1 mm thick axial tomographic slices was obtained for porcine aortic specimens during one simulated cardiac cycle. With an image acquisition sampling interval of 16.5 ms, the time sequences of CT slices were able to quantify the expansion and contraction of the aortic wall during each phase of the cardiac cycle. Through superficial tagging of the adventitial surface of the specimens with wire markers, measurement of wall strain in specific circumferential sectors and subsequent calculations of localised dynamic elastic modulus were possible. The precision of circumferential measurements made from the CT images utilising a cluster-growing segmentation technique was approximately ±0.25mm and allowed determination of the dynamic elastic modulus (E_dyn) with a precision of ±8kPa. Dynamic elastic modulus was resolved as a function of the harmonics of the physiological pressure waveform and as a function of the angular position around the vessel circumference. Application of this dynamic CT (DCT) technique to seven porcine thoracic aortic specimens produced a circumferential average (over all frequency components) E_dyn of 373±29kPa. This value was not statistically different (p<0.05) from the values of 430±77 and 390±47kPa obtained by uniaxial tensile testing and volumetric measurements respectively.     

Three-dimensional analysis of shear wave propagation observed by in vivo magnetic resonance elastography of the brain

Dynamic magnetic resonance elastography (MRE) is a non-invasive method for the quantitative determination of the mechanical properties of soft tissues in vivo. In MRE, shear waves are generated in the tissue and visualized using phase-sensitive MR imaging methods. The resulting two-dimensional (2-D) wave images can reveal in-plane elastic properties when possible geometrical biases of the wave patterns are taken into account. In this study, 3-D MRE experiments of in vivo human brain are analyzed to gain knowledge about the direction of wave propagation and to deduce in-plane elastic properties. The direction of wave propagation was determined using a new algorithm which identifies minimal wave velocities along rays from the surface into the brain. It was possible to quantify biases of the elastic parameters due to projections onto coronal, sagittal and transversal image planes in 2-D MRE. It was found that the in-plane shear modulus is increasingly overestimated when the image slice is displaced from narrow slabs of 2-5cm through the center of the brain. The mean shear modulus of the brain was deduced from 4-D wave data with about 3.5kPa. Using the proposed slice positions in 2-D MRE, this shear modulus can be reproduced with an acceptable error within a fraction of the full 3-D examination time.     

Directing osteogenesis of stem cells with hydroxyapatite precipitated electrospun eri–tasar silk fibroin nanofibrous scaffold

Stimulating stem cell differentiation without growth factor supplement offers a potent and cost-effective scaffold for tissue regeneration. We hypothesise that surface precipitation of nano-hydroxyapatite (nHAp) over blends of non-mulberry silk fibroin with better hydrophilicity and RGD amino acid sequences can direct the stem cell towards osteogenesis. This report focuses on the fabrication of a blended eri–tasar silk fibroin nanofibrous scaffold (ET) followed by nHAp deposition by a surface precipitation (alternate soaking in calcium and phosphate solution) method. Morphology, hydrophilicity, composition, and the thermal and mechanical properties of ET/nHAp were examined by field emission scanning electron microscopy, TEM, FT-IR, X-ray diffraction, TGA and contact angle measurement and compared with ET. The composite scaffold demonstrated improved thermal stability and surface hydrophilicity with an increase in stiffness and elastic modulus (778?±?2.4?N/m and 13.1?±?0.36?MPa) as compared to ET (160.6?±?1.34?N/m and 8.3?±?0.4?MPa). Mineralisation studies revealed an enhanced and more uniform surface deposition of HAp-like crystals, while significant differences in cellular viability and attachment were observed through 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and confocal microscopy study. The cell viability and expression of adhesion molecules (CD 44 and CD 29) are found to be optimum for subsequent stages of growth proliferation and differentiation. The rates of proliferation have been observed to decrease owing to the transition of MSC from a state of proliferation to a state of differentiation. The confirmation of improved osteogenic differentiation was finally verified through the alkaline phosphatase assay, pattern of gene expression related to osteogenic differentiation and morphological observations of differentiated cord blood human mesenchymal stem cells under fluorescence microscope. The results obtained showed the improved physicochemical and biological properties of the ET/nHAp scaffold for osteogenic differentiation without the addition of any growth factors.     

Predictive biomechanical analysis of ascending aortic aneurysm rupture potential

Aortic aneurysm is a leading cause of death in adults, often taking lives without any premonitory signs or symptoms. Adverse clinical outcomes of aortic aneurysm are preventable by elective surgical repair; however, identifying at-risk individuals is difficult. The objective of this study was to perform a predictive biomechanical analysis of ascending aortic aneurysm (AsAA) tissue to assess rupture risk on a patient-specific level. AsAA tissues, obtained intra-operatively from 50 patients, were subjected to biaxial mechanical and uniaxial failure tests to obtain their passive elastic mechanical properties. A novel analytical method was developed to predict the AsAA pressure-diameter response as well as the aortic wall yield and failure responses. Our results indicated that the mean predicted AsAA diameter at rupture was 5.6 ± 0.7 cm, and the associated blood pressure to induce rupture was 579.4 ± 214.8 mmHg. Statistical analysis showed significant positive correlation between aneurysm tissue compliance and predicted risk of rupture, where patients with a pressure-strain modulus ?100 kPa may be nearly twice as likely to experience rupture than patients with more compliant aortic tissue. The mechanical analysis of pre-dissection patient tissue properties established in this study could predict the “future” onset of yielding and rupture in AsAA patients. The analysis results implicate decreased tissue compliance as a risk factor for AsAA rupture. The presented methods may serve as a basis for the development of a pre-operative planning tool for AsAA evaluation, a tool currently unavailable.