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Work by Wu centres on SIMP , whereby in , solely the infills of 3D structures were optimized and the shell was represented by passive elements , whereas the approach in demonstrated a concurrent optimisation of shell and infill for 2D structures .
 concept or principle: 3D structures; machine or equipment: shell; material: elements; machine or equipment: shell; process parameter: infill; engineering feature: 2D structures
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: 3D structures; machine or equipment: shell; material: elements; machine or equipment: shell; process parameter: infill; engineering feature: 2D structures