A process for forming a surface modification on a polymer substrate and polymer substrates having such surface modifications. The process comprises the steps of absorbing a swelling monomer into the polymer substrate for a period of time in order to swell the polymer substrate; removing the swollen polymer from the swelling monomer; transferring the swollen polymer to a reaction mixture containing at least one functional monomer; polymerizing the functional monomer in the reaction mixture containing the swollen polymer substrate for a period of time; and removing the polymer from the reaction mixture. Because the surface modification produced by the process is a surface interpenetrating polymer network, the process is not sensitive to the reactive groups located on the surface of the polymer substrate. Further, the surface interpenetrating network bonds to the polymer substrate through caternary connections or other forms of chain entanglement and thus is quite stable. Polymer substrates having the surface modification of the present invention are capable of having a surface modification agent, such as heparin, adhere to the surface of the polymer substrate.