Morphology and adhesion mechanisms of newt <italic>Cynops orientalis</italic>’ foot pads

Abstract

Amphibian species such as tree frogs, torrent frogs, and arboreal salamanders possessing adhesive organs on their feet show superior wet adhesion abilities. In recent years, many research efforts have been carried out to understand the wet adhesion mechanisms. The surface tension (namely capillary force) from a thin film of liquid that is secreted into the contact zone between the pad and the substrate by organisms is considered to be the primary mechanism involved in wet adhesion. Meanwhile, other mechanisms such as close contact, dry adhesion, boundary friction, and suction effect may have synergic effect on wet adhesion. In this study, the newts Cynops orientalis were chosen to investigate their wet adhesion mechanisms, because they are able to adhere to and move over wet environments without falling. A tilting platform with controlled wetting conditions and dismountable substrates was built to test the adhesion ability of newts Cynops orientalis under different conditions. The platform can be manually rotated from 0° to 360° and was used to carry out the adhesion experiments by measuring the falling angle of the newts. And then a variety of microscopical techniques including scanning electron microscope (SEM), transmission electron microscope (TEM), and atomic force microscope (AFM) were used to investigate the morphology of newt foot pads. The experimental results show that the newts were able to adhere to the smooth surface of perspex with a tilting angle of more than 90° under dry and wet conditions, indicating strong adhesion and resistance against shear. Moreover, the young newts showed relatively stronger adhesive property of foot pads as compared with that of the sub-adult ones. The microscopic pictures of the foot pads show that the polygonal epidermal cells of foot pads of the sub-adult newts are mainly separated by narrow clefts and lots of mucous pores were observed at the boundaries between epidermal cells, while the epidermal cells of foot pads of the young newts are tightly packed and surrounded by clearly raised boundaries (337±82 nm in width and 180±55 nm in height) with middle narrow channels, and few mucous pores were observed on the foot pad surface. The epidermal cell surface of the sub-adult and young newts is covered by a layer of dense array of columnar nanopillars with density of about 60 μ m - 2. The nanopillars surrounded by small channels are semi-spherical at the apex and approximately as tall as their width (about 160 nm), which corresponds to an aspect ratio of about 1.0. The adhesive foot pad in the newts is very soft with an effective elastic modulus of 564.8±239.1 kPa, as measured by the atomic force microscope in nanoindentation mode. Based on the adhesive property of newt foot pads, the function of the characteristic morphology of foot pads and the adaptive adhesion mechanisms are discussed. The research indicates that the newts Cynops orientalis primarily utilize the capillary force generated by the fluid-filled joint between the pad and the substrate to achieve strong adhesion and resistance against shear. The young newts may also utilize the suction effect on smooth substrate surface as a result of the raised boundaries on their pads surface. The dense nanopillar arrays on the epidermal cells surface are compliant enough to possibly combine close contact, friction and dry adhesion together to enhance the effect of wet adhesion. This study will provide a useful suggestion for the design and development of new bio-inspired underwater adhesives.