There is a pervading presence of adhesive joints in nature. Adhesive secretions are used by organisms for attachment, construction, obstruction, defense, and predation. Most natural materials are hybrid materials combining organic and inorganic building blocks. Bioadhesives are able to build durable interfaces between hard and soft materials, often of disparate scales, and exhibit a certain number of characteristics that make them differ greatly from synthetic adhesives. The title of this chapter (Bioadhesives) includes a broad variety of different concepts: natural adhesives, biological adhesives, biocompatible adhesives, and biomimetic and bioinspired adhesives. The term natural adhesive describes substances that are formulated from partially or totally bio-based raw materials, which are employed as adhesives in man-made technology, but are not substances used by biological systems as glues. The term biological adhesive refers specifically to adhesive secretions of natural organisms in marine and other wet environments, or those produced on land. A different concept refers to what is named as biocompatible adhesive, including any natural or synthetic adhesive that interfaces with living tissues and biological fluids, and is suitable for short-/long-term biomedical applications. Specific mechanisms of adhesion found in nature are discussed – interlocking, suction, friction, dry and wet adhesion, gluing – to get inspiration for the development of new synthetic adhesives. Biomimetic adhesives are synthetic adhesives design to closely mimic the molecular structure and mechanisms of adhesion found in nature. Bioinspired adhesives are synthetic adhesives whose design is inspired in biological concepts, mechanisms, functions, and design features. Also the promising technology of self-healing polymers is reviewed, as an effective method for controlling crack propagation and debonding.
KeywordsBioadhesion bioadhesive biocompatible adhesive bioinspired adhesive biological adhesive biomimetic adhesive Dahlquist’s criterion dry adhesion friction gluing hierarchically structured adhesives interlocking natural adhesive patterned adhesives principle of contact splitting self-healing suction wet adhesion
Adhesives are used extensively in nature. Many biological systems, ranging from microorganisms through larger structures in vertebrates, use adhesive joints in structural and functional applications. They are composed mainly by polymers, but it is not uncommon to find examples where inorganic adhesives play an important role in the overall performance of some organisms. Their diversity is a source of inspiration for developing new man-made adhesives different from those currently available.
One reason for this pervading presence of adhesive joints in nature is that almost all natural materials are hybrid materials combining organic and inorganic building blocks. These units need to be joined together in order to get structural performance and functionality. Bioadhesives are able to build durable interfaces between hard and soft materials, often of disparate scales. Geometry and scale are also important in the final capabilities of the hybrid material. Nature is indeed a school for materials science and its associated disciplines such as chemistry, biology, physics, or engineering. Materials found in nature combine many inspiring properties such as sophistication, miniaturization, hierarchical organizations, hybridation, resistance, and adaptability.
Bioadhesives exhibit a certain number of characteristics that make them differ greatly from artificial adhesives. They show a sensitivity to – and critical dependence on – the presence of humidity, forming strong bonds under water. They make a recurrent use of molecular constituents, such that widely variable properties are attained from apparently similar elementary units. The non-specific nature of most bioadhesives is remarkable; they are able to form sound attachments with a wide diversity of substrates, irrespectively from the presence of detritus, humidity, or the roughness of the surface. Other interesting characteristic is that properties of bioadhesives vary in response to performance requirements, and improve fatigue resistance and resilience of the biological hybrid materials. The controlled orientation of structural elements, with a hierarchical organization and often complex shapes, demand from the bioadhesives the capacity of conferring to the materials a damage-tolerant design. The adhesion mechanisms in biology must be robust enough to function on rough surfaces; however, bioadhesion must be easily releasable when related to animal movement. A self-healing bioadhesive can stop minor damage from escalating to critical levels. Biomaterials are multifunctional and are produced in situ at room temperature and atmospheric pressure, although at slow rates. They are fabricated via highly coupled, often concurrent synthesis and assembly.
Some biological processes involve the adhesion of a particular molecule to a specific locus of a membrane or tissue (ligand–receptor interaction) referring to cellular interactions (cytoadhesion) or to mucosal adhesion (mucoadhesion), but this topic will not be covered in this chapter; see for example (Thomas and Peppas 2006).
Some terms have to be clarified in order to be precise when speaking about bioadhesives and bioadhesion. There are some natural substances, derived or extracted from a variety of organic or inorganic sources, which are used as adhesives: starch, casein, blood, soybean, bitumen, cements, etc. However, the main role of these substances in nature is not the bonding of substrates. The term natural adhesives will be used for these substances that are formulated from substantially or totally bio-based raw materials, which are used as adhesives in man-made technology, but are not substances used by biological systems as glues. On the other hand, the term biological adhesives includes the adhesive secretions used by organisms for attachment, construction, obstruction, defense, and predation, which are produced in marine and other wet environments (by fish, holothurians, mollusks, arthropods, worms, bacteria, algae, and fungi) or those produced on land (by amphibians, spiders, insects, mollusks, etc.). A different concept refers to what is named as biocompatible adhesive, including any natural or synthetic adhesive that interfaces with living tissues and biological fluids, and is suitable for short- /long-term biomedical applications.
Recent developments have been aimed to understand and make use of the exceptional properties of biological adhesives. Biomimetic adhesives are synthetic adhesives designed to closely mimic the molecular structure and mechanisms of adhesion found in nature. Bioinspired adhesives are synthetic adhesives whose design is inspired in biological concepts, mechanisms, functions, and design features. The aim is not to emulate any particular biological architecture or system, but to use such knowledge as a source of guiding principles and ideas.
Therefore, the title of this chapter (Bioadhesives) includes a broad variety of different types of adhesive: natural, biocompatible, biological, biomimetic, and bioinspired adhesives. All these categories are considered, and some examples are presented to illustrate every group, without the aim of presenting an exhaustive review; the references given help to complete the picture. In addition, specific mechanisms of adhesion found in nature are briefly discussed to get inspiration for the development of new synthetic adhesives. Also, the promising technology of self-healing polymers will be reviewed, as an effective method for controlling crack propagation and debonding.
As it was mentioned earlier, the term natural adhesives refers to substances that are formulated from substantially or totally bio-based raw materials, which are used as adhesives in man-made technology, but are not substances used by biological systems properly as glues. As a group, natural adhesives represent the oldest adhesives known to man, and they have had a widespread use till the World War II, when synthetic adhesives appeared as a more reliable option. A steady decrease of the consumption share of natural adhesives has followed. Nowadays, there is a renewed interest in natural feedstocks as the concept of “sustainability” is spreading worldwide. Applications like packaging and converting (P&C) are dominated primarily by waterborne adhesives, and a sizable percentage of these adhesives are natural based (Schwartz 2000). In many historical applications, these adhesives perform exceedingly well. New applications, especially in the medical fields, are also developing.
The reason behind the increase in the use of synthetic adhesives is the greater uniformity and control of the products, as compared to natural adhesives. However, adhesives based on natural formulations have certain advantages: easily available, stable quality, non-toxic and biodegradable, environmentally friendly, and relatively low cost. Natural adhesives are both from organic (starch, casein, blood, etc.) or inorganic (soluble silicates, cements, etc.) origin. The historical term for the natural adhesive of organic type has been glue, and this denomination has also been carried over to this day (Skeist 1990; Pizzi and Mittal 2003).
Natural Rubber and Gums
Natural rubber has an excellent flexibility, high initial tack, and good tack retention properties. Typical applications include its use as an adhesive in self-sealing envelopes, and, in general, as pressure sensitive adhesive. It also has wide applications in the footwear and upholstery industries, for the attachment of leather, fabrics, and rubber to one another or to wood and metal, bonding of paper, felt and textiles in the manufacturing of stationery, packaging, etc.
Natural rubber is obtained from rubber trees, and processed by solvent or water evaporation (latex) to manufacture adhesives. Shelf life is quite good and the adhesive is stable. However, latex is subjected to embrittlement by freezing. Natural rubber can be transformed into a more durable material via the addition of sulfur or other equivalent curatives, in a chemical process named vulcanization. These additives modify the natural polymer by cross-linking between the individual macromolecular chains. The vulcanized material is less sticky, but has superior mechanical properties and heat resistance. Natural rubber-based adhesives are not suitable for structural applications due to their low strength and permanent softening at elevated temperatures (over 90°C). Sunlight exposure can also damage the material. Resistance to water and to mold growth is adequate but poor regarding to oils, solvents, and oxidizing agents. Other compounds often appear in the formulation of natural rubber adhesives: calcium carbonate, zinc oxide, tackifiers, antioxidant, extenders, and curing agents.
Some natural adhesives can be obtained from gums (sticky resins poured from certain plants). The gum arabic is composed of a complex mixture of polysaccharides and proteins, mainly from acacia senegal. It is highly soluble in water and it has a low viscosity even for high concentrations. It is one of the oldest adhesives used without interruption in history (since 1600 BC). Gum tragacanth is a natural gum obtained from the dried sap of several Middle Eastern legumes of the genus Astragalus. It is highly viscous even in small concentrations in water, forming a gel that is odorless and tasteless. It is used as a binder in the pharmaceutical and food industries. Other vegetal gums are: guar gum, karaya gum, gatti gum, etc.
Starch and Dextrins
Adhesives based on starch and dextrin make up the single largest category amongst all natural adhesives. They have been extensively used in the packaging industry. They are easy to apply, hot or cold, from water dispersions. The mechanism of curing is by the loss of moisture, and the open time is ample enough to permit assembly during production. Other advantages are: stable quality, insoluble in oil and fats, non-toxic and biodegradable, heat resistant (thermosetting structure), and good adhesion to porous substrates. On the other hand, this class of adhesives is known to have a poor moisture resistance and mold growth.
Starch is a natural polysaccharide derived from seeds, roots, and leaves of certain plants, such as corn, wheat, potato, rice, etc. Starches are treated to have a viscosity and rheology more suitable for liquid adhesives, including alkali treatment, acid treatment, and oxidation (Baumann and Conner 2003). Oxidized or chlorinated starch has greater tack and is often used in adhesive applications. Dextrin is starch that is processed further, hydrolyzing the molecules with heat and acid in smaller fragments that are then repolymerized in highly branched chains, forming a readily soluble polymer. Depending on their molecular weight, and solid contents, dextrins are classified in three types: British gums (it is the strongest dextrin adhesive, ranking highest in molecular weight), Canary dextrins (lowest in molecular weight and viscosity), and White dextrins (with intermediate values of molecular weight).
Properties of starch and dextrin adhesives can be improved by addition of certain additives and modifiers. Borated dextrin includes borax (sodium tetraborate) to obtain high tack at moderate viscosities, with good aging characteristics. Plasticizers based on saps, polyglycols, and sulfonated oil derivatives act as lubricants to impart flexibility to the adhesive layer. Other plasticizers act as hygroscopic agents to decrease the drying rate of the film. Liquefiers are used to reduce viscosity, and also to control open time and speed of drying. Colloid stabilizers are used to delay degradation. To improve cold water resistance, but keeping the capacity of dissolution in hot water (bottle labels), polyvinyl alcohol or polyvinyl acetate blends are used. Optimal moisture resistance is possible through the addition of thermosetting resins (urea formaldehyde or resorcinol). Mineral fillers up to 50% of concentration are added to reduce costs and control penetration in porous substrates. Other additives include thixotropic agents, preservatives to prevent microbial activity, bleaches, and defoamers. Starch and dextrin adhesives are still in significant use today due to their availability and low cost.
Cellulose is another natural polymer used to prepare adhesives by chemical modification of the alcohol functional groups in its structure. The source for cellulose can be cotton or wastes from wood processing and related industries. They are thermoplastic, solvent-based adhesives.
Nitrocellulose adhesives have been in the market for long time, due to their versatility, good adherence to a variety of materials (glass, leather, metals, cloth, and some plastics), and strong bonding to polar substrates. Cellulose nitrate adhesives are water resistant and age fairly well, but discolor in sunlight. They have a high viscosity, good tack, and they develop a significant initial strength even before the complete evaporation of the solvent. Industrial applications have remained limited due to the high flammability of nitrocellulose films. Cellulose acetate adhesives have a higher resistance to heat than the nitrocellulose ones and exhibit a better aging, but are not as moisture resistant. Maximum service temperature is only about 50°C. Cellulose acetate flows even at low temperatures, and it is not acceptable for use in structural applications. It is resistant to oils, mold growth, weak acids, and certain solvents. Cellulose acetate butyrate adhesive is also a thermoplastic that sets by solvent evaporation, with improved resistance and heat stability, as compared with cellulose acetate. Ethyl cellulose adhesives are available as solvent solutions or as hot melts. They can vary from soft and tacky to strong and tough formulations. They adhere well to porous materials, such as paper and cloth, and exhibit a significant resistance to oil, mold, and fungus, but only moderate resistance to water.
Casein, animal and fish by-products, blood and soy are all used to make glues. Natural macromolecules in their formulations – long protein chains – perform the same role of polymers in synthetic adhesives. However, the proteins have to be processed, dried, and compounded for the intended application. Specific hydrolysis reactions, at controlled pH and temperature, denature the proteins and change their three-dimensional structure to adjust the molecular weights.
Casein is a dry protein obtained from cow’s milk. It is obtained by precipitation when lowering the pH of the milk. The raw product is light in color; it has a poor resistance to water, chemicals, and mold growth. Additives are commonly used to improve these properties. Water resistance is achieved through the use of urea formaldehyde resin or hexamethylenetetramine. Thickeners, thinners, inert organic fillers, nondrying oils, preservatives, humectants, and other additives are used. It is possible to blend the casein with other adhesive substances such as soybean meal, blood albumin, latex, and synthetic rubber.
Casein-based adhesives are used at room temperature. The powder form requires mixing with water before using, and set by loss of water and by a certain degree of chemical conversion. The mixture is non-tacky and a light pressure at the joint is required till the adhesive sets. Casein glues have a certain gap filling capacity. These adhesives are widely used for the labeling of glassware. For this application (e.g., labeling of beer bottles in breweries), the adhesive needs to be icewater-proof (IWR) and, when properly formulated, the films can be removed in alcohol alkaline soaker. IWR casein-based adhesives are fast and tacky, and the formulation includes a peptizer, a cross-linker, ammonia, some starch, preservatives, defoamers, etc. Wood is also an important application, especially in structural laminates for indoor use, not exposed to high humidity.
Collagen is the main connective protein in animals, and hence is used as the basic component of most animal glues. There are two main sources of collagen: hide and bone from cattle. It is hydrophilic in nature. The collagen-based adhesives are obtained by denaturing the high molecular weight proteins through an acid or basic hydrolysis at moderate temperature. For higher strength collagen adhesives, intramolecular cross-linking is achieved by oxidative removal of amine groups of certain amino acids in the proteins, to form aldehydes (Nesburn et al. 1991), a process also known as Schiff base protein cross-linking. Improvements in the control of cross-linking in protein adhesives can also go through UV controlled methods using heterobifunctional reagents.
Animal glues have limited resistance to water, mold growth, and vermin attack. Dry granulated products are prepared in warm water solution, and extended to form a tacky viscous film that on cooling gels to provide an immediate, moderately strong bond. Heating the adhesive above 60°C should be avoided because of degradation. Cold liquid glues are more adequate for immediate use; they are called high quality glues or technical jellies. Pre-plasticized cakes, jellies, are of higher molecular weight than glues. In order to optimize their properties for specific applications, certain additives are common in the commercial formulations: wetting agents, dispersing agents, gel depressants, chemical reactants, and plasticizers. It is also possible to use these adhesives in combination with other natural products, such as starches and dextrins. They are relatively cheap, and are used for bonding wood, paper, leather, textile, and as a binder for abrasive wheels and sand paper. Collagen-based adhesives have widespread application in the furniture and, in general, woodworking industries. They are also used for gummed remoistening tape, and in the sealing of shipping containers. For labeling of glassware, casein-based adhesives are far more interesting than collagen-based ones, mostly because they are easier to remove.
This natural adhesive is derived from fish skins, especially as a by-product of the cod industry. Fish glue is a weaker adhesive than animal glue, but it is interesting for certain applications, as a household adhesive that is able to adhere to glass, ceramics, metal, wood, cork, paper, and leather. One or both substrates have to be porous to allow the adhesive to dry or, alternatively, a remoistenable fish adhesive can be used.
It is prepared, after washing the fish skins to eliminate salt, by heating to extract the glue solids, and concentrated up to a 40–50% of solid content. Fish glue is water soluble, but gets insoluble by addition of aluminum sulfate, ferric sulfate, chrome alum, formaldehyde, and glyoxal. It is insoluble in organic solvents. The use of humectants can plasticize the relatively rigid dry films. The wet adhesive has high initial tack and a long open time. Water resistance is limited, but can be improved through the use of tanning agents. Also, preservatives are beneficial for protecting the adhesive against mold and fungus attack. Fish glue is used as an assembly adhesive in the woodworking industry, where enough time is needed for positioning before clamping. Handling strength is reached overnight. Fish glue is also used as a modifier of animal glue for the manufacturing of gummed tape. It can be used together with dextrin formulations for improving the tack in envelope seals, and in combination with polyvinyl acetate and latex.
Soluble dried beef blood is the main source to prepare blood glues. The main component is blood albumen, a by-product of meat packaging industry. Thermal resistance of blood glues is fair when set with heat, and also water resistance is outstanding. However, they are very sensitive to mold growth and attack by bacteria under damp conditions. Phenol formaldehyde is used as an antifungal agent, which also improves the strength of blood glues. Mixtures with animal glues, casein, and soybean are commonly used.
The adhesion of blood glues is good to paper, textiles, leather, cork and metals. They have been used in food packaging because they are odorless, non-toxic, and tasteless. However, blood glues are not in use in most countries, and they only have a limited commercial interest in certain regions for the manufacture of plywood (interior grade only).
Soybean flour can be used as the main component in adhesive formulations, but it is commonly added as extender for phenolic systems, or blended with casein or other adhesives. They are packaged as dry powders that contain both proteins and carbohydrates. The flour is dispersed in aqueous sodium hydroxide for preparing the adhesive, and also calcium hydroxide is added to extend the open time and to improve water resistance. Soybean glues have a limited water resistance, but recover their strength on drying. They are also susceptible to mold growth, and some fungicides need to be included in the commercial formulations. Fillers are used to reduce cost, but they also lower the performance of the adhesive. Some commonly used fillers are wood and walnut-shell flours, and also clay.
Soybean glues are mainly used in the cold for laminating of plywood, or at 140°C and high pressures for rapid online assemblies. Specifically designed for high output production, hybrid adhesives made by combining soy protein with phenol resorcinol formaldehyde are used due to their shorter cure time and resistance to moisture – essential for bonding of green lumber.
Bitumen adhesives are an example of natural adhesives that do not cure by cross-linking or chemical reaction, but become solid by entanglement of macromolecular chains. Bitumen or asphalt is indeed a complex mixture of polar and nonpolar hydrocarbons (Hefer and Little 2005). Some of them are long chain organic polymers and others are short chains that can act as solvents. Bitumen adhesives are of low cost and offer good chemical resistance to alkalis and water. However, they have poor strength, especially at elevated temperatures, and oils and many solvents can cause softening. They are available as water emulsions, hot melts, or in solvent solution. Applications include floor construction, laminating paper and foil, vibration dampers, etc. Adhesive and sealant formulations are suitable for adhesion to many types of materials: concrete, glass, metal, felt, and paper.
Thixotroping agents in bitumen adhesive formulations add more solvent but lowers the adhesive’s viscosity and mechanical strength, increasing the tack. Resistance to outdoor weathering is a common requirement for bitumens used in roofing applications. Natural rubber latex may be mixed with bitumen to improve the flexibility. It is possible to improve the tack at low temperatures without lowering the mechanical strength by adding ionomeric elastomers.
The term biocompatible adhesive includes any natural or synthetic adhesive that interfaces with living tissues and biological fluids, and is suitable for short-/long-term biomedical applications. These surgical glues are particularly useful as replacement or support for sutures that are sometimes difficult to manipulate during laparoscopic or microscopic procedures, or can be used as patches to aid in hemostatic and improve visibility of the operative field (Matsuda and Ikada 2004).
Biocompatible adhesives currently used for these surgical purposes include fibrin glues, cyanoacrilates, GRF (gelatin-resorcinol-formaldehyde), polyethylene glycol laser-activated albumin solders, chitosan adhesives, etc. All of them are applied to tissues in the liquid state (sol), followed by gel formation. This sol–gel transition takes place rapidly on the tissues. Cyanoacrilates-based adhesives are the primary candidates for medical applications. They are utilized mostly as tissue adhesives for external skin closure, rejoining veins, arteries, and intestines. Bleeding ulcers could also be sealed with a coating of cyanoacrilate adhesive, protecting the ulcer from stomach acids while healing progresses. In cosmetic surgery, cyanoacrilate adhesives replace stitches and allow skin grafts to heal with much less scarring. They are also useful in ophthalmological surgery, sealing punctures or lesions in the eyeball, and easing corneal transplants. Fibrin glue, which comprises fibrinogen and thrombin, is clinically used for the purpose of sealing, hemostasis and bonding. Thin coatings of biocompatible adhesive sprayed on the bleeding surface almost instantly stop the loss of blood. GRF glue has a unique application, which is to stop bleeding from acute aortic dissection. Hydrogels derived from alginate have been very useful in wound care and tissue engineering where they provide desirable mechanical properties in a hydrophilic environment. Citosan has been employed effectively as a tissue adhesive in which it was enzymatically cross-linked in situ.
Surgical adhesives have also disadvantages. For instance, fibrin glues have a potential risk of viral infection because fibrinogen and thrombin are harvested from human plasma. Concerns about the compatibility of cyanoacrilate-based adhesives with the human immunological system and toxicological effects have forced to go through long and expensive clinical evaluation due to regulatory approval procedures. Eventually, it has been demonstrated that hydrolytic degradation of the polymer causes break down into relatively harmless by-products, and it does at about the same rate at which the wounds heal.
Active programs are currently screening new candidates to develop a new generation of surgical adhesives, able to fulfill additional requirements: biocompatibility and biodegradability, mechanical compliance, incorporations of drugs, growth factors, or antibiotics, strong adhesion to the tissues under wet condition, and minimal inflammatory immune response. For this purpose, polymer adhesives based on entirely new chemistries (e.g., polyurethanes, polyglicerol-sebacate, proteins from living organisms) or biomimetic approaches (nanopatterned gecko-mimicking adhesives) are being developed (Langer et al. 2007).
The term biological adhesive includes the adhesive secretion used by organisms for attachment, construction, obstruction, defense, and predation (Smith and Callow 2006; Graham 2005). These natural glues are produced in marine and other wet environments by fish, holothurians, mollusks, arthropods, worms, bacteria, algae or fungi, and on land by amphibians, spiders, insects, etc.
Bacterial exopolymeric adhesives are present in biofilms, a common form of bacterial existence, where cells form highly hydrated cohesive masses that adhere to surfaces (Hagg 2006). Biofilms can be problematic and result in corrosion and fouling in industrial systems. These biological adhesives are composed of proteins – responsible for the initial adhesion processes, polysaccharides – responsible for the subsequent adhesive interaction and cohesive strength of the biofilm, and nucleic acids. The marine bacterium Alteromonas colwelliana (LST) produces a polysaccharide adhesive viscous exopolysaccharide (PAVE), which adhere strongly to surfaces under severe conditions in its natural environment. It also synthesizes dihydrosyphenylalanine (DOPA), tyrosinase, and related quinones, which participate in water-resistant adhesive production in higher organisms (e.g., mussel). The DOPA-based cross-linking is a productive approach to improve mechanical properties, in particular moisture resistance of adhesives derived from amine-functionalized polysaccharides.
Many fungi adhere tenaciously onto inert surfaces. The fungal-substratum adhesion is mediated by a glue secreted by the organism (Epstein and Nicholson 2006). Non-glue components in the extracellular matrix can alter the substrate by chemical etching, and increase the strength of attachment. Many fungi adhere more efficiently to hydrophobic than to hydrophilic surfaces. Fungi probably produce glycoprotein-based glues, which must be sufficiently nonsticky to be secreted, sticky during glue formation, and then typically nonsticky after setting. Fungal glues are extremely insoluble.
Intertidal green alga Ulva spores anchor to the substratum through the secretion of an adhesive secreted by the rhizoids (Callow and Callow 2006). Understanding interfacial adhesion in algae is necessary for controlling the undesired effects of soft fouling. The adhesive is discharged quickly from the attaching organism, wetting the substrate without dissolving in the water. During the bonding process, it has the capacity to exclude water molecules. Finally, the adhesive “cures” quickly to achieve a cohesive strength sufficient to hold the organism under turbulent conditions. All these processes are effective in an environment characterized by a wide range of substrates, temperatures, and salinities. Diatoms are a ubiquitous group of unicellular microalgae. The cell wall, or “frustule,” comprises the silicified wall and associated organic polymers. Adhesion to a substratum is related to the secretion of a mucilaginous substance that forms a biofilm when there is an accumulation of diatoms cells (Chiovitti et al. 2006). Some studies of the formation of adhesives in marine brown algae indicate that the mechanism is related to the oxidase-mediated polymerization of phenolic compounds (Potin and Leblanc 2006).
Adhesive gels (more than 95% water) are highly deformable, and they do not seem to be a priori an ideal candidate to become an adhesive. In fact, hydrogels are often excellent lubricants. Nevertheless, a wide array of animals use gels as powerful glues. For instance, snails use gels to attach to wet, untreated surfaces, with an adhesive strength that approaches the values of solid cements of mussels and barnacles. Many echinoderms, worm-like invertebrates, some species of frogs, and gastropods also adhere to gels. The normal mucous gel of these animals consists primarily of giant protein-carbohydrate complexes, which is not inherently sticky. The mucus that is used in adhesion, though, is different. A major difference is the presence of smaller glue proteins in the adhesive gel. They are able to stiffen gels by cross-linking of the polymer.
Sea stars, members of the phylum Echinodermata, use adhesive secretions extensively: in the tube feet or podia for attachment to a substrate, in the larval adhesive organs, and in the sticky defense organs of some species (Flammang 2006). Temporary adhesion is necessary for locomotion in the sea bottom. During attachment to the substrate, two types of cells release some of their granules whose contents coalesce and mix to form the adhesive material. During detachment, a different type of cell releases its de-adhesive secretion, which functions to jettison the outmost layer of the cuticle allowing the podium to detach. An adhesive footprint remains on the substrate.
Frogs of the genus Notaden secrete on their dorsal skin a yellow adhesive material (Graham et al. 2006). This adhesive bonds rapidly to a wide range of polar and nonpolar materials, including moist and cool surfaces. Notaden adhesive is compatible with cell attachment and growth, while forming an open porous structure that allow wound healing. The adhesive is a highly hydrated protein material, with small amounts of carbohydrates.
The siliceous skeletal system of the Western Pacific hexactinellid sponge, Euplectella aspergillum, is a complex hierarchically ordered composite (Weaver et al. 2007). The basic building blocks are laminated skeletal elements (spicules) that consist of a central proteinaceous axial filament surrounded by alternating concentric domains of consolidated silica nanoparticles and organic interlayers (adhesive). This animal also shows an interesting example of non-organic adhesive system. Bundled spicules are embedded in silica matrix that serves as a cement to consolidate and strengthen the entire skeletal system.
Mechanisms of Adhesion in Nature
Surface roughness plays an important role on the adhesional interaction between solids and is the main reason why macroscopic solids usually do not adhere to each other with any measurable strength. The interaction between neutral solids is very short ranged, becoming negligibly small already at separations of the order of a few atomic distances. Thus, strong interaction is only possible if at least one of the solids is elastically very soft so that the surface can bend and make atomic contact at the interface. In this case there will be a large area of real contact between the solids, and the stored elastic energy is small. The second criterion for strong adhesion is that the interaction between the solids should involve “long dissipative bonds.” That is, during pulloff, the effective adhesion bonds should elongate a long distance and the elastic energy stored in the bonds at the point where they break should be dissipated in the solids rather than used to break the other interfacial adhesion bonds (Persson 2007). Synthetic adhesives often satisfy both these criteria, and are based on thin elastically soft polymer films. However, biological adhesive systems –especially those used for locomotion – cannot be built on the same principles. Otherwise, the adhesive would wear rapidly, and during repeated use would rapidly be covered by small particles and would fail after just a few contact cycles. Nature has its own kind of mechanisms of adhesion. It is convenient to group them into six different categories (Barnes 2007) that act individually or combined: interlocking, friction, suction, wet adhesion, dry adhesion, and gluing.
Friction involves both microinterlocking and intermolecular forces between materials at points of contact. The relationship between friction and adhesion is one of the most fundamental issues in surface science (Ringlein and Robbins 2004). Friction is a mechanism that never acts independently, but coupled at the nanoscale with other adhesive mechanisms.
There are evidences of a combination of anisotropic friction and adhesion in the smooth adhesive pads of crickets (Jiao et al. 2000) and in toe pads of tree frogs, where friction would depend upon the viscoelastic properties of the intermediary fluid. This suggests that the static friction, which is biologically important to prevent sliding, is based on the non-Newtonian properties of the adhesive emulsion rather than on a direct contact between the cuticle and the substrate. However, to explain the adhesive capabilities of geckos, where there is no evidence of any adhesive substance at all, existing data suggest that friction at the seta level is about two to four times the adhesion (Autumn et al. 2000).
Disk-winged bats (family Thyropteridae) have well-developed suction disks on their wings, where adhesion is through reduced internal pressure (Riskin and Fenton 2001). Limpets can create strong suction adhesion mediated by mucus that is not inherently sticky. In this case, mucus is only used in the edges to maintain the pressure difference and to avoid leaks. The animal needs to contract the musculature to create suction; mucus strength in shear is low or non-existing.
Smooth adhesive pads are found among the arthropods (particularly insects), in amphibians (tree frogs and arboreal salamanders), and in a few mammals (arboreal possums). There does seem to be a best design for a toe pad, in tree frogs at least, and this is important for biomimetics: smooth adhesive pads have evolved several times independently –convergent evolution. The toe pad epithelium consists of an array of flat-topped cells separated by mucous filled grooves. The hexagonal array of channels that surround each epithelial cell presumably functions to spread mucus evenly over the pad surface, and under wet conditions remove surplus water. The most important property of smooth pads is their extreme softness. This gives them a unique ability to conform to irregularities in the surfaces to which they are adhering.
In all smooth adhesive pads, there is a thin intervening layer of fluid between the pad and substrate. In tree frogs it is watery mucus, in arboreal possums it is sweat, and in insects it is an emulsion consisting of a hydrophilic and a hydrophobic phase. The fluid provides the adhesive forces that hold the animal to its substrate; thus, many insects, tree frogs, and arboreal possums are said to attach by wet adhesion. Wet adhesion is made up of two components: capillarity, the surface tension forces generated at the air–fluid interface (meniscus) around the perimeter of the pad; and Stefan adhesion, viscous forces acting over the whole of the area of contact (Barnes 2007). The fluids involved in wet adhesion are not sticky at all, and gluing is not the mechanism of adhesion in this case; it is the force arising from the formation of liquid bridges between adhesive pads and substratum.
The gecko adhesion system has extraordinary properties: (1) is directional; (2) attaches strongly with minimal preload; (3) detaches quickly and easily; (4) sticks to nearly every material; (5) is self-cleaning; (6) does not self-adhere; and (7) is nonsticky by default (Autumn 2006). Gecko dry adhesion depends more on geometry than on chemistry of the substrate or the material of the fibrils – keratin proteins.
Van der Waals-type interaction of every single seta with the substrate is a sufficient mechanism to explain the uncommon properties of gecko adhesive system. Van der Waals attractive forces are intermolecular forces that exist between atoms or nonpolar molecules. They are extremely weak, typically less than 1% of the average covalent bond strength. Other mechanisms (e.g., friction, capillarity) could play a minor role.
Adhesive contact between elastic objects usually fail by propagation of crack-like flaws initiated at poor contact regions around surface asperities, impurities, trapped contaminants, etc. For a single fiber on substrate, the contact fails not by crack propagation, but always by uniform detachment at the theoretical strength of adhesion, a concept termed as “flaw tolerance” (Yao and Gao 2006). Bottom-up designed hierarchical structures of gecko allow the critical length for flaw tolerant adhesion to be extended to a larger scale. Structural hierarchy plays a key role in robust adhesion: it allows the work of adhesion to be exponentially enhanced with each added level of hierarchy.
An interesting observation is that, while geckos and some insects have adopted hairy tissues for robust and reversible adhesion, some other insects and tree frogs seem to have achieved this via smooth tissues. The main similarity of both designs is that the structured pad surfaces or particular properties of the pad materials guarantee a maximum real contact area with diverse substrates. Dahlquist’s criterion, which is based on empirical observations of pressure-sensitive adhesives, establishes an upper limit for Young’s modulus (E ≈ 100 kPa) of materials with tack properties (Dahlquist 1969). There is emerging evidence that an array of gecko setae can act like a tacky, deformable material –with an effective modulus, Eeff, close to the limit of tack – while individual setae and spatulae retain the structural integrity of stiff protein fibers –with a value of E ≈ 1 GPa for bulk beta-keratin.
Anisotropic attachment: Attachment force variable depending on the setal-spatula orientation with respect to the substrate, normal load, and parallel drag.
High adhesion coefficient: Ratio of preload to pull-off force, which represents the strength of adhesion as a function of the compressive load.
Low detachment force: Gecko manages to detach their feet in just 15 ms by simply increasing the angle of the setal shaft.
Material independent adhesion: Van der Waals-based interaction with the substrate depends basically on the geometry of the fibrillar structure and is not related to its chemistry.
Self-cleaning: Geckos feet are not contaminated by micron-scale particles. Spatulae are made in fact of an anti-adhesive material.
Anti-self-adhesion: Hierarchical fibrillar structure is designed to avoid the self-adhesion of individual seta.
Nonsticky default state: Gecko setae are nonsticky by default because only a very small contact fraction is possible without deforming the setal array.
Adhesion to a substratum may be permanent, transitory, temporary, and instantaneous. Permanent adhesion involves the secretion of a cement and is characteristic of sessile organisms staying at the same place throughout their adult life (e.g., the attachment of barnacles on rocks). Transitory adhesion allows simultaneous adhesion and locomotion: the animals attach by a viscous film they lay down between their body and the substratum, and creep on this film which they leave behind as they move. Instantaneous adhesion is related to very rapid attachment for prey capture or defensive reaction. Permanent adhesives consist almost exclusively of proteins, while non-permanent adhesives are made up of an association of proteins and carbohydrates.
Specifically for marine glues, charged and polar amino acids of their proteins are probably involved in adhesive interactions with the substratum through hydrogen and ionic bonding. Small side-chain amino acids, on the other hand, are often found in elastomeric proteins, which are able to withstand significant reversible deformations. Marine glues thus appear to be tailored for both high adhesive strength and high cohesive strength (Flammang 2003).
Biomimetic and Bioinspired Adhesives
Biomimetic adhesives are synthetic adhesives designed to closely mimic the molecular structure and mechanisms of adhesion found in nature. Bioinspired adhesives are synthetic adhesives whose design is inspired in biological concepts, mechanisms, functions, and design features. The aim is not to emulate any particular biological architecture or system, but to use such knowledge as a source of guiding principles and ideas.
A first approach is to obtain substances directly from organisms, to be used in the preparation of adhesive systems. Bacterial exopolymer adhesive have been isolated from several strains. PAVE (Polysaccharide Adhesive Viscous Exopolymer)-based adhesives are obtained from marine bacterium. The fermentation residues of the Ruminal cellulotic bacteria – consisting of incompletely fermented biomass, adherend bacterial cells, and associated exopolymers adhesins – have been found to be potentially useful as wood adhesives. Montana Biotech adhesive is a water-based bacterial exopolymer adhesive that is produced by an unidentified organism, and was developed as an alternative to VOC (volatile organic compounds)-containing adhesives. Specialty Biopolymers Corporation has developed another bacterial exopolymer-based adhesive of undisclosed structure. Most of the adhesives that have been studied are water based and appear to be best suited for bonding porous substrates such as wood, which readily allow for water evaporation or absorption. They exhibit strong adhesion to high energy surfaces and high cohesive strength, but are sensitive to moisture. Cross-linking ability or increased hydrophobic character of natural biopolymers can be improved using modern methods in bioengineering. By-products of the seaweed industry are rich in phenolic polymers, inspiring new uses for algae. When using algal phenolics as adhesives, an industrial catalysis process would require large amounts of enzymes. An international patent has succeeded in obtaining the industrial method for massive production of brown algae-derived adhesives (Vreeland 2002).
The challenge with all of the biological adhesives is to understand their mode of action sufficiently well that their essential features (e.g., specificity toward various substrata, speed of action, insolubilization of the adhesive, life expectancy of the attachment, etc.) can be mimicked, in the case of the design of new adhesives, or inhibited, in the case or biofouling control. Mimics of mussel adhesive proteins are used in the form of chemical conjugates with antifouling polymers for conferring fouling resistance to surfaces (Lee et al. 2006). These bioinspired polymers make outstanding surface anchors for antifouling polymers.
Gecko-inspired adhesives are booming, as illustrated by the growing number of papers published on this topic (Davies et al. 2009). One of the first developments was “gecko tape” (Geim et al. 2003), featuring flexible fibers of polyimide on the surface of a film of the same material using electron beam lithography and dry etching in oxygen plasma. Other approach involves the application of nanotechnology, notably the use of carbon nanotubes. The procedure consists in the deposition of multiwalled nanotubes by chemical vapor deposition onto quartz and silicon substrates (Yurdumakan et al. 2005). The nanotubes are typically 10–20 nm in diameter and around 65 μm long. The vertically aligned nanotubes are then encapsulated in poly(methylmethacrylate)-PMMA polymer before exposing the top 25 μm of the tubes by etching away some of the polymer. Unlike conventional adhesive adhesive tape, which eventually lose its stickiness, this new material sticks like a permanent glue but can be removed and reused. It can also adhere to a wider variety of materials, including glass and teflon. Recent improvements to this bioinspired adhesive (Ge et al. 2007) have succeeded for the first time to manufacture a flexible patch that can be used repeatedly with a peeling strength better than those of the natural gecko foot.
The structure of the feet of a beetle from the family Chrysomelidae has provided the inspiration for developing an adhesive that is produced from a mould, which has the required surface features – mushroom-shaped micro-hairs – embossed as a negative image. The mould is filled with a polymerizing mixture, which is allowed to cure, and then released (Daltorio et al. 2005). A selection of results for macroscale patches of gecko adhesive materials from a range of research groups can be found in Bogue (2008). To facilitate fabrication, most of the synthetic fibrillar structures developed and studied to date have had just a single level, consisting of simple micropillar arrays. Recently, structures with at least one additional element of complexity have been introduced, either a terminal thin plate or flared ends for separate fibrils (Greiner et al. 2009).
Whilst all of these developments concern dry adhesion, researchers are also now studying how derivatives of naturally occurring compounds from mollusks can be combined with gecko-type structures to yield adhesives that will operate in either dry or wet conditions (Lee et al. 2007). An array of gecko-mimetic silicone pillars fabricated by electron beam lithography and coated with a mussel-mimetic polymer. The adhesive properties of this bioinspired adhesive does not rely on van der Waals interactions; instead, it relies on the chemical interaction of the substrate with the hydroxyl groups in the mussel protein. The so-called geckel tape sticks strongly in both wet and dry environments.
Healing in biological systems is a pervading characteristic that would be very advantageous for man-made materials. Recent efforts have been directed to introduce this outstanding property in synthetic polymeric systems, including adhesives, based on three different approaches: self-healing polymers composed of microencapsulated healing agents (Brown et al. 2005), re-mendable polymers (Chen et al. 2003), and three-dimensional microvascular networks embedded in the polymer (Toohey et al. 2007).
Self-Healing Based on Microencapsulated Healing Agents
Self-Healing Based on Re-mendable Polymers
Re-mendable polymers can achieve multiple healing cycles, but require external intervention in the form of heat treatment and applied pressure. It is a powerful method of crack reparation for polymers that exhibit thermally reversible reactions for cross-linking linear chains. For example, a Diels-Alder polymerization product of reaction between a multi-furan and multi-maleimide has shown to be feasible and a recovery of about 57% of the original fracture load has been obtained. Other types of organic adhesives are not capable of repolymerizing, and alternative self-healing procedures should be employed.
Self-Healing Based on Embedded Microvascular Networks
Autonomic healing of structural damage is accomplished by delivery of healing agent to cracks in a polymer via a three-dimensional microvascular network embedded. Crack damage in the polymer is healed repeatedly. Recent advances in soft lithographic and direct-write assembly methods have enabled the creation of materials with complex embedded microvascular networks that emulate many of the key responses of biological vascular systems. The network is filled with a liquid healing agent, and solid catalyst particles are incorporated within the polymer. After damage occurs in the polymer, healing agent flows from the microchannels into the cracks through capillary action, and then are arrested at the adhesive–substrate interface. The healing agent interacts with the catalyst particles in the polymer to initiate polymerization, rebonding the crack faces. After a sufficient time period, the cracks are healed and the structural integrity of the adhesive is restored. As cracks reopen under subsequent loading, the healing cycle is repeated.
The healing agent possesses a low viscosity, which facilitates its flow into the crack plane. The solid-phase catalyst remains reactive during and after curing of the coating, and the catalyst particles quickly dissolve on contact with the monomer in the crack plane and polymerize under ambient conditions, producing a tough cross-linked polymer.
Direct-write assembly has been used to embed fully interconnected 3D microchannel network in an epoxy matrix. 3D scaffolds were fabricated with a fugitive organic ink using a robotic deposition apparatus in a layerwise scheme. As the presence of channels impacts the structural properties of the polymer, networks with maximum channel spacing and minimum channel diameter are desirable. Autonomic healing efficiency is evaluated on the basis of the ability of the healed polymer to recover fracture toughness.
Adhesives are used extensively in nature. Their diversity is a source of inspiration for developing new man-made adhesives different from those currently available.
Bioadhesives exhibit a certain number of characteristics that make them differ greatly from artificial adhesives: sensitivity to – and critical dependence on – the presence of humidity, recurrent use of molecular constituents, ability to form sound attachments with a wide diversity of substrates, variation in properties in response to performance requirements, improved fatigue resistance and resilience, and conferring to the biological hybrid materials a damage-tolerant design.
Adhesion mechanisms in biology are diverse, and robust enough to function on rough and wet surfaces. However, bio-adhesion must be easily releasable when related to animal movement.
Natural adhesives represent the oldest adhesives known to man. The general trend since World War II has been a steady decrease of the consumption share of natural adhesives. However, real volume has actually increased, and supposes around 30% of the market for adhesives that do not use volatile solvents.
Biocompatible adhesives are used as surgical glues for: external skin closure, rejoining veins, arteries, and intestines, sealing of bleeding ulcers and hemostasis, and ophthalmological surgery. Orthopedic surgical applications are envisaged for the near future. Toxicological and immunological effects have been forced to go through long and expensive clinical evaluation procedures.
Nature is indeed a school for materials science and its associated disciplines such as chemistry, biology, physics, or engineering. Biomimetic and bioinspired adhesives are being designed to take advantage of the many inspiring properties of materials found in nature, such as sophistication, miniaturization, hierarchical organizations, hybridation, resistance, and adaptability.
Self-healing bioadhesives can stop minor damage from escalating to critical levels. This characteristic is being introduced in man-made bioinspired adhesives.