Damaged cartilage can cause severe pain and restricted mobility, with few long term treatments available. The developing field of tissue engineering offers an alternative to the currently used full joint replacement. Restoring damaged cartilage through tissue engineering would enable an active lifestyle to be recovered and retained, without restrictions to joint mobility. This is increasingly important as the prevalence of osteoarthritis rises. Tissue engineering requires biomaterial scaffolds that mimic the function of the tissue while cells develop, and so the scaffold must provide the appropriate biological, chemical and mechanical stimuli. In this work, methods were developed to enable the design of scaffolds that mimic the microstructure and mechanical properties of articular cartilage. Electrospinning was investigated as a method to mimic the nanoscale collagen fibres within cartilage extracellular matrix. A parametric study was conducted to determine how changes to a gelatin solution affect the mechanical properties of the non-woven fibrous mesh. The solution properties had a clear impact on the morphology of the fibres, but the effect on the mesh mechanical properties was convoluted. The results demonstrated the need for greater understanding of the 3D morphology of electrospun meshes, to establish how these may be altered in order to design scaffolds with desirable mechanical properties. The fracture mechanics of soft materials are complex, and are generally overlooked when designing tissue engineering scaffolds. The complexities have led to a lack of standardised testing, making comparisons between studies impractical. In this work, fracture testing methods were compared, using a viscoelastic polymer to mimic some of the complexities of soft tissue mechanics. Mode III trouser tear tests and mode I pure shear tests were found to provide reliable measurements. Due to the ease of testing small samples, trouser tear testing was concluded to be the most advantageous for determining the fracture resistance of soft tissue engineering scaffolds. Finally, electrospun meshes were combined with hydrogels to create biomimetic scaffolds, which were characterised using tensile and trouser tear fracture tests. Fibre-reinforcement was shown to enhance the mechanical properties of a weak hydrogel, but diminished those of a strong, tough polyacrylamide (PAAm)-alginate hydrogel. The PAAm-alginate hydrogel exhibited mechanical properties close to those of natural articular cartilage, but without the microstructure that would enhance its suitability for use as a cartilage tissue engineering scaffold. An alternative method for reinforcing PAAm-alginate was proposed, which shows promise for producing a biocompatible scaffold that mimics both the mechanics and the microstructure of articular cartilage. Ultimately, this thesis aimed to improve the design of biomimetic scaffolds for cartilage tissue engineering, and advance mechanical characterisation techniques within this field.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Composites hydrogels were developed in film shape based on poly vinyl alcohol (PVA) using acidified attapulgite as crosslinking agent. The hydrogel composites were obtained in solution with amounts of attapulgite ranging from 0.05 to 2% in the polymer matrix in order to study its influence on the crystallinity, in the swelling properties and in the release of gentamicin sulphate. The obtained composites were characterized by XRD, FTIR and DSC. According to XRD data the polymer crystallinity is not affected by the clay addition, however, the presence of attapulgite modifies the melting and crystallization temperatures, behavior observed by DSC measures. In general, graphical profile of FTIR showed that there are interactions between PVA and attapulgite. The hydration kinetics at 37°C is independent of pH values and shown to obey the Fickian diffusion mechanism with values of n<0.5. Thus, the viability of hydrogels in drug delivery systems was evaluated by the swelling degree and by in vitro release. These two aspects were dependent on the concentration of the drug. Thus, although preliminaries, the results from release of gentamicin sulphate using PVA/attapulgite composites hydrogels proved be promising for future application. / Foram desenvolvidos hidrogéis compósito na forma de filme a base de poli (álcool vinílico) (PVA) usando a atapulgita acidificada como agente de reticulação. Os hidrogéis compósitos foram obtidos em solução variando a quantidade de atapulgita (0,05-2%) na matriz polimérica visando estudar sua influência na cristalinidade, nas propriedades de intumescimento e na liberação do sulfato de gentamicina. Os materiais preparados foram caracterizados por DRX, FTIR, DSC. Segundo os dados do DRX a cristalinidade do polímero não é afetada pela adição da argila, em contrapartida, a presença da atapulgita altera as temperaturas de fusão e cristalização, comportamento verificado pelo DSC. De maneira geral, o perfil gráfico do FTIR evidenciou que há interações entre a atapulgita e o PVA. A cinética de hidratação a 37°C mostrou ser independente do valor de pH e demostrou obedecer ao mecanismo de difusão fickiana com valores de n<0,5. Desse modo, a viabilidade dos hidrogéis em sistema de liberação de fármacos foi avaliada pelo grau de intumescimento e liberação in vitro. Estes dois aspectos foram dependentes da concentração do fármaco. Assim, apesar de preliminares, os resultados de liberação do sulfato de gentamicina utilizando hidrogéis compósito PVA/ atapulgita mostraram-se promissores para uma futura aplicação.

Content: Nowadays, protein-based nanoparticle as a biodegradable, biocompatible product attracts considerable interest for new uses in specialized technical areas. Gelatin is a denatured, biodegradable, and nonimmunogenic protein obtained by controlled hydrolysis of the triple-helix structure of collagen into single-strain molecules. As an amphiphilic biopolymer, gelatin can easily assemble into different kinds of aggregates under the defined pH and temperature and the resulting gelatin nanoparticles have been developed to be applied in the food industry and biomedical fields. Herein we report a novel macromolecular microsphere composites (MMC) hydrogels with the use of prepared methacrylated gelatin nanoparticles (MA-GNP) as the cross-linker. MA-GNP have the ability of chemical crosslinking by the polymerization of C=C bonds, such that the composite hydrogels can be formed by radical polymerization of acrylamide (AAm) on the surface of MA-GNP. The smooth spherical particles with an average size of ~100 nm have been synthesized through a modified two-step desolvation method as proved by atomic force microscopy (AFM). The results of nuclear magnetic resonance and dynamic light scattering further confirm the presence of reactive groups (C=C bonds) in the particles and its narrow sizes distribution. The resulting composite hydrogels (MA-GNP/PAAm) are porous materials with tunable pore sizes and exhibit enhanced compressive resistance and elasticity as well. Increasing appropriately the dosage of MA-GNP reduces the equilibrium swelling ratio and improves thermal stability of the gels. Moreover, all the hydrogels exhibit prolonged blood-clotting time, nonhemolytic nature and strong suitability for cell proliferation, indicating the improved antithrombogenicity and excellent cyto-compatibility. It suggests that the novel MA-GNP/PAAm hydrogels have potential application as tissue engineer scaffold materials, and the MA-GNP can be a promising macromolecular microsphere cross-linker for application in biomedical materials. The present work not only exploits new strategies to fabricate MMC hydrogels but also advance the potential application of biodegradable gelatin-based nanoparticles in biomedical fields. Take-Away: 1. A well-dispersed methacrylated gelatin nanoparticle (MA-GNP) with an average size of ~100 nm is presented by a modified two-step desolvation method. 2. MA-GNP is readily introduced into the polyacrylamide (PAAm) system as a cross-linker to prepare macromolecular microsphere composites (MMC) hydrogels via a free radical polymerization reaction. 3. MA-GNP is an effective cross-linker, improving both the compressive resistance and elasticity of MMC hydrogels as well as the biocompatibility.