Multivalent counterions diminish the lubricity of polyelectrolyte brushes

A brush with friction Polyelectrolyte brushes consist of charged polymer chains attached to a common backbone or surface. They provide excellent lubrication between two surfaces for both engineered and physiological materials. The packing of the brushes is sensitive to pH, temperature, or added salts. Yu et al. show that the presence of multivalent ions can cause brush collapse, similarly to monovalent ions (see the Perspective by Ballauff). Critically—and not observed with the addition of monovalent ions—very low concentrations of multivalent ions cause bridging between the brushes and increase friction between the surfaces to the extent that their value for biomedical devices is limited. Science , this issue p. 1434 ; see also p. 1399