Enhanced endothelial cell activity induced by incorporation of nano-thick tantalum layer in artificial vascular grafts

Highlights

• Ta-implanted surface layer on ePTFE was fabricated via sputtering-based plasma immersion ion implantation technique.

• Ta-implanted ePTFE exhibited excellent adhesion stability and an extremely low level of Ta ions released from the surface.

• Surface properties of the Ta-implanted ePTFE were highly favorable for endothelial cell adherence and growth.

• Ta-implanted ePTFE also possessed antithrombogenic properties, suppressing platelet adhesion and activation on the surface.

Abstract

Expanded polytetrafluoroethylene (ePTFE) has been successfully used as an artificial vascular graft material owing to its unique merits of fibrous structure, chemical stability, physical robustness, and nontoxicity. However, its insufficient endothelial cell affinity arising from its highly hydrophobic surface nature induces early thrombus formation and development of neointimal hyperplasia, leading to poor long-term patency rates. In this study, we demonstrate a novel rapid surface modification technique, termed as sputtering-based plasma immersion ion implantation (S-PIII), to elicit favorable vascular responses on the ePTFE surface. This technique enables rapid ion implantation of biologically compatible tantalum (Ta) into ePTFE surfaces, generating a nano-thick Ta-rich surface layer (<30 at.%) without any structural defects or loss of ePTFE’s fibrous morphology. Surface properties of ePTFE, such as its biologically inert chemical structure and strong hydrophobicity, are ameliorated considerably after the S-PIII treatment, which is more favorable for endothelial cell (EC) adherence, spreading, and proliferation. In particular, compared to a bare ePTFE surface, Ta-implanted ePTFE possesses an antithrombogenic property, suppressing platelet adhesion and activation on its surface. From an in vivo pilot study with a canine aortic bypass model, Ta-implanted ePTFE demonstrates substantially suppressed early thrombosis with rapid formation of EC monolayers covering the luminal surface.

Introduction

Recently, cardiovascular diseases (CVDs) such as myocardial infarction, arrhythmia, and peripheral ischemia have become leading diseases threatening human health, especially in ageing populations. The prevalence of cardiovascular risk factors including obesity, diabetes mellitus, and hypertension is causing CVDs to rapidly increase day by day; it is now responsible for the highest death toll in most industrialized countries [1]. In response to this problem, revascularization surgery through bypass grafting with autologous or allogenic grafts has been widely performed as a recommended treatment to restore blood flow by replacing diseased and damaged blood vessels. However, even though these surgical treatments exhibit high patency rates, problems including a lack of available autologous vessels in proper quality, donor shortages, and the risk of immune responses remain unsettled [2], [3], [4].

A possible intervention could be replacement with artificial vascular grafts made of biologically safe and compatible materials such as polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), or polyurethane (PU). In particular, the expanded form of PTFE (ePTFE) has been successfully used in clinical applications due to its unique merits of chemical stability, physical robustness, and nontoxicity. In addition, its highly porous structure, with an interconnected network of fibers, facilitates cell infiltration from the surrounding tissues and stimulates rapid tissue regeneration on artificial grafts [5]. Nevertheless, ePTFE vascular grafts can be substituted only for large-diameter blood vessels (>10 mm) with existing high blood flow rates. When used in medium- (6–10 mm) to small-diameter (<6 mm) vascular grafts, they are prone to early thrombosis and neointimal hyperplasia. Their innate hydrophobic characteristic with an extremely large surface area induces not only substantially increased adsorption of platelets and plasma proteins but also decreased endothelial cell (EC) adhesion and growth on its surface. This leads to a poor long-term patency rate with decreasing vascular graft diameter [6], [7].

In an effort to enhance the clinical outcomes of ePTFE vascular grafts in medium- and small-diameter applications, surface modifications have received substantial attention recently because of their versatile ability to elicit favorable cellular responses on modified surfaces. In particular, endothelialization, a crucial factor for long-term patency of vascular grafts [8], has been improved via immobilization of biomolecules such as DNA [9], cell adhesion peptide sequences [10], and growth factors [11], [12], [13]. However, pre-surface modification of ePTFE, which is essential for biomolecule coatings, involves lengthy and time-consuming processes, and is thus regarded as a negative factor in clinical treatment. Moreover, those modified surface layers have relatively low adhesion stability, especially in physiological conditions, and there is a high probability of inducing side effects in surrounding tissues, resulting in subsequent angiographic narrowing at the anastomosis site [14].

Plasma immersion ion implantation (PIII) is an attractive treatment for modifying the morphological and chemical properties of various polymers while maintaining sufficient adhesion stability. In particular, owing to its simple operation and non-line-of-sight characteristics, it is a promising candidate treatment for cylindrical vascular grafts [15]. In this process, a specimen is immersed in a plasma and several tens of kilovolts are applied to it. This induces uniform ion implantation over the surface of the whole specimen. However, under such an extremely high bias voltage, accelerated and injected ions may induce severe micro-structural damage and overall shape deformation of vascular grafts, involving bond breakage and molecular chain scission. Although ePTFE is well-known as a thermally and mechanically stable polymer substrate, submicron-scaled fibrous network structures may readily be destroyed, causing it to lose its proper structural functions. Therefore, there remains an urgent need for practical application of the PIII technique to ePTFE in order to facilitate rapid endothelialization and to suppress thrombus formation and graft failure.

Our continuous efforts to refine hardware and develop a simple and rapid ion implantation protocol have demonstrated that conventional direct current (DC) magnetron sputtering can induce extensive ion bombardment effects on a biomaterial’s surface when high negative bias voltages are applied to it [16], [17], [18], [19], [20], [21]. Numerous sputtered atoms from the surface of the target are immersed in the sputtering plasma and are instantly ionized as a result of energetic collisions with electrons and primary gaseous ions. When the substrate is negatively biased, neutral atoms and target ions are accelerated and implanted into the substrate surface via energetic collision cascades at markedly high doses and rates [18], [19], [20], [21]. This phenomenon is highly effective for the formation of an ion-implanted nano-thick skin layer of excellent interfacial strength due to its structural stability on the topmost surface of the substrate. Therefore, it can be free from general concerns of sputtering arising from thermal expansion and mechanical property mismatches between the target and substrate. We termed this technique as sputtering-based plasma immersion ion implantation (S-PIII) [20], [21]. Herein, to our knowledge for the first time, we introduce biologically compatible tantalum (Ta) for the vascular graft applications because of its excellent hemocompatibility and EC affinity [22], [23], [24]. We then implant it into an ePTFE surface using the proposed S-PIII method by applying a high negative bias voltage to a conducting metal core rod inserted into the ePTFE vascular graft. We then investigate and compare the surface morphology, chemistry, and stability of the ePTFE vascular graft after S-PIII (Ta-implanted ePTFE), normal Ta deposition (Ta-coated ePTFE), and non-treatment (bare ePTFE). We also estimated the biological properties of Ta-implanted ePTFE by assessing its affinity for ECs and platelets in vitro and in vivo with a canine aortic bypass model.