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3 Variable-density infills derived from density-based TO An emerging topic in structural optimization with a promising application for AM are TO-based compliance-driven infill strategies for porous material , which account for the principal stress directions imposed by the loading condition .
 concept or principle: structural optimization; manufacturing process: AM; process parameter: infill; material: porous material; machanical property: principal stress
Please list all entity words in the text that fit the category.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} 
Answer:
 concept or principle: structural optimization; manufacturing process: AM; process parameter: infill; material: porous material; machanical property: principal stress