Preparation of biomimetic photo-responsive polymer springs

This protocol describes the preparation of polymer springs that twist under irradiation with light, in a manner that mimics how plant tendrils twist and turn under the effect of differential expansion in different sections of the plant. The artificial springs are typically 1 mm in width, 50 μm in thickness and up to 10 mm in length, their length being limited by cell dimensions only. They are made from polymer networks that keep memory of a liquid crystalline order, and in which an azobenzene derivative is introduced covalently as a molecular photo-switch. This liquid crystal polymer is prepared by irradiation of a twist cell filled with a mixture of shape-persistent liquid crystals, liquid crystal having reactive end groups, molecular photo-switches, some chiral dopant and a small amount of photo-initiator. This cell is assembled out of two glass slides separated by a spacer and covered by a thin film of polyimide that was rubbed along the long axis of the cell for the bottom slide, and along the short axis of the cell for the top slide. Once the cell is filled by capillarity, photo-polymerization takes place at 48 ºC and takes approximately 1.5 h. The product is a photo-responsive liquid crystal polymer network that is characterised by optical microscopy, scanning electron microscopy and tensile strength measurements. The film is post-cured overnight at 60ºC. Removing the resulting soft polymer film and cutting out the desired spring-like shape takes ~45 min. The springs operate at ambient temperature, by mimicking the orthogonal contraction mechanism that is at the origin of plant coiling. They are shape shifting under irradiation with ultraviolet light and can be pre-programmed to either wind or unwind, as encoded in their geometry. Once illumination is stopped, the springs return to their initial shape in ambient light conditions