Swiss engineers have invented a pacemaker based on an clockwork wristwatch which is powered by the motion of the heart.     The prototype, which has been successfully tested in pigs, could mean  that patients no longer have to undergo repeat surgery when the  batteries in their device run out.     The invention, presented to the European Society of Cardiology Congress  in Barcelona, is based on the concept behind a self-winding wristwatch.    Currently pacemakers, which are used to help the heart beat more  regularly, need to be replaced on average every seven years, when  batteries fail   More than 40,000 people a year are fitted with pacemakers in England.     The devices are used for patients with unusually fast, slow or irregular heart rhythms.    The new model, stitched directly on to the heart, is based on the  concept of automatic clockwork developed by Swiss watchmakers in 1777  and generates electrical current using the movement of heart muscle.     Automat...