The viable and safe application of wireless power transfer for powering bioelectronic implants requires understanding the wave propagation in heterogeneous and dispersive media, the electromagnetic exposure assessment, and the optimum design of the system parameters to achieve a trade-off between efficiency and specific absorption rate levels. Therefore, based on the case study of a wirelessly charged deep-implanted pacemaker, a parametric analysis on the transmitter dimensions and electromagnetic properties is carried out to achieve such a trade-off. The results show that the system reaches the maximum efficiency without increasing SAR levels when the transmitter is composed of an electric source, an air-like substrate, and a superstrate matched to the wave impedance in the skin with a thickness of half the wavelength in this medium. Furthermore, this configuration is compared to a magnetic counterpart, and the reasons for its suboptimal performance are investigated in terms of near-field, reflection, and attenuation losses.