Over the past seven years, anin situchemical synthesis approach to biological systems has emerged, in which functional materials are assembled within tissues such as the brain – either constructed throughout the intact tissue (hydrogel-tissue chemistry/HTC), or genetically targeted to cell types (genetically-targeted chemical assembly/GTCA). Resulting hybrid materials are endowed with diverse capabilities, including anchoring and labeling of RNA and protein,in situsequencing, transparency, reversible size changes, and electrical insulation or conduction.