improved implantable microstimulators are covered with a biocompatible polymeric coating in order to provide increased strength to the capsule and to capture fragments of the microstimulator should it become mechanically disrupted . such coating also makes the microstimulator safer and easier to handle . the coating may include one or more diffusible chemical agents that are released in a controlled manner into the surrounding tissue . the chemical agents , such as trophic factors , antibiotics , hormones , neurotransmitters and other pharmaceutical substances , are selected to produce desired physiological effects , to aid , support or to supplement the effects of the electrical stimulation . further , microstimulators in accordance with the invention provide systems that prevent and / or treat various disorders associated with prolonged inactivity , confinement or immobilization of one or more muscles . such disorders include pressure ulcers , venous emboli , autonomic dysreflexia , sensorimotor spasticity and muscle atrophy . the microstimulator systems include external control for controlling the operation of the microstimulators . the control includes memory for programming preferred stimulation patterns for later activation by the patient or caregiver .