A drug that makes hearts repair themselves has been used in research on mice.The damage caused by a heart attack had previously been considered permanent.But a study in the journal Nature showed the drug, thymosin beta 4, if used in advance of a heart attack, was able to "prime" the heart for repair.The British Heart Foundation described repair as the "holy grail of heart research", but said any treatment in humans was years away.Due to advances in health care the number of people dying from coronary heart disease is falling.But those living with heart failure are on the rise - more than 750,000 people have the condition in the UK alone.