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Andrew Hoteling, Daniel Hook, Ivan Nunez, Joseph McGee, Joseph Hoff; Optimizing Physical Properties of a Silicone Hydrogel Material with Dual Phase Polymerization Processing. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4648.
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© ARVO (1962-2015); The Authors (2016-present)
A novel silicone hydrogel lens material, samfilcon A was designed to polymerize in two time resolved phases with the methacrylate monomers initiating the polymerization followed by NVP conversion to high molecular weight PVP. This reaction sequence enabled the formation of a silicone matrix (with high Dk/t and low bulk modulus) and a separate PVP phase that drives water content and surface wettability. The novel material was analyzed at various curing time points to confirm a dual phase polymerization achieving the targeted physical properties.
The reaction of samfilcon A was monitored using Photo Differential Scanning Calorimetry (Photo-DSC), Gas Chromatography-Flame Ionization Detector (GC-FID), and high resolution/accurate mass (HR/AM) Liquid Chromatography-Mass Spectrometry (LC-MS). Photo-DSC was used to monitor the reaction as a function of time and dose. GC-FID and LC-MS provided time dependent consumption of individual monomers, as well as formation of PVP polymer, to further the understanding of the reaction progress. Water content was determined using a gravimetric procedure while modulus was measured using ASTM method 1708. Modulus and % water were collected on 30 lenses. Dk/t (oxygen transmissibility) was calculated using the polarographic technique outlined in ANSI Z80.20:2010 and the mean central thickness on 4 lots of lenses, each of different thickness.
The DSC exotherm (Figure 1) demonstrated two distinct phases of polymerization for samfilcon A. Figure 1 shows phase 1 is initiated at 1 min with an exotherm of 0.5W/g while the second phase initiates at 8.5 minutes with an exotherm of 0.68W/g. GC-FID and LC-MS (Figure 2) demonstrated consumption of silicone monomers corresponding to phase 1, as well as NVP to PVP conversion corresponding to the phase 2 peak in Figure 1. Physical properties of the samfilcon A material demonstrated a mean modulus of 70 g/mm2, water content of 46%, and a Dk/t 163.
Photo DSC, GC-FID, and LC-MS techniques were used to confirm a dual phase curing process amenable to high speed manufacturing resulting in a silicone hydrogel material with a high water content (46%), high Dk/t (163) and low modulus (70 g/mm2) and a high concentration of PVP. The resulting samfilcon A material demonstrated the highest water content and Dk/t and lowest modulus compared to current leading silicone hydrogel lens materials.
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