The in vitro self‐assembly of multicellular spheroids generates highly organized structures in which the three‐dimensional structure and differentiated function frequently mimic that of in vivo tissues. This has led to their use in such diverse applications as tissue regeneration and drug therapy. Using Smoluchowski‐like rate equations, herein we present a model of the self‐aggregation of DU 145 human prostate carcinoma cells in liquid‐overlay culture to elucidate some of the physical parameters affecting homotypic aggregation in attachment‐dependent cells. Experimental results indicate that self‐aggregation in our system is divided into three distinct phases: a transient reorganization of initial cell clusters, an active aggregation characterized by constant rate coefficients, and a ripening phase of established spheroid growth. In contrast to the diffusion‐controlled aggregation previously observed for attachment‐independent cells, the model suggests that active aggregation in our system is reaction‐controlled. The rate equations accurately predict the aggregation kinetics of spheroids containing up to 30 cells and are dominated by spheroid adhesive potential with lesser con‐ tributions from the radius of influence. The adhesion probability increases with spheroid size so that spheroid–spheroid adhesions are a minimum of 2.5 times more likely than those of cell–cell, possibly due to the upregulation of extracellular matrix proteins and cell‐adhesion molecules. The radius of influence is at least 1.5 to 3 times greater than expected for spherical geometry as a result of ellipsoidal shape and possible chemotactic or Fröhlich interactions. Brownian‐type behavior was noted for spheroids larger than 30 μm in diameter, but smaller aggregates were more motile by as much as a factor of 10 for single cells. The model may improve spheroid fidelity for existing applications of spheroids and form the basis of a simple assay for quantitatively evaluating cellular metastatic potential as well as therapies that seek to alter this potential. © 2001 John Wiley & Sons, Inc. Biotechnol Bioeng 72: 579–591, 2001.

We show that protein unfolding on biomaterials may be dramatically reduced via tuning the chemical heterogeneity of the protein−material interface. Specifically, using dynamic single-molecule methods, we confirmed that the transient structure and dynamics of fibronectin (FN) may be mediated through varying the composition of random copolymer brushes. The brushes, which themselves represent an intriguing biomaterial, were composed of oligoethylene glycol and sulfobetaine methacrylate and presumably stabilized FN through partitioning and/or segregation of the copolymers. We further showed that, by controlling the transient structure and dynamics of FN, the secretion of TNF-α and IL-6 by RAW 264.7 was markedly diminished.

The surface diffusion of poly-L-lysine (PLL) in a planar nanoslit was studied using convex lens-induced confinement (CLiC) single-molecule tracking microscopy. Three surface chemistries were employed to understand the interplay of electrostatic and short-range interactions: an amine-functionalized silica surface, an oligo(ethylene oxide) (OEG)-modified surface, and a 1:1 mixture of the two ligands. Effective surface diffusion coefficients increased rapidly with slit height until saturating for slit heights <30 nm. While diffusion at a semi-infinite interface was significantly faster for OEG surfaces, the diffusion coefficient increased most rapidly with slit height for aminefunctionalized surfaces, resulting in surface diffusion within very thin slits being nearly independent of surface chemistry. Intermittent random walks were simulated within a planar slit geometry, using experimentally measured parameters obtained from diffusion at a single interface to account for the characteristic short-range interactions between PLL and each surface chemistry, and were in good agreement with experimental measurements.