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No resistance changes of Ag tracks printed on PA and CoPA were observed at lower strain amplitudes whereas at higher strain amplitudes these changes were the lowest for conductive tracks on CoPA .
 resistance, PA, strain, strain
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:
 resistance, PA, strain, strain