Core/shell microgels with �switchable� deformability

Abstract

Event Abstract Back to Event Core/shell microgels with “switchable” deformability Kimberly Clarke1, Erin Sproul2 and Ashley Brown2 1 Georgia Institute of Technology, School of Chemistry and Biochemistry, United States 2 North Carolina State University and University of North Carolina at Chapel Hill, Joint Department of Biomedical Engineering, United States Introduction: Microgels are colloidal hydrogels that have been utilized for a range of biomaterial applications due to their tunable mechanical and chemical properties. Thermoresponsive poly(N-isopropylacrylamide) (pNIPAM) microgels have been well characterized and are fabricated by precipitation polymerization, typically in the presence of a crosslinking agent, such as N,N’-methylenebisacrylamide (BIS). The mechanical properties of the resulting particles can be tuned by varying cross-linking density. Interestingly, pNIPAM microgels can be formed under "crosslinker free" conditions, which results in the formation of “ultra-low” crosslinked (ULC) particles. These particles are softer than traditionally crosslinked microgels, with a Young’s Modulus of ~10 kPA, and are highly deformable as evidenced by their ability to spread extensively on surfaces and translocate through nanopores significantly smaller than the hydrodynamic diameter of the particles [1]. Due to these unique properties, ULC particles are useful for a variety of biomedical applications, including a recent demonstration that this deformability enables clot contraction by ULC-based platelet-like particles [2]. Here, we aim to create microgels with “switchable” deformability. To that end, we synthesized core/shell microgels, comprised of a N,N’-(1,2-dihydroxyethylene)bisacrylamide (DHEA) crosslinked core, which can be degraded to produce a hollow microgel. We hypothesized that, like ULCs, hollow microgels would display high degrees of deformability. Materials and Methods: Core/shell microgels were constructed by first producing a core containing 90% N-Isopropylmethacrylamide (NIPMAM) and 10% DHEA through precipitation polymerization. Following purification of the core particles, shells containing 88% NIPAM, 2% BIS and 10% acrylic acid or 91% NIPAM, 4% BIS and 5% acrylic acid were added in a second precipitation polymerization reaction. To create hollow particles, cores were degraded by the addition of sodium periodate (NaIO4), which cleaves the DHEA crosslinks (diols). Particle hydrodynamic radii were then determined through dynamic light scattering (DLS) and particle deformability was determined with atomic force microscopy (AFM) to analyze spreading on a glass surface. Results and Discussion: DLS revealed an increase in hydrodynamic diameter in hollow microgels vs core/shell microgels. After core degradation, the diameter of core/shell microgels increased 23% (675±5 vs 829±9 nm) in 2% BIS crosslinked hollow microgels. Similarly, 4% BIS crosslinked hollow microgels exhibited a 34% increase in diameter (576±7 vs 774±19 nm). AFM demonstrated increased particle spreading in 2% BIS hollow microgels (Figure 1); a 119% increase in diameter (490±40 vs 1090±230 nm) with an 87% decrease in height (73±12 vs 10±1 nm) was observed. Similarly, AFM showed a 268% increase in diameter (404±76 vs 1490±423 nm) and a 93% decrease in height (86±18 vs 6±1 nm) in 4% BIS hollow microgels. These results demonstrate that hollow microgels display a high degree of deformability and spread much more extensively on surfaces than core/shell microgels. Conclusions: Like ULC microgels, hollow microgels display a high degree of deformability. Furthermore, unlike ULC microgels, because hollow microgels are created by degrading the core of core/shell microgels, this particle architecture allows for switchable deformability.