Poly(ethylene glycol) (PEG)-modified poly(lactic acid) (PLA) systems were created by physically entrapping the modifying species at the PLA surface. The surface characterization and biological performance of these materials are described. This modification strategy is performed by reversible gelation of the PLA surface following exposure to a solvent/nonsolvent mixture. PEG is then able to diffuse into the swollen surface region, before it is collapsed by the addition of more nonsolvent. This results in the localized physical entrapment of the diffused material. We have demonstrated by high-resolution X-ray photoelectron spectroscopy that control over the PEG surface density may be achieved by using predetermined process conditions, such as a particular solvent/nonsolvent ratio or a set polymer treatment time, and that surface coverage of around 75% is possible. Cell adhesion studies have shown that even in serum-containing media PEG entrapment will prevent attachment, with a 95% reduction in cell number compared to unmodified PLA. This modification strategy was also used to coentrap both PEG and poly(L-lysine)-RGD within the PLA surface region. The attachment of cells to this material shows that the entrapment approach may be used to create highly selective biomaterial surfaces that are able to prevent unwanted cell or protein adhesion yet actively promote specific cellular interaction.