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Sangly P. Srinivas, Giovanna Guidoboni, Lucia Carichino, Yiran Jiang, Joseph A. Bonanno; Rapid Measurement of Tear Oxygen Tension underneath Soft Contact Lenses by Frequency-Domain Phosphorimetry. Invest. Ophthalmol. Vis. Sci. 2012;53(14):6106.
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To measure the dynamics of tear oxygen tension (pO2) during soft contact lens wear using a non-invasive approach following a brief closure of the eyelids.
A slit-lamp fluorometer was modified for rapid measurement of the oxygen-sensitive phosphorescence lifetime of Pd-meso-tetra (4-carboxyphenyl) porphyrin (ex = 530 nm; em > 630 nm). The dye, bound to albumin, was coated on to the bottom surface of hydrogel lenses before use in healthy volunteers (Bonanno et al., IOVS, 43(2), 2002). The output of a green LED (Nichia Inc), which was modulated as a sine-wave at 1 KHz, was used as the excitation source. Phosphorescence, collected by the emission optics, held parfocal to that of excitation, was directed to the cathode of a photomultiplier tube (R928, Hamamatsu Inc). Oxygen-sensitive phase delay and demodulation were detected using a lock-in amplifier (SRS-830; 120 KHz bandwidth; Stanford Research Systems, Palo Alto, CA) and sampled at 50 Hz. The dynamics of pO2, calculated from phase delay using the Stern-Volmer equation, was compared to predictions from a 1-D transient O2 transport model governing diffusion and consumption across the lens + cornea matrix. The oxygen consumption across the cornea was assumed to follow Monod kinetics (Chhabra et al., J Biomed Mater Res B Appl Biomater., 90(1), 2009).
Rapid changes in the oxygen-sensitive phase delay and demodulations were observed following brief closure of the eyelids. The time to recover to a steady state increased with longer periods of eye closure (5 min). Thicker lenses (of low Dk/L) produced slower return to steady state (taking up to 35 secs) compared to that of the thin lens (< 5 secs). The corresponding dynamics of pO2 were similar to those predicted by the mathematical model using parameters reported earlier (Chhabra et al., 2009).
In contrast to the time-domain phosphorimetry (Bonanno et al., IOVS, 2002), the frequency-domain approach for phosphorescence lifetimes offers much higher sampling frequencies (50 Hz compared to 1 Hz). This high sampling rate will enable more accurate estimation of the oxygen consumption rate and otherparameters of interest in contact lens design.
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