Contact lens to measure individual ion concentrations in tears and applications to dry eye disease

Abstract

Dry eye disease (DED) affects millions of individuals in the United States and worldwide, and the incidence is increasing with an aging population. There is widespread agreement that the measurement of total tear osmolarity is the most reliable test, but this procedure provides only the total ionic strength and does not provide the concentration of each ionic species in tears. Here, we describe an approach to determine the individual ion concentrations in tears using modern silicone hydrogel (SiHG) contact lenses. We made pH (or H₃O⁺, hydronium cation,/OH^-, hydroxyl ion) and chloride ion (two of the important electrolytes in tear fluid) sensitive SiHG contact lenses. We attached hydrophobic C18 chains to water-soluble fluorescent probes for pH and chloride. The resulting hydrophobic ion sensitive fluorophores (H-ISF) bind strongly to SiHG lenses and could not be washed out with aqueous solutions. Both H-ISFs provide measurements which are independent of total intensity by use of wavelength-ratiometric measurements for pH or lifetime-based sensing for chloride. Our approach can be extended to fabricate a contact lens which provides measurements of the six dominant ionic species in tears. This capability will be valuable for research into the biochemical processes causing DED, which may improve the ability to diagnose the various types of DED.

Keywords: Chloride ion; Contact lens; Dry eye disease; Electrolytes; Fluorescence sensors; Hydrogels; Hydronium ion; Silicone hydrogels.

Figure 1

(A). Schematic of contact lens for measurement of a specific single ion. The schematic green shading indicates the location of the ion-sensitive fluorophore. (B) Schematic of a contact lens for complete electrolytes analysis in tears. Each color spot represents a different ion-specific fluorophores.

Figure 2

(A), Schematic cross section of non-silicone hydrogel (HG) contact lens with homogeneous interior structure. (B), Schematic of a small region of a SiHG showing the hydrophylic and hydrophobic interpenetrating polymer networks. The dimension is an approximation. The amphipathic H-ISFs are localized at the silicone-water interface throughout the contact lens by hydrophobic interactions.

Figure 3

(A), Iris identification security device. From Iris ID Inc. (B), 3D time-of-flight CMOS imaging detector for Laveno Tango cell phone announced in June 2016. (C), Pixel Sensor from Pixelteq.com providing eight band pass filters from 425 to 700 nm. (D), Pixel sensor in a functional housing.

Figure 4

Molecular structures of probes used for the present study.

Figure 5

(A), Typical monomer for a silicone hydrogel contact lens. (B), Water content and Dk values of HG and SiHG lenses. LA, Lotrafilcon A; LB, Lotrafilcon B; G, Galyfilcon A; C, Comfilcon A; S, Stenfilcon A; N, Nelficon A.

Figure 6

(A), Normalized emission spectra of 1,8-ANS in 1-hexanol, ethanol and Biofinity contact lens. ANS emission in Dailies-HG is too weak to measure emission spectrum, λex = 350 nm. (B), Fluorescence intensity decays of 1,8-ANS in 1-hexanol, ethanol and bound to a SiHG (Biofinity) and a HG (Dailies) contact lens. λex = 355 nm. Inset in (A) shows molecular structure of 1,8-ANS. (C), photographs of ANS doped contact lens with no emission filter, in room light and on a hand-held UV lamp. (D), Lifetime distribution analysis of 1,8-ANS in 1-hexanol, ethanol, and 1,8-ANS labeled Biofinity -SiHG or Dailies -HG.

Figure 7

(A), Normalized emission spectra with λex at 450 nm and (B), Intensity decays of NBD-C18 in THF, EtOH, Dailies-HG and a Biofinity SiHG contact lenses with λex at 473 nm. (C) NBD-C18 lifetime distribution in Biofinity-SiHG or Dailies-HG. The lifetime distribution in THF and EtOH shows single Gaussian distribution with a peak maximum at 10.5 and 6.9 ns, respectively.

Figure 8

Laser scanning confocal intensity images of NBD-C18 labeled Biofinity-SiHG lens at (A), 300; (B), 450 μm below the top center of lens and respective (C and D), phase shift and lifetime images (E and F). (G), Effect of the dwell time and incident intensities on the emission phase angles at 40 MHz. Phase angles are slightly offset for clarity. λex = 443 nm. Emission collected using a bandpass filter 525/50.

Figure 9

(A) pH dependent equilibrium between neutral and anionic form of 6HQ-C18. (B) Photographs of 6HQ-C18 labeled biofinity CL in pH 4.0 (left) and 10 (right) under room light or on a UV handlamp.

Figure 10

Excitation (A) and emission (B) spectra of 6HQ-C18 in Comfilcon A lens. (C), pH-dependent excitation wavelength ratio for 6HQ-C3 in buffer and 6HQ-C18 within Comfilcon A -SiHG lens. Emission monitored at 580 nm and λex = 350 nm.

Figure 11

Chloride quenching of SPQ-3 in water. A, emission spectra and B, Time-dependent decays. λex = 355 nm.

Figure 12

Chloride quenching of SPQ-18 in a Stenfilcon A (Aspire) contact lens. (A), emission spectra and (B), time-dependent decays. λex = 355 nm.

Figure 13

Comparison of lifetime Stern-Volmer plots for SPQ-C3 in water and SPQ-C18 in a Stenfilcon A (Aspire) contact lens.

Figure 14

(A), Lifetime distribution analysis of SPQ-C3 in water and (B) SPQ-C18 in Stenfilcon A (Aspire) with chloride quenching.