Pulsatile illumination for photobiology and optogenetics.

Living organisms exhibit a wide range of intrinsic adaptive responses to incident light. Likewise, in optogenetics, biological systems are tailored to initiate predetermined cellular processes upon light exposure. As genetically encoded, light-gated actuators, sensory photoreceptors are at the heart of these responses in both the natural and engineered scenarios. Upon light absorption, photoreceptors enter a series of generally rapid photochemical reactions leading to population of the light-adapted signaling state of the receptor. Notably, this state persists for a while before thermally reverting to the original dark-adapted resting state. As a corollary, the inactivation of photosensitive biological circuits upon light withdrawal can exhibit substantial inertia. Intermittent illumination of suitable pulse frequency can hence maintain the photoreceptor in its light-adapted state while greatly reducing overall light dose, thereby mitigating adverse side effects. Moreover, several photoreceptor systems may be actuated sequentially with a single light color if they sufficiently differ in their inactivation kinetics. Here, we detail the construction of programmable illumination devices for the rapid and parallelized testing of biological responses to diverse lighting regimes. As the technology is based on open electronics and readily available, inexpensive components, it can be adopted by most laboratories at moderate expenditure. As we exemplify for two use cases, the programmable devices enable the facile interrogation of diverse illumination paradigms and their application in optogenetics and photobiology.