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    Biosensors in contact lenses can monitor IOP and diabetes


    Diabetes stands out among the conditions that might lend themselves to longer term monitoring.

    Most often patients monitor their diabetes with enzyme-based finger pricks, which are painful and inconvenient and could lead to infections.

    One alternative approach is based on boronic acids, which bind to diol-containing species such as carbohydrates. One such approach uses boronic acid fluorophores (BAFs), which induce spectral changes in the presence of sugars through an excited state charge transfer mechanism. So far, though, the BAF response has not been sufficiently sensitive after incorporation in a contact lens.

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    Another fluorescence approach uses concanavalin A in a competitive assay. An attempt by Eyesense (Grossotheim, Germany) to commercialise this approach proved unsuccessful, the research reported.

    Other researchers have used enzyme-electrode-based mechanisms that take advantage of oxidation of glucose by glucose oxidase into hydrogen peroxide. Further oxidation of the hydrogen peroxide at the electrode releases free electrons, producing an electric current that can be coupled with a transmission system to produce a wireless sensing device, the authors wrote.

    Building on the research of others, Verily, a division of Alphabet, has developed this technology in partnership with Novartis. The system includes an antenna, a wireless sensor interface chip and a glucose sensor polymer substrate for the detection of tear glucose.

    This approach has shown high sensitivity and selectivity. But the enzyme has a short lifetime, and enzymatic sensors can also be affected by the typical sterilisation methods used in the contact lens industry, such as autoclaving, wrote Phan and colleagues.

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    The immobilisation of the enzyme on the substrate has posed a barrier to this approach and so has enzymatically generated water, which is susceptible to interference with redox-active molecules such as ascorbic acid, said the authors. For this reason, research is now focusing on novel nanostructures with glucose-sensing properties, but this technology remains in the early stages.

    Clinical difficulties have also imposed barriers to the development of a glucose-sensing contact lenses, said Phan and colleagues. To be effective the sensor must constantly monitor glucose levels and warn patients who are in danger of hyperglycemia so that they can inject themselves with insulin.

    But patients who depend on insulin injection also risk developing severe hypoglycaemia, which also need to be monitored in a timely manner, especially at night when blood glucose often drops. Yet blood glucose takes an average of 20 minutes to appear in tear fluid, and then the glucose must diffuse through the contact lens to reach the biosensor, the authors reported. This lag time could make this method of glucose monitoring insufficient for patients at high risk from hyperglycaemia or hypoglycaemia.

    Future obstacles

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