A paper, "Roles of epsilon-near-zero (ENZ) and mu-near-zero (MNZ) materials in optical metatronic circuit networks", recently published in Optics Express, has described methods of realising optical equivalents of capacitors and inductors in both serial and parallel combinations. This opens the door to the design of passive networks of such elements, such as filters. The use of ENZ and MNZ materials is crucial to obtain the required functionality. The issues of dispersion and loss are discussed. One of the key challenges for future implementations is the development of real MNZ materials at optical frequencies.