Hydroxypropyl methylcellulose (HPMC or hypromellose) is a derivative of cellulose in which some of the hydroxyl groups in the repeating glucose units have been methoxylated or hydroxypropylated. HPMC is a thickening and binding agent approved for food use in the USA and the EU. As a food additive, HPMC is an emulsifier, thickening and suspending agent, and an alternative to animal gelatin. HPMC is used in deep fried batter. Its Codex Alimentarius code (E number) is E464. Since HPMC solution is a non-Newtonian solution and exhibits pseudoplastic behaviour (specifically, thixotropic behaviour) various test methods are available, and the results of different methods and viscometers do not necessarily correspond to each other. Due to viscometer acceptable ranges of error, the viscosity is typically given as a mean or as a range. The degree of substitution is the average level of methoxy substitution on the cellulose chain. Since there is a maximum of three possible sites of substitution with each cellulose molecule, this average value is a real number between 0 and 3. However, the degree of substitution is often expressed in percentages. Hypromellose (INN) is a semisynthetic, inert, viscoelastic polymer used as an ophthalmic lubricant, as well as an excipient and controlled-delivery component in oral medicaments, found in a variety of commercial products. Hypromellose is a solid and slightly off-white to beige powder and may be formed into granules. The compound forms colloids when dissolved in water. Although non-toxic, it is combustible and can react vigorously with oxidizing agents. Hypromellose solutions were patented as a semisynthetic substitute for tear-film. Its molecular structure is predicated upon a base celluloid compound that is highly water-soluble. Post-application, celluloid attributes of good water-solubility reportedly aid in visual clarity. When applied, a hypromellose solution acts to swell and absorb water, thereby expanding the thickness of the tear-film. Hypromellose augmentation therefore results in extended lubricant time presence on the cornea, which theoretically results in decreased eye irritation, especially in dry climates, home, or work environments. On a molecular level, this polymer contains beta-linked D-glucose units that remain metabolically intact for days to weeks. On a manufacturing note, since hypromellose is a vegetarian substitute for gelatin, it is slightly more expensive to produce due to semisynthetic manufacturing processes. Aside from its widespread commercial and retail availability over the counter in a variety of products, hypromellose 2% solution has been documented to be used during surgery to aid in corneal protection and during orbital surgery (Wikipedia).