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Continuous fiber reinforcement is currently one of the biggest challenges researchers face in 3D printing of polymer composites .
 material: Continuous fiber reinforcement; concept or principle: face; manufacturing process: 3D printing; material: polymer composites
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: Continuous fiber reinforcement; concept or principle: face; manufacturing process: 3D printing; material: polymer composites