9.3 Adhesion mechanisms in nature and biomimetic adhesives

Nature's sticky secrets inspire amazing materials. Geckos use tiny hairs to cling to walls, while mussels make super-strong glue underwater. Scientists are copying these tricks to create new adhesives that work in tough conditions.

These bio-inspired adhesives could revolutionize medicine, robotics, and more. Imagine bandages that stick when wet or robots that climb like geckos. Nature's solutions are helping us solve complex engineering challenges in clever ways.

Gecko-Inspired Adhesives

Adhesion Mechanism and Microstructures

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Top images from around the web for Adhesion Mechanism and Microstructures

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  Frontiers | Adhesion of Individual Attachment Setae of the Spider Cupiennius salei to Substrates ... View original

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  Lamellae | The lamellae of a gecko are the thin strips under… | Flickr View original

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  Intermolecular Forces · Chemistry View original

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  Frontiers | Adhesion of Individual Attachment Setae of the Spider Cupiennius salei to Substrates ... View original

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  Lamellae | The lamellae of a gecko are the thin strips under… | Flickr View original

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• Gecko feet exhibit strong adhesion through van der Waals forces, which are weak intermolecular forces between molecules
  • These forces arise from temporary dipoles formed by fluctuations in electron density
  • Although individually weak, the cumulative effect of millions of setae and spatulae on gecko feet results in significant adhesion
• Gecko feet are covered with microscopic hair-like structures called setae, which are made of keratin (same material as human hair and nails)
  • Each seta is approximately 100 μm long and 5 μm in diameter
  • Setae are arranged in a dense array, with up to 14,000 setae per mm²
• At the tip of each seta are even smaller structures called spatulae, which are triangular-shaped pads
  • Spatulae are approximately 200 nm wide and 20 nm thick
  • The high density of spatulae (up to 1,000 per seta) maximizes contact area with the surface, enhancing adhesion

Biomimetic Adhesives and Applications

• Inspired by gecko feet, researchers have developed fibrillar adhesives that mimic the hierarchical structure of setae and spatulae
  • These adhesives consist of arrays of microscopic pillars or fibers made from polymers (polydimethylsiloxane) or carbon nanotubes
  • The high aspect ratio and dense packing of the fibers maximize contact area and adhesion strength
• A key feature of gecko-inspired adhesives is their reversible adhesion, allowing them to attach and detach from surfaces repeatedly
  • This reversibility is achieved through the control of fiber orientation and applied shear force
  • When the fibers are aligned parallel to the surface and a shear force is applied, adhesion is engaged; when the force is removed, the fibers return to their original orientation, releasing adhesion
• Potential applications of gecko-inspired adhesives include:
  • Climbing robots and grippers for manufacturing and space exploration
  • Biomedical devices such as skin patches and surgical tape
  • Reusable and residue-free adhesive tapes for various industries

Mussel-Inspired Adhesives

Adhesion Mechanism and Key Molecules

• Mussels secrete adhesive proteins called mussel foot proteins (Mfps) to attach themselves to surfaces underwater
  • These proteins contain a high concentration of the amino acid 3,4-dihydroxyphenylalanine (DOPA), which is derived from the catechol group
  • DOPA undergoes oxidation and cross-linking reactions, forming strong covalent bonds with surfaces and other proteins
• The adhesion mechanism of mussels relies on catechol chemistry, which involves the formation of hydrogen bonds, metal-ligand complexes, and covalent cross-links
  • Catechol groups can form hydrogen bonds with hydrophilic surfaces (glass, metal oxides) and π-π interactions with aromatic surfaces (graphite, polymers)
  • In the presence of metal ions (Fe³⁺, Cu²⁺), catechols form strong metal-ligand complexes, enhancing adhesion and cohesion
  • Oxidized catechols can also form covalent cross-links with other catechols or nucleophilic groups (amines, thiols), creating a cohesive network

Biomimetic Adhesives and Applications

• Mussel-inspired adhesives are designed to mimic the wet adhesion capabilities of mussels by incorporating catechol-functionalized polymers
  • These polymers are typically synthesized by grafting catechol groups onto the backbone of existing polymers (polyethylene glycol, chitosan) or by polymerizing catechol-containing monomers (dopamine methacrylamide)
  • The catechol content and polymer architecture can be tuned to optimize adhesion strength, curing time, and biocompatibility
• A major advantage of mussel-inspired adhesives is their ability to bond to various substrates in wet or humid environments
  • This property makes them suitable for applications in medicine, marine engineering, and water-resistant electronics
• Examples of mussel-inspired adhesives include:
  • Tissue adhesives for wound closure and surgical repair
  • Dental adhesives for restorative procedures and orthodontic brackets
  • Underwater adhesives for marine construction and ship hull repair
  • Conductive adhesives for flexible electronics and wearable sensors