Author's personal copy Contact Lens & Anterior Eye 35 (2012) 53–64 Contents lists available at SciVerse ScienceDirect Contact Lens & Anterior Eye jou rn al h om epa ge: www.elsevier.com/locate/clae Review Protein deposition on contact lenses: The past, the present, and the future Doerte Luensmann ∗ , Lyndon Jones Centre for Contact Lens Research, School of Optometry, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada a r t i c l e i n f o Keywords: Protein deposition Contact lens Hydrogel PolyHEMA Silicone hydrogel Biocompatibility a b s t r a c t Proteins are a key component in body fluids and adhere to most biomaterials within seconds of their exposure. The tear film consists of more than 400 different proteins, ranging in size from 10 to 2360 kDa, with a net charge of pH 1–11. Protein deposition rates on poly-2-hydroxyethyl methacrylate (pHEMA) and silicone hydrogel soft contact lenses have been determined using a number of ex vivo and in vitro experiments. Ionic, high water pHEMA-based lenses attract the highest amount of tear film protein (1300 g/lens), due to an electrostatic attraction between the material and positively charged lysozyme. All other types of pHEMA-based lenses deposit typically less than 100 g/lens. Silicone hydrogel lenses attract less protein than pHEMA-based materials, with <10 g/lens for non-ionic and up to 34 g/lens for ionic materials. Despite the low protein rates on silicone hydrogel lenses, the percentage of dena- tured protein is typically higher than that seen on pHEMA-based lenses. Newer approaches incorporating phosphorylcholine, polyethers or hyaluronic acid into potential contact lens materials result in reduced protein deposition rates compared to current lens materials. © 2012 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved. 1. Introduction Contact lenses represent a biomaterial that is widely used and relatively easy to study, due to its ease of removal from the ocular surface. Immediately after being placed on the eye, contact lenses are coated with a protein layer and most proteins attach strongly to the material, with typically less than 50% being removed by con- ventional care regimens [1–3]. The deposition of certain proteins to contact lenses has shown to increase the risk of microbial cell attachment to the lens material [4–6], and is also associated with inflammatory complications such as giant papillary conjunctivitis [7]. 2. Proteins in the tear film Protein deposition on contact lenses is substantially impacted by the lens material, and also by the protein concentration, protein structure and charge of the proteins within the tear film. Proteins are a major component of the human tear film and perform a vari- ety of important tasks, which include protecting the ocular surface from microorganisms, cell membrane transport/metabolism, regulating immune responses, protein folding, antioxidation, and act as protease inhibitors [8]. de Souza [9] identified 491 different ∗ Corresponding author at: Centre for Contact Lens Research, University of Water- loo, 200 University Ave West, Waterloo, N2l 3G1, ON, Canada. Tel.: +1 519 888 4567x37312; fax: +1 519 884 8769. E-mail address: dluensma@uwaterloo.ca (D. Luensmann). proteins and mucins in the tear film, ranging in size from 10 kDa to 2360 kDa [9]. Approximately 80% of the proteins have a size of <100 kDa [9] and they range in charge from isoelectric points (pI) of pH 1 to 11 [10]. Examples of proteins that have received the most attention in contact lens research include lysozyme (14.3 kDa, pI pH 11.4), lactoferrin (80 kDa, pI pH 8.7) and albumin (66 kDa, pI pH 5.2). The average pH of the tear film is 7.4, which results in lysozyme and lactoferrin being positively charged and albumin being nega- tively charged. Most proteins have a pI significantly above or below pH 7.4, which helps their solubility in the tear film, as proteins are least soluble if the environment is close to the protein’s pI, which would lead to increased aggregation and deposition rates [11,12]. The total protein concentration in the tear film ranges between 6.5 and 9.0 mg/mL and varies between individuals [13]. A variety of factors are known to influence the protein concentration in the tear film, including: time of the day [14,15], contact lens wear [16], age [17] and eye diseases such as Sjögrens syndrome [18]. Signif- icant differences in concentration are also seen when tearing is stimulated, compared to that seen with unstimulated tears [19]. 3. The principals of protein sorption Proteins adsorb to most surfaces, and while hydrophobic (non- polar) amino acids are typically protected inside the protein molecule, the hydrophilic (polar) amino acids, with and without charged side chains, interact freely with their environment [20]. If the charged side chains come into contact with an oppositely charged surface, the adsorption process is further reinforced. Each protein is typically folded in a three-dimensional structure, which 1367-0484/$ – see front matter © 2012 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.clae.2011.12.005