Kim Ragaert

Biomechanical properties of native and tissue engineered heart valve constructs

Due to the increasing number of heart valve diseases, there is an urgent clinical need for off-the-shelf tissue engineered heart valves. While significant progress has been made toward improving the design and performance of both mechanical and tissue engineered heart valves (TEHVs), a human implantable, functional, and viable TEHV has remained elusive. In animal studies so far, the implanted TEHVs have failed to survive more than a few months after transplantation due to insufficient mechanical properties. Therefore, the success of future heart valve tissue engineering approaches depends on the ability of the TEHV to mimic and maintain the functional and mechanical properties of the native heart valves. However, aside from some tensile quasistatic data and flexural or bending properties, detailed mechanical properties such as dynamic fatigue, creep behavior, and viscoelastic properties of heart valves are still poorly understood. The need for better understanding and more detailed characterization of mechanical properties of tissue engineered, as well as native heart valve constructs is thus evident. In the current review we aim to present an overview of the current understanding of the mechanical properties of human and common animal model heart valves. The relevant data on both native and tissue engineered heart valve constructs have been compiled and analyzed to help in defining the target ranges for mechanical properties of TEHV constructs, particularly for the aortic and the pulmonary valves. We conclude with a summary of perspectives on the future work on better understanding of the mechanical properties of TEHV constructs.

Mechanical and chemical recycling of solid plastic waste

This review presents a comprehensive description of the current pathways for recycling of polymers, via both mechanical and chemical recycling. The principles of these recycling pathways are framed against current-day industrial reality, by discussing predominant industrial technologies, design strategies and recycling examples of specific waste streams. Starting with an overview on types of solid plastic waste (SPW) and their origins, the manuscript continues with a discussion on the different valorisation options for SPW. The section on mechanical recycling contains an overview of current sorting technologies, specific challenges for mechanical recycling such as thermo-mechanical or lifetime degradation and the immiscibility of polymer blends. It also includes some industrial examples such as polyethylene terephthalate (PET) recycling, and SPW from post-consumer packaging, end-of-life vehicles or electr(on)ic devices. A separate section is dedicated to the relationship between design and recycling, emphasizing the role of concepts such as Design from Recycling. The section on chemical recycling collects a state-of-the-art on techniques such as chemolysis, pyrolysis, fluid catalytic cracking, hydrogen techniques and gasification. Additionally, this review discusses the main challenges (and some potential remedies) to these recycling strategies and ground them in the relevant polymer science, thus providing an academic angle as well as an applied one.

Influence of processing parameters and composition on the effective compatibilization of polypropylene–poly(ethylene terephthalate) blends

Abstract The effects of the addition of different functionalized compatibilizers on toughness, morphology and rheological properties of a polypropylene (PP) – poly(ethylene terephthalate) (PET) (85–15 wt%) blend were studied. The three compatibilizers compared were: (Styrene Ethylene Butylene Styrene)-grafted-(glycidyl methacrylate); (Styrene Ethylene Butylene Styrene) – grafted – (maleic anhydryde); (polyolefin) – grafted – (glycidyl methacrylate), abbreviated to: SEBS-g-GMA, SEBS-g-MA and POE-g-GMA respectively. The effective grafting content was the same for all three compatibilizers. Before the comparison of the different compatibilizers was done, first the effects of three different processing temperatures and three different compatibilizer contents were investigated, based on the addition of SEBS-g-GMA. The compatibilization effect was significantly improved with an increase in processing temperature from 250 to 300 °C. The toughness was increased with almost a factor two and a decrease in the average domain size of the dispersed phase was observed. An increase in compatibilizer content from 0.25 to 2.5 wt% resulted in a finer dispersity as well as in a steep increase in toughness, which was noted to approach the brittle-to-ductile transition. The comparison of the three compatibilizers was subsequently done at the most promising processing temperature and content: 300 °C and 2.5 wt%. The results showed that the addition of SEBS-g-MA and POE-g-GMA had a less significant positive effect on the compatibilization compared to SEBS-g-GMA. The difference is attributed to a higher reactivity for GMA compared to MA and a higher possibility for migration towards the PP-PET interface for the SEBS chain compared to the POE chain.

Machine design and processing considerations for the 3D plotting of thermoplastic scaffolds

3D plotting by micro-extrusion is a promising layer-wise fabrication method for the production of scaffolds in thermoplastic polymers. It is a solvent-free direct technique which permits extensive control over geometry and porosity. This paper highlights the complications that arise when using this technique for the processing of thermally sensitive polymers. It has been noted that the material is subject to extensive thermal load during processing, which may result in degradation by chain scission. This negatively affects scaffold (mechanical) properties as well as predictability and repeatability of the fabrication technique. A rationale is offered as to the main causes of this thermally induced degradation during processing and tentative ideas towards a solution are equally put forward.

Flexural mechanical properties of porcine aortic heart valve leaflets

Freshly excised porcine aortic heart valve cusps were subjected to a uni-axial flexural indentation test, from which the rupture characteristics and a functional stiffness parameter were determined. It was found that the flexural mechanical properties of aortic valve cusps (i) are unaffected by their coronary position and (ii) are sensitive to the effect of mechanical preconditioning. The resulting values of the cusps flexural mechanical properties are intended as a set of reference properties which scaffolds, meant for the tissue engineering of heart valves, must approximate in order to be considered as a functional replacement.

Predictive model for the Dutch post-consumer plastic packaging recycling system and implications for the circular economy

The Dutch post-consumer plastic packaging recycling network has been described in detail (both on the level of packaging types and of materials) from the household potential to the polymeric composition of the recycled milled goods. The compositional analyses of 173 different samples of post-consumer plastic packaging from different locations in the network were combined to indicatively describe the complete network with material flow analysis, data reconciliation techniques and process technological parameters. The derived potential of post-consumer plastic packages in the Netherlands in 2014 amounted to 341 Gg net (or 20.2 kg net.cap-1.a-1). The complete recycling network produced 75.2 Gg milled goods, 28.1 Gg side products and 16.7 Gg process waste. Hence the net recycling chain yield for post-consumer plastic packages equalled 30%. The end-of-life fates for 35 different plastic packaging types were resolved. Additionally, the polymeric compositions of the milled goods and the recovered masses were derived with this model. These compositions were compared with experimentally determined polymeric compositions of recycled milled goods, which confirmed that the model predicts these compositions reasonably well. Also the modelled recovered masses corresponded reasonably well with those measured experimentally. The model clarified the origin of polymeric contaminants in recycled plastics, either sorting faults or packaging components, which gives directions for future improvement measures.

The Effect of Injection Molding Temperature on the Morphology and Mechanical Properties of PP/PET Blends and Microfibrillar Composites

Within this research the effect of injection molding temperature on polypropylene (PP)/poly(ethylene terephthalate) (PET) blends and microfibrillar composites was investigated. Injection molding blends (IMBs) and microfibrillar composites (MFCs) of PP/PET have been prepared in a weight ratio 70/30. The samples were processed at three different injection molding temperatures (Tim) (210, 230, 280 °C) and subjected to extensive characterization. The observations from the fracture surfaces of MFCs showed that PET fibers can be achieved by three step processing. The results indicated that Tim has a big influence on morphology of IMBs and MFCs. With increasing the Tim, distinctive variations in particle and fiber diameters were noticed. The differences in mechanical performances were obtained by flexural and impact tests. Establishing relationships between the processing parameters, properties, and morphology of composites is of key importance for the valorization of MFC polymers.

Blending of recycled mixed polyolefins with recycled polypropylene : effect on physical and mechanical properties

The similar densities of polyethylene (PE) and polypropylene (PP), complicate the separation for these materials in the mechanical recycling of post-consumer plastics. Therefore, recycled mixed polyolefins (rMPO) are widely available as recycled (r) polymer material. These blends are commonly cheaper, but also inferior in properties to relatively pure waste streams of rPP or rPE. In order to improve these materials, blends of rMPO were made with more expensive, relatively high quality rPP. This approach was purposefully ‘bottom up’, exploring how to upcycle the lower grade rMPO instead of downcycling higher grade rPP with a measure of added (cheaper) rMPO. Blends were made with two types of rMPO, the one a HDPE-PP blends from hard plastic waste and the other a LDPE-HDPE-PP blends, based mostly on foil materials. Each of these materials had one type of mechanical property in common with the high quality rPP, these being Young’s modulus and toughness respectively. Blends were made at different concentration...

Development of crystalline morphology and its relationship with mechanical properties of PP/PET microfibrillar composites containing POE and POE-g-MA

The main goal of this research is to study the development of crystalline morphology and compare it to various mechanical properties of microfibrillar composites (MFCs) based on polypropylene (PP) and poly(ethylene terephthalate) (PET), by adding a functional compatibilizer and a non-functional rubber in two different steps in the processing sequence. The MFCs were prepared at a weight ratio of 80/20 PP/PET by twin screw extrusion followed by cold drawing and injection moulding. The non-functionalized polyolefin-based elastomer (POE) and the functional compatibilizer (i.e., POE grafted with maleic anhydride (POE-g-MA)) were added in a fixed weight percentage at two stages: during extrusion or during injection moulding. The morphology observations showed differences in crystalline structure, and the PP spherulite size was reduced in all MFCs due to the presence of PET fibrils. Their relationship with the mechanical performances of the composite was studied by tensile and impact tests. Adding the functional compatibilizer during extrusions showed better mechanical properties compared to MFCs. Overall, a clear relationship was identified between processing, structure and properties.

Influence of twin-screw configuration on the mechanical and morphological properties of polypropylene-clay composites

The influence of eight different twin-screw configurations, mainly varying in arrangement of restrictive (negative) kneading elements, on the mechanical and morphological properties of composites of polypropylene (PP) reinforced with organic modified montmorillonite (MMT) and coupling agent polypropylene-grafted-maleic anhydride (PP-g-MA) was studied. Pressure profile, maximum strain, fill rate and average viscosity were calculated using an existing twin-screw modelling software. Impact and tensile tests results, scanning electron microscopy (SEM) images and X-ray diffraction (XRD) measurements of the composites showed no significant influence of the position of the negative kneading blocks.