One of the most promising techniques to search for neutrinoless double beta decay are bolometers. Bolometers are low temperature detectors that are made of a crystal embedding the double-beta decay isotope and a sensor coupled to the crystal (usually glued). When a particle releases energy in the bolometric absorber (the crystal) it produces lattice vibrations that generate a temperature rise in the crystal measured by the sensor that works as a thermometer (germanium NTD ). This ensures the desired energy resolution, of the order of 5 keV FWHM in both cases. However, this simple configuration is not sufficient to achieve the sensitivities requested by the next-generation searches. Adding a light readout based on a simple bolometer improves dramatically the situation, but is not enough when looking at the direct-ordering region.