Wireless implantable bioelectronics and accurate in vivo characterization of path loss require efficient and robust in-body antennas. Loading electric antennas with the effective permittivities higher than that of surrounding tissues is a promising solution. In this study, proof-of-concept conformal microstrip antennas are proposed. The antennas are optimized for a standard impedance of 50 ${\Omega}$ and operate in all high-water-contents tissues. Integral ground plane shields the antenna from inner circuitry. The radiation efficiencies are 0.4%, 2.2%, and 1.2% for the (434, 868, and 1400)-MHz designs, respectively, when computed in a ${\phi}$ 100-mm spherical phantom with muscle-equivalent properties. The efficiencies are compared to the fundamental limitations and closely approaches them. Specific absorption rates are evaluated, and the corresponding maximum input power levels are established. Prototypes are fabricated and characterized to validate the design.