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An aluminum film was placed between two polydimethylsiloxane ( PDMS ) membranes to realize frequency multiplication by twice contact electrifications within one cycle of external force .
 Chemical: aluminum; Chemical: polydimethylsiloxane; Chemical: PDMS
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: aluminum; Chemical: polydimethylsiloxane; Chemical: PDMS