Apparatus, consisting of a catheter having a distal end and a predefined outer diameter. The apparatus also has a plurality of elastic filaments, each filament having at least one electrode fixed thereto and having two ends fixed within the catheter distal end to hold the filament as a loop. The loop intertwines with one or more other loops of the other filaments so that the plurality of the filaments forms an open lattice, which expands when uncompressed to a lattice diameter at least 5 times greater than the outer diameter of the catheter.
