  Optical wireless communications (OWCs) have been recognized as a candidate
enabler of next generation in-body nano-scale networks and implants. The
development of an accurate channel model capable of accommodating the
particularities of different type of tissues is expected to boost the design of
optimized communication protocols for such applications. Motivated by this,
this paper focuses on presenting a general pathloss model for in-body OWCs. In
particular, we use experimental measurements in order to extract analytical
expressions for the absorption coefficients of the five main tissues'
constitutions, namely oxygenated and de-oxygenated blood, water, fat, and
melanin. Building upon these expressions, we derive a general formula for the
absorption coefficient evaluation of any biological tissue. To verify the
validity of this formula, we compute the absorption coefficient of complex
tissues and compare them against respective experimental results reported by
independent research works. Interestingly, we observe that the analytical
formula has high accuracy and is capable of modeling the pathloss and,
therefore, the penetration depth in complex tissues.
