Background Decellularized tissues are expected to have major cellular immunogenic components removed and in the meantime maintain similar mechanical strength and extracellular matrix (ECM) structure. ...However, the decellularization processes likely cause alterations of the ECM structure and thus influence the mechanical properties. In the present study, the effects of different decellularization protocols on the (passive) mechanical properties of the resulted porcine aortic ECM were evaluated. Methods Decellularization methods using anionic detergent (sodium dodecyl sulfate), enzymatic detergent (Trypsin), and non-ionic detergent tert-octylphenylpolyoxyethylen (Triton X-100) were adopted to obtain decellularized porcine aortic ECM. Histologic studies and scanning electron microscopy were performed to confirm the removal of cells and to examine the structure of ECM. Biaxial tensile testing was used to characterize both the elastic and viscoelastic mechanical behaviors of decellularized ECM. Results All three decellularization protocols remove the cells effectively. The major ECM structure is preserved under sodium dodecyle sulfate (SDS) and Triton X-100 treatments. However, the structure of Trypsin treated ECM is severely disrupted. SDS and Triton X-100 decellularized ECM exhibits similar elastic properties as intact aorta tissues. Decellularized ECM shows less stress relaxation than intact aorta due to the removal of cells. Creep behavior is negligible for both decellularized ECM and intact aortas. Conclusion SDS and Triton X-100 decellularized ECM tissue appeared to maintain the critical mechanical and structural properties and might work as a potential material for further vascular tissue engineering.
Background Temporary cold storage is a common procedure for preserving tissues for a short time before using them in a clinical or experimental setting. The process of storing tissues at ...refrigeration or freezing temperatures can affect the mechanical properties of the tissue. Previous studies were limited to uniaxial tensile tests and did not have substantial explanations for what was structurally occurring in the tissue to account for the changes in mechanical properties. Materials and Methods This study involved biaxial tensile testing of bovine thoracic aortas that had been stored at common storage temperatures (4°C, −20°C, or −80°C) for three different time points (48 h, 1 wk, or 3 wk). The slopes of the initial and stiff region of the stress-strain curves were measured. The knee point of the stress-strain curves was also determined. Collagen content before and after storage was quantified using a Sircol collagen assay kit. Results The stored arteries showed decreased initial slope and increased stiff slope after 48 h of 4°C refrigeration. The −20°C and −80°C storage conditions had similar initial slopes compared with the fresh ones but an increase in the stiff slope. There was also a significant shift of the knee point to a higher strain and stress. The soluble and insoluble collagen content decreased significantly due to storage but the percentage of cross-linked collagen was unchanged. Conclusions Cold storage causes several changes in the mechanical properties due to structural and biochemical changes in the tissue. Overall, freezing protocols (either −20°C or −80°C) are suggested over refrigeration (4°C) for maintaining the initial stress-strain behavior.
The complex network structure of elastin and collagen extracellular matrix (ECM) forms the primary load bearing components in the arterial wall. The structural and mechanobiological interactions ...between elastin and collagen are important for properly functioning arteries. Here, we examined the elastin and collagen organization, realignment, and recruitment by coupling mechanical loading and multiphoton imaging. Two-photon excitation fluorescence and second harmonic generation methods were performed with a multiphoton video-rate microscope to capture real time changes to the elastin and collagen structure during biaxial deformation. Enzymatic removal of elastin was performed to assess the structural changes of the remaining collagen structure. Quantitative analysis of the structural changes to elastin and collagen was made using a combination of two-dimensional fast Fourier transform and fractal analysis, which allows for a more complete understanding of structural changes. Our study provides new quantitative evidence, to our knowledge on the sequential engagement of different arterial ECM components in response to mechanical loading. The adventitial collagen exists as large wavy bundles of fibers that exhibit fiber engagement after 20% strain. The medial collagen is engaged throughout the stretching process, and prominent elastic fiber engagement is observed up to 20% strain after which the engagement plateaus. The fiber orientation distribution functions show remarkably different changes in the ECM structure in response to mechanical loading. The medial collagen shows an evident preferred circumferential distribution, however the fiber families of adventitial collagen are obscured by their waviness at no or low mechanical strains. Collagen fibers in both layers exhibit significant realignment in response to unequal biaxial loading. The elastic fibers are much more uniformly distributed and remained relatively unchanged due to loading. Removal of elastin produces similar structural changes in collagen as mechanical loading. Our study suggests that the elastic fibers are under tension and impart an intrinsic compressive stress on the collagen.
The artery relies on interlamellar structural components, mainly elastin and collagen fibers, for maintaining its integrity and resisting dissection propagation. In this study, the contribution of ...arterial elastin and collagen fibers to interlamellar bonding was studied through mechanical testing, multiphoton imaging and finite element modeling. Steady-state peeling experiments were performed on porcine aortic media and the purified elastin network in the circumferential (Circ) and longitudinal (Long) directions. The peeling force and energy release rate associated with mode-I failure are much higher for aortic media than for the elastin network. Also, longitudinal peeling exhibits a higher energy release rate and strength than circumferential peeling for both the aortic media and elastin. Multiphoton imaging shows the recruitment of both elastin and collagen fibers within the interlamellar space and points to in-plane anisotropy of fiber distributions as a potential mechanism for the direction-dependent phenomena of peeling tests. Three-dimensional finite element models based on cohesive zone model (CZM) of fracture were created to simulate the peeling tests with the interlamellar energy release rate and separation distance at damage initiation obtained directly from peeling test. Our experimental results show that the separation distance at damage initiation is 80 μm for aortic media and 40 μm for elastin. The damage initiation stress was estimated from the model for aortic media (Circ: 60 kPa; Long: 95 kPa) and elastin (Circ: 9 kPa; Long: 14 kPa). The interlamellar separation distance at complete failure was estimated to be 3 - 4 mm for both media and elastin. Furthermore, elastin and collagen fibers both play an important role in bonding of the arterial wall, while collagen has a higher contribution than elastin to interlamellar stiffness, strength and toughness. These results on microstructural interlamellar failure shed light on the pathological development and progression of aortic dissection.
Many biological tissues are piezoelectric and pyroelectric with spontaneous polarization. Ferroelectricity, however, has not been reported in soft biological tissues yet. Using piezoresponse force ...microscopy, we discover that the porcine aortic walls are not only piezoelectric, but also ferroelectric, with the piezoelectric coefficient in the order of 1 pm/V and coercive voltage approximately 10 V. Through detailed switching spectroscopy mapping and relaxation studies, we also find that the polarization of the aortic walls is internally biased outward, and the inward polarization switched by a negative voltage is unstable, reversing spontaneously to the more stable outward orientation shortly after the switching voltage is removed. The discovery of ferroelectricity in soft biological tissues adds an important dimension to their biophysical properties, and could have physiological implications as well.
The prevalence of hypertension increases with aging and is associated with increased arterial stiffness. Resistant hypertension is presented when drug treatments fail to regulate a sustained ...increased blood pressure. Given that the mechanisms between the sympathetic nervous system and the kidney play an important role in blood regulation, renal denervation (RDN) has emerged as a therapeutic potential in resistant hypertension. In this study, we investigated the effects of RDN on the biomechanical response and microstructure of elastic arteries. Common carotid arteries (CCA) excised from 3-month, 8-month, and 8-month denervated rats were subjected to biaxial extension-inflation test. Our results showed that hypertension developed in the 8-month-old rats. The sustained elevated blood pressure resulted in arterial remodeling which was manifested as a significant stress increase in both axial and circumferential directions after 8 months. RDN had a favorable impact on CCAs with a restoration of stresses in values similar to control arteries at 3 months. After biomechanical testing, arteries were imaged under a multi-photon microscope to identify microstructural changes in extracellular matrix (ECM). Quantification of multi-photon images showed no significant alterations of the main ECM components, elastic and collagen fibers, indicating that arteries remained intact after RDN. Regardless of the experimental group, our microstructural analysis of the multi-photon images revealed that reorientation of the collagen fibers might be the main microstructural mechanism taking place during pressurization with their straightening happening during axial stretching.
In this paper, we studied the viscoelastic behaviors of isolated aortic elastin using combined modeling and experimental approaches. Biaxial stress relaxation and creep experiments were performed to ...study the time-dependent behavior of elastin. Experimental results reveal that stress relaxation preconditioning is necessary in order to obtain repeatable stress relaxation responses. Elastin exhibits less stress relaxation than intact or decellularized aorta. The rate of stress relaxation of intact and decellularized aorta is linearly dependent on the initial stress levels. The rate of stress relaxation for elastin increases linearly at stress levels below about 60 kPa; however, the rate changes very slightly at higher initial stress levels. Experimental results also show that creep response is negligible for elastin, and the intact or decellularized aorta. A quasi-linear viscoelasticity model was incorporated into a statistical mechanics based eight-chain microstructural model at the fiber level to simulate the orthotropic viscoelastic behavior of elastin. A user material subroutine was developed for finite element analysis. Results demonstrate that this model is suitable to capture both the orthotropic hyperelasticity and viscoelasticity of elastin.
Ferroelectric switching of elastin Liu, Yuanming; Cai, Hong-Ling; Zelisko, Matthew ...
Proceedings of the National Academy of Sciences - PNAS,
07/2014, Letnik:
111, Številka:
27
Journal Article
Recenzirano
Odprti dostop
Ferroelectricity has long been speculated to have important biological functions, although its very existence in biology has never been firmly established. Here, we present compelling evidence that ...elastin, the key ECM protein found in connective tissues, is ferroelectric, and we elucidate the molecular mechanism of its switching. Nanoscale piezoresponse force microscopy and macroscopic pyroelectric measurements both show that elastin retains ferroelectricity at 473 K, with polarization on the order of 1 μC/cm ², whereas coarse-grained molecular dynamics simulations predict similar polarization with a Curie temperature of 580 K, which is higher than most synthetic molecular ferroelectrics. The polarization of elastin is found to be intrinsic in tropoelastin at the monomer level, analogous to the unit cell level polarization in classical perovskite ferroelectrics, and it switches via thermally activated cooperative rotation of dipoles. Our study sheds light onto a long-standing question on ferroelectric switching in biology and establishes ferroelectricity as an important biophysical property of proteins. This is a critical first step toward resolving its physiological significance and pathological implications.
As major extracellular matrix components, elastin, and collagen play crucial roles in regulating the mechanical properties of the aortic wall and, thus, the normal cardiovascular function. The ...mechanical properties of aorta, known to vary with age and multitude of diseases as well as the proximity to the heart, have been attributed to the variations in the content and architecture of wall constituents. This study is focused on the role of layer-specific collagen undulation in the variation of mechanical properties along the porcine descending thoracic aorta. Planar biaxial tensile tests are performed to characterize the hyperelastic anisotropic mechanical behavior of tissues dissected from four locations along the thoracic aorta. Multiphoton microscopy is used to image the associated regional microstructure. Exponential-based and recruitment-based constitutive models are used to account for the observed mechanical behavior while considering the aortic wall as a composite of two layers with independent properties. An elevated stiffness is observed in distal regions compared to proximal regions of thoracic aorta, consistent with sharper and earlier collagen recruitment estimated for medial and adventitial layers in the models. Multiphoton images further support our prediction that higher stiffness in distal regions is associated with less undulation in collagen fibers. Recruitment-based models further reveal that regardless of the location, collagen in the media is recruited from the onset of stretching, whereas adventitial collagen starts to engage with a delay. A parameter sensitivity analysis is performed to discriminate between the models in terms of the confidence in the estimated model parameters.
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