Debra T. Auguste1, John E. Kirkwood2, Joachim Kohn3, Gerald Fuller2, and Robert K. Prud'homme4. (1) School of Engineering and Applied Sciences, Harvard University, 29 Oxford St., Cambridge, MA 02138, (2) Chemical Engineering, Stanford University, 381 North South Mall, Dept. Chemical Engineering- Stauffer 3- Room 113, Stanford, CA 94305, (3) Chemistry and Chemical Biology, Rutgers University and The New Jersey Center for Biomaterials, 145 Bevier Road, Piscataway, NJ 08854, (4) Chemical Engineering, Princeton University, A301 E-Quad, Princeton, NJ 08544
Surface rheology of irreversibly bound hydrophobically-modified PEG polymers (HMPEG) on a dipalmitoylphosphatidylcholine (DPPC) monolayer is investigated to determine attributes that may contribute to immune recognition. Previously, three comb-graft polymers (HMPEG136-DP3, HMPEG273-DP2.5 and HMPEG273-DP5) adsorbed on liposomes were examined for their strength of adsorption and protection from complement binding. The data supported an optimal ratio between the hydrophilicity of the PEG polymer and the number of hydrophobic anchors. The HMPEG polymers have different polymer brush thicknesses (4.2 to 5.9 nm) and levels of cooperativity (2.5 to 5 hydrophobes). The results indicate that an increased viscous force (above 0.25 mN*s/m) at the surface may enable the polymers to shield liposomes from protein interactions. Similar rheological behavior is shown for all polymer architectures at low polymer surface coverage (0.5 mg/m2, in the mushroom regime) whereas at high surface coverage (> 0.5 mg/m2, in the brush regime) we observe a structural dependence of the surface viscous forces at 40 mN/m. This threshold correlates with a 92% decrease in complement protein binding for liposomes coated with 1 mg/m2 HMPEG273-DP5. This may suggest that surface viscous forces play a role in reducing complement protein binding.