eyeTL2008 - Tribol Lett (2008) 30:1319 DOI...

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ORIGINAL PAPER Friction Coef±cient Measurement of Hydrogel Materials on Living Epithelial Cells Alison C. Dunn Æ Jessica A. Cobb Æ Ana N. Kantzios Æ Sung Jin Lee Æ Malisa Sarntinoranont Æ Roger Tran-Son-Tay Æ W. Gregory Sawyer Received: 18 September 2007 / Accepted: 20 February 2008 / Published online: 6 March 2008 Ó Springer Science+Business Media, LLC 2008 Abstract Soft biomaterials are often used in applications that involve contact and relative motion against biological tissues, as well as complicated and variable environments. The friction coef±cient of these contacts involving living human cells is of key importance in the analysis and suc- cess of these devices. This work measures the contacting friction coef±cient between soft hydrogel biomaterial sur- faces against live human corneal epithelial cells using a custom micro-tribometer. The friction coef±cients were of the order of l = 0.03 for contacts that did not cause gross destruction of the cell layer. Damage to the con²uent cell layer was assessed using a Trypan blue stain with optical microscopy. This damage was quanti±ed statistically using image-processing software. The damage was also corre- lated to in situ friction measurements, with the lowest friction values seen on undamaged cells and higher friction on damaged regions. Keywords Biotribology ± Hydrogel ± Friction 1 Introduction Hydrogel materials are used in many capacities in the human body, perhaps most commonly as optical correction devices. Hydrogels are polymer networks that contain substantial fractions of water, and they are typically hydrophilic (this depends on the hydration level). Hydro- gels are the primary material for soft contact lenses. These lenses move against both the cornea (epithelium) and the inner eyelid (palpebral conjunctiva); this is shown sche- matically in Fig. 1 . Though these soft materials are well suited to the wet environment of the eye, many patients suffer from extensive eye irritation after implantation due to mechanical friction, physiological factors, or both [ 1 ]. The mechanical properties of hydrogels are extremely sensitive to water content and humidity, and contact pres- sure is known to cause redistribution and/or reduction of the water content of hydrogel materials [ 2 , 3 ]. As such, any tribological testing of hydrogels must be done under rela- tively low contact pressure conditions, known solution, and controlled humidity environments [ 4 ]. The surface topog- raphy of these devices is a function of the material parameters and of molding conditions but have a typical R q of 5–10 nm [ 5 ]. Estimates of the contact pressure created by the eyelid during blinking range from 3.5 to 4.0 kPa and blinking speed average around 12 cm/s [ 6 9 ]. This combi- nation of low contact pressure and moderate sliding speed are challenging to match with tribological instruments.
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eyeTL2008 - Tribol Lett (2008) 30:1319 DOI...

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