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Colloid Surface Chemistry Critically Affects Multiple Particle Tracking Measurements of Biomaterials

M. T. Valentine ^*, Z. E. Perlman  , M. L. Gardel ^*, J. H. Shin , P. Matsudaira ^¶, T. J. Mitchison  and D. A. Weitz ^*

^* Department of Physics and Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138; Department of Cell Biology and Institute of Chemistry and Cell Biology, Harvard Medical School, Boston, Massachusetts 02115; Program in Biophysics, Harvard University, Cambridge, Massachusetts 02138; Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142; and ^¶ Department of Biology, Massachusetts Institute of Technology and the Whitehead Institute for Biomedical Research, Cambridge, Massachusetts 02142

Correspondence: Address reprint requests to Prof. David A Weitz, Physics Department, Harvard University, 29 Oxford St., Cambridge, MA 02138. Tel.: 617-496-2842; E-mail: weitz{at}deas.harvard.edu.

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.

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