Mechanical Innovations of a Climbing Cactus: Functional Insights for a New Generation of Growing Robots

Abstract

Climbing plants are being increasingly viewed as models for bioinspired growing robots capable of spanning voids and attaching to diverse substrates. We explore the functional traits of the climbing cactus Selenicereus setaceus (Cactaceae) from the Atlantic forest of Brazil and discuss the potential of these traits for robotics applications. The plant is capable of growing through highly unstructured habitats and attaching to variable substrates including soil, leaf litter, tree surfaces, rocks, and fine branches of tree canopies in wind-blown conditions. Stems develop highly variable cross-sectional geometries at different stages of growth. They include cylindrical basal stems, triangular climbing stems and apical star-shaped stems searching for supports. Searcher stems develop relatively rigid properties for a given cross-sectional area and are capable of spanning voids of up to 1 m. Optimization of rigidity in searcher stems provide some potential design ideas for additive engineering technologies where climbing robotic artifacts must limit materials and mass for curbing bending moments and buckling while climbing and searching. A two-step attachment mechanism involves deployment of recurved, multi-angled spines that grapple on to wide ranging surfaces holding the stem in place for more solid attachment via root growth from the stem. The cactus is an instructive example of how light mass searchers with a winged profile and two step attachment strategies can facilitate traversing voids and making reliable attachment to a wide range of supports and surfaces.

Structural performance of a climbing cactus: making the most of softness

“…Many woody vines and lianas produce a cylinder of stiff wood early in development or a thick layer of fibre tissue around the periphery of the stem [6]. The climbing cactus shows a different strategy by maintaining rigidity by enclosing a bulky but soft hydrogel-like tissue within a thin layer of hypodermal skin [8]. Our measurements of this indicate that its thickness and Young's modulus will not contribute significantly to stem rigidity directly as a stiffening element but acts as a bounding layer that contains swelling and de-swelling of soft cortical tissue essential for maintaining turgor driven rigidity.…”

“…A recent study on the South American species Selenicereus setaceus (Cactaceae) has highlighted how changes in overall stem geometry and structural Young's modulus can optimize stem rigidity for a searching-climbing habit across diverse substrates [8]. These vary from highly ribbed apical 'searcher stems' that reach and locate supports to more basal triangular and circular cross-sectional organizations of the attached and climbing stems (figure 1).…”

“…We then determined rheological properties of the mucilaginous tissue and compared transverse and longitudinal compression properties of the mucilaginous tissue and whole stem segments including wood cylinder tissue for all three stages of growth represented by searcher stems (highly lobed cross-section), climbing stems (triangular cross-sections) and older basal stems (rounded cross-sections) (figure 1) [8].…”

“…If so, they likely influence stem shape and geometry according to the degree of hydration of the bulky cortex especially for younger stages of growth before secondary growth of the wood cylinder. In a previous paper, we focused on the overall life history and strategies of this climbing cactus as a potential role model for robotic artefacts capable of growth [8]. In this paper, we analyse in detail the component parts of this model species and explored the following questions.…”

The Bio-Engineering Approach for Plant Investigations and Growing Robots. A Mini-Review

“…Early stem growth, where young shoots extend into spaces and search for support, are known as “searchers.” An outstanding example of a light-mass searcher can be found in the climbing catus Selenicereus setaceus (Soffiatti and Rowe, 2020 ). Searchers often have a light but stiff structure, and are capable to extend across voids and perform circumnutations to improve the probability of touching a support (Gallenmüller et al, 2004 ).…”