Developmental Themes of a Climbing Cactus—A Bio-Inspired Approach for New Actuator Technologies

We present a model cactus plant with developmental strategies, structures and means of colonizing its natural habitat that have been inspirational for the development of hydrogel-elastomer actuator systems. Climbing plants represent a rich source for ideas and concepts for the creation of new technologies and materials.<br/>Many climbing plants develop a mechanical architecture where apical branches (so called "searchers") have a stiffer structure compared to the basal climbing stem. This allows the apical branches to navigate and find a substrate to anchor themselves. The climbing cactus stands out from the usual organization present in most lianas. <i>Selenicereus setaceus</i> (Cactaceae) occurs in dry coastal forests of southeastern Brazil. Like most cacti, it has succulent stems adapted to arid and semi-arid conditions that undertake photosynthesis. Its leaves are extremely reduced and transformed into spines. Unlike close related species, individual plants are climbers. Initially they creep along the ground, then attach to tree trunks using specialized roots and then develop apical searchers that spread out from the tree canopy into the open light. <i>S. setaceus</i> can grow and develop in rough unstructured environments and is able to anchor on many kinds of substrates, keeping the stem firmly attached even in harsh climatic conditions.<br/>This climbing plant adapts its stem geometry to optimize rigidity. Creeping basal stems have circular cross-sections, giving rise to root-climbing stems (triangular cross sections), which produce searcher stems (star-shaped cross-sections). These maximize second moment of area and rigidity whilst retaining a physiologically “cheap” hydrogel-like internal structure. The searcher architecture is capable of crossing voids up to 1 m.<br/>The searcher stem is occupied by a hydrogel-like material: thin-walled water storage cells, surrounded by mucilage and enclosed by a stiff "skin". The skin is a composite material in three layers: an outer waterproofing layer, an inner layer of flattened cells (epidermis) and inside that several layers of spindle-shaped cells with hygroscopic cell walls. This arrangement ensures that the skin accommodates increases in volume due to water loss or gain, a condition that varies greatly in these plants. Swelling and de-swelling experiments showed significant increases and decreases of cross-sectional area in demineralized water or 0.5 M sucrose respectively, leading to profound changes in their geometry.<br/>Based on this cactus, two actuating systems have been developed, one based on the searcher geometry, in which movement is generated from external light or humidity stimuli, as in natural conditions. A second one was based in the developmental changes observed along the cactus stem, from cylindrical to triangular to star-shaped, using the materials swelling/deswelling capacity. In both systems, an artificial hydrogel mimicking the cactus soft cortex is employed as a self-actuating component, generating movement (Devo system 1) or changes in geometry (Devo system 2).<br/>This biological system provided insight for how variable developmental pathways —“devos”— in a biological model might be used to develop different kinds of technological actuator. These “variations on a developmental theme” are based on phases of development seen naturally in the cactus that are related to different “stages” of its climbing-attaching and searching behavior. Ongoing studies are exploring these traits further by investigating the autonomy, survival and growth of these skin-core, hydrogel-based plant systems, especially following catastrophic damage in harsh environmental settings. It is expected that these will give further bio-inspired clues for autonomy and survival of actuating technologies in challenging environments.