Sergio Mollá

Oxygen diffusion and edema with modern scleral rigid gas permeable contact lenses.

PURPOSE We defined the theoretical oxygen tension behind modern scleral contact lenses (CLs) made of different rigid gas permeable (RGP) materials, assuming different thickness of the tear layer behind the lens. A second goal was to show clinically the effect of the postlens tear film on corneal swelling. METHODS We simulated the partial pressure of oxygen across the cornea behind scleral CLs made of different lens materials (oxygen permeability Dk, 75-200 barrer) and different thickness (Tav, 100-300 μm). Postlens tear film thicknesses (Tpost-tear) ranging from 150 to 350 μm were considered. Eight healthy subjects were fitted randomly with a scleral lens with a thin and a thick postlens tear layer in two different sessions for a period of 3 hours under open-eye conditions. RESULTS The CLs with less than 125 barrer of Dk and a thickness over 200 μm depleted the oxygen availability at the lens-cornea interface below 55 mm Hg for a postlens tear film of 150 μm. For a postlens tear film thickness of 350 μm, no combination of material or lens thickness will meet the criteria of 55 mm Hg. Our clinical measures of corneal edema showed that this was significantly higher (P < 0.001, Wilcoxon signed ranks test) with the thicker compared to the thinner Tpost-tear (mean ± SD, 1.66 ± 1.12 vs. 4.27 ± 1.19%). CONCLUSIONS Scleral RGP CLs must be comprised of at least 125 barrer of oxygen permeability and up to 200 μm thick to avoid hypoxic effects even under open eye conditions. Postlens tear film layer should be below 150 μm to avoid clinically significant edema.

Influence of Pectin as a green polymer electrolyte on the transport properties of Chitosan-Pectin membranes

Novel blend membranes have been prepared from Chitosan (CH), Pectin (PEC) and their mixtures. The obtained samples were cross-linked and sulfonated before characterization. The results show that CH/PEC membranes display structural changes on the chemical and physical properties as a function of composition. DSC analysis reveals an endothermic peak due to the scission of the ionic pairs between carboxylic groups and ammonium groups, which produces a strong change on physical properties such as methanol permeability and proton conductivity. The methanol permeability decreases with the amount of Pectin from (4.24±0.04)×10-6cm2/s for pure Chitosan membrane to (1.51±0.03)×10-6cm2/s for blend CH/PEC membranes when the amount of Pectin is 50% (v/v). The proton conductivities of the blend membranes follow a similar behavior. For a pure CH membrane the conductivity is 2.44×10-3S/cm, decreasing with pectin content until the composition 50/50 (v/v), in which the conductivity drops almost one order of magnitude.

Diffusion and Monod kinetics model to determine in vivo human corneal oxygen-consumption rate during soft contact lens wear

PURPOSE We present an analysis of the corneal oxygen consumption Qc from non-linear models, using data of oxygen partial pressure or tension (P(O2) ) obtained from in vivo estimation previously reported by other authors. (1) METHODS: Assuming that the cornea is a single homogeneous layer, the oxygen permeability through the cornea will be the same regardless of the type of lens that is available on it. The obtention of the real value of the maximum oxygen consumption rate Qc,max is very important because this parameter is directly related with the gradient pressure profile into the cornea and moreover, the real corneal oxygen consumption is influenced by both anterior and posterior oxygen fluxes. RESULTS Our calculations give different values for the maximum oxygen consumption rate Qc,max, when different oxygen pressure values (high and low P(O2)) are considered at the interface cornea-tears film. CONCLUSION Present results are relevant for the calculation on the partial pressure of oxygen, available at different depths into the corneal tissue behind contact lenses of different oxygen transmissibility.

Polymers with Ionic Liquid Fragments as Potential Conducting Materials for Advanced Applications

During the last decades, the use of ionic liquids (ILs) has become one of the most useful techniques for the development of green chemistry tools.1 The potential use of liquid salts based on delocalized organic cations (ammonium or phosphonium) as designer green solvents represents one of the most significant contributions to modern chemistry. Thus, the high modularity of the structures of the ILs based on the proper selection of the cation and anion moieties has allowed the development of an almost infinite number of compounds well suited for each specific application. Nevertheless, in recent years, the application of ILs also needs to confront some important challenges in order to be able to develop the expected practical applications.2 Thus, some important drawbacks are limiting the general application of ILs in scientific and technological applications. First of all, the cost of ILs is clearly much higher than that for traditional solvents. Additionally, the benefits of the use of ILs as media for different chemical reactions and other applications is counterbalanced by the need of using traditional “non-green” solvents for the extraction of the desired products from the IL phase. Finally, recent studies have shown that, although ILs have been considered traditionally as environmentally friendly solvents because of the lack of any appreciable vapor pressure, some of the most usual ILs present some environmental concerns in particular in terms of their contact with aqueous media.3 Many of those drawbacks can be drastically reduced by the use of supported ILs. The immobilization of ILs onto a solid support provides a simple way for reducing the amount of IL required for a given application, reducing accordingly the associated cost; facilitates their handling and manipulation decreasing the need of using traditional solvents in the corresponding process, and finally, greatly reduces the potential leaching to the environment of the ILs.4 In order to accomplish this target two main approaches have been studied: 1. Non-covalent support of IL phases on the surface of inorganic or organic supports (SILPs). 2. Covalent attachment of IL-like phases on the surface of inorganic or organic supports (SILLPs).

Comments to paper entitled: Predicting scleral GP lens entrapped tear layer oxygen tensions

We have read with interest the article authored by Jaynes, J.M.; Edrington, T.B.; Weissman, B.A. recently published in Cont. Lens Anterior Eye, (2015; 38: 44–47) entitled “Predicting scleral GP lens entrapped tear layer oxygen tensions ” [1]. We found the topic very interesting. However, we think that the conclusion of the authors is not fully supported by their results. The authors alert to the fact that the cornea would suffer hypoxia for any combination under the best scenario they evaluated theoretically (300 microns lens with 140 barrer Dk and 50 microns corneal clearance). The authors indirectly infer that the cornea would undergo physiological effects. Similar results had already been found by Michaud et al. [2] and hypothesized that 250 microns lens with a Dk higher than 150 barrer and up to 200 microns of corneal clearance should be used to minimize corneal edema. However, none of these two studies provided clinical measures of edema. Having been working in this topic for the last 4 years we admit that this is at a first glance a logical thought. Indeed, when we first tried to publish our results back in 2011, reviewers raised the question whether such hypothesis would be verified considering the apparent absence of hypoxia in the clinical setting. Then we started a clinical pilot study to demonstrate the relationship between post-lens tear film thickness and physiological stress. Meanwhile, Michaud et al. [2] published their results, who already anticipated the results shown in the present study [1]. Both used constant oxygen consumption for the whole cornea which does not allow to estimate the accurate concentration of oxygen at different levels within the eye. The readers could find these estimations in our previous work [3]. Looking for a direct comparison between the three studies we can report the partial pressure of oxygen for 300–350 lens thickness (Dk = 100 barrer) and post-lens tear film of 300–350 microns. Michaud’s results would provide about 15 mmHg while Compan’s would predict about 23 mmHg. Extrapolated data from Jaynes results would render about 24 mmHg very close to our previous predictions. The outcomes from our clinical pilot study confirmed that edema (as an indirect sign of hypoxia stress) would indeed be