Pharmaceutical Sciences Claire Jarry and Matthew S. Shive
11.1 Smart Adhesives
One of the initial mentions of the term smart adhesives in the open literature was a review article with that title. Th e foci of the review were how to work smartly and the factors to consider when choosing an adhesive. Th e article did contain a table (Table 11.1) that is considered to be the forerunner of many concepts and applications of smart adhesives [2].
Another review discussed the uses of smart adhesives in the information technological fi elds. Such usages described the use of surface mount adhesives to fi x and hold soldering and other operations. Single-component heat-cured epoxies were used in this application. Other technology-related smart adhesives like conformal coatings provide thin dielectric coatings that extend circuit board lifetimes by protecting components and traces from corrosion, shorts, and mechanical damage. Solid systems such as UV curing acrylics and silicones were used in this function [3].
An interesting and comprehensive review of medical adhesives for use in products that are applied directly to the body or inside the body by surgical procedures has been reported. Th e article focused on pressure-sensitive adhesives and tissue adhesives.
Th ese two classes of adhesives were further divided into synthetic and biological adhesives. Th e report cited the progress gained in the understanding of wound management and the development and formulation of polymers for pressure-sensitive adhesives that facilitated the wound healing process. Adhesives with high- moisture vapor permeation allow wound dressings to control exudates more eff ectively. Trauma management has seen
advances made in smart pressure-sensitive adhesives that changed adhesive properties with external stimuli [4].
Researchers reported an example of smart adhesion at a poly- mer/metal (oxide) interface that responded reversibly to changes in temperature. Th e temperature dependence in this system arose from the rubber elasticity of the polymer, 1,4-polybutadiene, and mirrored the interfacial behavior of the same polymer against water. Th ese systems off er unique opportunities for designing responsive materials whose properties can be actively controlled [5].
Interesting uses of the waste from the chromium compound treatment of the collagen polymers resulting from the treatment of leather from the tanning process were reported in the literature.
Covalent intramolecular and intermolecular cross-links were formed between modifi ed lysine side chains within the collagen fi brils. High molecular weight versions expanded the potential commercial applications to the veterinary, medical, and cosmetic fi elds. It should be pointed out that the biopolymers isolated from chrome-tanned collagenic wastes were applied directly to the following industries: paper, wood, ink, textile and leather, as binders, biodegradable biopolymers, and protein-based self-feeding biodegradable fl ower pots, fi llers, retanning agents, and casein substitute in photographic adhesives. A second possibility would be to produce low formaldehyde content ecoadhesives and colla- gen-based smart fabric formed by chemical reaction with these biopolymers [6].
TABLE 11.1 Examples of Smart Adhesive Properties and Typical Applications
Smart Adhesive Properties Applications
Aesthetics—color surface fi nish Color matching
High contrast for visible security Cross-pigmentation for two-part security Opaque or translucent
Disassembly Disbond on demand for materials recycling
Materials mimicry Ability to be machined, drilled, or cut Magnetic
Th ermal conductivity Th ermal management in electronics and electric motors Heat sink attachment
Electrical conductivity Organic solder replacement Electrical resistance/insulation Electrical terminal potting Energy and vibration adsorption Th read locking
Gas seal in threading fi ttings Mirror bonding in transportation Joint disassembly
Heat resistance, fi re resistance, fi re extinguishing
Mass transport and aerospace interior trim
Hot joints in electrical devices or high-temperature environments Auto-surface coupling Minimizing surface treatment on aluminum and composites Self-indicating—presence of cured adhesive UV fl uorescence for automated quality
Color change on cure for hazardous gas fi ttings
Security Leak sealing and repair
Ultralow density Weight saving in transportation—particularly for aerospace duties Vibration adsorption Noise abatement, vibration damping
Regulatory compliance Intrinsically safe in use: medical devices, drug-delivery patches and food contact (direct or indirect).
Source: From Fakley, M., Chem. Ind., 21, 691, 2001. With permission.
In the realm of smart materials, smart adhesives, and other smart types, there exists a unique class of materials, which utilize a smart material in conjunction with a nonsmart material. Th ese loosely belong to the classifi cation of smart structures. Th ere are smart structures that monitor the behavior of adhesives or enhance the manufacture of adhesives.
Th e fi rst example of these types of smartness is the use of piezoelectrics to monitor the behavior of adhesives. Th e action of piezoelectrics is discussed in Chapter 9. Metalized poly(vinylid- ene fl uoride) fi lms were etched to provide multipoint sensors, which were imbedded within adhesive joints to measure the peel stresses. Peel stresses were thought to be the most critical stresses responsible for failure of a variety of adhesive joints. Th e tech- nique successfully demonstrated the peel stress trends expected in single and double lap joints and butt joints. Adhesive cure monitoring and void/porosity detection were also performed with ultrasound using the fi lm [7–9].
Other literature citations describing modifi cations or improvements to the use of piezoelectrics to monitor adhesive joints have been recently published. Th ese along with the previ- ously mentioned eff orts indicate the potential of having wires embedded with a bond to monitor the behavior, which in turn can lead to the prediction of the lifetime (or out of service time) of a structure [10–13]. Th is cited literature pertains to the through-life, nondestructive monitoring of adhesively bonded structures. It presented a discussion of the concept of microelec- tromechanical systems (MEMS) smart sensors. Th e MEMS smart sensors were permanently installed in such large struc- tures as ships, aircraft s, or land vehicles [14].
Another approach of incorporating smart materials with adhesives involves the addition of smart particles in the adhe- sive formulation to increase the reactivity of the adhesive.
Nano particles with crystal structure having properties of ferro- magnetic, ferrimagnetic, superparamagnetic, or piezoelectric materials were heated by electric, magnetic, or electromagnetic alternating fi elds aided in the hardening of adhesives [15,16].
As expected, the reverse has been accomplished. By heating nanoparticles with crystal structure having properties of ferro- magnetic, ferrimagnetic, superparamagnetic, or piezoelectric materials by electric, magnetic, or electromagnetic alternating fi elds, thermoplastic adhesives can be loosened [17].
Another concept that allows one to work smarter with adhe- sives consists of depositing adhesives or adhesion control agents in a manufacturing process using ink jet printing heads. Th e authors are advocates of this technique of applying minute amounts to confi ned or limited spaces. We would like to caution users of this application of chemistries that the consistence of the material is the same aft er deposition and is the same as formulated or initially manufactured [18].
Packaging of microelectronic devices has become more and more important. Applications for interconnection technologies range from highly specialized processors to be assembled on fl ex for mobile phones, highly defi ned packages as for hearing aids to low-cost transponders as for smart cards and smart labels. Each of these interconnection types has diff erent requirements in
technologies and costs. Th e use of isotropic conductive adhesive (ICA) for bumping and assembly for smart cards results in a number of advantages such as a simple versatile process, lower temperatures, and environment friendliness. Using an anisotro- pic conductive adhesive (ACA) or a nonconductive adhesive (NCA) for smart label production allows a high throughput from reel to reel [19].
One of the applications of special adhesives has been in the fabrication of smart cards. UV-curable ACAs were used for these mobile electronic products. Th e adhesives were subjected to dif- ferent mechanical or environmental attacks like warps, stretch- ing, changing humidity and temperature, which could aff ect the performance and reliability of the electronic products [20].
A simple-design Cu coil plated on polyethylene terephtha- late was used as the microstrip antenna of the smart card. Th e entire fabrication process of the UV-curable ACA was divided into three parts: high-power UV curing, chip-on-fl ex (COF) bonding, and postcuring. By varying the UV curing and postcuring parameters, a number of contactless smart cards were made [21].
A class of smart materials that has paralled the growth of and is closely related to the smart adhesives are the smart coat- ings. Th e advances of the smart coatings can be illustrated in the topics covered at a smart coating conference held in February 2005.
Examples of smart coatings included the formation of an enigmatic polymer with the unique property of being hydro- philic (water loving) when dry and hydrophobic (water hating) when wet. Th e researchers produced an antimicrobial coating by adding hydantoin into fl uorine-containing polymer chains. Th e researchers claimed that water-induced hydrophobic surfaces would result in new medicine devices, switching devices, and drag-reducing coatings [22].
A new fl uoropolymer additive for coating has been developed by Ausimont. Th is new coating additive assists in the removal of graffi ti from outside surfaces without aff ecting the coating and weathering properties of the surface [23].
Millenium Chemicals commercialized a polysiloxane silicone coating, Ecopaint. Th is unique coating in conjunction with either titanium dioxide or calcium carbonate or a combination of the two inorganics renders respirator-causing problems, nitrogen oxides (NOx gases), harmless. Buildings with this type of system will be more environmentally friendly [24].
In conclusion, one of the authors remembers an article by a group of French adhesive scientists in which they commented on the status of adhesion theories. Th ey indicated that there was still a great deal of information needed to explain why things stick together. Th e authors must take these remarks into account and apply similar methodologies to the develop- ment of smart adhesives. Th e growth of smart adhesives is in its early development stages. Most examples of the smart adhe- sives found in the open literature do not truly fi ll all the crite- ria of being smart that is having a “brain,” “nerves,” and
“muscles.” With time and research and development eff orts, this will change.
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