ARTICLE pubs.acs.org/Langmuir
Approach to Knowledge of the Interaction between the Constituents of Contact Lenses and Ocular Tears: Mixed Monolayers of Poly(methyl methacrylate) and Dipalmitoyl Phosphatidyl Choline M. Mi~nones Conde,† O. Conde,‡ J. M. Trillo,‡ and J. Mi~nones, Jr.*,‡ †
Department of Optometry, School of Optics and Optometry and ‡Department of Physical Chemistry, Faculty of Pharmacy, University of Santiago de Compostela, Campus Sur. 15706-Santiago de Compostela, Spain
bS Supporting Information ABSTRACT: Mixed monolayers of poly(methyl methacrylate) (PMMA), the main component of hard contact lenses, and dipalmitoyl phosphatidyl choline (DPPC), a characteristic phospholipidic constituent of ocular tear films, were selected as an in vitro model in order to observe the behavior of contact lenses on the eye. Using Langmuir monolayer and Brewster angle microscopy (BAM) techniques, the interaction between both components was analyzed from the data of surface pressure-area isotherms, compressional modulus-surface pressure, and relative film thickness versus time elapsed from the beginning of compression, together with BAM images. Regardless of the surface pressure at which the molecular/ monomer areas (Am) were recorded, the Am mole fractions of PMMA (XPMMA) plots show that the experimental results match the theoretical values calculated from additivity rule Am = XPMMAAPMMA þ XDPPCADPPC. The application of the Crisp phase rule to the phase diagram of the PMMA-DPPC system can explain the existence of a mixed monolayer made up of miscible components with ideal behavior at surface pressures below 25 mN/m. However, at very high surface pressures, when collapse is reached (at 60 mN/m), the single collapsed components are segregated into two independent phases. These results allows us to argue that PMMA hard contact lenses in the eye do not alter the structural characteristics of the phospholipid (DPPC) in tears.
1. INTRODUCTION According to the US FDA,1 contact lenses are currently classified into two main groups in accordance with their chemical composition and physical properties: hydrophilic (“filcon” or soft lenses), and hydrophobic (“focom” or hard lenses). Soft contact lenses are flexible, slightly cross-linked hydrogels that can absorb and retain large amounts of water (>35% w/w) in which oxygen can dissolve and diffuse toward the cornea. They are made up of acrylic polymers with very hydrophilic side chains and agents (ethylene glycol dimethacrylate, EDGMA) linking the polymer molecules in 3D networks. Thus, the hydrophilic groups contribute to the absorption of water into the polymer, whereas the hydrophobic bonds limit it; consequently, the hydration of a hydrogel can be modified by increasing or decreasing the degree of chain cross-linking.2-4 On the contrary, hydrophobic lenses do not allow this control of water content. In this case, their main component is the poly(methyl methacrylate) homopolymer (PMMA) obtained by the polymerization of monomer MMA, which has demonstrated excellent transparency but has a low water content (
