**Application of PEG-Covered Non-Biodegradable Polyelectrolyte Microcapsules in the Crustacean Circulatory System on the Example of the Amphipod** *Eulimnogammarus verrucosus*

**Ekaterina Shchapova 1,2, Anna Nazarova 1, Anton Gurkov 1,2, Ekaterina Borvinskaya 3, Yaroslav Rzhechitskiy 1, Ivan Dmitriev 1, Igor Meglinski 4,5,6 and Maxim Timofeyev 1,2,\***


Received: 15 May 2019; Accepted: 24 July 2019; Published: 27 July 2019

**Abstract:** Layer-by-layer assembled microcapsules are promising carriers for the delivery of various pharmaceutical and sensing substances into specific organs of different animals, but their utility in vivo inside such an important group as crustaceans remains poorly explored. In the current study, we analyzed several significant aspects of the application of fluorescent microcapsules covered by polyethylene glycol (PEG) inside the crustacean circulatory system, using the example of the amphipod *Eulimnogammarus verrucosus*. In particular, we explored the distribution dynamics of visible microcapsules after injection into the main hemolymph vessel; analyzed the most significant features of *E. verrucosus* autofluorescence; monitored amphipod mortality and biochemical markers of stress response after microcapsule injection, as well as the healing of the injection wound; and finally, we studied the immune response to the microcapsules. The visibility of microcapsules decreased with time, however, the central hemolymph vessel was confirmed to be the most promising organ for detecting the spectral signal of implanted microencapsulated fluorescent probes. One million injected microcapsules (sufficient for detecting stable fluorescence during the first hours after injection) showed no toxicity for six weeks, but in vitro amphipod immune cells recognize the PEG-coated microcapsules as foreign bodies and try to isolate them by 12 h after contact.

**Keywords:** biocompatibility; immunity; implantable sensors; invertebrate; layer-by-layer; primary cell cultures
