As described in Part 1, the base mucin layer is rather like interlocking Lego, so as we take away building blocks from the foundation layer of our Lego castle (the mucin layer), the structure becomes unstable. To counter-balance this tendency towards hyper-tonicity, many dry eye drops are formulated to be deliberately less concentrated with electrolytes, ie. hypo-tonic, so when they mix with the existing hypertonic film, an iso-tonic tear film results.
Earlier we mentioned that specific electrolytes in a product help it resemble human tears. These include;
- Potassium, chloride.
- Sodium, calcium.
Perhaps the most important electrolytes are potassium, to protect against epithelial oedema, and bicarbonate to promote epithelial repair following damage. The packaging of products containing bicarbonate has to guard against air contact which turns it into carbon dioxide, which easily permeates through normal plastic packaging.