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The gel is liquid as long as it is in contact with the acidity that is normal in a vagina, but will turn solid when it encounters semen, which is slightly alkaline. Any particles wider than 50 nanometres – including sperm, HIV and other viruses such as the herpes virus and the papilloma virus, which causes cervical cancer – are trapped, so the gel could double up as both a protectant and a contraceptive.