Contact lenses, hydrogels

Cast molding is an increasingly used manufacturing process for both rigid gas-permeable and hydrogel contact lenses. In this process, two molds, made from a variety of plastics, are used. A female mold forms the lens front surface (convex) and a male mold forms the lens back surface (concave). The plastic molds are made from metal tools or dies that are usually stainless steel, precision lathed, and polished to the specified lens design. A variety of mold materials are used. The polymerized, hardened lens is released from the mated molds and is processed in much the same way as the spin-cast lenses described above. 

Because fluorescein sodium can penetrate into many hydrogel contact lenses, the lenses become discolored, which raises bacterial growth issues and renders the lenses cosmetically objectionable. In addition, the boimd-ary between lens and tears becomes obscured, which precludes the use of fluorescein in soft contact lens fitting. Fluorexon, a molecule similar in fluorescent characteristics to that of fluorescein, is less readily absorbed by the soft lens material, which renders it useful in fitting and evaluating soft and hybrid design lenses. 

It has already been noted that hydrogel contact lenses are inherently more comfortable that rigid (RGP or PMMA) lenses. This is related to the former s superior flexibility and hydrophilic character which permits incorporation of substantial amounts of water (38-74%) into the lens material. However, after periods of wear time, some lenses may experience changes in hydration, that may be related to deposits, environmental (e.g., temperature and humidity) changes and improper care. In particrJar, dry spots may become evident on... 

Karlgard, C.C.S. Wong, N.S. Jones, L.W. Moresoli, C. in vitro uptake and release studies of ocular pharmaceutical agents by silicon-containing and p-HEM A hydrogel contact lenses. Int. J. Pharm. 2003, 257 (1), 141-151. 

Peng, C.C. Chauhan, A. Extended cyclosporine delivery by silicone-hydrogel contact lenses. J. Contr. Release 2011, 154 (3), 161-11 A. 

Ali, M. Byrne, M.E. Controlled release of high molecular weight hyaluronic acid from molecularly imprinted hydrogel contact lenses. Pharm. Res. 2009, 26 (3), 714-726. 

Malaekeh-Nikouei, B. Abbasi Ghaeni, F. Motamedshariaty, V.S. Mohajeri, S.A. Controlled release of prednisolone acetate from molecularly imprinted hydrogel contact lenses. J. Appl. Polym. Sci. 2012,126 (1), 387-394. 

V. Compan, A. Andrio, A. Lopez-Alemany, E. Riande, M. Refojo, Oxygen permeability of hydrogel contact lenses with organosilicon moieties. Biomaterials 23 (2002) 2767—2772. 

Harris MG, Sarver MD, Poise KA. Patient response to thin hydrogel contact lenses a comparative study. J Am Optom Assoc 1977 48(3) 295. 

N. Efron, and P.B. Morgan, Trends in use of silicone hydrogel contact lenses for daily wear. Contact Lens Anterior Eye, 31, 242-243,2008. 

Two kinds of soft hydrogel contact lenses were cleaned with different lens care solutions (ECS) and analyzed by XPS. The elemental composition did not vary strongly between the different systems but the 0=C/0—C concentration ratio varied by a factor of 5. It was inversely correlated with the initial deposition rate measured in adhesion tests of P. aeruginosa performed in a laminar flow chamber. This revealed the adverse effect of compounds rich in O-C moieties adsorbed from the LCS. " After the contact lenses were worn, the presence of nitrogen-rich compounds and a much higher O /O—C concentration ratio were observed. The initial deposition rate of bacteria was lower on worn than on unworn contact lenses. ... 

Taylor RL, Willcox MD, Williams TJ, Verran J. Modulation of bacterial adhesion to hydrogel contact lenses by albumin. Optom Vis Sci 1998 75 23-9. 

Maldonado-Codina, C., Efron, N. Impact of manufacturing technology and material composition on the mechanical properties of hydrogel contact lenses. Ophthalmic Physiol. Opt. 24(6), 551-561 (2004). doi 10.1111/j.l475-1313.2004.00236.x... 

---