Arnold Darbyshire

The anti-calcification potential of a silsesquioxane nanocomposite polymer under in vitro conditions: Potential material for synthetic leaflet heart valve

Calcification currently represents a major cause of failure of biological tissue heart valves. It is a complex phenomenon influenced by a number of biochemical and mechanical factors. Recent advances in material science offer new polymers with improved properties, potentially suitable for synthetic leaflets heart valves manufacturing. In this study, the calcification-resistance efficacy and mechanical and surface properties of a new nanocomposite polymeric material (polyhedral oligomeric silsesquioxane-poly(carbonate-urea)urethane; POSS-PCU) which has been developed by our group are assessed by means of in vitro testing. In particular, thin sheets of nanocomposite, glutaraldehyde-fixed bovine pericardium (BP) and polyurethane (PU) were exposed to a calcium solution into a specially designed in vitro accelerated physiological pulsatile pressure system for a period of 31days and a total of 4×10(7) cycles. The samples were investigated for signs of calcification after exposure to calcium solution by means of X-ray, microscopic and chemical inspections. Mechanical and surface properties were also studied using stress-strain behaviour and surface morphology and hydrophobicity. Comparison shows that, in the experimental conditions, the level of calcification for the nanocomposite is considerably lower than for the fixed BP (p=0.008) and PU samples (p=0.015). Also, mechanical properties were unchanged in POSS-PCU, while there was a significant deterioration in PU samples (p<0.05). Hydrophobicity was significantly reduced in both the POSS-PCU and PU samples (p<0.0001). However, the POSS-PCU nanocomposite remained more hydrophobic than the PU sample (p<0.0001). Less platelet adhered to the POSS-PCU compared to the PU (p<0.0001). These results indicate that the use of this nanocomposite in synthetic leaflets heart valves may lead to potential advantages in terms of long-term performances and durability.

A new biodegradable nanocomposite based on polyhedral oligomeric silsesquioxane nanocages: Cytocompatibility and investigation into electrohydrodynamic jet fabrication techniques for tissue-engineered scaffolds

Our group has developed a non‐biodegradable nanocomposite based on POSS (polyhedral oligomeric silsesquioxane) nanocages with PCU [poly(carbonate urethane)] and previous studies have shown good cell‐compatibility and antithrombogenic properties. The latest biodegradable formulation is a POSS‐modified poly(hexanolactone/carbonate)urethane/urea containing 80% hexanolactone (caprolactone) with the tradename UCL‐NanoBio™. The direct effect of the polymer on cells was investigated by seeding stem cells on to circular discs of the polymer in 24‐well plates; these discs were prepared mainly by electrohydrodynamic jetting. To assess the indirect effect of the polymer, various concentrations of the polymer powder were added to CCM (cell culture medium) and left on a shaker for 10 days. The precipitate was then removed and the CCM was used for culturing the cells seeded on to 24‐well plates. Cell viability and growth at 48 and 96 h were assessed using Alamar Blue™ and lactate dehydrogenase, and morphology was studied by scanning electron microscopy. Cells were shown to adhere well to the polymer, with cell metabolism being comparable with that found on TCP (tissue‐culture plastic). Indirect assessment demonstrated some decrease in cell viability with high concentrations of polymer, but showed no difference in cell death between polymer concentrations. The viability of cells seeded on to the polymer was comparable with that of those seeded on to TCP. Cell viability was comparable on both electrosprayed and electrospun scaffolds, but infiltration into the scaffold was much more evident on the electrospun scaffolds. It can be concluded that this new nanocomposite can support the growth and viability of stem cells and that scaffolds of this polymer nanocomposite fabricated by electrohydrodynamic jetting routes have potential use for tissue engineering in the future.

Effects of sterilization treatments on bulk and surface properties of nanocomposite biomaterials.

With the continuous and expanding use of implantable biomaterials in a clinical setting, this study aims to elucidate the influence of sterilization techniques on the material surface and bulk properties of two polyurethane nanocomposite biomaterials. Both solid samples and porous membranes of nondegradable polyhedral oligomeric silsesquioxane poly(carbonate-urea) urethane (POSS-PCU) and a biodegradable poly(caprolactone-urea) urethane (POSS-PCL) were examined. Sterilization techniques included conventional steam sterilization (autoclaving), gamma irradiation, and disinfection via incubating with ethanol (EtOH) for 10 min or 24 h. After treatment, the samples were examined using gel permeation chromatography (GPC), attenuated total reflectance Fourier transform infrared spectroscopy, and tensiometry. Cytotoxicity was evaluated through the culture of endothelial progenitor cells and the efficacy of sterilization method was determined by incubating each sample in tryptone soya broth and fluid thioglycollate medium for cultivation of microorganisms. Although EtOH did not affect the material properties in any form, the samples were found to be nonsterile with microbial growth detected on each of the samples. Gamma irradiation was not only effective in sterilizing both POSS-PCU and POSS-PCL but also led to minor material degradation and displayed a cytotoxic effect on the cultured cells. Autoclaving was found to be the optimal sterilization technique for both solid and porous membranes of the nondegradable POSS-PCU samples as it was successful in sterilizing the samples, displayed no cytotoxic side effects and did not degrade the material. However, the biodegradable POSS-PCL was not able to withstand the harsh environment during autoclaving, resulting in it losing all structural integrity.

A silver nanocomposite biomaterial for blood‐contacting implants

Cardiovascular implants must resist infection and thrombosis. A nanocomposite polymeric material [polyhedral-oligomeric-silsesquioxane-poly(carbonate-urea)urethane; POSS-PCU] demonstrates ideal properties for cardiovascular applications. Silver nanoparticles or nanosilver (NS) are recognized for efficient antibacterial properties. This study aims to determine the influence of NS integrated POSS-PCU on thrombogenicity. Silver nitrate was reduced with dimethylformamide and stabilized by the inclusion of fumed silica nanoparticles to prevent aggregation of NS and were incorporated into POSS-PCU to form a range of POSS-PCU-NS concentrations (by weight); 0.20% (NS16), 0.40% (NS32), 0.75% (NS64), and 1.50% (NS128). Surface wettability was determined with sessile-drop water contact angles. Platelets were introduced onto test samples and Alamar Blue (AB), mitochondrial-activity assay, quantified the degree of platelet adhesion whilst platelet-factor-4 (PF4) ELISA quantified the degree of platelet activation. Thromboelastography (TEG) determined the profiles of whole blood kinetics while hemolysis assay demonstrated the degree of blood compatibility. Increasing levels of NS induced greater hydrophilicity. A concentration dependant decrease in platelet adhesion and activation was observed with AB and PF4 readings, respectively. TEG demonstrated that the antithrombogenic properties of POSS-PCU were retained with POSS-PCU-NS16, and enhanced with POSS-PCU-NS32, but was reduced with POSS-PCU-NS64 and POSS-PCU-NS128. POSS-PCU-NS64 and POSS-PCU-NS128 demonstrated a hemolytic tendency, but no hemolysis was observed with POSS-PCU-NS16 and POSS-PCU-NS32. Overall, POSS-PCU-NS32 rendered potent antithrombogenic properties.

Polymeric coating of surface modified nitinol stent with POSS-nanocomposite polymer

Stent angioplasty is a successful treatment for arterial occlusion, particularly in coronary artery disease. The clinical communities were enthusiastic about the use of drug-eluting stents; however, these stents have a tendency to be a contributory factor towards late stage thrombosis, leading to mortality in a significant number of patients per year. This work presents an innovative approach in self-expanding coronary stents preparation. We developed a new nanocomposite polymer based on polyhedral oligomeric silsesquioxanes (POSS) and poly(carbonate-urea)urethane (PCU), which is an antithrombogenic and a non-biodegradable polymer with in situ endothelialization properties. The aim of this work is to coat a NiTi stent alloy with POSS-PCU. In prolonged applications in the human body, the corrosion of the NiTi alloy can result in the release of deleterious ions which leads to unwanted biological reactions. Coating the nitinol (NiTi) surface with POSS-PCU can enhance surface resistance and improve biocompatibility. Electrohydrodynamic spraying was used as the polymer deposition process and thus a few experiments were carried out to compare this process with casting. Prior to deposition the NiTi has been surface modified. The peel strength of the deposit was studied before and after degradation of the coating. It is shown that the surface modification enhances the peel strength by 300%. It is also indicated how the adhesion strength of the POSS-PCU coating changes post-exposure to physiological solutions comprised of hydrolytic, oxidative, peroxidative and biological media. This part of the study shows that the modified NiTi presents far greater resistance to decay in peel strength compared to the non-modified NiTi.

Electrohydrodynamic Jetting Behaviour of Polyhedral Oligomeric Silsesquioxane Nanocomposite

In this paper, we investigate in detail the electrohydrodynamic spraying of a nonbiodegradable nanocomposite polyhedral oligomeric silsesquioxane polymer developed in our laboratories and currently being explored for coating metallic stent materials. Different concentrations of the polymer have been dissolved to prepare, characterise, and electrohydrodynamically deposit the polymer on stainless steel. From the experiments, the solution containing 15 wt% polymer was selected for further investigation. The variation of film/ coating thickness as a function of spraying time was studied and the structural features of the film were assessed using microscopy. Films were also tensile tested. This study has identified a process and conditions which can be used in our stent coating research.

Next generation covered stents made from nanocomposite materials: A complete assessment of uniformity, integrity and biomechanical properties.

Covered stents are stents wrapped with a thin polymeric membrane, and are typically used to treat vessel aneurysms and seal perforated arteries. Current covered stents suffer from restenosis due to limitations in material and fabrication methods which leaves metallic struts directly exposed to blood. We have developed a biocompatible and haemocompatible nanocomposite polymer, polyhedral oligomeric silsesquioxane poly(carbonate-urea) urethane (POSS-PCU). We devised a novel combination of ultrasonic spray atomisation system and dip-coating process to produce small calibre covered stents with metal struts fully embedded within the membrane, which also yields greater coating uniformity. Stent-polymer bonding was enhanced via silanisation and coating of reactive pre-polymer. Platelet studies supported the non-thrombogenicity of POSS-PCU. Biomechanical performances including diametrical compliance, bending strength, radial strength and recoil were evaluated and optimised. This proof-of-principle manufacturing technique could lead to the development of next-generation small calibre adult and paediatric covered stents. These stents are currently undergoing preclinical trial. From the Clinical Editor: The use of stents to treat vascular diseases is now the standard of care in the clinical setting. Nonetheless, a major problem of the current stents is the risk of restenosis and thrombosis. The authors developed a nanocomposite material using polyhedral oligomeric silsesquioxane and poly(carbonate-urea) urethane (POSS-PCU) and incorporated into metallic stents. Preliminary data have already shown promising results. It is envisaged that this would further lead to better stent technology in the future.

A Material Conferring Hemocompatibility.

There is a need for biomimetic materials for use in blood-contacting devices. Blood contacting surfaces maintain their patency through physico-chemical properties of a functional endothelium. A poly(carbonate-urea) urethane (PCU) is used as a base material to examine the feasibility of L-Arginine methyl ester (L-AME) functionalized material for use in implants and coatings. The study hypothesizes that L-AME, incorporated into PCU, functions as a bioactive porogen, releasing upon contact with blood to interact with endothelial nitric oxide synthase (eNOS) present in blood. Endothelial progenitor cells (EPC) were successfully cultured on L-AME functionalized material, indicating that L-AME -increases cell viability. L-AME functionalized material potentially has broad applications in blood-contacting medical devices, as well as various other applications requiring endogenous up-regulation of nitric oxide, such as wound healing. This study presents an in-vitro investigation to demonstrate the novel anti-thrombogenic properties of L-AME, when in solution and when present within a polyurethane-based polymer.

An Aortic Model for the Physiological Assessment of Endovascular Stent-Grafts

BACKGROUND The aim of this study was to manufacture a new aortic model with physiological properties, which could be used for long-term durability testing of endovascular stent-grafts, as per the recommendations of the Food and Drug Administration. METHODS Porcine abdominal aortas were acquired to establish values for compliance. The aortic model was manufactured using a nanocomposite polymer. Latex mock aorta was used for comparison. A pulsatile flow phantom perfused the aortas and synthetic tubes at physiological pulse pressure and flow. Diametrical compliance and stiffness index were calculated over mean pressures from 30 to 120 mm Hg. Data were analyzed using one-way analysis of variance and Bonferronis test. RESULTS Flow circuit hemodynamic values were similar for porcine aorta and synthetic tubes. Compliance of aorta ranged from 2.97 ± 0.72 (mean ± SD) to 1.42 ± 0.37%/mm Hg × 10⁻². The polymer model showed significantly better compliance (range, 3.66 ± 1.05-2.72 ± 0.28%/mm Hg × 10⁻²; p < 0.05), with no significant difference in elastic stiffness index (range, 101.6 ± 28.9-51.3 ± 10.7 for aorta and 39.8 ± 8.5-34.2 ± 3.8 for polymer model; p > 0.05). It also showed anisotropic behavior similar to the aorta. Latex tubes showed compliance that was lower than that in aorta (range, 0.87 ± 0.24-0.86 ± 0.2%/mm Hg × 10⁻²) and failed by a significant distension on increase in pressure from mean of 90 mm Hg. CONCLUSIONS We have developed physiologically relevant aortic model showing compatible anatomy, compliance, and viscoelasticity, which could be used for long-term fatigue analysis of vascular stents and grafts. The latex mock aortas can fail at physiological pressures.

Biomimetic heterogenous elastic tissue development

There is an unmet need for artificial tissue to address current limitations with donor organs and problems with donor site morbidity. Despite the success with sophisticated tissue engineering endeavours, which employ cells as building blocks, they are limited to dedicated labs suitable for cell culture, with associated high costs and long tissue maturation times before available for clinical use. Direct 3D printing presents rapid, bespoke, acellular solutions for skull and bone repair or replacement, and can potentially address the need for elastic tissue, which is a major constituent of smooth muscle, cartilage, ligaments and connective tissue that support organs. Thermoplastic polyurethanes are one of the most versatile elastomeric polymers. Their segmented block copolymeric nature, comprising of hard and soft segments allows for an almost limitless potential to control physical properties and mechanical behaviour. Here we show direct 3D printing of biocompatible thermoplastic polyurethanes with Fused Deposition Modelling, with a view to presenting cell independent in-situ tissue substitutes. This method can expeditiously and economically produce heterogenous, biomimetic elastic tissue substitutes with controlled porosity to potentially facilitate vascularisation. The flexibility of this application is shown here with tubular constructs as exemplars. We demonstrate how these 3D printed constructs can be post-processed to incorporate bioactive molecules. This efficacious strategy, when combined with the privileges of digital healthcare, can be used to produce bespoke elastic tissue substitutes in-situ, independent of extensive cell culture and may be developed as a point-of-care therapy approach.3D Printing Artificial Elastic TissuesSolvent-free thermoplastic polyurethanes (TPU) could be used to 3D-print artificial tissues saving time and money. Achala de Mel and colleagues at University College London used open-source 3D-modelling software and commercially available 3D printers to fabricate a bespoke tracheal stent from custom-made TPU. The team was able to control the material’s porosity with 3D-design, which could facilitate its vascularisation if implanted. The trachea was mechanically and structurally similar to that of an adult, showing longitudinal elasticity and radial rigidity. When attached to a ventilator system, it responded well to pressures similar to those of inspiration, forced expiration, coughing or crying. 3D-printed trachea was treated with bioactive molecules so cells could potentially adhere to and proliferate on its surface. This method could be used to fabricate bespoke elastic tissue substitutes, avoiding costly and time-consuming cell-culture techniques.