  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.
