Diffusing colloidal probes of cell surfaces

Synergistic Polymer-Surfactant-Complex Mediated Colloidal Interactions and Deposition

Total internal reflection microscopy (TIRM) is used to directly, sensitively, and simultaneously measure colloidal interactions, dynamics, and deposition for a broad range of polymer-surfactant compositions. A deposition state diagram containing comprehensive information about particle interactions, trajectories, and deposition behavior is obtained for polymer-surfactant compositions covering four decades in both polymer and surfactant concentrations. Bulk polymer-surfactant phase behavior and surface properties are characterized to provide additional information to interpret mechanisms. Materials investigated include cationic acrylamide-acrylamidopropyltrimonium copolymer (AAC), sodium lauryl ether sulfate (SLES) surfactant, silica colloids, and glass microscope slides. Measured colloid-substrate interaction potentials and deposition behavior show nonmonotonic trends vs polymer-surfactant composition and appear to be synergistic in the sense that they are not easily explained as the superposition of single-component-mediated interactions. Broad findings show that at some compositions polymer-surfactant complexes mediate bridging and depletion attractions that promote colloidal deposition, whereas other compositions produce electrosteric repulsion that deters colloidal deposition. These findings illustrate mechanisms underlying colloid-surface interactions in polymer-surfactant mixtures, which are important to controlling selective colloidal deposition in multicomponent formulation applications.

Direct Measurements of kT-Scale Capsule-Substrate Interactions and Deposition Versus Surfactants and Polymer Additives

We report a novel approach to directly measure the interactions and deposition behavior of functional capsule delivery systems on glass substrates versus the concentration of an anionic surfactant sodium lauryl ether sulfate (SLES) and a cationic acrylamide-acrylamidopropyltrimonium copolymer (AAC). Analyses of three-dimensional optical microscopy trajectories were used to quantify lateral diffusive dynamics, deposition lifetimes, and potentials of mean force for different solution conditions. In the absence of additives, negatively charged capsule surfaces yield electrostatic repulsion with the negatively charged substrate, which inhibits deposition. With an increasing SLES concentration below the critical micelle concentration (CMC), capsule-substrate electrostatic repulsion is mediated by the charged surfactant solution that decreases the Debye length. Above the SLES CMC, depletion attraction causes enhanced deposition until eventually depletion repulsion inhibits deposition at concentrations ∼10 wt %. Addition of an ACC causes deposition via capsule-substrate bridging at all concentrations; the weakest deposition occurs at intermediate AAC concentrations from a competition of steric repulsion and attraction via a few extended bridges. The novel measurements and models of capsule interactions and deposition on substrates in this work provide a basis to fundamentally understand and rationally design complex rinse-off cleansing formulations with optimal characteristics.

Interfacial and Confined Colloidal Rod Diffusion

Optical microscopy is used to measure translational and rotational diffusion of colloidal rods near a single wall, confined between parallel walls, and within quasi-2D porous media as a function of rod aspect ratio and aqueous solution ionic strength. Translational and rotational diffusivities are obtained as rod particles experience positions closer to boundaries and for larger aspect ratios. Models based on position dependent hydrodynamic interactions quantitatively capture diffusivities in all geometries and indicate particle-wall separations in agreement with independent estimates based on electrostatic interactions. Short-time translational diffusion in quasi-2D porous media is insensitive to porous media area fraction, which appears to arise from a balance of hydrodynamic hindrance and enhanced translation due to parallel alignment along surfaces. Findings in this work provide a basis to interpret and predict interfacial and confined colloidal rod transport relevant to biological, environmental, and synthetic material systems.

Diffusing Colloidal Probes of kT-Scale Biomaterial-Cell Interactions

In the optimization of applied biomaterials, measurements of their interactions with cell surfaces are important to understand their influence on specific and nonspecific cell surface adhesion, internalization pathways, and toxicity. In this study, a novel approach using dark field video microscopy with combined real-time particle and cell tracking allows the trajectories of biomaterial-coated colloids to be monitored in relation to their distance from cell perimeters. Dynamic and statistical mechanical analyses enable direct measurement of colloid-cell surface association lifetimes and interaction potentials mediated by biomaterials. Our analyses of colloidal transport showed polyethylene glycol (PEG) and bovine serum albumin (BSA) lead to net repulsive interactions with cell surfaces, while dextran and hyaluronic acid (HA) lead to reversible and irreversible association to the cell surface, respectively. Our results demonstrate how diffusing colloidal probes can be used for nonobtrusive, sensitive measurements of biomaterial-cell surface interactions important to therapeutics, diagnostics, and tissue engineering.