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The carbon black and graphene filaments were brittle and fractured easily , but the copper-based filament could be bent at least 500 times with little change in its resistance .
 carbon black, graphene filaments, brittle, filament, be, resistance
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:
 carbon black, graphene filaments, brittle, filament, be, resistance