Development of a hybrid cardiovascular graft using a tissue engineering approach

Fabrication and Characterization of an Electro-Compacted Collagen/Elastin/Hyaluronic Acid Sheet as a Potential Skin Scaffold

The development of biomimetic structures with integrated extracellular matrix (ECM) components represents a promising approach to biomaterial fabrication. Here, an artificial ECM, comprising the structural protein collagen I and elastin (ELN), as well as the glycosaminoglycan hyaluronan (HA), is reported. Specifically, collagen and ELN are electrochemically aligned to mimic the compositional characteristics of the dermal matrix. HA is incorporated into the electro-compacted collagen-ELN matrices via adsorption and chemical immobilization, to give a final composition of collagen/ELN/HA of 7:2:1. This produces a final collagen/ELN/hyaluronic acid scaffold (CEH) that recapitulates the compositional feature of the native skin ECM. This study analyzes the effect of CEH composition on the cultivation of human dermal fibroblast cells (HDFs) and immortalized human keratinocytes (HaCaTs). It is shown that the CEH scaffold supports dermal regeneration by promoting HDFs proliferation, ECM deposition, and differentiation into myofibroblasts. The CEH scaffolds are also shown to support epidermis growth by supporting HaCaTs proliferation, differentiation, and stratification. A double-layered epidermal-dermal structure is constructed on the CEH scaffold, further demonstrating its ability in supporting skin cell function and skin regeneration.

Design and computational optimization of compliance-matching aortic grafts

Introduction: Synthetic vascular grafts have been widely used in clinical practice for aortic replacement surgery. Despite their high rates of surgical success, they remain significantly less compliant than the native aorta, resulting in a phenomenon called compliance mismatch. This incompatibility of elastic properties may cause serious post-operative complications, including hypertension and myocardial hypertrophy. Methods: To mitigate the risk for these complications, we designed a multi-layer compliance-matching stent-graft, that we optimized computationally using finite element analysis, and subsequently evaluated in vitro. Results: We found that our compliance-matching grafts attained the distensibility of healthy human aortas, including those of young adults, thereby significantly exceeding the distensibility of gold-standard grafts. The compliant grafts maintained their properties in a wide range of conditions that are expected after the implantation. Furthermore, the computational model predicted the graft radius with enough accuracy to allow computational optimization to be performed effectively. Conclusion: Compliance-matching grafts may offer a valuable improvement over existing prostheses and they could potentially mitigate the risk for post-operative complications attributed to excessive graft stiffness.

Glycosaminoglycans: From Vascular Physiology to Tissue Engineering Applications

Cardiovascular diseases represent the number one cause of death globally, with atherosclerosis a major contributor. Despite the clinical need for functional arterial substitutes, success has been limited to arterial replacements of large-caliber vessels (diameter > 6 mm), leaving the bulk of demand unmet. In this respect, one of the most challenging goals in tissue engineering is to design a "bioactive" resorbable scaffold, analogous to the natural extracellular matrix (ECM), able to guide the process of vascular tissue regeneration. Besides adequate mechanical properties to sustain the hemodynamic flow forces, scaffold's properties should include biocompatibility, controlled biodegradability with non-toxic products, low inflammatory/thrombotic potential, porosity, and a specific combination of molecular signals allowing vascular cells to attach, proliferate and synthesize their own ECM. Different fabrication methods, such as phase separation, self-assembly and electrospinning are currently used to obtain nanofibrous scaffolds with a well-organized architecture and mechanical properties suitable for vascular tissue regeneration. However, several studies have shown that naked scaffolds, although fabricated with biocompatible polymers, represent a poor substrate to be populated by vascular cells. In this respect, surface functionalization with bioactive natural molecules, such as collagen, elastin, fibrinogen, silk fibroin, alginate, chitosan, dextran, glycosaminoglycans (GAGs), and growth factors has proven to be effective. GAGs are complex anionic unbranched heteropolysaccharides that represent major structural and functional ECM components of connective tissues. GAGs are very heterogeneous in terms of type of repeating disaccharide unit, relative molecular mass, charge density, degree and pattern of sulfation, degree of epimerization and physicochemical properties. These molecules participate in a number of vascular events such as the regulation of vascular permeability, lipid metabolism, hemostasis, and thrombosis, but also interact with vascular cells, growth factors, and cytokines to modulate cell adhesion, migration, and proliferation. The primary goal of this review is to perform a critical analysis of the last twenty-years of literature in which GAGs have been used as molecular cues, able to guide the processes leading to correct endothelialization and neo-artery formation, as well as to provide readers with an overall picture of their potential as functional molecules for small-diameter vascular regeneration.

Hierarchical biofabrication of biomimetic collagen-elastin vascular grafts with controllable properties via lyophilisation

This article describes the development of a hierarchical biofabrication technique suitable to create large but complex structures, such as vascular mimicking grafts, using facile lyophilisation technology amenable to multiple other biomaterial classes. The combination of three fabrication techniques together, namely solvent evaporation, lyophilisation, and crosslinking together allows highly tailorable structures from the microstructure up to the macrostructure, and with the ability to independently crosslink each layer it allows great flexibility to match desired native mechanical properties independently of the micro/macrostructure. We have demonstrated the flexibility of this biofabrication technique by independently optimising each of the layers to create a multi-layered arterial structure with tailored architectural and biophysical/biochemical properties using a collagen-elastin composite. Taken together, the facile biofabrication methodology developed has led to the development of a biomimetic bilayered scaffold suitable for use as a tissue engineered vascular graft (for haemodialysis access or peripheral/coronary bypass), or as an in vitro test platform to examine disease progression, pharmacological toxicity, or cardiovascular medical device testing. STATEMENT OF SIGNIFICANCE: The ability to grow large complex tissues such as blood vessels for transplantation is often hampered by the limitations of the selected biofabrication technique. Here, we sought to overcome some of the fabrication limitations for naturally occurring cardiovascular polymers (collagen/elastin) via a hierarchical approach to fabrication where each layer is built upon the previous. This approach enabled the flexibility to modify and tailor each layer's properties independently via control over polymer concentration, microstructure, and crosslinking. This simple approach facilitated us to fabricate multi-layered vascular grafts which were remodelled into high-density vascular tissue after 21-days. The fabrication approach could be translated to a myriad of other tissues while the engineered vascular graft could also be used as a test platform for drugs/medical devices or as a tissue engineering scaffold for vascular grafting for different indications.

3D printing and characterization of a soft and biostable elastomer with high flexibility and strength for biomedical applications

Recent advancements in 3D printing have revolutionized biomedical engineering by enabling the manufacture of complex and functional devices in a low-cost, customizable, and small-batch fabrication manner. Soft elastomers are particularly important for biomedical applications because they can provide similar mechanical properties as tissues with improved biocompatibility. However, there are very few biocompatible elastomers with 3D printability, and little is known about the material properties of biocompatible 3D printable elastomers. Here, we report a new framework to 3D print a soft, biocompatible, and biostable polycarbonate-based urethane silicone (PCU-Sil) with minimal defects. We systematically characterize the rheological and thermal properties of the material to guide the 3D printing process and have determined a range of processing conditions. Optimal printing parameters such as printing speed, temperature, and layer height are determined via parametric studies aimed at minimizing porosity while maximizing the geometric accuracy of the 3D-printed samples as evaluated via micro-CT. We also characterize the mechanical properties of the 3D-printed structures under quasistatic and cyclic loading, degradation behavior and biocompatibility. The 3D-printed materials show a Young's modulus of 6.9 ± 0.85 MPa and a failure strain of 457 ± 37.7% while exhibiting good cell viability. Finally, compliant and free-standing structures including a patient-specific heart model and a bifurcating arterial structure are printed to demonstrate the versatility of the 3D-printed material. We anticipate that the 3D printing framework presented in this work will open up new possibilities not only for PCU-Sil, but also for other soft, biocompatible and thermoplastic polymers in various biomedical applications requiring high flexibility and strength combined with biocompatibility, such as vascular implants, heart valves, and catheters.

Chitosan/hyaluronic acid multilayer films are biocompatible substrate for Wharton's jelly derived stem cells

Background

Discovery of mesenchymal stem cells (MSCs) in various adult human tissues opened the way to new therapeutic strategies involving tissue engineering from these cells. More recently, vascular substitutes have opened the era of vascular engineering by making replacement vessels from purely biological material. The objective of our study was to create a vascular substitute from MSCs using a multilayer polyelectrolyte film based on natural polymers (Chitosan and Hyaluronic Acid).

Methods

Biocompatibility and cellular behavior were evaluated in this study using MSCs from the Wharton's jelly (WJ) of human umbilical cords. WJ-MSCs adherence was assessed and cells morphology was investigated by Scanning Electron Microscopy (SEM) and actin visualization (Phalloidin).

Results

The number of WJ-MSCs seeded on the (CHI/HA)₁₀ films was greater than the number of cells seeded on the collagen, as the spectrophotometric measurement showed a large cell proliferation on (CHI/HA)₁₀ in comparison with collagen. After adhesion, WJ-MSCs showed a fibroblastic morphology on CHI/HA as for control (collagen I). These results were confirmed by cytoskeleton staining.

Conclusions

The biocompatibility of WJ-MSCs and (CHI/HA)₁₀ showed the possibility to combine the use of WJ-MSCs and (CHI/HA)₁₀ films in vascular tissue engineering.

Biomaterial-Based Approaches to Address Vein Graft and Hemodialysis Access Failures

Veins used as grafts in heart bypass or as access points in hemodialysis exhibit high failure rates, thereby causing significant morbidity and mortality for patients. Interventional or revisional surgeries required to correct these failures have been met with limited success and exorbitant costs, particularly for the US Centers for Medicare & Medicaid Services. Vein stenosis or occlusion leading to failure is primarily the result of neointimal hyperplasia. Systemic therapies have achieved little long-term success, indicating the need for more localized, sustained, biomaterial-based solutions. Numerous studies have demonstrated the ability of external stents to reduce neointimal hyperplasia. However, successful results from animal models have failed to translate to the clinic thus far, and no external stent is currently approved for use in the US to prevent vein graft or hemodialysis access failures. This review discusses current progress in the field, design considerations, and future perspectives for biomaterial-based external stents. More comparative studies iteratively modulating biomaterial and biomaterial-drug approaches are critical in addressing mechanistic knowledge gaps associated with external stent application to the arteriovenous environment. Addressing these gaps will ultimately lead to more viable solutions that prevent vein graft and hemodialysis access failures.

A Material Conferring Hemocompatibility

There is a need for biomimetic materials for use in blood-contacting devices. Blood contacting surfaces maintain their patency through physico-chemical properties of a functional endothelium. A poly(carbonate-urea) urethane (PCU) is used as a base material to examine the feasibility of L-Arginine methyl ester (L-AME) functionalized material for use in implants and coatings. The study hypothesizes that L-AME, incorporated into PCU, functions as a bioactive porogen, releasing upon contact with blood to interact with endothelial nitric oxide synthase (eNOS) present in blood. Endothelial progenitor cells (EPC) were successfully cultured on L-AME functionalized material, indicating that L-AME -increases cell viability. L-AME functionalized material potentially has broad applications in blood-contacting medical devices, as well as various other applications requiring endogenous up-regulation of nitric oxide, such as wound healing. This study presents an in-vitro investigation to demonstrate the novel anti-thrombogenic properties of L-AME, when in solution and when present within a polyurethane-based polymer.

Blood-Vessel Mimicking Structures by Stereolithographic Fabrication of Small Porous Tubes Using Cytocompatible Polyacrylate Elastomers, Biofunctionalization and Endothelialization

Blood vessel reconstruction is still an elusive goal for the development of in vitro models as well as artificial vascular grafts. In this study, we used a novel photo-curable cytocompatible polyacrylate material (PA) for freeform generation of synthetic vessels. We applied stereolithography for the fabrication of arbitrary 3D tubular structures with total dimensions in the centimeter range, 300 µm wall thickness, inner diameters of 1 to 2 mm and defined pores with a constant diameter of approximately 100 µm or 200 µm. We established a rinsing protocol to remove remaining cytotoxic substances from the photo-cured PA and applied thio-modified heparin and RGDC-peptides to functionalize the PA surface for enhanced endothelial cell adhesion. A rotating seeding procedure was introduced to ensure homogenous endothelial monolayer formation at the inner luminal tube wall. We showed that endothelial cells stayed viable and adherent and aligned along the medium flow under fluid-flow conditions comparable to native capillaries. The combined technology approach comprising of freeform additive manufacturing (AM), biomimetic design, cytocompatible materials which are applicable to AM, and biofunctionalization of AM constructs has been introduced as BioRap(®) technology by the authors.

Fabrication and characterisation of biomimetic, electrospun gelatin fibre scaffolds for tunica media-equivalent, tissue engineered vascular grafts

It is increasingly recognised that biomimetic, natural polymers mimicking the extracellular matrix (ECM) have low thrombogenicity and functional motifs that regulate cell-matrix interactions, with these factors being critical for tissue engineered vascular grafts especially grafts of small diameter. Gelatin constitutes a low cost substitute of soluble collagen but gelatin scaffolds so far have shown generally low strength and suture retention strength. In this study, we have devised the fabrication of novel, electrospun, multilayer, gelatin fibre scaffolds, with controlled fibre layer orientation, and optimised gelatin crosslinking to achieve not only compliance equivalent to that of coronary artery but also for the first time strength of the wet tubular acellular scaffold (swollen with absorbed water) same as that of the tunica media of coronary artery in both circumferential and axial directions. Most importantly, for the first time for natural scaffolds and in particular gelatin, high suture retention strength was achieved in the range of 1.8-1.94 N for wet acellular scaffolds, same or better than that for fresh saphenous vein. The study presents the investigations to relate the electrospinning process parameters to the microstructural parameters of the scaffold, which are further related to the mechanical performance data of wet, crosslinked, electrospun scaffolds in both circumferential and axial tubular directions. The scaffolds exhibited excellent performance in human smooth muscle cell (SMC) proliferation, with SMCs seeded on the top surface adhering, elongating and aligning along the local fibres, migrating through the scaffold thickness and populating a transverse distance of 186 μm and 240 μm 9 days post-seeding for scaffolds of initial dry porosity of 74 and 83%, respectively.

Smooth muscle tissue engineering in crosslinked electrospun gelatin scaffolds

Crosslinked, multi-layer electrospun gelatin fiber scaffolds with generally ±45 degree fiber orientation have been used to grow human umbilical vein smooth muscle cells (HUVSMCs) to create a vascular tunica media graft. Scaffolds of different fiber diameter (2-5 μm in wet state), pore size, and porosity (16-21% in wet state) were assessed in terms of cell adherence and viability, cell proliferation, and migration in both in-plane and transverse directions through the scaffold as a function of time under static cell culture conditions. HUVSMC cell viability reached between 80 and 92% for all scaffolds after 9 days in culture. HUVSMCs adhered, elongated, and orientated in the fiber direction, and migrated through a scaffold thickness of 200-235 μm 9 days post-seeding under static conditions. The best scaffold was then used to assess the tissue engineering of HUVSMCs under dynamic conditions for a rotating, cell seeded, tubular scaffold in the bioreactor containing the culture medium. Dynamic conditions almost doubled the rate of cell proliferation through the scaffold, forming full tissue throughout a scaffold of 250-300 μm thickness 6 days post-seeding.

Insoluble elastin reduces collagen scaffold stiffness, improves viscoelastic properties, and induces a contractile phenotype in smooth muscle cells

Biomaterials with the capacity to innately guide cell behaviour while also displaying suitable mechanical properties remain a challenge in tissue engineering. Our approach to this has been to utilise insoluble elastin in combination with collagen as the basis of a biomimetic scaffold for cardiovascular tissue engineering. Elastin was found to markedly alter the mechanical and biological response of these collagen-based scaffolds. Specifically, during extensive mechanical assessment elastin was found to reduce the specific tensile and compressive moduli of the scaffolds in a concentration dependant manner while having minimal effect on scaffold microarchitecture with both scaffold porosity and pore size still within the ideal ranges for tissue engineering applications. However, the viscoelastic properties were significantly improved with elastin addition with a 3.5-fold decrease in induced creep strain, a 6-fold increase in cyclical strain recovery, and with a four-parameter viscoelastic model confirming the ability of elastin to confer resistance to long term deformation/creep. Furthermore, elastin was found to result in the modulation of SMC phenotype towards a contractile state which was determined via reduced proliferation and significantly enhanced expression of early (α-SMA), mid (calponin), and late stage (SM-MHC) contractile proteins. This allows the ability to utilise extracellular matrix proteins alone to modulate SMC phenotype without any exogenous factors added. Taken together, the ability of elastin to alter the mechanical and biological response of collagen scaffolds has led to the development of a biomimetic biomaterial highly suitable for cardiovascular tissue engineering.

Fabrication of poly (L-lactic acid)/gelatin composite tubular scaffolds for vascular tissue engineering

The in vitro fabrication of fully functional 3D vascular tissue construct represents one of the most fundamental challenges in vascular tissue engineering. Polymer blending is an effective method for developing, desirable bio-composites for tissue engineering. This study employs the blending of desired characteristics of a synthetic polymer, poly (L-lactic acid) (PLLA) and a biopolymer, gelatin for enhancing cell adhesion sites. Aligned and random PLLA/gelatin nanofibers were fabricated using electrospinning technique. Morphological and chemical characterization of the nanofibrous scaffolds was carried out and the size of fibers ranged from 100 to 500 nm. The SEM, fluorescent staining and viability assays revealed an increase in viability and proliferation of Human Umbilical Vein Endothelial Cells (HUVECs) and Smooth Muscle Cells (SMCs) proportional to gelatin content. The aligned fiber morphology helps cells to orient and elongate along their long axis. Thus the results were suggestive of the fact that topographically aligned nanofibrous scaffolds control cellular organization and possibly provide a good support for achieving the vital organization and physical properties of blood vessel.

Drug and cell delivery for cardiac regeneration

The spectrum of ischaemic cardiomyopathy, encompassing acute myocardial infarction to congestive heart failure is a significant clinical issue in the modern era. This group of diseases is an enormous source of morbidity and mortality and underlies significant healthcare costs worldwide. Cardiac regenerative therapy, whereby pro-regenerative cells, drugs or growth factors are administered to damaged and ischaemic myocardium has demonstrated significant potential, especially preclinically. While some of these strategies have demonstrated a measure of success in clinical trials, tangible clinical translation has been slow. To date, the majority of clinical studies and a significant number of preclinical studies have utilised relatively simple delivery methods for regenerative therapeutics, such as simple systemic administration or local injection in saline carrier vehicles. Here, we review cardiac regenerative strategies with a particular focus on advanced delivery concepts as a potential means to enhance treatment efficacy and tolerability and ultimately, clinical translation. These include (i) delivery of therapeutic agents in biomaterial carriers, (ii) nanoparticulate encapsulation, (iii) multimodal therapeutic strategies and (iv) localised, minimally invasive delivery via percutaneous transcatheter systems.

Effect of hyaluronic acid incorporation method on the stability and biological properties of polyurethane-hyaluronic acid biomaterials

The high failure rate of small diameter vascular grafts continues to drive the development of new materials and modification strategies that address this clinical problem, with biomolecule incorporation typically achieved via surface-based modification of various biomaterials. In this work, we examined whether the method of biomolecule incorporation (i.e., bulk versus surface modification) into a polyurethane (PU) polymer impacted biomaterial performance in the context of vascular applications. Specifically, hyaluronic acid (HA) was incorporated into a poly(ether urethane) via bulk copolymerization or covalent surface tethering, and the resulting PU-HA materials characterized with respect to both physical and biological properties. Modification of PU with HA by either surface or bulk methods yielded materials that, when tested under static conditions, possessed no significant differences in their ability to resist protein adsorption, platelet adhesion, and bacterial adhesion, while supporting endothelial cell culture. However, only bulk-modified PU-HA materials were able to fully retain these characteristics following material exposure to flow, demonstrating a superior ability to retain the incorporated HA and minimize enzymatic degradation, protein adsorption, platelet adhesion, and bacterial adhesion. Thus, despite bulk methods rarely being implemented in the context of biomolecule attachment, these results demonstrate improved performance of PU-HA upon bulk, rather than surface, incorporation of HA. Although explored only in the context of PU-HA, the findings revealed by these experiments have broader implications for the design and evaluation of vascular graft modification strategies.

Renovation of the injured heart with myocardial tissue engineering

Tissue engineering aims to create, repair and/or replace tissues and organs by using cells, scaffolds, biologically active molecules and physiologic signals. It is an interdisciplinary field that integrates aspects of engineering, chemistry, biology and medicine. One of the most challenging goals in the field of cardiovascular tissue engineering is the creation of a heart muscle patch. This review describes the principles, achievements and challenges of achieving this ambitious goal of creating contractile heart muscle. In addition, the new strategy of in situ and injectable tissue engineering for myocardial repair and regeneration is presented.

Is there an alternative to systemic anticoagulation, as related to interventional biomedical devices?

To reduce the toxic effects, related clinical problems and complications such as bleeding disorders associated with systemic anticoagulation, it has been hypothesized that by coating the surfaces of medical devices, such as stents, bypass grafts, extracorporeal circuits, guide wires and catheters, there will be a significant reduction in the requirement for systemic anticoagulation or, ideally, it will no longer be necessary. However, current coating processes, even covalent ones, still result in leaching followed by reduced functionality. Alternative anticoagulants and related antiplatelet agents have been used for improvement in terms of reduced restenosis, intimal hyperphasia and device failure. This review focuses on existing heparinization processes, their application in clinical devices and the updated list of alternatives to heparinization in order to obtain a broad overview, it then highlights, in particular, the future possibilities of using heparin and related moieties to tissue engineer scaffolds.

Biomechanics and biocompatibility of the perfect conduit-can we build one?

No currently available conduit meets the criteria for an ideal coronary artery bypass graft. The perfect conduit would combine the availability and complication-free harvest of a synthetic vessel with the long-term patency performance of the internal mammary artery. However, current polymer conduits suffer from inelastic mechanical properties and especially poor surface biocompatibility, resulting in early loss of patency as a coronary graft. Approaches to manufacture an improved conduit using new polymers or polymer surfaces, acellular matrices, or cellular constructs have to date failed to achieve a commercially successful alternative. Elastin, by mimicking the native extracellular environment as well as providing elasticity, provides the 'missing link' in vascular conduit design and brings new hope for realization of the perfect conduit.

Surface modification of a polyhedral oligomeric silsesquioxane poly(carbonate-urea) urethane (POSS-PCU) nanocomposite polymer as a stent coating for enhanced capture of endothelial progenitor cells

An unmet need exists for the development of next-generation multifunctional nanocomposite materials for biomedical applications, particularly in the field of cardiovascular regenerative biology. Herein, we describe the preparation and characterization of a novel polyhedral oligomeric silsesquioxane poly(carbonate-urea) urethane (POSS-PCU) nanocomposite polymer with covalently attached anti-CD34 antibodies to enhance capture of circulating endothelial progenitor cells (EPC). This material may be used as a new coating for bare metal stents used after balloon angioplasty to improve re-endothelialization. Biophysical characterization techniques were used to assess POSS-PCU and its subsequent functionalization with anti-CD34 antibodies. Results indicated successful covalent attachment of anti-CD34 antibodies on the surface of POSS-PCU leading to an increased propensity for EPC capture, whilst maintaining in vitro biocompatibility and hemocompatibility. POSS-PCU has already been used in 3 first-in-man studies, as a bypass graft, lacrimal duct and a bioartificial trachea. We therefore postulate that its superior biocompatibility and unique biophysical properties would render it an ideal candidate for coating medical devices, with stents as a prime example. Taken together, anti-CD34 functionalized POSS-PCU could form the basis of a nano-inspired polymer platform for the next generation stent coatings.

Improvement in endothelial cell adhesion and retention under physiological shear stress using a laminin-apatite composite layer on titanium

Apatite (Ap), laminin-apatite composite (L5Ap, L10Ap, L20Ap and L40Ap) and albumin-apatite (AlbAp) composite layers were prepared on titanium (Ti) using a supersaturated calcium phosphate solution supplemented with laminin (0, 5, 10, 20 and 40 μg ml(-1)) or albumin (800 μg ml(-1)). With an increase in the concentrations of laminin in the supersaturated calcium phosphate solutions, the amounts of laminin immobilized on the Ti increased. The number of human umbilical vein endothelial cells (HUVECs) adhered to the laminin-apatite composite layers were remarkably higher than those to the untreated Ti, Ap layer and AlbAp composite layer. The number of cells adhered to the L40Ap was 4.3 times the untreated Ti. Moreover, cells adhered to the laminin-apatite composite layers showed significantly higher cell retention under the physiological shear stress for 1 h and 2 h than those to the untreated Ti, Ap layer and AlbAp composite layer. The number of cells remaining on the L40Ap under the physiological shear stress for 2 h was 9.5 times that of the untreated Ti. The laminin-apatite composite layer is a promising interfacial layer for endothelialization of blood-contacting materials.

Biofunctionalized quantum dots for live monitoring of stem cells: applications in regenerative medicine

AIM

This study aimed to live monitor the degree of endothelial progenitor cell (EPC) integration onto tissue-engineering scaffolds by conjugating relevant antibodies to quantum dots (QDs).

MATERIALS & METHODS

Biocompatible mercaptosuccinic acid-coated QDs were functionalized with two different antibodies to EPC (CD133 with QDs of 640 nm wavelength [λ] and later-stage mature EPCs; and von Willebrand factor with QDs of λ595 and λ555 nm) using conventional carbomide and N-hydroxysuccinimide chemistry. Biofunctionalization was characterized with Fourier-transform infrared spectroscopy. Cell viability assays and gross morphology observations confirmed cytocompatibility and normal patterns of celluar growth. The antigens corresponding to each state of cell maturation were determined using a single excitation at λ488 nm.

RESULTS

The optimal concentrations of antibody-QD conjugates were biocompatible, hemocompatible and determined the state of EPC transformation to endothelial cells.

CONCLUSION

Antibody-functionalized QDs suggest new applications in tissue engineering of polymer-based implants where cell integration can potentially be monitored without requiring the sacrifice of implants.

A novel ovine ex vivo arteriovenous shunt model to test vascular implantability

The major objective of successful development of tissue-engineered vascular grafts is long-term in vivo patency. Optimization of matrix, cell source, surface modifications, and physical preconditioning are all elements of attaining a compatible, durable, and functional vascular construct. In vitro model systems are inadequate to test elements of thrombogenicity and vascular dynamic functional properties while in vivo implantation is complicated, labor-intensive, and cost-ineffective. We proposed an ex vivo ovine arteriovenous shunt model in which we can test the patency and physical properties of vascular grafts under physiologic conditions. The pressure, flow rate, and vascular diameter were monitored in real-time in order to evaluate the pulse wave velocity, augmentation index, and dynamic elastic modulus, all indicators of graft stiffness. Carotid arteries, jugular veins, and small intestinal submucosa-based grafts were tested. SIS grafts demonstrated physical properties between those of carotid arteries and jugular veins. Anticoagulation properties of grafts were assessed via scanning electron microscopy imaging, en face immunostaining, and histology. Luminal seeding with endothelial cells greatly decreased the attachment of thrombotic components. This model is also suture free, allowing for multiple samples to be stably processed within one animal. This tunable (pressure, flow, shear) ex vivo shunt model can be used to optimize the implantability and long-term patency of tissue-engineered vascular constructs.

Development of a compliant and cytocompatible micro-fibrous polyethylene terephthalate vascular scaffold

Bioengineering approaches have been intensively applied to create small diameter vascular grafts using artificial materials. However, a fully successful, high performing and anti-thrombogenic structure has not been achieved yet. In this study, we have designed and fabricated a novel non-woven fibrous vascular graft with biomechanical properties closely resembling those of native vessels. Vascular cell growth, preservation of cell phenotype, retention of vasoactive properties, as well as the effect of gelatin coating on the cellular interaction with the scaffolds under static and shear stress conditions were investigated. The non-woven fibrous scaffolds were made from melt blown polyethylene terephthalate fiber webs stacked by means of a consolidation technique. The scaffold variables were fiber diameter distribution and the number of consolidated web stacks. SEM analysis confirmed various fiber diameter and pore size ranges corresponding to the different conditions. The scaffolds showed burst pressure values of ∼1500 mmHg and compliance (8.4 ± 1.0 × 10(-2) % mmHg(-1) ) very similar to those of native arteries (∼8 × 10(-2) % mmHg(-1) ). The structure with the smallest fiber diameter range (1-5 μm) and pore size range (1-20 μm) was the most suitable for the growth of human brain endothelial cells and aortic smooth muscle cells. The cells maintained their specific cell phenotype, expressed collagen and elastin and produced cAMP in response to α-calcitonin gene-related peptide. However, under shear stress conditions (0.9 dyne cm(-2) ), only 30% of the cells were retained in both uncoated and gelatin-coated scaffolds indicating the need for improving the cell retention capacity of these structures, which is our future research direction. This study indicates that the biomechanical and biocompatible properties of this novel vascular scaffold are promising for the development of a vascular graft with similar characteristics to those of native vessels.

Vascular tissue engineering by computer-aided laser micromachining

Many conventional technologies for fabricating tissue engineering scaffolds are not suitable for fabricating scaffolds with patient-specific attributes. For example, many conventional technologies for fabricating tissue engineering scaffolds do not provide control over overall scaffold geometry or over cell position within the scaffold. In this study, the use of computer-aided laser micromachining to create scaffolds for vascular tissue networks was investigated. Computer-aided laser micromachining was used to construct patterned surfaces in agarose or in silicon, which were used for differential adherence and growth of cells into vascular tissue networks. Concentric three-ring structures were fabricated on agarose hydrogel substrates, in which the inner ring contained human aortic endothelial cells, the middle ring contained HA587 human elastin and the outer ring contained human aortic vascular smooth muscle cells. Basement membrane matrix containing vascular endothelial growth factor and heparin was to promote proliferation of human aortic endothelial cells within the vascular tissue networks. Computer-aided laser micromachining provides a unique approach to fabricate small-diameter blood vessels for bypass surgery as well as other artificial tissues with complex geometries.

Two ply tubular scaffolds comprised of proteins/poliglecaprone/polycaprolactone fibers

Electrospun bi-layer tubular hybrid scaffolds composed of poliglecaprone (PGC), polycaprolactone (PCL), elastin (E), and gelatin (G) were prepared and thereafter crosslinked by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC). Scanning electron microscopic (SEM) images revealed a highly porous micro-structure comprising randomly distributed non-woven fibers with the majority of fibers in submicron diameters. The EDC-crosslinking yielded an average crosslinking degree of 40%. Uni-axial tensile test of hydrated scaffolds in both longitudinal and circumferential directions revealed tensile properties, comparable to those of native arteries. The graft (PGC:PCL = 1:3) did not demonstrate significant difference before and after EDC-crosslinking in tensile strength or % strain in either longitudinal or circumferential directions. However, crosslinking increased the Young's modulus of the graft along the longitudinal direction (from 5.84 to 8.67 MPa). On the contrary, the graft (3:1) demonstrated a significant decrease in maximum strain in both directions. Cyto-assay using human umbilical vein endothelial cells (HUVECs) showed excellent cell viability.

Smooth muscle alpha-actin and calponin expression and extracellular matrix production of human coronary artery smooth muscle cells in 3D scaffolds

For a tissue-engineered coronary artery substitute to be a viable clinical option in the treatment of vascular diseases, it is necessary to use tissue-specific human cells. Coronary artery smooth muscle cells are the main resident cells in the tunica media of arteries. In this work, we examined the behavior and differentiation state of human coronary artery smooth muscle cells (HCASMCs) when cultured on 3D polyurethane scaffolds to fabricate hybrid vascular tissues. As the mechanical strength of the scaffold is an important element in engineered hybrid vascular substitutes, porous 3D polyurethane scaffolds fabricated using paraffin spheres and ammonium chloride particles were tested for their mechanical properties both in tension and in compression. The use of ammonium chloride particles as porogen generated scaffolds with superior mechanical properties, which are suitable for vascular tissue engineering. When seeded on uncoated, fibronectin-coated, and Matrigel-coated scaffolds, HCASMCs were well spread and started producing collagen as judged by histochemical analysis but appeared to lack elastin production. Fibronectin coating appeared to promote the infiltration of HCASMCs into the scaffold better than Matrigel coating but did not appear to affect the expression of collagen and elastin. Western blot analyses after successful cell recovery from the scaffolds indicated that HCASMCs, after culturing for 4 and 7 days, expressed similar amounts of smooth muscle alpha-actin and calponin regardless of extracellular matrix coating. Taken together, our data showed that the behavior and differentiation phenotype of HCASMCs can be analyzed after culture in 3D polyurethane scaffolds to establish appropriate conditions that will favor the fabrication of hybrid-engineered vascular substitutes.

Blood compatibility evaluation of poly(D,L-lactide-co-beta-malic acid) modified with the GRGDS sequence

Endothelialization is an ideal approach to improve the blood compatibility of synthetic polymers. However, cell detachment is inevitable under shear flow conditions. Therefore, the issue of blood compatibility needs to be addressed for both the bare and the endothelialized polymer. RGD-containing polymer P-GS5 was synthesized by modification of poly(D,L-lactide-co-beta-malic acid) (PLMA) with the peptide GRGDS. The compositions, molecular weights and hydrophilicities of poly(D,L-lactide) (PDLLA), PLMA, and P-GS5 were characterized by nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), gel-permeation chromatography (GPC) and water contact angle measurements, respectively. The blood compatibilities of the bare and the endothelialized polymers were evaluated by clotting time and platelet adhesion tests. The results showed that the coagulation pathways were not influenced before and after cell culture; the bare P-GS5 attracted less platelet adhesion and induced lower pseudopodia extension compared with PDLLA and PLMA, and the platelet adhesion on P-GS5 was almost completely eliminated after cell seeding. The results suggest that P-GS5 could be a potentially useful material in vascular tissue engineering.

Engineering high-density endothelial cell monolayers on soft substrates

This study demonstrates that a confluent monolayer of endothelial cells (ECs) can be tissue engineered on a soft substrate with a cell density and morphology that approximates in vivo conditions. We achieved formation of a confluent EC monolayer on polydimethylsiloxane (PDMS) elastomer by microcontact printing of fibronectin (FN) in a square lattice array of 3microm diameter circular islands at a 6microm pitch. Uniform coatings of FN or serum proteins on PDMS or on tissue-culture-treated polystyrene failed to support the equivalent EC density and/or confluence. The ECs on the FN micropatterned PDMS achieved a density of 1,536+/-247cellsmm(-2), close to the 3,215+/-336cellsmm(-2) observed in vivo from porcine pulmonary artery and significantly higher (2- to 5-fold) than EC density on other materials. The probable mechanism for enhanced EC adhesion, growth and density is increased focal adhesion (FA) formation between the ECs and the substrate. After 14days culture, the micropatterned FN surface increased the average number of FAs per cell to 35+/-10, compared to 7+/-6 for ECs on PDMS uniformly coated with FN. Thus, microscale patterning of FN into FA-sized, circular islands on PDMS elastomer promotes the formation of EC monolayers with in vivo-like cell density and morphology.

Biological relevance of ion energy in performance of human endothelial cells on ion-implanted flexible polyurethane surfaces

To improve the biocompatibility of polyurethane (PUR), we modified the surface by irradiation with different ions (Carbon; C, Oxygen; O, Nitrogen; N, or Argon; Ar) at 0.3-50 keV energy and doses of 1,00E+13 - 1,00E+15 ions/cm(2). The effects of ion implantation using different ion energies and densities were observed on adhesion, proliferation, and viability of human umbilical vein endothelial cells (HUVECs). The long-term in vitro stability of ion-implanted PUR was also investigated. Ion irradiation moderately affected the surface roughness (R(a)), but strongly enhanced the work of adhesion (W(a)). Cell adhesion was markedly improved on O-, N-, and Ar-, but not on C-implanted PUR surfaces. Medium ion energies and lower ion doses produced the best HUVEC attachment and proliferation, indicating the importance of choosing the proper range of energy applied during ion irradiation. In addition, apoptosis rates were significantly reduced when compared with unmodified PUR (uPUR). N implantation significantly protected the surface, although C implantation led to stronger surface erosions than on uPUR. In total, ion implantation on flexible PUR surfaces strongly improved the material surface characteristics and biocompatibility. Electron beam ion implantation within an appropriate energy window is thus a key to improving flexible PUR surfaces for clinical use to support endothelial cell performance. Thus, it can contribute to designing small-diameter grafts, which are in great demand, towards vascular tissue engineering applications.

Reendothelialization of tubular scaffolds by sedimentary and rotative forces: a first step toward tissue-engineered venous graft

PURPOSE

Uniform and tubular surface seeding is a prerequisite for tissue-engineered blood vessels to mature properly in a bioreactor. Our objective was to investigate reendothelialization of tubular scaffolds by the synergistic forces of sedimentation and rotation, so as to fabricate tissue-engineered venous grafts in vitro.

MATERIALS AND METHODS

Canine bone-marrow-derived endothelial progenitor cells were expanded in vitro. By means of a homemade horizontally rotative device, enzymatically decellularized porcine aortic scaffolds were tubularly seeded with the cells by precoating different matrices, under different rotation speeds, culture durations, and seeding techniques. Incorporation of lipoprotein and antiplatelet aggregation functions of seeded cells were evaluated. The seeding efficacies of various methods were compared by histology and scanning electronic microscopy.

RESULTS

Uniform distribution and a larger area of cell coverage were demonstrated by precoating with fibronectin (Fn) and a rotation speed of 2.5 revolutions per hour (rph). The efficacy of rotative seeding was comparable to its static counterpart at 4 h, but decreased at 72 h. The result of single-spin seeding was not different from that of three-spin seeding. The seeded cells showed their natural functions of lipoprotein uptake and antiplatelet aggregative properties. Based on these, we constructed 12 tissue-engineered venous grafts with a cell coverage area of 87.4+/-6.2%.

CONCLUSIONS

Efficient and reproducible endothelialization was demonstrated by precoating scaffolds with Fn and by performing single-spin seeding at a speed of 2.5 rph.

Elastase-sensitive elastomeric scaffolds with variable anisotropy for soft tissue engineering

PURPOSE

To develop elastase-sensitive polyurethane scaffolds that would be applicable to the engineering of mechanically active soft tissues.

METHODS

A polyurethane containing an elastase-sensitive peptide sequence was processed into scaffolds by thermally induced phase separation. Processing conditions were manipulated to alter scaffold properties and anisotropy. The scaffold's mechanical properties, degradation, and cytocompatibility using muscle-derived stem cells were characterized. Scaffold in vivo degradation was evaluated by subcutaneous implantation.

RESULTS

When heat transfer was multidirectional, scaffolds had randomly oriented pores. Imposition of a heat transfer gradient resulted in oriented pores. Both scaffolds were flexible and relatively strong with mechanical properties dependent upon fabrication conditions such as solvent type, polymer concentration and quenching temperature. Oriented scaffolds exhibited anisotropic mechanical properties with greater tensile strength in the orientation direction. These scaffolds also supported muscle-derived stem cell growth more effectively than random scaffolds. The scaffolds expressed over 40% weight loss after 56 days in elastase containing buffer. Elastase-sensitive scaffolds were complete degraded after 8 weeks subcutaneous implantation in rats, markedly faster than similar polyurethanes that did not contain the peptide sequence.

CONCLUSION

The elastase-sensitive polyurethane scaffolds showed promise for application in soft tissue engineering where controlling scaffold mechanical properties and pore architecture are desirable.

Engineering blood vessels by gene and cell therapy

Cardiovascular-related syndromes are the leading cause of morbidity and mortality worldwide. Arterial narrowing and blockage due to atherosclerosis cause reduced blood flow to the brain, heart and legs. Bypass surgery to improve blood flow to the heart and legs in these patients is performed in hundreds of thousands of patients every year. Autologous grafts, such as the internal thoracic artery and saphenous vein, are used in most patients, but in a significant number of patients such grafts are not available and synthetic grafts are used. Synthetic grafts have higher failure rates than autologous grafts due to thrombosis and scar formation within graft lumen. Cell and gene therapy combined with tissue engineering hold a great promise to provide grafts that will be biocompatible and durable. This review describes the field of vascular grafts in the context of tissue engineering using cell and gene therapies.

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.

Central gene transfer of interleukin-10 reduces hypothalamic inflammation and evidence of heart failure in rats after myocardial infarction

The expression of proinflammatory cytokines increases in hypothalamus of rats with myocardial infarction (MI) and heart failure. We used central gene transfer of human interleukin (IL)-10, a potent antiinflammatory cytokine, to counter the effects of brain proinflammatory cytokines and examine their functional significance. Sprague-Dawley rats underwent coronary ligation to induce MI or sham surgery (SHAM). One week later, adenoviral vectors encoding human IL-10 (AdIL-10) or beta-galactosidase (betaGal) were injected (30 microL over 30 minutes) into lateral ventricle. One week after injection, there was abundant expression of human IL-10 in the brain of MI+AdIL-10 and SHAM+AdIL-10 rats. Compared with SHAM+betaGal, MI+betaGal had increased (P<0.05) IL-1beta and cyclooxygenase-2 mRNA and protein and nuclear factor kappaB activity in the hypothalamus, cyclooxygenase-2 fluorescence in perivascular cells of the paraventricular nucleus of hypothalamus, prostaglandin E(2) in cerebrospinal fluid, and Fra-like activity (indicating neuronal excitation) in paraventricular nucleus. Plasma norepinephrine levels, lung/body weight, right ventricle/body weight, and left ventricular end-diastolic pressure were increased and maximal left ventricular dP/dt was decreased. All of these findings were ameliorated in MI rats treated with AdIL-10. Hypothalamic tumor necrosis factor-alpha and circulating tumor necrosis factor-alpha and IL-1beta levels, also increased in MI+betaGal, were not affected by AdIL-10 treatment. Rat native IL-10 was not affected by MI or AdIL-10. AdIL-10 had no effects on SHAM rats. The results demonstrate that cardiovascular and autonomic mechanisms leading to heart failure after MI can be modulated by manipulating the balance between proinflammatory and antiinflammatory cytokines in the brain.

Biodegradable elastomeric scaffolds with basic fibroblast growth factor release

Scaffolds that better approximate the mechanical properties of cardiovascular and other soft tissues might provide a more appropriate mechanical environment for tissue development or healing in vivo. An ability to induce local angiogenesis by controlled release of an angiogenic factor, such as basic fibroblast growth factor (bFGF), from a biodegradable scaffold with mechanical properties more closely approximating soft tissue could find application in a variety of settings. Toward this end biodegradable poly(ester urethane)urea (PEUU) scaffolds loaded with bFGF were fabricated by thermally induced phase separation. Scaffold morphology, mechanical properties, release kinetics, hydrolytic degradation and bioactivity of the released bFGF were assessed. The scaffolds had inter-connected pores with porosities of 90% or greater and pore sizes ranging from 34-173 microm. Scaffolds had tensile strengths of 0.25-2.8 MPa and elongations at break of 81-443%. Incorporation of heparin into the scaffold increased the initial burst release of bFGF, while the initial bFGF loading content did not change release kinetics significantly. The released bFGF remained bioactive over 21 days as assessed by smooth muscle mitogenicity. Scaffolds loaded with bFGF showed slightly higher degradation rates than unloaded control scaffolds. Smooth muscle cells seeded into the scaffolds with bFGF showed higher cell densities than for control scaffolds after 7 days of culture. The bFGF-releasing PEUU scaffolds thus exhibited a combination of mechanical properties and bioactivity that might be attractive for use in cardiovascular and other soft tissue applications.

Micropatterning with aerosols: Application for biomaterials

Adhesion and proliferation behaviors of bovine aortic endothelial cells (BAECs) were investigated on surfaces micropatterned with peptides using a novel approach. This micropatterning technique allows modification of macroscopic three-dimensional (3D) biomaterials surfaces and exploits the semi-random properties of aerosols and the principles of liquid atomization. The possibility to control cell behaviors on polytetrafluoroethylene (PTFE) surfaces tailored with this micropatterning approach was evaluated. CGRGDS and CWQPPRARI peptides were selected for their adhesive, migration and spreading properties. Culture of BAECs on patterned PTFE showed the possibility of modulating cell behaviors. The study showed that CGRGDS spots with a diameter of 10+/-2 microm over a background of CWQPPRARI peptides was the most effective combination to enhance endothelialization of PTFE. This micropatterning technique is innovative, easily adaptable, simple, and rapid for covering large 3D areas.

α2(VIII) Collagen Substrata Enhance Endothelial Cell Retention Under Acute Shear Stress Flow via an α2β1Integrin–Dependent Mechanism

BACKGROUND

Essential to tissue-engineered vascular grafts is the formation of a functional endothelial monolayer capable of resisting the forces of blood flow. This study targeted alpha2(VIII) collagen, a major component of the subendothelial matrix, and examined the ability of and mechanisms by which endothelial cells attach to this collagen under static and dynamic conditions both in vitro and in vivo.

METHODS AND RESULTS

Attachment of human endothelial cells to recombinant alpha2(VIII) collagen was assessed in vitro under static and shear conditions of up to 100 dyne/cm2. Alpha2(VIII) collagen supported endothelial cell attachment in a dose-dependent manner, with an 18-fold higher affinity for endothelial cells compared with fibronectin. Cell attachment was significantly inhibited by function-blocking anti-alpha2 (56%) and -beta1 (98%) integrin antibodies but was not RGD dependent. Under flow, endothelial cells were retained at significantly higher levels on alpha2(VIII) collagen (53% and 51%) than either fibronectin (23% and 16%) or glass substrata (7% and 1%) at shear rates of 30 and 60 dyne/cm2, respectively. In vivo studies, using endothelialized polyurethane grafts, demonstrated significantly higher cell retention rates to alpha2(VIII) collagen-coated than to fibronectin-coated prostheses in the midgraft area (P < 0.05) after 24 hours' implantation in the caprine carotid artery.

CONCLUSIONS

These studies demonstrate that alpha2(VIII) collagen has the potential to improve both initial cell attachment and retention of endothelial cells on vascular grafts in vivo, which opens new avenues of research into the development of single-stage endothelialized prostheses and the next generation of tissue-engineered vascular grafts.

Synthesis and Hemocompatibility of Biomembrane Mimicing Poly(carbonate urethane)s Containing Fluorinated Alkyl Phosphatidylcholine Side Groups

In this article, we designed and synthesized biomembrane mimicing segmented poly(carbonate urethane)s containing fluorinated alkyl phosphatidylcholine (PC) side groups. To obtain these novel poly(carbonate urethane)s, a new diol with a long side chain fluorinated alkyl phosphatidylcholine polar headgroup (2-[2-2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-hexadecafluoro-10-ethoxy-decyloxy-N-(2-hydroxy-1-hydroxymethyl-1-methyl-ethyl)-acetamide] phosphatidylcholine, HFDAPC) was first synthesized and characterized. Then a series of poly(carbonate urethane)s containing fluorinated alkyl phosphatidylcholine side groups were synthesized using methylenebis(phenylene isocyanate) (MDI), poly(1,6-hexyl-1,5-pentyl carbonate) diol (PHPCD), 1,4-butandiol (BDO), and HFDAPC. The obtained fluorinated phosphatidylcholine poly(carbonate urethane)s (FPCPCU) possessed high molecular weight, narrower molecular weight distribution, and good mechanical properties as characterized by GPC and Instron, showing an increased hydrophilicity and a possible arrangement of surface structure as characterized by water contact angle. XPS results indicated that the phosphatidylcholine polar headgroups have been indeed pulled out to the surface with the help of the migration of the fluorinated side chain that was directly connected with the phosphatidylcholine polar headgroup. A preliminary result by protein adsorption and platelet adhesion experiments suggested that only 5 approximately 12.5 mol % phosphatidylcholine could be enough for good hemocompatibility. The current work demonstrates a new synthetic approach that can be used to bring the bioactive PC groups to the surface of the PC-containing polyurethanes more effectively.

Surgical treatment of coronary multivessel disease

Coronary artery bypass grafting has had a significant impact on the treatment of angina, and has been the 'gold standard' since 1969. Its use and efficacy has been increased by revascularization in cardiac arrest and the use of the internal mammary artery. In parallel, catheter techniques have evolved by means of balloon dilatation and additional stenting. This has effected the referral to surgery despite the development of new arterialization techniques and minimally invasive surgery. As competing techniques, an acceptable equilibrium between surgery and stenting will be found within the next years.