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.

An historical perspective on cell mechanics

“…This represents some of the earliest uses of particle tracking in cell mechanics and is essentially a predecessor of modern-particle tracking and microrheology measurements [100,101]. Although this early work was carried out in the 1920s and suffers from an obvious lack of appropriate experimental and theoretical considerations, some of the same issues were being discussed as they are today, such as the influence of the size of the granule, the mesh size of the protoplasm, damage to the cell and the influence of temperature [3,4,102]. Similarly, an early magnetic microscope developed in 1923 [103] was used to oscillate nickel particles (~16 μm in diameter) inserted into living cells.…”

Spatially resolved microrheology of heterogeneous biopolymer hydrogels using covalently bound microspheres

“…Probes used in PTM that have greater interaction with the heterogeneous network samples are sensitive to network stiffness, while probes with minimal interaction measure viscosity of the fluid within the pores of the network (McGrath et al 2000;Valentine et al 2004;Chae and Furst 2005). To examine the effect of surface modification on the mobility of microspheres, NHS-and COOH-microspheres were embedded in polyacrylamide gels of two different total monomer concentrations with a fixed bis-acrylamide crosslinker concentration of 3 (w/w) %C.…”

“…The ideal probe for network rheology has been commonly defined as probe microspheres that are significantly larger than the mesh size of the network (Levine and Lubensky 2000;Valentine et al 2004;Squires and Mason 2010;Barhate et al 2011;He and Tang 2011). To understand how network mechanics probed by covalently bound microspheres of comparable size to the network compares with the mechanics obtained with large ideal microspheres as probes, we performed PTM using 0.5 µm red fluorescent NHS-microspheres and 0.2 µm green fluorescent NHSmicrospheres in polyacrylamide gel with 10.5 %(w/v)T. The MSD data for smaller microspheres were significantly larger than those of larger microspheres (Fig.…”

“…As the probes used are significantly smaller than the size of cells, the local mechanical properties can be measured with subcellular resolution. However, many factors, such as the probe surface chemistry and probe size (McGrath et al 2000;Valentine et al 2004;Chae and Furst 2005;Fu et al 2008;He and Tang 2011), have been found to complicate the interpretation of microrheological data. Current studies have advocated the use of probes that are larger than the network mesh size as a "gold standard" to measure the network mechanics due to physisorption of nearby network on the probe (He and Tang 2011).…”

“…However, the microenvironment around the cells is rich in microstructures with heterogeneous mesh sizes, and the need to predetermine appropriate probe size for PTM is impractical. While it is acknowledged that small probes have to physically interact with the network to effectively measure the network mechanics (He and Tang 2011), only probes with non-specific interactions with the network have been studied to date (McGrath et al 2000;Valentine et al 2004;Squires and Mason 2010;He and Tang 2011). To probe the network mechanics of materials where the length scales of heterogeneity are typically comparable to the probe size, such as biopolymer networks, an absence of probe/material interaction can lead to problems such as slippage (Fu et al 2008), cage-hopping (He and Tang 2011), and steric hindrance (Valentine et al 2004;Van Citters et al 2006), which would undermine the reliability and accuracy of such microrheological analysis.…”