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We explore elastic wave focusing and enhanced energy harvesting by means of a 3D-printed Gradient-Index Phononic Crystal Lens ( GRIN-PCL ) bonded on a metallic host structure .
 machanical property: elastic; concept or principle: energy harvesting; manufacturing process: 3D-printed; manufacturing process: Lens; material: metallic; concept or principle: structure
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: elastic, energy harvesting, 3D-printed, Lens, metallic, structure 
Answer:
 machanical property: elastic; concept or principle: energy harvesting; manufacturing process: 3D-printed; manufacturing process: Lens; material: metallic; concept or principle: structure