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Macrophages are specialized phagocytes that play central roles in immunity and tissue repair. Their diverse functionalities have led to an evolution of new allogenic and autologous macrophage products. However, realizing the full therapeutic potential of these cell‐based therapies requires development of imaging technologies that can track immune cell migration within tissues in real‐time. Such innovations will not only inform treatment regimens and empower interpretation of therapeutic outcomes but also enable prediction and early intervention during adverse events. Here, phase‐changing nanoemulsion contrast agents are reported that permit real‐time, continuous, and high‐fidelity ultrasound imaging of macrophages in situ. Using a de novo designed peptide emulsifier, liquid perfluorocarbon nanoemulsions are prepared and show that rational control over interfacial peptide assembly affords formulations with tunable acoustic sensitivity, macrophage internalization, and in cellulo stability. Imaging experiments demonstrate that emulsion‐loaded macrophages can be readily visualized using standard diagnostic B‐mode and Doppler ultrasound modalities. This allows on‐demand and long‐term tracking of macrophages within porcine coronary arteries, as an exemplary model. The results demonstrate that this platform is poised to open new opportunities for non‐invasive, contrast‐enhanced imaging of cell‐based immunotherapies in tissues, while leveraging the low‐cost, portable, and safe nature of diagnostic ultrasound. 

Abstract Fluorinated compounds, while rarely used by nature, are emerging as fundamental ingredients in biomedical research, with applications in drug discovery, metabolomics, biospectroscopy, and, as the focus of this review, peptide/protein engineering. Leveraging the fluorous effect to direct peptide assembly has evolved an entirely new class of organofluorine building blocks from which unique and bioactive materials can be constructed. Here, we discuss three distinct peptide fluorination strategies used to design and induce peptide assembly into nano‐, micro‐, and macro‐supramolecular states that potentiate high‐ordered organization into material scaffolds. These fluorine‐tailored peptide assemblies employ the unique fluorous environment to boost biofunctionality for a broad range of applications, from drug delivery to antibacterial coatings. This review provides foundational tactics for peptide fluorination and discusses the utility of these fluorous‐directed hierarchical structures as material platforms in diverse biomedical applications.