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Electrets have been increasingly investigated for their high piezoelectric sensitivity for sensing and energy harvesting applications , but fabricating complex 3D structures for optimum performance has remained challenging .
 material: Electrets; machanical property: piezoelectric sensitivity; application: sensing; concept or principle: energy harvesting; manufacturing process: fabricating; concept or principle: 3D structures; concept or principle: performance
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
 Entities: Electrets, piezoelectric sensitivity, sensing, energy harvesting, fabricating, 3D structures, performance 
Answer:
 material: Electrets; machanical property: piezoelectric sensitivity; application: sensing; concept or principle: energy harvesting; manufacturing process: fabricating; concept or principle: 3D structures; concept or principle: performance