Synthetic setae

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Stickybot, a climbing robot using synthetic setae Stickybot.jpg
Stickybot, a climbing robot using synthetic setae

Synthetic setae emulate the setae found on the toes of a gecko and scientific research in this area is driven towards the development of dry adhesives. Geckos have no difficulty mastering vertical walls and are apparently capable of adhering themselves to just about any surface. The five-toed feet of a gecko are covered with elastic hairs called setae and the ends of these hairs are split into nanoscale structures called spatulae (because of their resemblance to actual spatulas). The sheer abundance and proximity to the surface of these spatulae make it sufficient for van der Waals forces alone to provide the required adhesive strength. [2] Following the discovery of the gecko's adhesion mechanism in 2002, which is based on van der Waals forces, biomimetic adhesives have become the topic of a major research effort. These developments are poised to yield families of novel adhesive materials with superior properties which are likely to find uses in industries ranging from defense and nanotechnology to healthcare and sport.

Contents

Basic principles

Close view of a gecko's foot Gecko foot on glass.JPG
Close view of a gecko's foot

Geckos are renowned for their exceptional ability to stick and run on any vertical and inverted surface (excluding Teflon [3] ). However gecko toes are not sticky in the usual way like chemical adhesives. Instead, they can detach from the surface quickly and remain quite clean around everyday contaminants even without grooming.

Extraordinary adhesion

The two front feet of a tokay gecko can withstand 20.1 N of force parallel to the surface with 227 mm2 of pad area, [4] a force as much as 40 times the gecko's weight. Scientists have been investigating the secret of this extraordinary adhesion ever since the 19th century, and at least seven possible mechanisms for gecko adhesion have been discussed over the past 175 years. There have been hypotheses of glue, friction, suction, electrostatics, micro-interlocking and intermolecular forces. Sticky secretions were ruled out first early in the study of gecko adhesion since geckos lack glandular tissue on their toes. The friction hypothesis was also dismissed quickly because the friction force only acts in shear which cannot explain the adhesive capabilities of geckos on inverted surfaces. The hypothesis that the toe pads act as suction cups was dispelled in 1934 by experiments carried out in a vacuum in which the gecko's toes remained stuck. Similarly, the electrostatic hypothesis was refuted by an experiment showing that geckos could still adhere even when the build-up of electrostatic charge was impossible (such as on a metal surface in air ionized by a stream of x-rays). The mechanism of microinterlocking which suggested that the curved tips of setae could act as microscale hooks was also challenged by the fact that geckos generate large adhesive forces even on molecularly smooth surfaces.

Micro and nano view of gecko's toe Micro and nano view of gecko's toe.jpg
Micro and nano view of gecko's toe

The possibilities finally narrowed down to intermolecular forces, and the development of electron microscopy in the 1950s, which revealed the micro-structure of the setae on the gecko's foot, provided further proof to support this hypothesis. The problem was finally solved in 2000 by a research team led by biologists Kellar Autumn of Lewis & Clark College in Portland, Oregon, and Robert Full at the University of California at Berkeley. [6] They showed that the underside of a gecko toe typically bears a series of ridges, which are covered with uniform ranks of setae, and each seta further divides into hundreds of split ends and flat tips called spatulas (see figure on the right). A single seta of the tokay gecko is roughly 110 micrometers long and 4.2 micrometers wide. Each of a seta's branches ends in a thin, triangular spatula connected at its apex. The end is about 0.2 micrometers long and 0.2 micrometers wide. [5] The adhesion between gecko's foot and the surfaces is exactly the result of the Van der Waals force between each seta and the surface molecules. A single seta can generate up to 200 μN of force. [7] There are about 14,400 setae per square millimeter on the foot of a tokay gecko, which leads to a total number of about 3,268,800 setae on a tokay gecko's two front feet. From the equation for intermolecular potential:

where and are the number of contacts of the two surfaces, R is the radius of each contact and D is the distance between the two surfaces.

We find that the intermolecular force, or the van der Waals force in this case between two surfaces is greatly dominated by the number of contacts. This is exactly the reason why the gecko's feet can generate extraordinary adhesion force to different kinds of surfaces. The combined effect of millions of spatulae provides an adhesive force many times greater than the gecko needs to hang from a ceiling by one foot.

Attach and detach procedure of gecko's foot Lift-off mechanism.jpg
Attach and detach procedure of gecko's foot

Lift-off mechanism

The surprisingly large forces generated by the gecko's toes [8] raised the question of how geckos manage to lift their feet so quickly – in just 15 milliseconds – with no measurable detachment forces. Kellar Autumn and his research group found out the 'Lift-off mechanism' of the gecko's feet. Their discovery revealed that gecko adhesive actually works in a 'programmable' way that by increasing the angle between the setal shaft and the substrate to 30 degrees, no matter how big the perpendicular adhesive force is, geckos 'turn off' the stickiness since the increased stress at the trailing edge of the seta causes the bonds between seta and the substrate to break. The seta then returns to an unloaded default state. On the other hand, by applying preload and dragging along the surface, the geckos turn on the modulate stickiness. This 'Lift-off' mechanism can be shown in the figure on the right.

Self-cleaning ability

Unlike conventional adhesives, gecko adhesive becomes cleaner with repeated use, and thus stays quite clean around everyday contaminants such as sand, dust, leaf litter and pollen. In addition, unlike some plants and insects that have the ability of self-cleaning by droplets, geckos are not known to groom their feet in order to retain their adhesive properties – all they need is only a few steps to recover their ability to cling to vertical surfaces.

Model explaining self-cleaning ability Model for self cleaning.jpg
Model explaining self-cleaning ability

Kellar Autumn and his research group have conducted experiments to test and demonstrate this ability of the gecko. [9] They also use the contact mechanical model to suggest that self-cleaning occurs by an energetic disequilibrium between the adhesive forces attracting a dirt particle to the substrate and those attracting the same particle to one or more spatulae. In other words, the Van der Waals interaction energy for the particle-wall system requires a sufficiently great number of particle-spatula systems to counterbalance; however, relatively few spatulae can actually attach to a single particle, therefore the contaminant particles tend to attach to the substrate surface rather than the gecko's toe due to this disequilibrium. Figure on the right shows the model of interaction between N spatulas, a dirt particle and a planar wall.

It is important to know that this property of self-cleaning appears intrinsic to the setal nano-structure and therefore should be replicable in synthetic adhesive materials. In fact, Kellar Autumn's group observed how self-cleaning still occurred in arrays of setae when isolated from the geckos used.

Development and approaches

Number of papers published on "gecko adhesive" 2002~2007 Development of biomimetic adhesives.jpg
Number of papers published on "gecko adhesive" 2002~2007

The discoveries about gecko's feet led to the idea that these structures and mechanisms might be exploited in a new family of adhesives, and research groups from around the world are now investigating this concept. And thanks to the development of nano science and technology, people are now able to create biomimetic adhesive inspired by gecko's setae using nanostructures. Indeed, interest and new discoveries in gecko-type adhesives are booming, as illustrated by the growing number of papers published on this topic. [10] however, synthetic setae are still at a very early stage.

Effective design

Effective design of geckolike adhesives will require deep understanding of the principles underlying the properties observed in the natural system. These properties, principles, and related parameters of the gecko adhesive system are shown in the following table. [11] This table also gives us an insight into how scientists translate those good properties of gecko's setae (as shown in the first column) into the parameters they can actually control and design (as shown in the third column).

PropertiesPrinciplesparameters
1. Anisotropic attachment
2. High µ' (pulloff/preload)
Cantilever beamShaft length, radius, density, shaft angle
3. Low detachment forceLow effective stiffnessShaft modulus, spatular shape
4. Material independence stickinessVan der Waals mechanism
JKR-like* contact mechanics
Nanoarray (divided contact)
Spatular size, spatular shape, spatular density
5. Self-cleaning abilityNanoarray (divided contact)Spatular bulk modulus
6. Anti-self-stickinessSmall contact areaParticle size, shape, surface energy
7. Nonsticky default stateNonsticky spatulae, hydrophobic, Van der Waals forceSpatular size, shape, surface energy

*JKR refers to the Johnson, Kendall, Roberts model of adhesion [12]

In summary, the key parameters in the design of synthetic gecko adhesive include:

There is a growing list of benchmark properties that can be used to evaluate the effectiveness of synthetic setae, and the adhesion coefficient, which is defined as:

where is the applied preload force, and is the generated adhesion force. The adhesion coefficient of real gecko setae is typically 8~16.

Materials

In the first developments of synthetic setae, polymers like polyimide, polypropylene and polydimethylsiloxane (PDMS) are frequently used since they are flexible and easily fabricated. Later, as nanotechnology rapidly developed, Carbon Nanotubes (CNTs) are preferred by most research groups and used in most recent projects. CNTs have much larger possible length-to-diameter ratio than polymers, and they exhibit both extraordinary strength and flexibility, as well as good electrical properties. It is these novel properties that make synthetic setae more effective.

Fabrication techniques

A number of MEMS/NEMS fabrication techniques are applied to the fabrication of synthetic setae, which include photolithography/electron beam lithography, plasma etching, deep reactive ion etching (DRIE), chemical vapor deposition (CVD), and micro-molding, etc.

Examples

In this section, several typical examples will be given to show the design and fabrication process of synthetic setae. We can also gain an insight into the development of this biomimetic technology over the past few years from these examples.

Gecko tape

Micro view of gecko tape Micro view of Gecko Tape.jpg
Micro view of gecko tape
"Spider-Man test" of gecko tape Test of gecko tape.jpg
"Spider-Man test" of gecko tape

This example is one of the first developments of synthetic setae, which arose from a collaboration between the Manchester Centre for Mesoscience and Nanotechnology, and the Institute for Microelectronics Technology in Russia. Work started in 2001 and 2 years later results were published in Nature Materials. [13]

The group prepared flexible fibers of polyimide as the synthetic setae structures on the surface of a 5  μm thick film of the same material using electron beam lithography and dry etching in an oxygen plasma. The fibres were 2 μm long, with a diameter of around 500 nm and a periodicity of 1.6 μm, and covered an area of roughly 1 cm2 (see figure on the left). Initially, the team used a silicon wafer as a substrate but found that the tape's adhesive power increased by almost 1,000 times if they used a soft bonding substrate such as Scotch tape – This is because the flexible substrate yields a much higher ratio of the number of setae in contact with the surface over the total number of setae.

The result of this "gecko tape" was tested by attaching a sample to the hand of a 15 cm high plastic Spider-Man figure weighing 40 g, which enabled it to stick to a glass ceiling, as is shown in the figure. The tape, which had a contact area of around 0.5 cm2 with the glass, was able to carry a load of more than 100 g. However, the adhesion coefficient was only 0.06, which is low compared with real geckos (8~16).

Synthetic gecko foot hair

Micro view of the "Polypropylene Synthetic Gecko Foot Hair" Micro view of CNTs synthetic setae.jpg
Micro view of the "Polypropylene Synthetic Gecko Foot Hair"

As nanoscience and nanotechnology develop, more projects involve the application of nanotechnology, notably the use of carbon nanotubes (CNTs). In 2005, researchers from the University of Akron and Rensselaer Polytechnic Institute, both in the US, created synthetic setae structures by depositing multiwalled CNTs by chemical vapour deposition onto quartz and silicon substrates [15]

The nanotubes were typically 10–20 nm in diameter and around 65 μm long. The group then encapsulated the vertically aligned nanotubes in PMMA polymer before exposing the top 25 μm of the tubes by etching away some of the polymer. The nanotubes tended to form entangled bundles about 50 nm in diameter because of the solvent drying process used after etching. (As is shown in the figure on the right).

The results were tested with a scanning probe microscope, and it showed that the minimum force per unit area as 1.6±0.5×10−2 nN/nm2, which is far larger than the figure the team estimated for the typical adhesive force of a gecko's setae, which was 10−4 nN/nm2. Later experiments [16] with the same structures on Scotch tape revealed that this material could support a shear stress of 36 N/cm2, nearly four times higher than a gecko foot. This was the first time synthetic setae exhibited better properties than those of natural gecko foot. Moreover, this new material can adhere to a wider variety of materials, including glass and Teflon.

This new material has some problems, though. When pulled parallel to a surface, the tape releases, not because the CNTs lose adhesion from the surface but because they break, and the tape cannot be reused in this case. Moreover, unlike gecko's setae, this material only works for small area (approx. 1 cm2). The researchers are currently working on a number of ways to strengthen the nanotubes and are also aiming to make the tape reusable thousands of times, rather than the dozens of times it can now be used.

Geckel

Micro view of the geckel Micro view of geckel.jpg
Micro view of the geckel

While most developments concern dry adhesion, a group of researchers studied how derivatives of naturally occurring adhesive compounds from mollusks could be combined with gecko-type structures to yield adhesives that operate in both dry and wet conditions. [17]

The resulting adhesive, named 'geckel', was described to be an array of gecko-mimetic, 400 nm wide silicone pillars, fabricated by electron-beam lithography and coated with a mussel-mimetic polymer, a synthetic form of the amino acid that occurs naturally in mussels (left). [ clarification needed ].

Unlike true gecko glue, the material depends on van der Waals forces for its adhesive properties and on the chemical interaction of the surface with the hydroxyl groups in the mussel protein. The material improves wet adhesion 15-fold compared with uncoated pillar arrays. The so-called "geckel" tape adheres through 1,000 contact and release cycles, sticking strongly in both wet and dry environments.

So far, the material has been tested on silicon nitride, titanium oxide and gold, all of which are used in the electronics industry. However, for it to be used in bandages and medical tape, a key potential application, it must be able to adhere to human skin. The researchers tested other mussel-inspired synthetic proteins that have similar chemical groups and found that they adhere to living tissue. [17]

Geckel is an adhesive that can attach to both wet and dry surfaces. Its strength "comes from coating fibrous silicone, similar in structure to a gecko's foot, with a polymer that mimics the 'glue' used by mussels." [18]

The team drew inspiration from geckos, who can support hundreds of times their own body weight. Geckos rely on billions of hair-like structures, known as setae to adhere. Researchers combined this ability with the sticking power of mussels. Tests showed that "the material could be stuck and unstuck more than 1,000 times, even when used under water", retaining 85 percent of their adhesive strength. [19] [20] [21]

Phillip Messersmith, lead researcher on the team that developed the product, believes that the adhesive could have many medical applications, for example tapes that could replace sutures to close a wound and a water resistant adhesive for bandages and drug-delivery patches. [18]

Commercial production

Automated, high-volume fabrication techniques will be necessary for these adhesives to be produced commercially and were being investigated by several research groups. A group led by Metin Sitti from Carnegie Mellon University studied[ when? ] a range of different techniques which include deep reactive ion etching (DRIE), which has been used successfully to fabricate mushroom-shaped polymer fibre arrays, micro-moulding processes, direct self-assembly and photolithography.[ citation needed ]

In 2006, researchers at BAE Systems Advanced Technology Centre at Bristol, UK, announced that they had produced samples of "synthetic gecko" – arrays of mushroom-shaped hairs of polyimide – by photolithography, with diameters up to 100μm. These were shown to stick to almost any surface, including those covered in dirt, and a pull-off of 3,000 kg/m^2 was measured.[ citation needed ] More recently, the company has used the same technique to create patterned silicon moulds to produce the material and has replaced the polyimide with polydimethylsiloxane (PDMS). This latest material exhibited a strength of 220 kPa. Photo-lithography has the benefit of being widely used, well understood and scalable up to very large areas cheaply and easily, which is not the case with some of the other methods used to fabricate prototype materials.[ citation needed ]

In 2019, researchers from Akron Ascent Innovations, LLC, a company spun-out from University of Akron technology, announced the commercial availability of "ShearGrip" brand dry adhesives. [22] Rather than relying on photolithography or other micro-fabrication strategies, the researchers employed electrospinning to produce small diameter fibers based on the principle of contact splitting exploited by geckos. The product has reported shear strength greater than 80 pounds per square inch, with clean removal and reusability on many surfaces, and the ability to laminate the material to various face stocks in one or two sided constructions. [23] The approach is claimed to be more scalable than other strategies to produce synthetic setae and has been used to produce products for consumer markets under the brand name Pinless.

Applications

There have been a wide range of applications of synthetic setae, also known as "gecko tape," ranging from nanotechnology and military uses to health care and sport.

Nano tape

Nano tape Nano tape.jpg
Nano tape

"Nano tape" (also called "gecko tape") is often sold commercially as double-sided adhesive tape. It can be used to hang lightweight items such as pictures and decorative items on smooth walls without punching holes in the wall. The carbon nanotube arrays leave no residue after removal and can stay sticky in extreme temperatures. [24]

Robotics

No machine yet exists that can maneuver in the "scansorial" regime – that is, perform nimbly in general vertical terrain environments without loss of competence in level ground operation. Two major research challenges face the development scansorial robotics: First, they seek to understand, characterize and implement the dynamics of climbing (wall reaction forces, limb trajectories, surface interactions, etc.); and second, they must design, fabricate and deploy adhesive patch technologies that yield appropriate adhesion and friction properties to facilitate necessary surface interactions.

As progress continues in legged robotics, research has begun to focus on developing robust climbers. Various robots have been developed that climb flat vertical surfaces using suction, magnets, and arrays of small spines, to attach their feet to the surface.

RiSE platform

The RiSE platform was developed in Biomimetics and Dexterous Manipulation Laboratory, Stanford University. It has twelve degrees of freedom (DOF), with six identical two DOF mechanisms spaced equally in pairs along the length of the body. Two actuators on each hip drive a four bar mechanism, which is converted to foot motion along a prescribed trajectory, and positions the plane of the four bar mechanism angularly with respect to the platform. For the RiSE robot to succeed in climbing in both natural and man-made environments it has proven necessary to use multiple adhesion mechanisms. The RiSE robot does not, but will use dry adhesion in combination with spines. [25]

More recently, robots have been developed that utilize synthetic adhesive materials for climbing smooth surfaces such as glass.

These crawler and climbing robots can be used in the military context to examine the surfaces of aircraft for defects and are starting to replace manual inspection methods. Today's crawlers use vacuum pumps and heavy-duty suction pads which could be replaced by this material.

Stickybot

Researchers at Stanford University have also created a robot called Stickybot which uses synthetic setae in order to scale even extremely smooth vertical surfaces just as a gecko would. [26] [27]

Stickybot is an embodiment of the hypotheses about the requirements for mobility on vertical surfaces using dry adhesion. The main point is that we need controllable adhesion. The essential ingredients are:

  • hierarchical compliance for conforming at centimeter, millimeter and micrometer scales,
  • anisotropic dry adhesive materials and structures so that we can control adhesion by controlling shear,
  • distributed active force control that works with compliance and anisotropy to achieve stability.

Geckobot

Another similar example is "Geckobot" developed in Carnegie Mellon University, [28] which has climbed at angles of up to 60°.

Joint replacement

Adhesives based on synthetic setae have been proposed as a means of picking up, moving and aligning delicate parts such as ultra-miniature circuits, nano-fibres and nanoparticles, microsensors and micro-motors. In the macro-scale environment, they could be applied directly to the surface of a product and replace joints based on screws, rivets, conventional glues and interlocking tabs in manufactured goods. In this way, both assembly and disassembly processes would be simplified. It would also be beneficial to replace conventional adhesive with synthetic gecko adhesive in vacuum environment (e.g. in space) since the liquid ingredient in conventional adhesive would easily evaporate and causes the connection to fail.[ citation needed ]

Related Research Articles

<span class="mw-page-title-main">Adhesive</span> Non-metallic material used to bond various materials together

Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation.

<span class="mw-page-title-main">Gecko</span> Lizard belonging to the infraorder Gekkota

Geckos are small, mostly carnivorous lizards that have a wide distribution, found on every continent except Antarctica. Belonging to the infraorder Gekkota, geckos are found in warm climates throughout the world. They range from 1.6 to 60 centimetres.

<span class="mw-page-title-main">Biomimetics</span> Imitation of biological systems for the solving of human problems

Biomimetics or biomimicry is the emulation of the models, systems, and elements of nature for the purpose of solving complex human problems. The terms "biomimetics" and "biomimicry" are derived from Ancient Greek: βίος (bios), life, and μίμησις (mīmēsis), imitation, from μιμεῖσθαι (mīmeisthai), to imitate, from μῖμος (mimos), actor. A closely related field is bionics.

van der Waals force Interactions between groups of atoms that do not arise from chemical bonds

In molecular physics and chemistry, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and therefore more susceptible to disturbance. The van der Waals force quickly vanishes at longer distances between interacting molecules.

In biology, setae are any of a number of different bristle- or hair-like structures on living organisms.

<span class="mw-page-title-main">Byssus</span> Fibre secreted by some molluscs

A byssus is a bundle of filaments secreted by many species of bivalve mollusc that function to attach the mollusc to a solid surface. Species from several families of clams have a byssus, including pen shells (Pinnidae), true mussels (Mytilidae), and Dreissenidae.

<span class="mw-page-title-main">Adhesion</span> Molecular property

Adhesion is the tendency of dissimilar particles or surfaces to cling to one another.

<span class="mw-page-title-main">Lamella (surface anatomy)</span> Anatomical structure

In surface anatomy, a lamella is a thin plate-like structure, often one amongst many lamellae very close to one another, with open space between. Aside from respiratory organs, they appear in other biological roles including filter feeding and the traction surfaces of geckos.

<span class="mw-page-title-main">Hot-melt adhesive</span> Glue applied by heating

Hot-melt adhesive (HMA), also known as hot glue, is a form of thermoplastic adhesive that is commonly sold as solid cylindrical sticks of various diameters designed to be applied using a hot glue gun. The gun uses a continuous-duty heating element to melt the plastic glue, which the user pushes through the gun either with a mechanical trigger mechanism on the gun, or with direct finger pressure. The glue squeezed out of the heated nozzle is initially hot enough to burn and even blister skin. The glue is sticky when hot, and solidifies in a few seconds to one minute. Hot-melt adhesives can also be applied by dipping or spraying, and are popular with hobbyists and crafters both for affixing and as an inexpensive alternative to resin casting.

Scopulae, or scopula pads, are dense tufts of hair at the end of a spiders's legs. They are found mostly on hunting spiders, for example Salticidae and Sparassidae. Scopulae consist of microscopic hairs, known as setae, which are each covered in even smaller hairs called setules or "end feet", resulting in a large contact area.

Bioadhesives are natural polymeric materials that act as adhesives. The term is sometimes used more loosely to describe a glue formed synthetically from biological monomers such as sugars, or to mean a synthetic material designed to adhere to biological tissue.

Dry glue is an adhesion product based upon the adaptations of geckos' feet that allow them to climb sheer surfaces such as vertical glass. Synthetic equivalents use carbon nanotubes as synthetic setae on reusable adhesive patches.

<span class="mw-page-title-main">Gecko feet</span> Hairy feature allowing suction

The feet of geckos have a number of specializations. Their surfaces can adhere to any type of material with the exception of Teflon (PTFE). This phenomenon can be explained with three elements:

<span class="mw-page-title-main">Bio-inspired robotics</span>

Bio-inspired robotic locomotion is a fairly new subcategory of bio-inspired design. It is about learning concepts from nature and applying them to the design of real-world engineered systems. More specifically, this field is about making robots that are inspired by biological systems, including Biomimicry. Biomimicry is copying from nature while bio-inspired design is learning from nature and making a mechanism that is simpler and more effective than the system observed in nature. Biomimicry has led to the development of a different branch of robotics called soft robotics. The biological systems have been optimized for specific tasks according to their habitat. However, they are multifunctional and are not designed for only one specific functionality. Bio-inspired robotics is about studying biological systems, and looking for the mechanisms that may solve a problem in the engineering field. The designer should then try to simplify and enhance that mechanism for the specific task of interest. Bio-inspired roboticists are usually interested in biosensors, bioactuators, or biomaterials. Most of the robots have some type of locomotion system. Thus, in this article different modes of animal locomotion and few examples of the corresponding bio-inspired robots are introduced.

The chemistry of pressure-sensitive adhesives describes the chemical science associated with pressure-sensitive adhesives (PSA). PSA tapes and labels have become an important part of everyday life. These rely on adhesive material affixed to a backing such as paper or plastic film.

<span class="mw-page-title-main">Arthropod adhesion</span>

Arthropods, including insects and spiders, make use of smooth adhesive pads as well as hairy pads for climbing and locomotion along non-horizontal surfaces. Both types of pads in insects make use of liquid secretions and are considered 'wet'. Dry adhesive mechanisms primarily rely on Van der Waals' forces and are also used by organisms other than insects. The fluid provides capillary and viscous adhesion and appears to be present in all insect adhesive pads. Little is known about the chemical properties of the adhesive fluids and the ultrastructure of the fluid-producing cells is currently not extensively studied. Additionally, both hairy and smooth types of adhesion have evolved separately numerous times in insects. Few comparative studies between the two types of adhesion mechanisms have been done and there is a lack of information regarding the forces that can be supported by these systems in insects. Additionally, tree frogs and some mammals such as the arboreal possum and bats also make use of smooth adhesive pads. The use of adhesive pads for locomotion across non-horizontal surfaces is a trait that evolved separately in different species, making it an example of convergent evolution. The power of adhesion allows these organisms to be able to climb on almost any substance.

Self-cleaning surfaces are a class of materials with the inherent ability to remove any debris or bacteria from their surfaces in a variety of ways. The self-cleaning functionality of these surfaces are commonly inspired by natural phenomena observed in lotus leaves, gecko feet, and water striders to name a few. The majority of self-cleaning surfaces can be placed into three categories:

  1. superhydrophobic
  2. superhydrophilic
  3. photocatalytic.

Sticky pads are friction devices used to prevent objects from sliding on a surface, by effectively increasing the friction between the object and the surface.

Microsuction tape is a material for sticking objects to surfaces such as furniture, dashboards, walls, etc. One side is usually attached to the base surface by a classical adhesive. Objects are attached to the other side by pressing them against the tape. They stick to the tape due to small bubbles (cavities) on the surface of the tape. These contain air, which is squeezed out when the surface of an object is pressed against the surface of the tape. Due to sealing properties of the material, when the object is pulled off the surface, a vacuum is created in the cavities. Due to external air pressure, this creates a force that prevents the object from being removed from the surface, a mechanism similar to that of a suction cup.

<span class="mw-page-title-main">Nano tape</span> Synthetic adhesive tape

Nano tape, also called gecko tape; marketed under the name Alien Tape, is a synthetic adhesive tape consisting of arrays of carbon nanotubes transferred onto a backing material of flexible polymer tape. These arrays are called synthetic setae and mimic the nanostructures found on the toes of a gecko; this is an example of biomimicry. The adhesion is achieved not with chemical adhesives, but via van der Waals forces, which are weak electric forces generated between two atoms or molecules that are very close to each other.

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