Fundamental in-body limitations on achievable radiation efficiency could provide decision-making assistance to engineers working on antennas for implantable bioelectronics. In this study, proof-of-concept conformal microstrip antennas are proposed based on these theoretical foundations. In particular, maximizing the effective aperture and loading the antenna with materials having the permittivity higher than that of surrounding tissues is a promising solution for increasing the radiation efficiency. The operating frequencies are tuned to operate within the optimal range for deep-body implantation: 434, 868, and 1400 MHz. The achieved radiation efficiencies at these frequencies are 0.4%, 2.2%, and 1.2%, respectively, when simulated in a empty set100-mm spherical phantom with muscle-equivalent electromagnetic properties. The radiation performance at each frequency is compared to the fundamental limitations and closely approach them. Prototypes are characterized for the experimental validation.