Colloid surface chemistry critically affects multiple particle tracking measurements of biomaterials

Abstract

Characterization of the properties of complex biomaterials using microrheological techniques has the promise of providing fundamental insights into their biomechanical functions; however, precise interpretations of such measurements are hindered by inadequate characterization of the interactions between tracers and the networks they probe. We here show that colloid surface chemistry can profoundly affect multiple particle tracking measurements of networks of fibrin, entangled F-actin solutions, and networks of cross-linked F-actin. We present a simple protocol to render the surface of colloidal probe particles protein-resistant by grafting short amine-terminated methoxy-poly(ethylene glycol) to the surface of carboxylated microspheres. We demonstrate that these poly(ethylene glycol)-coated tracers adsorb significantly less protein than particles coated with bovine serum albumin or unmodified probe particles. We establish that varying particle surface chemistry selectively tunes the sensitivity of the particles to different physical properties of their microenvironments. Specifically, particles that are weakly bound to a heterogeneous network are sensitive to changes in network stiffness, whereas protein-resistant tracers measure changes in the viscosity of the fluid and in the network microstructure. We demonstrate experimentally that two-particle microrheology analysis significantly reduces differences arising from tracer surface chemistry, indicating that modifications of network properties near the particle do not introduce large-scale heterogeneities. Our results establish that controlling colloid-protein interactions is crucial to the successful application of multiple particle tracking techniques to reconstituted protein networks, cytoplasm, and cells.

FIGURE 1

Sketch illustrating several physical scenarios for the way colloidal particles can be embedded in a biopolymer network. (A) When chemically inert particles of radius a ≫ network mesh size ξ are used, the mean-squared displacement is directly related to the linear viscoelastic moduli. (B) When particles are resistant to protein adsorption and a ≤ ξ, tracers move within small microenvironments and their movements are sensitive to the viscosity of the solvent and hydrodynamic interactions with the network, but do not reflect the bulk viscoelasticity. (C) For a ≤ ξ, “sticky” tracers adsorb protein and recruit polymer strands to their surface, possibly modifying the local polymer concentration near the sphere. In this case, particle movements do reflect network fluctuations; however, the tracers may sample unusually and artifactually stiff regions of the network, leading to uncertainty in the interpretation of relationship of the particle dynamics to the network dynamics. (D) For a ≤ ξ, even a small amount of protein adsorption can cause particles to adhere to cavity walls, leading to unusual hydrodynamic interactions with the adsorbed network, and uncertainty in the interpretation of these data.

FIGURE 2

Bright-field (A) and fluorescence (B) images of 0.84-μm CML and BSA- and PEG-coated particles that have been incubated with R-BSA. The fluorescence intensity indicates the binding capacity of each particle. The CML and BSA-coated particles adsorb a significant amount of protein, whereas the adsorption on the PEG-coated particles is very small. There is a shift in the fields of view between the bright-field and fluorescence images due to different optics along the two paths; in some cases particles have diffused slightly between image acquisitions. (C) Normalized fluorescence intensity of the CML and BSA- and PEG-coated particles incubated with R-BSA. The intensity of the BSA-coated particles is 60% of that of the CML particles, indicating that the BSA coating does prevent some protein adsorption, but does not render the particles completely inert. The intensity of the PEG-coated particles is only 2% of that of the CML particles, indicating a significant improvement in protein resistivity.

FIGURE 3

Trajectories of 1-μm CML and BSA- and PEG-coated particles in a 0.43 mg/mL fibrin network with a/ξ ∼ 0.1–0.2. The BSA-coated and CML particles adhere to the stiff fibrin network and are completely immobile within the resolution of the measurement; circles indicate their static positions. By contrast, the PEG-coated particles are resistant to the nonspecific protein adsorption and remain mobile with trajectories that resemble random walks.

FIGURE 4

A sampling of the mean-squared displacements of individual PEG-coated particles moving in a fibrin network. Most particles diffuse, but some are locally constrained, leading to a plateau in their MSDs at long lag times.

FIGURE 5

Ensemble averaged mean-squared displacements of 1.0-μm CML (□), BSA-coated (○), and PEG-coated (▵) particles in an entangled F-actin solution. There is measurable decrease in the plateau value of the MSD of the CML particles as compared to those of the BSA- or PEG-coated particles; however, the overall effect is small, suggesting that the binding affinity of actin to bare CML particle surfaces is weak. We observe no difference between the plateau value of the MSDs for the BSA- and PEG-coated tracers.

FIGURE 6

The ensemble averaged MSDs for BSA-coated particles (solid symbols) and PEG-coated particles (open symbols) moving in actin networks cross-linked and bundled with the actin-binding protein scruin, at various ratios of scruin/actin: R = 1:30 (□), 1:15 (○), 1:10 (▿), 1:5 (▵), 1:2.5 (⋄), and 1 (◃). Representative error bars are shown for each surface coating. The BSA-coated particles are constrained for each R, reaching a plateau that decreases with increasing amounts of scruin. The ensemble averaged MSDs for PEG-coated particles show a completely different trend for R ranging from 1:30 to 1. At the lower concentrations of cross-linkers, the particles are constrained; however, the particle mobility increases with increasing amounts of scruin, until at R = 1, the particles are nearly diffusive at short lag times.

FIGURE 7

Two-particle mean-squared displacements for composite actin-scruin networks with (A) R = 1:30 and (B) R = 1:15; ∼70 particles are included in this calculation. The long wavelength elastic modes are unaffected by local coupling of the particles to the network, and differences between the BSA-coated particles (solid symbols) and PEG-coated particles (open symbols) measured with one-particle techniques are eliminated. In all cases, the MSDs show a plateau at long lag times, and the plateau values of are significantly smaller than those measured by one-particle techniques.