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The first part of this study focuses on characterization of key porous structure properties , i.e. , porosity , average mass density , average pore sizes and structures at microscopic scales .
 porous, properties, porosity, average, density, average
Please list all entity words in the text that fit the category. Output format is "word1, 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:
 porous, properties, porosity, average, density, average