It’s even easier to read than the books!
Monash describe it as “the first equipment-free, bioactive paper-based diagnostic device capable of reporting multiple conditions in written text.”
How does it work? Printed onto the surface is water-repelling chemicals, with gaps for the letters A, B and O, and the +/- sign. Into each of these letters, the device is embedded with antibodies that stick to antigens on blood cells. Antibody A sticks to antigen A, if the blood sample is A. Antibody B sticks to antigen B for B-type. AB blood contains both antigens, and will stick to both spots.
Blood is added onto the device, given time to stick to the antibodies in each letter, then rinsed off. Clumps of blood stuck to the surface antibodies remains, leaving the result displayed out in letters of blood. Pretty cool.
O-type has neither B nor A antigens on the cells, and so won’t stick at all. Only the preprinted O will be visible (see it’s in a slightly different colour to the A in the picture?) The cross inside the O must be embedded with both A and B antibodies, picking up either blood-type and crossing out the default option.
Rhesus factor, the +/-, works much the same way. The default is a preprinted horizontal -, with embedded Rh-antibodies on the vertical line to make a +.
They say the device was inspired by Tom Riddles diary in the Chamber of Secrets, where answers could be received in writing. I love the idea of this easy to use, easy to read blood test, but designing it from a Horcrux? Hm…