Design of Tree-Frog-Inspired Adhesives.

Julian K A Langowski,Peter Van Assenbergh,Dimitra Dodou,Johan L Van Leeuwen

doi:10.1093/icb/icaa037

Julian K A Langowski, Peter Van Assenbergh + Show 2 more

Open Access

https://doi.org/10.1093/icb/icaa037

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Abstract

SynopsisThe adhesive toe pads of tree frogs have inspired the design of various so-called ‘smooth’ synthetic adhesives for wet environments. However, these adhesives do not reach the attachment performance of their biological models in terms of contact formation, maintenance of attachment, and detachment. In tree frogs, attachment is facilitated by an interconnected ensemble of superficial and internal morphological components, which together form a functional unit. To help bridging the gap between biological and bioinspired adhesives, in this review, we (1) provide an overview of the functional components of tree frog toe pads, (2) investigate which of these components (and attachment mechanisms implemented therein) have already been transferred into synthetic adhesives, and (3) highlight functional analogies between existing synthetic adhesives and tree frogs regarding the fundamental mechanisms of attachment. We found that most existing tree-frog-inspired adhesives mimic the micropatterned surface of the ventral epidermis of frog pads. Geometrical and material properties differ between these synthetic adhesives and their biological model, which indicates similarity in appearance rather than function. Important internal functional components such as fiber-reinforcement and muscle fibers for attachment control have not been considered in the design of tree-frog-inspired adhesives. Experimental work on tree-frog-inspired adhesives suggests that the micropatterning of adhesives with low-aspect-ratio pillars enables crack arresting and the drainage of interstitial liquids, which both facilitate the generation of van der Waals forces. Our analysis of experimental work on tree-frog-inspired adhesives indicates that interstitial liquids such as the mucus secreted by tree frogs play a role in detachment. Based on these findings, we provide suggestions for the future design of biomimetic adhesives. Specifically, we propose to implement internal fiber-reinforcements inspired by the fibrous structures in frog pads to create mechanically reinforced soft adhesives for high-load applications. Contractile components may stimulate the design of actuated synthetic adhesives with fine-tunable control of attachment strength. An integrative approach is needed for the design of tree-frog-inspired adhesives that are functionally analogous with their biological paradigm.

Highlights

Bioadhesion is an interdisciplinary research field at the interface of biology, physics, and chemistry, which stimulates research on the fundamentals of adhesion and friction (Jagota and Hui 2011; Federle and Labonte 2019), provides insights into the evolution of biological adhesive systems (Buscher et al 2018; Gamel et al 2019; Russell and Gamble 2019), and generates inspiration for the design of synthetic micropatterned adhesive surfaces
We investigate which functional analogies have already been implemented in synthetic adhesives, and which hypotheses on the fundamental mechanisms of tree frog attachment are supported by experimental work on synthetic adhesives
The transfer of individual components into tree-frog-inspired adhesives may be sufficient for specific applications but likely leads to a reduced performance compared to the biological model

Summary

Introduction

Bioadhesion is an interdisciplinary research field at the interface of biology, physics, and chemistry, which stimulates research on the fundamentals of adhesion and friction (Jagota and Hui 2011; Federle and Labonte 2019), provides insights into the evolution of biological adhesive systems (Buscher et al 2018; Gamel et al 2019; Russell and Gamble 2019), and generates inspiration for the design of synthetic micropatterned adhesive surfaces ( referred to as ‘adhesives’; Li et al 2016; Eisenhaure and Kim 2017). Once a large area of close contact has been established, the generated contact needs to be maintained This requires sufficient mechanical strength of the soft pad to withstand external loads such as body weight and inertial forces (Bijma et al 2016). This scaling may be partially explained by a positive proportionality between shear load and contact area caused by the structural properties of the fiber-reinforced epidermis of frog pads, as observed in the fiber-reinforced adhesive pads of stick insects (Dirks et al 2012) To this geometric effect linking shear load with contact area, a release of elastic energy stored in the fiber-networks found in frog pads may facilitate contact area reduction, as described for various hairy attachment systems (Federle and Labonte 2019). Dhong and Frechette (2015) Iturri et al (2015) Chen et al (2015) Li et al (2015)

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Concluding remarks