Simulating piezoresistive effect in COMSOL4.1

Hi

I want to see the resistance change of a silicon beam due to piezoresistive effect when it undergoes stress on its terminals. Can anyone tell me how I can set the piezoresistive coefficient in COMSOL4.1? Is there any interface relating to piezoresistive effect in COMSOL4.1? I found that there were few materials about piezoresistive effect in this forum, so I posed this question.

Regards
James