A new study by the Chiba University, inspired by the colour changes in a peacock's feather, revealed how melanin-mimetic materials, such as polydopamine, could lead to anti-counterfeit and sensing applications.
By studying structural colour, which is a colour derived from optical interaction between light and a microstructure, often seen in nature, i.e. in peacock feathers, chemist Michinari Kohri of the Chiba University in Japan published a research about melanin-mimetic materials and their potential applications.
What is melanin
Melanin is a dark pigment that gives hair and skin its colour. It is also essential for the bright colours we see in some organisms. When light interacts with the structures of feathers, wings and shells of many organisms, like peacocks, butterflies and jewel beetles, it is scattered, appearing white. But when melanin is interspersed within these structures, some of the scattered light is absorbed, producing various colours.
Scientists are looking for ways to mimic these so-called 'structural colour' changes of living organisms in synthetic materials.
"Vivid structural colours can be obtained by constructing microstructures containing a light-absorbing black material made of natural or artificial melanin," says Kohri. "Research in this area is progressing rapidly worldwide."
The research on melanin-mimetic materials
One of the melanin-mimetic materials that scientists are studying is a compound called polydopamine (PDA), which is made of a material naturally found in the body, so it is biocompatible, and is also dark, so it absorbs light like melanin.
Scientists found they could control polydopamine's iridescence, that is, how much the colour changes as the angle of light hitting it shifts, similar to a peacock's feather. They achieved this by altering the particle size or by adding compounds that react to a magnetic field.
Scientists are also investigating particles formed of a polystyrene core and a polydopamine shell. Changing the diameter of the inner core, for example, leads to different colours. Making the polydopamine shell thicker causes the particles to be less closely packed, leading to non-iridescent structural colour, which remains the same regardless of the light angle.
Scientists have also toyed with controlling colour and angle-dependence by changing the shapes of polystyrene/polydopamine particles, making them hollow on the inside, and adding multiple coatings to the external shell.
The potential of melanin-mimetic materials
Polydopamine particles are showing potential for a variety of applications.
For example, since PDA is a highly biocompatible polymer that utilizes substances that are present in the living body, it may be useful to develop applications such as cosmetics that come into contact with the skin.
Tests have also been made using electrophoretic deposition (EPD). By performing EPD of silicon dioxide particles in the presence of the precursor of PDA (pre-PDA), a structural colour film was obtained on the flexible PET substrate. The pre-PDA enhanced the mechanical properties and enabled flexibility of the structural colour coating with bending resistance, showing the possibility of application to flexible displays.
In addition to being used in inks and colouring agents, also to dye fabrics, these melanin-mimetic materials could help prove a product is real versus counterfeit by shifting colour with strong light, wetting, or temperature changes.
Finally, scientists have found that adding these particles to rubber causes it to change colour when stretched or relaxed, which could be useful for sensing local stress and strain in bridges.