History of (micro) vascular surgery and the development of small-caliber blood vessel prostheses (with some notes on patency rates and re-endothelialization)

Current Strategies for Engineered Vascular Grafts and Vascularized Tissue Engineering

Blood vessels not only transport oxygen and nutrients to each organ, but also play an important role in the regulation of tissue regeneration. Impaired or occluded vessels can result in ischemia, tissue necrosis, or even life-threatening events. Bioengineered vascular grafts have become a promising alternative treatment for damaged or occlusive vessels. Large-scale tubular grafts, which can match arteries, arterioles, and venules, as well as meso- and microscale vasculature to alleviate ischemia or prevascularized engineered tissues, have been developed. In this review, materials and techniques for engineering tubular scaffolds and vasculature at all levels are discussed. Examples of vascularized tissue engineering in bone, peripheral nerves, and the heart are also provided. Finally, the current challenges are discussed and the perspectives on future developments in biofunctional engineered vessels are delineated.

Non-destructive 3D characterization of the blood vessel wall microstructure in different species and blood vessel types using contrast-enhanced microCT and comparison with synthetic vascular grafts

To improve the current treatment for vascular diseases, such as vascular grafts, intravascular stents, and balloon angioplasty intervention, the evaluation of the native blood vessel microstructure in full 3D could be beneficial. For this purpose, we used contrast-enhanced X-ray microfocus computed tomography (CECT): a combination of X-ray microfocus computed tomography (microCT) and contrast-enhancing staining agents (CESAs) containing high atomic number elements. In this work, we performed a comparative study based on staining time and contrast-enhancement of 2 CESAs: Monolacunary and 1:2 Hafnium-substituted Wells-Dawson polyoxometalate (Mono-WD POM and Hf-WD POM, respectively) for imaging of the porcine aorta. After showing the advantages of Hf-WD POM in terms of contrast enhancement, we expanded our imaging to other species (rat, porcine, and human) and other types of blood vessels (porcine aorta, femoral artery, and vena cava), clearly indicating microstructural differences between different types of blood vessels and different species. We then showed the possibility to extract useful 3D quantitative information from the rat and porcine aortic wall, potentially to be used for computational modeling or for future design optimization of graft materials. Finally, a structural comparison with existing synthetic vascular grafts was made. This information will allow to better understand the in vivo functioning of native blood vessels and to improve the current disease treatments. STATEMENT OF SIGNIFICANCE: Synthetic vascular grafts, used as treatment for some cardiovascular diseases, still often fail clinically, potentially because of a mismatch in mechanical behaviour between the native blood vessel and the graft. To better understand the causes of this mismatch, we studied the full 3D microstructure of blood vessels. For this, we identified Hafnium-substituted Wells-Dawson polyoxometalate as contrast-enhancing staining agent to perform contrast-enhanced X-ray microfocus computed tomography. This technique allowed to show important differences in the microstructure of different types of blood vessels and in different species, as well as with that of synthetic grafts. This information can lead to a better understanding of the functioning of blood vessels and will allow to improve current disease treatments, such as vascular grafts.

Acellular Tissue-Engineered Vascular Grafts from Polymers: Methods, Achievements, Characterization, and Challenges

Extensive and permanent damage to the vasculature leading to different pathogenesis calls for developing innovative therapeutics, including drugs, medical devices, and cell therapies. Innovative strategies to engineer bioartificial/biomimetic vessels have been extensively exploited as an effective replacement for vessels that have seriously malfunctioned. However, further studies in polymer chemistry, additive manufacturing, and rapid prototyping are required to generate highly engineered vascular segments that can be effectively integrated into the existing vasculature of patients. One recently developed approach involves designing and fabricating acellular vessel equivalents from novel polymeric materials. This review aims to assess the design criteria, engineering factors, and innovative approaches for the fabrication and characterization of biomimetic macro- and micro-scale vessels. At the same time, the engineering correlation between the physical properties of the polymer and biological functionalities of multiscale acellular vascular segments are thoroughly elucidated. Moreover, several emerging characterization techniques for probing the mechanical properties of tissue-engineered vascular grafts are revealed. Finally, significant challenges to the clinical transformation of the highly promising engineered vessels derived from polymers are identified, and unique perspectives on future research directions are presented.

Applications of Bacterial Cellulose as a Natural Polymer in Tissue Engineering

Choosing the material with the best regeneration potential and properties closest to that of the extracellular matrix is one of the main challenges in tissue engineering and regenerative medicine. Natural polymers, such as collagen, elastin, and cellulose, are widely used for this purpose in tissue engineering. Cellulose derived from bacteria has excellent mechanical properties, high hydrophilicity, crystallinity, and a high degree of polymerization and, therefore, can be used as scaffold/membrane for tissue engineering. In the current study, we reviewed the latest trends in the application of bacterial cellulose (BC) polymers as a scaffold in different types of tissue, including bone, vascular, skin, and cartilage. Also, we mentioned the biological and mechanical advantages and disadvantages of BC polymers. Given the data presented in this study, BC polymer could be suggested as a favorable natural polymer in the design of tissue scaffolds. Implementing novel composites that combine this polymer with other materials through modern or rapid prototyping methods can open up a great prospect in the future of tissue engineering and regenerative medicine.

Advancing tissue-engineered vascular grafts via their endothelialization and mechanical conditioning

Tissue engineering has garnered significant attention for its potential to address the predominant modes of failure of small diameter vascular prostheses, namely mid-graft thrombosis and anastomotic intimal hyperplasia. In this review, we described two main features underpinning the promise of tissue-engineered vascular grafts: the incorporation of an antithrombogenic endothelium, and the generation of a structurally and biomechanically mimetic extracellular matrix. From the early attempts at the in-vitro endothelialization of vascular prostheses in the 1970s through to the ongoing clinical trials of fully tissue-engineered vascular grafts, the historical advancements and unresolved challenges that characterize the current state-of-the-art are summarized in a manner that establishes a guide for the development of an effective vascular prosthesis for small diameter arterial reconstruction. The importance of endothelial cell purity and their arterial specification for the prevention of both diffuse neointimal hyperplasia and the accelerated development of atherosclerotic lesions is delineated. Additionally, the need for an extracellular matrix that recapitulates both the composition and structure of native elastic arteries to facilitate the protracted stability and patency of an engineered vasoactive conduit is described. Finally, the capacity of alternative sources of cells and mechanical conditioning to overcome these technical barriers to the clinical translation of an effective small diameter vascular prosthesis is discussed. In conclusion, this review provides an overview of the historical development of tissue-engineered vascular grafts, highlighting specific areas warranting further research, and commentating on the outlook of a clinically feasible and therapeutically efficacious vascular prosthesis for small diameter arterial reconstruction.

Artificial small-diameter blood vessels: materials, fabrication, surface modification, mechanical properties, and bioactive functionalities

Cardiovascular diseases, especially ones involving narrowed or blocked blood vessels with diameters smaller than 6 millimeters, are the leading cause of death globally. Vascular grafts have been used in bypass surgery to replace damaged native blood vessels for treating severe cardio- and peripheral vascular diseases. However, autologous replacement grafts are not often available due to prior harvesting or the patient's health. Furthermore, autologous harvesting causes secondary injury to the patient at the harvest site. Therefore, artificial blood vessels have been widely investigated in the last several decades. In this review, the progress and potential outlook of small-diameter blood vessels (SDBVs) engineered in vitro are highlighted and summarized, including material selection and development, fabrication techniques, surface modification, mechanical properties, and bioactive functionalities. Several kinds of natural and synthetic polymers for artificial SDBVs are presented here. Commonly used fabrication techniques, such as extrusion and expansion, electrospinning, thermally induced phase separation (TIPS), braiding, 3D printing, hydrogel tubing, gas foaming, and a combination of these methods, are analyzed and compared. Different surface modification methods, such as physical immobilization, surface adsorption, plasma treatment, and chemical immobilization, are investigated and are compared here as well. Mechanical requirements of SDBVs are also reviewed for long-term service. In vitro biological functions of artificial blood vessels, including oxygen consumption, nitric oxide (NO) production, shear stress response, leukocyte adhesion, and anticoagulation, are also discussed. Finally, we draw conclusions regarding current challenges and attempts to identify future directions for the optimal combination of materials, fabrication methods, surface modifications, and biofunctionalities. We hope that this review can assist with the design, fabrication, and application of SDBVs engineered in vitro and promote future advancements in this emerging research field.

Electrospinning of small diameter 3-D nanofibrous tubular scaffolds with controllable nanofiber orientations for vascular grafts

The control of nanofiber orientation in nanofibrous tubular scaffolds can benefit the cell responses along specific directions. For small diameter tubular scaffolds, however, it becomes difficult to engineer nanofiber orientation. This paper reports a novel electrospinning technique for the fabrication of 3-D nanofibrous tubular scaffolds with controllable nanofiber orientations. Synthetic absorbable poly-ε-caprolactone (PCL) was used as the model biomaterial to demonstrate this new electrospinning technique. Electrospun 3-D PCL nanofibrous tubular scaffolds of 4.5 mm in diameter with different nanofiber orientations (viz. circumferential, axial, and combinations of circumferential and axial directions) were successfully fabricated. The degree of nanofiber alignment in the electrospun 3-D tubular scaffolds was quantified by using the fast Fourier transform (FFT) analysis. The results indicated that excellent circumferential nanofiber alignment could be achieved in the 3-D nanofibrous PCL tubular scaffolds. The nanofibrous tubular scaffolds with oriented nanofibers had not only directional mechanical property but also could facilitate the orientation of the endothelial cell attachment on the fibers. Multiple layers of aligned nanofibers in different orientations can produce 3-D nanofibrous tubular scaffolds of different macroscopic properties.

Macromolecular biomaterials for scaffold-based vascular tissue engineering

Cardiovascular diseases are increasingly becoming the main cause of death all over the world, which has led to an increase in the economic and social burden of such diseases. Vascular tissue engineering (VTE) is providing a route towards interesting applications, mainly focussing on the in vitro, in vivo, or combined in vitro/in vivo regeneration of small-diameter blood vessels (<6 mm) for coronary or peripheral vascular substitutions. Although different approaches have been investigated in the past two decades to achieve this aim, the most common method uses a macromolecular-based structure to scaffold cells during the regeneration process. Therefore, the aim of this work is to comprehensively review macromolecular biomaterials that were designed, developed, fabricated, and tested for scaffolding VTE. In an effort to provide a comprehensive overview, this review will mainly focus on the mechanical properties of the construct and its biological performance that results from the scaffold colonization during cell growth.

UV surface modification of a new nanocomposite polymer to improve cytocompatibility

A novel modified nanocomposite was studied for the adhesion and proliferation of the human umbilical vein endothelial cell (HUVEC) line EA.hy926. The nanocomposite under investigation was poly(carbonate-urea)urethane with silsesquioxane nano-cages, here in the form of a mixture of two polyhedral oligomeric silsesquioxanes. The nanocomposite surfaces were exposed to ultraviolet (UV) light of a Xe(*)(2)-excimer lamp at a wavelength of 172 nm in an ammonia atmosphere. The effects of the irradiation were characterized by atomic force and scanning electron microscopy (AFM, SEM), X-ray photo-electron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FT-IR) using an attenuated total reflection (ATR) device and measurements of advancing water contact angle (CA). The irradiation resulted in the introduction of new hydrophilic N- and O-containing groups into the surface, which was initially amphiphilic, while surface morphology remained mainly unchanged. Slight chemical changes were also observed for the silsesquioxane nano-cages at the surface. Onto the untreated and irradiated samples HUVECs were seeded and grown for various durations in culture. Standard tissue-culture polystyrene (PS) was employed as a positive control to check the efficiency of the cell-culture methods. Viability and proliferation of the cells were then assessed using a non-radioactive assay. Compared to the untreated nanocomposite polymer, irradiation times of at least 5 min resulted in a significantly increased cell proliferation between 3 and 8 days after seeding with the HUVEC line EA.hy926.

A sandwich tubular scaffold derived from chitosan for blood vessel tissue engineering

Many materials have been investigated in blood vessel tissue engineering, such as PGA, PLGA, P4HB. However, chitosan is not mentioned in the arena. This study aimed to develop a chitosan-based tubular scaffold and examine its feasibility of being applied in this field. Briefly, a knitted chitosan tube was dipped into chitosan solution (2%, w/v) and dried, then its inner and outer surface was mantled with a layer of chitosan/gelatin (4:1, w/w) complex solution, and then freeze-dehydrated. In vitro characterization showed that the scaffold had a wall of 1.0 mm in thickness with a sandwich structure, and a porosity of 81.2%. The pore diameter was 50-150 microm and could be regulated by varying freezing conditions. The scaffold possessed proper swelling property, burst strength of almost 4000 mmHg, and high suture-retention strength. After degradation for 2 months, the scaffold could maintain enough mechanical strength with an average mass loss of 18.7%. Vascular smooth muscle cells could spread and grow very well on the scaffold. This study provided a novel method to fabricate chitosan and its complex into a tubular scaffold and demonstrated the feasibility of the scaffold employed in the field of blood vessel tissue engineering.

Mechanical properties of bacterial cellulose and interactions with smooth muscle cells

Tissue engineered blood vessels (TEBV) represent an attractive approach for overcoming reconstructive problems associated with vascular diseases by providing small calibre vascular grafts. The aim of this study has been to evaluate a novel biomaterial, bacterial cellulose (BC), as a potential scaffold for TEBV. The morphology of the BC pellicle grown in static culture was investigated with SEM. Mechanical properties of BC were measured in Krebs solution and compared with the properties of porcine carotid arteries and ePTFE grafts. Attachment, proliferation and ingrowth of human smooth muscle cells (SMC) on the BC were analysed in vitro. The BC pellicle had an asymmetric structure composed of a fine network of nanofibrils similar to a collagen network. The shape of the stress-strain response of BC is reminiscent of the stress-strain response of the carotid artery, most probably due to the similarity in architecture of the nanofibrill networks. SMC adhered to and proliferated on the BC pellicle; an ingrowth of up to 40 microm was seen after 2 weeks of culture. BC exhibit attractive properties for use in future TEBV.

[Microsurgery: History of instrumental vascular anastomoses, our experience with eversion-stapling using VCS forceps]

One century, after Carrel in 1906, technics of vascular surgery are the same. After two world wars, peace surgery has been improved by war surgery. Microscopy surgery gave a new way for vascular surgery which became microsurgery with specific instrumentation. We have move from the developing period of microsurgery in the 1970s, to the fully matured period of microsurgery in the 1980s and the the development of clinical free flaps. The 1990s must be the turning point from autogenous tissue transplantation to allogenic transplantation. Ethic comity keeps keys of future! About microvascular anastomoses, many instrumental technics are explored but no-one is better than the classic manual suture. For us, the best instrumental technic is the anastomose with titanium clips VCS((R)) but we only use it in good situation without difficulties.

Manufacture of elastic biodegradable PLCL scaffolds for mechano-active vascular tissue engineering

A soft and very elastic poly(lactide-co-epsilon-caprolactone) (PLCL)(50:50, Mn 185 x 10(3)) was synthesized. Tubular scaffolds were prepared by an extrusion-particulate leaching method for mechano-active vascular tissue engineering. The copolymer was very flexible but completely rubber-like elastic. Even the high porous PLCL scaffolds (90% salt wt) exhibited 200% elongation, but recovery over 85% in a tensile test. Moreover, the PLCL scaffolds maintained their high elasticity also in culture media under cyclic mechanical strain conditions. The highly porous scaffold (90% salt wt) withstood for an initial 1 week without any deformation and sustained for 2 weeks in culture media under cyclic stress of 10% amplitude and at 1 Hz frequency which are similar to the natural vascular conditions. Vascular smooth muscle cells (VSMCs) were seeded on to the PLCL scaffolds. The cell adhesion and proliferation on the scaffolds of various pore-size were increased with increasing pore size. For the pore sizes of 50-100 microm, 100-150 microm, 150-200 microm and 200-250 microm, the ratios of cell numbers were about 1:1.2:1.9:2.2, respectively, at both 12 h and 5 days. Similarly, the higher porous scaffolds exhibited more cell adhesion and proliferation compared to lower porous one, where the effect was more pronounced in the longer proliferation period. SMC-seeded scaffolds were implanted subcutaneously in athymic nude mice to confirm the biocompatibility. Such a high elastic property and proper biocompatibility to SMCs of PLCL scaffolds prepared in this study will be very useful to engineer SM-containing tissues such as blood vessels under mechanically dynamic environments (mechano-active tissue engineering).

Clinical experience with non-penetrating vascular clips in free-flap reconstructions

To date, the gold standard for performing a microvascular anastomosis has been the penetrating suture with attached needle. During the last two decades, non-penetrating techniques have been introduced, including the Unilink system for end-to-end anastomoses, and the VCS clip-applier system for both end-to-end and end-to-side anastomoses. The aim of this study was to compare the results of different techniques used to create microvascular anastomoses in free-flap reconstructions. Between January 1995 and October 1999, we performed 474 microvascular anastomoses in 216 consecutive free-tissue transfers. The anastomosis techniques included manual sutures (42%), Unilink rings (34%) and VCS clips (24%). Seven combined sutured-clipped anastomoses were excluded from further analysis. The mean anastomotic time when rings were applied was significantly shorter than when using clips (P 0.0001) or sutures (P 0.0001). Venous anastomoses using clips took less time than those using sutures (P 0.05). There were 19 anastomotic failures, all of which lead to early flap failure. Ten flaps were salvaged by early reoperation; nine flaps were lost. Three more flaps were lost as a result of other causes, bringing the flap survival rate down to 94.4%. Early flap failure was caused by failure of the arterial anastomosis in eight cases; all of them were sutured (these represented 5% of all arterial anastomoses with sutures). None of the clipped arterial anastomoses failed. Early flap failure was caused by failure of the venous anastomosis in 11 patients. Three of these anastomoses were sutured (representing 6% of all venous anastomoses with sutures), seven were anastomosed with rings (representing 5% of all venous anastomoses with rings) and one was clipped (representing 2% of all venous anastomoses with clips). Both the VCS clip-applier system and the Unilink system are easy to handle and allow fast microvascular anastomoses without intraluminal penetration. The patency rate of clipped vessels is at least as good as the patency rates of vessels anastomosed using sutures or rings.

Improving vascular grafts: the importance of mechanical and haemodynamic properties

In the last 40 years, as techniques and materials have improved, the success rate of vascular prostheses with a diameter greater than 6mm has risen steadily, 5-year survival rates exceeding 95% in most centres. With smaller grafts no comparable improvement has occurred, the majority failing within 5 years, usually as a result of intimal hyperplasia and, ultimately atherosclerosis, in and around the downstream anastomosis. Clinical evidence suggests that the patency rates of small grafts are improved by matching the elastic properties of the graft to that of the artery into which it is placed. Although there is little reliable evidence that 'elastic mismatch' per se is the cause of intimal hyperplasia, it is generally accepted that mechanical factors are important in its genesis. These include disturbed flow at the anastomosis leading to fluctuations in shear stress at the endothelium (a known cause of intimal hyperplasia in normal arteries), injury due to suturing and stress concentration at the anastomosis. Few suitable materials or techniques have yet been developed to improve the long-term survival rates of small grafts. Recent advances in tissue engineering in which prostheses are manufactured by culturing vascular smooth muscle cells on a tubular scaffold of biodegradable polymer may ultimately make it possible to manufacture biologically and haemodynamically compatible grafts with diameters as small as 1mm.

Influence of luminal pore size on the patency rate and endothelialization of polymeric microvenous prostheses

In microvenous prosthetic surgery a continuous search for better patency rates is necessary to enable a clinical application. In this search for better patencies, modifications in the wall structure are being made. Directions found in the literature suggest that pore size plays an important role in achieving better patencies. Thus far, no study has been conducted to evaluate the influence of pore size on the patency rate of polyurethane microvenous prostheses. Since polyurethane is known to yield good patency rates, we conducted this study in which we compared different luminal pore sizes with regard to patency. Pore size varied from 0.6 to 20 microns in microvenous polyurethane-based prostheses (length 5-6 mm, internal diameter 1 mm). The results showed a favorable patency rate in the pore sizes larger than 5.0 microns (patency 75%) when compared to pore sizes smaller than 2.0 microns (patency 50%). This study demonstrates that microvenous polyurethane-based prostheses with a luminal pore size larger than 5.0 microns may yield better patency rates than prostheses with a luminal pore size smaller than 5.0 microns. Further studies are currently being performed to elucidate the very reasons for this effect.

Peripheral hemodynamic stability during prolonged anesthesia in the rat

The present study tests the hypothesis that peripheral hemodynamics in the rat femoral artery remain relatively stable during 3 hours of general anesthesia. Hemodynamic variables (pulse period, maximal centerline velocity, lumen diameter, and mean volumetric flow) were measured by the 20 MHz-pulsed ultrasonic Doppler velocimeter method at 30-minute intervals. The hemodynamic variables were not statistically different during the duration of the study. The mean values were pulse period 160 +/- 2 msec, maximal centerline velocity 37.0 +/- 1.9 cm/sec, lumen diameter 0.94 +/- 0.03 mm, and mean volumetric flow 1.92 +/- 0.19 cc/min.

Hemodynamics after autogenous, interpositional grafting in small arteries

This investigation initiates the quantitative, hemodynamic assessment of interpositional grafts in small rat arteries. The left femoral arteries in 16 Sprague-Dawley rats were transected at two levels, and were then repaired using interrupted suturing technique. This effectively provided an ideally sized and histologically matched interposition graft. The 20-MHz PUDVM method was used to measure blood velocities, and derive values for vessel lumen area, temporal mean of the spatial mean velocity (Vsm), and volumetric flows. Measurements were completed distal to the interposed graft. Variables were quantitated in the preoperative and at the 5-, 15-, and 30-minute postoperative intervals. Statistical analysis of data indicated that the interpositional grafting procedure resulted in increased vessel lumen area and decreased Vsm, but, importantly, volumetric flow (about 8.20 ml/min) remained unchanged. This study demonstrated that, although hemodynamic characteristics are altered, volumetric flow can be restored after an experimental, interpositional grafting procedure in small arteries.

Sodium hyaluronate as an aid in microvascular surgery

The purpose of this study was to compare the suture of a microvascular anastomosis with and without the aid of sodium hyaluronate. The divided femoral arteries of ten Sprague-Dawley rats were sutured using sodium hyaluronate on one side. Operating time, bleeding, and patency rates were studied and compared. No significant differences were found in the measured parameters. However, the clinical impression is that the use of sodium hyaluronate facilitates the suture of a microanastomosis.