| http://www.w3.org/ns/prov#value | - A device according to claim 13, wherein the water-soluble material is selected from a water-soluble polymer such as polyvinyl alcohol, collagen, gelatin, pectin or sugar paper or any water-soluble organic or inorganic sold such as a sugar or an ionic salt such as an alkali metal chloride, hydroxymethyl cellulose or hydroxypropyl cellulose, any of said materials optionally being in admixture with a
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