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: Considering the small increase in the patient ' s independence , the high costs of bilateral implants , and the difficulty most patients experience in handling the devices , the question arises as to whether bilateral pallidal stimulation is a real alternative to pallidotomy .
### Output Text:
Considering the small increase in the patient ' s independence , the high costs of bilateral implants , and the difficulty most patients experience in handling the devices , the question arises as to whether bilateral pallidal stimulation is a real alternative to pallidotomy . 