Biodegradable polymer which, edible films have gained more attention due to their environmentally-friendly characteristics, vast variety and availability, non-toxicity, and low cost. Biodegradable polymers have a long history, and since many are natural products, the precise timeline of their discovery and use cannot be accurately traced. One of the first medicinal uses of a biodegradable polymer was the catgut suture, which dates back to at least 100 AD. The first catgut sutures were made from the intestines of sheep, but modern catgut sutures are made from purified collagen extracted from the small intestines of cattle, sheep, or goats. The concept of synthetic
Musical instrument classification ways, having finger-holes to control pitch, rather than valves. Keyboard instruments do not fit easily into this scheme. For example, the piano has strings, but they are struck by hammers, so it is not clear whether it should be classified as a string instrument or a percussion instrument. For this reason, keyboard instruments are often regarded as inhabiting a category of their own, including all instruments played by a keyboard, whether they have struck strings (like the piano), plucked strings (like the harpsichord) or no strings at all (like the celesta). It might be said that with these extra categories,