A microbicide is a topical agent that, when applied to a mucosal surface, serves as a barrier to infection. Microbicides can come in the form of a gel, cream, foam, or ring. There is a lot of work being done to see if a microbicides can be developed that protects against HIV, from either vaginal or rectal contact.
A significant trial called VOICE (Vaginal and Oral Interventions to Control the Epidemic) has discontinued the part of the study relating to tenofovir-containing gel used as a microbicide for women, since it was found to be ineffective.
More information can be found here.
Whilst disappointing, this is not the end of the road for microbicides – other studies are still ongoing. Microbicides for anal use may need to have entirely different formulations to those used vaginally,as the area of mucosal tissue at risk in the rectum is much larger and is open-ended rather than a closed space as in the vagina.