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Long Term Evaluation of Nanofibrous, Bioabsorbable Polycarbonate Urethane Grafts for Small Diameter Vessel Replacement in Rodents

  • Magdalena Eilenberg
    Affiliations
    Department of Surgery, Medical University of Vienna, Vienna, Austria

    Centre for Biomedical Research, Medical University of Vienna, Vienna, Austria

    Ludwig Boltzmann Cluster for Cardiovascular Research, Medical University of Vienna, Vienna, Austria
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  • Marjan Enayati
    Affiliations
    Centre for Biomedical Research, Medical University of Vienna, Vienna, Austria

    Ludwig Boltzmann Cluster for Cardiovascular Research, Medical University of Vienna, Vienna, Austria
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  • Daniel Ehebruster
    Affiliations
    Centre for Biomedical Research, Medical University of Vienna, Vienna, Austria
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  • Christian Grasl
    Affiliations
    Ludwig Boltzmann Cluster for Cardiovascular Research, Medical University of Vienna, Vienna, Austria

    Centre of Medical Physics and Biomedical Engineering, Medical University of Vienna, Vienna, Austria
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  • Ingrid Walter
    Affiliations
    Department of Pathobiology, Veterinary University, Vienna, Austria
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  • Barbara Messner
    Affiliations
    Surgical Research Laboratories-Cardiac Surgery, Department of Surgery, Medical University of Vienna, Vienna, Austria
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  • Stefan Baudis
    Affiliations
    Institute of Applied Synthetic Chemistry, Vienna University of Technology, Vienna, Austria

    Austrian Cluster for Tissue Regeneration, Vienna, Austria
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  • Paul Potzmann
    Affiliations
    Institute of Applied Synthetic Chemistry, Vienna University of Technology, Vienna, Austria

    Austrian Cluster for Tissue Regeneration, Vienna, Austria
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  • Christoph Kaun
    Affiliations
    Ludwig Boltzmann Cluster for Cardiovascular Research, Medical University of Vienna, Vienna, Austria

    Department of Internal Medicine II, Medical University of Vienna, Vienna, Austria
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  • Bruno K. Podesser
    Affiliations
    Centre for Biomedical Research, Medical University of Vienna, Vienna, Austria

    Ludwig Boltzmann Cluster for Cardiovascular Research, Medical University of Vienna, Vienna, Austria

    Austrian Cluster for Tissue Regeneration, Vienna, Austria
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  • Johann Wojta
    Affiliations
    Ludwig Boltzmann Cluster for Cardiovascular Research, Medical University of Vienna, Vienna, Austria

    Department of Internal Medicine II, Medical University of Vienna, Vienna, Austria
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  • Helga Bergmeister
    Correspondence
    Corresponding author. Center for Biomedical Research, Ludwig Boltzmann Cluster for Cardiovascular Research, Medical University of Vienna, Waehringer Guertel 18–20, A-1090, Vienna, Austria.
    Affiliations
    Centre for Biomedical Research, Medical University of Vienna, Vienna, Austria

    Ludwig Boltzmann Cluster for Cardiovascular Research, Medical University of Vienna, Vienna, Austria
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Open ArchivePublished:December 21, 2019DOI:https://doi.org/10.1016/j.ejvs.2019.11.004

      Objective

      Biodegradable materials for in situ vascular tissue engineering could meet the increasing clinical demand for sufficient synthetic small diameter vascular substitutes in aortocoronary bypass and peripheral vascular surgery. The aim of this study was to design a new degradable thermoplastic polycarbonate urethane (dPCU) with improved biocompatibility and optimal biomechanical properties. Electrospun conduits made from dPCU were evaluated in short and long term follow up and compared with expanded polytetrafluoroethylene (ePTFE) controls.

      Methods

      Both conduits were investigated prior to implantation to assess their biocompatibility and inflammatory potential via real time polymerase chain reaction using a macrophage culture. dPCU grafts (n = 28) and ePTFE controls (n = 28) were then implanted into the infrarenal abdominal aorta of Sprague–Dawley rats. After seven days, one, six, and 12 months, grafts were analysed by histology and immunohistochemistry (IHC) and assessed biomechanically.

      Results

      Anti-inflammatory signalling was upregulated in dPCU conduits and increased significantly over time in vitro. dPCU and ePTFE grafts offered excellent long and short term patency rates (92.9% in both groups at 12 months) in the rat model without dilatation or aneurysm formation. In comparison to ePTFE, dPCU grafts showed transmural ingrowth of vascular specific cells resulting in a structured neovessel formation around the graft. The graft material was slowly reduced, while the compliance of the neovessel increased over time.

      Conclusion

      The newly designed dPCU grafts have the potential to be safely applied for in situ vascular tissue engineering applications. The degradable substitutes showed good in vivo performance and revealed desirable characteristics such as biomechanical stability, non-thrombogenicity, and minimal inflammatory response after long term implantation.

      Keywords

      This study provides long term results of a novel, degradable, small diameter vascular graft in a rat model. The Degradable polycarbonate urethane (dPCU) exhibits appropriate biomechanical properties and improved biocompatibility with reduced secondary inflammation. dPCU conduits promoted rapid complete endothelialisation, increased and sustained transmural cellular ingrowth, proliferation of cells and microvessel formation with minimal inflammatory response.

      INTRODUCTION

      Autologous saphenous vein, internal mammary artery (IMA), and the radial artery (RA) are currently the gold standard for small diameter vascular reconstructions. The limited availability of autologous graft material is aggravated by prior use or insufficient graft quality. These shortages are becoming more frequent as our ageing society suffers from metabolic syndrome, arteriosclerosis, and varicosities. However, successful long term replacement of small diameter vascular grafts (SDVG) by a synthetic ready to use material remains challenging.
      • Desai M.
      • Seifalian A.M.
      • Hamilton G.
      Role of prosthetic conduits in coronary artery bypass grafting.
      Currently used synthetic materials such as expanded polytetrafluoroethylene (ePTFE) and Dacron perform well as large diameter conduits. In grafts with 2–6 mm inner diameter suitable for peripheral arterial or coronary artery revascularisation, patency is very low because of high thrombogenicity, intimal hyperplasia (IH), inflammatory processes, or graft infection.
      • Pashneh-Tala S.
      • MacNeil S.
      • Claeyssens F.
      The tissue-engineered vascular graft-past, present, and future.
      After implantation, inflammatory cells migrate onto the graft and are important mediators of graft remodelling, switching from the initial pro-inflammatory signal to tissue remodelling and repair. Monocytes/macrophages are present until degradation and produce cell proliferation and remodelling factors such as interleukin (IL)-6, IL-10, and matrix metalloproteases. Long term presence of synthetic material may lead to extended foreign body reaction and fibrosis/scar tissue formation.
      • Crupi A.
      • Costa A.
      • Tarnok A.
      • Melzer S.
      • Teodori L.
      Inflammation in tissue engineering: the Janus between engraftment and rejection.
      Furthermore a graft infection may cause disrupted graft healing, and anastomotic failure or rupture as a result of chronic inflammation.
      • Pashneh-Tala S.
      • MacNeil S.
      • Claeyssens F.
      The tissue-engineered vascular graft-past, present, and future.
      Many materials and approaches for small diameter vessel (SDV) replacement have been studied;
      • Pashneh-Tala S.
      • MacNeil S.
      • Claeyssens F.
      The tissue-engineered vascular graft-past, present, and future.
      the ideal ready to use material should be designed to match the native vessel in compliance, dimensions, haemodynamic factors, anti-thrombotic and anti-inflammatory (non-toxic) qualities, and should promote fast endothelial cell adherence.
      • Chang W.G.
      • Niklason L.E.
      A short discourse on vascular tissue engineering.
      From the investigated synthetic materials, non-degradable electrospun polycarbonate urethane (PCU) has shown promise to meet the criteria. Thermoplastic PCUs are block copolymers with hard and soft segments (semi)crystalline and amorphous domains, consisting of (poly)carbonate based precursors, which form physically cross linked networks with superior mechanical properties. Non-degradable, thermoplastic PCU with various modifications has been tested in small and large animal models with sound results, and pilot studies have been performed successfully in human arteriovenous fistulae.
      • Ahmed M.
      • Hamilton G.
      • Seifalian A.M.
      The performance of a small-calibre graft for vascular reconstructions in a senescent sheep model.
      • Qiu X.
      • Lee B.L.
      • Ning X.
      • Murthy N.
      • Dong N.
      • Li S.
      End-point immobilization of heparin on plasma-treated surface of electrospun polycarbonate-urethane vascular graft.
      • Ferraresso M.
      • Bortolani E.M.
      • Amnon G.
      A two-year experience with a rapid access, self-sealing, polycarbonate urethane nanofiber vascular access graft for hemodialysis.
      These PCUs often consist of poly (hexamethylene carbonate), combined with aromatic diisocyanates, for example 4,4′-methylene bisphenyl diisocyanate (MDI) with 1,4-butane diol as chain extenders
      • Sobczak M.
      • Dębek C.
      • Olędzka E.
      • Nałęcz-Jawecki G.
      • Kołodziejski W.L.
      • Rajkiewicz M.
      Segmented polyurethane elastomers derived from aliphatic polycarbonate and poly(ester-carbonate) soft segments for biomedical applications.
      and, although categorised as non-degradable, can be subject to uncontrolled biodegradation, especially under oxidative stress.
      • Dempsey D.K.
      • Carranza C.
      • Chawla C.P.
      • Gray P.
      • Eoh J.H.
      • Cereceres S.
      • et al.
      Comparative analysis of in vitro oxidative degradation of poly(carbonate urethanes) for biostability screening.
      For this reason, the present study group aimed to further improve the qualities of PCUs by design and synthesis of a material with specific biodegradability for in situ vascular tissue engineering. The advantage of a biodegradable approach includes continuous remodelling of graft until an endogenous neovessel has formed. Prerequisites for degradable materials are adequate biomechanical stability until sufficient vascular specific, host dependent, tissue formation has occurred to avoid leakage, rupture, and aneurysm formation.
      • Elomaa L.
      • Yang Y.P.
      Additive Manufacturing of vascular grafts and vascularized tissue constructs.
      The advantage of this newly synthesised thermoplastic degradable polycarbonate urethane (dPCU) is its slow degradation rate by the use of cleavable carbonate based chain extenders, resulting in controlled graft reduction without development of acidic or toxic byproducts. The biocompatibility of the polyurethanes has been improved by the use of aliphatic rather than aromatic organic compounds.
      • Baudis S.
      • Ligon S.C.
      • Seidler K.
      • Weigel G.
      • Grasl C.
      • Bergmeister H.
      • et al.
      Hard-block degradable thermoplastic urethane-elastomers for electrospun vascular prostheses.
      Furthermore, because of the excellent biomechanical characteristics of dPCU, a slim graft wall, matching that of the native vessel, was designed.
      • Pashneh-Tala S.
      • MacNeil S.
      • Claeyssens F.
      The tissue-engineered vascular graft-past, present, and future.
      The aim of the study was to characterise the graft material with regard to biomechanical properties and biocompatibility and further analyse the short and long term performance of this new synthesised dPCU in a rodent model.

      METHODS

      Material and biomechanical testing

      A pre-polymer method was used to synthesise the thermoplastic polyurethane polymer.
      • Baudis S.
      • Ligon S.C.
      • Seidler K.
      • Weigel G.
      • Grasl C.
      • Bergmeister H.
      • et al.
      Hard-block degradable thermoplastic urethane-elastomers for electrospun vascular prostheses.
      ,
      • Enayati M.
      • Eilenberg M.
      • Grasl C.
      • Riedl P.
      • Kaun C.
      • Messner B.
      • et al.
      Biocompatibility assessment of a new biodegradable vascular graft via in vitro co-culture approaches and in vivo model.
      In contrast to the previously described polymers, the dPCU in this study was based on poly (hexamethylene carbonate), hexamethylene diisocyanate (HMDI), and bis(3-hydroxypropyl) carbonate. An average molecular weight of 860 Da for poly (hexamethylene carbonate) (which corresponds to a typical chain length in thermoplastic polyurethanes) and a molar ratio of the above mentioned components of 1:2:1 resulted in a good trade off between tensile strength and toughness of the material.
      • Ochiai B.
      • Amemiya H.
      • Yamazaki H.
      • Endo T.
      Synthesis and properties of poly(carbonate-urethane) consisting of alternating carbonate and urethane moieties.
      dPCU grafts were fabricated by the electrospinning method and characterised morphologically and mechanically by scanning electron microscopy (SEM; JEOL JSM-5400, Japan) and micro-computed tomography (μCT) (μCT-35, SCANCO Medical, Zurich, Switzerland).
      • Bergmeister H.
      • Schreiber C.
      • Grasl C.
      • Walter I.
      • Plasenzotti R.
      • Stoiber M.
      • et al.
      Healing characteristics of electrospun polyurethane grafts with various porosities.
      ,
      • Bergmeister H.
      • Seyidova N.
      • Schreiber C.
      • Strobl M.
      • Grasl C.
      • Walter I.
      • et al.
      Biodegradable, thermoplastic polyurethane grafts for small diameter vascular replacements.
      Additionally, dPCU's fibre distribution was studied by atomic force microscopy (AFM) using a JPK NanoWizard III (JPK Instruments AG, Germany) connected to an inverted optical microscope (Axio Observer Z1, Zeiss). AFM silicon nitride probes (DNP-S10, Bruker, USA) were applied as cantilevers with a spring constant of 0.3 N/m.
      • Iturri J.
      • Vianna A.C.
      • Moreno-Cencerrado A.
      • Pum D.
      • Sleytr U.B.
      • Toca-Herrera J.L.
      Impact of surface wettability on S-layer recrystallization: a real-time characterization by QCM-D.
      The suture retention of dPCU was tested prior to implantation using a polypropylene thread (7/0 Prolene, BV 176–8, Ethicon, USA) with a tensile tester (Messphysik Beta 10–2,5, Messphysik Materials Testing GmbH, Altenmarkt, Austria).
      • Bergmeister H.
      • Schreiber C.
      • Grasl C.
      • Walter I.
      • Plasenzotti R.
      • Stoiber M.
      • et al.
      Healing characteristics of electrospun polyurethane grafts with various porosities.
      The maximum tensile force and compliance in the physiological range were investigated before and after six and 12 months of implantation and compared with ePTFE (n = 4 per group and time, details in supplements) using a uniaxial BOSE ElectroForce LM1 test bench system (Bose Corp. MN, USA).
      • Bergmeister H.
      • Schreiber C.
      • Grasl C.
      • Walter I.
      • Plasenzotti R.
      • Stoiber M.
      • et al.
      Healing characteristics of electrospun polyurethane grafts with various porosities.
      ,
      • Bergmeister H.
      • Seyidova N.
      • Schreiber C.
      • Strobl M.
      • Grasl C.
      • Walter I.
      • et al.
      Biodegradable, thermoplastic polyurethane grafts for small diameter vascular replacements.
      The porosity of dPCU was determined by the liquid intrusion method.
      • de Valence S.
      • Tille J.C.
      • Giliberto J.P.
      • Mrowczynski W.
      • Gurny R.
      • Walpoth B.H.
      • et al.
      Advantages of bilayered vascular grafts for surgical applicability and tissue regeneration.
      Before proceeding with any in vivo or in vitro experiment samples were sterilised with ethylene oxide.

      In vitro evaluation by macrophage markers and inflammatory cytokines

      All experiments involving animals or animal tissues were conducted in compliance with European and national legislation and were approved by the Austrian Federal Ministry of Education, Science and Research (reference number: BMWF-66.009/0097-II/3b/2013). The animals’ care was in accordance with institutional guidelines.
      Macrophages were isolated from adult male Sprague–Dawley rats (350–400 g, Centre for Biomedical Research, Vienna, Austria) by a peritoneal lavage technique as described elsewhere.
      • Enayati M.
      • Eilenberg M.
      • Grasl C.
      • Riedl P.
      • Kaun C.
      • Messner B.
      • et al.
      Biocompatibility assessment of a new biodegradable vascular graft via in vitro co-culture approaches and in vivo model.
      Isolated macrophages at a density of 2 × 105 cells/well (24 well plate) were seeded on the luminal side of the grafts (10 mm* 1.8 mm) and incubated for one, three, seven, and 21 days. All experiments were repeated independently three times (n = 3 per time point).
      Real time polymerase chain reaction (RT-PCR) was used to identify expression of CD68 for pan-macrophages, CCR7 and CD80 for pro-inflammatory (M1) and CD163 and CD206 for anti-inflammatory (M2) macrophage markers. The ratio of CD80/CD163 gene expression was calculated as an indicator of M1/M2 response. Furthermore, pro-inflammatory cytokines IL-1α and tumour necrosis factor (TNF)-α and anti-inflammatory IL-10 were studied in the presence of the dPCU and ePTFE. RT-PCR was performed using the RNeasy Mini Kit (Qiagen, Valencia, CA, USA) via QIAcube system (Qiagen).
      • Enayati M.
      • Eilenberg M.
      • Grasl C.
      • Riedl P.
      • Kaun C.
      • Messner B.
      • et al.
      Biocompatibility assessment of a new biodegradable vascular graft via in vitro co-culture approaches and in vivo model.
      Data were analysed using LightCycler Software (LightCycler Software Version 3.5, Roche, Basel, Switzerland).

      In vivo evaluation

      Electrospun dPCU and ePTFE prostheses were implanted as interposition grafts into the infrarenal aorta of 56 inbred Sprague–Dawley rats (male, body weight 300–400 g) (n = 7 per time point and group) using an operating microscope (Zeiss OPMI 9-FC, Zeiss, Germany).
      • Bergmeister H.
      • Seyidova N.
      • Schreiber C.
      • Strobl M.
      • Grasl C.
      • Walter I.
      • et al.
      Biodegradable, thermoplastic polyurethane grafts for small diameter vascular replacements.
      The group allocation was randomised. Prostheses were anastomosed end to end using an interrupted suture technique (Monosof 10/0, Tyco, Norwalk, CT, USA) by one experienced surgeon. Neither anti-coagulation nor anti-platelet drugs were administered to the animals. Grafts remained in situ for seven days, one, six, or 12 months (dPCU n = 7, ePTFE n = 7, for each time point, respectively). In the six month and 12 month implantation groups, digital subtraction angiography was performed using 1 mL/kg iopamidol (Jopamiro®, 300 mg/mL, Bracco, Vienna, Austria).

      Histology, immunohistochemistry (IHC), and immunofluorescence

      Histological samples were obtained from all time points, processed and evaluated regarding endothelialisation (von Willebrand Factor, vWF), cell proliferation and invasion (Ki67, haematoxylin eosin (HE), smooth muscle actin (SMA), calponin, vimentin, collagen), and inflammatory potential (ED1/CD68, ED2/CD163) (detailed information in the Supplementary material). To assess occurrence of degradation, stenosis, or aneurysm formation, five consecutive histological slide samples from both anastomotic sites and midgraft regions were evaluated regarding their respective inner diameter and wall thicknesses. Samples after seven days implantation were excluded from statistical evaluation regarding quantitative histological parameters, because of difficult tissue fixation on the graft.

      Statistical analysis

      Median (quartile) values or mean and standard deviation, if applicable, were given to describe continuous variables, and absolute numbers and percentages were used to describe categorical variables. The number of microvessels, foreign body giant cells, SMA, calponin, ED1, ED2 positive cells in the media, and absolute numbers of cells within the graft were set in relation to the graft area. Differences in continuous variables between different time points and materials were tested using analysis of variance (ANOVA), Scheffé’s post hoc analysis, and the two sample t test. Non-normally distributed variables were compared by the Wilcoxon rank sum test. Correlations of continuous variables were characterised using the Spearman correlation coefficient. Patency rates were calculated using the Kaplan–Meier analysis. All p values are results of two sided tests and p values < .05 were considered to be statistically significant. SPSS software version 24.0 (IBM corporations Inc. 1989–2016; Armonk, NY, USA) was used for statistical analyses.

      RESULTS

      Graft characteristics

      Fabricated grafts had the following characteristics: fibre diameter: 1.66 ± 0.77 μm; porosity: 54.9 ± 1.5%; pore size: 3.8 ± 1 μm; inner diameter: 1.8 ± 0.1 mm; wall thickness: 84 ± 12 μm; graft length: 18 mm (Fig. 1). ePTFE grafts (inner diameter: 1.5 mm; wall thickness: 100 μm; graft length: 18 mm; internodal distance: 5–25 μm, Zeus, Orangeburg, SC, USA) were used as controls.
      Figure 1
      Figure 1Morphological features of degradable polycarbonate urethane (dPCU). The dPCU graft morphology was assessed prior to implantation by microcomputed tomography (A, B, C, G), scanning electron microscopy (D, F, H), and atomic force microscopy (E) (n = 4 for each technique). Cross section of the graft (A), cross section through the graft's wall (B, D), luminal surface (E, F, H) and view from the adventitial side (G), angled view showing both the luminal (above) and the adventitial side (below) (C).

      Surgical handling and biomechanics

      dPCU grafts were easy to handle and remained open by their inherent wall tension, whereas ePTFE grafts, similar in wall thickness, collapsed prior to implantation. Markedly improved suturability was observed in dPCU (suture retention dPCU 1.87 ± 0.17 N vs. ePTFE 0.09 ± 0.01 N, p < .001). The ultimate tensile force was higher in dPCU grafts than in ePTFE throughout the experiment (prior implantation: 3.79 ± 0.64 vs. 1.49 ± 0.26 N, p < .001; after six months: 4.79 ± 1.13 N vs. 2.05 ± 0.49 N, p < .001; and after one year: 5.02 ± 0.13 N vs. 1.98 ± 0.36 N, p < .001). The compliance of dPCU was similar to that of ePTFE grafts before (3.19 ± 0.75%/100 mmHg vs. 3.50 ± 1.4%/100 mm Hg, n.s.) and after 180 days of implantation (3.72 ± 0.95%/100 mmHg vs. 3.54 ± 0.89%/100 mmHg, n.s). After one year the compliance increased significantly in dPCU compared with baseline, but remained unaffected in ePTFE (dPCU 5.0 ± 1.62%/100 mmHg, p = .026; ePTFE 3.66 ± 1.13%/100 mmHg, n.s.).

      In vitro testing

      In vitro macrophage culture showed a predominant pro-inflammatory response within the first three days in both materials. After seven days there was a clear transition from M1 pro-inflammatory (CD80) to M2 anti-inflammatory (CD163) macrophages in dPCU. In ePTFE the pro-inflammatory response decreased significantly; however, the transition of M1 to M2 did not occur in these grafts. In detail, pro-inflammatory signalling in ePTFE was significantly higher regarding CCR7 (d3 p < .001) and CD80 (d3,7,21 p < .001). By contrast, anti-inflammatory signalling was upregulated and increased significantly over time in dPCU (p < .001), while there was only a moderate increase in ePTFE (n.s.). After 21 days CD163 and CD206 expression was significantly higher in dPCU in comparison to ePTFE (p < .001).
      In ePTFE, pro-inflammatory cytokines were significantly increased at an early phase, but downregulated significantly at later stages (IL-1α d1 p < .001; TNF-α d3 p < .001). IL-10 as anti-inflammatory cytokine was increased significantly in dPCU (d1 p = .037, d3 p = .004, and d21 p < .0001) (Fig. 2, Supplementary tables).
      Figure 2
      Figure 2In vitro evaluation of inflammatory behaviour. The expression of pro- and anti-inflammatory markers were studied in a macrophage culture exposed to expanded polytetrafluoroethylene (ePTFE) (grey) and degradable polycarbonate urethane (dPCU) (black) over the course of 21 days. Interleukin (IL)-1α (A) and tumour necrosis factor (TNF)-α (B) were used as markers for pro-inflammatory, and IL-10 (C) as marker for anti-inflammatory cytokine expression. Macrophage differentiation to pro-inflammatory M1- (D: CCR7 and E: CD80), anti-inflammatory M2- (F: CD163, G: CD206), and pan-macrophages (H: CD68) as well as the M1/M2 ratio (I) were evaluated (n = 3 per time-point per group). *p < .05, **p < .005, ****p ≤ .0001 (complete data in the supplementary material).

      In vivo results

      In one dPCU animal, occlusion occurred after 100 days because of intimal hyperplasia, and two rats died after early thrombosis within the first week in the ePTFE group. Therefore, patency was 92.9% for dPCU and ePTFE according to Kaplan–Meier estimates (Fig. 7, Table S5). At days seven, 30, 180, and 365, the individual patency was 100%, 100%, 92.9%, and 92.9% for dPCU and 100%, 92.9%, 92.9%, and 92.9% for ePTFE, respectively.
      Figure 3
      Figure 3Macroscopic comparison of degradable polycarbonate urethane (dPCU) (A, B, C, D) and expanded polytetrafluoroethylene (ePTFE) (E, F, G) as infrarenal conduit (A) Digital subtraction angiography 12 months after infrarenal aortic graft implantation (graft edges marked with arrows). Macroscopic pictures of grafts at the time of implantation immediately after reperfusion (B, E) and after 12 months of implantation (C, F). Comparison of luminal surfaces after longitudinal opening of the grafts (D, G) (n = 7 per time point and group.)
      Figure 4
      Figure 4Architecture of the integrated degradable polycarbonate urethane (dPCU) graft in direct comparison to the native abdominal aorta. The natural vessel's structure (A) consists of the intima (arrow), media (m), and adventitia (a). After implantation of the dPCU graft (g) the architecture was soon restored and the layers discernible (B). The graft was incorporated between neointima and neomedia (12 months of implantation, n = 7). Stainings from left to right: haematoxylin eosin, smooth muscle actin, elastica, anilin blue, von Willebrand factor.
      Figure 5
      Figure 5Colocalisation of vascular proteins by immunofluorescent staining. Representative confocal images of host cell localisation in the native infrarenal rat aorta (A) and degradable polycarbonate urethane (dPCU) grafts (F, K) after 12 months of implantation (n = 7, respectively). Colocalisation of vascular proteins is indicated by immunofluorescent staining of von Willebrand factor (vWF; C, H), smooth muscle actin (SMA; D, I, M), and calponin (N). 4′,6-diamidino-2-phenylindole (DAPI) for cell nuclei (B, G, L); Combinations of DAPI, vWF, and SMA in (E) and (J) or DAPI, SMA, and calponin in (O). The residual graft is marked by *.
      Figure 6
      Figure 6Expanded polytetrafluoroethylene (ePTFE) vs. degradable polycarbonate urethane (dPCU): characterisation by different immunohistochemical staining. While there were smooth muscle actin (SMA; E) and calponin (F) positive dominant neomedia in dPCU (E–H), in ePTFE grafts (A–D) predominantly vimentin positive cells were found at the adventitial side (C). Intimal hyperplasia (D) marked by * and regular intima (H) are shown by von Willebrand factor (vWF) stain. Immunohistochemical stainings: SMA (A, E); calponin (B, F); vimentin (C, G); vWF (D, H).
      Figure 7
      Figure 7Cumulative Kaplan–Meier estimate of the patency of degradable polycarbonate urethane (dPCU) and expanded polytetrafluoroethylene (ePTFE) grafts. Tick marks indicate censored animals because of their group allocation according to the implantation time (7, 30, 180, 365 days; n = 7/group/material). The numbers for animals at risk are given for the individual implantation times and at the event of occlusion (days 2 and 100).
      Peri-operative mortality was 8.9% because of bleeding and complications from anaesthesia and these animals were not considered in further calculations. In all animals reaching the endpoints of six and 12 months respectively, angiography showed a patent graft without signs of aneurysm or stenosis. The inner diameter according to digital subtraction angiography and histology did not change significantly over time. On longitudinal opening of the samples, all grafts were patent, displaying a shiny regular luminal surface without signs of plaque, thrombus, or calcification (Fig. 3).

      Microscopic evaluation

      A decrease in average graft wall thickness was observed after 12 months, but was not significant compared with pre-operative values (84 ± 12 μm vs. 59.1 ± 16.7 μm, n.s.). Full endothelialisation of all dPCU grafts was accomplished within 30 days, while overall 14.3% of ePTFE had no coverage (p = .05) or were only partially covered (17.9%, p = .02). Small areas of intimal hyperplasia [median 12 cell layers (7; 13)] were found in proximity to the anastomosis in three dPCU grafts, while IH covering the entire length of the graft was only found in one dPCU sample. By contrast, in eight ePTFE grafts [median of 11 cell layers (8; 17)] IH covered the total graft surface. The total number of cells within the dPCU graft remained stable over time (p = .58), in ePTFE conduits cell numbers decreased (p = .03). Overall, more cells migrated into dPCU than ePTFE grafts (437.2 ± 257.9 cells/mm2 vs. 110.7 ± 121.6 cells/mm;
      • Pashneh-Tala S.
      • MacNeil S.
      • Claeyssens F.
      The tissue-engineered vascular graft-past, present, and future.
      p < .001). Proliferation remained constant in dPCU, while it started at a higher level in ePTFE (Ki67, 35.4 ± 34.3 cells/mm2 vs. 636.3 ± 436.7 cells/mm2 after 30 days, p = .001) and dropped significantly over time (p < .001), until there were no significant differences between materials at six (p = .2) and 12 months (p = .17). Within the first month, neovessel formation with the regular composition of (single layer) endothelium, intermediate graft, neomedia, and adventitia was visible in all dPCU grafts (Fig. 4). SMCs aligned, forming the typical layer of the media. Furthermore, colocalisation of SMA and calponin was present at six and 12 months (r = .75, p = .005) (Fig. 5). The total number of vasa vasorum was higher in dPCU grafts (dPCU: 67.0 ± 66.9 vessels/mm,
      • Pashneh-Tala S.
      • MacNeil S.
      • Claeyssens F.
      The tissue-engineered vascular graft-past, present, and future.
      ePTFE: 32.4 ± 28.7 vessels/mm,
      • Pashneh-Tala S.
      • MacNeil S.
      • Claeyssens F.
      The tissue-engineered vascular graft-past, present, and future.
      p = .03). The presence of vimentin positive cells within the periconduit layer (indicating fibrous tissue) was more scarce in dPCU grafts (104 ± 57.8 cells/mm2 vs. 241.7 ± 190.8 cells/mm,
      • Pashneh-Tala S.
      • MacNeil S.
      • Claeyssens F.
      The tissue-engineered vascular graft-past, present, and future.
      p = .01) (Fig. 6). There were few signs of inflammation in either graft, although overall pan-macrophage infiltration was significantly higher in ePTFE grafts (151.6 ± 138.9 cells/mm2 vs. 370.2 ± 355 cells/mm,
      • Pashneh-Tala S.
      • MacNeil S.
      • Claeyssens F.
      The tissue-engineered vascular graft-past, present, and future.
      p = .02). In vivo ED1 and ED2 positive cells or foreign body giant cells were exceedingly rare and did not differ between grafts.

      DISCUSSION

      The present study comprised an investigation of a slowly biodegradable small diameter vascular conduit from a newly designed dPCU regarding its in vivo characteristics and in vitro biocompatibility behaviour. The graft is highly promising, as it showed superiority in surgical handling, biomechanical properties, and endothelialisation, and lower in vitro inflammatory potential compared with the control ePTFE material.
      Non-availability of autologous veins or arteries may be a limitation for performing peripheral or coronary revascularisation. Ready made, off the shelf SDVG have yet to reach adequate performance. The first commercially available and CE certified, non-degradable PCU prosthesis (AVflo™) for haemodialysis showed 56% primary and 82% secondary patency rate in 12 patients after 24 months.
      • Ferraresso M.
      • Bortolani E.M.
      • Amnon G.
      A two-year experience with a rapid access, self-sealing, polycarbonate urethane nanofiber vascular access graft for hemodialysis.
      Polyhedral oligomeric silsesquioxane poly (carbonate-urea)urethane (POSS-PCU) reached a patency of 64% after nine months in a senescent sheep model of carotid artery replacement.
      • Ahmed M.
      • Hamilton G.
      • Seifalian A.M.
      The performance of a small-calibre graft for vascular reconstructions in a senescent sheep model.
      To meet surgical and biomechanical prerequisites, wall thickness was frequently increased to many times the host vessel dimensions,
      • Ahmed M.
      • Hamilton G.
      • Seifalian A.M.
      The performance of a small-calibre graft for vascular reconstructions in a senescent sheep model.
      ,
      • de Valence S.
      • Tille J.C.
      • Giliberto J.P.
      • Mrowczynski W.
      • Gurny R.
      • Walpoth B.H.
      • et al.
      Advantages of bilayered vascular grafts for surgical applicability and tissue regeneration.
      ,
      • Wu W.
      • Allen R.A.
      • Wang Y.
      Fast-degrading elastomer enables rapid remodeling of a cell-free synthetic graft into a neoartery.
      resulting in a compliance mismatch and shear stress at the anastomosis. In this study, the dimensions of the graft were adjusted to be as close as possible to those of the host vessel. In the left anterior descending (LAD) arteries of healthy volunteers, the luminal diameter was 2.2 ± 0.6 mm, as measured by high resolution transthoracic echocardiography.
      • Perry R.
      • Joseph M.X.
      • Chew D.P.
      • Aylward P.E.
      • De Pasquale C.G.
      Coronary artery wall thickness of the left anterior descending artery using high resolution transthoracic echocardiography-normal range of values.
      However, a marginally smaller diameter was chosen to avoid mismatch to the rat's aorta. The outstanding properties of dPCU made possible matching to the host dimensions and biomechanical performance, which reflects the excellent overall performance and superiority in surgical handling of this graft. Remarkably, the compliance of dPCU increased significantly after 12 months of implantation and was comparable or somewhat inferior to that of human LAD or IMA, but superior to that of other synthetic grafts.
      • van Haaften E.E.
      • Bouten C.V.C.
      • Kurniawan N.A.
      Vascular mechanobiology: towards control of in situ regeneration.
      ,
      • Kumar V.A.
      • Brewster L.P.
      • Caves J.M.
      • Chaikof E.L.
      Tissue engineering of blood vessels: functional requirements, progress, and future challenges.
      As expected, because of the slow degrading nature of the material with a calculated full degradation after two years, there was no significant decrease in wall thickness. Nonetheless, the structural thinning of the material by bulk degradation and concomitant regeneration of tissue may reflect in increasing compliance over time. In the absence of anticoagulant therapy, the patency rate of dPCU was 92.9% at a follow up of six to 12 months compared with the previously mentioned plasma heparin treated PCU and POSS-PCU (86% and 64%, respectively).
      • Ahmed M.
      • Hamilton G.
      • Seifalian A.M.
      The performance of a small-calibre graft for vascular reconstructions in a senescent sheep model.
      ,
      • Qiu X.
      • Lee B.L.
      • Ning X.
      • Murthy N.
      • Dong N.
      • Li S.
      End-point immobilization of heparin on plasma-treated surface of electrospun polycarbonate-urethane vascular graft.
      ePTFE controls reached an overall patency of 92.9%, which in previous studies ranged from 67% to 100% in the infrarenal interposition model.
      • Bergmeister H.
      • Seyidova N.
      • Schreiber C.
      • Strobl M.
      • Grasl C.
      • Walter I.
      • et al.
      Biodegradable, thermoplastic polyurethane grafts for small diameter vascular replacements.
      ,
      • Pektok E.
      • Cikirikcioglu M.
      • Tille J.-C.
      • Kalangos A.
      • Walpoth B.H.
      Alcohol pretreatment of small-diameter expanded polytetrafluoroethylene (ePTFE) grafts: quantitative analysis of graft healing characteristics in the rat abdominal aorta interposition model.
      ,
      • Mugnai D.
      • Tille J.-C.
      • Mrówczyński W.
      • de Valence S.
      • Montet X.
      • Möller M.
      • et al.
      Experimental noninferiority trial of synthetic small-caliber biodegradable versus stable vascular grafts.
      A subtle equilibrium is required between degradation of polymers and regeneration of tissue, to ensure safe remodelling and to prevent rupture or aneurysm formation.
      • Soletti L.
      • Nieponice A.
      • Hong Y.
      • Ye S.H.
      • Stankus J.J.
      • Wagner W.R.
      • et al.
      In vivo performance of a phospholipid-coated bioerodable elastomeric graft for small-diameter vascular applications.
      Slow
      • Yokota T.
      • Ichikawa H.
      • Matsumiya G.
      • Kuratani T.
      • Sakaguchi T.
      • Iwai S.
      • et al.
      In situ tissue regeneration using a novel tissue-engineered, small-caliber vascular graft without cell seeding.
      ,
      • Hashi C.K.
      • Derugin N.
      • Janairo R.R.R.
      • Lee R.
      • Schultz D.
      • Lotz J.
      • et al.
      Anti-thrombogenic modification of small-diameter microfibrous vascular grafts.
      and fast degrading polymers
      • Wu W.
      • Allen R.A.
      • Wang Y.
      Fast-degrading elastomer enables rapid remodeling of a cell-free synthetic graft into a neoartery.
      have been tested for this purpose by different groups. Prolonged exposure to foreign material has been associated with an extended foreign body response through activation of the inflammation cascade.
      • Anderson J.M.
      • Rodriguez A.
      • Chang D.T.
      Foreign body reaction to biomaterials.
      In vivo studies of degradable materials such as poly (lactic) acid, poly (glycolic) acid, and polycaprolactone often showed a distinctive inflammatory response.
      • de Valence S.
      • Tille J.C.
      • Giliberto J.P.
      • Mrowczynski W.
      • Gurny R.
      • Walpoth B.H.
      • et al.
      Advantages of bilayered vascular grafts for surgical applicability and tissue regeneration.
      ,
      • Wu W.
      • Allen R.A.
      • Wang Y.
      Fast-degrading elastomer enables rapid remodeling of a cell-free synthetic graft into a neoartery.
      ,
      • Kurobe H.
      • Maxfield M.W.
      • Tara S.
      • Rocco K.A.
      • Bagi P.S.
      • Yi T.
      • et al.
      Development of small diameter nanofiber tissue engineered arterial grafts.
      The present study shows that, despite the slow degradation and persistent presence of foreign material, infiltration of macrophages at the interface and into the graft was exceedingly rare. No acidic degradation byproducts were expected,
      • Artham T.
      • Doble M.
      Biodegradation of aliphatic and aromatic polycarbonates.
      which was indirectly confirmed by both the in vivo and in vitro studies. The in vitro study indicated an initial pro-inflammatory upregulation, found also in other degradable grafts,
      • Enayati M.
      • Eilenberg M.
      • Grasl C.
      • Riedl P.
      • Kaun C.
      • Messner B.
      • et al.
      Biocompatibility assessment of a new biodegradable vascular graft via in vitro co-culture approaches and in vivo model.
      ,
      • Wu W.
      • Allen R.A.
      • Wang Y.
      Fast-degrading elastomer enables rapid remodeling of a cell-free synthetic graft into a neoartery.
      followed by a shift towards anti-inflammatory M2 positive macrophages and cytokines to promote healing and remodelling. In the ePTFE group, the pro-inflammatory response remained upregulated. Successful graft endothelialisation determines short and long term patency and is host dependent.
      • Yow K.H.
      • Ingram J.
      • Korossis S.A.
      • Ingham E.
      • Homer-Vanniasinkam S.
      Tissue engineering of vascular conduits.
      Here, a significantly quicker, full endothelialisation of the dPCU grafts was observed. Endothelialisation occurred mainly through transanastomotic ingrowth and additionally by means of fallout endothelialisation as described by Pennel et al.
      • Pennel T.
      • Bezuidenhout D.
      • Koehne J.
      • Davies N.H.
      • Zilla P.
      Transmural capillary ingrowth is essential for confluent vascular graft healing.
      Areas of intimal hyperplasia were rare, showed no luminal restriction and as in human applications were mostly confined to anastomotic sites.
      • Zilla P.
      • Bezuidenhout D.
      • Human P.
      Prosthetic vascular grafts: wrong models, wrong questions and no healing.
      dPCU provided a more liveable environment for cells, expressed by the significantly higher level of cells in the graft and at the interface. After only one month the vessel anatomy was partially restored, showing over time an increasingly organised neomedia and collagen rich neo-adventitia accompanied by a high number of microvessels. Despite the presence of collagen, no evidence was found for elastin formation as in the native aorta. Kurobe et al. detected formation of minimal amounts of elastin surrounding their electrospun polylactic acid grafts in a mouse model.
      • Kurobe H.
      • Maxfield M.W.
      • Tara S.
      • Rocco K.A.
      • Bagi P.S.
      • Yi T.
      • et al.
      Development of small diameter nanofiber tissue engineered arterial grafts.
      The positive colocalisation in confocal imaging and significantly positive correlation of calponin and SMA positive SMCs may indirectly indicate functionality of this newly formed media.
      • Lee K.W.
      • Stolz D.B.
      • Wang Y.
      Substantial expression of mature elastin in arterial constructs.
      A comparably structured neovessel was, to the present authors’ knowledge, only reported by Hashi et al. by testing a slowly degrading PLA graft.
      • Hashi C.K.
      • Derugin N.
      • Janairo R.R.R.
      • Lee R.
      • Schultz D.
      • Lotz J.
      • et al.
      Anti-thrombogenic modification of small-diameter microfibrous vascular grafts.
      In ePTFE and other investigated materials, mainly unorganised cell clusters were found.
      • Soletti L.
      • Nieponice A.
      • Hong Y.
      • Ye S.H.
      • Stankus J.J.
      • Wagner W.R.
      • et al.
      In vivo performance of a phospholipid-coated bioerodable elastomeric graft for small-diameter vascular applications.
      ,
      • Kurobe H.
      • Maxfield M.W.
      • Tara S.
      • Rocco K.A.
      • Bagi P.S.
      • Yi T.
      • et al.
      Development of small diameter nanofiber tissue engineered arterial grafts.
      Therefore, it may be hypothesised that slow degradation without inflammatory stimulus as found in the present material encourages ordered cell alignment and tissue regeneration.

      Limitations

      Small rodent models may have limitations regarding their immunological response, cardiovascular physiology, and haemostasis mechanisms compared with humans. However, these models are an essential tool in vascular graft development when limitations are seriously considered. Results obtained from this study have to be confirmed in a long term large animal model.

      CONCLUSION

      In this study, dPCU grafts acted as temporary scaffolds for the newly forming artery and supported formation of a structured smooth muscle cell layer, microvessel formation, and full endothelialisation. The main issue of slow degrading polymers perpetuating an extended foreign body reaction has not been confirmed in dPCU. Slow degrading and almost inert dPCU conduits appear to be a promising, safe, and long term approach for in situ tissue engineering of small diameter vessels.

      conflict of Interest

      The authors declare no conflict of interest.

      FUNDING

      This work was supported by the Ludwig-Boltzmann-Cluster for Cardiovascular Research and the Austria Wirtschaftservice (PRIZE).

      Acknowledgements

      We would like to thank all supporting staff of the Centre of Biomedical Research, Claudia Höchsmann for her meticulous support with the immunohistochemistry, Dr Jagoba Iturri for the AFM imaging, and Matthew Di Franco for the μCT imaging and proof-reading.

      Appendix A. Supplementary data

      The following is the Supplementary data to this article:

      References

        • Desai M.
        • Seifalian A.M.
        • Hamilton G.
        Role of prosthetic conduits in coronary artery bypass grafting.
        Eur J Cardiothorac Surg. 2011; 40: 394-398
        • Pashneh-Tala S.
        • MacNeil S.
        • Claeyssens F.
        The tissue-engineered vascular graft-past, present, and future.
        Tissue Eng B Rev. 2016; 22: 68-100
        • Crupi A.
        • Costa A.
        • Tarnok A.
        • Melzer S.
        • Teodori L.
        Inflammation in tissue engineering: the Janus between engraftment and rejection.
        Eur J Immunol. 2015; 45: 3222-3236
        • Chang W.G.
        • Niklason L.E.
        A short discourse on vascular tissue engineering.
        NPJ Regen Med. 2017; 2 (pii: 7)https://doi.org/10.1038/s41536-017-0011-6
        • Ahmed M.
        • Hamilton G.
        • Seifalian A.M.
        The performance of a small-calibre graft for vascular reconstructions in a senescent sheep model.
        Biomaterials. 2014; 35: 9033-9040
        • Qiu X.
        • Lee B.L.
        • Ning X.
        • Murthy N.
        • Dong N.
        • Li S.
        End-point immobilization of heparin on plasma-treated surface of electrospun polycarbonate-urethane vascular graft.
        Acta Biomater. 2017; 51: 138-147
        • Ferraresso M.
        • Bortolani E.M.
        • Amnon G.
        A two-year experience with a rapid access, self-sealing, polycarbonate urethane nanofiber vascular access graft for hemodialysis.
        J Vasc Access. 2016; 17: 210-214
        • Sobczak M.
        • Dębek C.
        • Olędzka E.
        • Nałęcz-Jawecki G.
        • Kołodziejski W.L.
        • Rajkiewicz M.
        Segmented polyurethane elastomers derived from aliphatic polycarbonate and poly(ester-carbonate) soft segments for biomedical applications.
        J Polym Sci A Polym Chem. 2012; 50: 3904-3913
        • Dempsey D.K.
        • Carranza C.
        • Chawla C.P.
        • Gray P.
        • Eoh J.H.
        • Cereceres S.
        • et al.
        Comparative analysis of in vitro oxidative degradation of poly(carbonate urethanes) for biostability screening.
        J Biomed Mater Res A. 2014; 102: 3649-3665
        • Elomaa L.
        • Yang Y.P.
        Additive Manufacturing of vascular grafts and vascularized tissue constructs.
        Tissue Eng Part B Rev. 2017; 23: 436-450
        • Baudis S.
        • Ligon S.C.
        • Seidler K.
        • Weigel G.
        • Grasl C.
        • Bergmeister H.
        • et al.
        Hard-block degradable thermoplastic urethane-elastomers for electrospun vascular prostheses.
        J Polym Sci A Polym Chem. 2012; 50: 1272-1280
        • Enayati M.
        • Eilenberg M.
        • Grasl C.
        • Riedl P.
        • Kaun C.
        • Messner B.
        • et al.
        Biocompatibility assessment of a new biodegradable vascular graft via in vitro co-culture approaches and in vivo model.
        Ann Biomed Eng. 2016; 44: 3319-3334
        • Ochiai B.
        • Amemiya H.
        • Yamazaki H.
        • Endo T.
        Synthesis and properties of poly(carbonate-urethane) consisting of alternating carbonate and urethane moieties.
        J Polym Sci A Polym Chem. 2006; 44: 2802-2808
        • Bergmeister H.
        • Schreiber C.
        • Grasl C.
        • Walter I.
        • Plasenzotti R.
        • Stoiber M.
        • et al.
        Healing characteristics of electrospun polyurethane grafts with various porosities.
        Acta Biomater. 2013; 9: 6032-6040
        • Bergmeister H.
        • Seyidova N.
        • Schreiber C.
        • Strobl M.
        • Grasl C.
        • Walter I.
        • et al.
        Biodegradable, thermoplastic polyurethane grafts for small diameter vascular replacements.
        Acta Biomater. 2015; 11: 104-113
        • Iturri J.
        • Vianna A.C.
        • Moreno-Cencerrado A.
        • Pum D.
        • Sleytr U.B.
        • Toca-Herrera J.L.
        Impact of surface wettability on S-layer recrystallization: a real-time characterization by QCM-D.
        Beilstein J Nanotechnol. 2017; 8: 91-98
        • de Valence S.
        • Tille J.C.
        • Giliberto J.P.
        • Mrowczynski W.
        • Gurny R.
        • Walpoth B.H.
        • et al.
        Advantages of bilayered vascular grafts for surgical applicability and tissue regeneration.
        Acta Biomater. 2012; 8: 3914-3920
        • Wu W.
        • Allen R.A.
        • Wang Y.
        Fast-degrading elastomer enables rapid remodeling of a cell-free synthetic graft into a neoartery.
        Nat Med. 2012; 18: 1148-1153
        • Perry R.
        • Joseph M.X.
        • Chew D.P.
        • Aylward P.E.
        • De Pasquale C.G.
        Coronary artery wall thickness of the left anterior descending artery using high resolution transthoracic echocardiography-normal range of values.
        Echocardiography. 2013; 30: 759-764
        • van Haaften E.E.
        • Bouten C.V.C.
        • Kurniawan N.A.
        Vascular mechanobiology: towards control of in situ regeneration.
        Cells. 2017; 6
        • Kumar V.A.
        • Brewster L.P.
        • Caves J.M.
        • Chaikof E.L.
        Tissue engineering of blood vessels: functional requirements, progress, and future challenges.
        Cardiovasc Eng Technol. 2011; 2: 137-148
        • Pektok E.
        • Cikirikcioglu M.
        • Tille J.-C.
        • Kalangos A.
        • Walpoth B.H.
        Alcohol pretreatment of small-diameter expanded polytetrafluoroethylene (ePTFE) grafts: quantitative analysis of graft healing characteristics in the rat abdominal aorta interposition model.
        Artif Organs. 2009; 33: 532-537
        • Mugnai D.
        • Tille J.-C.
        • Mrówczyński W.
        • de Valence S.
        • Montet X.
        • Möller M.
        • et al.
        Experimental noninferiority trial of synthetic small-caliber biodegradable versus stable vascular grafts.
        J Thorac Cardiovasc Surg. 2013; 146 (e1): 400-407
        • Soletti L.
        • Nieponice A.
        • Hong Y.
        • Ye S.H.
        • Stankus J.J.
        • Wagner W.R.
        • et al.
        In vivo performance of a phospholipid-coated bioerodable elastomeric graft for small-diameter vascular applications.
        J Biomed Mater Res A. 2011; 96: 436-448
        • Yokota T.
        • Ichikawa H.
        • Matsumiya G.
        • Kuratani T.
        • Sakaguchi T.
        • Iwai S.
        • et al.
        In situ tissue regeneration using a novel tissue-engineered, small-caliber vascular graft without cell seeding.
        J Thorac Cardiovasc Surg. 2008; 136: 900-907
        • Hashi C.K.
        • Derugin N.
        • Janairo R.R.R.
        • Lee R.
        • Schultz D.
        • Lotz J.
        • et al.
        Anti-thrombogenic modification of small-diameter microfibrous vascular grafts.
        Arterioscler Thromb Vasc Biol. 2010; 30: 1621-1627
        • Anderson J.M.
        • Rodriguez A.
        • Chang D.T.
        Foreign body reaction to biomaterials.
        Semin Immunol. 2008; 20: 86-100
        • Kurobe H.
        • Maxfield M.W.
        • Tara S.
        • Rocco K.A.
        • Bagi P.S.
        • Yi T.
        • et al.
        Development of small diameter nanofiber tissue engineered arterial grafts.
        PLoS One. 2015; 10e0120328
        • Artham T.
        • Doble M.
        Biodegradation of aliphatic and aromatic polycarbonates.
        Macromol Biosci. 2008; 8: 14-24
        • Yow K.H.
        • Ingram J.
        • Korossis S.A.
        • Ingham E.
        • Homer-Vanniasinkam S.
        Tissue engineering of vascular conduits.
        Br J Surg. 2006; 93: 652-661
        • Pennel T.
        • Bezuidenhout D.
        • Koehne J.
        • Davies N.H.
        • Zilla P.
        Transmural capillary ingrowth is essential for confluent vascular graft healing.
        Acta Biomater. 2018; 65: 237-247
        • Zilla P.
        • Bezuidenhout D.
        • Human P.
        Prosthetic vascular grafts: wrong models, wrong questions and no healing.
        Biomaterials. 2007; 28: 5009-5027
        • Lee K.W.
        • Stolz D.B.
        • Wang Y.
        Substantial expression of mature elastin in arterial constructs.
        Proc Natl Acad Sci U S A. 2011; 108: 2705-2710

      Linked Article

      • Off the Shelf Bioabsorbable Grafts: Meeting the Unmet Need
        European Journal of Vascular and Endovascular SurgeryVol. 59Issue 4
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          Synthetic grafts are associated with excellent long term patency when they are used to replace large diameter arteries where the flow is high and the resistance is low. Conversely, their performance is disappointing when they are used to bypass small diameter arteries such as the coronary and infragenicular vessels. The construction of an artificial blood vessel with biomechanical properties identical to those of native vessels, including the ability to contract, to secrete, to heal, and even to grow, has been elusive.
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