Colloidal aggregation can be made self-limiting by controlling the ratio of
reactive groups (ligands such as biotin coupled to phospholipids and incor
porated in a vesicle membrane) on the colloid surface to crosslinking agent
s (multifunctional receptors such as avidin or streptavidin) in solution. A
distinct transition occurs between limited and complete aggregation as a f
unction of the ligand-to-receptor ratio. The "limited" aggregates formed ar
e spherical in nature with a fractal dimension of 2.9. The size of the aggr
egates depends on the overall concentration of surface accessible biotin-li
gands, which can be controlled either by the biotin-lipid fraction in the b
ilayer at fixed vesicle concentration, or by increasing the vesicle concent
ration at fixed biotin-lipid fraction. The spherical shapes and concentrati
on dependence are the result of the free diffusion of the ligands on the ve
sicle surfaces; A simple model of the process based on Smolukowski aggregat
ion kinetics coupled with a Langmuir-type surface reaction is consistent wi
th experiment. This process might be generalized to any system of colloids
with surface reactive groups that can be coupled by a soluble cross-linking
agent.