Like a column collapsing under the burden of a heavy roof, erectile dysfunction is a classical mechanical engineering problem, says a US urologist. Using mathematical models of penis geometry and hydrostatic pressure, doctors can predict when penises will fail – and in which vagina – he says. The most widely investigated parameter of penile rigidity is intracavernosal pressure (ICP) – the fluid pressure achieved by blood build up in the two expandable "caverns" of the penis. For a healthy man, the erect ICP is between 60 and 90 millimetres of mercury (mmHg), but can drop to just 30 mmHg, in...