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The mechanical properties of natural materials , particularly the superior fracture toughness , make biological structures highly suitable for biomimetic studies .
 concept or principle: mechanical properties; concept or principle: materials; concept or principle: fracture; engineering feature: biological structures; concept or principle: biomimetic studies
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: mechanical properties; concept or principle: materials; concept or principle: fracture; engineering feature: biological structures; concept or principle: biomimetic studies