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Adhesives
1- INTRODUCTION
During the 20th century, diagnosis and treatment of injury and illness improved significantly. This was partially made possible through the development of biomaterials which save lives, relieve suffering and help maintain quality of life for a population enjoying ever-increasing longevity.
The first generation biomaterials have been developed by technology transfer from other market sectors such as mechanical engineering and electronics. As a result, the complexity of replacing the nature and function of the natural tissue has placed limitations on this approach. However, today materials science, nanotechnology and molecular biology techniques permit design and preparation of precisely engineered surfaces.
One of the most interesting and extensively studied example of biomaterials is the Bioadhesives, which will be presented in this paper. But prior to starting with the adhesives, let us consider the surface and interface reactions that give rise to adhesion phenomena.
1-i) The Historical Perspective
The Greeks found that they needed only two fundamental forces to account for all the natural phenomena; love and hate! Love brought things together while hate caused them to part. This theory was first proposed by Empedocles around 450 B.C. and was much improved by Aristotle; forming the basis of chemical theory for 2000 years [1].
Figure 1. The natural forces [1].
Towards the second part of the nineteenth century, the modern concept of surface tension forces was firmly established. According to this theory, the surface forces are the same as those that hold molecules together in solids and liquids. In both cases they arise from interactions acting over very short distances. In addition, it was shown that these very short range surface tension forces can account for such phenomena as capillarity, the shapes of macroscopic liquid droplets on surfaces, the contact angle between coalescing soap bubbles and the break up of a jet of water into spherical droplets.
Thus, it was established that very short range forces can lead to very long range (i.e., macroscopic) effects [2]. Now let us consider those intermolecular forces.
1-ii) Classification of Forces
Intermolecular forces can be loosely classified into three categories. First, there are forces that are purely electrostatic in origin arising from the Coloumb force between charges. The interaction between charges, permanent dipoles, quadrupoles, etc. fall into this category. Second, there are polarization forces that arise from the dipole moments induced in atoms and molecules by the electric fields of nearby charges and permanent dipoles. All interactions in a solvent medium involve polarization effects. Third, there are forces that are quantum mechanical in nature. Such forces give rise to covalent or chemical bonding and to the repulsive steric or exchange interactions that balance the attractive forces at very short distances. A number of examples of interactions are given in Figure 2. [1].
Figure 2. İnteractions between molecules [1].
The fundamental forces involved in particle-surface interactions are the same as those which were already mentioned. However, those forces can manifest themselves in quite different ways and lead to qualitatively new features when acting between large particles or extended surfaces. This condition describes the phenomena of surface energies, which are determined by intermolecular forces between two surfaces. Surface energy (for solids. surface tension is used for liquids) implies the interaction potentials of molecules belonging to surfaces. It is evident that the intermolecular forces that determine the melting point of a substance are the same as those that determine the surface energy.
1-iii) Adhesion
Surface energy is the free energy change when the surface area of a medium is increased by a unit area. For solids it is denoted by gs and is given in units of energy per unit area (m.J.m-2 , erg.cm-2 ) For liquids it is denoted by gl and is usually given in units of tension per unit length (m.N.m-1, dyne.cm-1) which is numerically and dimensionally the same as the unit energy.
Simply, the concept of surface energy describes the interaction potentials of molecules of different surfaces developed due to disrupted equilibrium bonding arrangements. This disruption leads to excess energy of surface which is minimized by either attracting foreign materials (adsorption), or by bonding with the adsorbate (chemisorption).
The free energy change, or the reversible work done to separate unit areas of two surfaces from contact to infinity in vacuum is called the ‘work of adhesion’ if the surfaces are different and ‘work of cohesion’ if the surfaces are the same. Therefore, adhesion is the welding of two unlike surfaces.
The adhesion phenomena can be explained by examining the behavior of macroscopic liquid droplets when they come into contact with a surface. The Figure 3. demonstrates three of possible situations resulting from settlement of spherical liquid droplets on a rigid surface.
Figure 3. Concepts of ‘wetting’ and ‘nonwetting’ [1].
Note that in the Figure 3., the upper and lower drawings in each box are formally equivalent. The media 2 and 3 are interchanged so that θbottom = 180- θtop
The θ is the contact angle between the droplet and the surface. It explains compatibility of two surfaces. Smaller the θ, more compatible are the surfaces and it is more probable that adhesion will occur.
When θ is zero, the liquid spreads on the surface completely and it is said to wet the surface. Wetting is a nonreciprocal property, i.e; if A spreads on B, B does not necessarily spread on A. A contact angle between zero and ninety degrees shows that the droplet wets the surface partially. However any contact angle with a degree greater than ninety, demonstrates the nonwetting phenomena.
While the above results are derived from the specific case of a spherically shaped droplet on a flat surface, the contact angle is independent of surface geometry [3].
1-iv) Adhesives
Contact angles can often be changed by chemically modifying substances or by addition of certain solute molecules into the medium, which are called surfactants. Adhesives are materials that enable welding of two surfaces by providing strong interactions which are unlikely to be formed otherwise. The terms adherent and substrate are used for a body or material to be bonded by an adhesive.
Intimate molecular contact is necessary for the success of adhesives and relies largely on the following:
1- Adhesive must exhibit zero or near zero contact angle when liquid.
2- Adhesive must have a low viscosity during bonding.
3- Adhesive must be able to displace air and contaminants during application.
Four main mechanisms of adhesion have been proposed:
1- Mechanical interlocking theory involves the penetration of the bonding agent into the surface irregularities or porosity in the substrate surface as means of adhesion.
2- Adsorption theory relies on the fact that if intimate interfacial molecular contact is achieved, interatomic and intermolecular forces will establish a strong joint.
3- Diffusion theory states that the adhesion of polymers to substrates and to eachother requires a mutual diffusion of polymer molecules or segments across the interface.
4- Electronic theory suggests that electronic transfer between adhesive and adherent may lead to electrostatic forces that result in highly intrinsic adhesion.
All adhesives either contain polymers or polymers are formed within the adhesive bonds. Polymers give the adhesives cohesive strength and can be thought of as strings of beads which may be either linear, branched or crosslinked. Crosslinked polymers will not flow when heated, and may swell but not dissolve in solvents [3]. All biological adhesives should be crosslinked because this eliminates deformation under constant load, i. e; creeping. Creeping is very likely to occur in such aqueous environments as biological systems.
The ‘special’ environment of tissues and their regenerative capacities make the development of an ideal adhesive difficult. As well as the general requirements of an adhesie, bioadhesives should also meet the needs of :
☻ being capable of rapid polymerization without production of excessive heat or toxic products,
☻ being biocompatible, without interfering with the biological tissues.
Many adhesives contain additives that are not polymers. These include stabilizers against degradation by oxygen and UV, plasticizers which increase the flexibility and lower the glass transition temperature. As well as stabilizers and plasticizers, powdered mineral fibers are also used for :
☺ reducing shrinkage on hardening
☺ lowering the cost
☺ modifying flow properties
☺ modifying final mechanical properties of the adhesives.
2- CLASSIFICATION OF BIOADHESIVES
Bioadhesive materials can be divided into two categories according to their application sites in the body ;
1- Soft Tissue Adhesives : They are used both externally for temporarily fixation of devices such as colostomy bags and internally for wound closure and sealing.
2- Hard Tissue Adhesives : They are used to bond prosthetic materials to calcified tissues such as bone, tooth, enamel.
2-1) Soft Tissue Adhesives
2-1) a) CYANOACRYLATES
Cyanoacrylates are commonly used adhesives both in everyday use as super glue and as a biomaterial. Its molecular structure is shown in Figure 3. As the figure implies, they contain two strongly electron withdrawing groups, CN- and COO-, which makes them very susceptible to anionic polymerization.
CN
┃
CH2 = C
┃
COOC2H5
Figure 3. Ethylcyanoacrylate
Polymerization is initiated by water, which is adsorbed on all surfaces in the atmosphere [3], and is complete within seconds. This process is inhibited by oxygen. Therefore, the curing reaction will not happen until the oxygen supply is cut off.
While the cyanoacrylates are incredibly strong adhesives, they are not bioabsorbable, biocompatible, and they have been found to be carcinogenic in some cases. Another drawback of this type adhesives is the fact that they hydrolize quickly in the body, leaving behind formaldehyde, which causes acute inflammatory response [4]. Current studies have lead the development of two derivatives of cyanoacrylates, which are 2-octylcyanoacrylate and n-butylcyanoacrylate. These materials degrade more slowly than their predecessor and polymerize more rapidly in blood [4].
Recently the uses for cyanoacrylates have mostly been involved in treating smaller lacerations. It is an alternative to using sutures in plastic surgery and is very effective in treating simple lacerations in children [5].
2-1) b) FIBRIN
The agents in fibrin sealants are used to produce a process similar to the physiological coagulation cascade to form a fibrin clot [6]. They can be used for hemostasis, wound closure, and tissue sealing. The fibrin clot is then reabsorbed throughout the wound’s natural healing process.
The fibrin sealants are advantageous because they are both biocompatible and biodegradable, and are not associated with inflammation, foreign body reactions, tissue necrosis, or extensive fibrosis [7].
One disadvantage of fibrin sealants was thought to be the high associated risk of hepatitis and AIDS transmission from the pooled human plasma during the in vitro production of fibrinogen. This risk is greatly reduced by individual units of screened donor blood plasma. Therefore, the appropriate use use of fibrin sealants can improve blood conservation and reduce intraoperative bleeding in certain procedures [6].
2- 1 ) c) GELATIN – HYDROGEL SEALANTS
These type of adhesives provide alternative resorbable biological glues that have greater bonding strength than fibrin based glues. The first such hydrogel glues developed and used clinically were GRF (gelatin + resorcinol + formaldehyde) and GRFG (gelatin + resorcinol + formaldehyde + gluteraldehyde) glues. Despite their efficiency, the use of GRF and GRFG is limited because of their cytotoxicity, resulting from the formaldehyde release during degradation. Substential late complications have been reported following GRF use in cardiothoracic surgery [8].
The second generation of gelatin – hydrogel glues are less toxic because the formaldehyde is substituted with other crosslinking agents, e.g., carbodiimide or genipin [9].
Photochemically activated tissue bonding technology involves using photoreactive gelatins and a water soluble macromer (polyethylene glycol diacrylate PEDGA). Photoreactive groups, such as UV – reactive benzophenone or visible light – reactive xanthene dyes, are incorporated into gelatins which are then suspended in a saline solution. This solution will form an adhesive hydrogel within 1 min. When irradiated with the appropriate light. The in situ photogelation will result in prompt, safe and effective hemostasis.
2- 2) Hard Tissue Adhesives
Adhesion to calcified tissues (bone, enamel, dentin) can be achieved by mechanical interlocking to machined surfaces. Orthopedic implants have been succesfully fixed by using room temperature – polymerizing methyl methacrylate, however conditions in the mouth are much more stringent due to changing environmental temperature, mechanochemical stress on the bond, and the presence of bacteria. Therefore any other type of adhesives in terms of polymerization conditions were required to be developped. Considerable developments in new dental cements and adhesive systems have occured in recent years in an attempt to provide a leak – proof bond to attach fillings, crowns, and veneers to the tooth [10].
Zinc phosphate cement is traditional, hard, rigid cement which penetrates into mechanically produced surface irregularities. The cement is gradually soluble in oral fluids and has been known to irritate pulp but has a general effectiveness of 10-20 years.
Zinc polyacrylate calcified tissue cements are formed from zinc oxide and a polyacrylic acid solution. The metal ion, cross-links the polymer via carboxyl groups to form a complex with calcium ions on the surface of the tissue. Zinc polycarboxylate cements demonstrate adequate physical properties, excellent biocompatibility and proven adhesion to enamel and dentin.
Glass ionomer cements are also based on polyacrylic acid or copolymers of itaconic or maleic acid. But they utilize calcium aluminosilicate glass powder instead of zinc oxide. Set structures and residual glass particles yield stronger and more rigid cement while exhibiting adhesive properties similar to those of the zinc polyacrylate cements. Glass ionomer cements are formed by the reaction of alumina and silicate, which are chelated with carboxylate groups to crosslink the polyacids [11, 12].
The resin cements are fluid or paste like monomer systems based on aromatic or urethane dimethylacrylates polymerized by chemical reactions.
3- RECENT STUDIES INVOLVING BIOADHESIVES
In this part, I want to focus on the improvements on adhesives used in clinical dentistry. But first of all let us have a look at the dentin composition which is the actual portion of the teeth that is exposed to the complex environment of mouth.
Dentin
The dentin is a vital, mineralised tissue formed in a collagen network. Dentinal tubules, which contain odontoblast processes, are radiating from the pulp to the enamel. The odontoblast responds to irritation with mineral deposition in dentinal tubules and formation of new dentine on the pulp walls.
Approximately half the dentine volume is mineral as hydroxyapatite crystals . The crystals are smaller than the ones found in enamel and contain more carbonates.
Dentin collagen is mainly type I and the amino acids proline, glycine and hydroxyproline are common. The collagen molecules form fibrils, which are stabilised by covalent crosslinks. The crosslinking increases resistance to degradation.
The Dentine Caries Lesion
The caries lesion progression in the dentine is a process governed by the intensity of the bacterial acid challenge, the structure of the tissue and the response of the tissue [13]. The carious process in the dentine starts with bacterial acids affecting the mineral crystals. The crystals become smaller during the dissolution process, and ”porous” areas are formed. Diffusion of acids and dissolved minerals are facilitated in the slightly demineralised dentinal structure. Diffusion of bacterial products into the dentinal tubules induces a vital response via the odontoblasts, and minerals are deposited in the tubules. Re- and demineralisation also leads to deposition of irregular crystals in tubules and dentine.
When bacterial acids attack dentine, the mineral gradually becomes dissolved exposing the collagen network. During the demineralisation the crosslinks in exposed collagen reversibly shift to precursors. This reversible denaturation of collagen implies that upon removal of the acid challenge, reorganisation and thus remineralisation can take place.
As the process continues, collagen exposed to bacterial acids becomes less resistant to enzymatic degradation, and the denaturation is not longer reversible. When the mineral is completely lost, further enzymatic breakdown of collagen occurs. The dentinal structure in this necrotic part of the lesion is lost.
The caries process can progress with different speed depending on the metabolic activity of the bacteria. A rapidly progressing lesion will differ from a slow progressing lesion. However, in general the dentine caries lesion exhibits different zones of tissue destruction and tissue defence reactions [14].
In conclusion, the caries lesion contains an outer zone of necrotic material. The inner zone is demineralised to various degrees, but has the potential to remineralise if the acid challenge is removed.
Traditional caries removal
The dental drill in various shapes is designed to cut enamel, dentine and restorative materials quickly and with little effort. It is a necessary tool in many aspects of operative dentistry, but in removing dentine caries it is much too non-selective. A radical surgery with the drill results in unnecessary removal of healthy tissue or tissue that can recover (i.e. can remineralise), pain, pulpal complications and weakening of the remaining tooth [15].
A question rises here: 'in what way should the cements be improved so that they both treat and prevent formation of such tooth decays and dental caries without causing such pain?'
A variety of cements have been used in clinical dentistry as liners and bases for cavities, for fixing orthodontic appliances, and for luting crowns and bridges [16].
It is possible to find a wide range of studies performed for improvement of dental cements. John W. Nicholson and his colleagous studied the behavior of several cements in lactic acid solution [17].
The fact that caries is known to be arrested by relatively minor changes in pH suggests that the presence of a cement capable of neutralizing the lactic acid is likely to be beneficial for prevention of further caries.
The traditional cements used in clinical dentistry are glass- ionomers, zinc phosphate and zinc polycarboxylate. All of these cements fall into acid- base class of cements which set by neutralization reaction. Setting is actually hardening of cement into a fully integrated mass without any phase separation. At the end of setting, salt is formed [18].
The study mentioned above made use of the below cements (Table I):
CEMENT |
TYPE |
Chemfil superior |
Glass ionomer |
Chelonfil |
Glass ionomer |
Aqua Cem |
Glass ionomer |
Vitremer luting |
Glass ionomer Resin modified |
Poly F plus |
Zinc polycarboxylate |
Zinc phosphate |
Zinc phosphate |
Table I. Names and types of cements used in experiment. (adapted from Nicholson J. W. et al,)
They placed all above cements in equal amounts and concentrations of lactic acid solution at room temperature the initial pH being 2.60.
After one week, the below results were obtained:
CEMENT |
pH |
None |
2.58 |
Chemfil superior |
4.42 |
Chelonfil |
4.25 |
Aqua Cem |
4.55 |
Vitremer luting |
4.14 |
Poly F plus |
5.25 |
Zincphosphate |
4.79 |
Table II. Local changes in pH of lactic acid solution. (adapted from Nicholson J. W. et al,)
After 1 week, the pH of control solution was recorded to be 2.58 which is not a significantly different value than the initial one. By contrast, all of the cements showed effects on solution pH which are highly significant. As the results imply, the least 1.54 units of pH have changed during this time.
It was also stated in the paper that cement specimens showed no apparent adverse effects following the exposure to lactic acid for 1 week. The surfaces of cements had recorded to be smooth. So we can conclude that glass – ionomer cements are not severely damaged by the acidic conditions.
If the similar changes in pH occur in vivo, the cements will be clinically significant that they may prevent acid mediated decay by locally increasing the pH of their environment.
CONCLUSION
The possibility of neutralization of storage media by cements does not seem to gain that much attention. Applications of these kind of cements, which are capable of neutralizing their acidic environments would not only inhibit development of dental caries but also prevent acid mediated decay of both tooth and the cement itself.
Further studies should be performed to understand more fully the influence of cements on solution pH for given exposure conditions, i. e; the nature, duration and frequency of exposure as well as the effect of size and shape of the cement samples to the effectiveness of neutralization. Also, the extend to which these effects occur under clinical conditions should also be determined.
REFERENCES:
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Academic Press, New York, 1991
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16- Mount G.J., Color atlas of glass _ ionomer cements , 2nd edition, London : Martin Dunitz, 1994
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A variety of scientific articles and clinical studies can be found on the following web address : http://www.mediteam.se/node167.asp |
