Novel thromboresistant materials

Fabrication of nanoparticle-reinforced composite hydrogel for improved durability, antifouling, and thrombosis-resistance in arteriovenous grafts

Arteriovenous grafts are routinely designed to provide a deliberate connection between an artery and vein in patients during hemodialysis. The commonly used grafts present significant drawbacks such as thrombosis, bacterial infection, and biofouling which prevents their functionality. To endow hydrogels with improved anti-thrombosis, stable antifouling, and strong mechanical strength, a surface-modified nanoparticle-reinforced nanohybrid hydrogel is developed. In brief, zwitterionic sulfobetaine methacrylate (SBMA) is coated on bentonite clay (BC) nanoparticles via a simple method. BC-SBMA nanoparticles were then loaded onto sodium alginate /polyvinyl alcohol hydrogel composite. Calcium chloride (Ca2+) crosslinking is employed to form stable network and optimize polyvinyl alcohol/sodium alginate (PS) hydrogel composite. BC-SBMA particles were dispersed into PS hydrogel and crosslinked to form nanohybrid hydrogel (PS@BC-SBMA). The nanohybrid hydrogel was characterized for its morphological, mechanical, physicochemical, antibacterial, biocompatibility, antifouling, ex-vivo anti-thrombogenic, and in-vivo anti-inflammatory properties. The results revealed that the presence ofBC-SBMA particles boosted the mechanical strength and facilitated biocompatibility. The presence of zwitterionic polymers provided excellent antifouling properties toward blood platelets, unnecessary proteins, and bacterial strains. Hence, the cooperative effects of the nanohybrid hydrogel such as biocompatibility, antifouling, and mechanical properties lead to a desirable candidate for blood-contacting implants.

Surface modification strategies for improved hemocompatibility of polymeric materials: a comprehensive review

Polymeric biomaterials are a widely used class of materials due to their versatile properties. However, as with all other types of materials used for biomaterials, polymers also have to interact with blood. When blood comes into contact with any foreign body, it initiates a cascade which leads to platelet activation and blood coagulation. The implant surface also has to encounter a thromboinflammatory response which makes the implant integrity vulnerable, this leads to blood coagulation on the implant and obstructs it from performing its function. Hence, the surface plays a pivotal role in the design and application of biomaterials. In particular, the surface properties of biomaterials are responsible for biocompatibility with biological systems and hemocompatibility. This review provides a report on recent advances in the field of surface modification approaches for improved hemocompatibility. We focus on the surface properties of polysaccharides, proteins, and synthetic polymers. The blood coagulation cascade has been discussed and blood - material surface interactions have also been explained. The interactions of blood proteins and cells with polymeric material surfaces have been discussed. Moreover, the benefits as well as drawbacks of blood coagulation on the implant surface for wound healing purposes have also been studied. Surface modifications implemented by other researchers to enhance as well as prevent blood coagulation have also been analyzed.

Unravelling Surface Modification Strategies for Preventing Medical Device-Induced Thrombosis

The use of biomaterials in implanted medical devices remains hampered by platelet adhesion and blood coagulation. Thrombus formation is a prevalent cause of failure of these blood-contacting devices. Although systemic anticoagulant can be used to support materials and devices with poor blood compatibility, its negative effects such as an increased chance of bleeding, make materials with superior hemocompatibility extremely attractive, especially for long-term applications. This review examines blood-surface interactions, the pathogenesis of clotting on blood-contacting medical devices, popular surface modification techniques, mechanisms of action of anticoagulant coatings, and discusses future directions in biomaterial research for preventing thrombosis. In addition, this paper comprehensively reviews several novel methods that either entirely prevent interaction between material surfaces and blood components or regulate the reaction of the coagulation cascade, thrombocytes, and leukocytes.

Functionalization of in vivo tissue-engineered living biotubes enhance patency and endothelization without the requirement of systemic anticoagulant administration

Vascular regeneration and patency maintenance, without anticoagulant administration, represent key developmental trends to enhance small-diameter vascular grafts (SDVG) performance. In vivo engineered autologous biotubes have emerged as SDVG candidates with pro-regenerative properties. However, mechanical failure coupled with thrombus formation hinder translational prospects of biotubes as SDVGs. Previously fabricated poly(ε-caprolactone) skeleton-reinforced biotubes (PBs) circumvented mechanical issues and achieved vascular regeneration, but orally administered anticoagulants were required. Here, highly efficient and biocompatible functional modifications were introduced to living cells on PB lumens. The 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-N-methoxy (DMPE)-PEG-conjugated anti-coagulant bivalirudin (DPB) and DMPE-PEG-conjugated endothelial progenitor cell (EPC)-binding TPS-peptide (DPT) modifications possessed functionality conducive to promoting vascular graft patency. Co-modification of DPB and DPT swiftly attained luminal saturation without influencing cell viability. DPB repellent of non-specific proteins, DPB inhibition of thrombus formation, and DPB protection against functional masking of DPT's EPC-capture by blood components, which promoted patency and rapid endothelialization in rat and canine artery implantation models without anticoagulant administration. This strategy offers a safe, facile, and fast technical approach to convey additional functionalization to living cells within tissue-engineered constructs.

A retrospective, comparative, clinical study of occlusion rate of peripherally inserted central catheters fabricated of poly(vinyl alcohol)-based hydrogel composite

Thrombotic accumulation is associated with surface interactions between blood proteins and vascular access devices. Catheter occlusion results from this process, and is a costly, common, occurrence with peripherally inserted central catheters (PICCs). Hydrophilic catheter materials exhibit antithrombotic properties. This retrospective study evaluates the occurrence of catheter occlusion of PICCs constructed of a poly(vinyl alcohol)-based hydrogel composite known as hydrophilic biomaterial (HBM), compared to thermoplastic polyurethane (TPU) control devices. A total of 121 PICCs, 60 HBM and 61 TPU, were placed in patients with a clinical need and were reviewed for the occurrence of catheter occlusion. The records review found that occlusions occurred in 0/60 (0.0%) of the HBM PICCs and 13/61 (21.3%) of TPU PICCs (p = 0.001). HBM exhibits favorable qualities for vascular access, most importantly its extreme hydrophilicity. Clinically, this may be responsible for the reduction in PICC occlusions, which could improve patient outcomes.

Endothelium-Mimetic Surface Modification Improves Antithrombogenicity and Enhances Patency of Vascular Grafts in Rats and Pigs

We first identified thrombomodulin (TM) and endothelial nitric oxide (NO) synthase as key factors for the antithrombogenic function of the endothelium in human atherosclerotic carotid arteries. Then, recombinant TM and an engineered galactosidase responsible for the conversion of an exogenous NO prodrug were immobilized on the surface of the vascular grafts. Surface modification by TM and NO cooperatively enhanced the antithrombogenicity and patency of vascular grafts. Importantly, we found that the combination of TM and NO also promoted endothelialization, whereas it reduced adverse intimal hyperplasia, which is critical for the maintenance of vascular homeostasis, as confirmed in rat and pig models.

Advancements in left ventricular assist devices to prevent pump thrombosis and blood coagulopathy

Mechanical circulatory support (MCS) devices, such as left ventricular assist devices (LVADs) are very useful in improving outcomes in patients with advanced-stage heart failure. Despite recent advances in LVAD development, pump thrombosis is one of the most severe adverse events caused by LVADs. The contact of blood with artificial materials of LVAD pumps and cannulas triggers the coagulation cascade. Heat spots, for example, produced by mechanical bearings are often subjected to thrombus build-up when low-flow situations impair washout and thus the necessary cooling does not happen. The formation of thrombus in an LVAD may compromise its function, causing a drop in flow and pumping power leading to failure of the LVAD, if left unattended. If a clot becomes dislodged and circulates in the bloodstream, it may disturb the flow or occlude the blood vessels in vital organs and cause internal damage that could be fatal, for example, ischemic stroke. That is why patients with LVADs are on anti-coagulant medication. However, the anti-coagulants can cause a set of issues for the patient-an example of gastrointestinal (GI) bleeding is given in illustration. On account of this, these devices are only used as a last resort in clinical practice. It is, therefore, necessary to develop devices with better mechanics of blood flow, performance and hemocompatibility. This paper discusses the development of LVADs through landmark clinical trials in detail and describes the evolution of device design to reduce the risk of pump thrombosis and achieve better hemocompatibility. Whilst driveline infection, right heart failure and arrhythmias have been recognised as LVAD-related complications, this paper focuses on complications related to pump thrombosis, especially blood coagulopathy in detail and potential strategies to mitigate this complication. Furthermore, it also discusses the LVAD implantation techniques and their anatomical challenges.

Control of Blood Coagulation by Hemocompatible Material Surfaces-A Review

Hemocompatibility of biomaterials in contact with the blood of patients is a prerequisite for the short- and long-term applications of medical devices such as cardiovascular stents, artificial heart valves, ventricular assist devices, catheters, blood linings and extracorporeal devices such as artificial kidneys (hemodialysis), extracorporeal membrane oxygenation (ECMO) and cardiopulmonary bypass. Although lower blood compatibility of materials and devices can be handled with systemic anticoagulation, its side effects, such as an increased bleeding risk, make materials that have a better hemocompatibility highly desirable, particularly in long-term applications. This review provides a short overview on the basic mechanisms of blood coagulation including plasmatic coagulation and blood platelets, as well as the activation of the complement system. Furthermore, a survey on concepts for tailoring the blood response of biomaterials to improve the hemocompatibility of medical devices is given which covers different approaches that either inhibit interaction of material surfaces with blood components completely or control the response of the coagulation system, blood platelets and leukocytes.

Tissue engineering and 3D printing of bioartificial pancreas for regenerative medicine in diabetes

Diabetes is a severe chronic disease worldwide. In various types of diabetes, the pancreatic beta cells fail to secrete sufficient insulin, at some point, to regulate blood glucose levels. Therefore, the replacement of dysfunctional pancreas, islets of Langerhans, or even the insulin-secreting beta cells facilitates physiological regulation of blood glucose levels. However, the current lack of sufficient donor human islets for cell replacement therapy precludes a routine and absolute cure for most of the existing diabetes cases globally. It is envisioned that tissue engineering of a bioartificial pancreas will revolutionize regenerative medicine and the treatment of diabetes. In this review, we discuss the anatomy and physiology of the pancreas, and identify the clinical considerations for engineering a bioartificial pancreas. Subsequently, we dissect the bioengineering problem based on the design of the device, the biomaterial used, and the cells involved. Last but not least, we highlight current tissue engineering challenges and explore potential directions for future work.

Blood Oxygenation Using Fluoropolymer-Based Artificial Lung Membranes

Artificial lung (AL) membranes are used for blood oxygenation for patients undergoing open-heart surgery or acute lung failures. Current AL technology employs polypropylene and polymethylpentene membranes. Although effective, these membranes suffer from low biocompatibility, leading to undesired blood coagulation and hemolysis over a long term. In this work, we propose a new generation of AL membranes based on amphiphobic fluoropolymers. We employed poly(vinylidene-co-hexafluoropropylene), or PVDF-co-HFP, to fabricate macrovoid-free membranes with an optimal pore size range of 30-50 nm. The phase inversion behavior of PVDF-co-HFP was investigated in detail for structural optimization. To improve the wetting stability of the membranes, the fabricated membranes were coated using Hyflon AD60X, a type of fluoropolymer with an extremely low surface energy. Hyflon-coated materials displayed very low protein adsorption and a high contact angle for both water and blood. In the hydrophobic spectrum, the data showed an inverse relationship between the surface free energy and protein adsorption, suggesting an appropriate direction with respect to biocompatibility for AL research. The blood oxygenation performance was assessed using animal sheep blood, and the fabricated fluoropolymer membranes showed competitive performance to that of commercial polyolefin membranes without any detectable hemolysis. The data also confirmed that the bottleneck in the blood oxygenation performance was not the membrane permeance but rather the rate of mass transfer in the blood phase, highlighting the importance of efficient module design.

Dynamic Luminal Topography: A Potential Strategy to Prevent Vascular Graft Thrombosis

AIM

Biologic interfaces play important roles in tissue function. The vascular lumen-blood interface represents a surface where dynamic interactions between the endothelium and circulating blood cells are critical in preventing thrombosis. The arterial lumen possesses a uniform wrinkled surface determined by the underlying internal elastic lamina. The function of this structure is not known, but computational analyses of artificial surfaces with dynamic topography, oscillating between smooth and wrinkled configurations, support the ability of this surface structure to shed adherent material (Genzer and Groenewold, 2006; Bixler and Bhushan, 2012; Li et al., 2014). We hypothesized that incorporating a luminal surface capable of cyclical wrinkling/flattening during the cardiac cycle into vascular graft technology may represent a novel mechanism of resisting platelet adhesion and thrombosis.

METHODS AND RESULTS

Bilayer silicone grafts possessing luminal corrugations that cyclically wrinkle and flatten during pulsatile flow were fabricated based on material strain mismatch. When placed into a pulsatile flow circuit with activated platelets, these grafts exhibited significantly reduced platelet deposition compared to grafts with smooth luminal surfaces. Constrained wrinkled grafts with static topography during pulsatile flow were more susceptible to platelet accumulation than dynamic wrinkled grafts and behaved similar to the smooth grafts under pulsatile flow. Wrinkled grafts under continuous flow conditions also exhibited marked increases in platelet accumulation.

CONCLUSION

These findings provide evidence that grafts with dynamic luminal topography resist platelet accumulation and support the application of this structure in vascular graft technology to improve the performance of prosthetic grafts. They also suggest that this corrugated structure in arteries may represent an inherent, self-cleaning mechanism in the vasculature.

Single and multi-functional coating strategies for enhancing the biocompatibility and tissue integration of blood-contacting medical implants

Device-associated clot formation and poor tissue integration are ongoing problems with permanent and temporary implantable medical devices. These complications lead to increased rates of mortality and morbidity and impose a burden on healthcare systems. In this review, we outline the current approaches for developing single and multi-functional surface coating techniques that aim to circumvent the limitations associated with existing blood-contacting medical devices. We focus on surface coatings that possess dual hemocompatibility and biofunctionality features and discuss their advantages and shortcomings to providing a biocompatible and biodynamic interface between the medical implant and blood. Lastly, we outline the newly developed surface modification techniques that use lubricant-infused coatings and discuss their unique potential and limitations in mitigating medical device-associated complications.

Titanium Dioxide Nanotube Arrays for Cardiovascular Stent Applications

Efficient stent implantation among others depends on avoiding the aggregation of platelets in the blood vessels and appropriate proliferation of endothelial cells and controlled proliferation of smooth muscle cells, which reduces the development of pathology, such as neointimal hyperplasia, thrombosis, and restenosis. The current article provides an elegant solution for prevention of platelet and smooth muscle cell adhesion and activation on stent surfaces while obtaining surface conditions to support the growth of human coronary artery endothelial cells. This was achieved by surface nanostructuring and chemical activation of the surface. Specific nanotopographies of titanium were obtained by electrochemical anodization, while appropriate chemical properties were attained by treatment of titanium oxide nanotubes by highly reactive oxygen plasma. Surface properties were studied by scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. Wettability was evaluated by measuring the water contact angle. The influence of nanostructured morphology and plasma modification on in vitro biological response with human coronary artery endothelia and smooth muscle cells as well as whole blood was studied. Our results show that a combination of nanostructuring and plasma modification of the surfaces is an effective way to achieve desired biological responses necessary for implantable materials such as stents.

Regenerating Antithrombotic Surfaces through Nucleic Acid Displacement

Blood-contacting devices are commonly coated with antithrombotic agents to prevent clot formation and to extend the lifespan of the device. However, in vivo degradation of these bioactive surface agents ultimately limits device efficacy and longevity. Here, a regenerative antithrombotic catheter surface treatment is developed using oligodeoxynucleotide (ODN) toehold exchange. ODN strands modified to carry antithrombotic payloads can inhibit the thrombin enzyme when bound to a surface and exchange with rapid kinetics over multiple cycles, even while carrying large payloads. The surface-bound ODNs inhibit thrombin activity to significantly reduce fibrinogen cleavage and fibrin formation, and this effect is sustained after ODN exchange of the surface-bound strands with a fresh antithrombotic payload. This study presents a unique strategy for achieving a continuous antithrombotic state for blood-contacting devices using an ODN-based regeneration method.

Arteriovenous Access Graft Infection: Standards of Reporting and Implications for Comparative Data Analysis

There is presently a lack of organization and standardized reporting schema for arteriovenous graft (AVG) infections. The purpose of this article is to evaluate the various types of treatment modalities for access site infections through an analysis of current publications on AVG. Key proposals are made to support standardization in a data-driven manner to make infection reporting more uniform and thereby facilitate more meaningful comparisons between various dialysis modalities and AVG technologies.

Novel high-strength thromboresistant poly(vinyl alcohol)-based hydrogel for vascular access applications

Adherence of proteins, cells, and microorganisms to the surface of biomaterials used for vascular access contribute to device failure by thrombosis, occlusions, and infections. Current technologies for inhibiting these complications are limited to coatings and additives that are limited in duration of efficacy and often induce adverse side effects. In this work, we developed a novel composite hydrogel structure comprising of a porous poly(vinyl alcohol) (PVA) that is impregnated with poly(acrylic acid) (PAA) and heat treated to create a physically cross-linked high-strength hydrogel material. The swelling and mechanical properties can be controlled by the temperature and duration of heat treatment to increase the cross-link density of the matrix. The heat treated composite PVA/PAA hydrogel exhibits both the mechanical strength and durability of thermoplastic polyurethanes (TPUs) and the inherently non-thrombogenic surface functionality of PVA-based hydrogels without the use of chemical cross-linking agents. The composite hydrogels were found to maintain their mechanical integrity and surface functionality after accelerated aging in a simulated-use in vitro model for 162.5 days real-time equivalent. Relative to commercial catheter materials, the composite PVA/PAA hydrogel exhibited up to an average of 97% reduction in platelet adhesion when exposed to an in vitro blood loop model and a lower rate of tip occlusion due to thrombosis. This high-strength thromboresistant hydrogel could have a major impact as a novel biomaterial for use in vascular access applications to improve patient health.

Endothelialization of cardiovascular devices

Blood-contacting surfaces of cardiovascular devices are not biocompatible for creating an endothelial layer on them. Numerous research studies have mainly sought to modify these surfaces through physical, chemical and biological means to ease early endothelial cell (EC) adhesion, migration and proliferation, and eventually to build an endothelial layer on the surfaces. The first priority for surface modification is inhibition of protein adsorption that leads to inhibition of platelet adhesion to the device surfaces, which may favor EC adhesion. Surface modification through surface texturing, if applicable, can bring some hopeful outcomes in this regard. Surface modifications through chemical and/or biological means may play a significant role in easy endothelialization of cardiovascular devices and inhibit smooth muscle cell proliferation. Cellular engineering of cells relevant to endothelialization can boost the positive outcomes obtained through surface engineering. This review briefly summarizes recent developments and research in early endothelialization of cardiovascular devices. STATEMENT OF SIGNIFICANCE: Endothelialization of cardiovascular implants, including heart valves, vascular stents and vascular grafts is crucial to solve many problems in our health care system. Numerous research efforts have been made to improve endothelialization on the surfaces of cardiovascular implants, mainly through surface modifications in three ways - physically, chemically and biologically. This review is intended to highlight comprehensive research studies to date on surface modifications aiming for early endothelialization on the blood-contacting surfaces of cardiovascular implants. It also discusses future perspectives to help guide endothelialization strategies and inspire further innovations.

The blood compatibility challenge. Part 4: Surface modification for hemocompatible materials: Passive and active approaches to guide blood-material interactions

Biomedical devices in the blood flow disturb the fine-tuned balance of pro- and anti-coagulant factors in blood and vessel wall. Numerous technologies have been suggested to reduce coagulant and inflammatory responses of the body towards the device material, ranging from camouflage effects to permanent activity and further to a responsive interaction with the host systems. However, not all types of modification are suitable for all types of medical products. This review has a focus on application-oriented considerations of hemocompatible surface fittings. Thus, passive versus bioactive modifications are discussed along with the control of protein adsorption, stability of the immobilization, and the type of bioactive substance, biological or synthetic. Further considerations are related to the target system, whether enzymes or cells should be addressed in arterial or venous system, or whether the blood vessel wall is addressed. Recent developments like feedback controlled or self-renewing systems for drug release or addressing cellular regulation pathways of blood platelets and endothelial cells are paradigms for a generation of blood contacting devices, which are hemocompatible by cooperation with the host system. STATEMENT OF SIGNIFICANCE: This paper is part 4 of a series of 4 reviews discussing the problem of biomaterial associated thrombogenicity. The objective was to highlight features of broad agreement and provide commentary on those aspects of the problem that were subject to dispute. We hope that future investigators will update these reviews as new scholarship resolves the uncertainties of today.

Surface modification of polyvinyl alcohol (PVA)/polyacrylamide (PAAm) hydrogels with polydopamine and REDV for improved applicability

Developing a small-diameter vascular graft with a satisfactory performance in terms of mechanical and biological properties remains a challenging issue because of comprehensive requirements from clinical applications. Polyvinyl alcohol (PVA)/polyacrylamide (PAAm) hydrogels exhibit many desirable characteristics for small-diameter vascular grafts because of their tunable mechanical properties, especially high compliance. However, poor cells adhesion hinders their application for endothelialization in situ. Therefore, in the present work, polydopamine (PDA) and tetrapeptide Arg-Glu-Asp-Val (REDV) were used to functionalize the hydrogels surface and improve cells adhesion. A series of characterizations were systematically conducted to examine the applicability of coated hydrogels to small-diameter vascular grafts. Results showed that bare and coated hydrogels have appropriate structural stability, and no significant differences in tensile properties could be found after being coated with PDA or PDA-REDV. The hydrophilicity of the hydrogels decreased with the coatings of PDA and especially PDA-REDV to improve protein adsorption, porcine iliac artery endothelial cells (PIECs) adhesion, viability, proliferation, and spreading on the hydrogels. Lower hemolysis percentages and higher blood clotting index values were attained for the hydrogels, suggesting their satisfactory hemocompatibility. Overall, the present work provided insights into the development of a novel hydrogel-based small-diameter vascular graft. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 108B:117-127, 2020.

Blood-Compatible Surfaces with Phosphorylcholine-Based Polymers for Cardiovascular Medical Devices

For the acquisition of blood-compatible materials, various hydrophilic polymers for surface modification have been examined. Among them, polymers with a representative phospholipid polar group, the phosphorylcholine (PC) group, are a successful example. These polymers were designed from inspiration of the cell membrane surface and provide protein adsorption resistance even following contact with plasma. This important property is based on the unique hydration state of water molecules surrounding hydrated polymer; in other words, water molecules weakly interact with the polymers and maintain their favorable cluster structure through hydrogen bonding. These polymers are not only hydrophilic, but also electrically neutral, important characteristics which make hydrogen bonding with water molecules less likely to occur and avoid hydrophobic interactions. Phosphorylcholine groups and other zwitterionic structures are significant as hydrophilic functional groups meeting these important requirements. In this review, blood compatibility of a polymer having a PC group is introduced in relation to its hydration structure, followed by a description of the applications of this polymer to cardiovascular medical devices.

Vascular response and intrastent thrombus in the early phase after drug-eluting versus bare-metal stent implantation in patients with ST-segment elevation myocardial infarction: An observational, single-center study

BACKGROUND AND OBJECTIVES

Second-generation drug-eluting stents (G2-DES) are associated with a lower rate of acute and subacute stent thrombosis compared with bare-metal stent (BMS) in the setting of ST-segment elevation myocardial infarction (STEMI). In this study, our aim was to compare the vascular response and thrombus burden between G2-DES and BMS in early-phase STEMI.

METHODS

Between May 2010 and August 2014, a total of 41 STEMI patients treated by either G2-DES (n = 26; everolimus-eluting stent [EES]: n = 15, zotarolimus-eluting stent [ZES]: n = 11) or BMS (n = 15) and, with multivessel disease requiring additional percutaneous coronary intervention (PCI), were prospectively enrolled. Optical coherence tomography (OCT) imaging was performed at 1 month after stent implantation.

RESULTS

Baseline clinical characteristics, except for age (61.5 ± 9.3 vs 69.3 ± 9.8, P = 0.01, t test), were comparable between patients with drug-eluting stent (DES) and BMS. The incidence of residual thrombus after the stent implantation for STEMI was comparable between DES and BMS (7.7% vs 6.7%, P = 0.88, χ 2 test). At 1 month, thrombus burden, defined as the mean thrombus area divided by the mean lumen area, was significantly smaller with DES than with BMS (median interquartile range (IQR), 1.2 (0.0, 1.0) vs 1.2 (0.0, 2.2), P = 0.04, Mann-Whitney U test), despite a similar percentage of malapposed (median (IQR), 6.2 (2.4, 9.0) vs 2.6 (0.0, 5.8)%, P = 0.07, Mann-Whitney U test) or uncovered struts (median (IQR), 6.8 (1.8, 13.1) vs 6.14 (2.8, 18.5)%, P = 0.45, Mann-Whitney U test). No significant difference in thrombus burden was observed between EES and ZES.

CONCLUSIONS

Thrombus burden was significantly smaller with DES than with BMS at 1-month follow-up in STEMI cases, although the percentage of malapposed or uncovered struts was similar between the groups. This may partly explain the lower rate of acute and subacute stent thrombosis in G2-DES that has previously been reported in the literature.

Rapid and simple purification of elastin-like polypeptides directly from whole cells and cell lysates by organic solvent extraction

Elastin-like polypeptides (ELP) are a well-known class of proteins that are being increasingly utilized in a variety of biomedical applications, due to their beneficial physicochemical properties. A unifying feature of ELP is their demonstration of a sequence tunable inverse transition temperature (Tt) that enables purification using a simple, straightforward process called inverse transition cycling (ITC). Despite the utility of ITC, the process is inherently limited to ELP with an experimentally accessible Tt. Since the underlying basis for the ELP Tt is related to its high overall hydrophobicity, we anticipated that ELP would be excellent candidates for purification by organic extraction. We report the first method for rapidly purifying ELP directly from whole E. coli cells or clarified lysates using pure organic solvents and solvent mixtures, followed by aqueous back extraction. Our results show that small ELP and a large ELP-fusion protein can be isolated in high yield from whole cells or cell lysates with greater than 95% purity in less than 30 min and with very low levels of LPS and DNA contamination.

Discovery and Characterization of a Potent and Specific Peptide Ligand Targeting Endothelial Progenitor Cells and Endothelial Cells for Tissue Regeneration

Endothelial progenitor cells (EPCs) and endothelial cells (ECs) play a vital role in endothelialization and vascularization for tissue regeneration. Various EPC/EC targeting biomolecules have been investigated to improve tissue regeneration with limited success often due to their limited functional specificity and structural stability. One-bead one-compound (OBOC) combinatorial technology is an ultrahigh throughput chemical library synthesis and screening method suitable for ligand discovery against a wide range of biological targets, such as integrins. In this study, using primary human EPCs/ECs as living probes, we identified an αvβ3 integrin ligand LXW7 discovered by OBOC combinatorial technology as a potent and specific EPC/EC targeting ligand. LXW7 overcomes the major barriers of other functional biomolecules that have previously been used to improve vascularization for tissue regeneration and possesses optimal stability, EPC/EC specificity, and functionality. LXW7 is a disulfide cyclic octa-peptide (cGRGDdvc) containing unnatural amino acids flanking both sides of the main functional motif; therefore it will be more resistant to proteolysis and more stable in vivo compared to linear peptides and peptides consisting of only natural amino acids. Compared with the conventional αvβ3 integrin ligand GRGD peptide, LXW7 showed stronger binding affinity to primary EPCs/ECs but weaker binding to platelets and no binding to THP-1 monocytes. In addition, ECs bound to the LXW7 treated culture surface exhibited enhanced biological functions such as proliferation, likely due to increased phosphorylation of VEGF receptor 2 (VEGF-R2) and activation of mitogen-activated protein kinase (MAPK) ERK1/2. Surface modification of electrospun microfibrous PLLA/PCL biomaterial scaffolds with LXW7 via Click chemistry resulted in significantly improved endothelial coverage. LXW7 and its derivatives hold great promise for EPC/EC recruitment and delivery and can be widely applied to functionalize various biological and medical materials to improve endothelialization and vascularization for tissue regeneration applications.

Nanofibers of Human Tropoelastin-inspired peptides: Structural characterization and biological properties

Regenerative medicine is taking great advantage from the use of biomaterials in the treatments of a wide range of diseases and injuries. Among other biomaterials, self-assembling peptides are appealing systems due to their ability to spontaneously form nanostructured hydrogels that can be directly injected into lesions. Indeed, self-assembling peptide scaffolds are expected to behave as biomimetic matrices able to surround cells, to promote specific interactions, and to control and modify cell behavior by mimicking the native environment as well. We selected three pentadecapeptides inspired by Human Tropoelastin, a natural protein of the extracellular matrix, expected to show high biocompatibility. Moreover, the here proposed self-assembling peptides (SAPs) are able to spontaneously aggregate in nanofibers in biological environment, as revealed by AFM (Atomic Force Microscopy). Peptides were characterized by XPS (X-ray Photoelectron Spectroscopy) and IRRAS (Infrared Reflection Absorption Spectroscopy) both as lyophilized (not aggregated) and as aggregated (nanofibers) samples in order to investigate some potential differences in their chemical composition and intermolecular interactions, and to analyze the surface and interface of nanofibers. Finally, an accurate investigation of the biological properties of the SAPs and of their interaction with cells was performed by culturing for the first time human Mesenchymal Stem Cells (hMSCs) in presence of SAPs. The final aim of this work was to assess if Human Tropoelastin-inspired nanostructured fibers could exert a cytotoxic effect and to evaluate their biocompatibility, cellular adhesion and proliferation.

Anti-thrombotic technologies for medical devices

Thrombosis associated with medical devices may lead to dramatic increases in morbidity, mortality and increased health care costs. Innovative strategies are being developed to reduce this complication and provide a safe biocompatible interface between device and blood. This article aims to describe the biological phenomena underlying device-associated thrombosis, and surveys the literature describing current and developing technologies designed to overcome this challenge. To reduce thrombosis, biomaterials with varying topographical properties and incorporating anti-thrombogenic substances on their surface have demonstrated potential. Overall, there is extensive literature describing technical solutions to reduce thrombosis associated with medical devices, but clinical results are required to demonstrate significant long-term benefits.

Device thrombosis and pre-clinical blood flow models for assessing antithrombogenic efficacy of drug-device combinations

Thrombosis associated with blood-contacting devices is a complex process involving several component interactions that have eluded precise definition. Extensive investigations of individual biological modules such as protein adsorption, coagulation cascade activation and platelet activation/adhesion/aggregation have provided an initial foundation for developing biomaterials for blood-contacting devices, but a material that is intrinsically non-thrombogenic is yet to be developed. The well-recognized association between fluid dynamics parameters such as shear stress, vortices, stagnation and thrombotic processes such as platelet aggregation and coagulation aggravate thrombosis on most device geometries that elicit these flow disturbances. Thus, antithrombotic drugs that were developed to treat thrombosis associated with vascular diseases such as atherosclerosis have also been adapted to mitigate the risk of device thrombosis. However, balancing the risk of bleeding with the antithrombotic efficacy of these drugs continues to be a challenge, and surface modification of devices with these drug molecules to mitigate device thrombosis locally has been explored. Pre-clinical blood flow models to test the effectiveness of these drug-device combinations have also evolved and several in-vitro, ex-vivo, and in-vivo test configurations are available with their attendant merits and limitations. Despite considerable efforts toward iterative design and testing of blood contacting devices and antithrombogenic surface modifications, device thrombosis remains an unsolved problem.

Improved Hemocompatibility of Multilumen Catheters via Nitric Oxide (NO) Release from S-Nitroso-N-acetylpenicillamine (SNAP) Composite Filled Lumen

Blood-contacting devices, such as intravascular catheters, suffer from challenges related to thrombus formation and infection. Nitric oxide (NO) is an endogenous antiplatelet and antimicrobial agent. Exogenous release of NO from various polymer matrices has been shown to reduce thrombosis and infection of/on implantable medical devices. However, the clinical applications of such materials have been hindered due to factors such as NO donor leaching and thermal instability. In this study, a novel approach is demonstrated in which one lumen of commercial dual lumen catheters is dedicated to the NO release chemistry, allowing the other lumen to be available for clinical vascular access. A composite consisting of poly(ethylene glycol) (PEG) and S-nitroso-N-acetylpenicillamine (SNAP) is used to fill the NO-releasing lumen of commercial 7 French silicone catheters. Physiological levels of NO are released from the SNAP-PEG catheters for up to 14 d, as measured by chemiluminescence NO analyzer (in PBS buffer at 37 °C). PEG facilitates the NO release from SNAP within the lumen by increasing the water absorption and slowly dissolving the solid SNAP-PEG composite. In a CDC biofilm bioreactor, the SNAP-PEG catheters are found to reduce >97% bacterial adhesion as compared to the PEG controls for single bacterial species including E. coli and S. aureus. SNAP-PEG and PEG control catheters were implanted in rabbit veins for 7 h (single lumen) and 11 d (dual lumen) to evaluate their hemocompatibility properties. Significant reductions in thrombus formation on the SNAP-PEG vs PEG controls were observed, with ca. 85% reduction for 7 h single lumen catheters and ca. 55% reduction for 11 d dual lumen catheters.

Recent advances in thromboresistant and antimicrobial polymers for biomedical applications: just say yes to nitric oxide (NO)

Biomedical devices are essential for patient diagnosis and treatment; however, when blood comes in contact with foreign surfaces or homeostasis is disrupted, complications including thrombus formation and bacterial infections can interrupt device functionality, causing false readings and/or shorten device lifetime. Here, we review some of the current approaches for developing antithrombotic and antibacterial materials for biomedical applications. Special emphasis is given to materials that release or generate low levels of nitric oxide (NO). Nitric oxide is an endogenous gas molecule that can inhibit platelet activation as well as bacterial proliferation and adhesion. Various NO delivery vehicles have been developed to improve NO's therapeutic potential. In this review, we provide a summary of the NO releasing and NO generating polymeric materials developed to date, with a focus on the chemistry of different NO donors, the polymer preparation processes, and in vitro and in vivo applications of the two most promising types of NO donors studied thus far, N-diazeniumdiolates (NONOates) and S-nitrosothiols (RSNOs).

Photo-assisted generation of phospholipid polymer substrates for regiospecific protein conjugation and control of cell adhesion

Novel photo-reactive phospholipid polymers were synthesized for use in the preparation of nonfouling surfaces with protein conjugation capacity. Poly[2-methacryloyloxyethyl phosphorylcholine (MPC)-ran-N-methacryloyl-(l)-tyrosinemethylester (MAT)] (P(MPC/MAT)) was synthesized by conventional radical polymerization, with the MAT units capable of being oxidized by 254nm UV irradiation. Because of this photo-oxidation, active species such as catechol and quinone were alternately generated in the copolymer. A silicon wafer was subjected to surface modification through spin coating of P(MPC/MAT) from an aqueous solution for use as a model substrate. The surface was then irradiated several times with UV light. The thickness of the polymer layers formed on the Si wafers was influenced by various parameters such as polymer concentration, UV irradiation time, and composition of the MAT units in P(MPC/MAT). Oxidized MAT units were advantageous not only for polymer adhesion to a solid surface but also for protein conjugation with the adhered polymers. The amount of protein immobilized on UV-irradiated P(MPC/MAT) was dependent on the composition of the MAT units in the polymer. Furthermore, it was confirmed that protein immobilization on the polymer occurred through the oxidized MAT units because the protein adsorption was significantly reduced upon blocking these units through pretreatment with glycine. Conjugation of regiospecific protein could also be achieved through the use of a photomask. In addition, nonspecific protein adsorption was reduced on the non-irradiated regions whose surface was covered with physisorbed P(MPC/MAT). Therefore, P(MPC/MAT) can be used in the preparation of nonfouling substrates, which enable micrometer-sized manipulation of proteins through photo-irradiation. Function of proteins immobilized on MPC copolymers was also confirmed by cell adhesion test. As such, photo-reactive MPC copolymers are suitable for performing controlled protein conjugation and preparing polymer-protein hybrid platforms for use in biomedical and diagnostic devices.

STATEMENT OF SIGNIFICANCE

Novel photo-reactive phospholipid polymers have been synthesized for immobilization on solid surfaces and regiospecific protein conjugation. Tyrosine residues embedded in 2-methacryloyloxyethyl phosphorylcholine (MPC) copolymers could be photo-oxidized, resulting in polymers able to form layers on a solid surface and conjugate with proteins. Moreover, nonspecific biofouling on the surface significantly reduced when the oxidized tyrosine units in the polymer layers were blocked. Upon UV irradiation through a photomask, the UV-exposed tyrosine units were selectively oxidized, forming the only specific regions in which protein conjugation could occur.

Equivalent outcomes with standard and heparin-bonded expanded polytetrafluoroethylene grafts used as conduits for hemodialysis access

BACKGROUND

Obtaining and maintaining dialysis access after failure of autologous access sites remains a significant concern for patients on hemodialysis. Polytetrafluoroethylene (PTFE) is the most common conduit used. Heparin-bonded expanded PTFE (HB-PTFE) grafts have recently been introduced as an improved conduit, with suggestions that HB offers benefits because of its resistance to thrombosis. In this retrospective study, the outcomes of HB-PTFE were compared with standard wall PTFE (S-PTFE) arteriovenous grafts (AVGs).

METHODS

From January 2004 to December 2014, 483 adults (46% male; mean age, 60 years; range, 25-87 years) with end-stage renal disease underwent placement of AVGs (234 HB-PTFE and 248 S-PTFE). The two groups did not differ significantly in demographics or access history. Patency, reintervention, infection, and functional dialysis rates were examined.

RESULTS

Technical success was 99% in HB-PTFE and 98% in S-PTFE. The 30-day major adverse cardiovascular events were 2% in HB-PTFE and 3% in S-PTFE. Mean time to access was 5.1 ± 1.8 weeks for HB-PTFE and 6.9 ± 1.9 weeks for S-PTFE (P = .0001). Median follow-up was 23 months. The 2-year primary, assisted primary, and secondary patency rates were 20% ± 7% vs 18% ± 8% (P = .85), 35% ± 8% vs 28% ± 7% (P = .51), and 38% ± 6% vs 36% ± 7% (P = .83) for HB-PTFE vs S-PTFE, respectively. Both groups underwent a similar number of secondary interventions (2.1 and 1.9 interventions per person-year of follow-up for HB-PTFE vs S-PTFE respectively; P = .87). There were no significant differences in infection (11% vs 12%) or pseudoaneurysm formation (5% vs 6%) between HB-PTFE and S-PTFE groups. Functional dialysis durations were equivalent between HB-PTFE and S-PTFE groups.

CONCLUSIONS

HB-PTFE grafts offer no distinct advantage over S-PTFE grafts for hemodialysis and should not be considered a preferential conduit for AVG.

In situ regeneration of bioactive coatings enabled by an evolved Staphylococcus aureus sortase A

Surface immobilization of bioactive molecules is a central paradigm in the design of implantable devices and biosensors with improved clinical performance capabilities. However, in vivo degradation or denaturation of surface constituents often limits the long-term performance of bioactive films. Here we demonstrate the capacity to repeatedly regenerate a covalently immobilized monomolecular thin film of bioactive molecules through a two-step stripping and recharging cycle. Reversible transpeptidation by a laboratory evolved Staphylococcus aureus sortase A (eSrtA) enabled the rapid immobilization of an anti-thrombogenic film in the presence of whole blood and permitted multiple cycles of film regeneration in vitro that preserved its biological activity. Moreover, eSrtA transpeptidation facilitated surface re-engineering of medical devices in situ after in vivo implantation through removal and restoration film constituents. These studies establish a rapid, orthogonal and reversible biochemical scheme to regenerate selective molecular constituents with the potential to extend the lifetime of bioactive films.

Bioactive coatings offer a strategy to modulate host response to implants, but their translation to the clinic is hampered by their fast in vivo degradation. Here, the authors use an engineered bacterial protein to regenerate an anti-thrombogenic film in vitro and in situ after device implantation.

Enhanced Patency and Endothelialization of Small-Caliber Vascular Grafts Fabricated by Coimmobilization of Heparin and Cell-Adhesive Peptides

The clinical utility of a small-caliber vascular graft is still limited, owing to the occlusion of graft by thrombosis and restenosis. A small-caliber vascular graft (diameter, 2.5 mm) fabricated by electrospinning with a polyurethane (PU) elastomer (Pellethane) and biofunctionalized with heparin and two cell-adhesive peptides, GRGDS and YIGSR, was developed for the purpose of preventing the thrombosis and restenosis through antithrombogenic activities and endothelialization. The vascular grafts showed slightly reduced adhesion of platelets and significantly decreased adsorption of fibrinogen. In vitro studies demonstrated that peptide treatment on a vascular graft enhanced the attachment of human umbilical vein endothelial cells (HUVECs), and the presence of heparin and peptides on the graft significantly increased the proliferation of HUVECs. In vivo implantation of heparin/peptides coimmobilized graft (PU-PEG-Hep/G+Y) and PU (control) grafts was performed using an abdominal aorta rabbit model for 60 days followed by angiographic monitoring and explanting for histological analyses. The patency was significantly higher for the modified PU grafts (71.4%) compared to the PU grafts (46.2%) at 9 weeks after implantation. The nontreated PU grafts showed higher levels of α-SMA expression compared to the modified grafts, and for both samples, the proximal and distal regions expressed higher levels compared to the middle region of the grafts. Moreover, immobilization of heparin and peptides and adequate porous structure were found to play important roles in endothelialization and cellular infiltration. Our results strongly encourage that the development of small-caliber vascular grafts is feasible.

Biodegradable Polymers Influence the Effect of Atorvastatin on Human Coronary Artery Cells

Drug-eluting stents (DES) have reduced in-stent-restenosis drastically. Yet, the stent surface material directly interacts with cascades of biological processes leading to an activation of cellular defense mechanisms. To prevent adverse clinical implications, to date almost every patient with a coronary artery disease is treated with statins. Besides their clinical benefit, statins exert a number of pleiotropic effects on endothelial cells (ECs). Since maintenance of EC function and reduction of uncontrolled smooth muscle cell (SMC) proliferation represents a challenge for new generation DES, we investigated the effect of atorvastatin (ATOR) on human coronary artery cells grown on biodegradable polymers. Our results show a cell type-dependent effect of ATOR on ECs and SMCs. We observed polymer-dependent changes in IC50 values and an altered ATOR-uptake leading to an attenuation of statin-mediated effects on SMC growth. We conclude that the selected biodegradable polymers negatively influence the anti-proliferative effect of ATOR on SMCs. Hence, the process of developing new polymers for DES coating should involve the characterization of material-related changes in mechanisms of drug actions.

Polysaccharide-based antibiofilm surfaces

Surface treatment by natural or modified polysaccharide polymers is a promising means to fight against implant-associated biofilm infections. The present review focuses on polysaccharide-based coatings that have been proposed over the last ten years to impede biofilm formation on material surfaces exposed to bacterial contamination. Anti-adhesive and bactericidal coatings are considered. Besides classical hydrophilic coatings based on hyaluronic acid and heparin, the promising anti-adhesive properties of the algal polysaccharide ulvan are underlined. Surface functionalization by antimicrobial chitosan and derivatives is extensively surveyed, in particular chitosan association with other polysaccharides in layer-by-layer assemblies to form both anti-adhesive and bactericidal coatings.

STATEMENT OF SIGNIFICANCE

Bacterial contamination of surfaces, leading to biofilm formation, is a major problem in fields as diverse as medicine, first, but also food and cosmetics. Many prophylactic strategies have emerged to try to eliminate or reduce bacterial adhesion and biofilm formation on surfaces of materials exposed to bacterial contamination, in particular implant materials. Polysaccharides are widely distributed in nature. A number of these natural polymers display antibiofilm properties. Hence, surface treatment by natural or modified polysaccharides is a promising means to fight against implant-associated biofilm infections. The present manuscript is an in-depth look at polysaccharide-based antibiofilm surfaces that have been proposed over the last ten years. This review, which is a novelty compared to published literature, will bring well documented and updated information to readers of Acta Biomaterialia.

Haemostatic Response of Polyethylene Terephthalate Treated by Oxygen and Nitrogen Plasma Afterglows

Samples of polymer polyethylene terephthalate were coated with heparin and the haemostatic response has been determined by optical imaging of samples after incubation with fresh blood from a healthy donor. Prior to coating the samples were treated by neutral reactive particles of the oxygen or nitrogen plasma flowing afterglow. X-ray photoelectron spectroscopy analysis showed intensive functionalization of the polymer foils upon treatment with afterglows; however, the concentration of sulphur from heparin remained below the detection limit. The optical imaging showed densely distributed blood platelets in highly activated forms on untreated samples, whereas treatment with both afterglows revealed improved hemocompatibility. Best results were obtained for oxygen-functionalized polymer, whereas additional coating with heparin caused moderate loss of hemocompatibility, that was explained by deactivation of surface functional groups upon incubation with heparin.

Recombinant Technology in the Development of Materials and Systems for Soft-Tissue Repair

The field of biomedicine is constantly investing significant research efforts in order to gain a more in-depth understanding of the mechanisms that govern the function of body compartments and to develop creative solutions for the repair and regeneration of damaged tissues. The main overall goal is to develop relatively simple systems that are able to mimic naturally occurring constructs and can therefore be used in regenerative medicine. Recombinant technology, which is widely used to obtain new tailored synthetic genes that express polymeric protein-based structures, now offers a broad range of advantages for that purpose by permitting the tuning of biological and mechanical properties depending on the intended application while simultaneously ensuring adequate biocompatibility and biodegradability of the scaffold formed by the polymers. This Progress Report is focused on recombinant protein-based materials that resemble naturally occurring proteins of interest for use in soft tissue repair. An overview of recombinant biomaterials derived from elastin, silk, collagen and resilin is given, along with a description of their characteristics and suggested applications. Current endeavors in this field are continuously providing more-improved materials in comparison with conventional ones. As such, a great effort is being made to put these materials through clinical trials in order to favor their future use.

Early Experience With a Novel Hybrid Vascular Graft for Hemodialysis Access Creation in Patients With Disadvantaged Anatomy

Purpose: To describe the use of the Hybrid vascular graft in disadvantaged anatomy for hemodialysis access creation and compare outcomes to standard-wall polytetrafluoroethylene (PTFE) grafts. Methods: In a retrospective analysis, 25 patients (mean age 65±14 years; 13 men) who received the Hybrid graft were compared with 35 contemporaneous patients (mean age 63±12 years; 20 men) who received a standard PTFE graft for hemodialysis access over a 2-year period. Criteria for Hybrid graft placement were (1) exhausted or inadequate peripheral veins for arteriovenous fistula (AVF) creation and concomitant small target veins that precluded conventional PTFE graft placement, (2) previous graft anastomosis or a stent in the venous target at the level of the axilla, or (3) failed brachial-basilic or brachial-brachial upper arm transposition AVF with a small target vein at the axilla. Efficacy, anatomic and clinical considerations, and technique were reviewed; patency rates, complications, and reinterventions were examined. Results: Technical success was achieved in all cases, and all grafts were usable for hemodialysis. Seven of 25 Hybrid patients required stent-graft extensions and 3 patients required angioplasty to improve venous outflow at the time of Hybrid graft insertion. Three of 35 standard PTFE graft patients required angioplasty to improve venous outflow at the time of graft insertion. There was no perioperative mortality or procedure-related morbidity in either group. Median follow-up was 21 months. The patient survival estimate was 66% at 2 years. Estimated primary patency (24% vs 18%, p>0.05), assisted primary patency (34% vs 28%; p>0.05), and secondary patency rates (40% vs 38%, p≥0.05) at 24 months were equivalent for Hybrid vs PTFE grafts, respectively. Venous hypertension was not a complication following Hybrid graft implantation but was seen in 2 patients with the standard PTFE graft. Conclusion: The Hybrid graft offers a safe, technically effective alternative for patients with disadvantaged anatomy requiring hemodialysis access and has comparable outcomes to standard PTFE grafts. Further clinical experience and long-term data are required for determining the proper utility of this device in chronic dialysis-dependent patients.

Fibrinogen matrix deposited on the surface of biomaterials acts as a natural anti-adhesive coating

Adsorption of fibrinogen on the luminal surface of biomaterials is a critical early event during the interaction of blood with implanted vascular graft prostheses which determines their thrombogenicity. We have recently identified a nanoscale process by which fibrinogen modifies the adhesive properties of various surfaces for platelets and leukocytes. In particular, adsorption of fibrinogen at low density promotes cell adhesion while its adsorption at high density results in the formation of an extensible multilayer matrix, which dramatically reduces cell adhesion. It remains unknown whether deposition of fibrinogen on the surface of vascular graft materials produces this anti-adhesive effect. Using atomic force spectroscopy, single cell force spectroscopy, and standard adhesion assays with platelets and leukocytes, we have characterized the adhesive and physical properties of the contemporary biomaterials, before and after coating with fibrinogen. We found that uncoated PET, PTFE and ePTFE exhibited high adhesion forces developed between the AFM tip or cells and the surfaces. Adsorption of fibrinogen at the increasing concentrations progressively reduced adhesion forces, and at ≥2 μg/ml all surfaces were virtually nonadhesive. Standard adhesion assays performed with platelets and leukocytes confirmed this dependence. These results provide a better understanding of the molecular events underlying thrombogenicity of vascular grafts.

New agents for thromboprotection. A role for factor XII and XIIa inhibition

Blood coagulation is essential for hemostasis, however excessive coagulation can lead to thrombosis. Factor XII starts the intrinsic coagulation pathway and contact-induced factor XII activation provides the mechanistic basis for the diagnostic aPTT clotting assay. Despite its function for fibrin formation in test tubes, patients and animals lacking factor XII have a completely normal hemostasis. The lack of a bleeding tendency observed in factor XII deficiency states is in sharp contrast to deficiencies of other components of the coagulation cascade and factor XII has been considered to have no function for coagulation in vivo. Recently, experimental animal models showed that factor XII is activated by an inorganic polymer, polyphosphate, which is released from procoagulant platelets and that polyphosphate-driven factor XII activation has an essential role in pathologic thrombus formation. Cumulatively, the data suggest to target polyphosphate, factor XII, or its activated form factor XIIa for anticoagulation. As the factor XII pathway specifically contributes to thrombosis but not to hemostasis, interference with this pathway provides a unique opportunity for safe anticoagulation that is not associated with excess bleeding. The review summarizes current knowledge on factor XII functions, activators and inhibitors.

Cell-mimetic coatings for immune spheres

Extrinsically induced or engineered cells are providing new therapeutic means in emerging fields such as cell therapeutics, immunomodulation and regenerative medicine. We are demonstrating a spatial induction method using lipid coatings, which can change signal presentation strength from material surface to adherent macrophage cells, that induce early cell-cell interaction leading to organotypic morphology. For that, we have developed a cell mimetic lipid coating with a rafts size to the order of transmembrane proteins (<10 nm) with enhanced lateral elastic properties. Such surface coatings are capable of reducing adherent macrophage spreading, while enabling early induction of cell-cell interaction to form organotypic macrophage colonies or "spheres" (M-spheres).

Elastin-coated biodegradable photopolymer scaffolds for tissue engineering applications

One of the main open issues in modern vascular surgery is the nonbiodegradability of implants used for stent interventions, which can lead to small caliber-related thrombosis and neointimal hyperplasia. Some new, resorbable polymeric materials have been proposed to substitute traditional stainless-steel stents, but so far they were affected by poor mechanical properties and low biocompatibility. In this respect, a new material, polypropylene fumarate (PPF), may be considered as a promising candidate to implement the development of next generation stents, due to its complete biodegradability, and excellent mechanical properties and the ease to be precisely patterned. Besides all these benefits, PPF has not been tested yet for vascular prosthesis, mainly because it proved to be almost inert, while the ability to elicit a specific biological function would be of paramount importance in such critical surgery applications. Here, we propose a biomimetic functionalization process, aimed at obtaining specific bioactivation and thus improved cell-polymer interaction. Porous PPF-based scaffolds produced by deep-UV photocuring were coated by elastin and the functionalized scaffolds were extensively characterized, revealing a stable bound between the protein and the polymer surface. Both 3T3 and HUVEC cell lines were used for in vitro tests displaying an enhancement of cells adhesion and proliferation on the functionalized scaffolds.

Reprogramming the specificity of sortase enzymes

Staphylococcus aureus sortase A catalyzes the transpeptidation of an LPXTG peptide acceptor and a glycine-linked peptide donor and has proven to be a powerful tool for site-specific protein modification. The substrate specificity of sortase A is stringent, limiting its broader utility. Here we report the laboratory evolution of two orthogonal sortase A variants that recognize each of two altered substrates, LAXTG and LPXSG, with high activity and specificity. Following nine rounds of yeast display screening integrated with negative selection, the evolved sortases exhibit specificity changes of up to 51,000-fold, relative to the starting sortase without substantial loss of catalytic activity, and with up to 24-fold specificity for their target substrates, relative to their next most active peptide substrate. The specificities of these altered sortases are sufficiently orthogonal to enable the simultaneous conjugation of multiple peptide substrates to their respective targets in a single solution. We demonstrated the utility of these evolved sortases by using them to effect the site-specific modification of endogenous fetuin A in human plasma, the synthesis of tandem fluorophore-protein-PEG conjugates for two therapeutically relevant fibroblast growth factor proteins (FGF1 and FGF2), and the orthogonal conjugation of fluorescent peptides onto surfaces.

Recent advancements in the cardiovascular drug carriers

Cardiovascular disease is the disease that affects the cardiovascular system, vascular diseases of the brain and kidney, and peripheral arterial disease. Despite of all advances in pharmacological and clinical treatment, heart failure is a leading cause of morbidness and mortality worldwide. Many new therapeutic advance strategies, including cell transplantation, gene delivery or therapy, and cytokines or other small molecules, have been research to treat heart failure. The main aim of this review article is to focus on nano carriers advancement and addressing the problems associated with old and modern therapeutics such as nonspecific effects and poor stability.