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A self-writing diary in one of J K Rowling's books on Harry Potter has inspired researchers to create a paper that spells out a person's blood type. A team from Monash University in Australia has developed a paper-based sensor that writes blood type as text. The sensor may help non-experts to interpret the results rapidly, especially in emergency situations and during humanitarian disasters. The study appears in the journal Angewandte Chemie. The device works according to the so-called ABO system, classing blood samples according to A, B, AB or O types, and also spelling out whether the type is Rhesus positive or negative. According to the system, an A or B letter indicates which antigens are present in red blood cells. So someone who has, for instance, blood type A has A antigens, and if blood type is AB, then both A and B antigens are present. People with O blood type have no antigens at all.