The invention provides compositions that include crosslinking agents having multiple photoactivatable groups, such as diaryl ketones, or a diaryl ketone, such as benzophenone, and at least one polymerizable monomer, such as a zwitterionic monomer. The compositions are useful as surface coating agents that provide brush type polymeric coatings. These polymeric coatings can be used on medical devices, such as artificial joints, to reduce wear and tear between the components of the joint and thus reduce or eliminate debris generated by friction between the joint components.