Mechanics and morphology of permanent attachment systems in plants

Permanent attachment pads of climbing plants are an example of highly efficient attachment structures, which have been evolved, tested and optimized in the course of evolution. However, information about the attachment mechanisms is still scarce, although the excellent mechanical performance of attachment structures has been recognized. In this study, the morphology and the biomechanics of attachment pads of Boston Ivy (Parthenocissus tricuspidata) as well as the interface between the pad and different substrates were investigated. The self-clinging liana P. tricuspidata develops swollen tips at the end of its tendrils which form into attachment pads. Attached and non-attached structures were analyzed using microscopical and mechanical testing methods. The overall strength of the interface was studied using tensile tests on a large number of individual pads attached to different substrates. On the micrometer-scale, the mechanical properties of the constituent materials were studied using nanoindentation. Ontogenetic variations in the morphology of attached and non-attached structures were observed. Cell size, cell orientation and grade of lignification vary over the pad cross-section normal to the interface. Furthermore, cell size, cell wall thickness as well as cell orientation show variations in the plane of the interfacial region. The cells in the edge regions appear to secrete a strong adhesive fluid. The morphological variations are related to changes in mechanical properties. For example, the region close to the pad edge is up to four times stiffer than the regions closer the pad center. A single lignified pad withstands normal stresses at the interface of up to 4 MPa while failure never occurred only at the interface. Four different failure modes were observed: failure of the substrate, internal failure of the attachment pad, a combination of the two modes or in case of non-porous materials mixed failure of plant components and interface. The attachment strength of young pads has been found to be significantly smaller than the attachment strength of older, lignified pads. It was on average only 28% of the value found for the older pads. The attachment process can be described at least as a two step process with a pre-attachment and a final attachment. Microscopical investigations show a perfect form closure between pad and substrate. Overall, attachment structures of climbers can be considered as a composite material and these natural interfaces are promising models for new technological concepts.