Biological reaction to small-diameter vascular grafts made of silk fibroin implanted in the abdominal aortae of rats.

BACKGROUND: Bombyx mori silk fibroin (SF) is biocompatible and degradable and has been proposed as a new material for small-diameter vascular grafts. We compared biological reactions to vascular grafts made of SF and polyethylene terephthalate (PET) to reveal the potential ability of SF as a base and/or coating materials for vascular prostheses. METHODS: SF was combined with PET or gelatin (G) to make 4 types of vascular grafts (SF/SF, SF/G, PET/SF, and PET/G, shown as "base/coating material," respectively), which are 1.5 mm in diameter and 10 mm in length. The 4 types of grafts (n = 6, respectively) were implanted into rat abdominal aortae and explanted 2 weeks or 3 months later. RESULTS: Two weeks after implantation, there are no significant differences among the 4 kinds of grafts in biological reactions evaluated by histopathologic examination. However, a remarkable difference was observed after 3 months. The area of tissue infiltration into the inside of the graft wall was approximately 2.5 times larger in SF/SF than that in PET/G. The endothelialization was achieved almost 100% in SF/SF, despite only 50% was achieved in PET/G. CONCLUSIONS: Results show that SF has a higher potential as a base of vascular grafts than the commercially available PET/G graft. The larger tissue infiltration area in PET/SF compared with that in PET/G also indicates the potential of SF as a coating material. In the present study, SF delivered promising results as base and coating materials for small-diameter vascular prostheses.

An aortic model for the physiological assessment of endovascular stent-grafts.

BACKGROUND: The aim of this study was to manufacture a new aortic model with physiological properties, which could be used for long-term durability testing of endovascular stent-grafts, as per the recommendations of the Food and Drug Administration. METHODS: Porcine abdominal aortas were acquired to establish values for compliance. The aortic model was manufactured using a nanocomposite polymer. Latex mock aorta was used for comparison. A pulsatile flow phantom perfused the aortas and synthetic tubes at physiological pulse pressure and flow. Diametrical compliance and stiffness index were calculated over mean pressures from 30 to 120 mm Hg. Data were analyzed using one-way analysis of variance and Bonferroni's test. RESULTS: Flow circuit hemodynamic values were similar for porcine aorta and synthetic tubes. Compliance of aorta ranged from 2.97 ± 0.72 (mean ± SD) to 1.42 ± 0.37%/mm Hg × 10⁻². The polymer model showed significantly better compliance (range, 3.66 ± 1.05-2.72 ± 0.28%/mm Hg × 10⁻²; p < 0.05), with no significant difference in elastic stiffness index (range, 101.6 ± 28.9-51.3 ± 10.7 for aorta and 39.8 ± 8.5-34.2 ± 3.8 for polymer model; p > 0.05). It also showed anisotropic behavior similar to the aorta. Latex tubes showed compliance that was lower than that in aorta (range, 0.87 ± 0.24-0.86 ± 0.2%/mm Hg × 10⁻²) and failed by a significant distension on increase in pressure from mean of 90 mm Hg. CONCLUSIONS: We have developed physiologically relevant aortic model showing compatible anatomy, compliance, and viscoelasticity, which could be used for long-term fatigue analysis of vascular stents and grafts. The latex mock aortas can fail at physiological pressures.

Matching the diameter of ePTFE bypass prosthesis with a native artery improves neoendothelialization.

BACKGROUND AND AIM: The undersizing of the bypass graft diameter compared to native artery changes blood flow characteristics and velocity which may affect conduit neo-endothelialization, intimal hyperplasia reaction and patency. The aim of this study was to evaluate conduit neoendothelialization, intimal hyperplasia reaction and patency results between undersized and matched ePTFE grafts. MATERIAL AND METHODS: In 16 male Sprague-Dawley rats, undersized (1-mm internal diameter) and matched (2-mm internal diameter) ePTFE grafts were anastomosed end-to-end in the infrarenal abdominal aorta. Blood flow volume per minute was measured and wall shear stress was calculated for each group. After 3 weeks of follow-up, angiography was performed via the left carotid artery just before sacrifice. Conduit neoendothelialization and intimal hyperplasia reaction were measured by computer-assisted morphometry. RESULTS: Wall shear stress was 8 times higher for the undersized group (840.56 vs. 105.07 mPa). Three weeks after implantation, conduit neoendothelialization was better in matched grafts compared to undersized grafts (441 vs. 574 microm, p = 0.008). Intimal hyperplasia reaction was similar for both groups (8.7 vs. 6.7 microm(2)/microm for undersized and matched grafts, respectively). Patency rate was 7/8 for undersized and 8/8 for matched ePTFE grafts. CONCLUSION: Although the graft patency and the intimal hyperplasia reaction were not different between the two groups after 3 weeks, matched grafts had a significantly better endothelialization compared to undersized grafts. This short-term beneficial effect may influence long-term patency results.

Biodegradable and elastomeric vascular grafts enable vascular remodeling.

Implanted grafts, including vascular substitutes, inevitably experience remodeling by host cells. The design of grafts capable of promoting constructive remodeling remains a challenge within regenerative medicine. Here, we used a biodegradable elastic polymer, poly (l-lactide-co-ε-caprolactone) (PLCL), to develop a vascular graft with circumferentially aligned microfibers. The grafts exhibited excellent handling properties and resistance to deformation. Upon implantation in rat abdominal aorta, graft-guided neoartery regeneration was achieved in a short period (4 weeks) as evidenced by rapid cell infiltration and alignment, and complete endothelialization. During vascular remodeling, a high ratio of M2/M1 macrophage was detected, and the expression of pro-inflammatory and anti-inflammatory cytokines first increased and then decreased to normal level for the follow-up period. By 12 months, the PLCL grafts were almost completely degraded and a well-integrated neoartery was formed with characteristics comparable to native arteries, such as transparent appearance, synchronous pulsation, dense and orderly extracellular matrix (ECM) arrangement, strong and compliant mechanical properties, and vasomotor response to pharmacologic agents. Taken together, our strategy represents a new avenue for guided tissue regeneration by designing the grafts to promote tissue remodeling via controlling structure, degradation and mechanical properties of the scaffolds.

Direct surface modification of metallic biomaterials via tyrosine oxidation aiming to accelerate the re-endothelialization of vascular stents.

Rapid in-situ re-endothelialization of coronary stents is one of the most effective approaches to inhibit late thrombosis and restenosis. Strut surfaces allowing excellent adhesion and migration of endothelial cells and endothelial progenitor cells may accelerate in-situ re-endothelialization. Here, a well-known endothelial cell adhesive peptide, Arg-Glu-Asp-Val (REDV), was directly immobilized onto metallic surfaces by means of single-step tyrosine oxidation with copper chloride (II) and hydrogen peroxide, which we recently reported as a new biomaterial modification technique. REDV immobilization on a 316L stainless steel plate improved endothelial cell adhesion and effectively suppressed platelet adhesion in vitro. In addition, a Co-Cr stent immobilized with Ac-Tyr-Gly-Gly-Gly-Arg-Glu-Asp-Val (Y-REDV) was implanted into a rabbit abdominal aorta. On 7 days postimplantation, 80% of the strut surface of the Y-REDV-immobilized stent was covered by a thin neointimal layer and was similar in appearance to native endothelium. Restenosis and late thrombosis were not observed in the Y-REDV-immobilized stent for 42 days. These findings suggest that direct immobilization of Y-REDV peptide onto metallic biomaterials by tyrosine oxidation is effective for promoting in-situ re-endothelialization in vascular stents. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 491-499, 2018.

The effect of a silk Fibroin/Polyurethane blend patch on rat Vessels.

Patch grafts are widely used in various kind of vascular surgeries such as detect repair or dilation of vascular stenosis. Expanded polytetrafluoroethylene (ePTFE) patches are flexible and handle well, but have shown problems with calcification as they are non-bioabsorbable and therefore permanently remain in the body. It is important to develop an alternative biocompatible patch. Silk fibroin (SF) was developed as a biocompatible material, but it lacks of the elasticity required for surgery as a patch. Polyurethane (PU) is also a well-known elastomer so this study focused on the SF and the PU blend materials with a weight ratio of 5:5 (SF/PU). To evaluate the SF/PU patch, the patches were implanted into the abdominal aortas of rats, using the ePTFE patch in the control group. Because it was more flexible the SF/PU patch was easier to implant than the ePTFE patch. At 1 week after implantation, the SF/PU patch had been infiltrated with cells and collagen fiber. The ePTFE control patch did not accumulate collagen fiber until 3 months and calcification occurred at 4 weeks. The SF/PU patch did not present any signs of calcification for 3 months. This study addressed the problems associated with using SF in isolation and showed that the SF/PU patch can be considered as a useful alternative to the ePTFE to overcome the problem of calcification.

Mechanical responses of a compliant electrospun poly(L-lactide-co-epsilon-caprolactone) small-diameter vascular graft.

To design a "mechano-active" small-diameter artificial vascular graft, a tubular scaffold made of elastomeric poly(L-lactide-co-epsilon-caprolactone) fabrics at different wall thicknesses was fabricated using an electrospinning (ELSP) technique. The wall thickness of the fabricated tube (inner diameter; approximately 2.3-2.5 mm and wall thickness; 50-340 microm) increased proportionally with ELSP time. The wall thickness dependence of mechanical responses including intraluminal pressure-induced inflation was determined under static and dynamic flow conditions. From the compliance-related parameters (stiffness parameter and diameter compliance) measured under static condition, the smaller the wall thickness, the more compliant the tube. Under dynamic flow condition (1 Hz, maximal/minimal pressure of 90 mmHg/45 mmHg) produced by a custom-designed arterial circulatory system, strain, defined as the relative increase in diameter per pulse, increased with the decrease in wall thickness, which approached that of a native artery. Thus, a mechano-active scaffold that pulsates synchronously by responding to pulsatile flow was prepared using elastomeric PLCL as a base material and an ELSP technique.

Cathepsin D activity of venous and polyester vascular grafts implanted in the abdominal aorta of the dog.

Autogenous veins and polyester prostheses were implanted in the abdominal aorta of dogs. The venous or polyester grafts were removed together with sections of the normal arteries and veins 1 month, 4 months, and 3 years after the operation. This material was then dissected into the intimal, medial, and adventitial layers, and homogenized after which their cathepsin D activity was determined. It was found that the cathepsin D activity was far higher in all the layers of the venous grafts, particularly in the polyester grafts, than in the corresponding layers of the normal arteries and veins.

Sequential studies of arterial wall regeneration in microporous, compliant, biodegradable small-caliber vascular grafts in rats.

Microporous, compliant, biodegradable vascular grafts prepared from a mixture of polyurethane (95% weight) and poly-L-lactic acid (5% weight) can function as a temporary scaffold for the regeneration of the arterial wall in small-caliber arteries. This study was undertaken to document the sequential events leading to this regeneration. Therefore, polyurethane/poly-L-lactic acid vascular grafts were implanted into the abdominal aorta of rats (N = 28) and were harvested at regular intervals from 1 hour up to 12 weeks after implantation. The implants were evaluated by means of light and electron microscopy. At each time of harvesting, the implants were patent and showed arterial pulsations. No stenosis or dilatation was observed. Endothelial cells grew from the adjacent aortic intima across the anastomoses, from day 6 onward, to form an almost complete neointima after 6 weeks of implantation. Smooth muscle cells also grew from the adjacent aortic media over the graft lattice through the platelet-fibrin coagulum from day 6 onward. The smooth muscle cells, predominantly longitudinally arranged at week 6, but also circularly arranged in some areas at week 12, formed a neomedia in which elastic laminae regenerated. Polymorphonuclear leukocytes and monocytes initially invaded the graft lattices. Fibroblasts, histiocytes, and capillaries grew from the perigraft tissue into the polyurethane/poly-L-lactic acid lattices from day 6 onward, which resulted in the formation of a neoadventitia. The polyurethane/poly-L-lactic acid lattices started to disintegrate from day 12 onward. The regenerative processes in the disintegrating polyurethane/poly-L-lactic acid grafts resulted in the formation of neoarteries, which were of sufficient strength, compliance, and thromboresistance to function as small-caliber arterial substitutes.

Pore size, tissue ingrowth, and endothelialization of small-diameter microporous polyurethane vascular prostheses.

Small-diameter microporous polyurethane vascular prostheses with an average pore size of between 5 and 30mum at the outer surfaces and 30mum at the luminal surface were prepared. Thirty-two PU and 8 expanded polytetrafluoroethylene (ePTFE) prostheses were implanted into the abdominal aorta of rats for periods ranging from 1 to 8 weeks. Harvested prostheses were analysed histologically and morphologically. The progress of endothelial-like cells and the extent of infiltration of perigraft tissues were quantified. All of the prostheses showed fast growth of endothelial-like cells in the second week, with the PU prostheses having an external pore size of 30mum producing the highest rate. It was also during the second week that perigraft tissue grew most significantly into the prosthetic structure. This coincident may suggest the importance of rapid tissue regeneration for the early endothelial healing. The role of the ingrowth perigraft tissues is likely to support and stabilize the neointima. The thickening of neointima was mainly located at the vicinity of the proximal anastomoses of some of the PU prostheses and was unrelated with the extent of perigraft tissue infiltration. In the PU prostheses, a complete lining of endothelial-like cells was achieved by the end of 4 weeks. Expanded PTFE prostheses displayed smooth, thin intima, very limited tissue ingrowth, and incomplete coverage of endothelial-like cells.

Vascugraft microporous polyesterurethane arterial prosthesis as a thoraco-abdominal bypass in dogs.

In their progression towards clinical acceptance, any new synthetic vascular grafts under development must undisputedly prove that the chemistry and structure used in the construction of the prostheses is safe and that their biocompatibility and performance as arterial substitutes are satisfactory without degradation or weakening of the device. This study was conducted to evaluate the safety of the microporous polyesterurethane Vascugraft by investigating its biocompatibility in terms of cellular proliferation, morphology and adhesion of human fibroblasts on virgin and blood-soaked Vascugraft prostheses, and its performance in vivo as a large calibre graft in a canine thoraco-abdominal bypass model for periods of implantation ranging from 4 h to 6 months. After 3 d incubation, better cell proliferation and adhesion were observed on blood-soaked Vascugraft than on a non-porous polyurethane graft, Mitrathane, and two other polytetrafluorethylene prostheses, Impra and Goretex. Furthermore, no leachable cytotoxic contaminants were released from the prostheses. In vivo, the Vascugraft has demonstrated a good performance with the development of an endothelialised internal capsule at both anastomoses 2 weeks after implantation, reaching the medial portion of the graft at 4 months. During this period, the prostacyclin I2/thromboxane A2 ratio increased and was higher than 1.0 at 2 months. In addition, the Vascugraft exhibited low surface thrombogenicity in terms of radiolabelled platelets and fibrin deposited. Chemically, as revealed by ESCA and FTIR analyses, a slight decrease in carbonate content was observed on the external surface of the Vascugraft during the early post-implantation periods. Breaks in the microfibrous structure were also observed at 4 and 6 months, occurring mainly in the anastomotic regions and believed to be stress-related. This study shows that the polymer used in the Vascugraft is biocompatible in terms of fibroblast proliferation and promotes fair healing characteristics. However, the chemical and structural surface modifications noted in this study are disturbing and question the total inocuity of the Vascugraft. Consequently, the decision by B. Braun Melsungen AG to end this project is both highly conscientious and professional.

Compliance and biodegradation of vascular grafts stimulate the regeneration of elastic laminae in neoarterial tissue: an experimental study in rats.

Microporous vascular grafts that are compliant and biodegradable can function as scaffolds for the regeneration of the arterial wall in small-caliber arteries. The purpose of this study was to determine the specific influence of both compliance and biodegradation of microporous vascular grafts on this regeneration, especially on the regeneration of elastic laminae. Therefore we implanted three different types of microporous vascular grafts into the abdominal aorta of rats. These grafts were (I) compliant, biodegradable (group I; n = 6), (II) compliant, biostable (group II; n = 8), and (III) noncompliant, biodegradable (group III; n = 8). Six weeks after implantation the implants were evaluated by means of light microscopy and electron microscopy. The compliance of the implants, as indicated by arterial pulsations, was well maintained in group I but not in group II. In all groups a neomedia had regenerated, composed of smooth muscle cells that were predominantly longitudinally arranged. Elastic laminae were present almost throughout the neomedia in group I, restricted to the luminal layers of the neomedia in group II, and totally absent in the neomedia of group III. These results demonstrate that both compliance and biodegradation stimulate the regeneration of elastic laminae in neoarterial tissue. Because of the compliance of microporous vascular grafts, smooth muscle cells are mechanically stimulated by the arterial pulsations to produce elastin arranged in laminae. Because of the biodegradation of these grafts, compliance is maintained, which therefore favors the regeneration of elastic laminae.

Microporous, complaint, biodegradable vascular grafts for the regeneration of the arterial wall in rat abdominal aorta.

Microporous, complaint, biodegradable vascular grafts prepared from mixtures of polyurethane (PU) and poly-L-lactic acid (PLLA) can function as temporary scaffolds for the regeneration of the arterial wall in small-caliber arteries. This study was undertaken to determine the most suitable composition for PU/PLLA vascular grafts to ensure an optimal regeneration. Four types of PU/PLLA vascular grafts differing in percent weight of the PU/PLLA mixture, molecular weight of PLLA, and pore size were implanted into the abdominal aorta of rats (n = 32). Six weeks after implantation two implants of each graft type were evaluated by means of scanning electron microscopy and six implants were evaluated by means of light microscopy. In two types of the PU/PLLA vascular grafts, both of which were prepared from a 95%/5% weight PU/PLLA mixture with PLLA of molecular weight 500,000 but which had a different pore size, there was (I) absence of aneurysm formation and maintenance of arterial implant pulsations, (II) regeneration of a complete antithrombogenic neointima, (III) regeneration of a neomedia of comparable thickness to the media of normal rat abdominal aorta with the regeneration of elastic laminae almost throughout its thickness, and (IV) regeneration of a sufficiently supporting neoadventitia. These results demonstrate that a 95%/5% weight PU/PLLA mixture with PLLA of molecular weight 500,000 is the most suitable composition for PU/PLLA vascular grafts to ensure an optimal regeneration of a neoarterial wall that is of sufficient strength, compliance, and thromboresistance to function as a small-caliber arterial substitute. Pore size of these PU/PLLA grafts does not affect regeneration.

Tissue engineering of autologous aorta using a new biodegradable polymer.

BACKGROUND: Ovine pulmonary valve leaflets and pulmonary arteries have been tissue-engineered (TE) from autologous cells and biodegradable polyglycolic acid (PGA)-polyglactin copolymers. Use of this cell-polymer construct in the systemic circulation resulted in aneurysm formation. This study evaluates a TE vascular graft in the systemic circulation which is based on a new copolymer of PGA and polyhydroxyalkanoate (PHA). METHODS: Ovine carotid arteries were harvested, expanded in vitro, and seeded onto 7-mm diameter PHA-PGA tubular scaffolds. The autologous cell-polymer vascular constructs were used to replace 3-4 cm abdominal aortic segments in lambs (group TE, n = 7). In a control group (n = 4), aortic segments were replaced with acellular polymer tubes. Vascular patency was evaluated with echography. All control animals were sacrificed when the grafts became occluded. Animals in TE group were sacrificed at 10 days (n = 1), 3 (n = 3), and 5 months (n = 3). Explanted TE conduits were evaluated for collagen content, deoxyribonucleic acid (DNA) content, structural and ultrastructural examination, mechanical strength, and matrix metalloproteinase (MMP) activity. RESULTS: The 4 control conduits became occluded at 1, 2, 55, and 101 days. All TE grafts remained patent, and no aneurysms developed by the time of sacrifice. There was one mild stenosis at the anastomotic site after 5 months postoperatively. The percent collagen and DNA contents approached the native aorta over time (% collagen = 25.7%+/-3.4 [3 months] vs 99.6%+/-11.7 [5 months], p < 0.05; and % DNA = 30.8%+/-6.0 [3 months] vs 150.5%+/-16.9 [5 months], p < 0.05). Histology demonstrated elastic fibers in the medial layer and endothelial specific von Willebrand factor on the luminal surface. The mechanical strain-stress curve of the TE aorta approached that of the native vessel. A 66 kDa MMP-2 was found in the TE and native aorta but not in control group. CONCLUSIONS: Autologous aortic grafts with biological characteristics resembling the native aorta can be created using TE approach. This may allow the development of "live" vascular grafts.

The role of platelet-derived growth factor in intraluminal stented graft healing.

BACKGROUND: Intraluminally placed polytetrafluoroethylene grafts are associated with enhanced graft endothelialization and diminished intimal hyperplasia when compared with interposition grafts. This study determined the role of platelet-derived growth factor in intraluminal graft healing. STUDY DESIGN: Thirty dogs underwent infrarenal abdominal aorta polytetrafluoroethylene interposition (control, n = 15) or intraluminal stented (n = 15) grafting. Grafts were explanted at 1, 3, and 6 weeks. The percent of graft area endothelialization and intima to media height ratios were calculated. By using protein electrophoresis and the Western blot technique, platelet-derived growth factor, identified by immunolabeling with anti-platelet-derived growth factor antibody, was isolated from proximal, mid-, and distal graft regions and was quantified using densitometry. RESULTS: Graft area endothelialization was 0 +/- 3.3 percent, 2.3 +/- 3.3 percent, and 19.0 +/- 3.3 percent for 1-, 3-, and 6-week controls; and 4.7 +/- 3.7 percent, 30.5 +/- 3.3 percent, and 86.8 +/- 3.3 percent for 1-, 3-, and 6-week stented grafts. Endothelialization was greater in stented grafts at 3 and 6 weeks (p < .01). Proximal anastomosis intima to media height ratios were 1.61 +/- 0.15, 1.54 +/- 0.14, and 1.48 +/- 0.15 for 1-, 3-, and 6-week control grafts, and 0.42 +/- 0.18, 0.41 +/- 0.15, and 0.47 +/- 0.14 for 1-, 3-, and 6-week stented grafts. Similar intima to media height ratio values were present at the distal anastomosis. Lower intima to media height ratios were observed in all stented grafts (p < .01). The platelet-derived growth factor content at 1-, 3-, and 6-weeks was lower in all stented grafts when compared with controls. The content of platelet-derived growth factor was greatest in 3-week controls, with a significant difference noted in the mid-graft region (p < .05). The content of platelet-derived growth factor remained stable in all stented graft regions over 6 weeks. An inverse correlation between stented graft platelet-derived growth factor content and endothelialization (r = -0.43) and a positive correlation with proximal anastomotic intimal hyperplasia (r = 0.73) were identified. CONCLUSIONS: Lower platelet-derived growth factor content is associated with decreased intimal hyperplasia and improved healing in intra-arterial polytetrafluoroethylene grafts.

The development of mechanical strength of surgically anastomosed arteries sutured with Dexon.

The strength of surgically anastomosed arteries of the rat sutured with Dexon thread is studied. The abdominal aortas and the carotid arteries were severed, sutured and then the wounds were closed and the animal healed. After a specific period of time up to 13 months, the vessels were taken out and tested in uniaxial loading condition. The stress-strain relationship of the vessels was measured, and then the vessels were pulled to failure. It was found that the strength of the anastomosis was the lowest in about 4 months. In the first day, the force at failure was about the same as that of the control. Then the strength decreased with time, until a minimum was reached in 4-6 months. The tensile force to failure was about 25% of the control for the carotid artery and 49% of the control for the abdominal aorta. The corresponding values of the tensile stress at failure were 17 and 11%, respectively. The different percentages of forces and stresses were caused by the thickening of the vessel wall in the neighborhood of the suture line in the healing process. After 4-6 months, the strength gained again. At 13 months, the strength of the anastomosis was about the same as that of the control. The stretch ratio at failure was approximately constant through all periods.

The influence of competitive flow on graft patency.

To evaluate the effects of competitive flow, bilateral aorto-iliac ePTFE grafts, 10 cm in length and with an internal diameter of 6 mm, were placed in ten mongrel dogs. On one side the external iliac artery was ligated (the control side); on the opposite side (the experimental side) flow continued through the native aorto-iliac system, as well as the graft, to allow a situation of competitive flow. Flow measurements showed the experimental graft carried 38.6% of the total blood flow going to the ipsilateral external iliac artery. After 30 days the grafts were exposed and patency and flow were evaluated by Doppler ultrasound. The control side showed 60% patency; all of the grafts on the experimental side were occluded. In addition, complete endothelial healing was observed at the occluded ostia of all experimental grafts. We conclude that competitive flow does influence graft patency and that graft thrombosis appears to be a relatively early phenomenon, as evidenced by the endothelial healing which occurred.

Evaluation of the capabilities of a hemoglobin vesicle as an artificial oxygen carrier in a rat exchange transfusion model.

Encapsulation of hemoglobin within a liposome is one of the strategies in the development of artificial oxygen carriers. It maintains the oxygen transporting properties of hemoglobin and, at the same time, eliminates the side effects of cell free hemoglobin. Hemoglobin vesicles (HbV) are a type of liposome encapsulated hemoglobin. They have a particle size of approximately 250 nm, a hemoglobin concentration of 10 g/dl, and the oxygen affinity, P50, is regulated to 32 Torr. In this study the authors examined the oxygen transporting capability of HbV in vivo, by performing exchange transfusions in rats. Exchange transfusion (90% of the estimated circulatory volume) with HbV suspended in 5% albumin (containing 160 mEq/L, sodium and 107 mEq/L, chloride) was carried out in male Wistar rats. Mean arterial pressure and heart rate were monitored through the arterial catheter. Arterial blood samples for gas analyses were also obtained from the arterial catheter. Abdominal aortic blood flow was measured by an ultrasonic pulsed Doppler flowmeter as an indicator of cardiac output. The oxygen tension of blood withdrawn from the right atrium was measured as an indicator of mixed venous oxygen tension. These values were employed to calculate oxygen delivery and consumption. Renal cortical and skeletal muscle tissue oxygen tensions were monitored as indicators of tissue perfusion. Five percent albumin and washed rat red blood cells suspended in 5% albumin containing 10 g/dl of hemoglobin; were employed as controls. At the completion of a 90% exchange transfusion, renal cortical and skeletal muscle tissue oxygen tensions, along with oxygen delivery and consumption, were sustained almost equally well with the HbV suspension compared to the washed rat red blood cell suspension, but declined significantly with the albumin suspension. The results indicate that the oxygen transporting capability of HbV was almost equivalent to that of rat red blood cells.

New experimental protocols for tensile testing of abdominal aortic analogues.

This work proposes an in vitro tensile testing protocol that is able to characterize abdominal aortic (AA) analogues under physiologically inspired mechanical loadings. Kinematic parameters are defined in agreement with in vivo measurements of aortic dynamics. A specific focus is given to the choice of the applied loading rates, deriving from the knowledge of aortic Peterson modulus and blood pressure variations from diastolic to systolic instants. The influence of physiological elongation rates has been tested on both porcine AAs and a thermoplastic polyurethane (TPU) material used to elaborate AA analogues. The diastolic and systolic elongation rates estimates vary between orders of magnitude O(10(-2)) and O(10(-1))s(-1). Negligible differences are obtained when comparing stress-elongation responses between both physiological elongation rates. In contrast, a noticeable stiffening of the TPU mechanical response is observed compared to that obtained under the common low traction rate of O(10(-3))s(-1). This work shows how relevant physiological elongation rates can be evaluated as a function of age, gender and pathological context.

Flow patterns and preferred sites of atherosclerotic lesions in the human aorta - II. Abdominal aorta.

OBJECTIVE: As in Part I, to elucidate the role of fluid mechanical factors in the localized genesis and development of atherosclerotic lesions in man, here in the abdominal aorta. METHODS: Flow patterns and preferred sites of atherosclerotic lesions in the aorta were studied in detail using the same isolated transparent aortic trees prepared from humans postmortem and the flow visualization and cinemicrographic techniques as in Part I. RESULTS: Under steady flow simulating mid-systole, the flow was found to be disturbed at the aorto-celiac and aorto-superior mesenteric artery junctions by the formation of complex secondary and adverse flows along the lateral and posterior walls of the abdominal aorta. More complex secondary and adverse flows formed at the branching sites of the left and right renal arteries. Furthermore, considerable interactions occurred between the secondary and adverse flows formed at the branching sites of the above four arteries, resulting in the formation of a large and long recirculation zone along the lateral and posterior walls of the abdominal aorta corresponding to these branches. The velocity profile was almost flattened throughout the entire length of the descending aorta. CONCLUSIONS: Atherosclerotic lesions were found mainly at the posterior and lateral walls of the abdominal aorta where slow adverse and recirculation flows formed and where wall shear stress was low.