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The function of HA crystals and collagen molecules are like the steel rod and cement to concrete : one part provides flexibility and the other provides strength and toughness .
 material: HA crystals; material: collagen molecules; material: steel; machine or equipment: rod; material: cement; material: concrete; machanical property: flexibility; machanical property: strength; machanical property: toughness
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:
 material: HA crystals; material: collagen molecules; material: steel; machine or equipment: rod; material: cement; material: concrete; machanical property: flexibility; machanical property: strength; machanical property: toughness