Engineering colloidal assembly via biological adhesion

Due to highly specialized recognition properties, biological receptor-ligand interactions offer valuable tools for engineering the assembly of novel colloidal materials. A unique sub-class of these macromolecules, called selectins, was exploited to develop binary suspensions where particles are programmed to associate reversibly or irreversibly via specific biomolecular cross-linking. Flow cytometry and videomicroscopy were used to examine factors controlling suspension assembly and structure, including biomolecular affinity and density, and individual and total particle volume fractions. By functionalizing small (R_A = 0.47 μm) and larger (R_B = 2.75 μm) particles with high surface densities of complementary E-selectin/sialyl Lewis X (sLeX) carbohydrate chemistry, a series of structures, from colloidal micelles (large particle coated with smaller particles) and clusters, to rings and elongated chains, was synthesized by decreasing the number ratio, N_A/N_B, of small (A) to large (B) particles (2 ≤ N_A/N_B ≤ 200) at low total volume fraction (10−4 ≤ &phis;_T ≤ 10−3 ). Using significantly lower surface densities, the low affinity binding between E-selectin and sLeX was exploited to create particles that interact reversibly, and average particle interaction lifetimes were tuned from minutes down to single selectin-carbohydrate bond lifetimes (≈1 s) by reducing sLeX density, a significant step toward assembling ordered microstructures. Particle binding lifetimes were analyzed with a receptor-ligand binding model, yielding estimates for molecular parameters, including on rate, 10−2 s−1 < k_on < 10−1 s−1, and unstressed off rate, 0.25 s−1 ≤ $kor$ ≤ 1.0 s−1, that characterize the docking dynamics of particles. Finally, at significantly higher volume fraction (&phis; _T ≥ 10−1) and low number ratio, the rheology of space-filling networks crosslinked by high affinity streptavidin-biotin chemistry was probed to acquire knowledge on bulk properties of biocolloidal suspensions. Flow curves (apparent viscosity (η) versus shear rate ()) exhibited non-Newtonian behavior, and the viscosity and extent of shear thinning increased with an increase in total volume fraction. Micrographs suggest that shear thinning originated from a breakdown of the binary network into smaller flocs, and ultimately a fluid-like suspension, with increasing shear rate. With such remarkable control over particle interactions, biologically-mediated colloidal assembly holds promise for engineering new materials with biophysically tunable properties and applications.