Biomaterials Science PAPER Cite this: Biomater. Sci., 2019, 7, 1411 Received 30th November 2018, Accepted 9th January 2019 DOI: 10.1039/c8bm01545g rsc.li/biomaterials-science Disparate effects of PEG or albumin based surface modification on the uptake of nano- and micro-particles† Preeti Sharma, Devashish Sen, Varsha Neelakantan, Vinidhra Shankar and Siddharth Jhunjhunwala * Surface modification of particulate systems is a commonly employed strategy to alter their interaction with proteins and cells. Past studies on nano-particles have shown that surface functionalization with polyethylene glycol (PEG) or proteins such as albumin increases circulation times by reducing their pha- gocytic uptake. However, studies on surface functionalized micro-particles have reported contradictory results. Here, we investigate the effects of surface functionalization using polystyrene particles with 4 different diameters ranging from 30 nm to 2.6 μm and coating them with either albumin or PEG. Our results show that with increasing particle size, surface functionalization has less to no effect on altering phagocytic uptake. The data also suggest that these differences are observed with a dense arrangement of molecules on the surface (dense brush conformation for PEG conjugation), appear to be independent of the serum proteins adsorbing on particle surfaces, and are independent of the endocytic uptake pathway. These results provide insight into the differences in the ability of surface modified nano- and micro-particles to avoid phagocytic uptake. 1. Introduction Nano- and micro-particle systems are being developed for a wide range of biological applications that include, but are not limited to, in vivo imaging, 1,2 drug delivery 3,4 and vaccination strategies. 5–8 Such diverse uses are achieved by modulating both physical (shape and size) and chemical (bulk material and surface properties) characteristics of the particulate system. 9,10 Importantly, while the bulk composition of par- ticles may determine their function, the surface characteristics are a major determinant of biological compatibility. 11,12 Surface properties of particulates dictate their interaction with plasma proteins, in vivo circulation times, drug release kinetics, and phagocytosis by immune cells. 13–17 Of the many surface characteristics, the most commonly studied are charge and hydrophobicity–hydrophilicity. For example, it has been shown that particles with highly hydrophobic 18 or positively charged surfaces 19,20 are taken up in larger quantities by pha- gocytic cells, while coating nano-particle surfaces with hydro- philic molecules such as PEG is known to reduce their phago- cytic uptake. 21–24 Similarly, studies have shown the dysopsoni- zation effects of albumin coatings. 25,26 However, a majority focus on nano-particles, and the few that study surface functionalization of micro-particles show contradictory results. Some of these reports show lowered phagocytosis following PEGylation of micro-particles. 22,24,27 In contrast, others observe no effect of micro-particle PEGylation on the uptake levels. 28,29 The aforementioned studies relied on qualitative microscopy and error-prone flow cytometry-based side-scatter measurements for their analysis of phagocytosis. To under- stand these conflicting results and re-assess the effect of dec- orating micro-particle surfaces with albumin or PEG, a sys- tematic and quantitative study needs to be performed. Utilizing fluorescently tagged particles and albumin/PEG molecules, here, we studied the effect of surface modification on phagocytosis. While surface modification affects the uptake of nano-particles as expected, 15,30 it does not affect the uptake of micro-particles. The altered uptake is not a result of changes in the surface charge or density of molecules on the surface. We also assessed the effect of serum proteins and inhibitors of various endocytic pathways on the uptake of surface modified nano- and micro-particles, and observed no major differences. Further similar effects were observed in ex vivo and in vivo uptake studies with surface-modified micro- particles being taken up at levels similar to non-modified particles. † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c8bm01545g Centre for BioSystems Science and Engineering, 3rd Floor C Wing Biological Sciences Building, Indian Institute of Science, Bengaluru-560012, India. E-mail: siddharth@iisc.ac.in This journal is © The Royal Society of Chemistry 2019 Biomater. Sci. , 2019, 7, 1411–1421 | 1411 Published on 09 January 2019. Downloaded by Indian Institute of Science on 4/25/2019 6:10:01 AM. View Article Online View Journal | View Issue