This article discusses the fundamental architectures for optical wireless systems for biomedical applications. After summarizing the main applications and reporting their requirements, {we describe the characteristics of the transdermal and in-body optical channels as well as the challenges that they impose in the design of communication systems.} In more detail, we provide three possible architectures for transdermal communications, namely electro-optical (EO) monitoring, opto-electrical (OE), and all-optical (AO) for neural stimulation, which are currently under investigation, whereas for in-body communications, we provide a nano-scale AO (NAO) concept. For each architecture, we discuss the main operation principles, the technology enablers, and research directions for their development. Finally, we highlight the necessity of designing an information-theoretic framework for the analysis and design of the physical (PHY) and medium access control (MAC) layers, which takes into account the channels~characteristics.
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