He trains on patients by withdrawing blood from them
Question

                A doctor must learn how to withdraw blood from his patient. In order to perfect this he must train a lot. Some patients sometimes are in full and chronic coma and their cases are hopeless. Will we be sinful if we train by withdrawing little quantity of blood from them that will not harm them but at the same time will benefit us? Is this considered necessity?.
Praise be to Allah.
    It is not permissible to withdraw blood from patients for the 
    purpose of learning and training, even if there is no hope of the patients 
    recovering, because that is harming them and wronging them. But if the 
    patient is compos mentis and gives permission for blood to be taken in the 
    interests of others, there is nothing wrong with that, so long as he will 
    not be harmed thereby. 

    And Allaah is the Source of strength, may Allaah send 
    blessings and peace upon our Prophet Muhammad and his companions and family. 
    End quote.