An apparatus comprises a sensor for measuring a physiological parameter of a subject, wherein the physiological parameter sensor is adapted to be worn by the subject; an actuator comprising an electro-active polymer material, EAP, portion for adjusting the position of the physiological parameter sensor relative to the subject; a feedback sensor for measuring movement of the physiological parameter sensor and/or the subject; a controller configured to process the measurements of the feedback sensor and to adjust the position of the actuator based on information from the feedback sensor.