Scientists have designed microscopic "grenades" that can explode their cancer-killing payload in tumours.

The team will present its findings at the National Cancer Research Institute conference next week.

They plan to use liposomes - tiny bubbles of fat which carry materials round the body - to release toxic drugs when their temperature is raised.

The "grenades" are intended to avoid side-effects by ensuring the drugs target only the tumour.

Experts said such technology, which has been effective in animal experiments, was the "holy grail of nanomedicine".

Cancer scientists are trying to harness the transporting abilities of these fatty spheres by getting them to carry toxic drugs to tumours.

"The difficulty is, how do you release them when they reach their target?" Prof Kostas Kostarelos, from the University of Manchester, told the BBC News website.

The Nanomedicine Lab in Manchester has designed liposomes that are water-tight at normal body temperature. But when the temperature increases to 42C they become leaky.

"The challenge for us is to try to develop liposomes in such a way that they will be very stable at 37C and not leak any cancer drug molecules and then abruptly release them at 42C," Prof Kostarelos added.

Warming

He suggests heat pads could be used to warm tumours on the body surface such as skin, head or neck cancers.

Probes can heat tumours inside the body, and there is also discussion about using ultra sound to warm tumours.

In early tests on mice with melanoma there was "greater uptake" of drugs in tumours using the thermal grenades. And that resulted in a "moderate improvement" in survival rates.

Prof Kostarelos said similar techniques were being trialled in patients and this "is not a fantasy."

Prof Charles Swanton, the chairman of the conference, said targeted liposomes were a "holy grail of nanomedicine".

He added: "These studies demonstrate for the first time how they can be built to include a temperature control, which could open up a range of new treatment avenues.

"This is still early work but these liposomes could be an effective way of targeting treatment towards cancer cells while leaving healthy cells unharmed."