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Peptides expressing the furan moiety were attached to poly ( ethylene glycol ) ( PEG ) hydrogels through a thermally reversible covalent bond formed by a Diels - Alder reaction .
 Chemical: furan; Chemical: poly ( ethylene glycol ); Chemical: PEG
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: furan; Chemical: poly ( ethylene glycol ); Chemical: PEG