Methods and devices including implantable micro-sensors used to detect tissue-dissolved inert gas and to detect microbubble formation to avoid Caisson disease are described. The disclosed methods and devices are based on measuring the refractive index changes in hydrophobic liquids after absorbing an inert gas such as nitrogen. The changes in the refractive index are based on implementing one of an interferometry, optical microcavity resonance shift, a photonic crystal resonance, a beam deflection, a resonance tuning or detuning, an amplitude change, or an intensity change method.