Nature, Published online: 19 November 2025; doi:10.1038/s41586-025-09729-xThe fast skin-permeable polyzwitterion poly[2-(N-oxide-N,N-dimethylamino)ethyl methacrylate] may enable non-invasive transdermal delivery of insulin, and potentially facilitate the use of other protein- and peptide-based therapeutics through transdermal delivery.