The ability to deliver light into the brain for the purpose of controlling neural activity or other biological processes, optogenetics, has opened up new frontiers in both basic neuroscience and neuroengineering. One arena of great activity is in the use of microbial opsins such as channelrhodopsins, halorhodopsins, and archaerhodopsins to make neurons activatable or silenceable by light, thus enabling assessment of the causal contribution of specific neurons, brain regions, and neural pathways to normal and abnormal behaviors and neural computations. To date, numerous in vivo studies have used optical fibers, and fiber-electrode hybrids, to deliver visible light into brain targets in which neurons express opsins, but an optical fiber can target just a single region. An implantable probe capable of delivering light to arbitrary points in a 3-dimensional volume would enable more versatile optical control, opening up the ability to deliver patterned light to manipulate neural activity in distributed brain circuits.
The Boyden lab recently developed a linear probe comprising a set of integrated microwaveguides running in parallel to each other, microfabricated on a single substrate and capable of delivering light independently to multiple brain targets along the probe axis. In a paper published in Optics Letters, they now extend this design to the case of 3-dimensional light delivery to a set of targets distributed throughout the brain, by first fabricating waveguide combs containing many linear probes parallel to one another, then aligning multiple combs in a custom engineered baseplate for coupling to a digital micromirror device (DMD) for arbitrary light patterning. Each waveguide is about 10 microns x 10 microns wide, very small, and thus hundreds can be packed in the space occupied by a typical optical fiber. In addition, a 1024 x 768 pixel DMD, with a laser coupled, could easily enable the control of up to ~1000 points in 3-D space (even pooling many DMD mirrors, e.g. 800, per waveguide to increase power). The power out the end of the waveguide can be up to ~150 mW/mm^2, comparable to the highest powers used in optogenetics.