Nanotribology of biopolymer brushes in aqueous solution using dissipative particle dynamics simulations: an application to PEG covered liposomes in a theta solvent

Abstract

We undertake the investigation of sheared polymer chains grafted on flat surfaces to model liposomes covered with polyethylene glycol brushes as a case study for the mechanisms of efficient drug delivery in biologically relevant situations, for example, as carriers for topical treatments of illnesses in the human vasculature. For these applications, specific rheological properties are required, such as low viscosity at high shear rate to improve the transport of the liposomes. Therefore, extensive non-equilibrium, coarse -grained dissipative particle dynamics simulations of polymer brushes of various lengths and shear rates are performed to obtain the average viscosity and the friction coefficient of the system as functions of the shear rate and polymerization degree under theta -solvent conditions, and find that the brushes experience considerable shear thinning at large shear rates. The viscosity () and † Corresponding author. Electronic mail: agama@alumni.stanford.edu 2 the friction coefficient () are shown to obey the scaling laws ~̇− 0.31 , and ~̇0 .69 at high shear rate () in theta solvent, irrespective of the brushes degree of polymerization.These results confirm recent scaling predictions and reproduce very well trends in measurements of the viscosity at high (̇) of red blood cells in a liposome containing medium.

Friction coefficient and viscosity of polymer brushes with and without free polymers as slip agents

“…has been shown to lead to the correct prediction of scaling exponents, among other phenomena [41,42]. Another advantage of DPD is that its thermostat, defined by the dissipative and random forces, remains stable even under non -equilibrium conditions [43].…”

Brushes of semiflexible polymers in equilibrium and under flow in a super-hydrophobic regime

“…This means that the relation between the total force exerted on the walls and the shear rate is independent of the polymer brush, and only depends on the liquid. It was possible to extract the viscosity of the simple liquid from the relation: F x = η·A·γ, obtaining a value of η = 1.28ετ σ −3 [18,21]. This essentially shows that the liquid behaves as a simple (newtonian) liquid with a well defined viscosity for the whole range of shear rates, and that the different morphologies of brush-liquid interface or gas content inside the brush layer does not have a significative role in the diffusion of momenta in the system.…”

“…The first bonds of the grafted polymers, in this reference, are directed perpendicularly to the grafting plane, as it is the case in our work. Using soft potentials within the Dissipative Particle Dynamics (DPD) simulation scheme various groups studied different aspects of a polymer brush exposed to flow [10,21,25]. Deng et al [10] included bending rigidity in their model to account for semiflexible polymers in the context of glycocalyx and studied brush height and slip length for Couette (shear-driven) and Poiseuille (pressure-driven) flows.…”

“…However, the vast majority of work was devoted to fully flexible polymer chains. Computer simulation of coarse-grained systems studied friction of bearing brushes [14][15][16][17] and the flow of simple liquids or polymer melts confined in brush-coated polymer channels as function of grafting density, interface properties and flow intensity [18][19][20][21][22]. Milchev and Binder studied the structure of brushes formed by semiflexible chains by Monte Carlo and molecular dynamics simulations [23].…”

Selected Topics of Computational and Experimental Fluid Mechanics

“…Finally, another clear example are the scaling laws observed between the viscosity (η) and the friction coefficient (µ). This behaviour was reproduced by non-equilibrium DPD simulations for sheared polymer chains grafted onto flat surfaces [42]. The scaling laws η ∼ γ −0.31 and µ ∼ γ −0.69 at high shear rates γ were obtained [42].…”