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Chapter 30

Physics of Friction Applied to Medical Devices Andrea Liebmann-Vinson

Downloaded by COLUMBIA UNIV on June 15, 2013 | http://pubs.acs.org Publication Date: December 10, 1999 | doi: 10.1021/bk-2000-0741.ch030

Becton Dickinson Technologies, P. O. Box 12016/21 Davis Drive, Research Triangle Park, NC 27709-2016

Physics of friction in lubricated medical devices is discussed using the surprisingly complexfrictionand lubrication issues in disposable plastic syringes as a single but illustrative example. Syringes represent a particular important example of medical devices due to the ubiquitous application in modern drug delivery and in a variety of nosocomial procedures. Frictional performance in syringes is found to strongly depend on handling characteristics, namely the velocity at which the plunger is moved inside the syringe barrel. Three distinct regimes of lubricated friction, in analogy to a Stribeck curve, are revealed when studying this velocity dependence of syringe performance: (1) boundary lubrication, (2) mixed lubrication regimes and (3) hydrodynamic shear. Forces to initiate stopper motion and stick-slip motion observed when syringes are used in connection with mechanical dispensing devices (syringe pumps) are associated with the boundary and mixed lubrication regime. Hydrodynamic shear forces determine "feel" of a syringe when operated manually by hand. The velocity range of all three regimes is strongly coupled with lubricant mobility. Immobilizing lubricant treatments, such as exposure to high-energy radiation or an oxidizing plasma, cause the velocity ranges of each lubrication regime to shift. The fundamentals of friction and lubrication are thus shown to be embodied in common but important medical devices.

Introduction Historical Perspective. Friction is a fundamental physical phenomenon occurring whenever two contacting surfaces are in relative motion. Mankind has struggled against and has used friction since ancient times, as is apparentfromthe earliest recorded

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© 2000 American Chemical Society

In Microstructure and Microtribology of Polymer Surfaces; Tsukruk, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

475 engineering projects in history. For example, a mural painting in a grotto at El Beresh dated from about 1900 B.C. shows a colossus being pulled along on a sledge while one man, standing in front of the sledge, pours a lubricating liquid in its path (1). Many centuries later, Leonardo da Vinci deduced two empirical basic laws of friction: (1) friction is proportional to normal load acting on the contact pair and (2) friction is independent of size of contacting bodies. These laws were lost until rediscovered about 200 years later by Guillaume Amontons, who carried out the first scientific friction studies in 1699 (2). About 1780, Coulomb added another basic rule by saying that (3) friction is independent of sliding velocity (3). However, with improved experimental techniques, it was found later that "Amontons* rules' may be regarded only as crude approximations (4-7). Elastomers and polymers are prime examples of materials for which thefrictionalbehavior fail to follow Amonton's rules (8-14). Downloaded by COLUMBIA UNIV on June 15, 2013 | http://pubs.acs.org Publication Date: December 10, 1999 | doi: 10.1021/bk-2000-0741.ch030

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Theories of Friction. Many theories were put forth to explain the mechanism of friction (12, 15-18). For example, Amontons and Coulomb believed friction to originate from surface roughness. But Desaguliers discovered that smoother surfaces do not necessarily exhibit lower friction, and he was first to associate friction with adhesion occurring between contacting surfaces (2). All early theories had in common that friction was described as being a purely surface-related phenomenon. Later, it was realized that friction is not a process confined to the surface but can cause distortions propagating substantially into the sample bulk (4, 19). Bowden and Tabor's "adhesion and ploughing theory of friction" explained friction between metals as being the consequence of adhesion between areas of real contact, resulting in junction formation which have to be sheared during sliding, and plowing of asperities on the harder surface through the surface of the softer material (72, 17). The same theory was applied to explain the sliding friction of polymers. Adhesion forces between two surfaces in contact were claimed to be the dominating contribution to the friction force for sliding of unlubricated polymer surfaces at relatively slow constant velocities. Plowing of harder asperities on the softer counterface was found to be the source offrictionalforces occurring in rolling and lubricated sliding friction of polymers where formation of interfacial contacts or junctions can be prevented (72, 77, 20-23). Sliding friction of polymers and especially rubbers was found to deviate from "Amontons* (and Coulomb's) rules" in that friction force was found to be velocity dependent and, thus, reflect viscoelastic properties of these materials (8, 10,14, 20, 24-47). Controversy about mechanisms offriction(7, 48, 49) and especially the role of adhesion in friction (50-53) exists to this day and the study of simplified model systems utilizing modern techniques of nanotribology, especially in combination with advanced computational methods, are hoped to resolve much of the existing confusion (54-59). Lubrication. Friction in the presence of a lubricant, which is commonly used to mitigate high friction contacts, is a velocity dependent phenomenon that can be illustrated with the so-called. Stribeck curve (7, 60, 61). Three regimes of lubricated friction are identified: (1) boundary, (2) mixed, and (3) hydrodynamic lubrication. Frictional problems, such as wear orfriction-forcefluctuations, so-called stick-slip motion, are usually associated with the mixed and boundary lubrication regime. Either velocity of the moving counterfaces is too low, pressure on the friction contact is too


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high, or lubricant viscosity is too low to sustain the necessary hydrostatic pressure in the lubricant film to allow for complete separation of the moving counterfaces. It is in these regimes that physico-chemical properties of all three components, lubricant and both opposing surfaces, are important. Lubrication is essential for adequate performance of most machinery and devices comprised of moving parts. The objective of this contribution is to demonstrate that medical devices are no exception to this rule. Medical Devices. The term "medical device" covers a broad range of instruments, implements, or implants which are used in medical care in hospitals, the physician office or in home care. Examples include wheel chairs, contact lenses, pacemakers, endoscopes, vascular prostheses, extracorporeal blood circuits, joint or bone prostheses, thermometers, scalpels, catheters and syringes, just to name a few. As a consequence of this diversity in application, many different materials (biomaterials), including metals, ceramics, glasses, carbons, composites and plastics are used. And since many of these devices have moving parts, all kinds of different friction contacts, both lubricated and unlubricated, are encountered in medical devices. Syringes are a particularly important example of medical devices because of the ubiquitous application in modern drug delivery. Disposable plastic syringes have essentially replaced the old-fashioned glass syringes, except in some special procedures, and are today the most prevalent parenteral drug delivery device worldwide. A threepiece syringe, illustrated in Figure I, typically consists of a polypropylene barrel and an elastomeric stopper. The stopper is affixed to a plastic plunger rod and is frequently made out of vulcanized natural rubber, isoprene rubber or styrene-butadiene rubber (62). The liquid-tight seal necessary to contain drugs or bodilyfluidswithin a syringe is provided by an interference fit between elastomeric stopper and barrel. This interference fit creates a sealing pressure sufficient to withstand therigorsoffillingand injection. A drawback, of course, is that the sealing pressure acts as a normal load on the frictional contact and thus increases the already high frictional resistance to movement of the elastomeric stopper in the barrel (63). Lubricant, usually a medical grade silicone oil (64-66), is required to mitigate this high coefficient of friction between elastomeric stopper and plastic barrel. Sterilization is required for most medical devices in order to prevent delivery of pathogenic organism. Common sterilization methods for disposable plastic syringes are exposure to ethylene oxide gas or exposure to high energy radiation, such as an electron beam or γ-radiation from a Co source (67). Such treatments have the potential to alter the friction properties of a syringe as will be discussed briefly in this contribution. 60

Organization. This review is divided into three main parts and is focused on the physics of friction in disposable plastic syringes. In the first section, selected fundamentals of friction essential tofrictionin disposable plastic syringes are reviewed. The second and third section are concerned with frictional contacts occurring in parenteral drug administration: cutting and penetration of skin and tissue by hypodermic needles; friction between an elastomeric stopper and a plastic barrel in the presence of a lubricant. In the third section, friction related problems occurring in two typical syringe applications (manual hand and syringe pump operation) are used to


477 illustrate the applicability of basic physical concepts to explain friction in lubricated medical devices.


Physics of Friction Dry Friction. Three steps, shown schematically in Figure 2, are involved in sliding a macroscopic body, which may be called slider, against a stationary substrate: (1) bringing the bodies into contact, (2) applying a normal load (in many cases this is simply the weight of the slider), where "normal" refers to the normal (perpendicular) direction with respect to the substrate surface, and (3) applying a tangential load to initiate motion, where "tangential" means parallel to the substrate surface. These three steps are discussed in more detail in the following with emphasis on the sliding friction of polymers. Step 1: Contacting. In the first step, slider and substrate are brought into contact along the normal direction. As illustrated in Figure 2, contact will only be made in discrete spots where surface asperities from both surfaces meet, due to roughness that almost all surfaces possess (49, 68-70). In those so-called "areas of real contact", molecules from both surfaces are in intimate contact, allowing for "stickiness" to develop (71). This "stickiness" is commonly referred to as adhesion because it is due to attractive intermolecular force fields originating from all molecules situated on the surface of either substrate or slider (72-74). Attractive forces acting across the contact area (Figure 2) cause the bodies to be pulled together more closely, leading to spontaneous elastic or even plastic deformation of the contact zone and, thus, create finite contact areas even under zero external load (71, 75-78). Contact area and stress distribution between two elastic spheres under a given external load wasfirstcalculated by Hertz in 1881 (79). His calculations were based on elastic properties of the materials and geometrical arguments, ignoring any interactions taking place across the contact area. In 1971, Johnson, Roberts, and Kendall (JRK) extended Hertz theory to include attractive interactions acting across the contact area of contacting elastic bodies (75). Later this theory was further extended by Derjaguin, Muller and Toporov (DMT) to include interactions acting just outside the contact area, in regions where the two surfaces are still in close proximity (80). The most important differences between the earlier Hertz and later JRK and DMT theories are: (1) the contact area at zero load is zero in Hertz theory, but afinitevalue in the JRK and DMT theories (even for slightly negative loads); (2) the contact area predicted by JRK and DMT theories is always larger than that predicted by Hertz for a given load. This phenomenon of increasing contact area due to attractive intermolecular interactions across interfaces of contacting bodies, or simply said, the "sticking together" of surfaces in contact, is commonly called "adhesion" and represents the adhesion part of the previously mentioned Bowden and Tabor "adhesion and ploughing theory of friction". This choice of nomenclature is an unfortunate one, since adhesion means different things to different scientists (according to the Dictionary of Scientific and Technical Terms, the term "adhesion" finds application with mostly different meanings in a variety of fields, such as botany, electromagnetism, engineering, mechanics, medicine, and physics (81)) (51, 82, 83). It should thus be emphasized that adhesion in context offrictionis synonymous for the effective force of attraction between surfaces


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-Plunger Uncompressed Stopper Outside of Barrel

Inge tnge

Barrel Wall Compressed Stopper within Barrel


-Stopper -Barrel

Dla. A Interference Fit (A

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