Tailored one-way and two-way shape memory capabilities of poly(ε-caprolactone)-based systems for biomedical applications

Shape Memory Polymers (SMPs) are a class of smart materials, capable of significant shape variations on the application of an environmental stimulus. SMPs present various advantages with respect to their metallic and ceramic counterparts, such as processing at a low costs, large recoverable strains and mechanical properties close to those of soft biological tissues. SMPs can be potentially employed in biomedical applications, and minimally invasive surgery devices, where a single deployment is required (“one-way” shape memory effect), are the most frequently investigated. Also the “two-way” shape memory behavior, i.e. the triggered change between two distinguished shapes on the application of an on-off stimulus, is recently a desired feature, that may fulfill the requirements for the development of actuators or artificial muscles. To this end polymers like poly(ε-caprolactone) (PCL) are considered promising systems, due to their biodegradability and to the possibility to present, when a crosslinked structure is provided, reversible actuation under specific thermo-mechanical conditions. In this work we have explored the one- and two-way shape memory capabilities of a novel type of covalently crosslinked semicrystalline systems, prepared by a sol-gel approach from alkoxysilane- terminated PCL precursors. Silica-like crosslink points were generated under mild reaction conditions, carried out at room temperature and allowing to avoid additives such as organic peroxides typically used for free-radical thermal- or UV-curing. As biological tests have shown, the materials present lack of toxicity and are able to sustain cell adhesion and proliferation. Further by varying the molecular weight of the PCL precursors it is possible to tailor the crosslink density, the crystallization and the melting temperature. The one-way shape memory behavior, investigated in optimized thermo-mechanical cycles, show that the materials are able to fully fix the applied shape and to restore from 90% to 100% of their original shape by heating them on a narrow region close to Tm. To investigate the application of the materials as self-expandable stents, shape memory experiments are currently carried out on tubular specimens with tailored Tm at about 37°C (i.e. close to the human body temperature). The materials also display two-way shape memory capabilities when subjected to a constant load and heated and cooled on a temperature region spanning from below the crystallization temperature to above the melting temperature, leading to maximum strain changes of about 80%. The effects of the applied load and of the crosslink density were explored and revealed them as key-parameter to obtain a tailored actuation.