SeeThrough: a rationally designed skull clearing technique for in vivo brain imaging.

Light scattering in the skull limits optical access to the brain. Here we present SeeThrough, a skull-clearing technique that enables simple, high-resolution, and minimally-invasive brain imaging without skull removal. Through systematic screening of over 1600 chemicals, we rationally developed a refractive index-matching solution that combines water- and organic solvent-based components, achieving both high clearing efficiency and biocompatibility. The reagents exhibit minimal brain penetration, maintain tissue integrity, and avoid inflammatory responses. Notably, SeeThrough provides imaging sensitivity and contrast comparable to open-skull window imaging, while permitting minimally-invasive monitoring of brain border macrophages as well as blood and cerebrospinal fluid dynamics. Combined with two-photon imaging, SeeThrough enables spatially and temporally scalable imaging applications in the mouse brain, including ~400 µm deep imaging, one-month longitudinal imaging, and mesoscale, cellular-resolution monitoring of brain activity for network-level analysis. Thus, SeeThrough offers a broadly accessible platform for high-throughput, physiology-preserving imaging of the brain parenchyma and brain-skull interface.