William M. Carroll

The evolution of cardiovascular stent materials and surfaces in response to clinical drivers: A review

This review examines cardiovascular stent materials from the perspective of a range of clinical drivers and the materials that have been developed in response to these drivers. The review is generally chronological and outlines how stent materials have evolved from initial basic stainless steel devices all the way through to the novel biodegradable devices currently being explored. Where appropriate, pre-clinical or clinical data that influenced decisions and selections along the way is referenced. Opinions are given as to the merit and direction of various ongoing and future developments.

Passivation of nitinol wire for vascular implants--a demonstration of the benefits.

This study investigated a passivation process for polished nitinol wires and vascular stent components, after being given a typical shape setting heat treatment. Heat treated samples were passivated in a nitric acid solution and a series of corrosion tests, surface analysis and chemical analysis was performed. Potentiodynamic polarization tests demonstrated a significant increase in breakdown potential for passivated samples, compared to heat treated surfaces. Surface analysis indicated that the passivation reduces Ni and NiO content in the oxide and increases TiO2 content. Chemical analysis of passivation solutions suggests that the improvement in corrosion resistance is proportional to the quantity of nickel removed. Long term immersion tests demonstrate that nickel release from the surface of the material decreases with time and the quantity of nickel released is lower for passivated samples. The improved corrosion resistance is maintained after extended periods of immersion in saline solution.

A platinum–chromium steel for cardiovascular stents

The desire to reduce the strut thickness of cardiovascular stents has driven the development of a new high strength radiopaque alloy, based on additions of platinum to a chromium-rich iron based matrix. This paper reports on initial development of the alloy and the rationale for selection of the composition. Data is presented for tensile and microstructural characterization, surface oxide analysis, corrosion resistance and endothelial cell response of the alloy. The results demonstrate the solid solution strengthening effect of the platinum, with an average yield strength of 480 MPa achieved. The material surface consists of primarily chromium oxide which contributes to the high corrosion resistance observed. The cell assay result suggests that surfaces of this Pt-enhanced alloy endothelialize in a manner comparable to stainless steel.

Studies on the passivation of aluminium in chromate and molybdate solutions

Abstract Results on the electrochemical behaviour of aluminium in aggressive chloride solutions containing molybdate and dichromate anions are presented and discussed. An ennoblement in the pitting potential of aluminium was observed in dichromate-containing solutions and in non-acidic molybdate-containing solutions. It was proposed that reduction of the molybdate and dichromate species occurred at flawed regions in the passive film, but that the larger polymeric molybdate species, formed in acid solutions, could not be accommodated at the flawed regions. An oxidation peak, observed at −1200 to −1400 mV(SCE), in the molybdate-containing solution was attributed to the growth of a mixed aluminium-molybdenum passive layer. The complete passivation of an aluminium electrode, undergoing meta-stable pitting, was achieved by the introduction of dichromate anions into the test solution. This was attributed to a transformation process whereby chloride-containing aluminium hydroxy complexes were converted into hydrated aluminium oxides.

The activation of aluminium by indium ions in chloride, bromide and iodide solutions

The corrosion and passive behaviour of aluminium in chloride, bromide and iodide solutions in the absence and presence of indium, as activator ions, was investigated using electrochemical techniques. Pit formation mechanisms for aluminium in chloride-, bromide- and iodide-containing solutions appeared to be similar with the caveat that, in the case of iodide anions, liberated iodine attacked the base metal leading to destabilization of the forming film. In addition, the formation of iodic and hypoiodous acids gave rise to the build-up of a highly aggressive pit solution which inhibited repassivation. The difference in the activity of the halide anions was evident also on activation of the aluminium surface by indium ions introduced into the working solution. Greater adsorption of bromide and iodide anions at flawed areas reduced the rate and efficiency of activator deposition at the aluminium surface, with the result that the rate at which the surface was activated was reduced. Activation appeared, also, to be more permanent in chloride-containing electrolytes.

Activation of aluminium in halide solutions containing ‘activator ions’

Abstract The activation of 99.999% aluminium by the ‘activator’ elements—indium, mercury, gallium, zinc and tin—was studied using potentiostatic current-time techniques. These ions, dissolved in the test electrolyte, plate on to the aluminium surface giving rise to localized attack which was manifested by current fluctuations or surges. The induction period τ, prior to the onset of attack, was shown to depend on (i) passivation potential; (ii) halide type and concentration; (iii) quality of the passive film; (iv) temperature. The data obtained with mercury ions support the oxide-detachment mechanism advanced by Reboul et al. An alternative mechanism based on the surface-charge characteristics of the new-formed oxide is proposed for activation by indium and gallium species.

The electrochemical behaviour of aluminium activated by gallium in aqueous electrolytes

Abstract An electrochemical study of the effects of gallium, as an alloying component and present as ions in solution, on the passive behaviour of 99.999% aluminium is reported. It was possible to deposit gallium onto the aluminium surface by a cathodic polarization at −2.0 V(SCE) in halide solutions containing gallium ions. This was achieved by an alkalization process which favoured the formation of hydrolysed gallium species of the form [HGaO 3 ] 2− and [GaO 2 ] − which subsequently led to the formation of metallic gallium at the surface. Al-Ga alloys with low levels of gallium displayed, also, more active behaviour following cathodic polarization. This enrichment of gallium at the surface had an activating effect on the aluminium substrate. Complete activation could be achieved only if a certain amount of gallium was present as a solid solution. It is proposed that the role of gallium is to bias the surface charge in the direction of activation, and that activation occurs only when sufficient bias has been achieved. This will depend on the surface concentration of gallium and the concentration of gallium in solid solution. Once this condition is satisfied halide adsorption occurs more readily at the biased surface and attack proceeds through the formation of a number of aluminium complex species.

Straightforward, One-Step Fabrication of Ultrathin Thermoresponsive Films from Commercially Available pNIPAm for Cell Culture and Recovery

The use of thermoresponsive surfaces as platforms for cell culture and cell regeneration has been explored over the last couple of decades. Poly-N-isopropylacrylamide (pNIPAm) is a well characterized thermoresponsive polymer which has an aqueous lower critical solution temperature (LCST) in a physiologically useful range, which allows it to reversibly attract (T < 32 °C) and repel water (T > 32 °C). It is this phenomenon that is exploited in temperature-controlled cell harvesting. pNIPAm coatings are generally poorly cell compatible and a number of complex or expensive techniques have been developed in order to overcome this issue. This study seeks to design a simple one-step system whereby commercially sourced pNIPAm is used to achieve similar results. Films were deposited using the operationally simple but rheologically complex spin coating technique. Reversible temperature modulated cell adhesion was achieved using a variety of different cell lines. This system offers a simplistic and cheaper alternative to methods used elsewhere.

Stability of passive films formed on aluminium in aqueous halide solutions

Abstract Potentiostatic current-time plots (i=f(t) curves) are a useful means of assessing the quality and stability of passive oxide layersformed on aluminium. For chloride and bromide solutions in particular, the current decay profiles are dominated by peaks and transients provided the selected potential in the passive region is within 70–100 mVof the potentiodynamically determined pitting potential. Outside this region smooth decay profiles are always obtained. Films improve with time under most test conditions. For iodide solutions the current-time curves indicate a gradual destabilisation of the passive film due to 12 production. The pH of the halide solution was found to exert a greater influence than previously thought. Addition of In3+ or Hg2+ ions to the test electrolyte after a prepassivation period leads to activation which in the case of indium is manifested in the form of localised attack. For mercury the attack is more general in nature. The induction time before activation was shown to depe...

Ultra-thin spin coated crosslinkable hydrogels for use in cell sheet recovery—synthesis, characterisation to application

The advantages of detaching adherent cells from thermoresponsive platforms over conventional cell detachment protocols has been well documented. This study focuses on the development of an alternative method to produce thermoresponsive surfaces for cell and cell sheet regeneration to already established techniques which are complex and expensive and may be inaccessible to many laboratories. A photcrosslinkable poly-N-isopropylacrylamide (pNIPAm) copolymer was synthesised and thin films of the copolymer were deposited using the operationally simple spin coating technique which were subsequently crosslinked upon exposure to ultraviolet (UV) irradiation. Characterisation of hydrogel properties and behaviour was achieved using UV spectroscopy, atomic force microscopy (AFM), advancing contact angle, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and white light interferometry analyses. Results indicate that 3T3 fibroblast cells adhere and proliferate to confluence on hydrogels of 30 nm was comparatively poor. Confluent cell sheets were harvested from the sub 30 nm ultra-thin hydrogels upon temperature reduction within 10 min. Spin coating allows for the facile control of film thickness via variation of the depositing polymer solution concentration and therefore the routine fabrication of ultra-thin hydrogels capable of hosting cells to confluence is easily achievable.