Robert E. Baier

A review of adhesion science.

OBJECTIVE Adhesion or cohesion includes an adherend, adhesive, and intervening interface. Adhesive joints may include one or more interfaces. Adhesion science focuses on understanding the materials properties associated with formation of the interfaces, changes in the interfaces with time, and events associated with failure of the interfaces. METHODS The key principles for good interface formation are creation of a clean surface, generation of a rough surface for interfacial interlocking, good wetting of the substratum by the adhesive/cohesive materials, adequate flow and adaptation for intimate interaction, and acceptable curing when phase changes are required for final joint formation. RESULTS Much more effort is needed in the future to carefully assess each of these using available testing methods that attempt to characterize the energetics of the interfaces. Bonding involves potential contributions from physical, chemical, and mechanical sources but primarily relies on micro-mechanical interaction for success. Characterization of the interface before adhesion, during service, and after failure would be much more useful for future investigations and remains as a great challenge. SIGNIFICANCE Scientists should more rigorously apply techniques such as comprehensive contact angle analysis (rather than simple water wettability) for surface energy determination, and AFM in addition to SEM for surface texture analysis.

Characterization of oral in vivo films formed on different types of solid surfaces

Studies were made of oral films formed in vivo, which had been allowed to form on fused silica and Ge-prisms during periods between 2 s and 2 h using a variety of physico-chemical methods. To produce surfaces of different qualities the silica and Ge-prisms had either been detergent-washed, glow discharge treated or covered with polydimethylsiloxane. The following simultaneous analytical techniques were performed on the adsorbed films: a. internal reflection infrared spectroscopy, b. ellipsometry, c. contact potential measurements, d. contact angle measurements, e. scanning electron microscopy and f. energy-dispersive x-ray analysis. The results of these studies show that the formation of oral films proceeds at high speed and is of a certain qualitative selectivity. The formed films were found to be stable over long periods of time, and only showed patches of adhering micro-organisms on some of the prisms which had been exposed in the oral cavity for 2 h.

THE ORGANIZATION OF BLOOD COMPONENTS NEAR INTERFACES

The arrangement of blood elements near their border with intact vascular endothelial cells is poorly understood. However, the order and succession of blood components arriving at nonphysiologic boundaries has been experimentally determined. After essentially instantaneous transitions occur in the interfacially bound layer of water and ions, specific blood proteins, usually fibrinogen, adsorb immediately. The strange interface is coated by a 50-A thick film within five seconds, “converting” to a more complex mixture of components with increasing contact time. Blood platelets arrive continuously, but little cell adhesion occurs until the “conditioning” film of protein achieves a criticalbut nonequilibrium-state, about 200-A deep. Within two minutes of first blood contact, most platelets adhere in a nearly random and discontinuous monolayer. White cells, mainly neutrophils, are chemotactically activated toward the adherent platelets (and aggregates) within five minutes, adding to the growing white thrombus mass, in most cases. Thrombi extend into the blood flow, accelerating fibrin formation and red cell entrapment to result in a typical red clot. At some interfaces, white cells assist in detaching platelets to leave a “passive” bloodcompatible proteinaceous film.

Spectroscopic analysis of polypeptide conformation in polymethyl glutamate monolayers.

Abstract Monolayers of poly-γ-methyl- l -glutamate spread on water from chloroform solution haveplateaux in their surface pressure-area isotherms and large limiting areas, thus differing from those spread from pyridine-rich solution, which have no plateaux and much smaller limiting areas. Infrared spectra and isotherms of monolayers from a graded series of spreading solutions showed corresponding changes. Samples were obtained both by transfer at constant surface pressures onto multiple internal reflection prisms and by collapse on to transparent plates. The spectra were independent of the degree of compression, even through the plateau region and collapse. Films from chloroform-rich solutions showed spectra whose amide I and II absorbtion bands were typical of α-helical or random conformations, whereas films spread from pyridine-rich solutions had a component characteristic of the β-conformation, which increased with increasing pyridine concentration. The conformation of the polypeptide in the surface film was also related to its conformation in the spreading solution as determined by optical rotation. Correlation of large monolayer areas with the absence of the extended-chain β-structure is not consistent with older concepts of protein conformation at interfaces but agrees with more recent studies of films and intact membranes.

Effect of cleaning and sterilization on titanium implant surface properties and cellular response

Titanium (Ti) has been widely used as an implant material due to the excellent biocompatibility and corrosion resistance of its oxide surface. Biomaterials must be sterile before implantation, but the effects of sterilization on their surface properties have been less well studied. The effects of cleaning and sterilization on surface characteristics were bio-determined using contaminated and pure Ti substrata first manufactured to present two different surface structures: pretreated titanium (PT, Ra=0.4 μm) (i.e. surfaces that were not modified by sandblasting and/or acid etching); (SLA, Ra=3.4 μm). Previously cultured cells and associated extracellular matrix were removed from all bio-contaminated specimens by cleaning in a sonicator bath with a sequential acetone-isopropanol-ethanol-distilled water protocol. Cleaned specimens were sterilized with autoclave, gamma irradiation, oxygen plasma, or ultraviolet light. X-ray photoelectron spectroscopy (XPS), contact angle measurements, profilometry, and scanning electron microscopy were used to examine surface chemical components, hydrophilicity, roughness, and morphology, respectively. Small organic molecules present on contaminated Ti surfaces were removed with cleaning. XPS analysis confirmed that surface chemistry was altered by both cleaning and sterilization. Cleaning and sterilization affected hydrophobicity and roughness. These modified surface properties affected osteogenic differentiation of human MG63 osteoblast-like cells. Specifically, autoclaved SLA surfaces lost the characteristic increase in osteoblast differentiation seen on starting SLA surfaces, which was correlated with altered surface wettability and roughness. These data indicated that recleaned and resterilized Ti implant surfaces cannot be considered the same as the first surfaces in terms of surface properties and cell responses. Therefore, the reuse of Ti implants after resterilization may not result in the same tissue responses as found with never-before-implanted specimens.

Conditioning surfaces to suit the biomedical environment: recent progress.

One major determinant of the suitability of various engineering materials for use in biological settings is the relative strength of adhesion obtained between those materials and their contacting viable phases. Maximal adhesive strength and immobility are desired for orthopedic and dental implants, for example, while minimal bioadhesion is critical to preventing unwanted thrombus formation in cardiovascular devices, plaque buildup on dental prostheses, and bacterial fouling of heat exchangers. This article reviews the principles of adhesive phenomena in such harsh environments, introduces some novel test methods and materials available for sensitive analysis of the earliest interfacial events, and provides a brief illustration of their use in the study of surface fouling of food processing equipment. Attention is then directed to adhesive phenomena in the oral environment, examining new surface conditioning methods for the prevention of micro-organism deposits, as well as the promotion of excellent tissue bonding to implanted prosthetic devices. Other bioadhesive phenomena considered include those important to the safe and effective function of new cardiovascular devices, such as the artificial heart and substitute blood vessels, and the prevention of biological fouling of materials in the sea. Comparisons of the primary interfacial events in these diverse systems--all wet, salty, and biochemically active--illustrate that Nature has been very conservative in accommodating encounters with strange boundaries. As with a classic drama, the players and timing may vary, but the script remains the same.

Surface analysis of fouling-resistant marine coatings

Generally applicable techniques for surface analysis are described for prepared coating surfaces, with the recommendation that they be utilized prior to any fouling‐release field trials. The effectiveness of these techniques to predict nontoxic fouling‐release is supported by citation of results from the performance of commercial coatings in both seawater and freshwater environments, and comparison with standard reference materials. It is no longer necessary to rely on assumptions about the types of surfaces that will most easily allow physical release of accumulated biofouling, since the field results now show why analysing the surfaces in advance of their aqueous exposure gives important clues about their likely fouling performance. Written test protocols that do not include this level of surface inspection limit understanding and do not aid it, since ambiguities in fouling results are not otherwise traceable to characterized initial coating qualities.

Degradative effects of conventional steam sterilization on biomaterial surfaces

Prior to implantation trials in animals, the effect of steam sterilization on the surface properties of metallic and coated biomaterials was studied. Pure germanium plates and cast surgical Vitallium discs and subperiosteal implants were treated to present three standard types of biomaterials surfaces prior to steam sterilization, ranging from scrupulously clean, high-energy metals to uniformly low-energy organic layers. Both before and after sterilization, the sample surfaces were characterized by a variety of nondestructive physiochemical techniques. The results indicate that steam sterilization is likely to compromise the properties of otherwise carefully prepared biomedical implants by depositing hydrophobic organic and hygroscopic salt contaminants over the implant surfaces.

Rapid and inexpensive quantification of the combined polar components of surface wettability : application to biofouling

A technique is presented (the SHM method) that rapidly and inexpensively quantifies surface wettability using aqueous methanol solutions. The SHM method, which can be performed using basic, generally available laboratory equipment, yields a single value that is strongly correlated with the combined polar (acid‐base) components of surface wettability. In laboratory studies employing silanised glass surfaces, larval settlement rates of the bryozoan Bugula neritina and the ascidian Ascidia nigra are negatively correlated with surface wettability as quantified by SHM (r = ‐0.79, P < 0.02 for both B. neritina and A. nigra). In contrast, settlement of newly‐metamorphosed cyprid larvae of the acorn barnacle Batanus amphitrite is positively correlated with SHM (r = +0.75, P<0.05). The SHM method is potentially useful to biologists who require initial data regarding the influence of surface wettability on biological processes before proceeding to more sophisticated, in‐depth studies involving collaboration with su...

Potential invasion of microorganisms and pathogens via ‘interior hull fouling’: biofilms inside ballast water tanks

Surfaces submerged in an aquatic milieu are covered to some degree with biofilms – organic matrices that can contain bacteria, microalgae, and protozoans, sometimes including disease-causing forms. One unquantified risk of aquatic biological invasions is the potential for biofilms within ships’ ballast water tanks to harbor pathogens, and, in turn, seed other waters. To begin to evaluate this vector, we collected biofilm samples from tanks’ surfaces and deployed controlled-surface sampling units within tanks. We then measured a variety of microbial metrics within the biofilms to test the hypotheses that pathogens are present in biofilms and that biofilms have higher microbial densities compared to ballast water. Field experiments and sampling of coastwise and oceangoing ships arriving at ports in Chesapeake Bay and the North American Great Lakes showed the presence of abundant microorganisms, including pathogens, in biofilms. These results suggest that ballast-tank biofilms represent an additional risk of microbial invasion, provided they release cells into the water or they are sloughed off during normal ballasting operations.