Regenerative medicine involves processes of replacing, engineering or regenerating human cells, tissues or organs to restore or establish normal function. A major technology of regenerative medicine is tissue engineering, which has variously been defined as "an interdisciplinary field that applies the principles of engineering and the life sciences toward the development of biological substitutes that restore, maintain, or improve tissue function", or "the creation of new tissue by the deliberate and controlled stimulation of selected target cells through a systematic combination of molecular and mechanical signals. Tissue engineering emerged during the 1990s as a potentially powerful option for regenerating tissue and research initiatives were established in various cities in the USA and in European countries including the UK, Italy, Germany and Switzerland, and also in Japan