We have developed novel photopolymer gels to function as separators in blood collection tubes. By incorporating antioxidants such as α-tocopherol and nitroxides (TEMPO and TEMPOL), the new formulation can be sterilized with electron beam or gamma rays at a dose level of 17 kGy, without inducing premature curing of the photopolymers. For the blood separator gels that contain α-tocopherol, our results show that α-tocopherol plays a decisive role in impeding C-centered free radical propagation reactions through an H-transfer mechanism. This mechanism involves the transfer of an H-atom from the hydroxyl group (OH) of α-tocopherol to the propagating C-centered radical leading to the termination of the polymerization. The sterilization radiation-induced premature curing of the photopolymer was also prevented in the blood separator gel containing nitroxides. For the gels containing TEMPO or TEMPOL, inhibition of the premature curing was achieved through an addition reaction or an H-transfer reaction, respectively. Our results also show that while α-tocopherol is not a contributing factor in the subsequent (time-of-use) UV curing of the gels, nitroxides enhance the UV curing process through nitroxide-mediated living free radical polymerization reactions leading to a decrease in UV curing time. The photopolymer separator gels are shown to function advantageously in clinical laboratory testing, especially for cell-free DNA measurements in blood.