The phenomenon of adhesion is of cardinal importance in the pharmaceutical, biomedical and dental fields. A few eclectic examples will suffice to underscore the importance/relevance of adhesion in these three areas. For example, the adhesion between powdered solids is of crucial importance in tablet manufacture. The interaction between biodevices (e.g., stents, bio-implants) and body environment dictates the performance of such devices, and there is burgeoning research activity in modifying the surfaces of such implements to render them compatible with bodily components. In the field of dentistry, the modern trend is to shift from retaining of restorative materials by mechanical interlocking to adhesive bonding.
This unique book addresses all these three areas in an easily accessible single source. The book contains 15 chapters written by leading experts and is divided into four parts: General Topics; Adhesion in Pharmaceutical Field; Adhesion in Biomedical Field; and Adhesion in Dental Field. The topics covered include:
- Theories or mechanisms of adhesion.
- Wettability of powders.
- Role of surface free energy in tablet strength and powder flow behavior.
- Mucoadhesive polymers for drug delivery systems.
- Transdermal patches.
- Skin adhesion in long-wear cosmetics.
- Factors affecting microbial adhesion.
- Biofouling and ways to mitigate it.
- Adhesion of coatings on surgical tools and bio-implants.
- Adhesion in fabrication of microarrays in clinical diagnostics.
- Antibacterial polymers for dental adhesives and composites.
- Evolution of dental adhesives.
- Testing of dental adhesives joints.
Kashmiri Lal Mittal was employed by the IBM Corporation from 1972 through 1993 Currently, he is teaching and consulting worldwide in the broad areas of adhesion as well as surface cleaning. He is the editor of more than 135 books and the Founding Editor of the journal Reviews of Adhesion and Adhesives.
Frank Etzler earned his PhD in physical chemistry from the University of Miami. He is currently an Associate Professor of Pharmaceutics in the School of Pharmacy at the Lake Erie College of Osteopathic Medicine (LECOM).
Theories and Mechanisms of Adhesion in the Pharmaceutical, Biomedical and Dental Fields
Douglas J. Gardner
University of Maine, Advanced Structures and Composites Center, Orono, ME., U.S.A
Corresponding author: email@example.com
Adhesion is an important attribute of material behavior in the pharmaceutical, biomedical, and dental fields that influences the interactions among different substances in the human body, and it is also important as it plays an important role in various processes, including, but not limited to, the manufacture of drugs, medical devices and dental care. Adhesive bonding is an important area focusing on the creation of joined substrates and composite materials. Based on the wide variety of adhesive bonding situations, the concept of adhesion can be broadly applied across different material types and interactions. Mechanisms of adhesion fall into two broad areas: those that rely on mechanical interlocking or entanglement and those that rely on charge interactions. There are seven accepted theories of adhesion. These are: mechanical interlocking; electrostatic theory; adsorption (thermodynamic) or wetting theory; diffusion theory; chemical bonding theory; acid-base theory; and theory of weak boundary layers. In addition, elastomeric-based adhesives exhibit a characteristic adhesion behavior described as tackiness or stickiness that aids in the creation of an almost instantaneous adhesive bond. This chapter provides an overview of adhesion theories and mechanisms relative to applications in the pharmaceutical, biomedical and dental fields.
Keywords: Adhesion, mechanisms, theories, adhesives, bonding, mechanical interlocking, electrostatic, adsorption, wetting, diffusion, chemical, acid-base, weak boundary layers, tackiness
Adhesion mechanisms in the pharmaceutical, biomedical, and dental fields are similar to those encountered in other fields of materials science. However, the biggest challenge is that the adhesion mechanisms will typically occur in or will be influenced by the environment of the human body. The primary challenges facing adhesion in the environment of the human body include: creation of an adhesive bond in contact with various bodily fluids, blood, saliva, etc.; durability of an adhesive bond when exposed to various bodily fluids; the biochemical onslaught related to the body's immune response and cellular regeneration; and exposure to inherent bodily microorganisms such as bacteria and fungi. Common examples of adhesion in the pharmaceutical, biomedical, and dental fields include the manufacture of respiratory inhalants such as albuterol; the application of medical bandages such as Band-aids® used to cover wounds; and the use of denture adhesives to secure false teeth. It is the goal of this Chapter to provide an overview of the current theories and mechanisms of adhesion with reference to applications in the pharmaceutical, biomedical, and dental fields.
1.1.1 Adherend Material Properties Relevant to Adhesion
In the adhesion science and technology community, most materials to be adhesively bonded or glued are referred to as adherends. Adherends in the human body being bonded are usually in a solid form while adhesives are typically in the liquid form (Table 1.1).
Table 1.1 Examples of adherend and adhesive types in the human body. Adherend type Examples Adhesive type Examples
Dense Solid Teeth Low and medium viscosity liquid Acrylate adhesives Porous Solid Bone Low viscosity liquid or viscous filled-adhesive Poly(methyl methacrylate) Soft Solid Skin Low viscosity liquid Cyanoacrylate adhesives for surgical sutures
The processes of joining materials through adhesive bonding to form a bonded assembly in the pharmaceutical, biomedical, and dental fields are quite variable in terms of adherend types and bonding processes including the strength and durability requirements of the resulting adhesive bond. To better understand adhesive bonding processes, adhesion scientists have characterized adhesion mechanisms or theories based on the fundamental behavior of materials being bonded (adherends) as well as the adhesives used to bond the materials. Understanding adhesion requires a close familiarity with the bulk and surface material properties of the adherend and the material property characteristics of the adhesive being used. A list of general material property features to be considered in studying or assessing adhesion is shown in Table 1.2. Surface properties of interest related to adhesion include topography, surface thermodynamics, chemical functionality, hardness, and surface charge. Adhesive features to be considered include: molecular weight, rheology, curing characteristics, thermal transition of polymers, and viscoelasticity. For the bonded assembly, the ultimate mechanical properties, durability, and biological compatibility characteristics are of major importance. In addition, when considering adhesion in the pharmaceutical, biomedical, and dental fields, one also needs to consider cell adhesion. Cellular adhesion is involved with the bonding of a cell to a surface, extracellular matrix or another cell using cell adhesion molecules . Cell adhesion continues to receive considerable attention in the adhesion field.
Table 1.2 General materials related to adhesion and their assessment methods. Material Assessment methods
Adherend Topography, wettability, chemical functionality, hardness, surface charge Adhesive Molecular weight, rheology, curing characteristics, thermal transitions, viscoelasticity Bonded Assembly Mechanical properties, durability, creep behavior, biological compatibility
1.1.2 Length Scale of Adherend-Adhesive Interactions
The prevailing adhesion theories can be assembled into two types of interactions: 1) those that rely on interlocking or entanglement; and 2) those that rely on charge interactions. Furthermore, it is beneficial to know the length scale(s) over which the adhesion interactions occur. The comparisons of adhesion interactions relative to length scale are listed in Table 1.3. It is obvious that the adhesion interactions relying on interlocking or entanglement, mechanical and diffusion, can occur over larger length scales than the adhesion interactions relying on charge interactions. Most charge interactions involve interactions on the molecular level or nano length scale.
Table 1.3 Comparison of adhesion interactions relative to length scale. Category of adhesion mechanism Type of interaction Length scale
Mechanical Interlocking or entanglement 0.01-1000 µm Diffusion Interlocking or entanglement 10 nm-2 µm Electrostatic Charge 0.1-1 µm Covalent Bonding Charge 0.1-0.2 nm Acid-Base interaction Charge 0.1-0.4 nm Hydrogen Bonding Charge 0.235-0.27 nm Lifshitz-van der Waals Charge 0.5-1 nm
The length scale of adherend-adhesive interactions is also of importance in understanding adhesion mechanisms because although many practical aspects of adhesion occur on the macroscopic length scale (millimeter to centimeter), many of the basic adhesion interactions occur on a much smaller length scale (nanometer to micrometer) (Table 1.4). Wound protection using a Band-Aid® typically occurs on the cm length scale. Interactions between inhaler droplets in the lung occur on the millimeter length scale, and typical microscopic evaluation of the adherend-adhesive bondlines is performed at the 100 µm length scale. Bacteria are on the order of 0.5 to 5 µm in diameter. Nanoparticles are generally in the scale of 10 to 100 nm in diameter.
Table 1.4 Orders of scale for adherend-adhesive interactions in the pharmaceutical, biomedical and dental fields.* Scale Test specimen or material characteristics for determining adherend-adhesive interactions
1 cm, 10 mm Wound protection using a Band-Aid® 10-3 meter, 1 mm Inhaler droplet interactions in the lung 10-4 meter, 100 µm Microscopic evaluation of adherend-adhesive bondline 10-6 meter, 1-4 µm Size of bacteria 10-7 meter, 100 nm Scale of nanoparticles
*Adapted from Gardner et al. .
1.2 Mechanisms of Adhesion
There are seven...