Is the leaf structure in anyway necessary for the chemical reactions or is it just a nice, familiar form that makes the project seem more natural than some array of microfluidic chambers?
I would guess that while the leaf shape in particular is not required, the loose space-filling curve is a good shape for the tube to get the most use out of the block. Could it perhaps be slightly more space-efficient with a rectangle and a heater-coil shape? Maybe, but I'd expect not by much, and I would expect other resource constraints (the reactants) to matter before you've filled all available sunny spots and need to maximize medicine-per-square-meter, if only because sunny rooftops are not exactly high-demand space.