21210
While the basic nanofibrous morphology is retained in all cases , selection of the particular core residues results in switching from antiparallel hydrogen bonding to parallel hydrogen bonding in addition to changes in nanofiber morphology and in hydrogel rheological properties .
 hydrogen, hydrogen
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 hydrogen, hydrogen