Common plastic, a polymer formed by the concatenation of many ethylene molecules to form a larger structure. The length and branching frequency of the polymer chain influences the physical and mechanical properties of the plastic. Broad classes of polyethylene include high-density polyethylene (HDPE), used to make cups and mugs and relatively rigid items, and low-density polyethylene (LDPE), which is much more flexible and stretchy.

Don't forget about Ultra High Molecular Weight Polyethylene (UHMWPE). This fantastic stuff is used in surgical implants, such as replacement knees and hips. It is tough enough to replace sections of bone, but has enough cushion and low enough friction to also replace the cartilage in a joint. It has incredible wear characteristics (think of how much abuse your knee takes each day) and is relatively easy to machine. The only downside is that sterilization generally leads to the formation of free radicals, which adversely affects performance.

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