Fluorescent contact lens for continuous non-invasive measurements of sodium and chloride ion concentrations in tears

Abstract

Rapid and non-invasive measurement of hydration status is medically important because even mild levels of dehydration can have a significant impact on physical and cognitive performance. Despite the potential value of determining whole-body hydration based on the electrolytes found in tears, very few tests are available. An area of intense interest is the development of a contact lens which could measure ion concentrations in tears, specifically that of sodium (Na+) and chloride (Cl−) ions, the dominant electrolytes in blood plasma and tears. Here, we describe a method to make fluorescent contact lenses which allow determination of Na+ and Cl− ion concentrations in tears. Fluorophores known to be sensitive to Na+ and Cl− were derivatized to bind non-covalently to two commercially-available silicone hydrogel (SiHG) contact lenses—the Biofinity (Comfilcon A) or MyDay (Stenfilcon A) lenses. The sodium- and chloride-sensitive fluorophores displayed spectral changes in the physiological range for Na+ and Cl− ions in tears. The lenses for both Na+ and Cl− ions were completely reversible. The sodium responses were not sensitive to protein interference including human lysozyme, human serum albumin and mucin type 2. The chloride sensitivity was similar with both lenses, but the sodium-sensitive range was different in the Biofinity and MyDay lenses. We also fabricated a lens with both the Na+ and Cl− probes in a single MyDay lens resulting in a contact lens that independently measured Na+ and Cl− concentrations without physical separation of the fluorophores. Our findings indicated that a sodium and chloride-sensitive contact lens (NaCl-lens) could be used for rapid non-invasive detection of whole-body hydration, as well as associated diseases or other infections.