Monday's paper in the journal Basic and Clinical Andrology showed that injecting a gel into male rhesus monkeys' reproductive parts was almost 100 percent effective in preventing pregnancy in their sexual partners. The gel, also known as Vasalgel, works by physically blocking sperm from moving through the vas deferens during ejaculation. This is similar to how a vasectomy works—mechanical blockage, as opposed to some hormonal method—but Vasalgel is more easily reversed than a surgical procedure, because the gel can be flushed out of the vas deferens with a simple injection. Researchers at California National Primate Research tested the gel in 16 adult male rhesus monkeys by performing the procedure, then allowing the monkeys to live amongst fertile females for one to two years. There were no pregnancies.