Cross-linked fluorescent supramolecular nanoparticles as finite tattoo pigments with controllable intradermal retention times

Objectives: Tattooing has been utilized by the medical community for precisely demarcating anatomic landmarks. This practice is especially important for identifying biopsy sites of nonmelanoma skin cancer (NMSC) due to the long interval (i.e., up to 3 months) between the initial diagnostic biopsy and surgical treatment. In this study, we aim to introduce cross-linked fluorescent supramolecular nanoparticles (c-FSNPs) as a “finite tattoo” pigment, with optimized photophysical properties and intradermal retention time.

Methods: Fluorescent supramolecular nanoparticles were encapsulated with a fluorescent conjugated polymer, poly[5-methoxy-2-(3-sulfopropoxy)-1,4-phenylenevinylene] (MPS-PPV), into a core via a supramolecular synthetic approach. FSNPs which possess fluorescent properties superior to those of the free MPS-PPV were obtained through a combinatorial screening process. A cross-linking reaction of FSNPs was further conducted to generate micrometer-sized c-FSNPs. The intradermal retention times of different sized c-FSNPs along with various FSNPs were then examined by tattooing on the back of nu/nu mice.

Results: A small combinatorial library composed of 27 different formulations of FSNPs was prepared by changing the ratios among three molecular building blocks. The fluorescence spectra confirmed that the emission of 670 nm sized FSNP was ca. 10-fold greater than that of free MPS-PPV. By controlling the quantity of the cross-linker, we successfully produced c-FSNPs with sizes up to 1456 nm. After tattooing, no visible signal from FSNPs and c-FSNPs was observed under ambient light irradiation. The tattooed FSNPs and c-FSNPs exhibit a size dependent intradermal retention time in mice. As a result, 1456 nm sized c-FSNPs show the slowest fluorescent signal decay and maintain their fluorescent signal up to 84 days (12 weeks, ca. 3 months). At day 94, all fluorescent signals from the tattooed FSNPs and c-FSNPs diminished to undetectable levels. In contrast, the commercially available tattoo pigment (i.e., ZnO, ca. 5 μm) exhibited a consistent fluorescent intensity over 94 days. In addition, a pathological study of animal skin confirmed that no obvious inflammatory cells were observed after c-FSNP tattooing as compared to normal skin.

Conclusion: We have developed a finite tattoo pigment, c-FSNPs, with strong fluorescent properties, a controllable lifetime, and biocompatibility to achieve successful in vivo finite tattooing. The translation of our c-FSNPs into NMSC patients is underway with the hope of providing the correct information on the biopsied malignant sites. Research Support: