Biomechanics of Marine Suction Cups and Applications to Artificial Suction Technology

Table of Contents

Abstract

Suction cups are important adhesive adaptations for many marine animals, allowing for locomotion, predation, stability, and grasping of objects. Interesting morphological adaptations have allowed the suction cups of distinct species to best accommodate the purpose of the suctioning mechanism as well as the unique environment that the organism inhabits. The structures of natural suction cups are often far more sophisticated than any human-made homolog, and their specific features have increasingly provided inspiration for many mechanical advancements. The suctioning concept has been applied to wall-climbing robots, mechanical gripping mechanisms, and even underwater adhesive sensing skins. This paper explores the specific morphology and physical mechanisms of suction cups in remoras, octopuses, and clingfish, as well as the mechanical advancements they have inspired in bio-inspired synthetic suction technology.

Background

Although the morphology of suction cups differs between species, suction cups work through the same basic mechanical mechanism: a force generated between the suction cup and the surface by a pressure differential. Atmospheric or fluid pressure acts upon the surface of all objects exposed to air, with the force directed perpendicularly from the surface to the center of the object’s mass. As a suction cup is compressed against a surface, the fluid inside the suction cup is evacuated, creating a vacuum with zero pressure inside of the suction cup. Assuming that the surface that the suction cup is attached to is rigid, smooth, and uniform, the net force on the suction cup is the force created by the external pressure pushing the suction cup downwards at the center of the suction cup and securing it on the surface. Then, the force needed to dislodge the suction cup is proportional to the product of the external pressure with the contact area of the suction cup (Purdue, 2019).

The basic suctioning mechanism has been enhanced in nature by several phenomena, notably viscous flow. Viscous flow is a powerful mechanism of adhesion useful for animals too large to rely on capillary action or intermolecular forces alone for adhesion or locomotion, such as slugs, and plays a significant role in predation for frogs. In the case of slugs, the force that allows the slugs to displace themselves is generated by the combination of their body deformation as well as the forces caused when their viscous, non-Newtonian slime is set in motion. In the case of frogs, whose tongues eject and retract quickly to capture small insects, their sticky saliva creates an adhesive force as it is set in motion, dictated by the laws of viscous flow (Chan & Carlson, 2019).

In simple terms, Newtonian fluids possess a constant viscosity regardless of the stress applied to the fluid. On the contrary, non-Newtonian fluids behave with a nonlinear viscosity response when placed under some non-constant strain or normal stress, caused by motion of the fluid. For example, slugs can attach themselves securely to walls with their glue-like slime, but when a force is generated to move them forward, their slime becomes liquified and smooth. In the case of adhesion mechanisms, we can think of this motion caused when two plates (such as the frog’s tongue or slug’s body and the surface to which it is adhering) move away from each other, causing some shear strain on the liquid between them (Chan & Carlson, 2019). The additive nature of the traditional suction force and the new force created by non-Newtonian gels is illustrated in Figure 1 below.

Fig. 1 Viscous flow (Source: Tsukagoshi & Osada, 2021).

The viscous flow adhesive force, synonymous to Stefan adhesion in this case, is directly proportional to the area of contact between the two surfaces, as well as the viscosity of the substance, and indirectly proportional to the distance between the two surfaces. Additionally, as expected, the faster the surfaces move away from each other, the greater the stress produced in the fluid, and thus the greater the adhesive force. Described quantitatively, the force generated between the sucker and the surface is reduced to Equation 1 below. R4 represents sucker pad size, μ describes the viscosity of the fluid in the cavity, 1/D3(t) is gap size, and dD(t)dt is the speed at which the surfaces move away from each other (Chan & Carlson, 2019).

(1)

In many marine organisms such as remoras, octopuses, and clingfish, this phenomenon of viscous flow has been used to enhance the traditional pressure-based suction cups discussed in the following portion (Chan & Carlson, 2019). The sticky gel or mucus excreted by these organisms not only creates an additional adhesive force but also allows for a more airtight seal between the suction cup and a rough surface and can even be used to release the suction cups more easily (Tsukagoshi & Osada, 2021).

Octopus Suction Cups

The octopus suction cup, more often referred to as a ‘sucker’, is an incredibly versatile tool that allows octopuses to anchor themselves onto both flat and irregular surfaces, hold prey, and execute complicated maneuvers (Packard, 1988). This natural biological feature has been of great interest to researchers as inspiration for new technological advancements. In order to understand possible octopus-inspired mechanical advancements, we must first understand the intricate mechanisms of the octopus sucker.

The sucker is a muscular hydrostat, a biological structure used to manipulate items. It is composed of two main parts: the acetabulum, an upper hollow cup, and the infundibulum, a lower disk-like portion, as seen in Figure 2. When the sucker is adhesively active, the infundibulum is flattened against the surface onto which it wants to attach. A rim of loose and folded dermis and epithelium outlines the infundibulum and separates it from the acetabulum with a circumferential groove. This rim can bend on the sides of thin filaments and sheets in order to help strengthen the grip. The lining that coats the infundibulum extends through an opening and coats the inner surface of the acetabulum. A short muscular base attaches the suckers to the arm which allows the sucker to rotate in any direction and can extend its reach up to twice its resting length. This composition of the octopus sucker makes it extremely efficient and effective in maneuvering and attaching itself to objects and surfaces (Kier & Smith, 2002). 

Fig. 2 Octopus suction cup (Grasso & Setlur, 2007).

The acetabulum and the infundibulum, also referred to as the ‘intrinsic sucker musculature’, consist of a tightly packed, three-dimensional array of muscle (Nachtigall, 1974). Within the sucker wall, there are three main muscle fibers: radial, circular, and meridional.  Radial fibers run throughout the thickness of the sucker, circular fibers run parallel to the opening of the infundibulum, and meridional fibers run from the apex to the bottom of the sucker. This breakdown is shown in Figure 3. The roof of the acetabulum is mainly composed of radial muscle fibers which originate from connective tissue capsules that cover the inner and outer wall surface (Girod, 1884). The roof also consists of meridional muscle fibers which are positioned adjacent to the outer surfaces of the roof and extend between the radial muscle fibers.

Fig. 3 Octopus sucker tissue breakdown (Bagheri et al., 2020).

The wall of the acetabulum has a similar musculature and consists of similar radial and meridional fiber bundles. The third main muscle fiber, circular muscle fibers, are oriented parallel to the surface of the infundibulum. There is a particularly sturdy bundle of these circumferential fibers that are positioned adjacent to the inner surface, therefore forming a large sphincter (a ring-shaped muscle that tightens and relaxes to open/close a passage in the body) at the level of the narrow opening that connects the acetabulum and the infundibulum. The suckers attach to the arms by a series of extrinsic muscle bundles that stem from a connective tissue layer (the previously mentioned ‘base’) that surrounds the arm musculature and then extend downwards to connect with the sucker and insert on the outer connective tissue capsule of the acetabulum. These extrinsic muscle bundles bend the base and allow the sucker to orient itself through selective contraction of a bundle or group of bundles of muscle fibers (Kier & Smith, 1990). 

The fine details of the musculature and overall composition of the sucker are critical to its biomechanical function. There are two major mechanisms that play a role in creating and maintaining suction: pressure reduction in the acetabulum, and formation of a seal with the surface of attachment by the infundibulum. 

Reducing the pressure in the acetabular cavity depends heavily on the muscular mechanism of the sucker. Unlike most organs, the sucker lacks hardened internal and external skeletal material. Instead, the radial muscle of the acetabular wall displays auxetic properties, contracting and generating a force which will in turn decrease the thickness of the wall. Since the wall consists of mostly solid muscle and connective tissue, its volume capacity remains relatively constant. This means that the reduction in thickness of the wall must expand the surface area of the infundibulum, which therefore increases the volume of the acetabular cavity causing water to flow into the cavity through the orifice if a proper seal has not been formed. However, if the sucker is sealed to the surface, the strong interactions between water molecules in the cavity prevent substantial expansion. (Kier & Smith, 1990).

In order to form a watertight seal, the infundibulum must be quite agile. Like the acetabulum, a muscular-hydrostat system controls the infundibulum’s motions. When the radial muscle contracts, the infundibular wall thins, and the infundibular surface expands. The circumference and surface area of the infundibulum are also reduced when the meridional muscles act and antagonize the radial muscles upon contraction. This simultaneous action of contraction flattens the infundibular surface and aids in bending the rim of the infundibulum towards the acetabulum. This mechanism is especially advantageous for the formation of the seal since, with the appropriate neuromuscular control, incredibly complicated bends and deformations can occur at any location to adapt to varied surfaces (Graziadei, 1962). Mucus produced by the epithelium may also be necessary to help form a watertight seal, as discussed in the section on viscous flow. 

A close inspection of the chitinous cuticle that covers the infundibulum’s surface reveals “denticles” as well as numerous grooves that extend from the infundibulum’s center to its rim (Nixon and Dilly, 1977). These 3–4 nanometer-diameter denticles, shown in Figure 4, help with sucker attachment by supplying a network of interconnected water-filled pockets that can create the initial sub-ambient pressure. When the sucker is attached to a surface, it must resist both perpendicular forces trying to lift the sucker from the surface, as well as the forces trying to make the sucker slide along the surface (Denny, 1988). Since octopuses are most often seen with their arms parallel to the line of force, denticles may serve an additional purpose of increasing the force of friction between the rim and the surface. The gradual wear and tear from this exerted friction may contribute to periodic shedding of the sucker linings.

Fig. 4 Infundibulum surface with denticles (Kier & Smith, 2002).

Octopuses can stay attached to surfaces for an extended period which suggests that the acetabular roof, responsible for contraction, may have a mechanism for storing elastic energy and maintaining pressure. When the acetabular roof is thickened by contraction, it is theorized that there is elastic energy stored in the meridional and circular muscle fibers, allowing for long-term attachment. When these muscles relax, the energy stored in the fibers produces a force which thins the wall and decreases the pressure in the acetabular cavity, allowing the octopus to detach voluntarily and quickly (Kier & Smith, 2002).

Remora Suction Cups

Remora fish are any of the eight species that belong to the Echeneid family. The Remora remora species is most well-known for the suction cup that is located on the top of its head, and for its mutualistic relationships with large marine animals. The remora fish attaches itself to large predators such as sharks, dolphins and turtles, gaining access to food, protection from predators, and transportation. The host benefits from this relationship as the remora feeds off the dangerous parasites that try to harm the host (Leao, 2002).

The remora fish’s suction cup, as seen in Figure 5, allows it to attach itself to the very irregular surface of the skin of its host and remain rigidly attached despite water resistance and the inertia created by the predator’s movements. Furthermore, the remora is also able to detach itself voluntarily in an impressively brief time span to be able to feed. The dorsal suction pad is a disk-shaped, adapted first dorsal fin that, through evolution, was able to develop a suction mechanism which has inspired many research initiatives due to its impressive features (Gamel et al, 2019).

Fig. 5 View from below of remora suction disk with magnified spinule image (Wang et al., 2022).

An anatomical analysis of the composition of the suction pad is needed to understand the complexity of the attachment mechanism of the remora. The lip tissue that surrounds the suction pad is composed of three layers. The outer skin layer, which encounters the skin of the predator, is made of connective tissue with dense, coarse collagen fibers. The middle layer contains vertically aligned collagen fibers, giving the elastic lips a high tensile modulus (resistance to deformation by stretching) and low compression modulus (easily deformable through compression). The lowermost layer, or the lip, is composed of compact collagen bundles (Su et al, 2020). This composition allows the lips to act as the suction seal for the adhesion process. During the first contact of the suction cup and the surface, the space between the two is reduced, which expels the water from the cavity. When the suction cup tries to go back to its original shape, a vacuum is created between the surface and the apparatus. This low-pressure zone creates a substantial pressure-differential, allowing the lips to remain attached to the surface. In general, the adhesion due to a suction cup is equal to the product between the pressure differential generated by the cup and sucker area. This gives the formula F = PA. However, this equation does not account for the multiple other factors that go into the mechanism, and thus, a more in-depth analysis of the remora adhesion is required (Chen, 2020).

To regulate the adhesion and separation from the host, push-rod mechanoreceptor complexes are found in the epithelium of the remora’s fish adhesive disc. These take a dome shape that pushes upwards under the upper-most layer of the skin, as shown in Figure 6. Three vesicle chains contain the sensory nerves, which are connected by spinal nerves and arranged in such a way that stretching or compression of the skin does not alter nerve communication. The pushrod mechano-receptors have been found mostly in the anterior side of the suction mechanism, leading to the conclusion that both sticking and unsticking are most efficient when initiated from the anterior side of the fish (Cohen et al, 2020). The collection of mechanoreceptors is used to regulate the rotation of the pectinate lamellae found inside the suction disk through erector and depressor muscles, which change the pitch of the spinules.

Fig. 6 (a) Schematic of overall lip histology.  (b) Zoom-in on push-rod receptor complex (Cohen et al., 2020).

The lamellae are a combination of adapted pterygiophores, and range in number from 18 to 28. Three large erector muscles attach to the ventral processes and manage to increase the angle between the lamellae and the surface (Fulcher and Motta, 2006). This mechanism is essential for the creation of sub-ambient pressure which gives the remoras their impressive attachment capabilities. Furthermore, the lamellae are covered with two to three rows of small spines called spinules, cone-shaped structures involved in the creation of negative pressure. In fact, when the erector muscles are engaged, the spinules become more vertical with respect to the skin of the host. The change in spinule pitch has two major consequences: the increase in volume of the space between the suction cup and the target’s skin, and the creation of friction by the spinules over the irregular shape of the host. The increased volume allows for a higher vacuum to be created, which increases suction strength, while spinule friction enhances the adhesion by allowing the suction cup to resist external forces from strong currents in any direction (Beckert et al, 2015).

When the remora is attached to a host, it is subjected to ever-changing water drag and shear loads at exceedingly high speeds. Furthermore, the irregular nature of the surface of, for example, shark skin would make any synthetic suction cup ineffective. Therefore, without the presence of spinules, remora fish would be unable to survive. The shape of the spinule tip is the main factor that contributes to the creation of friction. The ‘wavelength’ is a property of the spinule binding that defines the distance between asperity peaks on the surface the remora attaches to, and this property is directly correlated to tip shape. Maximum friction is generated exactly at the minimum wavelength of each spinule tip. Varying spinules tips with different sharpness are found all over the surface of the lamellae. This guarantees that for any given surface the fish might want to attach to, there are enough spinules available to create the necessary friction (Beckert et al, 2015). The suction created by the soft lips also helps increase the affinity between the spinules and the skin of the host, lifting the dendrites of the marine animal’s skin for a tighter seal. Figure 7 shows a spinule interacting with the shark’s dendrites. This mechanism is so effective that the force required to separate a remora from plexiglass can be half of what is required to detach it from shark skin. 

Fig. 7 Schematic of a spinule making contact with the dendrites of a shark’s epidermis (Beckert 2015).

When the suction due to the lips lifts the denticles of the shark’s epidermis, the spinule tip can better fit in-between the dendrites, enhancing friction. The friction generated by the spinule will not only depend on the coefficient of friction between the tip and the dermis, but also on how well the spinule tip penetrates the spaces between dendrites (Beckert, 2015).

The erector muscles and mechanoreceptors allow for constant tuning of the normal and tangential adhesion. This means that a remora can rapidly switch between stages of no attachment, low-friction, and robust attachment. The lips are responsible for normal adhesion control, while the rotating papillae (and the spinules on its surface) tune the tangential adhesion. For the detachment process, the soft lips fold outwards, until the seal is broken. This process can be done in less than ~0.2s (Wang, 2022).

While a remora is attached to a host, the forces at play can be analyzed through a free body diagram. As shown in Figure 8, there are four main forces in action. These forces are drag generated by water resistance (FD), friction force (FF), the adhesion force generated by the pad (FS), and the normal force (FN).

Fig. 8 Free body diagram of a remora attached to an underwater host (Beckert, 2015).

The entire mechanism of the remora fish adhesion has evolved to be able to balance out these forces over extended periods of time. Looking at the horizontal axis, one sees that friction (FF) opposes drag (FD). In this case, the force of drag is dependent on the drag coefficient (CD), the density of the surrounding fluid (ρ), the area occupied by the remora (A), and the speed of the host (U). On the other hand, friction is dependent on the coefficient of friction between the surfaces (µ) and the normal force (FN). It becomes evident that detachment can occur when the drag force overcomes the friction generated by the remora. As seen previously, spinules are used to create friction between the irregular surface of the host and the remora. Through this diagram, it becomes clear how important friction enhancement by spinules is to the remora’s hitchhiking technique. Furthermore, in the vertical axis, the adhesion force (FS) and the normal force interact. The adhesion force is dependent on the sub-ambient pressure generated by the pad (P) and the area of the remora pad (Apad). The soft lips of the remora in junction with the spinules must generate enough constant adhesion force so that the remora can remain attached to its host. Constant regulation of adhesion is also necessary as movement by the host continuously affects force equilibrium  (Beckert, 2015).

Northern Clingfish Suction Cups

The northern clingfish (Figure 9) is a small fish that lives on the North American pacific coast (Aquarium Bay Aquarium, n.d.). Its small size would make it prone to get carried away by a strong current. However, they have evolved a suction disk that is able to withstand a pull force that is 80-250 times stronger than the weight of the fish (Huie & Summers, 2020). They can also use their suction disk for hunting food by using their suction cup to rip limpets off the rocks they are attached to (SciFri, 2014). To resist the strong current forces, their suction cups must resist strong pull forces as well as sliding forces that would eventually allow water inside a suction cup and break the suction. Therefore, northern clingfish had to evolve a mechanism for creating high friction between the suction disc and the substrate. 

Fig. 9 Northern clingfish attached to a rock (Ditsche & Summers, 2019).

Clingfish have a single suction disc on the ventral side of their body. It is believed that this suction disc has evolved from pectoral and pelvic fins. In the clingfish’s suction disc, these fins now form suction chambers, cavities where negative pressure is maintained. They also possess two additional cavities, one anterior and one posterior to the suction chamber, that contribute to suction by maintaining sub-ambient pressure (Sandoval et. al., 2020). The suction disc itself is made up of soft tissues that allow the disc to bulge during the adhesion process to generate negative pressure (Arita, 1967). Moreover, the softness of the disc allows it to adapt to a wide variety of surfaces to create a proper seal (Ditsche & Summers, 2019).

Fig. 10 Smaller hexagonal papillae (p) on a clingfish suction cup (c); microscopic rods (r) that cover papillae (d); filaments at the tips of the rods (e) (Ditsche & Summers, 2019).

The suction caused by the negative pressure generated in these suction chambers provides the main method of adhesion, but they also rely on friction on a microscopic level. The suction disc’s margin is covered by a hierarchy of increasingly small structures (Figure 10) that are theorized to contribute to friction (Sandoval et. al., 2019). The largest structures are the epidermal papillae which can be seen with the naked eye. These papillae are covered by tiny rods of approximately 5 μm that are themselves divided into filaments of approximately 0.2 μm (Ditsche & Summers, 2019).

In addition to creating friction between the suction disc’s margin and the substrate, this hierarchy of structures allows the suction disc to adapt to microscopic irregularities in a substrate’s surface to create a watertight seal. This particularity is extremely important to clingfish because they need to be able to stick to rough and irregular surfaces like rocks. Because of this unique feature, experiments on dead fish and fish with severed nerves have shown that clingfish are able to maintain adhesion without muscle involvement, minimizing energy usage (Arita, 1967).

The process of creating suction starts before the fish contacts the substrate.  They first use their ventral muscles to flatten the suction cup to reduce its volume. An interesting difference between the clingfish suction disc and other suction cups is the fact that the margins have two slits. J. A. Sandoval et. al. (2019) used finite element analysis (FEA) to study the effect of these slits on this initial compression step to reduce volume as shown in Figure 11. 

Fig. 11 FEA of the two suction cup models used. a) i) and ii)  Show the simulation with an equivalent stress (Pa) while being constrained radially. iii) and iv) Show the unconstrained simulation for an equivalent stress (Pa) (Sandoval et. Al., 2019).

For the comparison, they used the geometry of an artificial circular suction cup and the geometry of a bioinspired suction cup with four slits, their best-performing bioinspired suction cup. The FEA showed that the suction cup with the four slits had 17% less total strain energy. The lower strain energy means that it is easier to lower the volume until the clingfish contacts the surface and can relax its muscles. After this, the natural tendency of the disc’s tissues to revert to their original shape returns the cup to its usual cup shape which will increase its volume (Arita, 1967). If the soft cup in combination with the collection of small surface structures succeeded at creating a watertight seal with the surface, then, this increase in volume will result in a decrease in pressure.

If the clingfish is modeled as a rigid body, the current applies a horizontal force that tries to slide the fish away from its initial position as well as a vertical force that tries to lift it off the substrate. The friction generated by the margin’s microstructure and the normal force is what prevents the fish from sliding. In addition to that, the external pressure applies a vertical force that prevents the suction cup from lifting off when a vertical pulling force is applied. For suction cups, this force can usually be approximated as the product of the external pressure and the margin’s area. It is also believed that the mucus that covers the clingfish’s papillae helps it  adhere to surfaces through Stefan adhesion, the force generated by viscous fluids to resist the separation with the substrate (Wainwright et. al., 2013). Therefore, the total vertical force that resists pulling would be equal to the sum of the force generated by external pressure and Stefan adhesion when the fish is modeled as a rigid body. However, experimental data shows that the maximum vertical load that a clingfish suction cup can resist grows with the friction between the suction cup and the surface (P. Ditsche & A. Summer, 2019). Because the suction cup is a deformable body, the pulling force will try to slide the margins toward the center of the suction cup (Figure 12).  

Fig. 12 Main failure points for traditional suction cups on smooth and irregular surfaces (Wainwright et al., 2013).

This would create bulging and break the watertight seal, causing water to flow inside, and eliminating the pressure differential. The bones that support the clingfish’s suction cup are in part responsible for preventing the margin from slipping inward, and the secondary friction-based adhesion of the papillae helps counteract leakage. This combination of factors makes it difficult to get a direct equation for the maximum load that can be sustained by a suction cup. Moreover, the combination of Stefan adhesion with the passive friction-based adhesion means it does not take much energy to maintain adhesion. However, the fish can increase adhesion by contracting their dorsal muscles which expands the volume up to a certain point, and muscles are used to release the suction disc from the substrate. On the other hand, the releasing process heavily involves the activation of the dorsal muscles that push down on the inside of the suction chamber to reduce the volume and the pressure difference. Then, they pull on the disc’s margin to break the seal. This mechanism is supported by the fact that fish whose nerves had been severed such that they would not be able to use their dorsal muscles were not able to release themselves normally (Arita, 1967).

Bio-Inspired Advancements in Suction Technology

Remora-Inspired Robot

Inspired by the remora’s remarkable suction disk, Siqi Wang from the University of Beijing and a group of scientists developed a robot that could mimic the adhesion capabilities of the remora (Wang  et al., 2022). The focus of the design was not only to achieve a suction force like that of the remora, but to also allow for full control over the attachment and detachment mechanisms. This robot provides the groundwork needed for untethered, autonomous robots that could perform underwater expeditions to study marine life and collect delicate biological samples through remora-like “hitchhiking” in areas too difficult or unsafe for humans to reach (Wang et al., 2022).

To study remora anatomy, the group dissected a euthanized remora’s suction cup, which was later embedded in wax and sectioned. This allowed for examination of the morphology of the pectinate lamellae and of the spinules. Additionally, locomotive and force analysis was done on a living remora. The remora was trained to attach to a smooth acrylic plate at the bottom of the tank, which had a highly accurate, water-resistant six-axis force sensor.

To test the effect of the disc lisp modulus on the strength of the suction, the team utilized three different silicon-based materials with varying moduli (69, 337 and 662 kPa). Lamellae and a disc base were 3D printed on a soft mold. Laser technology was used to create spinules, which were 200 μm thick, later embedded on the lamellae. Finally, the compression-rotation and compression-extension mechanisms for the lips and the lamellae, respectively, were attached to the dorsal side of the robot. It was found that the 69 kPa modulus material for the soft lip was the most effective in terms of friction and for adhesion capabilities.

After the design was complete, the robot was tasked with imitating the way a remora attaches to another being. For this, the robot must be able to have three different gluing states: zero adhesion (used for skimming), in which the fish is separated by a noticeably short distance from its target; low-friction adhesion, meaning that the lips have attached to the surface but the lamellae have are not yet erect, which allows for sliding; and finally robust adhesion, which is when the remora finally settles and the lamellae are fully erect. The three stages of adhesion are illustrated in Figure 13. The design of the robot requires only 2 actuators which operate under only one degree of freedom, which gives greater accessibility and easier reproducibility for future design iterations.

Fig. 13 Photographs of the biomimetic disc prototype show mode switching between three friction states (Wang et al., 2022).

The robot’s mechanism allowed it to control the curling of the lips, which is the major factor for the first part of the adhesion process and for detachment. The lamellae actuators controlled their spin, which in turn decided when robust adhesion or low-friction adhesion was performed. The maximum pull-up stress for the mechanism was measured to be 47 kPa, and a load of only 0.1 N was required for the mechanism to be able to attach to a surface. Frictional force control ranges were measured to be between 10-1  and 10 –11 N.  This research also helped demonstrate that changing the stiffness of the lips had a significant impact on the adhesion forces. There is an evident trade-off between the stiffness of the lips and the suction strength. An incredibly soft lip can compress and attach itself more easily to surfaces, but it is also less resistant to forces pulling upwards. In the remora’s case however, this is circumvented by a system of blood vessels that can change the stiffness of the lips.

Octopus-Inspired Underwater Attachment

Based on the cephalopods’ impressive reversible underwater attachment mechanism, Sean T. Frey and other scientists have developed intelligent adhesive skins that work underwater and are rapidly switchable (Frey et al., 2022).

The biggest challenge for the design team was to incorporate the numerous sensing mechanisms that the octopuses utilize that allow them to control their gluing to different surfaces, and that give the adhesion reversibility. A soft, adaptable silicone stem covered by a pneumatically activated membrane controls adhesion. Micro-light detection and ranging optical proximity sensors were used in conjunction with the soft elements to give real-time micro control of adhesion.  The network of sensors and mechanisms emulated the nervous system of a cephalopod, providing dexterity and control both underwater and in dry environments. 

The apparatus can reliably attach and detach from surfaces multiple times without a decrease in effectiveness, and the adhesives can be turned from fully on to fully off in less than 0.1 second. The ability to change the geometry of the stalk (for toughness) and the geometry of the membrane (for strength and release) gives the mechanism active and constant control over the suction force being applied. Toughness gives the mechanism the opportunity to deform while still maintaining a firm grip on the object it is attached to. Strength of the adhesion allows for heavy objects to be lifted, and the release mechanism allows for rapid freeing from attachment despite the toughness and strength. 

Fig. 14 (A) Wearable adhesive glove with integrated adhesives, sensors and processors (B) Cross-section of a finger illustrating the embedded sensor and adhesive element (Frey et al., 2022).

The suction cup mechanism was later integrated into a glove, as shown in Figure 14, to allow for manipulation. Every fingertip is dotted with an active adhesive element and a micro-Lidar optical sensor. When the sensors detect an object nearby, the active adhesive mechanisms are triggered. The mechanism can be fine-tuned to varying sizes of objects by altering the way the sensors detect nearby objects. By determining a threshold for the number of sensors that must sense an object before triggering the attachment mechanism, larger or smaller objects can be picked more easily and more precisely. Therefore, the object was shown to be able to attach and detach, underwater, to objects of varying surfaces and shapes. The progression of the sensing stages as a function of time is shown in Figure 15.

Fig. 15 Sequence of the sensorized adhesive followed by switched release (Frey et al., 2022).

The ability to attach with considerable force to numerous surfaces even under imperfect conditions is a great advancement in underwater technology. Through testing, it was demonstrated that the suction cups were at their most effective when ninety percent of their surface was in contact with the object. This wiggle room allows them to glue onto irregular surfaces at non-ideal angles while still maintaining a constant grip. Future research on the ideal indices of strength and toughness can further improve on this attachment to irregular surfaces, and other types of sensing (chemical or mechanical), and different mechanisms for the stalk and membrane changes could also greatly advance this technology.

Clingfish-Inspired Artificial Suction Cups

Two interesting studies have attempted to replicate the advantages of clingfish adhesive discs in passive suction cups, each focusing on a specific evolutionary advantage of the clingfish suction cups. Biomimicking clingfish suction cups may be useful in certain applications where objects need the ability to adhere to surfaces with varying levels of roughness both underwater and above the surface. For instance, J. A. Sandoval et. al (2019) show that their biomimetic suction cups (Figure 16) were able to adhere to objects while in the air and to everyday items in water. The only action required by a robot would be to flatten the suction cup to create a pressure differential. This could be particularly useful in remotely operated underwater vehicles where the strong grip of robotic arms could be replaced with soft suction cups, reducing the risk of breaking objects such as delicate archeological artifacts.

Fig. 16 Bioinspired suction cups where the seal of the disc margin (dm) and the suction chamber (sc) are made of different materials (scale bar 5mm). c) Shows a suction cup with two slits to more closely mimic a clingfish suction cup while b) shows the best performing geometry with 4 slits (Sandoval et. al. 2019).

The first factor that gives a mechanical advantage to clingfish suction cups compared to simple artificial ones is the support given by the bones that prevents buckling and subsequent leakage. To replicate this, one can design a suction cup that uses a soft material that replicates the lips and a stiffer material that replicates the bones. In the study of P. Ditsche & A. Summers (2019), this kind of suction cup was better able to adhere to rougher surfaces than simple suction cups but with a slightly lower tenacity . A similar result was observed with J. A. Sandoval et. al. (2019) where they found that soft lips with any kind of geometry outperformed single material suction cups. Moreover, their work shows that adding slits inspired by a clingfish sucker cup can improve performance, since the addition of slits helps the sucker adapt to the substrate without buckling.

The second factor to replicate is the additional friction generated by papillae and their hierarchically smaller structures. P. Ditsche & A. Summers (2019) offer two separate ways of creating friction with a suction cup. The first method is to mold the silicon of the outer lips on sandpaper. This simple approach works for generating additional friction but is limited by the impossibility of creating such complex microstructures. Their second method involved the use of a soft material with stiffer inclusions that increase friction. Another approach is tested by J. A. Sandoval et. al. (2019), the researchers molded square prisms on the surface of the soft lips. They made two prototypes with these micropillars, one made of stiff silicon, and one made of soft silicon. They found that soft micropillars outperformed hard micropillars and theorized that it is because softness helps the microstructures create a better seal. The suction cups with soft micropillars still reduced performance, but the authors noted that more inquiry may be needed in this method because the geometry of papillae was not fully replicated.

Conclusion

The specialized suction cups of octopuses, the remora fish, and the northern clingfish represent sophisticated natural feats of engineering that have allowed each organism to adapt seamlessly to their unique environment and competitive needs. The traditional pressure-differential suctioning function of the octopus is enhanced by a coordinated symphony of radial muscle fibers controlled by neuromuscular feedback, allowing for complicated deformations on uneven surfaces. In the remora fish, the changing pitch of an intricate system of sharp spinules produces a friction-enhanced suction mechanism tailored for attachment to the skin of large predators. The small northern clingfish, prone to being carried away by strong tides, also utilizes a hierarchical system of friction-creating filaments in combination with a soft and moldable suction cup, adapted specifically to attaching itself to highly irregular surfaces. Such marine animals showcase some of the most powerful suctioning mechanisms in the natural world, far surpassing human-made suction designs of the past. Studying the biomechanics of these natural devices allows us to better understand how evolution has improved these mechanisms through the addition of interesting new adaptations. Taken together, this study increases our understanding of the physiology of these different aquatic mammals and how their suction mechanisms provide competitive advantages tailored to their specific environment, in turn informing future biomimetic applications in the field of suction technology.

References

  1. Aquarium Bay Aquarium. (n.d.). Northern clingfish.  https://www.montereybayaquarium.org/animals/animals-a-to-z/northern-clingfish
  2. Arita, G. S. (1967). A comparative study of the structure and function of the adhesive apparatus of the Cyclopteridae and Gobiesocidae (T). University of British Columbia. Retrieved from https://open.library.ubc.ca/collections/ubctheses/831/items/1.0104391
  3. Bagheri, H., Hu, A., Cummings, S., Roy, C., Casleton, R., Wan, A., Erjavic, N., Berman, S., Peet, M. M., Aukes, D. M., He, X., Pratt, S. C., Fisher, R. E., & Marvi, H. (2020). New Insights on the Control and Function of Octopus Suckers. Advanced Intelligent Systems, 2(6), 1900154. https://doi.org/10.1002/aisy.201900154
  4. Beckert, M., Flammang, B. E., & Nadler, J. H.. (2015). Remora attachment is enhanced by spinule friction. Journal of Experimental Biology, 218(22), 3551–3558. http://doi.org/10.1242/jeb.123893
  5. Beckert, M. Mechanics of remora adhesion, Georgia Institute of technology, August 2015. book-37703.pdf (mobt3ath.com)
  6. Chan, T.S., Carlson, A. Physics of adhesive organs in animals. Eur. Phys. J. Spec. Top. 227, 2501–2512 (2019). https://doi.org/10.1140/epjst/e2019-800131-2
  7. Chen, Y., Meng, J., Gu, Z., Wan, X., Jiang, L. et Wang, S.. (2020). Bioinspired Multiscale Wet Adhesive Surfaces: Structures and Controlled Adhesion. Advanced functional materials, 30(5), 1905287. http://doi.org/10.1002/adfm.201905287
  8. Cohen, K. E., Crawford, C. H., Hernandez, L. P., Beckert, M., Nadler, J. H. et Flammang, B. E.. (2020). Sucker with a fat lip: The soft tissues underlying the viscoelastic grip of remora adhesion. Journal of anatomy, 237(4), 643‑654. https://doi.org/10.1111/joa.13227
  9. Cohen, K. E., Flammang, B. E., Crawford, C. H., & Hernandez, L. P.. (2020). Knowing when to stick: touch receptors found in the remora adhesive disc. Royal Society Open Science, 7(1), 190990. http://doi.org/10.1098/rsos.190990
  10. Denny, M. W. 1988. Biology and the mechanics of the wave-swept environment. Princeton University Press, Princeton, New Jersey.
  11. Ditsche, P., & Summers, A.. (2019). Learning from Northern clingfish ( Gobiesox maeandricus ): bioinspired suction cups attach to rough surfaces. Philosophical Transactions of the Royal Society B: Biological Sciences, 374(1784), 20190204. http://doi.org/10.1098/rstb.2019.0204
  12. Frey, S. T., Haque, A. B. M. T., Tutika, R., Krotz, E. V., Lee, C., Haverkamp, C. B., Markvicka, E. J., & Bartlett, M. D. (2022). Octopus-inspired adhesive skins for intelligent and rapidly switchable underwater adhesion. Science Advances, 8(28), eabq1905. https://doi.org/doi:10.1126/sciadv.abq1905
  13. Fulcher, B.A. ,  Motta, P.J. (2006) Suction disk performance of echeneid fishes. Can. J. Zool. 84: 42–50. https://doi.org/10.1139/Z05-167
  14. Gamel, K. M., Garner, A. M., & Flammang, B. E.. (2019). Bioinspired remora adhesive disc offers insight into evolution. Bioinspiration & Biomimetics, 14(5), 056014. http://doi.org/10.1088/1748-3190/ab3895
  15. Girod, P. 1884. Recherches sur la peau des céphalopodes. La ventouse. Arch. Zool. Exp. Gen, 2379-401.
  16. Grasso, Frank & Setlur, Pradeep. (2008). Inspiration, simulation and design for smart robot manipulators from the sucker actuation mechanism of cephalopods. Bioinspiration & biomimetics. 2. S170-81. 10.1088/1748-3182/2/4/S06. 
  17. Graziadei, P. P. C. 1962. Receptors in the suckers of Octopus. Nature (London), 19557-59.
  18. Huie, J. M., & Summers, A. P.. (2022). The effects of soft and rough substrates on suction-based adhesion. Journal of Experimental Biology, 225(9). http://doi.org/10.1242/jeb.243773
  19. Kier, W. M., and A. M. Smith. 1990. The morphology and mechanics of octopus suckers. Biol. Bull. (Woods Hole), 178126-136
  20. Kier, W. M., & Smith, A. M. (2002). The Structure and Adhesive Mechanism of Octopus Suckers. Academic OUP; Department of Biology, University of North Carolina, Chapel Hill. https://tinyurl.com/4rdmevc9 
  21. Leao, M. (2002). Remora remora Brown sucker. Animal Diversity Web. https://animaldiversity.org/accounts/Remora_remora/
  22. Nachtigall, W. 1974. Biological mechanisms of attachment: The comparative morphology and bioengineering of organs for linkage, suction, and adhesion. Springer-Verlag, New York.
  23. Nixon, M., and P. N. Dilly. 1977. Sucker surfaces and prey capture. Symp. Zool. Soc. Lond, 38447-511.
  24. Packard, A. 1988. The skin of cephalopods (Coleoids): General and special adaptations. In E. R. Trueman and M. R. Clarke (eds.), The Mollusca, form and function, 11, pp. 37–67. Academic Press, Inc., San Diego.
  25. Purdue University. (2019). Demos: 2A-01 Suction Cups. Department of Physics and Astronomy. https://www.physics.purdue.edu/demos/display_page.php?item=2A-01 
  26. S. Su, S. Wang, L. Li, et al. (2020). Vertical Fibrous Morphology and Structure-Function Relationship in Natural and Biomimetic Suction-Based Adhesion Discs, Matter-US, 2 1–15. https://doi.10.1016/j.matt.2020.01.018
  27. Sandoval, J. A., Jadhav, S., Quan, H., Deheyn, D. D., & Tolley, M. T.. (2019). Reversible adhesion to rough surfaces both in and out of water, inspired by the clingfish suction disc. Bioinspiration & Biomimetics, 14(6), 066016. http://doi.org/10.1088/1748-3190/ab47d1
  28. Sandoval, J. A., Sommers, J., Peddireddy, K. R., Robertson-Anderson, R. M., Tolley, M. T., & Deheyn, D. D.. (2020). Toward Bioinspired Wet Adhesives: Lessons from Assessing Surface Structures of the Suction Disc of Intertidal Clingfish. ACS Applied Materials & Interfaces, 12(40), 45460–45475. http://doi.org/10.1021/acsami.0c10749
  29. SciFri. “This Fish Sucks: The Northern Clingfish” Youtube, 27 Feb. 2014, https://www.youtube.com/watch?v=SySkRqjEb5w.
  30. Tsukagoshi, H., & Osada, Y. (2021). Soft Hybrid Suction Cup Capable of Sticking to Various Objects and Environments. Actuators. 10(3), 50.  https://www.mdpi.com/2076-0825/10/3/50 
  31. Wainwright, D. K., Kleinteich, T., Kleinteich, A., Gorb, S. N., & Summers, A. P.. (2013). Stick tight: suction adhesion on irregular surfaces in the northern clingfish. Biology Letters, 9(3), 20130234. http://doi.org/10.1098/rsbl.2013.0234
  32. Wang, S. et al. (2022) A biomimetic remora disc with tunable, reversible adhesion for surface sliding and skimming, Biomimetics, 17, 036001. https://doi.org/10.1088/1748-3190/ac4e7a
  33. Wang, Y., Yang, X., Chen, Y., Wainwright, D. K., Kenaley, C. P., Gong, Z., Liu, Z., Liu, H., Guan, J., Wang, T., Weaver, J. C., Wood, R. J., & Wen, L. (2017). A biorobotic adhesive disc for underwater hitchhiking inspired by the remora suckerfish. Science robotics, 2(10), eaan8072. https://doi.org/doi:10.1126/scirobotics.aan8072
  34. William M. Kier, Andrew M. Smith, The Structure and Adhesive Mechanism of Octopus Suckers, Integrative and Comparative Biology, Volume 42, Issue 6, December 2002, Pages 1146–1153, https://doi.org/10.1093/icb/42.6.1146