A hydrophilic PDMS (polydimethylsiloxane) surface was formed by the synthesis of an interpenetrating polymer network (IPN) in a two-step process. In the first step, PDMS was loaded with crosslinker and initiator using a solvent that swells the PDMS. In the second step, the PDMS sample was submerged into a solution containing the hydrophilic monomer followed by a UV-polymerization step. The choice of solvent in the second step is critical to obtain a hydrophilic surface. It can be concluded that the solubility parameter of the solvent should be above a threshold value. Hence, in the second step only sufficiently polar solvents will result in hydrophilic PDMS-IPNs. These principles are illustrated by using N-vinyl-2-pyrrolidone as the hydrophilic monomer forming PVP/PDMS-IPNs. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 3059-3067, 2008