Gecko-Inspired Adhesive Mechanisms and Adhesives for Robots—A Review

Abstract

Small living organisms such as lizards possess naturally built functional surface textures that enable them to walk or climb on versatile surface topographies. Bio-mimicking the surface characteristics of these geckos has enormous potential to improve the accessibility of modern robotics. Therefore, gecko-inspired adhesives have significant industrial applications, including robotic endoscopy, bio-medical cleaning, medical bandage tapes, rock climbing adhesives, tissue adhesives, etc. As a result, synthetic adhesives have been developed by researchers, in addition to dry fibrillary adhesives, elastomeric adhesives, electrostatic adhesives, and thermoplastic adhesives. All these adhesives represent significant contributions towards robotic grippers and gloves, depending on the nature of the application. However, these adhesives often exhibit limitations in the form of fouling, wear, and tear, which restrict their functionalities and load-carrying capabilities in the natural environment. Therefore, it is essential to summarize the state of the art attributes of contemporary studies to extend the ongoing work in this field. This review summarizes different adhesion mechanisms involving gecko-inspired adhesives and attempts to explain the parameters and limitations which have impacts on adhesion. Additionally, different novel adhesive fabrication techniques such as replica molding, 3D direct laser writing, dip transfer processing, fused deposition modeling, and digital light processing are encapsulated.

Review of Vegetable Nanolubricants for Tribological Applications

“…In order to further stress the lubricant and obtain tribological properties from them, it is observed that researchers prefer to use the 4-ball method or the pin on disk method, together they are more than 50% of the tests reported in the literature, for example Reeves et al [8] and Shafi et al [11] both investigated avocado oil with similar pin on disk equipment, Shafi et al [11] reported abrasive wear traits but primarily scuffing wear, on the other hand Reeves et al [8] reported a wear mechanism very similar to that reported by Shafi et al [11], this tells us that the reproducibility in tribological equipment can be reliable if standard methods of measurement of tribological characteristics are followed. As can be seen in Table 3, important reductions in COF and wear are obtained thanks to the action of NPs with very low concentrations of weight % (less than 1.5%), for example, for canola oil Omrani et al [21] and Sikdar et al [22] achieved important reductions in the coefficient of friction of up to 84% and both report a scuffing type wear using NPs of GNP which coincides with the ball milled shape. Both used a tribotester with rotary sliding motion such as ball on disk and pin on disk.…”

“…Table 3 presents various studies found in the literature of vegetable lubricants filled with NP additives and their impact in COF and wear. For canola oil, GNPs and hBN were able to lower COF by 52% and 40%, respectively [22]. NPs of hBN in jatropha oil also reduced wear by 20.28% [26], whereas in olive oil the enhancement in wear was of 13% [17].…”

“…5b, the NPs form a thin protective layer between the friction surfaces, reducing friction and wear. Several authors have reported this lubrication mechanism in vegetable oils modified by the addition of different NPs [11,17,[38][39][40][41]46,47,21,22,24,26,27,29,33,37]. The other main category is related to the presence of NPs on surface enhancement, including the mending and polishing effects.…”

“…In the polishing effect, Fig. 5d, the hard NPs act as an abrasive reducing the roughness of the rubbing surfaces, resulting in a decrease in friction [10, [21][22][23][24]42]. Finally, Laminar-shape NPs such as GNPs and hBN are weakly held together by van de Waals bond, therefore, they exfoliate during friction and the layers are transferred to asperities providing a tribofilm [48] (Fig.…”

Bioinspired Dry Adhesives for Highly Adaptable and Stable Manipulating Irregular Objects under Vibration

“…More importantly, structural design only on the micro-end layer cannot promote the performance of adhesion stability. Although a great deal of work has been done to improve adaptability by introducing soft foam backing into adhesion-based robotic systems, [11,[37][38][39] it is still difficult to avoid the problem of cracks spreading easily due to continuous interfaces. In this case, even if an ideal contact is established based on the compliance of the soft backing, the low tolerance of the continuous adhesion interface to defects results instable gripping of target objects with complex abrupt contours.…”