Various light sources have been developed for the application of optical stimulation in the optogenetics field. Light transmission inside living tissue is limited to a distance of a few millimeters; hence, it is necessary to insert the light source near the nerve tissue to be stimulated. If a device is rigid, it causes mechanical stimulation to act on the nerve tissue. The application of mechanical stimulation may induce inflammation, obstructing neural activity. Fabricating such a device out of a soft material can prevent mechanical stimulation of cells and mitigate biological reactions such as inflammation or encapsulation. Minimizing the sizes of LED and other light sources as much as possible and mounting them on a flexible substrate can provide the entire device with flexibility. Micro-LEDs can be reduced to a size almost comparable to that of a cell and it has even been reported that some have been mounted on the tip of needle-shaped devices inserted into living tissue. A device using organic semiconductors is sufficiently soft to be bent, which is a characteristic not observed in inorganic semiconductors. Using organic LEDs can realize wide-area flexible light-emitting surfaces and they are widely anticipated to be the next generation of light sources. This chapter introduces technologies used to manufacture these soft light sources and examples of optical stimulation devices that incorporate them.
Keywords: Flexible electronics; Light source; Optogenetics; Organic light-emitting diode; Thin-film device.