Background and Objective: In-body biotelemetry devices enable wireless monitoring of a wide range of physiological parameters. These devices rely on antennas to interface with external receivers, yet existing systems suffer from impedance detuning caused by the substantial differences in electromagnetic properties among various tissues. In this paper, we propose an immune-to-detuning in-body biotelemetry platform featuring a novel tissue-independent antenna design. Methods: Our approach uses a novel slot-patch conformal antenna integrated into a flexible polyimide printed circuit board containing the device circuitry and encapsulated within a 17.7 mm x empty set 8.9 mm biocompatible shell. The antenna is synthesized and optimized using a hybrid analytical-numerical approach and, then, characterized numerically and experimentally in terms of impedance stability. Results: The proposed platform shows stable impedance, whereas operating in any mammalian tissue as well as in air. The system is optimized for the 434-MHz industrial, scientific, and medical band and can easily be returned for any MedRadio band in the 401-457-MHz spectrum. Conclusion: Ultrarobust impedance characteristics were achieved. Without any modifications, the proposed biotelemetry platform can be used, for instance, as an ingestible for humans or as an implantable for a wide range of animals: from rodents to cattle.