The role of hairs in the adhesion of octopus suckers: a hierarchical peeling approach

Bio-inspired switchable soft adhesion for the boost of adhesive surfaces and robotics applications: A brief review

In nature, millions of creatures, such as geckos, tree frogs, octopuses, etc., have evolved fantastic switchable adhesion capabilities to climb swiftly on vertical even inverted surfaces or hunt for prey easily, adapting to harsh and unpredictable environments. Notably, these fascinating adhesive behaviors depend on interfacial forces (friction, van der Waals force, capillary force, vacuum suction, etc.), which primarily originate from the interactions between the soft micro/nanostructures evolved in the natural creatures and objects. Over the past few decades, these biological switchable adhesives have inspired scientists to explore and engineer desirable artificial adhesives. In this review, we summarized the state-of-the-art research on the ultra-fast adhesive motion of three types of biological organisms (gecko, tree frog, and octopus). Firstly, the basic adhesion principles in the three representative organisms, including micro/nanostructures, interfacial forces, and fundamental adhesion models, are reviewed. Then, we discussed the adhesion mechanisms of the prominent organisms from the perspective of soft contacts between micro/nanostructures and the substrates. Later, the mechanics-guided design principles of artificial adhesive surfaces, as well as the smart adhesion strategies, are summarized. The applications of these bio-inspired switchable adhesives are demonstrated, including wearable electronic devices, soft grippers, and climbing robots. The challenges and opportunities in this fast-growing field are also discussed.

Advanced Bionic Attachment Equipment Inspired by the Attachment Performance of Aquatic Organisms: A Review

In nature, aquatic organisms have evolved various attachment systems, and their attachment ability has become a specific and mysterious survival skill for them. Therefore, it is significant to study and use their unique attachment surfaces and outstanding attachment characteristics for reference and develop new attachment equipment with excellent performance. Based on this, in this review, the unique non-smooth surface morphologies of their suction cups are classified and the key roles of these special surface morphologies in the attachment process are introduced in detail. The recent research on the attachment capacity of aquatic suction cups and other related attachment studies are described. Emphatically, the research progress of advanced bionic attachment equipment and technology in recent years, including attachment robots, flexible grasping manipulators, suction cup accessories, micro-suction cup patches, etc., is summarized. Finally, the existing problems and challenges in the field of biomimetic attachment are analyzed, and the focus and direction of biomimetic attachment research in the future are pointed out.

Analysis of suction-based gripping strategies in wildlife towards future evolutions of the obstetrical suction cup

The design of obstetrical suction cups used for vacuum assisted delivery has not substantially evolved through history despite of its inherent limitations. The associated challenges concern both the decrease of risk of soft tissue damage and failure of instrumental delivery due to detachment of the cup. The present study firstly details some of the suction-based strategies that have been developed in wildlife in order to create and maintain an adhesive contact with potentially rough and uneven substratum in dry or wet environments. Such strategies have permitted the emergence of bioinspired suction-based devices in the fields of robotics or biomedical patches that are briefly reviewed. The objective is then to extend the observations of such suction-based strategies toward the development of innovative medical suction cups. We firstly conclude that the overall design, shape and materials of the suction cups could be largely improved. We also highlight that the addition of a patterned surface combined with a viscous fluid at the interface between the suction cup and scalp could significantly limit the detachment rate and the differential pressure required to exert a traction force. In the future, the development of a computational model including a detailed description of scalp properties should allow to experiment various designs of bioinspired suction cups.

A New Concept for an Adhesive Material Inspired by Clingfish Sucker Nanofilaments

Underwater adhesive materials are in high demand in various fields, and fish species with sucker disks have attracted attention due to their superior performance and interesting structures. The clingfish, in particular, is widely known for using hierarchical sucker disk structures to demonstrate rapid and strong adhesion to rocky surfaces under strong currents. We examined the combination of nanofilaments and mucus in the clingfish sucker disk. Nanofilaments reinforce mucus adhesion force by reducing the compliance without affecting the contact area. We prepared structures from hard polymers and soft polydimethylsiloxane (PDMS) that mimicked clingfish sucker nanofilaments and mucus, with these biomimetic structures showing significant adhesion force underwater. Furthermore, the hardness and length of the nanofilaments and Young's modulus and thickness of the mucus-mimicking PDMS layer had critical effects on the adhesion force. According to the results, clingfish nanofilaments act as hard bracing for the soft mucus, and the structural combination of the conflicting characteristics of hardness and softness, thus achieved, is crucial for strong adhesion.

Self-adhesive hydrogels for tissue engineering

Hydrogels consisting of a three-dimensional hydrophilic network of biocompatible polymers have been widely used in tissue engineering. Owing to their tunable mechanical properties, hydrogels have been applied in both hard and soft tissues. However, most hydrogels lack self-adhesive properties that enable integration with surrounding tissues, which may result in suture or low repair efficacy. Self-adhesive hydrogels (SAHs), an emerging class of hydrogels based on a combination of three-dimensional hydrophilic networks and self-adhesive properties, continue to garner increased attention in recent years. SAHs exhibit reliable and suitable adherence to tissues, and easily integrate into tissues to promote repair efficiency. SAHs are designed either by mimicking the adhesion mechanism of natural organisms, such as mussels and sandcastle worms, or by using supramolecular strategies. This review summarizes the design and processing strategies of SAHs, clarifies underlying adhesive mechanisms, and discusses their applications in tissue engineering, as well as future challenges.

Air-encapsulating elastic mechanism of submerged Taraxacum blowballs

In this article, we report the observation of an air-encapsulating elastic mechanism of Dandelion spherical seed heads, namely blowballs, when submerged underwater. This peculiarity seems to be fortuitous since Taraxacum is living outside water; nevertheless, it could become beneficial for a better survival under critical conditions, e.g. of temporary flooding. The scaling of the volume of the air entrapped suggests its fractal nature with a dimension of 2.782 and a fractal air volume fraction of 4.82 × 10⁻² m^0.218, resulting in nominal air volume fractions in the range of 14–23%. This aspect is essential for the optimal design of bioinspired materials made up of Dandelion-like components. The miniaturization of such components leads to an increase in the efficiency of the air encapsulation up to the threshold (efficiency = 1) achieved for an optimal critical size. Thus, the optimal design is accomplished using small elements, with the optimal size, rather than using larger elements in a lower number. The described phenomenon, interesting per se, also brings bioinspired insights toward new related technological solutions for underwater air-trapping and air-bubbles transportation, e.g. the body surface of a man could allow an apnea (air consumption of 5–10 l/min) of about 10 min if it is covered by a material made up of a periodic repetition of Dandelion components of diameter ≅18 μm and having a total thickness of about 3–6 cm.

Toward Bioinspired Wet Adhesives: Lessons from Assessing Surface Structures of the Suction Disc of Intertidal Clingfish

The clingfish attaches to rough surfaces with considerable strength using an intricate suction disc, which displays complex surface geometries from structures called papillae. However, the exact role of these structures in adhesion is poorly understood. To investigate the relationship between papillae geometry and adhesive performance, we developed an image processing tool that analyzed the surface and structural complexity of papillae, which we then used to model hydrodynamic adhesion. Our tool allowed for the automated analysis of thousands of papillae in specimens across a range of body sizes. The results led us to identify spatial trends in papillae across the complex geometry of the suction disc and to establish fundamental structure-function relationships used in hydrodynamic adhesion. We found that the surface area of papillae changed within a suction disc and with fish size, but that the aspect ratios and channel width between papillae did not. Using a mathematical model, we found that the surface structures can adhere considerably when subjected to disturbances of moderate to high velocities. We concluded that a predominant role of the papillae is to leverage hydrodynamic adhesion and wet friction to reinforce the seal of the suction disc. Overall, the trends in papillae characteristics provided insights into bioinspired designs of surface microstructures for future applications in which adhesion is necessary to attach to diverse surfaces (in terrestrial or aquatic environments), even when subjected to disturbance forces of randomized directionality.