Non-linear dissolution mechanisms of sodium calcium phosphate glasses as a function of pH in various aqueous media

© 2020 Elsevier Ltd Phosphate glasses for bioresorbable implants display dissolution rates that vary significantly with composition, however currently their mechanisms of dissolution are not well understood. Based on this systematic study we present new insights into these mechanisms. Two-stage dissolution was observed, with time dependence initially parabolic and later linear, and a two-stage model was developed to describe this behaviour. Dissolution was accelerated by lower Ca concentration in the glass, and lower pH in the dissolution medium. A new dissolution mechanism is proposed, involving an initial stage where diffusion-controlled formation of a conversion layer occurs. Once the conversion layer is stabilised, layer dissolution reactions become rate-limiting. Under this mechanism the transition time is sensitive to the nature of the conversion layer and solution conditions. These results reveal the dependence of P2O5–CaO–Na2O glass dissolution on solution pH, and provide new insight into the dissolution mechanisms, particularly regarding the transition between the two dissolution stages

Effect of chemical–electrochemical surface treatment on the roughness and fatigue performance of porous titanium lattice structures

Additive manufacturing (AM) has enabled the fabrication of extremely complex components such as porous metallic lattices, which have applications in aerospace, automotive, and in particular biomedical devices. The fatigue resistance of these materials is currently an important limitation however, due to manufacturing defects such as semi-fused particles and weld lines. In this work a chemical–electrochemical surface treatment (Hirtisation®) is used for post-processing of Ti-6Al-4V lattices, reducing the strut surface roughness (Sa) from 12 to 6 μm, removing all visible semi-fused particles. The evenness of this treatment in lattices with relative density up to 18.3% and treatment depth of 6.5 mm was assessed, finding no evidence of reduced effectiveness on internal surfaces. After normalising to quasi-static mechanical properties to account for material losses during hirtisation (34%–37% reduction in strut diameter), the fatigue properties show a marked improvement due to the reduction in surface roughness. Normalised high cycle fatigue strength increased from around 0.1 to 0.16-0.21 after hirtisation, an average increase of 80%. For orthopaedic implant devices where matching the stiffness of surrounding bone is crucial, the fatigue strength to modulus ratio is a key metric. After hirtisation the fatigue strength to modulus ratio increased by 90%, enabling design of stiffness matched implant materials with greater fatigue strength. This work demonstrates that hirtisation is an effective method for improving the surface roughness of porous lattice materials, thereby enhancing their fatigue performance

Tuning structural relaxations, mechanical properties, and degradation timescale of PLLA during hydrolytic degradation by blending with PLCL-PEG

Poly-L-lactide (PLLA) is a popular choice for medical devices due to its bioresorbability and superior mechanical properties compared with other polymers. However, although PLLA has been investigated for use in bioresorbable cardiovascular stents, it presents application-specific limitations which hamper device therapies. These include low toughness and strength compared with metals used for this purpose, and slow degradation. Blending PLLA with novel polyethylene glycol functionalised poly(L-lactide-co-$\varepsilon$-caprolactone) (PLCL-PEG) materials has been investigated here to tailor the mechanical properties and degradation behaviour of PLLA. This exciting approach provides a foundation for a next generation of bioresorbable materials whose properties can be rapidly tuned. The degradation of PLLA was significantly accelerated by addition of PLCL-PEG. After 30 days of degradation, several structural changes were observed in the polymer blends, which were dependent on the level of PLCL-PEG addition. Blends with low PLCL-PEG content displayed enthalpy relaxation, resulting in embrittlement, while blends with high PLCL-PEG content displayed crystallisation, due to enhanced chain mobility brought on by chain scission, also causing embrittlement. Moderate PLCL-PEG additions (10% PLCL(70:30)-PEG and 20 - 30% PLCL(80:20)-PEG) stabilised the structure, reducing the extent of enthalpy relaxation and crystallisation and thus retaining ductility. Compositional optimisation identified a sweet spot for this blend strategy, whereby the ductility was enhanced while maintaining strength. Our results indicate that blending PLLA with PLCL-PEG provides an effective method of tuning the degradation timescale and mechanical properties of PLLA, and provides important new insight into the mechanisms of structural relaxations that occur during degradation, and strategies for regulating these.Lucideon Lt

Shear yielding and crazing in dry and wet amorphous PLA at body temperature

Understanding the inelastic, rate-dependent mechanical response of biodegradable polymers is important for the design of load-bearing biodegradable structures with controlled deformation and failure response. In this study, we investigate the mechanical response of amorphous polylactic acid (PLA) in dry and wet conditions prior to the onset of degradation at body temperature. The presence of water decreases the glass transition temperature by 4.5 °C, the storage modulus by 21%, and the compressive and tensile yield strengths by about 10%, despite a small water uptake of 0.93 wt%. The tensile response of PLA is dominated by craze yielding, rather than shear plasticity, and is stable against necking despite pronounced strain softening and local strain heterogeneities measured by Digital Image Correlation (DIC). Further analysis of the DIC strain fields in dry and wet samples suggests a transition from pure craze yielding in dry samples to a coexistence of craze yielding and shear plasticity in wet samples. The mechanism shift between tension and compression behaviour of dry and wet PLA has implications for the design of load-bearing structures and for constitutive modelling

Effect of chemical–electrochemical surface treatment on the roughness and fatigue performance of porous titanium lattice structures

Additive manufacturing (AM) has enabled the fabrication of extremely complex components such as porous metallic lattices, which have applications in aerospace, automotive, and in particular biomedical devices. The fatigue resistance of these materials is currently an important limitation however, due to manufacturing defects such as semi-fused particles and weld lines. In this work a chemical–electrochemical surface treatment (Hirtisation®) is used for post-processing of Ti-6Al-4V lattices, reducing the strut surface roughness (Sa) from 12 to 6 μm, removing all visible semi-fused particles. The evenness of this treatment in lattices with relative density up to 18.3% and treatment depth of 6.5 mm was assessed, finding no evidence of reduced effectiveness on internal surfaces. After normalising to quasi-static mechanical properties to account for material losses during hirtisation (34%–37% reduction in strut diameter), the fatigue properties show a marked improvement due to the reduction in surface roughness. Normalised high cycle fatigue strength increased from around 0.1 to 0.16-0.21 after hirtisation, an average increase of 80%. For orthopaedic implant devices where matching the stiffness of surrounding bone is crucial, the fatigue strength to modulus ratio is a key metric. After hirtisation the fatigue strength to modulus ratio increased by 90%, enabling design of stiffness matched implant materials with greater fatigue strength. This work demonstrates that hirtisation is an effective method for improving the surface roughness of porous lattice materials, thereby enhancing their fatigue performance

A technique for improving dispersion within polymer-glass composites using polymer precipitation

Particulate reinforcement of polymeric matrices is a powerful technique for tailoring the mechanical and degradation properties of bioresorbable implant materials. Dispersion of inorganic particles is critical to achieving optimal properties, however established techniques such as twin-screw extrusion or solvent casting can have significant drawbacks including excessive thermal degradation or particle agglomeration. We present a facile method for production of polymer-inorganic composites that reduces the time at elevated temperature and the time available for particle agglomeration. Glass slurry was added to a dissolved PLLA solution, and ethanol was added to precipitate polymer onto the glass particles. Characterisation of parts formed by subsequent micro-injection moulding of composite precipitate revealed a significant reduction in agglomeration, with d0.9 reduced from 170 to 43 μm. This drastically improved the ductility (ɛB) from 7% to 120%, without loss of strength or stiffness. The method is versatile and applicable to a wide range of polymer and filler materials

The evolution of the structure and mechanical properties of fully bioresorbable polymer-glass composites during degradation

Fully bioresorbable polymer matrix composites have long been considered as potential orthopaedic implant materials, however their combination of mechanical strength, stiffness, ductility and bioresorbability is also attractive for cardiac stent applications. This work investigated reinforcement of polylactide-based polymers with phosphate glasses, addressing key drawbacks of current polymer stents, and examined the often-neglected evolution of structure and mechanical properties during degradation. Incorporation of 15–30 wt% phosphate glass led to modulus increases of up to 80% under simulated body conditions, and 15 wt% glass composites retained comparable ductility to pure polymers, crucial for stent applications where ductility and stiffness are required. Two-stage degradation was observed, dominated by interfacial water absorption and glass dissolution. Polymer embrittlement mechanisms (crystallisation, enthalpy relaxation) were suppressed by glass addition, allowing composites to achieve a more controlled loss of mechanical properties during degradation, which could allow gradual transfer of loading to newly healed tissue. These results provide a valuable new system for understanding the structural and mechanical changes occurring during degradation of fully bioresorbable polymer matrix composites, providing important new data to underpin the design of effective cardiac stent materials

The evolution of the structure and mechanical properties of fully bioresorbable polymer-glass composites during degradation

Fully bioresorbable polymer matrix composites have long been considered as potential orthopaedic implant materials, however their combination of mechanical strength, stiffness, ductility and bioresorbability is also attractive for cardiac stent applications. This work investigated reinforcement of polylactide-based polymers with phosphate glasses, addressing key drawbacks of current polymer stents, and examined the often-neglected evolution of structure and mechanical properties during degradation. Incorporation of 15–30wt.% phosphate glass led to modulus increases of up to 80% under simulated body conditions, and 15wt.% glass composites retained comparable ductility to pure polymers, crucial for stent applications where ductility and stiffness are required. Two-stage degradation was observed, dominated by interfacial water absorption and glass dissolution. Polymer embrittlement mechanisms (crystallisation, enthalpy relaxation) were suppressed by glass addition, allowing composites to achieve a more controlled loss of mechanical properties during degradation, which could allow gradual transfer of loading to newly healed tissue. These results provide a valuable new system for understanding the structural and mechanical changes occurring during degradation of fully bioresorbable polymer matrix composites, providing important new data to underpin the design of effective cardiac stent materials.The authors thank Lucideon Ltd. for providing materials and financial suppor

Non-linear dissolution mechanisms of sodium calcium phosphate glasses as a function of pH in various aqueous media

Phosphate glasses for bioresorbable implants display dissolution rates that vary significantly with composition, however currently their mechanisms of dissolution are not well understood. Based on this systematic study we present new insights into these mechanisms. Two-stage dissolution was observed, with time dependence initially parabolic and later linear, and a two-stage model was developed to describe this behaviour. Dissolution was accelerated by lower Ca concentration in the glass, and lower pH in the dissolution medium. A new dissolution mechanism is proposed, involving an initial stage where diffusion-controlled formation of a conversion layer occurs. Once the conversion layer is stabilised, layer dissolution reactions become rate-limiting. Under this mechanism the transition time is sensitive to the nature of the conversion layer and solution conditions. These results reveal the dependence of P2O5–CaO–Na2O glass dissolution on solution pH, and provide new insight into the dissolution mechanisms, particularly regarding the transition between the two dissolution stages