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Dynamic biological adhesion: mechanisms for controlling attachment during locomotion - PubMed

  • ️Tue Jan 01 2019

Review

. 2019 Oct 28;374(1784):20190199.

doi: 10.1098/rstb.2019.0199. Epub 2019 Sep 9.

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Review

Dynamic biological adhesion: mechanisms for controlling attachment during locomotion

Walter Federle et al. Philos Trans R Soc Lond B Biol Sci. 2019.

Abstract

The rapid control of surface attachment is a key feature of natural adhesive systems used for locomotion, and a property highly desirable for man-made adhesives. Here, we describe the challenges of adhesion control and the timescales involved across diverse biological attachment systems and different adhesive mechanisms. The most widespread control principle for dynamic surface attachment in climbing animals is that adhesion is 'shear-sensitive' (directional): pulling adhesive pads towards the body results in strong attachment, whereas pushing them away from it leads to easy detachment, providing a rapid mechanical 'switch'. Shear-sensitivity is based on changes of contact area and adhesive strength, which in turn arise from non-adhesive default positions, the mechanics of peeling, pad sliding, and the targeted storage and controlled release of elastic strain energy. The control of adhesion via shear forces is deeply integrated with the climbing animals' anatomy and locomotion, and involves both active neuromuscular control, and rapid passive responses of sophisticated mechanical systems. The resulting dynamic adhesive systems are robust, reliable, versatile and nevertheless remarkably simple. This article is part of the theme issue 'Transdisciplinary approaches to the study of adhesion and adhesives in biological systems'.

Keywords: active and passive control; directional adhesion; peeling; strain energy.

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Conflict of interest statement

We declare we have no competing interests.

Figures

Figure 1.
Figure 1.

Animal adhesive systems range from permanent to temporary and highly dynamic. While permanent adhesive systems are glue-based, slow temporary adhesive systems use releasable glues or suction, and the most dynamic adhesive systems employ interfacial forces. Image sources are provided in the electronic supplementary material. (Online version in colour.)

Figure 2.
Figure 2.

In all non-aquatic animals climbing with adhesive pads tested to date, adhesion, FA, is approximately half of the shear force, FS, acting during detachment (see panel (d)); the dashed line visualizes this approximate ‘rule of thumb’, which appears to hold for (a) geckos (Gekko gecko, seta, array and toe-level data; [59,60]), (b) tree frogs (Litoria caerulea, whole-body data; [66]), (c) dock beetles (Gastrophysa viridula, single-pad data, D Labonte & JMR Bullock 2015, unpublished data), (e) stick insects (Carausius morosus, single-pad data; [62]), (f) cockroaches (Nauphoeta cinerea, single-pad data; [68]) and (g) ants (Oecophylla smaragdina, single-pad data; [69]). Shear force therefore appears to be a universal control mechanism independent of pad morphology (smooth or hairy), adhesive mechanism (wet or dry) or contact size. Detailed regression results for (a–c) and (e–g) can be found in the electronic supplementary material. (Online version in colour.)

Figure 3.
Figure 3.

(a) Owing to the sprawled-leg posture of most climbing animals, externally applied attachment forces result in the application of both a normal and a shear force at the level of individual pads (illustrated in (b); climbing animals can also apply shear forces actively). These shear forces make it harder to detach the pads, and this ‘shear-sensitivity’ can be partially understood through peeling models (c), which liken adhesive pads to thin strips of adhesive tape, with width w, peeled at an angle ϕ. (d) As a unit length L0 of the tape is peeled, the point of force application moves by a fraction of this distance. Because this fraction decreases with decreasing peeling angle (or increasing shear force component), more force needs to be supplied to do the required work, leading to an apparent ‘strengthening’ of the contact. (e) Biological adhesive pads are thin and soft, and therefore probably stretch upon detachment (strain ɛ). This stretching increases the work done upon detachment, so reducing the effect outlined in (d). (f) The negative effect of pad stretching can be circumvented if the tape is stretched prior to detachment (‘pre-strain’ ɛ0). Storing strain energy in attached parts of the tape can not only make involuntary detachment less likely, but also aid rapid voluntary detachment. A more detailed discussion is provided in the text. (Online version in colour.)

Figure 4.
Figure 4.

‘Stability envelopes’ for tape peeling at varying tape pre-strains and peel angles ϕ. If the strain exceeds a minimum strain ɛmin (dashed, vertical blue line), stable attachment requires the application of the minimum force to stabilize the contact (dotted red line). In this regime, spontaneous detachment occurs whenever the applied force drops below this lower bound, providing a rapid and efficient detachment mechanism. (Online version in colour.)

Figure 5.
Figure 5.

Comparison of adhesive pads and friction pads in insects. Both pad types are specialized for resisting forces in different directions, and thereby serve fundamentally different functions. Adhesive pads are located distally on the foot, and produce high adhesion when activated by shear (pulling) forces, whereas friction pads are located proximally on the foot, and produce high coefficients of friction even when pressed only gently against the substrate. (Online version in colour.)

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