A rubber-band heat engine - does not violate the second law of thermodynamics, at least apparently. The system is regularly heated and cooled:

http://readingpenrose.files.wordpres...and-engine.gif

An analogous pH-sensitive-polymer heat engine - almost obviously violates the second law of thermodynamics. Hydrogen ions are regularly added to and removed from the system:

http://www.gsjournal.net/old/valev/val3.gif

Pentcho Valev