Researchers have discovered a new and unexpected mode of self-assembly involving a polymer (hyaluronic acid) and a small molecule (peptide amphiphiles). When brought together, the two instantly assemble into a flexible but strong sac in which the researchers can grow human stem cells, creating a sort of miniature laboratory. The sacs can survive for weeks in culture, and their membranes are permeable to proteins. The method also can produce thin films whose size and shape can be tailored.