2287
Crosslinked polyethylenimines ( PEIs ) have been frequently examined over the past decade since they can maintain the transfection efficiency of commercially available , 25k branched PEI , but exhibit less cytotoxicity .
 polyethylenimines, PEIs, PEI
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 polyethylenimines, PEIs, PEI