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Periodic structures of scaffolds with the maximal effective diffusivity aimed at providing an ideal environment for nutrient transportation were studied by a level-set based optimization method .
 engineering feature: scaffolds; process characterization: diffusivity; concept or principle: optimization
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:
 engineering feature: scaffolds; process characterization: diffusivity; concept or principle: optimization