Researchers have shown that they can co-opt specific cells in biological systems to orchestrate the construction of functional materials. In turn, those materials can alter cell behavior, in this case influencing the firing pattern of neurons. Zhenan Bao and Karl Deisseroth of Stanford University and coworkers harnessed neurons to make a conductive polyaniline mesh coating (Science 2020, DOI: 10.1126/science.aay4866). To make the polymer coating, the researchers used viral vectors to deliver the gene for an ascorbate peroxidase called Apex2 to specific neurons. Hydrogen peroxide triggers the Apex2 to polymerize polyaniline from a 1:1 mixture of aniline monomers and dimers. The key was getting the right ratio of ingredients to ensure enough hydrogen peroxide for the reaction to proceed without damaging the cells. With the 1:1 mixture, a polyaniline mesh forms on the outer surface of the cell membrane of neurons expressing Apex2. The firing pattern changed for neurons with the polyaniline coating. The method could be used to engineer neural circuits in cultured neurons, brain slices, and intact Caenorhabditis elegans worms. Further, the researchers demonstrated that the approach works with other cell types by targeting cultured human embryonic kidney cells. The researchers want to extend the method to other types of cells and materials.