Architectural control of ice-templated collagen scaffolds for cardiac regeneration
A range of biomaterials and fabrication methods have been explored to produce biomimetic scaffolds to facilitate cardiac tissue regeneration. Ice-templated collagen scaffolds have demonstrated translational success in other clinical applications. The ice templating technique utilizes phase separation dynamics during solidification and subsequent sublimation of ice to produce scaffolds with interconnected porosity. Although composition has been found to be key to determining cellular response, both nano-scale and micro-scale surface features of ice templated collagen scaffolds have also been found to encourage cellular ingrowth and attachment. Previous research has introduced techniques to control pore size and anisotropy. To date, however, ice-templating has not been shown to allow control of architecture to the extent that it is possible to replicate the structure of more complex tissue morphologies such as the myocardium. In this thesis, the underpinning physics of ice formation is leveraged to determine the final architecture of ice-templated collagen scaffolds. A controllable directional freezing apparatus was designed and built to enable fine control of the thermal environment during solidification. A relationship between the set thermal parameters and final pore architecture was established. This relationship enabled the production of structures with controlled pore alignment and size. The direct control and monitoring capabilities of the freezing apparatus enabled observations of intrinsic freezing kinetics. This insight allowed the solidification processes of anisotropic and isotropic ice-templating to be compared, and a link between the previously distinct fields was hypothesized. A novel thermal control technique was developed that dictated ice growth directions and achieved complex lamellar orientation of ice-templated collagen scaffolds. A new mould design was produced, with a heat-sink at the base and heat sources in the mould walls, which afforded three-dimensional thermal control during the solidification process. Ultimately, this created complex lamellar orientation of ice-templated collagen scaffolds. The technique is presented alongside a finite element model, developed as a predictive tool for the design of final lamellar orientation. Heat source moulds were used to introduce controlled thermal gradients during the solidification phase of the ice-templating process. Various heat source profiles were implemented and simulated. It was found that by introducing controlled complex thermal gradients during solidification, scaffolds with multidirectional pore orientations were produced, and the finite element simulation was found to accurately predict lamellar orientation. Taken together, the model and heat source freezing technique provide the opportunity for design and production of regenerative collagen scaffolds with tailored architectural morphologies. After establishing a control protocol for producing structures with tailored local lamellar architecture, patches were tested by observing the effects of scaffold architecture on cellular behaviour and mechanical conformation to the dynamic movements of the heart. Cardiomyocytes (H9 hESCs) were seeded onto scaffolds with aligned and isotropic pore structures and the cell signalling patterns were then compared. It was determined that the biomimetic accuracy of the aligned scaffold improved the uniformity of calcium signalling in cardiomyocytes when compared with those on isotropic structures. These results indicate that myocardial function is enhanced by defined scaffold orientation. The application of an ex vivo ovine cardiac perfusion model enabled direct observation of the native myocardial movement during the cardiac cycle. Through direct optical imaging and digital image correlation, collagen scaffolds were tested to assess their response to native myocardial deformation patterns. The strain dynamics of aligned and isotropic scaffold architectures were compared, and the efficacy of both glue and suture fixation methods were explored. It was determined that aligned scaffolds adhered with suture fixation complied with the native physio-mechanical environment. Similarly adhered isotropic scaffolds and patches adhered with glue, however, resulted in reduced deformation relative to the native myocardium. The work in this thesis has established a novel freeze casting technique to afford specific three dimensional control of collagen scaffold alignment. The resulting scaffolds with directionally aligned pore architectures were found to enhance cellular and mechanical dynamics to better replicate the native behaviour of myocardial tissue.
Cardiac tissue engineering, Ice templating, freeze casting, regenerative scaffold, tissue engineering
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