Search

This article describes 1D extension tests on bovine collagen samples (8% collagen in water). At such a high collagen concentration, the mechanical properties of semi-solid samples can be approximated by hyperelastic models (two-parametric HGO and Misof models were used), or simply by Hooke’s law and the modulus of elasticity E. The experiments confirm a significant increase in the E-modulus of the samples irradiated with high-energy electrons. The modulus E ~ 9 kPa of non-irradiated samples increases monotonically up to E ~ 250 kPa for samples absorbing an e-beam dose of ~3300 Gy. This amplification is attributed to the formation of cross-links by irradiation. However, E-modulus can be increased not only by irradiation but also by exposure to a high strain rate. For example, soft isotropic collagen extruded through a 200 mm long capillary increases the modulus of elasticity from 9 kPa to 30 kPa, and the increase is almost isotropic. This stiffening occurs when the corrugated collagen fibers are straightened and are aligned in the flow direction. It seems that the permanent structural changes caused by extrusion mitigate the effects of the ex post applied irradiation. Irradiation of extruded samples by 3300 Gy increases the modulus of E-elasticity only three times (from 30 kPa to approximately 90 kPa). Extruded and ex post irradiated samples show slight anisotropy (the stiffness in the longitudinal direction is on an average greater than the transverse stiffness).

Aortic dissection is a biomechanical phenomenon associated with a failure of internal cohesion, which manifests itself through the delamination of the aortic wall. The goal of this study is to deepen our knowledge of the delamination strength of the aorta. To achieve this, 661 peeling experiments were carried out with strips of the human aorta collected from 46 cadavers. The samples were ordered into groups with respect to (1) anatomical location, (2) orientation of the sample, and (3) extension rate used within the experiment. The obtained results are in accordance with the hypothesis that delamination resistance is not sensitive to the extension rates 0.1, 1, 10, and 50 mms

The biofunctionalization of scaffolds for tissue engineering is crucial to improve the results of regenerative therapies. This study compared the effect of platelet-functionalization of 2D electrospun and 3D centrifugal spun scaffolds on the osteogenic potential of hMSCs. Scaffolds prepared from poly-e-caprolactone, using electrospinning and centrifugal spinning technology, were functionalized using five different concentrations of platelets. Cell proliferation, metabolic activity and osteogenic differentiation were tested using hMSCs cultured in differential and non-differential medium. The porous 3D structure of the centrifugal spun fibers resulted in higher cell proliferation. Furthermore, the functionalization of the scaffolds with platelets resulted in a dose-dependent increase in cell metabolic activity, proliferation and production of an osteogenic marker – alkaline phosphatase. The effect was further promoted by culture in an osteogenic differential medium. The increase in combination of both platelets and osteogenic media shows an improved osteoinduction by platelets in environments rich in inorganic phosphate and ascorbate. Nevertheless, the results of the study showed that the optimal concentration of platelets for induction of hMSC osteogenesis is in the range of 900–3000 × 109 platelets/L. The study determines the potential of electrospun and centrifugal spun fibers with adhered platelets, for use in bone tissue engineering.

Our study presents a novel collagen wound dressing prepared from freshwater fish skin ( Cyprinus Carpio) collagen type I. Half of the sponges were cross-linked with carbodiimide. The cross-linked and non-cross-linked collagen sponges were subsequently impregnated with gentamicin and lyophilized thus allowing for the attainment of the appropriate gentamicin content without the removal thereof during the cross-linking stage. The structure was evaluated via micro-CT and infrared spectrometry and the structural stability and gentamicin release properties were evaluated in phosphate buffer solution. The sponges were further tested via a rat model of an infected wound with Pseudomonas aeruginosa inoculation and compared with a reference commercial product. The sponges thus prepared provided a degree of open porosity that was comparable to or higher than that of the reference commercial product. Spectrometry analysis revealed that the cross-linked collagen sponge and reference commercial product sponge preserved their secondary collagen structure after 168 h while early accelerated degradation was observed with respect to the non-cross-linked collagen sponge. Gentamicin was released rapidly from all the sponges. Compared to those animals with gentamicin-containing sponges or gentamicin administered intramuscularly, the animals with the cross-linked collagen sponge without gentamicin exhibited marked clinical and laboratory infection signs. Both the administration routes (intramuscular and via gentamicin-containing sponges) provided similar gentamicin plasma levels. The resulting highly homogeneous product which was characterized by excellent structural and clinical properties proved effective in terms of the treatment of a surgical wound infection in a rat model. We demonstrated that all the gentamicin was released from the sponge and was absorbed in the systemic circulation. This is the first time that Cyprinus Carpio collagen has been used in the preparation of wound dressings. Thus, gentamicin-containing sponges provide a promising tool for the treatment and prevention of surgical site infections.

Objectives. Surgical wounds resulting from biofilm-producing microorganisms represent a major healthcare problem that requires new and innovative treatment methods. Rifampin is one of a small number of antibiotics that is able to penetrate such biofilms, and its local administration has the potential to serve as an ideal surgical site infection protection and/or treatment agent. This paper presents two types (homogeneous and sandwich structured) of rifampin-releasing carbodiimide-cross-linked fresh water fish collagen wound dressings. Methods. The dressings were prepared by means of the double-lyophilization method and sterilized via gamma irradiation so as to allow for testing in a form that is able to serve for direct clinical use. The mechanical properties were studied via the uniaxial tensile testing method. The in vivo rifampin-release properties were tested by means of a series of incubations in phosphate-buffered saline. The microbiological activity was tested against methicillin-resistant staphylococcus aureus (MRSA) employing disc diffusion tests, and the in vivo pharmacokinetics was tested using a rat model. A histological examination was conducted for the study of the biocompatibility of the dressings. Results. The sandwich-structured dressing demonstrated better mechanical properties due to its exhibiting ability to bear a higher load than the homogeneous sponges, a property that was further improved via the addition of rifampin. The sponges retarded the release of rifampin in vitro, which translated into at least 22 hours of rifampin release in the rat model. This was significantly longer than was achieved via the administration of a subcutaneous rifampin solution. Microbiological activity was proven by the results of the disc diffusion tests. Both sponges exhibited excellent biocompatibility as the cells penetrated into the scaffold, and virtually no signs of local irritation were observed. Conclusions. We present a novel rifampin-releasing sandwich-structured fresh water fish collagen wound dressing that has the potential to serve as an ideal surgical site infection protection and/or treatment agent.

The study presents a novel vancomycin-releasing collagen wound dressing derived from Cyprinus carpio collagen type I cross-linked with carbodiimide which retarded the degradation rate and increased the stability of the sponge. Following lyophilization, the dressings were subjected to gamma sterilization. The structure was evaluated via scanning electron microscopy images, micro-computed tomography, and infrared spectrometry. The structural stability and vancomycin release properties were evaluated in phosphate buffered saline. Microbiological testing and a rat model of a wound infected with methicillin-resistant Staphylococcus aureus (MRSA) were then employed to test the efficacy of the treatment of the infected wound. Following an initial mass loss due to the release of vancomycin, the sponges remained stable. After 7 days of exposure in phosphate buffered saline (37°C), 60% of the material remained with a preserved collagen secondary structure together with a high degree of open porosity (over 80%). The analysis of the release of vancomycin revealed homogeneous distribution of the antibiotic both across and between the sponges. The release of vancomycin was retarded as proved by in vitro testing and further confirmed by the animal model from which measurable concentrations were observed in blood samples 24 hours after the subcutaneous implantation of the sponge, which was more than observed following intraperitoneal administration. The sponge was also highly effective in terms of reducing the number of colony-forming units in biopsies extracted from the infected wounds 4 days following the inoculation of the wounds with the MRSA solution. The presented sponges have ideal properties to serve as wound dressing for prevention of surgical site infection or treatment of already infected wounds.

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.

This paper presents a new dynamic inflation experiment method for identifying the viscoelastic parameters of blood vessels and grafts. The material parameters are estimated only from the recorded damped natural pressure oscillation and from the initial geometry of the tested specimens. The identified parameters are dynamic compliance, the damping ratio, fictitious fluid viscosity, frequency, and, for tubular samples, the elastic incremental modulus and the wall viscosity. Several experiments with latex tubular samples were performed to check the suitability of this unique experimental method, and to estimate the limits of its applicability (effects of operational parameters, e.g. the geometry of the samples and the viscosity of the working fluids).

Aim. To assess the systemic and local immunological response to subcutaneous implants of a vascular graft covered with collagen extracted from the European carp (freshwater fish) or with collagen of bovine origin. Methods. Pieces of a vascular graft covered by pure bovine (Bos taurus, BOV, n=14) or carp (Cyprinus carpio, CYP, n=14) collagen 5 mm in size were implanted subcutaneously in the dorsum of a Balb/cOla mice. A sham operation group of 12 animals served as the control. At 7 and 14 days after the operation, one-half of each group was terminated and blood for serum, spleen, and implant with surrounding tissue were collected. Mean cytokine (TNF-α, IL-10, IL-4, IL-1β, IL-13, and IFN-γ) levels in serum were determined using ELISA. Spleen cell cultures were used for in vitro testing of lymphocyte proliferation and cytokine secretion. Local expressions of IL-6, IL-10, TNF-α, TGF-β, CCL-2, and CCL-3 were determined using PCR. Results. We found no significant difference among control, BOV, and CYP groups in mean cytokine serum levels at seven days. At day 14, the BOV group had higher levels of TNF-α (P=.018) and both the BOV and CYP groups had lower levels of IL-4 (P=.011 and P=.047, respectively) compared with the control group. Both tested implants showed only a minimal effect on the production of selected cytokines. Cell proliferation in the CYP group stimulated by CYP gel at 14 days was significantly lower than by BOV gel in BOV group (P=.0031) or by CYP gel in the control group (P=.041). The difference between the groups in the local RNA expression of all the tested mediators both at 7 and at 14 days was not significant apart from a lower level of TNF-α in the BOV group compared to CYP at 14 days (P=.013). Conclusions. Implants covered with carp collagen induce an immunological response that is comparable to that of bovine collagen covered implants in a mouse model.

Infections of the musculoskeletal system present a serious problem with regard to the field of orthopedic and trauma medicine. The aim of the experiment described in this study was to develop a resorbable nanostructured composite layer with the controlled elution of antibiotics. The layer is composed of collagen, hydroxyapatite nanoparticles, and vancomycin hydrochloride (10 wt%). The stability of the collagen was enhanced by means of cross-linking. Four cross-linking agents were studied, namely an ethanol solution, a phosphate buffer solution of N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride/N-hydroxysuccinimide, genipin, and nordihydroguaiaretic acid. High performance liquid chromatography was used so as to characterize the in vitro release rates of the vancomycin and its crystalline degradation antibiotically inactive products over a 21-day period. The maximum concentration of the released active form of vancomycin (approximately 265 mg/L) exceeded the minimum inhibitory concentration up to an order of 17 times without triggering the burst releasing effect. At the end of the experiment, the minimum inhibitory concentration was exceeded by up to 6 times (approximately 100 mg/L). It was determined that the modification of collagen with hydroxyapatite nanoparticles does not negatively influence the sustainable release of vancomycin. The balance of vancomycin and its degradation products was observed after 14 days of incubation.

The inflation-extension tests were performed with developed hybrid composite manufactured from biological collagenous matrix and reinforced with polyester mesh. One sample was made without the mesh. This control specimen showed compliant and almost linear behavior. Tests of samples with integrated mesh in the matrix demonstrated that mechanical properties of final composite could be modulated by placing of its reinforced component in the matrix. It was shown that we can obtain response from stiff and linear to nonlinear and very compliant depending on the position of the mesh within biological matter.

Image analysis of C/PPS samples was done to detect the orientation of randomly oriented pellets on the surface. Binary image was discretized on facets with rotating abscissa with length of N pixels. The frequency of detected peaks depending on the orientation angle was analyzed and material isotropy was confirmed. Mechanical characteristics were calculated with modified lamination theory for randomly reinforced composites. FE simulation of the tensile test was verified with tests based on ASTM D3039. Isotropy which was confirmed from image processing analysis has not been fully confirmed from the verifying tensile tests due to great dispersion in measured values.

Introduction: The aim of this study was to develop a prototype of an artificial blood vessel which has similar mechanical properties to a human saphenous vein graft and to experimentally verify the function of the prosthesis via ovine carotid bypass implantation. Material and methods: The prototype of an artificial graft prosthesis for low flow was developed and manufactured from a collagenous matrix and reinforcing polyester mesh. We compared the results of both the pressurisation and the mechanical stress evaluation tests of VSM with four types of hybrid vascular graft. The most similar graft (type II) was chosen for the first ovine model implantation. Results: Dominant behavior e.g. mechanical response of VSM graft in plots of circumferential and axial stress during loading is observed in circumferential direction. Average results of used VSM showed area of ideal mechanical response and the properties of artificial blood vessels were fitted into this area. Developed graft remained patent after 161 days of follow up in ovine model. Conclusions: The mechanical properties of the graft were designed and adjusted to be similar to the behaviour of human saphenous veins. This approach showed promising results and enhanced the final performance of the prosthesis.

Aim. The aim of this study was to evaluate short-term patency of the new prosthetic graft and its structural changes after explantation. Methods. The study team developed a three-layer conduit composed of a scaffold made from polyester coated with collagen from the inner and outer side with an internal diameter of 6 mm. The conduit was implanted as a bilateral bypass to the carotid artery in 7 sheep and stenosis was created in selected animals. After a period of 161 days, the explants were evaluated as gross and microscopic specimens. Results. The initial flow rate (median ± IQR) in grafts with and without artificial stenosis was 120±79 ml/min and 255±255 ml/min, respectively. Graft occlusion occurred after 99 days in one of 13 conduits (patency rate: 92%). Wall-adherent thrombi occurred only in sharp curvatures in two grafts. Microscopic evaluation showed good engraftment and preserved structure in seven conduits; inflammatory changes with foci of bleeding, necrosis, and disintegration in four conduits; and narrowing of the graft due to thickening of the wall with multifocal separation of the outer layer in two conduits. Conclusions. This study demonstrates good short-term patency rates of a newly designed three-layer vascular graft even in low-flow conditions in a sheep model.

In the present study, inflation tests with free axial extension of 15 human vena saphena magna were conducted ex vivo to obtain data suitable for multi-axial constitutive modeling at overloading conditions (pressures up to approximately 15 kPa). Subsequently the data were fitted with a hyperelastic, nonlinear and anisotropic constitutive model based on the theory of the closed thick-walled tube. It was observed that initial highly deformable behavior (up to approximately 2.5 kPa) in the pressure–circumferential stretch response is followed by progressive large strain stiffening. Contrary to that, samples were much stiffer in longitudinal direction, where the observed stretches were in the range 0.98–1.03 during the entire pressurization in most cases. The effect of possible residual stress was evaluated in a simulation of the intramural stress distribution with the opening angle prescribed to 0°, 10°, 20°, 30°, 40°, and 50°. The result suggests that the optimal opening angle making the stress distribution through the wall thickness uniform is about 40°. The material parameters presented here are suitable for use in mechanobiological simulations describing the adaptation of the autologous vein wall after bypass surgery.

The effects of the polyvinyl alcohol (PVA) concentration on mechanical properties of hydrogels based on blends of native or denatured collagen / PVA were examined. Blends of PVA with collagen were obtained by mixing the solutions in different ratios, using glycerol as a plasticizer. The solutions were cast on polystyrene plates and the solvent was allowed to evaporate at room temperature. Uniaxial tensile tests were performed in order to obtain the initial modulus of elasticity (up to deformation 0.1), the ultimate tensile stress and the deformation at failure of the material in the water-saturated hydrogel form. It was found that the material was elastic and the addition of PVA helped to enhance both the ultimate tensile stress and modulus of elasticity of the films. Samples prepared from denaturated collagen showed the higher ultimate tensile stress and the deformation at failure in comparison with those prepared from native collagen. The results suggest that we could expect successful application of the collagen/PVA biomaterial for tissue engineering.

Background The primary objective of Tissue engineering is a regeneration or replacement of tissues or organs damaged by disease, injury, or congenital anomalies. At present, Tissue engineering repairs damaged tissues and organs with artificial supporting structures called scaffolds. These are used for attachment and subsequent growth of appropriate cells. During the cell growth gradual biodegradation of the scaffold occurs and the final product is a new tissue with the desired shape and properties. In recent years, research workplaces are focused on developing scaffold by bio-fabrication techniques to achieve fast, precise and cheap automatic manufacturing of these structures. Most promising techniques seem to be Rapid prototyping due to its high level of precision and controlling. However, this technique is still to solve various issues before it is easily used for scaffold fabrication. In this article we tested printing of clinically applicable scaffolds with use of commercially available devices and materials. Research presented in this article is in general focused on “scaffolding” on a field of bone tissue replacement. Results Commercially available 3D printer and Polylactic acid were used to create originally designed and possibly suitable scaffold structures for bone tissue engineering. We tested printing of scaffolds with different geometrical structures. Based on the osteosarcoma cells proliferation experiment and mechanical testing of designed scaffold samples, it will be stated that it is likely not necessary to keep the recommended porosity of the scaffold for bone tissue replacement at about 90%, and it will also be clarified why this fact eliminates mechanical properties issue. Moreover, it is demonstrated that the size of an individual pore could be double the size of the recommended range between 0.2–0.35 mm without affecting the cell proliferation. Conclusion Rapid prototyping technique based on Fused deposition modelling was used for the fabrication of designed scaffold structures. All the experiments were performed in order to show how to possibly solve certain limitations and issues that are currently reported by research workplaces on the field of scaffold bio-fabrication. These results should provide new valuable knowledge for further research.

The significance of studying of human pericardial mechanics arises from a wide range of possible applications. For instance, tissue engineers study potential use of a human pericardium in a design of homograft valve bioprosthesis. In cooperation with the Institute of the Physiology of Academy of Sciences of the Czech Republic, human pericardium tissue has been modified by different cross-linking agents. The function and performance of the potential future bioprostheses depends closely on sources material. Thus characterization of the mechanical properties of the human pericardial tissue is an important problem. Several samples of pericardial tissue as well as cross-linked tissue underwent uniaxial loading and unloading. Their nonlinear mechanical response was modeled using hyperelastic theory, assuming incompressible isotropic deformation. Considering future possible tissue application, mechanical properties of the material under stress corresponding to the loading in aortic valve were evaluated.

The inflation-extension test was performed in order to obtain the mechanical response (stress-strain curves) of the human vein - vena saphena magna (usually used for coronary artery bypass graft surgery). Tubular samples of the vein were inflated four times up to the pressure approx. 4 kPa (vein pressure) and then four times up to approx. 16 kPa (systolic pressure). The experiments were recorded by the CCD camera. The longitudinal and circumferential deformations of the tube were evaluated using the edge detection method. The experimental data were fitted by anisotropic, nonlinear, constitutive model in order to obtain model parameters, especially the parameter which can be explained as collagen fibres orientation approximation. This parameter was then compared with the findings from histology. The histology analyses based on label-free imaging were performed additionally to the mechanical testing. Collagen (most important load-bearing component of the vein wall) was visualized using second harmonic generation imaging (SHG, excitation at 860 nm by a tunable IR pulse laser, detection at 430±10 nm). This method enabled us to observe collagen through the vein wall. It was found that the collagen fibres are helically aligned within the vein at an angle 37±6° measured from circumferential axis. The results of collagen orientation angle show a good agreement of findings obtained from histology and from constitutive model.

Intracoronary stenting (PCI) has become standard revascularization technique to reopen blocked arteries. Although significant progress in stenting technology and implantation techniques has been made a number of problems remain. Specifically, stent sizing and inflation pressures are still a matter of scientific debates. Despite a large number of biomechanical computational simulations experimental data are rare, likely due to technical difficulties to measure dilatation pressures and coronary dimensions in the same settings. Our study shows that valuable data can be obtained by employing digital image correlation for 3D strain measurement during stent inflation ex-vivo that can provide further insight into the stent–artery wall interactions.

The longitudinal prestrain of arteries facilitates their physiological function. Remodeling, adaptation and aging result in an age-dependent magnitude of the pretension. Although the phenomenon is known, detailed statistics, especially for human arteries, are lacking. This study was designed to propose the regression model capable of estimating the prestrain of the human abdominal aorta. The length of the abdominal aorta before, l, and after excision from the body, L, the diameter, heart weight, thickness of left ventricle and degree of atherosclerosis were collected in autopsies of 156 male cadavers of known age. Longitudinal prestrain was quantified by means of the stretch ratio λ=l/L. Statistical analysis revealed significant dependence between age, prestrain, diameter and atherosclerosis, which were best fitted to the power law equation. Longitudinal prestretch reduced with age significantly; λmean=1.30±0.07 for age30 (n=29), whereas λmean=1.06±0.03 for age59 (n=31) with p-value0.0001. Raw data gave linear correlation coefficients as follows: λ-age (R=-0.842); l-age (R=0.023); L-age (R=0.476); (l-L)-age (R=-0.811). It was concluded that longitudinal prestrain decreases nonlinearly with age and both age and diameter are suitable predictors of the prestrain. Data suggests that unloaded length elongates with age in contrast to the elastic retraction.

Remodelling is a dynamic process occurring during growth and it includes sensing of environmental changes, tissue resorbance, i.e. the removal of existing old bone, and formation of new tissue. The biomechanical remodelling process is relatively well formulated for bones and can be divided into three stages: (1) bone resorption based on the osteoclast activity, (2) bone deposition based on the osteoblast activity and (3) bone growth control established on RANK/RANKL/OPG pathway-RANKL/OPG balance. The main driving force of remodelling process is a dynamic loading (cyclic compression and expansion, e.g. walking or running), which strongly influences the rate of chemical reactions. The evolution from the homogeneous density distribution to the corticalis and cancellous bone formation is shown. An inevitable influence of a dynamic mechanical loading and osteoprotegerin (OPG) concentration is demonstrated. Deformations were calculated by commercial code ANSYS. The clinical experience indicates that the dynamic loading (above the threshold level 1500-2500microstains/s), especially walking with a characteristic time approximately 1s, influences the whole process of bone remodelling after a time period of approximately 3 months. The numerical simulation shows that the concentration of the new bone and the bone elastic constants substantially depend on history and intensity of the loading, drug delivery and nutrition.

In the present study, the inflation test of human vena saphena magna was conducted to obtain data suitable for multi-axial constitutive modeling at overloading conditions (pressures up to approximately 18 kPa). Subsequently the data were fitted with a hyperelastic, nonlinear and anisotropic constitutive model based on the theory of the closed thick-walled tube. It was observed that initial highly deformable behavior in the pressure–circumferential stretch response is followed by progressive large strain stiffening, which is in contrast to the pressure–axial stretch response. The effect of possible residual stress was evaluated in a simulation of the intramural stress distribution with the opening angle prescribed to 0°, 10°, 20°, 30°, 40°, and 50°. The result suggests that the optimal opening angle making the stress distribution through the wall thickness uniform is about 20°.

This paper presents a new fluid transient inflation experiment applied on a physical model of vein (short latex tube, 5mm diameter). Aim of experiments is assessment of wall viscous behaviour from attenuated pulsation of the tested sample. Experimental data obtained from dynamic test are compared with numerical simulation and a viscoelastic parameter of Haslach constitutive model is identified. It is verified that the viscoelasticity of wall has a greater impact to the damping of pulsation than the viscosity of water filling the sample and the attached capillary. Volume of sample depends on internal pressure measured by a pressure transducer. The maximum dissipation constitutive model of viscoelastic wall sample was employed for description of viscoelastic behaviour. Frequency of natural oscillation of pressure is determined by inertia of water column within the tested sample and attached capillary and by the tested specimen stiffness. The pressure pulsations are initiated by a sudden pressure drop at water surface.

The aim of the study was to verify whether constitutive model parameters obtained from quasistatic inflation-extension test could be used for in-vivo computational simulation of venous graft used as bypass at arterial conditionns. Inflation tests of human vena saphena magna at three loading rates were performed: a quasistatic (slow), moderate, and fast, which was similar to human heart rate. The experimental data were fitted by hyperelastic nonlinear anisotropic constitutive model (Holzapfel-Gasser-Ogden) in order to obtain material parameters. It was found that parameters differ significantly with respect to loading rate. Especially the parameter related to large strain stiffening (k2) increased with loading rate. The results suggest that constitutive parameters are strongly influenced by experimental conditions which we interpret as viscoelasticity of the vein grafts.

This study describes stiffness of arterial wall near aortic bifurcation. Uni-axial tensile tests with specimens cut out from human abdominal aorta, aortic bifurcation and common iliac arteries were carried out. Material anisotropy was verified as well as site-specific stiffness. The stiffness was described by initial secant modulus of elasticity (EINIT). This parameter was gained from stress/strain data at 12% of deformation. The tissue samples in various regions and directions were concluded as described as follows: region of aorto-iliac bifurcation is significantly stiffer in longitudinal direction than in the circumferential direction in all of three examined parts of arterial tree. The comparison between tissue samples cut and loaded in longitudinal direction indicates that arterial tree stiffens in caudal direction. No significant change in material properties was found in the apex of bifurcation in comparison with adjacent tissue. Stiffening of arterial tree during ageing was verified.

Biological prostheses of human heart valves are prepared from autologous heart valves and from xenogeneic heart valves or pericardium. Xenogenous and allogenous biological prostheses are associated with adverse immune reactions, thrombosis and degeneration, and thus they have a high rate of reoperation. An optimum autologous tissue for heart valve grafts therefore needs to be found. Human pericardium was assessed as a potential source, and was compared with native human heart valves and porcine and bovine pericardium and heart valves. The tissues were evaluated for their mechanical properties, their collagen content and structure, and their histological structure. We observed differences in mechanical properties, collagen fibre orientation, the shape of the collagen bundles, and the collagen content among the pericardium samples and heart valves of different origin.

The significance of studying human pericardial mechanics arises from a wide range of possible applications. For instance, tissue engineers study the potential use of a human pericardium in a design...

The main problem of currently used xenogeneic biological heart valves is the development of degenerative changes leading to valve failure. Reoperation is required in approximately 65% of patients at 15 years after implantation. The challenge of heart valve tissue engineering is to create a new type of biological prosthesis for clinical use. The aim of our study is to construct a living autologous human pericardial heart valve that will have optimal mechanical properties and a similar histological structure as the normal aortic heart valve. Three leaflet heart valve constructs made from human pericardium were attached onto a plastic holder and cultured under dynamic conditions for up to four weeks. After this time conditioned pericardial samples were compared to control unconditioned pericardial samples from the same patient and to that of the normal aortic heart valve. Histological, immunohistochemical and biomechanical assessments were performed. Pericardial interstitial cells (PICs) are able to respond to mechanical stresses by proliferating and differentiating into an active (myofibroblast-like) phenotype and are able to produce new extracellurar matrix (ECM). A threefold increase in PIC number and a twofold increase in smooth muscle actin (SMA) positive cells was observed after dynamic conditioning. These measurements were statistically significant (p

The objective of our study was to compare the histological structure and cellular organization of autologous human pericardium to that of the human aortic heart valve. Collagen, elastin and glycosaminoglycans are responsible for the mechanical properties of aortic heart valve leaflets. Aortic valve leaflets are composed primarily of collagen representing 50% of total extracellular matrix (ECM). The main collagen types in the aortic heart valve are collagen I (74% of total collagen) and collagen III (24% of total collagen). Elastin represents 13% and is responsible for the elastic properties of the valve leaflet. Collagen has a specific architecture that endows heart valve tissue with the ability to withstand circulatory forces over the course of a lifetime. Its fiber orientation, density and cell associations are very important for this purpose. Normal aortic heart valves were obtained during heart transplantation and compared to autologous human pericardium before and after dynamic conditioning using classical histological assessment, immunohistochemical analysis and confocal microscopy. The architecture of pericardial tissue is very similar to that of the normal aortic heart valve possessing well organized collagen fibers with embedded pericardial interstitial cells (PICs) forming a three dimensional network. Instead of the trilaminar histological organization present in the aortic heart valve, the pericardium possesses one layer whose densely packed collagen bundles closely resemble that of the lamina fibrosa of the native aortic heart valve by confocal microscopy. Elastin fibers are evenly distributed throughout the entire thickness of the pericardium in comparison to the specialized elastin containing layer in the lamina ventricularis of the aortic heart valve. PICs are also evenly distributed throughout the pericardium. In the inner pericardial layer facing the heart these cells have a more spindle-like shape similar to that of valvular interstitial cells (VICs) in the lamina fibrosa, while in the outer part of the pericardium these cells have a more spread-out cytoplasmic morphology interacting with more loosely distributed collagen bundles. Like in the aortic heart valve, PICs show cell-cell interactions in addition to cell-matrix interactions. Our study confirmed similarities in the cellular and ECM organization of human pericardium and the native aortic heart valve. It may be concluded that human autologous pericardium may be a favorable tissue for heart valve replacement. Autologous pericardial tissue may also avoid a negative immune system response that could adversely affect recipient graft uptake and downstream ECM remodeling events.

This study introduces a new quantity, the combined arteriosclerotic index (CAI), which is defined as the ratio between the diameter and the longitudinal prestrain of an artery. The longitudinal prestrain has been adopted as the ratio between the in situ length and the excised length of the abdominal aorta, and is a measure of arterial elasticity. During ageing, arteriosclerosis is manifested by the loss of pretension and by enlargement of the diameter of the artery. CAI combines these two effects. A sample of 61 female and 194 male autopsy measurements of human abdominal aortas shows that CAI correlates significantly with chronological age (R = 0.916/0.921; female/male). The sample had the following parameters: age 53 ± 19/48 ± 16 years; diameter of the abdominal aorta 12.4 ± 2.2/13.4 ± 2.1 mm; and longitudinal prestrain 1.13 ± 0.10/1.15 ± 0.10 (mean ± sample standard deviation; female/male). The resulting CAI was 11.2 ± 2.7/11.9 ± 2.6 mm. The classical linear regression model was employed for age estimation by CAI. The model gave a residual standard deviation of 7.6/6.3 years and a 95% prediction interval range of ± 15.4/12.5 years (female/male). A two-sample t-test confirmed that there are significant differences between the female and male population during ageing, reflected by CAI, unlike longitudinal prestrain. It was concluded that CAI is a suitable predictor of age at time of death and is easily obtainable in the autopsy room.

This study was designed to assess the biomechanical interactions between the coronary artery wall and intracoronary stents following implantation in vitro. Balloon expandable stent was deployed in vitro into the sample of the left anterior descending coronary artery of a 67 years old female with multiple atherosclerotic lesions. The stent was selected to match approximately the internal diameter of the healthy segment of the target artery. The stent implantation procedure was recorded with CCD camera. Digital images were subsequently processed with the edge detector based on Canny algorithm. Obtained coordinates of the surface contours were used in the deformation analysis. For the sake of simplicity the deformation was considered as the ratio between the distances of deformed and reference contour points at the same longitudinal position. We found that the stent expansion induced significant over-stretching of the external coronary artery. We have concluded that optical tracking of the external surface of the artery during the stent deployment provides sufficiently accurate deformation analysis potentially useful in the assessment of biomechanical interactions during intracoronary stenting.

Arteries in vivo undergo significant longitudinal pretension which helps to optimize their mechanical function. This pretension is changed within the life and is involved in remodeling and adaptation processes. There is, however, surprisingly small number of papers reporting systematic statistics of the age-related distribution of the longitudinal prestrain in human arteries. This study was designed to fill this gap. Statistics of the longitudinal prestrain were gained within the autopsy measurements of abdominal aortic segments’ retraction. The study involved 18 female (51 male) subjects aged 41.6±15.3 (37.3±14.1) years, who gave the prestrain 0.16±0.1 (0.19±0.1); mean±SD. The observation proved that the longitudinal prestrain decreases significantly with increased age. The linear correlation coefficient was R=-0.866 in female group (R=-0.832 male). The linear equation was employed as the regression model and the prediction intervals were also computed. It was concluded that, despite rather small number of observations, the age is a suitable predictor of the longitudinal prestrain in human abdominal aorta.

It is a well-known fact that the length of an artery in situ and the length of an excised artery differs. Retraction of blood vessels is usually observed. This pre-tension plays crucial role in arterial biomechanics. It augments an artery wall load-bearing capacity. This paper presents the longitudinal pre-strain of the human aorta as an index of human age. The length of abdominal aortas was measured during autopsies before and after segment resection. The longitudinal pre-strain was calculated in 130 donors; 100 male and 30 female bodies. The pre-strain was defined as the ratio between in situ length and the length after the excision. The mean pre-strain was found to be 1.18 ± 0.10 for male and 1.14 ± 0.10 for female sample (mean ± standard deviation). The age in the male group was 41.6 ± 15.9 years; and 47.7 ± 17.7 years in the female group. Statistical analysis revealed the correlation coefficient between age and pre-strain r = −0.821 and r = −0.839 in male and female group, respectively. The analysis also confirmed close correlation between aortic circumference and age; and between circumference and pre-strain. Linear and power law regression equations were employed and prediction intervals were computed. The power law estimates the age more accurately than linear one model. Nevertheless, especially for small values of the pre-strain (aged individuals) the linear model can be advantageous.

Distributions of collagen fibrils and smooth muscle cells nuclei (SMC) orientations were investigated in histological sections obtained from medial layer of human thoracic aorta. The sections were stained with van Gieson. Digital image of the sections was converted to binary pixel map with target component represented in white (logical unity). Selected image was indicated to elucidate the sensitivity to threshold conditions and three different binary conversions were performed. Consecutively images were processed by in house developed software BinaryDirections which uses an algorithm of the rotation line segment to determine significant directions in digital images. The algorithm operates in the way that in each target pixel a line segment is rotated step by step to explore neighborhood of the pixel. Exploring the neighborhood the number of unity pixels in each rotating step is determined. The distribution of orientations in the entire image is gained after normalization either as averaged density distribution from all pixels or as an histogram of the most abundant directions in the image. It was found that both collagen fibrils and SMC nuclei analyses give significant peak in distributions. Its value ranges between 45° - 65° (defined as declination from longitudinal axis of an artery in a tubular configuration) depending on the method. It implies that preferred direction in aortic medial layer was oriented circumferentially rather than longitudinally. This conclusion was almost independent of the threshold setup. Results suggest that the orientations of SMC nuclei and collagen fibrils are mutually correlated and determination of collagen fibril orientations, which may be stained in insignificant manner, could be supported with SMC nuclei orientations to obtain more realistic models.

Cyclic uni-axial tensile tests with samples of human aorta were performed with an aim to obtain data describing the Mullins effect of arterial tissue. According to presumed anisotropy, reported widely, both samples oriented longitudinally and circumferentially were tested. Each of tested samples underwent cyclic tension up to a particular value of a stretch four times, consecutively maximum limit of reached stretch was increased and subsequent four cycles were performed. Significant stress softening of aortic tissue and residual strains were confirmed. An idealization was made in such a way that reloading and unloading curves are coincident. It was hypothesized that the stress softening observed within reloading of previously loaded tissue may be described by an evolution of material parameters. These parameters should be related to an alternation of internal structure. We proposed a model based on changes in limiting fiber extensibility of fibrillar component of the aortic wall, primarily represented by a collagen. The arterial wall was assumed to be hyperelastic transversely isotropic material with different response under primary loading and unloading. A stored energy function was additively split into isotropic and anisotropic part. Preferred direction in continuum, defined in referential configuration, was assumed to be unchanged with cyclic loading. Every straining level in the cyclic test had its own value of fiber extensibility related to strain maximum previously reached. The isotropic matrix response was modeled using Neo-Hooke term with shear modulus values different under primary loading and reloading, however all reloading values were held the same. The predictions of the model described above were in good agreement with observations.

Implantation of coronary artery bypass graft (CABG) is still one of frequent revascularization methods. Biomechanics of a CABG wall may significantly affect its successful function, nevertheless, papers which concern with a constitutive modeling of the CABG are lacking in literature. The purpose of this study is to describe a constitutive behavior of the CABG tissue. A sample of the CABG underwent three years of remodeling under arterial conditions. This sample was analyzed in an inflation–extension test (vertically aligned closed-end tube was loaded by an internal overpressure and by varying axial force). Displacements of the specimen were recorded by CCD cameras and subsequently evaluated by digital image cross-correlation. The experiment comprised preconditioning cycles and measurement periods. Mechanical response of the CABG was studied using four different values of an axial force (weight). A pressure – circumferential stretch and an axial force – axial stretch data are presented. Presence of the so called inversion point in the pressure – axial stretch data was observed, manifested by an elongation of a pressurized tube under some critical value of axial stretch and by a shortening as soon as the axial stretch exceeds the critical value. This interesting phenomenon was previously reported for iliac arteries. Now we may confirm it for saphenous vein graft also. Selected data were used to fit a material model. The tissue was modeled as a one-layered composite reinforced by two families of helical fibers. The material was assumed to be locally orthotropic, nonlinear, incompressible and hyperelastic. Material parameters were estimated for the strain energy function based on a limiting fiber extensibility assumption. Model parameters are fitted by optimization based on radial and axial equilibrium equation in the thick–walled tube. Material model fits selected data successfully. Further work will be aimed at extension of material model domain on all measured data.

Composite tube fabricated from elastomer matrix reinforced with NiTi wires was tested within simultaneous inflation and extension. Four different longitudinal weights were applied (m=41g; 188g; 488g; and 785g) to induce initial prestrain of the tube. The inflation was induced with repeating pressurization up to 200kPa; four times with each weight. Displacements and loading pressure were recorded into PC.

An anisotropic strain energy density function based on limiting fiber extensibility assumption was suggested. The function was deduced directly from isotropic Gent model. A material was modeled as a composite reinforced with two families of helical fibers. The anisotropy of the strain energy function was incorporated via pseudo-invariants I 4 and I 6. Mathematical expression includes three material parameters. Suitability of the model for a description of arterial mechanical response was verified by regression analysis of experimental data. Computational model based on a cylindrical thick-walled tube with residual strains was used to estimate material parameters. Identified material model fits pressure-radius data of an aortic inflation test successfully. Further upgrades of the model are discussed.