Nitric Oxide Releasing Polyurethanes with Covalently Linked Diazeniumdiolated Secondary Amines

A dual-action nitric oxide-releasing slippery surface for extracorporeal organ support: Dynamic in vitro hemocompatibility evaluation

Numerous biomaterials have been developed for application in blood-contacting medical devices to prevent thrombosis; however, few materials have been applied to full-scale devices and evaluated for hemocompatibility under clinical blood flow conditions. We applied a dual-action slippery liquid-infused (LI) nitric oxide (NO)-releasing material modification (LINO) to full-scale blood circulation tubing for extracorporeal lung support and evaluated the tubing ex vivo using swine whole blood circulated for 6 h at a clinically relevant flow. LINO tubing was compared to unmodified tubing (CTRL) and isolated LI and NO-releasing modifications (n = 9/group). The primary objective was to evaluate safety and blood compatibility of this approach, prior to progression to in vivo testing of efficacy in animal models. The secondary objective was to evaluate coagulation outcomes relevant to hemocompatibility. No untoward effects of the coating, such as elevated methemoglobin fraction, were observed. Additionally, LINO delayed platelet loss until 6 h versus the reduction in platelet count in CTRL at 3 h. At 6 h, LINO significantly reduced the concentration of platelets in an activated P-selectin expressing state versus CTRL (32 ± 1% decrease, p = .02). Blood clot deposition was significantly reduced on LINO blood pumps (p = .007) and numerically reduced on tubing versus CTRL. Following blood exposure, LINO tubing continued to produce a measurable NO-flux (0.20 ± 0.06 × 10^-10  mol cm^-2  min^-1 ). LINO is a potential solution to reduce circuit-related bleeding and clotting during extracorporeal organ support, pending future extended testing in vivo using full-scale extracorporeal lung support devices.

Opportunities for Nitric Oxide in Potentiating Cancer Immunotherapy

Despite nearly 30 years of development and recent highlights of nitric oxide (NO) donors and NO delivery systems in anticancer therapy, the limited understanding of exogenous NO's effects on the immune system has prevented their advancement into clinical use. In particular, the effects of exogenously delivered NO differing from that of endogenous NO has obscured how the potential and functions of NO in anticancer therapy may be estimated and exploited despite the accumulating evidence of NO's cancer therapy-potentiating effects on the immune system. After introducing their fundamentals and characteristics, this review discusses the current mechanistic understanding of NO donors and delivery systems in modulating the immunogenicity of cancer cells as well as the differentiation and functions of innate and adaptive immune cells. Lastly, the potential for the complex modulatory effects of NO with the immune system to be leveraged for therapeutic applications is discussed in the context of recent advancements in the implementation of NO delivery systems for anticancer immunotherapy applications. SIGNIFICANCE STATEMENT: Despite a 30-year history and recent highlights of nitric oxide (NO) donors and delivery systems as anticancer therapeutics, their clinical translation has been limited. Increasing evidence of the complex interactions between NO and the immune system has revealed both the potential and hurdles in their clinical translation. This review summarizes the effects of exogenous NO on cancer and immune cells in vitro and elaborates these effects in the context of recent reports exploiting NO delivery systems in vivo in cancer therapy applications.

Bio-inspired hemocompatible surface modifications for biomedical applications

When blood first encounters the artificial surface of a medical device, a complex series of biochemical reactions is triggered, potentially resulting in clinical complications such as embolism/occlusion, inflammation, or device failure. Preventing thrombus formation on the surface of blood-contacting devices is crucial for maintaining device functionality and patient safety. As the number of patients reliant on blood-contacting devices continues to grow, minimizing the risk associated with these devices is vital towards lowering healthcare-associated morbidity and mortality. The current standard clinical practice primarily requires the systemic administration of anticoagulants such as heparin, which can result in serious complications such as post-operative bleeding and heparin-induced thrombocytopenia (HIT). Due to these complications, the administration of antithrombotic agents remains one of the leading causes of clinical drug-related deaths. To reduce the side effects spurred by systemic anticoagulation, researchers have been inspired by the hemocompatibility exhibited by natural phenomena, and thus have begun developing medical-grade surfaces which aim to exhibit total hemocompatibility via biomimicry. This review paper aims to address different bio-inspired surface modifications that increase hemocompatibility, discuss the limitations of each method, and explore the future direction for hemocompatible surface research.

Electrochemical Generation of Nitric Oxide for Medical Applications

Over the past 30 years, the significance of nitric oxide (NO) has become increasingly apparent in mammalian physiology. It is biosynthesized by three isoforms of nitric oxide synthases (NOS): neuronal (nNOS), endothelial (eNOS), and inducible (iNOS). Neuronal and eNOS both produce low levels of NO (nM) as a signaling agent and vasodilator, respectively. Inducible (iNOS) is present in activated macrophages at sites of infection to generate acutely toxic (μM) levels of NO as part of the mammalian immune defense mechanism. These discoveries have led to numerous animal and clinical studies to evaluate the potential therapeutic utility of NO in various medical operations/treatments, primarily using NO gas (via gas-cylinders) as the NO source. In this review, we focus specifically on recent advances in the electrochemical generation of NO (E-NOgen) as an alternative means to generate NO from cheap and inert sources, and the fabrication and testing of biomedical devices that utilize E-NOgen to controllably generate NO for medical applications.

Rational design of biodegradable thermoplastic polyurethanes for tissue repair

As a type of elastomeric polymers, non-degradable polyurethanes (PUs) have a long history of being used in clinics, whereas biodegradable PUs have been developed in recent decades, primarily for tissue repair and regeneration. Biodegradable thermoplastic (linear) PUs are soft and elastic polymeric biomaterials with high mechanical strength, which mimics the mechanical properties of soft and elastic tissues. Therefore, biodegradable thermoplastic polyurethanes are promising scaffolding materials for soft and elastic tissue repair and regeneration. Generally, PUs are synthesized by linking three types of changeable blocks: diisocyanates, diols, and chain extenders. Alternating the combination of these three blocks can finely tailor the physio-chemical properties and generate new functional PUs. These PUs have excellent processing flexibilities and can be fabricated into three-dimensional (3D) constructs using conventional and/or advanced technologies, which is a great advantage compared with cross-linked thermoset elastomers. Additionally, they can be combined with biomolecules to incorporate desired bioactivities to broaden their biomedical applications. In this review, we comprehensively summarized the synthesis, structures, and properties of biodegradable thermoplastic PUs, and introduced their multiple applications in tissue repair and regeneration. A whole picture of their design and applications along with discussions and perspectives of future directions would provide theoretical and technical supports to inspire new PU development and novel applications.

Stearic acid modified nano CuMOFs used as a nitric oxide carrier for prolonged nitric oxide release

Nitric oxide (NO) shows high potential in the cardiovascular system with anticoagulant and antibacterial efficacy. Cu based metal organic frameworks with amino modification (CuMOFs) were found to have an extraordinary high NO loading, but at the expense of framework stability in ambient moisture. Nano CuMOFs was synthesized by hydrothermal method in this work, and treated with stearic acid (SA) creating a hydrophobic form. It was found that the structure of the particles was not affected after treatment with SA, and the treated CuMOFs had tunable hydrophobicity. Both CuMOFs and SA modified CuMOFs adsorbed NO with the reaction of the amino group and NO to form a NONOate. SA modification enhanced stability of the CuMOFs in phosphate buffer solution (PBS, pH = 7.4), slowed down the interaction between the NO loading unit and H₂O, and thus NO releasing was prolonged. The resulting NO-loaded CuMOFs inhibited platelet activation dramatically, prolonged the coagulation time and displayed excellent antibacterial properties. They could be envisioned as a good candidate for application in blood contacting implants.

Nitric oxide (NO) shows high potential in the cardiovascular system with anticoagulant and antibacterial efficacy.

Nitric Oxide Releasing Delivery Platforms: Design, Detection, Biomedical Applications, and Future Possibilities

Gasotransmitters belong to the subfamily of endogenous gaseous signaling molecules, which find a wide range of biomedical applications. Among the various gasotransmitters, nitric oxide (NO) has an enormous effect on the cardiovascular system. Apart from this, NO showed a pivotal role in neurological, respiratory, and immunological systems. Moreover, the paradoxical concentration-dependent activities make this gaseous signaling molecule more interesting. The gaseous NO has negligible stability in physiological conditions (37 °C, pH 7.4), which restricts their potential therapeutic applications. To overcome this issue, various NO delivering carriers were reported so far. Unfortunately, most of these NO donors have low stability, short half-life, or low NO payload. Herein, we review the synthesis of NO delivering motifs, development of macromolecular NO donors, their advantages/disadvantages, and biological applications. Various NO detection analytical techniques are discussed briefly, and finally, a viewpoint about the design of polymeric NO donors with improved physicochemical characteristics is predicted.

Functional-modified polyurethanes for rendering surfaces antimicrobial: An overview

Antimicrobial surfaces and coatings are rapidly emerging as primary components in functional modification of materials and play an important role in addressing the problems associated with biofouling and microbial infection. Polyurethane (PU) consisting of alternating soft and hard segments has been one of the most important coating materials that have been widely applied in many fields due to its versatile properties. This review attempts to provide insight into the recent advances in antimicrobial polyurethane coatings or surfaces. According to different classes of antimicrobial components along with their antimicrobial mechanism, the synthesis pathways are presented systematically herein to afford polyurethane with antimicrobial properties. Also, the challenges and opportunities of antimicrobial PU coatings and surfaces are also discussed. This review will be beneficial to the exploitation and the further studies of antimicrobial polyurethane materials for a variety of applications.

Enzyme-Activated Nitric Oxide-Releasing Composite Material for Antibacterial Activity Against Escherichia coli

Bacterial infections occurring on medical devices are incredibly difficult to treat, highlighting the urgency for progress in developing antibiotics and antibacterial materials. This work describes the preparation of an antibacterial prodrug polymer composite material for use as an antibacterial coating for medical devices to prevent infections. Polyvinyl chloride and polyurethane films are prepared containing a bacterial nitroreductase enzyme-activated diazeniumdiolate that releases nitric oxide (NO), a known potent antimicrobial agent. Characterization of the surface of the composite materials by scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDS) reveals that the surface of the materials is composed of high amounts of nitrogen due to incorporation of the NO donor compound, up to 13.2% nitrogen on the surface of the 2.5% w/v diazeniumdiolate composite. NO release from the composite films is observed only after metabolism by a bacterial nitroreductase enzyme isolated from E. coli, demonstrating the prodrug nature of the polymer composite materials. Antibacterial efficacy experiments resulted in up to a 66% reduction in E. coli after exposure to the diazeniumdiolate-composite materials. This work details the first illustration of an antibacterial enzyme-activated NO-releasing polymer, a material with potential application as a medical device coating to prevent device-associated infections and improve patient outcomes.

Mimicking the Endothelium: Dual Action Heparinized Nitric Oxide Releasing Surface

The management of thrombosis and bacterial infection is critical to ensure the functionality of medical devices. While administration of anticoagulants is the current antithrombotic clinical practice, a variety of complications, such as uncontrolled hemorrhages or heparin-induced thrombocytopenia, can occur. Additionally, infection rates remain a costly and deadly complication associated with use of these medical devices. It has been hypothesized that if a synthetic surface could mimic the biochemical mechanisms of the endothelium of blood vessels, thrombosis could be reduced, anticoagulant use could be avoided, and infection could be prevented. Herein, the interfacial biochemical effects of the endothelium were mimicked by altering the surface of medical grade silicone rubber (SR). Surface modification was accomplished via heparin surface immobilization (Hep) and the inclusion of a nitric oxide (NO) donor into the SR polymeric matrix to achieve synergistic effects (Hep-NO-SR). An in vitro bacteria adhesion study revealed that Hep-NO-SR exhibited a 99.46 ± 0.17% reduction in viable bacteria adhesion compared to SR. An in vitro platelet study revealed Hep-NO-SR reduced platelet adhesion by 84.12 ± 6.19% compared to SR, while not generating a cytotoxic response against fibroblast cells. In a 4 h extracorporeal circuit model without systemic anticoagulation, all Hep-NO-SR samples were able to maintain baseline platelet count and device patency; whereas 66% of SR samples clotted within the first 2 h of study. Results indicate that Hep-NO-SR creates a more hemocompatible and antibacterial surface by mimicking two key biochemical functions of the native endothelium.

S-Nitroso-N-acetylpenicillamine impregnated endotracheal tubes for prevention of ventilator-associated pneumonia

The chances of ventilator-associated pneumonia (VAP) increases 6-20 folds when an endotracheal tube (ETT) is placed in a patient. VAP is one of the most common hospital-acquired infections and comprises 86% of the nosocomial pneumonia cases. This study introduces the idea of nitric oxide-releasing ETTs (NORel-ETTs) fabricated by the incorporation of the nitric oxide (NO) donor S-nitroso-N-acetylpenicillamine (SNAP) into commercially available ETTs via solvent swelling. The impregnation of SNAP provides NO release over a 7-day period without altering the mechanical properties of the ETT. The NORel-ETTs successfully reduced the bacterial infection from a commonly found pathogen in VAP, Pseudomonas aeruginosa, by 92.72 ± 0.97% when compared with the control ETTs. Overall, this study presents the incorporation of the active release of a bactericidal agent in ETTs as an efficient strategy to prevent the risk of VAP.

Versatile biomimetic medical device surface: hydrophobin coated, nitric oxide-releasing polymer for antimicrobial and hemocompatible applications

In medical device design, there is a vital need for a coating that promotes treatment of the patient and simultaneously prevents fouling by biomacromolecules which in turn can progress to infections, thrombosis, and other device-related complications. In this work, hydrophobin SC3 (SC3), a self-assembling amphiphilic protein, was coated on a nitric oxide (NO) releasing medical grade polymer to provide an antifouling layer to work synergistically with NO's bactericidal and antiplatelet activity (SC3-NO). The contact angle of SC3 samples were ∼30% lesser than uncoated control samples and was maintained for a month in physiological conditions, demonstrating a stable, hydrophilic coating. NO release characteristics were not adversely affected by the SC3 coating and samples with SC3 coating maintained NO release. Fibrinogen adsorption was reduced over tenfold on SC3 coated samples when compared to non-SC3 coated samples. The viable cell count of adhered bacteria (Staphylococcus aureus) on SC3-NO was 79.097 ± 7.529% lesser than control samples and 49.533 ± 18.18% lesser than NO samples. Platelet adherence on SC3-NO was reduced by 73.407 ± 14.59% when compared to control samples and 53.202 ± 25.67 when compared to NO samples. Finally, the cytocompatibility of SC3-NO was tested and proved to be safe and not trigger a cytotoxic response. The overall favorable results from the physical, chemical and biological characterization analyses demonstrate the novelty and importance of a naturally-produced antifouling layer coated on a bactericidal and antiplatelet polymer, and thus will prove to be advantageous in a multitude of medical device applications.

Nitric oxide-releasing semi-crystalline thermoplastic polymers: preparation, characterization and application to devise anti-inflammatory and bactericidal implants

Semi-crystalline thermoplastics are an important class of biomaterials with applications in creating extracorporeal and implantable medical devices. In situ release of nitric oxide (NO) from medical devices can enhance their performance via NO's potent anti-thrombotic, bactericidal, anti-inflammatory, and angiogenic activity. However, NO-releasing semi-crystalline thermoplastic systems are limited and the relationship between polymer crystallinity and NO release profile is unknown. In this paper, the functionalization of poly(ether-block-amide) (PEBA), Nylon 12, and polyurethane tubes, as examples of semi-crystalline polymers, with the NO donor S-nitroso-N-acetylpenicillamine (SNAP) within, is demonstrated via a polymer swelling method. The degree of crystallinity of the polymer plays a crucial role in both SNAP impregnation and NO release. Nylon 12, which has a relatively high degree of crystallinity, exhibits an unprecedented NO release duration of over 5 months at a low NO level, while PEBA tubing exhibits NO release over days to weeks. As a new biomedical application of NO, the NO-releasing PEBA tubing is examined as a cannula for continuous subcutaneous insulin infusion. The released NO is shown to enhance insulin absorption into the bloodstream probably by suppressing the tissue inflammatory response, and thereby could benefit insulin pump therapy for diabetes management.

VV extracorporeal life support for the Third Millennium: will we need anticoagulation?

Since the late 1600's medicine and science have entertained the idea of extracorporeal circulation. With this technology to allow for cardiac and pulmonary support came the development of anticoagulation. Although this advanced the technology and capabilities of extracorporeal life support, it was not without complications and risks. The most common complications in extracorporeal life support (ECLS) present day are related to hemorrhage and thrombus due to the need for systemic anticoagulation and the challenges associated with it. This review focuses on present day techniques for anticoagulation for ECLS and what future surface modifications may do to obviate the use of systemic anticoagulation entirely.

Controlling the Uptake and Regulating the Release of Nitric Oxide in Microporous Solids

Representative compounds from three classes of microporous solids, namely, metal-organic frameworks (MOFs), hybrid ultra-microporous materials (HUMs), and porous-organic polymers (POPs), were investigated for their nitric oxide gas uptake and release behavior. Low-pressure sorption studies indicated strong chemisorption of NO on the free amine groups decorating the MOF UiO-66-NH₂ when compared to its non-amine-functionalized parent. The HUMs demonstrated reversible physisorption within the low-pressure regime, but interestingly in one case there was evidence for chemisorption following pressurization with NO at 10 bar. Significant release of chemisorbed NO from the UiO-66-NH₂ and one of the HUMs was triggered by addition of acid to the medium, a pH change from 7.4 to 5.4 being sufficient to trigger NO release. An imidazole-based POP exhibited chemisorption of NO at high pressure wherein the ring basicity facilitated both NO uptake and spontaneous release upon contact with the aqueous release medium.

Thromboprophylaxis in Extracorporeal Circuits: Current Pharmacological Strategies and Future Directions

The development of extracorporeal devices for organ support has been a part of medical history and progression since the late 1900s. These types of technology are primarily used and developed in the field of critical care medicine. Unfractionated heparin, discovered in 1916, has really been the only consistent form of thromboprophylaxis for attenuating or even preventing the blood-biomaterial reaction that occurs when such technologies are initiated. The advent of regional anticoagulation for procedures such as continuous renal replacement therapy and plasmapheresis have certainly removed the risks of systemic heparinization and heparin effect, but the challenges of the blood-biomaterial reaction and downstream effects remain. In addition, regional anticoagulation cannot realistically be applied in a system such as extracorporeal membrane oxygenation because of the high blood flow rates needed to support the patient. More recently, advances in the technology itself have resulted in smaller, more compact extracorporeal life support (ECLS) systems that can-at certain times and in certain patients-run without any form of anticoagulation. However, the majority of patients on ECLS systems require some type of systemic anticoagulation; therefore, the risks of bleeding and thrombosis persist, the most devastating of which is intracranial hemorrhage. We provide a concise overview of the primary and alternate agents and monitoring used for thromboprophylaxis during use of ECLS. In addition, we explore the potential for further biomaterial and technologic developments and what they could provide when applied in the clinical arena.

Liquid-infused nitric oxide-releasing (LINORel) silicone for decreased fouling, thrombosis, and infection of medical devices

Recent reports on liquid-infused materials have shown promise in creating ultra-low fouling surfaces, but are limited in their ability to prevent bacterial proliferation and prevent platelet activation in blood-contacting applications. In this work, a liquid-infused nitric oxide-releasing (LINORel) material is created by incorporating the nitric oxide (NO) donor S-nitroso-acetylpenicillamine (SNAP) and silicone oil in commercial medical grade silicone rubber tubing through a solvent swelling process. This combination provides several key advantages over previous NO-releasing materials, including decreased leaching of NO donor, controlled release of NO, and maintenance of ultra-low fouling property of liquid-infused materials. The LINORel tubing reduces protein adhesion as observed using fluorescence imaging, and platelet adhesion (81.7 ± 2.5%) in vitro over a 2 h period. The LINORel combination greatly reduces bacterial adhesion and biofilm formation of two most common pathogens responsible for hospital acquired infections: gram-positive Staphylococcus aureus and gram-negative Pseudomonas aeruginosa (99.3 ± 1.9% and 88.5 ± 3.3% respectively) over a 7-day period in a CDC bioreactor environment. Overall, the LINORel approach provides a synergistic combination of active and passive non-fouling approaches to increase biocompatibility and reduce infection associated with medical devices.

Tunable Nitric Oxide Release from S-Nitroso-N-acetylpenicillamine via Catalytic Copper Nanoparticles for Biomedical Applications

The quest for novel therapies to prevent bacterial infections and blood clots (thrombosis) is of utmost importance in biomedical research due to the exponential growth in the cases of thrombosis and blood infections and the emergence of multi-drug-resistant strains of bacteria. Endogenous nitric oxide (NO) is a cellular signaling molecule that plays a pivotal role in host immunity against pathogens, prevention of clotting, and regulation of systemic blood pressure, among several other biological functions. The physiological effect of NO is dose dependent, which necessitates the study of its tunable release kinetics, which is the objective of this study. In the present study, polymer composites were fabricated by incorporating S-nitroso-N-acetylpenicillamine (SNAP) in a medical-grade polymer, Carbosil, and top-coated with varying concentrations of catalytic copper nanoparticles (Cu-NPs). The addition of the Cu-NPs increased the NO release, as well as the overall antimicrobial activity via the oligodynamic effect of Cu. SNAP (10 wt %) composites without Cu-NP coatings showed a NO flux of 1.32 ± 0.6 × 10^-10 mol min^-1 cm^-2, whereas Cu-NP-incorporated SNAP films exhibited fluxes of 4.48 ± 0.5 × 10^-10, 4.84 ± 0.3 × 10^-10, and 11.7 ± 3.6 × 10^-10 mol min^-1 cm^-2 with 1, 3, and 5 wt % Cu-NPs, respectively. This resulted in a significant reduction (up to 99.8%) in both gram-positive and gram-negative bacteria, with very low platelet adhesion (up to 92% lower) as compared to that of the corresponding controls. Copper leachates from the SNAP films were detected using the inductively coupled plasma-mass spectrometry technique and were found to be significantly lower in concentration than the recommended safety limit by the FDA. The cell viability test performed on mouse fibroblast 3T3 cells provided supportive evidence for the biocompatibility of the material in vitro.

A hemocompatible polyurethane surface having dual fibrinolytic and nitric oxide generating functions

Thrombus formation remains a serious problem in developing blood compatible materials. Despite continuous, intensive efforts over many years to prepare surfaces that prevent clotting, such surfaces have not been achieved; indeed it seems that surface-induced clotting is inevitable. An alternative approach is to accept that clotting will occur and to design surfaces so that small, nascent clots will be lysed before they can cause harm. The generation of plasmin, as in the fibrinolytic system, may be adopted for this purpose. The vascular endothelium (the inner surface of intact blood vessels) releases nitric oxide (NO) on a continuous basis. NO protects against platelet activation and aggregation, and also has an anti-proliferative effect on smooth muscle cells (SMCs). Based on these two important functions of the vascular system, the approach of constructing a fibrinolytic surface that generates NO is developed in the present work. Poly(oligo(ethylene glycol) methyl ether methacrylate-co-6-amino-2-(2-methacylamido)-hexanoic acid) (poly(OEGMA-co-LysMA)) was attached to a vinyl-functionalized polyurethane (PU) surface by graft polymerization giving a surface (PU-POL) with protein-resistant properties (via poly(OEGMA)) and clot lysing properties (via poly(LysMA)). Selenocystamine, which catalyzes S-nitrosothiol decomposition to generate NO in the vasculature, was then immobilized on the PU-POL surface via covalent attachment. A dual functioning surface with fibrinolytic activity (lysis of nascent clots) and NO releasing ability (inhibition of platelet adhesion and SMC adhesion as well as proliferation) was thereby constructed.

Characterization and in vivo performance of nitric oxide-releasing extracorporeal circuits in a feline model of thrombogenicity

Infection and thrombosis are the two leading complications associated with blood contacting medical devices, and have led to the development of active materials that can delivery antibiotics or antithrombotic agents. Two key characteristics of these materials are the ability to produce controlled delivery, as well as minimal systemic delivery of the agent outside of the device site. Nitric oxide (NO) releasing materials are attractive as NO plays pivotal roles in the body's natural defense against bacterial infection, as well as regulation of platelet adhesion and activation. This work characterizes an NO-releasing extracorporeal circuit (ECC) under flow conditions for the first time, examining the effect of incubation and application of the top coating on leaching of NO donor and NO-release kinetics. Top coated ECCs with incubation delivered ca. 1% of the total NO potential over the 4-h period, whereas uncoated ECCs delivered over 4.5% of the total NO. Incubated ECC loops maintained a flux of 1.83 ± 0.50 × 10^-10 mol min^-1 cm^-2 for the full 4 h duration. The NO-releasing ECC loops significantly increased the time-to-clot as compared to the corresponding control (11 ± 3.6 min control, 132 ± 93.0 min NO-releasing) when evaluated in vivo in a feline animal model. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 539-546, 2017.

A chemical stability study of trimethylsilane plasma nanocoatings for coronary stents

Trimethylsilane (TMS) plasma nanocoatings were deposited onto stainless steel coupons in direct current (DC) and radio frequency (RF) glow discharges and additional NH₃/O₂ plasma treatment to tailor the coating surface properties. The chemical stability of the nanocoatings were evaluated after 12 week storage under dry condition (25 °C) and immersion in simulated body fluid (SBF) at 37 °C. It was found that nanocoatings did not impact surface roughness of underlying stainless steel substrates. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy were used to characterize surface chemistry and compositions. Both DC and RF nanocoatings had Si- and C-rich composition; and the O- and N-contents on the surfaces were substantially increased after NH₃/O₂ plasma treatment. Contact angle measurements showed that DC-TMS nanocoating with NH₃/O₂ treatment generated very hydrophilic surfaces. DC-TMS nanocoatings with NH₃/O₂ treatment showed minimal surface chemistry change after 12 week immersion in SBF. However, nitrogen functionalities on RF-TMS coating with NH₃/O₂ post treatment were not as stable as in DC case. Cell culture studies revealed that the surfaces with DC coating and NH₃/O₂ post treatment demonstrated substantially improved proliferation of endothelial cells over the 12 week storage period at both dry and wet conditions, as compared to other coated surfaces. Therefore, DC nanocoatings with NH₃/O₂ post treatment may be chemically stable for long-term properties, including shelf-life storage and exposure to the bloodstream for coronary stent applications.

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).

Transport of Nitric Oxide (NO) in Various Biomedical grade Polyurethanes: Measurements and Modeling Impact on NO Release Properties of Medical Devices

Nitric oxide (NO) releasing polymers are promising in improving the biocompatibility of medical devices. Polyurethanes are commonly used to prepare/fabricate many devices (e.g., catheters); however, the transport properties of NO within different polyurethanes are less studied, creating a gap in the rational design of new NO releasing devices involving polyurethane materials. Herein, we study the diffusion and partitioning of NO in different biomedical polyurethanes via the time-lag method. The diffusion of NO is positively correlated with the PDMS content within the polyurethanes, which can be rationalized by effective media theory considering various microphase morphologies. Using catheters as a model device, the effect of these transport properties on the NO release profiles and the distribution around an asymmetric dual lumen catheter are simulated using finite element analysis and validated experimentally. This method can be readily applied in studying other NO release medical devices with different configurations.

Reprint of: Nitric oxide-releasing polysaccharide derivative exhibits 8-log reduction against Escherichia coli, Acinetobacter baumannii and Staphylococcus aureus

Health-care associated infections (HAIs) and the increasing number of antibiotic-resistant bacteria strains remain significant public health threats worldwide. Although the number of HAIs has decreased by using improved sterilization protocols, the cost related to HAIs is still quantified in billions of dollars. Furthermore, the development of multi-drug resistant strains is increasing exponentially, demonstrating that current treatments are inefficient. Thus, the quest for new methods to eradicate bacterial infection is increasingly important in antimicrobial, drug delivery and biomaterials research. Herein, the bactericidal activity of a water-soluble NO-releasing polysaccharide derivative was evaluated in nutrient broth media against three bacteria strains that are commonly responsible for HAIs. Data confirmed that this NO-releasing polysaccharide derivative induced an 8-log reduction in bacterial growth after 24h for Escherichia coli, Acinetobacter baumannii and Staphylococcus aureus. Additionally, the absence of bacteria after 72 h of exposure to NO illustrates the inability of the bacteria to recover and the prevention of biofilm formation. The presented 8-log reduction in bacterial survival after 24h is among the highest reduction reported for NO delivery systems to date, and reaches the desired standard for industrially-relevant reduction. More specifically, this system represents the only water-soluble antimicrobial to reach such a significant bacterial reduction in nutrient rich media, wherein experimental conditions more closely mimic the in vivo environment than those in previous reports. Furthermore, the absence of bacterial activity after 72 h and the versatility of using a water-soluble compound suggest that this NO-releasing polysaccharide derivative is a promising route for treating HAIs.

Nitric oxide generating/releasing materials

Harnessing the impressive therapeutic potential of nitric oxide (NO) remains an ongoing challenge. This paper describes several of the current strategies both with respect to the underlying chemistry and physics and to the applications where they have shown promise. Included in this overview are molecular systems such as NONOates that release NO through chemical reactions and delivery vehicles such as nanoparticles that can generate, store, transport and deliver NO and related bioactive forms of NO such as nitrosothiols. Although there has been much positive movement, it is clear that we are only at the early stages of knowing how to precisely produce, transport and deliver to targeted sites therapeutic levels of NO and related molecules.

Immobilization of Metal-Organic Framework Copper(II) Benzene-1,3,5-tricarboxylate (CuBTC) onto Cotton Fabric as a Nitric Oxide Release Catalyst

Immobilization of metal-organic frameworks (MOFs) onto flexible polymeric substrates as secondary supports expands the versatility of MOFs for surface coatings for the development of functional materials. In this work, we demonstrate the deposition of copper(II) benzene-1,3,5-tricarboxylate (CuBTC) crystals directly onto the surface of carboxyl-functionalized cotton capable of generating the therapeutic bioagent nitric oxide (NO) from endogenous sources. Characterization of the CuBTC-cotton material by XRD, ATR-IR, and UV-vis indicate that CuBTC is successfully immobilized on the cotton fabric. In addition, SEM imaging reveals excellent surface coverage with well-defined CuBTC crystals. Subsequently, the CuBTC-cotton material was evaluated as a supported heterogeneous catalyst for the generation of NO using S-nitrosocysteamine as the substrate. The resulting reactivity is consistent with the activity observed for unsupported CuBTC particles. Overall, this work demonstrates deposition of MOFs onto a flexible polymeric material with excellent coverage as well as catalytic NO release from S-nitrosocysteamine at therapeutic levels.

Biodegradable citrate-based polyesters with S-nitrosothiol functional groups for nitric oxide release

Nitric oxide (NO) is a biologically-active free radical involved in numerous physiological processes such as regulation of vasodilation, promotion of cell proliferation and angiogenesis, and modulation of the inflammatory and immune responses. Furthermore, NO has demonstrated the ability to mitigate the foreign body response that often results in the failure of implanted biomedical devices. Although NO has promising therapeutic value, the short physiological half-life of exogenous NO complicates its effective delivery. For this reason, the development of NO-releasing materials that permit the localized delivery of NO is an advantageous method of utilizing this molecule for biomedical applications. Herein, we report the synthesis and characterization of biodegradable NO-releasing polyesters prepared from citric acid, maleic acid, and 1,8-octanediol. NO release was achieved by incorporation of S-nitrosothiol donor groups through conjugation of cysteamine and ethyl cysteinate to the polyesters, followed by S-nitrosation with tert-butyl nitrite. The extent of NO loading and the release properties under physiological conditions (pH 7.4 PBS, 37 °C) were determined by chemiluminesence-based NO detection. The average total NO content of poly(citric-co-maleic acid-co-1,8-octanediol)-cysteamine was determined to be 0.45 ± 0.07 mol NO g^-1 polymer, while the NO content for poly(citric-co-maleic acid-co-1,8-octanediol)-ethyl cysteinate was 0.16 ± 0.04 mol NO g^-1 polymer. Continuous NO release under physiological conditions was observed for at least 6 days for the cysteamine analog and 4 days for the ethyl cysteinate analog. Cell viability assays and morphological studies with human dermal fibroblasts indicated an absence of toxic leachates at a cytotoxic level, and suggested that these citrate-based polyesters may be suitable for future biomedical applications.

Origin of Long-Term Storage Stability and Nitric Oxide Release Behavior of CarboSil Polymer Doped with S-Nitroso-N-acetyl-D-penicillamine

The prolonged and localized delivery of nitric oxide (NO), a potent antithrombotic and antimicrobial agent, has many potential biomedical applications. In this work, the origin of the long-term storage stability and sustained NO release mechanism of S-nitroso-N-acetyl-D-penicillamine (SNAP)-doped CarboSil 20 80A polymer, a biomedical thermoplastic silicone-polycarbonate-urethane, is explored. Long-term (22 days) localized NO release is achieved by utilizing a cross-linked silicone rubber as topcoats, which can greatly reduce the amount of SNAP, NAP, and NAP disulfide leaching from the SNAP-doped CarboSil films, as measured by LC-MS. Raman spectroscopy and powder X-ray diffraction characterization of SNAP-doped CarboSil films demonstrate that a polymer-crystal composite is formed during the solvent evaporation process when SNAP exceeds its solubility in CarboSil (ca. 3.4-4.0 wt %). Further, when exceeding this solubility threshold, SNAP exists in an orthorhombic crystal form within the bulk of the polymer. The proposed mechanism of sustained NO release in SNAP-doped CarboSil is that the solubilized SNAP in the polymer matrix decomposes and releases NO, primarily in the water-rich regions near the polymer/solution interface, and the dissolved SNAP in the bulk polymeric phase becomes unsaturated, resulting in the dissolution of crystalline SNAP within the bulk of the polymer. This is a very slow process that ultimately leads to NO release at the physiological flux levels for >3 weeks. The increased stability of SNAP within CarboSil is attributed to the intermolecular hydrogen bonds between the SNAP molecules that crystallize. This crystallization also plays a key role in maintaining RSNO stability within the CarboSil polymer for >8 months at 37 °C (88.5% remains). Further, intravascular catheters fabricated with this new material are demonstrated to significantly decrease the formation of Staphylococcus aureus biofilm (a leading cause of nosocomial bloodstream infections) (in vitro) over a 7 day period, with 5 log units reduction of viable cell count on catheter surfaces. It is also shown that the NO release catheters can greatly reduce thrombus formation on the catheter surfaces during 7 h implantation in rabbit veins, when compared to the control catheters fabricated without SNAP. These results suggest that the SNAP-doped CarboSil system is a very attractive new composite material for creating long-term NO release medical devices with increased stability and biocompatibility.

A computational study of the effect of the metal organic framework environment on the release of chemically stored nitric oxide

The use of copper based metal organic frameworks as a vehicle for the storage and delivery of chemically stored nitric oxide has been proposed based on recent experiments [J. Am. Chem. Soc., 2012, 134, 3330-3333]. In these experiments copper based metal organic frameworks (MOFs) suspended in ethanol catalytically convert chemically stored nitric oxide (in the S-nitrosothiol or RSNO form) to free nitric oxide at a slow and sustained rate, as compared to a quick release in a solution of ethanol containing free copper ions. In order to gain insight on the effect of the MOF environment on the catalytic activity, a combination of electronic structure calculations on representative clusters and classical simulations using a force-field (partly parameterized on the above calculations) is used to study a simple RSNO species, S-nitrosomethane (CH3SNO) as well as the biologically compatible S-nitrosocysteine, both in the MOF and free copper solution. The free energy profiles of bringing the RSNO species to the catalytic centers have been compared and related to the different solvation environments of the copper catalyst in the complex solvated MOF and in free copper solution. Surprisingly, in the case of the simple CH3SNO moiety as well as the S-nitrosocysteine case, the free energy profile of bringing the first RSNO from the center of one of the pores to the catalytic site in the pore is very similar to the free solution case. On the other hand, bringing a second RSNO molecule to the same catalytic site or to the adjacent catalytic copper site show relatively higher barriers. These studies help shed light on the sustained nitric oxide release in the MOF environment.

Nitric oxide-releasing polysaccharide derivative exhibits 8-log reduction against Escherichia coli, Acinetobacter baumannii and Staphylococcus aureus

Health-care associated infections (HAIs) and the increasing number of antibiotic-resistant bacteria strains remain significant public health threats worldwide. Although the number of HAIs has decreased by using improved sterilization protocols, the cost related to HAIs is still quantified in billions of dollars. Furthermore, the development of multi-drug resistant strains is increasing exponentially, demonstrating that current treatments are inefficient. Thus, the quest for new methods to eradicate bacterial infection is increasingly important in antimicrobial, drug delivery and biomaterials research. Herein, the bactericidal activity of a water-soluble NO-releasing polysaccharide derivative was evaluated in nutrient broth media against three bacteria strains that are commonly responsible for HAIs. Data confirmed that this NO-releasing polysaccharide derivative induced an 8-log reduction in bacterial growth after 24h for Escherichia coli, Acinetobacter baumannii and Staphylococcus aureus. Additionally, the absence of bacteria after 72h of exposure to NO illustrates the inability of the bacteria to recover and the prevention of biofilm formation. The presented 8-log reduction in bacterial survival after 24h is among the highest reduction reported for NO delivery systems to date, and reaches the desired standard for industrially-relevant reduction. More specifically, this system represents the only water-soluble antimicrobial to reach such a significant bacterial reduction in nutrient rich media, wherein experimental conditions more closely mimic the in vivo environment than those in previous reports. Furthermore, the absence of bacterial activity after 72h and the versatility of using a water-soluble compound suggest that this NO-releasing polysaccharide derivative is a promising route for treating HAIs.

Nitric oxide release from polydimethylsiloxane-based polyurethanes

Localized nitric oxide (NO) release from polymeric materials holds much promise for the prevention of coagulation often associated with implantable and extracorporeal blood-contacting devices. Films of polyurethane (PU) containing incorporated polyethyleneimine were thus exposed to NO gas to form diazeniumdiolates (NONOates) in situ. Donor incorporation and NO gas exposure did not affect the mechanical properties of the films. The NO release capacity increased with increasing polydimethylsiloxane (PDMS) content in the soft segment of the PU: total capacity could be more than doubled (P<0.05) from 0.093 ± 0.028 to 0.225 ± 0.004 mmol/g when the PDMS content was increased from 0 to 100%. Release kinetics were best approximated using a modified Korsemeyer-Peppas power law (R²=0.95-0.99). Despite the resultant rapid initial decrease in NO release rates, values above that observed for quiescent endothelial cells (0.83 pmol·cm⁻²·s⁻¹) were maintained for extended periods of 5-10 days, while rates above that of a stimulated endothelium (2.7-6.8 pmol·cm⁻²·s⁻¹) were achieved for the first 24 hours. This method of NONOate formation may be advantageous, as potential premature NO release by exposure of diazeniumdiolated donors during incorporation, processing and storage, can be avoided by in situ diazoniumdiolation closer to the time of implantation.

Accelerating in situ endothelialisation of cardiovascular bypass grafts

The patency of synthetic cardiovascular grafts in the long run is synonymous with their ability to inhibit the processes of intimal hyperplasia, thrombosis and calcification. In the human body, the endothelium of blood vessels exhibits characteristics that inhibit such processes. As such it is not surprising that research in tissue engineering is directed towards replicating the functionality of the natural endothelium in cardiovascular grafts. This can be done either by seeding the endothelium within the lumen of the grafts prior to implantation or by designing the graft such that in situ endothelialisation takes place after implantation. Due to certain difficulties identified with in vitro endothelialisation, in situ endothelialisation, which will be the focus of this article, has garnered interest in the last years. To promote in situ endothelialisation, the following aspects can be taken into account: (1) Endothelial progenital cell mobilization, adhesion and proliferation; (2) Regulating differentiation of progenitor cells to mature endothelium; (3) Preventing thrombogenesis and inflammation during endothelialisation. This article aims to review and compile recent developments to promote the in situ endothelialisation of cardiovascular grafts and subsequently improve their patency, which can also have widespread implications in the field of tissue engineering.

The effect of a polyurethane coating incorporating both a thrombin inhibitor and nitric oxide on hemocompatibility in extracorporeal circulation

Nitric oxide (NO) releasing (NORel) materials have been extensively investigated to create localized increases in NO concentration by the proton driven diazeniumdiolate-containing polymer coatings and demonstrated to improve extracorporeal circulation (ECC) hemocompatibility. In this work, the NORel polymeric coating composed of a diazeniumdiolated dibutylhexanediamine (DBHD-N2O2)-containing hydrophobic Elast-eon™ (E2As) polyurethane was combined with a direct thrombin inhibitor, argatroban (AG), and evaluated in a 4 h rabbit thrombogenicity model without systemic anticoagulation. In addition, the immobilizing of argatroban to E2As polymer was achieved by either a polyethylene glycol-containing (PEGDI) or hexane methylene (HMDI) diisocyanate linker. The combined polymer film was coated on the inner walls of ECC circuits to yield significantly reduced ECC thrombus formation compared to argatroban alone ECC control after 4 h blood exposure (0.6 ± 0.1 AG/HMDI/NORel vs 1.7 ± 0.2 cm(2) AG/HMDI control). Platelet count (2.8 ± 0.3 AG/HMDI/NORel vs 1.9 ± 0.1 × 10(8)/ml AG/HMDI control) and plasma fibrinogen levels were preserved after 4 h blood exposure with both the NORel/argatroban combination and the AG/HMDI control group compared to baseline. Platelet function as measured by aggregometry remained near normal in both the AG/HMDI/NORel (63 ± 5%) and AG/HMDI control (58 ± 7%) groups after 3 h compared to baseline (77 ± 1%). Platelet P-selectin mean fluorescence intensity (MFI) as measured by flow cytometry also remained near baseline levels after 4 h on ECC to ex vivo collagen stimulation (16 ± 3 AG/HMDI/NORel vs 11 ± 2 MFI baseline). These results suggest that the combined AG/HMDI/NORel polymer coating preserves platelets in blood exposure to ECCs to a better degree than AG/PEGDI/NORel, NORel alone or AG alone. These combined antithrombin, NO-mediated antiplatelet effects were shown to improve thromboresistance of the AG/HMDI/NORel polymer-coated ECCs and move potential nonthrombogenic polymers closer to mimicking vascular endothelium.

Nitric oxide donors for cardiovascular implant applications

In an era of increased cardiovascular disease burden in the ageing population, there is great demand for devices that come in to contact with the blood such as heart valves, stents, and bypass grafts that offer life saving treatments. Nitric oxide (NO) elution from healthy endothelial tissue that lines the vessels maintains haemostasis throughout the vasculature. Surgical devices that release NO are desirable treatment options and N-diazeniumdiolates and S-nitrosothiols are recognized as preferred donor molecules. There is a keen interest to investigate newer methods by which NO donors can be retained within biomaterials so that their release and kinetic profiles can be optimized. A range of polymeric scaffolds incorporating microparticles and nanomaterials are presenting solutions to current challenges, and have been investigated in a range of clinical applications. This review outlines the application of NO donors for cardiovascular therapy using biomaterials that release NO locally to prevent thrombosis and intimal hyperplasia (IH) and enhance endothelialization in the fabrication of next generation cardiovascular device technology.

Electromodulated Release of Nitric Oxide Through Polymer Material from Reservoir of Inorganic Nitrite Salt

Herein, we report a new approach to electromodulate the release of NO at physiological levels through polymeric materials from a stable nitrite electrolyte reservoir, with potential application in controlling biofilm formation and clotting on intravascular catheters. The NO flux can be turned 'on' and 'off' electrochemically, on demand.

Fabrication of biodegradable polymeric nanofibers with covalently attached NO donors

Many common wound healing aids are created from biodegradable polymeric materials. Often, these materials are unable to induce complete healing in wounds because of their failure to prevent infection and promote cell growth. This study reports the development of therapeutic materials aimed at overcoming these limitations through the release of a naturally occurring antimicrobial agent from a porous, polymeric fiber scaffold. The antimicrobial character was achieved through the release of nitric oxide (NO) while the porous structure was fabricated through electrospinning polymers into nanofibers. Three variations of the polymer poly(lactic-co-glycolic-co-hydroxymethyl propionic acid) (PLGH) modified to include thiol and NO groups were investigated. Fibers of the modified polymers exhibited smooth, bead free morphologies with diameters averaging between 200 and 410 nm. These fibers were deposited in a random manner to create a highly porous fibrous scaffold. The fibers were found to release NO under physiological pH and temperature and have the capacity to release 0.026 to 0.280 mmol NO g(-1). The materials maintained their fibrous morphological structure after this exposure to aqueous conditions. The sustained morphological stability of the fiber structure coupled to their extended NO release gives these materials great potential for use in wound healing materials.

Nitric oxide release: part I. Macromolecular scaffolds

The roles of nitric oxide (NO) in physiology and pathophysiology merit the use of NO as a therapeutic for certain biomedical applications. Unfortunately, limited NO payloads, too rapid NO release, and the lack of targeted NO delivery have hindered the clinical utility of NO gas and low molecular weight NO donor compounds. A wide-variety of NO-releasing macromolecular scaffolds has thus been developed to improve NO's pharmacological potential. In this tutorial review, we provide an overview of the most promising NO release scaffolds including protein, organic, inorganic, and hybrid organic-inorganic systems. The NO release vehicles selected for discussion were chosen based on their enhanced NO storage, tunable NO release characteristics, and potential as therapeutics.

Biomimetic modification of metallic cardiovascular biomaterials: from function mimicking to endothelialization in vivo

Biosystem-surface interactions play an important role in various biological events and determine the ultimate functionality of implanted devices. Endothelialization or mimicking of endothelium on the surface of cardiovascular materials is a promising way to solve the problems of material-induced thrombosis and restenosis. Meanwhile, a multifunctional surface design is needed as antithrombotic properties should be considered in the period when the implants are not yet completely endothelialized. In this article, we summarize some successful approaches used in our laboratory for constructing multifunctional endothelium-like surfaces on metallic cardiovascular biomaterials through chemical modification of the surface or by the introduction of specific biological molecules to induce self-endothelialization in vivo. Some directions on future research in these areas are also presented.

Metal organic frameworks as nitric oxide catalysts

The use of metal organic frameworks (MOFs) for the catalytic production of nitric oxide (NO) is reported. In this account we demonstrate the use of Cu(3)(BTC)(2) as a catalyst for the generation of NO from the biologically occurring substrate, S-nitrosocysteine (CysNO). The MOF catalyst was evaluated as an NO generator by monitoring the evolution of NO in real time via chemiluminescence. The addition of 2, 10, and 15-fold excess CysNO to MOF-Cu(II) sites and cysteine (CysH) resulted in catalytic turnover of the active sites and nearly 100% theoretical yield of the NO product. Control experiments without the MOF present did not yield appreciable NO generation. In separate studies the MOF was found to be reusable over successive iterations of CysNO additions without loss of activity. Subsequently, the MOF catalyst was confirmed to remain structurally intact by pXRD and ATR-IR following reaction with CysNO and CysH.

Polymer-Based Nitric Oxide Therapies: Recent Insights for Biomedical Applications

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 (2006-2011) 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.

Synthesis of Nitric Oxide-Releasing Polyurethanes with S-Nitrosothiol-Containing Hard and Soft Segments

Nitric oxide (NO)-releasing polyurethanes capable of releasing up to 0.20 μmol NO cm(-2) were synthesized by incorporating active S-nitrosothiol functionalities into hard and soft segment domains using thiol group protection and post-polymerization modifications, respectively. The nitrosothiol position within the hard and soft segment domains of the polyurethanes impacted both the total NO release and NO release kinetics. The NO storage and release properties were correlated to both chain extender modification and ensuing phase miscibility of the polyurethanes. Thorough material characterization is provided to examine the effects of hard and soft segment modifications on the resultant polyurethane properties.