Peripheral arterial disease is a leading cause of morbidity and mortality. The most commonly utilized prosthetic material for peripheral bypass grafting is expanded polytetrafluoroethylene (ePTFE) yet it continues to exhibit poor performance from restenosis due to neointimal hyperplasia, especially in femoral distal bypass procedures. Recently, we demonstrated that periadventitial delivery of all-trans retinoic acid (atRA) immobilized throughout porous poly(1,8 octamethylene citrate) (POC) membranes inhibited neointimal formation in a rat arterial injury model. Thus, the objective of this study was to investigate whether atRA immobilized throughout the lumen of ePTFE vascular grafts would inhibit intimal formation following arterial bypass grafting. Utilizing standard ePTFE, two types of atRA-containing ePTFE vascular grafts were fabricated and evaluated: grafts whereby all-trans retinoic acid was directly immobilized on ePTFE (atRA-ePTFE) and grafts where all-trans retinoic acid was immobilized onto ePTFE grafts coated with POC (atRA-POC-ePTFE). All grafts were characterized by SEM, HPLC, and FTIR and physical characteristics were evaluated in vitro. Modification of these grafts, did not significantly alter their physical characteristics or biocompatibility, and resulted in inhibition of intimal formation in a rat aortic bypass model, with atRA-POC-ePTFE inhibiting intimal formation at both the proximal and distal graft sections. In addition, treatment with atRA-POC-ePTFE resulted in increased graft endothelialization and decreased inflammation when compared to the other treatment groups. This work further confirms the biocompatibility and efficacy of locally delivered atRA to inhibit intimal formation in a bypass setting. Thus, atRA-POC-ePTFE grafts have the potential to improve patency rates in small diameter bypass grafts and warrant further investigation.

The recent development of “continuous projection microstereolithography” also known as CLIP technology has successfully alleviated the main obstacles surrounding 3D printing technologies: production speed and part quality. Following the same working principle, we further developed the μCLIP process to address the needs for high-resolution 3D printing of biomedical devices with micron-scale precision. Compared to standard stereolithography (SLA) process, μCLIP fabrication can reduce fabrication time from several hours to as little as a few minutes. μCLIP can also produce better surface finish and more uniform mechanical properties than conventional SLA, as each individual “fabrication layer” continuously polymerizes into the subsequent layer. In this study, we report the process development in manufacturing high-resolution bioresorbable stents using our own μCLIP system. The bioresorbable photopolymerizable biomaterial (B-ink) used in this study is methacrylated poly(1, 12 dodecamethylene citrate) (mPDC). Through optimization of our μCLIP process and concentration of B-ink components, we have created a customizable bioresorbable stent with similar mechanical properties exhibited by nitinol stents. Upon optimization, fabricating a 2 cm tall vascular stent that comprises 4000 layers was accomplished in 26.5 minutes.

3D-printed stents are fabricated with high speed and resolution by micro-continuous liquid interface production (microCLIP) of a bioresorbable, antioxidant, photopolymerizable polydiolcitrate biomaterial. Printed stents can match the mechanical properties of bare metal stents and strengthen porcine arteries after deployment. This technology is a big step forward toward on-the-spot and on-demand printing of patient-specific stents.

Event Abstract Back to Event 3D-printing bioresorbable vascular stents with metal stent-matching recoil strength Evan Baker1, Henry Ware1, Robert Van Lith2, Jian Yang2, Fan Zhou1, Cheng Sun1 and Guillermo Ameer1, 2, 3, 4 1 Northwestern University, Mechanical Engineering, United States 2 Northwestern University, Biomedical Engineering, United States 3 Northwestern University, Simpson Querrey Institute, United States 4 Northwestern University, Surgery, United States Introduction: The placement of a vascular stent is a common intervention to address the obstruction of blood flow as a result of atherosclerotic disease[1],[2]. Problems associated with metal stents led to the development of bioresorbable stents (BRS)[3]. BRSs allow resistance against late stent thrombosis[4] and restoration of natural vasomotion following stent resorption. Poly(L-lactide) (PLLA) or its copolymer with glycolic acid have been investigated for BRS[6],[6]. However, degradation is very slow, potentially affecting long-term tissue remodeling, while the polymer degradation products are oxidative stress-inducing[7]-[9], which could exacerbate tissue inflammation and intimal hyperplasia[10],[11]. Polymer properties also limit strut designs, complicating BRS manufacture. Finally, none of these stents allow for patient-specific customization, which could improve vessel patency. In this work we investigated a UV-curable, antioxidant polydiolcitrate together with projection microstereolithography (PμSL) to fabricate bioresorbable stents that can be customized for each patient. This flexible additive manufacturing could potentially be used for on-the-spot printing, e.g. in the operating room[12],[13]. Materials and Methods: Biomaterial Ink: Citric acid and 1,12-dodecanediol were melted in a 2:1 ratio, co-polymerized (140°C, 30 min), purified and freeze-dried. This pre-polymer (22g) was dissolved in tetrahydrofuran (180 mL) with imidazole (816 mg) and glycidyl methacrylate (17.04 g), heated (60°C, 6 hrs) and purified to yield methacrylated poly(1,12-dodecamethylene citrate) (mPDC). To formulate B-InkTM, 52% mPDC was mixed with 2.2% Irgacure 819 (photo-initiator), 0.2% Sudan I (UV absorber to control curing depth) and 46% diethyl fumarate (solvent to control viscosity) (Fig. 1A). Degradation: UV-cured films were incubated in PBS, collected at time points, washed with water, freezedried and weighed (Fig. 1B). Antioxidant properties: Cured films were incubated in free radical ABTS solution. The absorption peak at 734 nm can be monitored and color change is a measure of free radical scavenging (Fig. 1C). Cell compatibility: Cured mPDC was gas sterilized, incubated in DMEM media to remove unreacted monomers and seeded with human aortic smooth muscle cells (HASMC). After 3 days, viability was assessed with calcein AM (Fig.1D). Projection microstereolithography (PμSL): Stents were built from the B-InkTM in a 20 µm layer-by-layer fashion directly from a 3D CAD design (Fig. 2B). Each layer was cured by single exposure using a liquid crystal display panel as a dynamic mask for UV light. After the final layer, stents were further polymerized with additional UV exposure (Fig. 2A). Morphology: Morphology of the printed stents was observed via scanning electron microscopy (SEM) (Fig. 2C, D). Mechanical testing: Radial compression tests of stents to 25% of the stents' outer diameter were performed on an Instron 5544 mechanical tester (Fig. 2E). Results and Discussion: Synthesized mPDC could be cured by UV exposure (Fig. 1A). mPDC is degradable, antioxidant and cell compatible (Fig. 1B-D). Formulation into B-InkTM enabled UV-based 3D printing of stents with 20 µm layers using projection microstereolithography (Fig. 2A-D). The mechanical properties of 3D-printed stents with struts of 400 µm were comparable to those of a control nitinol stent (Fig. 2E). These results hold great promise for on-the-spot patient-customized stent manufacture. References:[1] Kudagi, V. S.; White, C. J. Am J Cardiovasc Drugs 2013, 13, 199-212. DOl:10.1007/s40256-013-0023-6[2] Litsky, J.; Chanda, A.; Stilp, E.; Lansky, A.; Mena, C. Medical Devices (Auckland, N.Z.) 2014, 7, 149-156. DOl:10.2147/MDER.S45472[3] Kassimis, G.; Spiliopoulos, S.; Katsanos, K.; Tsetis, D.; Krokidis, M. E. Expert Rev Cardiovasc Ther 2014, 12, 443-50. DOl:10.1586/14779072.2014.897226[4] Bosiers, M.; Cagiannos, C.; Deloose, K.; Verbist, J.; Peeters, P. Vascular Health and Risk Management 2008, 4, 553-559.[5] Onuma, Y.; Serruys, P. W. Circulation 2011, 123, 779-797. DOl:10.1161/circulationaha.110.971606[6] Iqbal, J.; Onuma, Y.; Ormiston, J.; Abizaid, A.; Waksman, R.; Serruys, P. Eur Heart J 2014, 35, 765-76. DOl:10.1093/eurheartj/eht542[7] Abbott, D. A.; Suir, E.; Duong, G. H.; de Hulster, E.; Pronk, J. T.; van Maris, A. J. Appl Environ Microbiol 2009, 75, 2320-5. DOl:10.1128/aem.00009-09[8] Selvam, S.; Kundu, K.; Templeman, K. L.; Murthy, N.; Garcia, A. J. Biomaterials 2011, 32, 7785-92. DOl:10.1016/j.biomaterials.2011.07.020[9] Zhou, J.; Tsai, Y. T.; Weng, H.; Tang, L. Free Radic Biol Med 2012, 52, 218-26. DOl:10.1016/j.freeradbiomed.2011.10.452[10] Kawamoto, R.; Yamashita, A.; Nishihira, K.; Furukoji, E.; Hatakeyama, K.; Ishikawa, T.; Imamura, T.; Itabe, H.; Eto, T.; Asada, Y. Pathol Res Pract 2006, 202, 447-56. DOl:10.1016/j.prp.2005.12.011[11] Juni, R. P.; Duckers, H. J.; Vanhoutte, P. M.; Virmani, R.; Moens, A. L. J Am Coll Cardiol 2013, 61, 1471-81. DOl:10.1016/j.jacc.2012.11.068[12] Mota, C.; Puppi, D.; Chiellini, F.; Chiellini, E. Journal of Tissue Engineering and Regenerative Medicine 2015, 9, 174-190. DOl:10.1002/term.1635[13] Bose, S.; Vahabzadeh, S.; Bandyopadhyay, A. Materials Today 2013, 16, 496-504. DOl:http://dx.doi.org/10.1016/j.mattod.2013.11.017 Keywords: blood vessel, material design, Bioprinting, Biodegradable material Conference: 10th World Biomaterials Congress, Montréal, Canada, 17 May - 22 May, 2016. Presentation Type: New Frontier Oral Topic: Biomaterials in printing Citation: Baker E, Ware H, Van Lith R, Yang J, Zhou F, Sun C and Ameer G (2016). 3D-printing bioresorbable vascular stents with metal stent-matching recoil strength. Front. Bioeng. Biotechnol. Conference Abstract: 10th World Biomaterials Congress. doi: 10.3389/conf.FBIOE.2016.01.02062 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 27 Mar 2016; Published Online: 30 Mar 2016. Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Evan Baker Henry Ware Robert Van Lith Jian Yang Fan Zhou Cheng Sun Guillermo Ameer Google Evan Baker Henry Ware Robert Van Lith Jian Yang Fan Zhou Cheng Sun Guillermo Ameer Google Scholar Evan Baker Henry Ware Robert Van Lith Jian Yang Fan Zhou Cheng Sun Guillermo Ameer PubMed Evan Baker Henry Ware Robert Van Lith Jian Yang Fan Zhou Cheng Sun Guillermo Ameer Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Intimal hyperplasia (IH) is a type of scarring that involves complex pathophysiological responses of the vasculature to injury, including overproliferation and migration of vascular smooth muscle cells (VSMCs), adventitial fibroblasts, and the activation of macrophages. The objective of this research was to develop a biodegradable polymer with intrinsic properties that would combat the cellular processes that contribute to IH. Citric acid, 1,8-octanediol, and all-trans retinoic acid (atRA) were incorporated into a polyester network via a condensation reaction to form the thermoset poly(1,8-octamethylene-citrate-co-retinate) (POCR). POCR was chemically characterized and assessed for the presence of antioxidant and retinoidlike properties. HNMR and ATR-FTIR confirmed the incorporation of atRA into the backbone of the polymer network. POCR was able to scavenge radicals and inhibit lipid peroxidation. The proliferation and migration of vascular smooth muscle cells cultured on POCR were inhibited, whereas endothelial cell proliferation and migration were not. These results are consistent with the biological effects of atRA. These results are the first to demonstrate the synthesis of a polymer with intrinsic antirestenotic properties for potential use in the fabrication of vascular devices such as stents and vascular grafts.

In this chapter, an overview will be given of progress in the development of antioxidant polymers and their therapeutic use. Oxidative stress has been identified as a key player in many disease conditions, as well as a result of commonly used implantable biomaterials. Systemic delivery of antioxidants is hampered by poor absorption, loss of bioactivity, limited time frame of effectiveness, and nontargeted lowering of oxidative stress. For this reason, polymers are being developed for localized, sustained antioxidant activity. Antioxidant entrapment in polymers will be discussed, as well as development of polymers with intrinsic antioxidant activity, with an emphasis on the latter.

After vascular interventions, endothelial cells are typically injured or lacking, resulting in decreased NO synthesis to maintain vascular health. Moreover, inflammation as a result of the tissue injury and/or the presence of an implanted foreign polymer such as a vascular graft causes excessive generation of reactive oxygen species (ROS) (e.g., superoxide), which can react with NO. The combination of the above creates a general decline in NO bioavailability, as well as oxidative stress due to less available NO to scavenge ROS. Localized NO delivery is an attractive solution to alleviate these issues; however, NO donors typically exhibit unpredictable NO payload release when using nitrosothiols or the risk of nitrosamine formation for synthetic diazeniumdiolates. The objective of this study was therefore to synthesize an NO donor from a biological peptide that could revert to its native form upon NO release. To this effect, protamine sulfate (PS), an FDA-approved peptide with reported vasodilator and anticoagulant properties, was diazeniumdiolated to form PS/NO. PS/NO showed diazeniumdiolate-characteristic UV peaks and NO release in physiological solutions and was capable of scavenging radicals to decrease oxidative stress. Furthermore, PS/NO selectively inhibits the proliferation of smooth muscle cells and adventitial fibroblasts, thereby reversing reported mitogenic properties of PS. Endothelial cell growth, on the other hand, was promoted by PS/NO. Finally, PS retained its anticoagulant properties upon diazeniumdiolation at clinically relevant concentrations. In conclusion, we have synthesized an NO prodrug from a biological peptide, PS/NO, that selectively inhibits proliferation of smooth muscle cells and fibroblasts, retains anticoagulant properties, and reverts back to its native PS form upon NO payload release.

Oxidative stress plays an important role in the limited biological compatibility of many biomaterials due to inflammation, as well as in various pathologies including atherosclerosis and restenosis as a result of vascular interventions. Engineering antioxidant properties into a material is therefore a potential avenue to improve the biocompatibility of materials, as well as to locally attenuate oxidative stress-related pathologies. Moreover, biodegradable polymers that have antioxidant properties built into their backbone structure have high relative antioxidant content and may provide prolonged, continuous attenuation of oxidative stress while the polymer or its degradation products are present. In this report, we describe the synthesis of poly(1,8-octanediol-co-citrate-co-ascorbate) (POCA), a citric-acid based biodegradable elastomer with native, intrinsic antioxidant properties. The in vitro antioxidant activity of POCA as well as its effects on vascular cells in vitro and in vivo were studied. Antioxidant properties investigated included scavenging of free radicals, iron chelation and the inhibition of lipid peroxidation. POCA reduced reactive oxygen species generation in cells after an oxidative challenge and protected cells from oxidative stress-induced cell death. Importantly, POCA antioxidant properties remained present upon degradation. Vascular cells cultured on POCA showed high viability, and POCA selectively inhibited smooth muscle cell proliferation, while supporting endothelial cell proliferation. Finally, preliminary data on POCA-coated ePTFE grafts showed reduced intimal hyperplasia when compared to standard ePTFE grafts. This biodegradable, intrinsically antioxidant polymer may be useful for tissue engineering application where oxidative stress is a concern.

Oxidative stress in tissue can contribute to chronic inflammation that impairs wound healing and the efficacy of cell-based therapies and medical devices. We describe the synthesis and characterization of a biodegradable, thermoresponsive gel with intrinsic antioxidant properties suitable for the delivery of therapeutics. Citric acid, poly(ethylene glycol) (PEG), and poly-N-isopropylacrylamide (PNIPAAm) were copolymerized by sequential polycondensation and radical polymerization to produce poly(polyethylene glycol citrate-co-N-isopropylacrylamide) (PPCN). PPCN was chemically characterized, and the thermoresponsive behavior, antioxidant properties, morphology, potential for protein and cell delivery, and tissue compatibility in vivo were evaluated. The PPCN gel has a lower critical solution temperature (LCST) of 26 °C and exhibits intrinsic antioxidant properties based on its ability to scavenge free radicals, chelate metal ions, and inhibit lipid peroxidation. PPCN displays a hierarchical architecture of micropores and nanofibers, and contrary to typical thermoresponsive polymers, such as PNIPAAm, PPCN gel maintains its volume upon formation. PPCN efficiently entrapped and slowly released the chemokine SDF-1α and supported the viability and proliferation of vascular cells. Subcutaneous injections in rats showed that PPCN gels are resorbed over time and new connective tissue formation takes place without signs of significant inflammation. Ultimately, this intrinsically antioxidant, biodegradable, thermoresponsive gel could potentially be used as an injectable biomaterial for applications where oxidative stress in tissue is a concern.

Prosthetic vascular grafts do not mimic the antithrombogenic properties of native blood vessels and therefore have higher rates of complications that involve thrombosis and restenosis. We developed an approach for grafting bioactive heparin, a potent anticoagulant glycosaminoglycan, to the lumen of ePTFE vascular grafts to improve their interactions with blood and vascular cells. Heparin was bound to aminated poly(1,8-octanediol-co-citrate) (POC) via its carboxyl functional groups onto POC-modified ePTFE grafts. The bioactivity and stability of the POC-immobilized heparin (POC-Heparin) were characterized via platelet adhesion and clotting assays. The effects of POC-Heparin on the adhesion, viability and phenotype of primary endothelial cells (EC), blood outgrowth endothelial cells (BOECs) obtained from endothelial progenitor cells (EPCs) isolated from human peripheral blood, and smooth muscle cells were also investigated. POC-Heparin grafts maintained bioactivity under physiologically relevant conditions in vitro for at least one month. Specifically, POC-Heparin-coated ePTFE grafts significantly reduced platelet adhesion and inhibited whole blood clotting kinetics. POC-Heparin supported EC and BOEC adhesion, viability, proliferation, NO production, and expression of endothelial cell-specific markers von Willebrand factor (vWF) and vascular endothelial-cadherin (VE-cadherin). Smooth muscle cells cultured on POC-Heparin showed increased expression of α-actin and decreased cell proliferation. This approach can be easily adapted to modify other blood contacting devices such as stents where antithrombogenicity and improved endothelialization are desirable properties.

Since the discovery of nitric oxide (NO) in the 1980s, this cellular messenger has been shown to participate in diverse biological processes such as cardiovascular homeostasis, immune response, wound healing, bone metabolism, and neurotransmission. Its beneficial effects have prompted increased research in the past two decades, with a focus on the development of materials that can locally release NO. However, significant limitations arise when applying these materials to biomedical applications. This Feature Article focuses on the development of NO-releasing and NO-generating polymeric materials (20062011) with emphasis on recent in vivo applications. Results are compared and discussed in terms of NO dose, release kinetics, and biological effects, in order to provide a foundation to design and evaluate new NO therapies.

Despite the enormous advances realized in cardiology and cardiovascular surgery, approximately 500,000 deaths due to coronary heart disease occur annually in the US [166].

BACKGROUND: S-nitrosothiols (SNO) release nitric oxide (NO) through interaction with ascorbic acid (AA). However, little is known about their combined effect in the vasculature. The aim of this study was to investigate the effect of AA on SNO-mediated NO release, proliferation, cell cycle progression, cell death, and oxidative stress in vascular cells.METHODS: Vascular smooth muscle cells and adventitial fibroblasts harvested from the aortae of Sprague-Dawley rats were treated with AA, +/- S-nitrosoglutathione (GSNO), or +/- diethylenetriamine NONOate (DETA/NO). NO release, proliferation, cell cycle progression, cell death, and oxidative stress were determined by the Griess reaction, [H-3]-thymidine incorporation, flow cytometry, trypan blue exclusion, and 5-(and-6)chloromethyl-2',7'dichlorodihydrofluorescein staining, respectively.RESULTS: AA increased NO release from GSNO 3-fold (P < .001). GSNO and DETA/NO significantly decreased proliferation, but AA abrogated this effect (P < .05). Mirroring the proliferation data, changes in cell cycle progression induced by GSNO and DETA/NO were reversed by the addition of AA. GSNO- and DETA/NO-mediated increases in oxidative stress were significantly decreased by the addition of AA (P < .001).CONCLUSIONS: Despite causing increased NO release from GSNO, AA reduced the antiproliferative and cell cycle effects of GSNO and DETA/NO through the modulation of oxidative stress. (C) 2011 Elsevier Inc. All rights reserved.