Caudicles in vandoid orchids: A carotenoid-based soft material with unique properties

130 years ago, Darwin observed that caudicles in vandoid orchids possess considerable elasticity and further hypothesized that their elasticity serves to improve pollination efficiency. However, there has been no study that seeks to either quantitatively backup Darwin's hypothesis or characterize this natural material for practical use. Here we show that vandoid caudicles are a novel kind of soft material with extremely high extensibility (1190%), low modulus (160 kPa) and density lower than that of water. Vandoid caudicles contain carotenoids that attach to basal polymers through noncovalent interactions. Inspired by the chemical structure of caudicles, we synthesize calcium-alginate/polyacrylamide hydrogels supplemented with carotenoids and demonstrate that their strength as well as stretchability are enhanced two-fold. Our findings identify a new carotenoid-based material system with unique properties that approach the current boundaries of the Ashby chart, demonstrating potential application of carotenoids as biocompatible reinforcing agent for hydrogels. STATEMENT OF SIGNIFICANCE: We have investigated the microstructure, mechanical properties and chemical components of vandoid caudicles as an elastic plant tissue and demonstrated a bio-inspired design that can enhance the elasticity of hydrogels. Existing research on vandoid caudicles are very few and mainly focus on their phylogenetics and developmental process, and the potential application of caudicles in the field of material sciences remains unexplored. Our results showed that caudicles are more stretchable than most natural and synthetic elastomers and have a modulus similar to hydrogels. Carotenoids, an important chemical component of caudicles, can be used as supplements to hydrogels to improve their strength and stretchability.