19334
Transient network hydrogels cross - linked through histidine - divalent cation coordination bonds were studied by conventional rheologic methods using histidine - modified star poly ( ethylene glycol ) ( PEG ) polymers .
 Chemical: histidine; Chemical: histidine; 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: histidine; Chemical: histidine; Chemical: poly ( ethylene glycol ); Chemical: PEG