Contact Lens & Anterior Eye 35 (2012) 260–265 Contents lists available at SciVerse ScienceDirect Contact Lens & Anterior Eye j ourna l ho me p ag e: ww w.elsevier.com/locate/clae Aqueous salt transport through soft contact lenses: An osmotic-withdrawal mechanism for prevention of adherence Colin Cerretani a , Cheng-Chun Peng a,b , A. Chauhan b , C.J. Radke a,c,∗ a Department of Chemical and Biomolecular Engineering, University of California, Berkeley, CA 94720-1462, United States b Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, United States c Vision Science Group, University of California, Berkeley, CA 94720, United States a r t i c l e i n f o Article history: Received 21 January 2012 Received in revised form 4 June 2012 Accepted 3 July 2012 Keywords: Soft-contact-lens binding Threshold salt permeability Squeeze and osmotic-withdrawal flows Attractive thin-film forces Tear-film evaporation a b s t r a c t In addition to improving oxygen permeability, modern silicone-hydrogel (SiHy) soft contact lenses (SCLs) exceed a limiting diffusive ion permeability to aqueous sodium chloride. Below the ion-permeability threshold, siloxane-based SCLs are prone to bind against the corneal epithelium. Salt permeability is argued to reflect indirectly water hydraulic permeability. However, no quantitative explanation is avail- able to date for a threshold salt permeability. We hypothesize that molecular salt diffusion through a SCL supports the postlens tear film (PoLTF) by enhancing water flow into the PoLTF from the cornea. Higher salt concentrations in the PoLTF raise the osmotic pressure there relative to that in the cornea increasing osmotic water withdrawal from the cornea. The proposed osmotic-withdrawal mechanism successfully predicts a self-consistent threshold lens salt permeability when thin-film attractive binding forces are introduced. For the first time, we present a quantitative picture for the possible origin of a threshold salt permeability in SCL manufacture. © 2012 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved. 1. Introduction Incorporation of siloxane moieties into hydrogels to achieve oxygen permeability exceeding about 100 Barrer has revolution- ized soft-contact-lens manufacture. High oxygen permeability minimizes corneal hypoxia and permits extended lens wear [1–3]. Concomitantly, an aqueous sodium-chloride diffusive permeabil- ity in excess of about 2 × 10 -7 cm 2 /s appears necessary to prevent lens adhesion to the epithelium and permit lens movement during blinking [1]. Extensive reviews of SiHy contact-lens synthesis and behavior emphasize the importance of a critical salt permeability [2,3]. Fig. 1 from the recent work of Guan et al. [4] demonstrates that all current commercial soft contact lenses (SCLs), both siloxane- based and HEMA-based, exceed the threshold NaCl permeability (see also Table III in [4]). This observation has not been generally recognized, although the importance of a threshold salt permeabil- ity to prevent lens binding was documented experimentally over a decade ago [1–5]. Unfortunately, no satisfactory physical explana- tion is currently available. Nicolson et al. [1] argue that salt permeability indirectly gauges water transport rate through a SCL. Specifically, these authors ∗ Corresponding author at: Department of Chemical and Biomolecular Engineer- ing, University of California, 101E Gilman, Berkeley, CA 94720-1462, United States. Tel.: +1 510 642 5204; fax: +1 510 642 4778. E-mail address: radke@berkeley.edu (C.J. Radke). hypothesize that water is compressed out of a lens during a blink and replenished during an interblink. The squeezed-out water maintains a thick enough postlens tear film (PoLTF) to avoid lens adhesion. Thus, a threshold water hydraulic permeability is neces- sary to maintain lens movement. According to the review chapters of Tighe [2,3], a simple calculation (not presented) confirms that above a critical hydraulic permeability, water flow in a SCL is suf- ficient to maintain an adequate PoLTF boundary-layer thickness. Monticelli et al. [6], however, disagree pointing out that neither a squeeze-out nor a squeeze-through mechanism can sustain the PoLTF because hydraulic permeabilities of SCLs are miniscule. Upon neglect of gel-relaxation kinetics under lid-blink compres- sion, the flow rate of tear expelled from a SCL obeys Darcy’s law [6,7] J L = k H   p L  (1) where J L is the volumetric flux of water through the hydrogel, k H is the hydraulic permeability,  is the Newtonian viscosity of tear, L is the harmonic mean thickness of the lens, and p is the applied lid force per unit area. Typical values of the parameters are: k H = 10 -8 m 2 [6],  = 1 mPa s, L = 100 m, and p ∼ 2 kPa [8–10]. With these values, the water volumetric flux is 10 -4 m/s. For a 0.1- s blink, the amount of tear exuded from the lens per blink, expressed as an increase in PoLTF thickness, is about 10 -5 m. This estimate is conservative because the applied lid force corresponds to that at the lid margin where the lid force is largest. Further, and consistent 1367-0484/$ – see front matter © 2012 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.clae.2012.07.003