Biomimetic photo-actuation: progress and challenges

Abstract

Photo-actuation, such as that observed in the reversible sun-tracking movements of heliotropic plants, is produced by a complex, yet elegant series of processes. In the heliotropic leaf movements of the Cornish Mallow, photo-actuation involves the generation, transport and manipulation of chemical signals from a distributed network of sensors in the leaf veins to a specialized osmosis driven actuation region in the leaf stem. It is theorized that such an arrangement is both efficient in terms of materials use and operational energy conversion, as well as being highly robust. We concern ourselves with understanding and mimicking these light driven, chemically controlled actuating systems with the aim of generating intelligent structures which share the properties of efficiency and robustness that are so important to survival in Nature. In this work we present recent progress in mimicking these photo-actuating systems through remote light exposure of a metastable state photoacid and the resulting signal and energy transfer through solution to a pH-responsive hydrogel actuator. Reversible actuation strains of 20% were achieved from this arrangement, with modelling then employed to reveal the critical influence hydrogel pKa has on this result. Although the strong actuation achieved highlights the progress that has been made in replicating the principles of biomimetic photo-actuation, challenges such as photoacid degradation were also revealed. It is anticipated that current work can directly lead to the development of high-performance and low-cost solartrackers for increased photovoltaic energy capture and to the creation of new types of intelligent structures employing chemical control systems.

Photoacidity of Indolinospirobenzopyrans in Water

“…MCHs have been shown to provide a means for biomimetic photoactuation (Figure 2a). For example, Faul and co-workers [76] engineered a device [77] capable of replicating the behaviour of the Cornish mallow -a plant that reorients its leaves according to the position of the light source. Their device features a MCHcontaining hydrogel matrix which contracts/expands depending on light-induced pH changes.…”

Hydrogel-based biosensors and sensing devices for drug delivery

“…To better understand the kinetic response of hydrogel-based sensors, we must understand the kinetic response of the hydrogel itself. The swelling and shrinking of hydrogels requires transport pH Hydrogel swollen volume Cationic Anioni of the stimulus into the network followed by water transport into or out of the network [14]. An interesting complexity in this system is that shrinking of the gel can generally occur more rapidly than its swelling.…”