The sustainability of the healthcare and elderly care systems has become a critical issue because of aging population and demographic decline. In this context, cybernetic avatars (CAs) have been proposed as a means to extend human capabilities beyond physical constraints. While conventional studies have mainly focused on socio-CAs based on remote robots, increasing attention has been directed toward in-body CAs, which perform sensing, actuation, and information processing within the human body. In-body CAs exist in diverse forms ranging from the nano- to millimeter-scale, each with distinct communication requirements and design constraints. Correspondingly, various communication modalities have been investigated, including molecular communication, terahertz communication, optical communication, radio frequency communication, and coupling-based methods. However, these technologies have largely evolved in isolation with no unified design framework available for in-body CA systems. This study considered in-body communication as a design problem and investigated a taxonomy based on three axes: the scale, communication medium, and function. Using this framework, the properties and applicability of different communication modalities were systematically characterized, and design guidelines for selecting and combining them according to application requirements were derived. Furthermore, structural gaps across scales were identified, highlighting the necessity of hierarchical architectures and gateway mechanisms for integrating heterogeneous communication domains.

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