Design of self-folding shape-memory elastomer composites for biomedical applications

Nowadays, the strong demand for advanced devices able to hold specific functions implies the continuous development of innovative materials. Imparting groundbreaking properties to existing materials, such as the ability to degrade after use, is one example of improvement which paves the way to unlock specific and advanced applications, especially in the biomedical field. In addition, the development of innovative strategies for fine-tuning the design of polymer composites makes it possible to synergistically combine advanced materials into functional devices. The present thesis aims to develop functional sheets presenting simultaneously degradability, mechanical self-folding, elastomer behavior and shape-memory properties. This goal was successfully achieved by developing sheets of shape-memory elastomer composites based on fully degradable materials and specifically designed to afford composition asymmetry of the sheet sides providing self-folding ability. Firstly, a new concept to design a composite sheet able to self-fold in various shapes depending on the orientation of the uniaxial stretching was proven by the embedding of a honeycomb-structured poly(ε-caprolactone) (PCL) electrospun nanofiber mat into a commercially available polydimethylsiloxane (PDMS) as elastomer matrix allowing large-scale testing. Remarkably, these self-folding sheets were obtained by a one-step process being an upfront alternative to conventional multi-layered laminates reported in the literature. They are also characterized by a different roughness on each side. Then, to achieve a completely degradable composite sheet, the thermo-cured PDMS was substituted by photo-crosslinkable polyphosphoester (PPE) copolymers for the matrix. We first evidenced that the scattering of the UV light by the nanofibers allows a fast (few minutes) and a high homogeneity of the PPE matrix crosslinking. Then, we showed that the distribution of the UV reactive moieties along the PPE backbone tremendously influences the ultimate elongation (max). Locating them at both chain-ends in a triblock architecture, rather than randomly, and optimizing the crosslinking conditions (use of solvent and sonication) led to the first PPE elastomer exhibiting max up to 230% and then allowed the fabrication of the targeted self-folding degradable shape-memory elastomer sheet. Based on preliminary in vitro results, biomedical applications can be foreseen for temporary implants in blood vessels such as internal bandage or as potential degradable candidate to conventional stents.