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Csaba Kotsmar, Teresa Nadolski, Nicole Taylor, Kevin Yeh, Clayton J. Radke; Solute Release From Soft-contact-lens Hydrogels. Invest. Ophthalmol. Vis. Sci. 2012;53(14):6100.
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Transport of solute molecules through soft-contact-lens (SCL) hydrogels, such as salts, wetting agents, nutrients, and drugs, is important to on-eye behavior. Typically, solutes are loaded into the gel and released over time. This work measures the mesh size of contact-lens materials and demonstrates that solute absorption and release times decrease with increasing mesh size. For solutes that interact strongly with the gel matrix, release times are much slower than loading times and can approach infinity for irreversible interactions.
Two-photon confocal microscopy detects the transient concentration profiles of fluorescently tagged dextrans and proteins with different charges in HEMA/MAA hydrogels during both loading and release (see Figure 1 for sodium fluorescein). Diffusion theory gives the solute effective diffusion coefficient D. Oscillatory shear rheometry gives the gel mesh size.
For uncharged dextrans of molecular weight near 10 kDa, D increases from 3.24±0.1 x 10-8 cm2/s to 4.02 ± 0.4 x 10-7 cm2/s both for absorption and desorption as the mesh size increases from 3 to 9 nm. These solutes interact reversibly with the gel. For protein oppositely charged to the gel network, the diffusion coefficient in the loading direction greatly decreases to 5.2±0.3 x 10-9 cm2/s. Desorption of the anionic protein is extremely slow indicating strong interaction with the gel strands.
Using rheometry and polymer physics, we successfully measure mesh sizes of SCL materials. For reversibly interacting solutes, absorption and release rates increase with gel mesh size and are identical in both loading and release directions. For solutes that strongly interact with the gel, uptake rates are diminished and release rates are extremely slow. It is not possible to predict solute release rates from hydrogels using studies conducted in the loading direction only.
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