3D printed-poly(lactic acid) scaffolds with improved bioactivity for bone tissue engineering

PLA scaffolds produced by 3D printing are a promising alternative to bone tissue engineering. A simple mussel inspired method, polydopamine (PDA) surface functionalization, has being proposed as an efficient way to enhance the bioactivity of biomaterials, such as PLA, and to promote covalent immobilization of a variety of proteins such as collagen (COL) type I. This study aimed to characterize PLA scaffolds produced by FDM with different pore size, concomitantly to the influence of PDA coating as platform to enhance the bioactivity and the immobilization of COL I onto printed PLA. Scaffolds geometry influenced directly in the porosity and compressive strength of printed parts. The PDA layer improved the COL immobilization onto the surface of PLA scaffolds in 92% and enhanced the adhesion of porcine Bone Marrow Stem Cells (MSCs). The combination of PDA and COL layers provided the best conditions for early stage cell adhesion and proliferation. Cells cultured onto PDA/COL scaffolds produced substantially higher amounts of Alkaline Phosphatase (ALP), a marker of osteogenesis, by 21 days in culture. Scaffolds with projected pores of 700 µm in size (strut spacing = 1 mm) and coated with PDA plus COL I provides appropriate porosity, mechanical properties and biological conditions to allow bone tissue regeneration.Arcabouços de poli(ácido láctico) (PLA) produzidos por impressão 3D são uma alternativa promissora na engenharia tecidual. A funcionalização da superfície baseada no modelo de mexilhão – Polidopamina (PDA), foi proposta como uma maneira eficiente de aumentar a bioatividade e promover a imobilização covalente de biomoléculas, tais como o colágeno (COL) tipo I, nas superfícies. Este estudo teve como objetivo caracterizar arcabouços de PLA produzidos por impressão 3D, com diferentes espaçamentos entre os filamentos, concomitantemente à influência do recobrimento com PDA como plataforma para aumentar a bioatividade e a imobilização de COL I na superfície dos arcabouços. A geometria influenciou diretamente na porosidade e na resistência à compressão das peças impressas. A camada de PDA melhorou a imobilização de COL I na superfície dos arcabouços em 92% e aumentou a adesão de células-tronco mesenquimais. A sobreposição de PDA e COL I forneceu as melhores condições para adesão celular e proliferação no estágio inicial da cultura celular. Além disso, nessa condição, células produziram quantidade expressivamente maior de fosfatase alcalina, um conhecido marcador de osteogênese, após 21 dias em cultura. Os arcabouços com poros projetados de 700 µm (espaçamento intermediário = 1 mm) e revestidos com PDA/COL proporcionam porosidade, tamanho de poro, propriedades mecânicas e condições biológicas adequadas para permitir a regeneração do tecido ósseo

The portuguese active labour market policy during the period 1998-2003 - a comprehensive conditional difference-in-difference application

In most studies in the literature only the participation in a single programme versus non-participation is evaluated. This approach, however, does not address the needs of a comprehensive evaluation of an active public intervention in the labour market. Active labour market programmes, like the Portuguese, are not restricted to a particular measure. Rather, in most cases, the public employment service offers a wide variety of programmes to the universe of potential participants. In this context, the issue is participation in one programme versus participation in an alternative programme. In particular, it is appropriate to investigate which programme presents a higher causal effect. Imbens (2000) and Lechner (2001) extended the traditional matching methodology to a context of multiple programme participation. The approach followed in this paper intends to go even further. Indeed, since the traditional matching methodology, which considers the conditional independence assumption, is not appropriate in the context of the Portuguese labour market analysis, we will adopt the assumption of the bias stability. Taking into consideration the selection on unobservables, the matching methodology, combined with the difference-in-differences methodology, will be then our selected evaluation approach. The paper presents the estimation of the average treatment effects on the treated in six distinct states (the non-participation state, plus five “active” programmes). The results, in terms of employability, are not identical across the different states in the short-run (i.e. in the first six to twelve months after participation), but they do seem to converge in the long-run (i.e. after two and a half years)

Laying foundations for successful customer relationship management - the case of landing jobs

The purpose of this study is to lay the foundations forLanding.jobs tobuildsuccessful customer relationship strategies, dueto the need for automation in dealing with customersrecently created by the company’s expansion.After reviewing Customer Relationship Management (CRM) literature focused on success factors and challenges of implementing CRM strategies, a descriptive analysis of Landing.jobs’ user base was performed, as well as an in-depth review of its customer journey and assessment of pains, finalized with an online survey to gather talent insights. Overall, Landing.jobs needs focusing on gathering quality knowledge on talent, which was concluded to be eased by implementing a CRM software to manage more efficiently the relationship with the customer base

Investigation of near and post stall behaviour of axial compression systems

The design of modern gas-turbine engines is continuously being improved towards better performance, better efficiency and reduced cost. This trend of aero-engine design requires compression systems which produce higher pressure ratios and thus, have higher loaded blades and a closer spacing between blade rows. Such designs are more prone to aerodynamic instabilities and consequent stall and surge can be catastrophic. The majority of the research conducted on compressor stall and surge is limited to old designs with lower pressure ratios or single stage compression systems. In this thesis, the near and post stall behaviour of a modern multi-stage high-speed intermediate pressure compressor rig and an aero-engine three-shaft compression system are studied in detail. The main objective is to develop and validate reliable CFD models to predict surge and rotating stall and shed light on the underlying physical mechanism of the phenomena. CFD computations were performed to gain understanding of the current capability to model the flow behaviour of a multi-stage compressor rig near stall condition. Two turbulence models were tested and an extensive grid discretization study was performed. In order to improve the prediction of the compressor's stability boundary, a modification in the widely known Spalart-Allmaras turbulence model is proposed. Subsequently, unsteady CFD computations were carried out to evaluate the impact of flow unsteadiness in the performance prediction of this compressor rig. It was found that for operating conditions characterized by non-axisymmetric flow features, an unsteady full annulus model is required to predict the compressor performance. For low speeds, these flow features develop over a wide range of operating conditions. When the compressor operates at high speeds, these flow features are limited to operating conditions near the stability boundary. The above findings were validated against experimental results. Early stages of this research revealed that numerical calibration of a CFD surge computation in a three-shaft engine is a challenging task due to compressor matching. Hence, an iterative methodology for matching the compressors was introduced and validated against experimental data. This study considered a surge event where the engine was initially operating at mid power condition. When comparing the numerical result with measured data, it was found that the engine bleed system has a major impact on the aerodynamic loading predictions in the core system. Therefore, this system needs to be considered by component designers when accounting for robustness to surge loads. The post stall response of a three-shaft engine compression system which is initially operating at design was investigated. It was found that the maximum surge over-pressures are caused by a combined effect between a surge induced shock wave and high pressure gas travelling towards the core inlet during the surge blow-down period. Furthermore, it was demonstrated that the maximum surge loads are obtained for a surge event initiated by fuel-spike. Finally, a cheaper computational approach to model surge in axial compression systems is proposed. This approach consisted of using an unsteady single passage model to predict the flow behaviour during the surge event. After comparison with full annulus results for three different scenarios, it was concluded that the single passage is capable of predicting the blow-down period of surge which is characterized by a long period of flow reversal. This model fails to predict the correct time and length scales during surge onset and flow transition between reverse to forward flow at the beginning of recovery. These time instants are characterized by non-axisymmetric flow features. However, the single passage model shows a good correlation with the results obtained using a full annulus model for estimation of average values of static pressure and mass flows during surge. This can drastically reduce simulations times from months to days during compressor surge analysis.Open Acces