In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: This percentage decreased to 45 . 45 , 44 . 4 , and 14 . 28 % at 1 year , 2 years , and 3 years postimplant , respectively .
### Output Text:
This percentage decreased to 45 . 45 , 44 . 4 , and 14 . 28 % at 1 year , 2 years , and 3 years postimplant , respectively . 