Deposition and characterisation of RF magnetron sputtered phosphate based glasses

Phosphate based glasses are emerging in the field of biomaterials for their potential to resorb in biological environments, fulfilling applications from the fibre reinforcement of resorbable polymeric matrices to carriers for therapeutic drug delivery. Here we show the optimisation and characterisation of thin film glasses deposited by RF magnetron sputtering onto medical implant materials such as Ti6Al4V to function as ion leaching coatings to promote osseointegration or inhibit bacterial attachment. Vaporisation of the target preform occurred by momentum exchange interactions leading to non stoichiometric transfer to the condensed coating, sputtering in the order Na > Mg > Ca > Fe > P. Structural analyses revealed short range variation between compositionally equivalent glasses such that coatings with 32.5, 34 and 37 mol% P2O5 showed bulk polymerisation increasing in Q2 species by (23% to 45%) versus (9% to 32%) in quenched glasses. P O P bridging oxygens on the surface of coatings formed (PO3) metaphosphates (Q2), compared to (PO4)3 orthophosphates (Q0) and (P2O7)4 pyrophosphates (Q1) in quenched glasses. Quinternary coatings of up to 2.67 μm, containing Fe3+ and Ti4+ intermediate and cross linking elements were degraded in distilled water and phosphate buffered saline. Fe2O3 was increased from 4 to 8 mol% to stabilise dissolution, however an observed increase was attributed to variable condensation energies leading to inequivalent enthalpy and internal stress states. A comparison of a compositionally equivalent condensed and quenched glass suggested that the surface ratios of P O P to (P=O and PO ) were 34.2% to 65.8% versus 20.5% to 79.5% respectively leading to more soluble coating surfaces, exhibiting an exponential degradation dependence in the first 2 h in distilled water, followed by a linear profile. Post deposition heat treatments at 500, 550 and 610 °C were employed to stabilise dissolution and to tailor mechanical properties. All phosphate glass coatings showed interfacial tensile adhesion in excess of 73.6 MPa; surpassing ISO and FDA requirements for HA coatings. The initial exponential degradation from 0 2 h was stabilised via heat treatment. From 2 24 h coatings exhibited linear ion release rates ordering P > Na > Mg > Ca > Fe whilst dissolution rates reduced by factors of 2.44 to 4.55, attributed the formation of crystals and the depletion of hydrophilic P O P bonds within the surface layer. Vapour deposition has shown its ability to condense tailorable compositions of glasses, maintaining their amorphous tetrahedral structures whilst demonstrating exceptional adhesion to Ti6Al4V substrates. Coatings have demonstrated linear ion release capabilities and the ability to accommodate a vast array of potentially therapeutic ions to promote osteogenic or antimicrobial capabilities

Polymer-based Thermoelectric Devices

Currently, over 50% of all energy generated in the US is lost as waste heat, and thermoelectric generators offer a promising means to recoup some of this energy, if their efficiency is improved. While organic thermoelectric materials lack the efficiency of their inorganic counterparts, they are composed of highly abundant resources and have low temperature processing conditions. Recently, a new class of redox-active polymers, radical polymers, has exhibited high electrical conductivity in an entirely amorphous medium. In addition, these radical polymers have a simple synthetic scheme and can be highly tunable to provide desired electrical properties. In this study, the thermoelectric properties of a nitroxide radical-based polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), is evaluated in a doped state. 4-ethylbenzenesulfonic acid (EBSA) is used to dope PTMA solutions. The Seebeck coefficient and conductivity measurements were collected to calculate the thermoelectric power factor of the material at an average temperature of 40 ˚C. We expect to find that doped PTMA has a peak power factor of ~10-2 μW m-1 K-2. While these power factor values would not exceed a state-of-the-art organic semiconductor, they would show that radical polymers are a viable alternative to pi-conjugated semiconducting polymers. These redox-active polymers are still a new type of semiconducting polymer; therefore, this study could suggest that further research is necessary to determine their full capabilities and the radical solutions they may have to offer

Deposition and characterisation of RF magnetron sputtered phosphate based glasses

Phosphate based glasses are emerging in the field of biomaterials for their potential to resorb in biological environments, fulfilling applications from the fibre reinforcement of resorbable polymeric matrices to carriers for therapeutic drug delivery. Here we show the optimisation and characterisation of thin film glasses deposited by RF magnetron sputtering onto medical implant materials such as Ti6Al4V to function as ion leaching coatings to promote osseointegration or inhibit bacterial attachment. Vaporisation of the target preform occurred by momentum exchange interactions leading to non stoichiometric transfer to the condensed coating, sputtering in the order Na > Mg > Ca > Fe > P. Structural analyses revealed short range variation between compositionally equivalent glasses such that coatings with 32.5, 34 and 37 mol% P2O5 showed bulk polymerisation increasing in Q2 species by (23% to 45%) versus (9% to 32%) in quenched glasses. P O P bridging oxygens on the surface of coatings formed (PO3) metaphosphates (Q2), compared to (PO4)3 orthophosphates (Q0) and (P2O7)4 pyrophosphates (Q1) in quenched glasses. Quinternary coatings of up to 2.67 μm, containing Fe3+ and Ti4+ intermediate and cross linking elements were degraded in distilled water and phosphate buffered saline. Fe2O3 was increased from 4 to 8 mol% to stabilise dissolution, however an observed increase was attributed to variable condensation energies leading to inequivalent enthalpy and internal stress states. A comparison of a compositionally equivalent condensed and quenched glass suggested that the surface ratios of P O P to (P=O and PO ) were 34.2% to 65.8% versus 20.5% to 79.5% respectively leading to more soluble coating surfaces, exhibiting an exponential degradation dependence in the first 2 h in distilled water, followed by a linear profile. Post deposition heat treatments at 500, 550 and 610 °C were employed to stabilise dissolution and to tailor mechanical properties. All phosphate glass coatings showed interfacial tensile adhesion in excess of 73.6 MPa; surpassing ISO and FDA requirements for HA coatings. The initial exponential degradation from 0 2 h was stabilised via heat treatment. From 2 24 h coatings exhibited linear ion release rates ordering P > Na > Mg > Ca > Fe whilst dissolution rates reduced by factors of 2.44 to 4.55, attributed the formation of crystals and the depletion of hydrophilic P O P bonds within the surface layer. Vapour deposition has shown its ability to condense tailorable compositions of glasses, maintaining their amorphous tetrahedral structures whilst demonstrating exceptional adhesion to Ti6Al4V substrates. Coatings have demonstrated linear ion release capabilities and the ability to accommodate a vast array of potentially therapeutic ions to promote osteogenic or antimicrobial capabilities

Generation and characterisation of gallium titanate surfaces through hydrothermal ion-exchange processes

Infection negation and biofilm prevention are necessary developments needed for implant materials. Furthermore, an increase in publications regarding gallium (Ga) as an antimicrobial ion has resulted in bacterial-inhibitory surfaces incorporating gallium as opposed to silver (Ag). The authors present the production of novel gallium titanate surfaces through hydrothermal ion-exchange reactions. Commercially-pure Ti (S0: Cp-Ti) was initially suspended in NaOH solutions to obtain sodium titanate (S1: Na2TiO3) layers ca. 0.5–1 μm in depth (2.4 at.% Na). Subsequent suspension in Ga(NO3)3 (S2: Ga2(TiO3)3), and post-heat-treatment at 700 °C (S3: Ga2(TiO3)3-HT), generated gallium titanate layers (9.4 and 4.1 at.% Ga, respectively). For the first time, RHEED analysis of gallium titanate layers was conducted and demonstrated titanate formation. Degradation studies in DMEM showed S2: Ga2(TiO3)3 released more Ga compared to S3: Ga2(TiO3)3-HT (2.76 vs. 0.68 ppm) over 168 h. Furthermore, deposition of Ca/P in a Ca:P ratio of 1.71 and 1.34, on S2: Ga2(TiO3)3 and S3: Ga2(TiO3)3-HT, respectively, over 168 h was seen. However, the study failed to replicate the antimicrobial effect presented by Yamaguchi who utilised A. baumannii, compared to S. aureus used presently. The authors feel a full antimicrobial study is required to assess gallium titanate as a candidate antimicrobial surface

Layered Al2O3-SiO2 and Al2O3-Ta2O5 thin-film composites for high dielectric strength, deposited by pulsed direct current and radio frequency magnetron sputtering

Multilayer thin films have the potential to act as high dielectric strength insulation for wire and microelectronics. In this study, films consisting of 2, 4 or 8 layers, composed of Al2O3 with SiO2 or Ta2O5, were prepared via pulsed direct current and radio frequency magnetron sputtering to a thickness of between 152 and 236 nm. The dielectric strengths of all films exceeded the 310 Vμm−1 achieved for PDC Al2O3. Maximum dielectric strengths were obtained for four layer composites; Al2O3-SiO2-Al2O3-SiO2 (466 Vμm−1) and Al2O3-Ta2O5-Al2O3-Ta2O5 (513 Vμm−1), each containing two PDC-Al2O3 and two RF-SiO2/Ta2O5 layers. Whilst the average dielectric strength was higher in the Ta2O5 composites, they suffered from higher leakage prior to breakdown with ca. 6.5 nA compared to ca. 0.1 nA for SiO2 composites. The mechanical properties of the composites were poorer due to increased intrinsic coating stress. Samples exhibited complete interfacial delamination with maximum coating adhesion strengths of 22 and 25 MPa. The variance resulted from larger coefficient of thermal expansion for Ta2O5 compared to SiO2. Sputtered composites of Al2O3 and either SiO2 or Ta2O5 had high breakdown strength with reasonable adhesion and could be suitable for insulating copper conductors in the aerospace and automotive industries

Degradation and characterization of resorbable phosphate-based glass thin-film coatings applied by radio-frequency magnetron sputtering

Quinternary phosphate-based glasses of up to 2.67 μm, deposited by radio-frequency magnetron sputtering, were degraded in distilled water and phosphate-buffered saline (PBS) to investigate their degradation characteristics. Magnetron- sputtered coatings have been structurally compared to their compositionally equivalent melt-quenched bulk glass counterparts. The coatings were found to have structurally variable surfaces to melt-quenched glass such that the respective bridging oxygen to nonbridging oxygen bonds were 34.2% to 65.8% versus 20.5% to 79.5%, forming metaphosphate (PO3)−(Q2) versus less soluble (P2O7)4− (Q1) and (PO4)3− (Q0), respectively. This factor led to highly soluble coatings, exhibiting a t1/2 degradation dependence in the first 2 h in distilled water, followed by a more characteristic linear profile because the subsequent layers were less soluble. Degradation was observed to preferentially occur, forming voids characteristic of pitting corrosion, which was confirmed by the use of a focused ion beam. Coating degradation in PBS precipitated a (PO3)−metaphosphate, an X-ray amorphous layer, which remained adherent to the substrate and seemingly formed a protective diffusion barrier, which inhibited further coating degradation. The implications are that while compositionally similar, sputter-deposited coatings and melt-quenched glasses are structurally dissimilar, most notably, with regard to the surface layer. This factor has been attributed to surface etching of the as-deposited coating layer during deposition and variation in the thermal history between the processes of magnetron sputtering and melt quenching

Developing highly nanoporous titanate structures via wet chemical conversion of DC magnetron sputtered titanium thin films

© 2020 The Authors Titanate structures have been widely investigated as biomedical component surfaces due to their bioactive, osteoinductive and antibacterial properties. However, these surfaces are limited to Ti and its alloys, due to the nature of the chemical conversion employed. The authors present a new method for generating nanoporous titanate structures on alternative biomaterial surfaces, such as other metals/alloys, ceramics and polymers, to produce bioactive and/or antibacterial properties in a simple yet effective way. Wet chemical (NaOH; 5 M; 60 °C; 24 h) conversion of DC magnetron sputtered Ti surfaces on 316L stainless steel were investigated to explore effects of microstructure on sodium titanate conversion. It was found that the more equiaxed thin films (B/300) generated the thickest titanate structures (ca. 1.6 μm), which disagreed with the proposed hypothesis of columnar structures allowing greater NaOH ingress. All film parameters tested ultimately generated titanate structures, as confirmed via EDX, SEM, XPS, XRD, FTIR and Raman analyses. Additionally, the more columnar structures (NB/NH & B/NH) had a greater quantity of Na (ca. 26 at.%) in the top portion of the films, as confirmed via XPS, however, on average the Na content was consistent across the films (ca. 5–9 at.%). Film adhesion for the more columnar structures (ca. 42 MPa), even on polished substrates, were close to that of the FDA requirement for plasma-sprayed HA coatings (ca. 50 MPa). This study demonstrates the potential of these surfaces to be applied onto a wide variety of material types, even polymeric materials, due to the lower processing temperatures utilised, with the vision to generate bioactive and/or antibacterial properties on a plethora of bioinert materials

A comparison of outcomes between bovine pericardial and porcine valves in 38 040 patients in England and Wales over 10 years

OBJECTIVES: Biological valves are the most commonly implanted prostheses for aortic valve replacement (AVR) surgery in the UK. The aim of this study was to compare performance of porcine and bovine pericardial valves implanted in AVR surgery with respect to survival and reintervention-free survival in a retrospective observational study. METHODS: Prospectively collected clinical data for all first-time elective and urgent AVRs with or without concomitant coronary artery bypass graft (CABG) surgery performed in England and Wales between April 2003 and March 2013 were extracted from the National Institute for Cardiovascular Outcomes Research database. Patient life status was tracked from the Office for National Statistics. Time-to-event analyses were performed using log-rank tests and Cox proportional hazards regression modelling with random effects/grouped frailty for responsible cardiac surgeons. RESULTS: A total of 38 040 patients were included (64.9 % bovine pericardial; 35.1 % porcine). Patient characteristics were similar between the groups. The median follow-up was 3.6 years. There was no statistically significant difference in survival (P = 0.767) (the 10-year surviva

Developing alkaline titanate surfaces for medical applications

Improving the surface of medical implants by plasma spraying of a hydroxyapatite coating can be of critical importance to their longevity and the patient’s convalescence. However, residual stresses, cracking, undesired crystallisation and delamination of the coating compromise the implants lifetime. A promising alternative surface application is an alkali-chemical treatment to generate bioactive surfaces, such as sodium and calcium titanate and their derivatives. Such surfaces obviate the need for high temperatures and resulting micro-crack formation and potentially improve the bioactive and bone integration properties through their nanoporous structures. Also, metallic ions such as silver, gallium and copper can be substituted into the titanate structure with the potential to reduce or eliminate the infections. This review examines the formation and mechanisms of bioactive/antibacterial alkaline titanate surfaces, their successes and limitations, and explores the future development of implant interfaces via multifunctional titanate surfaces on Ti-based alloys and on alternative substrate materials