In our recently published research in Advancement of scienceMy student Ben Alessio and I have proposed a possible mechanism explaining how these distinctive patterns form, which could be applied to medical diagnostics and synthetic materials.
A thought experiment can help visualize the challenge of achieving distinct color patterns. Imagine that you are gently adding a drop of blue and red dye to a cup of water. The droplets will slowly distribute throughout the water due to the diffusion process, where molecules move from an area of higher concentration to an area of lower concentration. Eventually, the water will contain an equal concentration of blue and red pigments and turn purple. Hence, diffusion tends to create color uniformity.
Naturally, a question arises: How can distinctive color patterns form under diffusion?
Movement and boundaries
Mathematician Alan Turing first addressed this question in his 1952 paper “The Chemical Basis of Morphology.” Turing showed that under the right conditions, the chemical reactions involved in producing color could interact with each other in a way that prevented diffusion. This makes it possible for colors to self-organize and create interconnected regions of different colors, forming what are now called Turing patterns.
However, in mathematical models, the boundaries between color regions are fuzzy due to diffusion. This is in contrast to nature, where borders are often sharp and colors are well separated.
Our team believed that the key to discovering how animals create distinctive color patterns could be found in laboratory experiments on micron-sized molecules, such as the cells involved in producing animal skin colors. My work and that of other labs have found that micron-sized particles form banded structures when placed between an area of high concentration of other solutes and an area of low concentration of other solutes.
In the context of our thought experiment, changes in the concentration of blue and red pigments in water can cause other molecules in the liquid to move in certain directions. When the red dye moves to an area where it is in lower concentration, nearby molecules will be carried with it. This phenomenon is called diffusiophoresis.
You benefit from the process of diffusion electrophoresis when you wash your clothes: dirt particles move away from your clothes while soap molecules diffuse from your shirt into the water.
Draw sharp boundaries
We wondered whether Turing patterns composed of regions with concentration differences could also move micron-sized particles. If so, would the patterns produced by these particles be sharp rather than fuzzy?
To answer this question, we ran computer simulations of Turing patterns—including hexagons, lines, and double spots—and found that diffusion electrophoresis makes the resulting patterns significantly more distinct in all cases. These diffusion simulations were able to replicate the complex patterns on the skin of the ornate boxfish and the moray jewel eel, something not possible by Turing’s theory alone.
To further support our hypothesis, our model was able to reproduce the results of a laboratory study on how bacteria work coli bacteria They transport molecular cargo within themselves. Diffusion electrophoresis resulted in clearer movement patterns, confirming its role as a physical mechanism behind biological pattern formation.
Because the cells that produce the pigments that make up animal skin colors are also micron-sized, our findings suggest that diffusion photophoresis may play a key role in creating distinctive color patterns on a larger scale in nature.
Learn nature’s trick
Understanding how nature’s software functions specifically can help researchers design artificial systems that perform similar tasks.
Laboratory experiments have shown that scientists can use diffusion electrophoresis to create membrane-less water filters and low-cost tools for drug development.
Our work suggests that the combination of conditions that form Turing patterns and diffusion electrophoresis could also form the basis for artificial skin patches. Just like adaptive skin patterns in animals, when Turing patterns change – for example from hexagons to lines – this indicates fundamental differences in chemical concentrations inside or outside the body.
Skin patches that can sense these changes can diagnose medical conditions and monitor a patient’s health by detecting changes in biochemical markers. These skin patches can also sense changes in the concentration of harmful chemicals in the environment.
The work is ahead of us
Our simulations focused exclusively on spherical particles, while the cells that make pigments in the skin come in different shapes. The influence of shape on the formation of complex patterns remains unclear.
Moreover, melanocytes move in a complex biological environment. More research is needed to understand how this environment inhibits movement and possibly freezes patterns in place.
Besides animal skin patterns, Turing patterns are also essential for other processes such as embryonic development and tumor formation. Our work suggests that diffusion photophoresis may play an underappreciated but important role in these natural processes.
Studying how biological patterns form will help researchers move one step closer to mimicking their functions in the laboratory, an ancient endeavor that could benefit society.