Tribological Classification of Contact Lenses: From Coefficient of Friction to Sliding Work

The coefficient of friction (CoF) has been reported to correlate with clinical comfort of soft contact lenses (SCL). However, a classification in terms of a CoF is not always applicable to soft materials, such as hydrogels, due to the frequently observed nonlinearity between the lateral and the normal forces. An alternative methodology is presented to quantify the tribological characteristics of soft materials under boundary lubrication in terms of average work. Average work was derived from knowledge of the area of contact, the interfacial shear stress, and sliding distance. To illustrate the work concept, three commercially available SCL (n = 10) and rabbit corneas (n = 10) were characterized with regard to lateral force against a biomimetic mucin-coated glass disk in a tear-like fluid, by means of microtribometry. The contact area between the glass disk and the SCL was measured in situ and fitted to an elastic-foundation model of the material. On the cornea, the contact area was observed via the expulsion of a fluorescent marker from the contact region. All SCL materials had significantly (p < 0.05) different values for average work. Furthermore, the interfacial shear stress on the cornea was found to be at least an order of magnitude lower than on any of the SCL. Average work represents a single figure of merit for the lubricious properties of soft materials, such as SCL, that do not show a linear relationship between lateral and normal forces.ISSN:1023-8883ISSN:1573-271

Single proteins that serve linked functions in intracellular and extracellular microenvironments

Maintenance of organ homeostasis and control of appropriate response to environmental alterations requires intimate coordination of cellular function and tissue organization. An important component of this coordination may be provided by proteins that can serve distinct, but linked, functions on both sides of the plasma membrane. Here we present a novel hypothesis in which non-classical secretion can provide a mechanism through which single proteins can integrate complex tissue functions. Single genes can exert a complex, dynamic influence through a number of different processes that act to multiply the function of the gene product(s). Alternative splicing can create many different transcripts that encode proteins of diverse, even antagonistic, function from a single gene. Posttranslational modifications can alter the stability, activity, localization, and even basic function of proteins. A protein can exist in different subcellular localizations. More recently, it has become clear that single proteins can function both inside and outside the cell. These proteins often lack defined secretory signal sequences, and transit the plasma membrane by mechanisms separate from the classical ER/Golgi secretory process. When examples of such proteins are examined individually, the multifunctionality and lack of a signal sequence are puzzling - why should a protein with a well known function in one context function in such a distinct fashion in another? We propose that one reason for a single protein to perform intracellular and extracellular roles is to coordinate organization and maintenance of a global tissue function. Here, we describe in detail three specific examples of proteins that act in this fashion, outlining their specific functions in the extracellular space and in the intracellular space, and we discuss how these functions may be linked. We present epimorphin/syntaxin-2, which may coordinate morphogenesis of secretory organs (as epimorphin) with control of protein secretion (as syntaxin-2), amphoterin/high mobility group box-1 (HMGB1), which may link inflammation (as amphoterin) with regulation of gene expression (as HMGB1), and tissue transglutaminase, which affects delivery of and response to apoptotic signals by serving a related function on both sides of the plasma membrane. As it is notable that all three of these proteins have been reported to transit the plasma membrane through non-classical secretory mechanisms, we will also discuss why coordinated inside/outside functions may be found in some examples of proteins which transit the plasma membrane through non-classical mechanisms and how this relationship can be used to identify additional proteins that share these characteristics