Skin cancer might seem like the easiest cancer to deal with – after all, it's right there on the outside of the body, so removing it should be simple and safe, right? Unsurprisingly the reality is not so simple and treatment often still requires chemotherapy, which is delivered intravenously and can cause a whole range of unpleasant side effects. Now, researchers have made the first steps towards a kind of chemo that can be "painted" onto the skin.

As the deadliest form of skin cancer, melanoma is a particularly cruel illness, sitting right out in the open air but spreading its harm inwards, through the lymphatic system. By the time it makes itself known on the skin it might be too late, and chemotherapy is still our best bet to keep it under control after it's started spreading. But of course, that shotgun blast of a treatment affects healthy cells throughout the body too.

Finding ways to apply chemo or other treatments directly to tumor cells is a key area of cancer research. Recent breakthroughs include a technique where nanoparticles are injected and then only activated at the tumor site by way of x-ray pulses, which creates singlet oxygen to destroy the cancer cells. Similar studies have used infrared light to heat up gold nanoparticles, which get into tumors by hitchhiking on white blood cells.

But these techniques are used to single out cancer cells that might be hiding deep inside the body, where it's hard to reach. Melanomas should be easier to target, given their appearance on the skin. With that in mind, the researchers on the new study set out to develop a chemotherapy-loaded hydrogel that could be applied topically.

The gel was designed to help the active cancer-fighting ingredients penetrate deep into the skin. In this case, that was the chemotherapy drug paclitaxel, which was wrapped in a surfactant and then a few phospholipid layers. The resulting nanoparticles are called "transfersomes," and the surfactants let them slip through the skin more easily where they can then get to work fighting the cancer cells.

In tests on mice, the researchers applied the gel onto melanoma tumors once a day, as well as giving the animals injections of paclitaxel every few days, while a second group received only the injections. After 12 days, the tumors in those that had received the gel were about half the size of those that were just getting injections of the drug, suggesting that the topical treatment was helping to slow down the cancer's growth.

As interesting as those results are, it's of course far too early to get too excited. The approach might not necessarily translate to humans, and there's not yet any data on whether the gel would ever be enough by itself, meaning injections might still be necessary. That said, the researchers believe they've made the first steps towards the goal of a chemo gel that patients could just rub on their skin at home by themselves.

The research was published in the journal ACS Nano.