Fat intra-body communication (Fat-IBC) is a novel concept of communication through the low-loss fat tissue layer of the body. This new data transmission approach is promising for the implementation of wireless, in-body, bidirectional Brain-Machine-Body connectivity, which will profoundly impact different fields, such as brain implants with read/write capability and human-like artificial sensors and limbs. In this paper, we apply an evolutionary algorithm combined with a full-wave electromagnetic solver to optimize a flat epidermal antenna, to improve the radiation coupling into the body and favor the signal transmission in the subcutaneous fat layer. The application of this method to a skin-fat communication channel shows satisfactory results when both the antennas of the link are considered and materials with high relative permittivity are used. A good radiation coupling into the body is observed, and transmission losses lower than 5 dB/cm are achieved.