A system for applying a polymer (hydrogel) to the inner surface of a vessel by photopolymerization includes an inner catheter received within an outer catheter and a fluid channeling structure mounted on the catheters with its proximal end attached to the outer catheter and its distal end attached to the inner catheter. The fluid channeling structure includes an elastomeric sleeve sandwiched between an inner elastic coil and an outer light emission coil. Light emission coil loops are interspaced with elastic coil loops. The light emission coil has a smaller diameter than the elastic coil, drawing the elastomeric sleeve down between loops of the elastic coil to form a helical channel. The coils and sleeve are stretched to provide a narrowed crossing profile for delivery, and are contracted at a treatment site to form a helical cavity between the sleeve and the vessel wall within which to perform interfacial photopolymerization.