Australasian Science Magazine

iStockphoto / 4FR

By Ben Powell

Melanin protects us from the Sun’s radiation, but as it also conducts electricity it could be used in bioelectronic devices and prostheses.

Skin cancer is the most common form of cancer in Australia and New Zealand; two out of three Australians will be diagnosed with skin cancer before their 70th birthday. Melanoma, the most deadly form of skin cancer, is cancer of the cells responsible for producing melanin.

Melanin colours our skin, hair and eyes, and protects us from the sun’s harmful radiation. It is also found in our brains, but its function there is less well-understood.

If we want to understand how our bodies use melanin then it is important to understand the physical and chemical properties of melanin. This seemingly routine task has led to several surprises and mysteries.

One of these surprises is that it conducts electricity very well. Most of the conductors we encounter in everyday life are inorganic – either metals like copper or semiconductors like silicon. But there is a growing interest in making electronic devices using organic molecules.

Many biological systems communicate by sending electronic signals. For example, to turn the page of this magazine you must send an electrical signal from your brain, through your nervous system to the muscles you want to move. In many ways this is just like a computer controlling a robotic arm.

The analogy between biology and robotics has led many people to ask whether a computer could control a real biological arm, or could we invent a chip that, when implanted into your body, would allow you to control a robotic arm by just thinking?

This may sounds like a science fiction nightmare, but there are many reasons to do this, such as the development of artificial arms for amputees or enabling paraplegics to walk. The first steps in this new field of “bioelectronics” are just being taken.

One of the major problems in bioelectronics is that man-made electronic devices move electrons to carry electrical signals, while biology achieves the same end by shuffling ions around.

We have recently discovered that melanin uses both ions and electrons to conduct electricity. This suggests that melanin could be the vital ingredient in future bioelectronic devices, translating between the electrical signals used in both electronic devices and biological systems.

Why Is Melanin Black?

There are several different types of melanin. People with ginger hair only produce pheomelanin, which contains iron, giving its distinctive reddish hue. People with dark hair also produce eumelanin, which is very black.

When you shine light on eumelanin it is absorbed, and the associated energy dissipates as heat. This is not just true for visible light, but also for ultraviolet light. Thus melanin protects us from the sun by absorbing ultraviolet light.

Clearly there is a major evolutionary advantage for melanin to absorb light so well, but how does it absorb this light?

In the 1960s Barnett Rosenberg and co-workers suggested that melanin is a naturally occurring organic semiconductor. The semiconductor hypothesis seemed to be confirmed in the early 1970s when John McGinness and collaborators made an electrical switch from melanin. This switch is just like the millions of tiny switches that form the basis of every digital device, such as computers or smart phones. McGinness’ celebrated device, which is now housed at the Smithsonian Institute, was the first example of an organic electronic device.

In the late 1970s Alan Heeger, Alan MacDiarmid and Hideki Shirakawa found that another organic material, polyacetylene, is a semiconductor with a high conductivity. This paved the way for a massive growth in research into organic electronics. Recently the first organic electronic devices have reached the consumer market – most notably in low power displays for mobile phones. In 2000 Heeger, MacDiarmid and Shirakawa were awarded the Nobel Prize in chemistry for their contributions to this field.

Nevertheless, several problems lingered with the theory that melanin is a semiconductor. In a semiconductor, electrons move according to the rules of quantum mechanics, and this quantum transport can be characterised by a single energy scale known as the “band gap”, which describes how hard it is to move an electron. Importantly, if melanin is a semiconductor then photons whose frequency is less than its band gap will not be absorbed. By measuring the absorption of light with different frequencies, one should be able to determine what the band gap is.

This is important, as the band gap is a key parameter required to understand how a semiconductor conducts electricity. But despite many attempts, no one has ever managed to see the band gap in melanin.

The Structure Problem

Another major problem in the field was the structure of melanin. Biological materials typically rely on a carefully controlled structure to achieve the tasks that they have evolved for. Indeed, some diseases, such as “mad cow” disease, are caused when biomolecules fold in the wrong way. Therefore understanding a material’s structure is an important first step in understanding its properties and, from there, its biological function.

A great deal of effort around the world has gone into discovering the structure of melanin. While the basic building blocks of melanin have been known for decades, no clear understanding of how they fit together has emerged.

In 2006 my collaborators and I realised that we could solve both these problems by introducing one simple idea: that melanin does not have a structure. Instead it is just a mess of different chemicals. This would clearly explain why no one had been able to determine the structure but it also explains why melanin absorbs light of different colours (or frequencies) so well.

Each different molecule absorbs light of certain colours well, but these colours will be different for different molecules, so as a collection they will absorb light of all colours. All chemists know that everything you make is brown before you purify it. Nature has just taken advantage of this to make a very low cost sunscreen!

The basic building blocks of melanin are the chemicals di­hydroxyindole (DHI) and dihydroxyindole-carboxylic acid (DHICA). There are many ways of joining even just two of these molecules. I calculated that each different way of pairing these molecules leads to the dimer absorbing light of a slightly different colour – as needed for our hypothesis.

My collaborators then made a dilute solution of DHICA and measured its absorption as the DHICA molecules joined together slowly. At first the absorption looked just like what you’d expect for isolated DHICA molecules, but as the DHICA molecules joined together the spectrum broadened and smoothed out to look like the spectrum of melanin. This confirmed our hypothesis.

However, our “chemical disorder hypothesis”, as our idea has become known, had a major problem – if melanin is not a semiconductor, why should it conduct electricity? Did nature need melanin to conduct electricity or is it just a by-product of melanin’s main role as a sunscreen?

Hot, Wet and Dirty

An interesting quirk of McGinness’ switch is that it only works in a humid environment. In fact, the conductivity of melanin changes dramatically as the humidity changes. Rosenberg had predicted how these changes should occur in the semiconductor model, so we made a series of careful measurements of melanin’s conductivity at different humidities and found that the theory could not explain the data. This was another nail in the coffin for the semiconductor theory of melanin, but it still did not explain why melanin conducts electricity so well.

Looking closely again at the melanin building blocks we noticed that they contain hydrogen ions that could leave the molecule relatively easily. This would leave the hydrogen ion free to carry electricity and also an electron on the molecule that could also carry electricity. This would also explain why increasing the humidity increases the conductivity, as a more humid environment would cause more hydrogen ions to leave the molecule. But does it really happen?

We carried out experiments in which we inject subatomic particles called muons into the melanin and watch how the muons decay. The muons behave like a proton as far as chemistry is concerned, but they are radioactive.

By analysing the muon decay we can see that as the humidity increases the muons move from the quiet environment that comes with being chemically bonded to the melanin into the noisy environment of freely moving around the material. This shows that the protons, which move in the same way as the muons, must contribute to the conductivity of melanin as predicted by our hypothesis.

Our work has therefore shown that melanin is a “mixed conductor” where both ions and electrons carry electrical current. This is a key ingredient that has not previously been available for bioelectronic applications, and promises to open many new doors in this emerging field of research.