Superhydrophobicity of the gecko toe pad: biological optimization versus laboratory maximization

Abstract

While many gecko-inspired hierarchically structured surfaces perform as well as or better than the natural adhesive system, these designs often fail to function across a variety of contexts. For example, the gecko can adhere to rough, wet and dirty surfaces; however, most synthetic mimics cannot maintain function when faced with a similar situation. The solution to this problem lies in a more thorough investigation of the natural system. Here, we review the adhesive system of the gecko toe pad, as well as the far less-well-studied anti-adhesive system that results from the chemistry and structure of the toe pad (superhydrophobicity). This paradoxical relationship serves as motivation to study functional optimization at the system level. As an example, we experimentally investigate the role of surface lipids in adhesion and anti-adhesion, and find a clear performance trade-off related to shear adhesion in air on a hydrophilic surface. This represents the first direct investigation of the role of surface lipids in gecko adhesion and anti-adhesion, and supports the argument that a system-level approach is necessary to elucidate optimization in biological systems. Without such an approach, bioinspired designs will be limited in functionality and context, especially compared to the natural systems they mimic.This article is part of the themed issue 'Bioinspired hierarchically structured surfaces for green science'.

Direct evidence of acid-base interactions in gecko adhesion

“…However, the observed high-frequency shift (105 ± 16 cm −1 ) indicates that the surface of contacting setal tips (i.e., spatulae) must be covered with unbound lipids exposing polar headgroups, instead of the nonpolar tails. This arrangement of unbound lipids on the setal surface inferred from our SFG results is not contradictory to the previously observed high water contact angles on the toe pad, as recent work by Stark et al ( 15 ) demonstrated that superhydrophobicity is a consequence of the hierarchical structure of the toe pad rather than the presence or absence of lipids. Moreover, this lipid arrangement is similar to the arrangement of lipids in mammalian stratum corneum ( 45 , 46 ).…”

“…First, early studies showed that gecko adhesion to hydroxylated surfaces such as glass and alumina was particularly high, which cannot be explained solely by vdW forces because vdW forces are insensitive to surface chemistry. Lower setal adhesion to a silicon wafer coated with a hydrophobic coating compared to a bare silicon wafer further corroborates these results and suggests that forces other than, or in addition to, vdW interactions may contribute to gecko adhesion ( 4 , 15 ). Second, gecko adhesion enhances with increasing humidity at the setal and whole-animal level, implying that capillary forces also play a role in gecko adhesion in humid environments ( 3 , 4 , 7 ).…”

“…However, the orientation of these lipids (i.e., headgroups or tails) at the setal surface remains unclear. For instance, the superhydrophobicity of the adhesive gecko toe pad and SFG peaks corresponding to lipid molecules at the contact interface led Hsu et al to conclude that nonpolar tails of unbound lipids were exposed at the setal surface ( 8 , 15 , 31 , 33 ). However, the water contact angle of gecko setae does not change even after delipidization (chemical treatment to remove unbound lipids), which raises questions about the orientation of the unbound lipids ( 15 ).…”

“…For instance, the superhydrophobicity of the adhesive gecko toe pad and SFG peaks corresponding to lipid molecules at the contact interface led Hsu et al to conclude that nonpolar tails of unbound lipids were exposed at the setal surface ( 8 , 15 , 31 , 33 ). However, the water contact angle of gecko setae does not change even after delipidization (chemical treatment to remove unbound lipids), which raises questions about the orientation of the unbound lipids ( 15 ). Orientation of unbound lipids on the setal surface is important because if headgroups are exposed on the setal surface, the setae could interact with substrates using hydrogen bonding, capillary forces, and electrostatic forces, in addition to vdW forces.…”

A bioinspired strategy for designing well-ordered nanotubular structures by templateless electropolymerization of thieno[3,4- b ]thiophene-based monomers

“…Given that these structures cause unexpected and not yet theoretically understood surface behaviour, further study and characterization of this class of interfaces are needed. Beyond their overall impact on surface properties and applications in interfacial engineering, vertically aligned structures are also omnipresent in nature (gecko toe pads, plant and leaf surfaces) and biology (tissue architectures) [11][12][13][14][15][16][17][18][19][20]. Therefore, identifying methods to replicate these naturally formed structures provides unique avenues to study natural systems and also to implement bioinspiration when addressing engineering challenges.…”