(308a) Award Submission: Water Content and Solute Partitioning in Hybrid Silicone-Hydrogels

With roughly six times the amount of corneal oxygen supply compared to HEMA-based soft contact lenses, silicone-hydrogel (SiHy) soft contact lenses (SCLs) provide comfort for extended wear, while maintaining ocular health [1-3]. Despite their commercial importance, the structure and morphology of SiHy SCLs is not well understood. It is, however, widely accepted that silicone-based SCLs should contain co-continuous hydrophilic and hydrophobic polymer phases [3]. The hybrid structure of SiHys allows for sufficient salt transport through the water phase [4] localized in hydrophilic domains, as well as, sufficient oxygen permeability through the silicone domains [2,3]. Additionally, partitioning of care-solution and wetting agents in SCLs for improved on-eye lens performance [5, 6] is dictated largely by the water content [7] and, thus, the SiHy microstructure. To our knowledge, no studies attempt prediction of water uptake or solute partitioning in silicone-based hydrogels.

We investigate equilibrium water and solute uptake in SiHys both experimentally and theoretically.  Specifically, we focus on quantitatively predicting how initial SiHy formulation design affects equilibrium water content and partitioning of aqueous and oleophilic solutes of similar size (~100-300 Da). The SiHy materials studied were synthesized with free radical polymerization and consist of varying hydrophilic fraction (HEMA, MAA) and silicone fraction (Tris, mPDMS) with a block-copolymer macromer to ensure a co-continuous structure. Equilibrium water content was measured gravimetrically. UV/vis spectrophotometry and fluorescence confocal laser scanning microscopy were employed to measure partition coefficients for aqueous and oleophilic solutes respectively.  Water content is predicted assuming that water is confined to the hydrophilic phase in the gel.  Likewise, aqueous solutes have a high affinity for water, and dominantly partition in the hydrophilic phase as well. Conversely, oleophilic solutes predominantly partition into the silicone phase due to their low water solubility. To account for aqueous-solute exclusion and oloephilic-solute-adsorption in the silicone phase, extended Enhancement Factor Partitioning Theory [7] is adopted. Comparison of theory for water content and solute partitioning to the experimental data is excellent.

[1] Tighe B. Silicone hydrogels: structure properties and behavior. In: Silicone Hydrogels: Continuous Wear Contact Lenses; Sweeney DF, Ed. Butterworth-Heinemann: Oxford, 2004; pp.1-27

[2] Stapleton F, et al. Silicone hydrogel contact lenses and the ocular surface. The Ocular surface. 2006; 4; 24.

[3] Nicolson PC, Vogt J. Soft contact lens polymers: an evolution. Biomaterials. 2001; 22; 3273.

[4] Guan L, et al. Permeability and partition coefficient of aqueous sodium chloride in soft contact lenses. Journal of Applied Polymer Science. 2011; 122; 1457.

[5] Jones L, et al. The TFOS international workshop on contact lens discomfort: report of the contact lens materials, design, and care subcommittee. Invest Ophthalmol Vis Sci 2013; 54:TFOS37-70.

[6] Ketelson HA, Meadows DL, Stone RP. Dynamic wettability properties of a soft contact lens hydrogel. Colloid Surface B. 2005; 50; 1.

[7] Dursch TJ, et al. Water-soluble drug partitioning and adsorption in HEMA/MAA hydrogels. Biomaterials. 2014; 35; 620.