Influence of collagen fibril alignment in collagen scaffolds mineralization

Bone extracellular matrix (ECM) gives bone its unique mechanical properties, thus being important in bone tissue engineering to mimic its native composition and microstructure. The main component in bone ECM is Type-I collagen, which works as a scaffold for bone cell attachment and mineral deposition. There are two locations where mineralization takes place: the intrafibrillar and interfibrillar spaces of collagen fibrils. Several theories try to explain mineral formation in bone tissue. However, none of them was proven right, and so, the mechanism for mineralization facilitated by the collagen lattice is still unknown. This way, it is essential to study such a mechanism and develop bone tissue engineering approaches to mimic bone ECM. In this work, we hypothesized that collagen fibrils alignment would promote scaffolds mineralization. To prove it, we 3D printed fibrillar collagen hydrogels scaffolds using the suspension 3D printing technique, in which a gelatin slurry was used as a suspension bath. First, an optimization of the printing process was performed. Then scaffolds were printed, choosing the finest (123 ± 25 µm) and largest fiber diameter (215 ± 66 µm) obtained. Fiber diameter size was proven to affect the collagen fiber alignment inside the scaffolds and, consequently, affect mineral precipitation. The smallest fiber diameter scaffold showed signs of mineralization after one day inside a mineralization solution. In contrast, the largest fiber scaffold only presented mineralization signs after three days of submersion. Moreover, suspended electrowriting was introduced in this dissertation, obtaining a 40 µm jet diameter inside a castor oil bath, proving to be a promising additive manufacturing technology to achieve higher resolution constructs when 3D printing hydrogels.A matrix extra celular (ECM) do osso confere-lhe as suas únicas propriedades mecânicas e, portanto, torna-se importante mimetizar a composição e microestrutura nativa da ECM em engenharia de tecidos ósseos. O colagénio tipo-I é o principal componente da ECM e funciona como suporte para adesão celular e deposição de minerais, podendo a mineralização ocorrer nos espaços Intra e interfibrilares do colagénio. Existem algumas teorias que tentam explicar os mecanismos de formação de minerais no tecido ósseo. Contudo, nenhuma foi comprovada, fazendo com que, o mecanismo pelo qual a mineralização facilitada por colagénio ocorre, seja ainda desconhecido. Desta forma, estudar este mecanismo e desenvolver técnicas, em engenharia de tecidos, para que seja possível mimetizar a ECM do osso, torna-se essencial. Neste projeto, teorizámos que o alinhamento das fibrilas de colagénio em scaffolds iriam promover a sua mineralização. Deste modo, hidrogéis de colagénio fibrilar foram impressos recorrendo à técnica de impressão em suspensão, na qual foi utilizado um banho de suspensão à base de gelatina. Inicialmente, a otimização do processo de impressão foi efetuada. De seguida, as fibras mais estreitas (123 ± 25 µm) e largas (215 ± 66 µm) foram escolhidas para realizar a impressão das estruturas desejadas. Foi provado que o diâmetro das fibras influencia o alinhamento das fibrilas de colagénio nas estruturas e, consequentemente, a sua mineralização. As fibras de menor dimensão apresentaram sinais de mineralização após um dia de submersão na solução de mineralização, enquanto que, as fibras mais largas exibiram sinais de deposição de minerais apenas após três dias. Adicionalmente, a técnica de suspended electrowriting foi introduzida nesta dissertação, obtendo-se um diâmetro de jato de 40 µm dentro de um banho de óleo de ricínio, provando que esta é uma técnica promissora de fabrico aditivo, que nos permite atingir melhores resoluções na impressão 3D de hidrogéis

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements