There are many eye-catching colors in the colorful nature, such as the tail feathers of peacocks, the wings of butterflies, the feathers of the head of the scarlet hummingbird, the elytra wings of the weevil, and the skin of chameleons. The study found that the periodic micro-nano structures in these organisms with a variety of different structures are actually a type of photonic crystal that can selectively reflect visible light while filtering (absorbing or scattering) visible light in other bands.
It is reported that the bright colors of the feathers, skin and organs of these animals come from the selective reflection of light by the periodic structure of photon crystals, which is the result of a physical interaction. In response to this, Professor Huang Shaoming and Associate Professor Yang Dongpeng of the School of Materials and Energy of Guangdong University of Technology called it structural color. Although traditional organic dyes, heavy metal pigments and other color materials also have bright colors, there are problems such as easy photobleaching and environmental pollution.
Compared with traditional coloring materials, structural colors have many advantages such as photobleach resistance, long-term storage, and environmental friendliness. Artificial preparation of micro-nano structure bionic materials with similar functions and regulation of their structural colors have become research hotspots in the fields of colloid chemistry, intraocular crystallography, and photophysics.
Chameleons are able to change their structural colors according to changes in their surrounding environment, and materials with bionic properties have a wide range of application prospects in display, printing, anti-counterfeiting, sensors, biological and optical devices. The study found that the iris cells of the chameleon's skin have a large number of guanine nanoparticles arranged in a non-tightly stacked ordered structure that can selectively reflect visible light, such as red. When the surrounding environment changes (such as blue), the chameleon adjusts the distance between the guanine nanoparticles by stretching the skin, thereby changing its own structural color to make it consistent with the surrounding color.
This special, sensitive force-causogenic discoloration property depends mainly on the non-dense accumulation structure between guanine particles and the large particle spacing (D), which leaves a lot of room for deformation, so that the chameleon skin takes on a rich color during stretching. In addition, the bright color of the chameleon's skin is derived from the large refractive index contrast (Δn) between a large number of guanine nanoparticles and other parts, and Δn is a key parameter that determines the reflectivity and brightness of photon crystals. Therefore, building large D and Δn is the key to achieving the skin of the bionic chameleon.
At present, the preparation strategy of force-induced photochromic photonic crystals is mostly based on swelling, two-step filling and self-assembly. Among them, the force-induced photochromic photonic crystals prepared by the swelling method cannot obtain a large particle spacing (D), resulting in a narrow discoloration range (Δλ<120 nm) and poor sensitivity (<200). The force-color-changing photonic crystals prepared by the two-step filling strategy take into account both large particle spacing (D: 110 nm) and refractive index contrast (Δn), and the resulting force-chemtogenic photonic crystals have bright structural colors, a wide color change range (200 nm) and high sensitivity (1000).
However, similar to other force-causing photonic crystalline gels, it is easy to lose its force-induced color-changing properties due to solvent volatilization in the gel under dry or high temperature conditions. Self-assembly of colloidal particles to prepare force-induced photochromic photonic crystals is a simple and efficient strategy. However, the prepared force-chemtochromic photonic crystals have limited sensitivity (300) and wavelength difference (150 nm) due to insufficient particle spacing.
Recently, the team produced force-induced photochromic photonic crystals with high sensitivity (921) and large wavelength differences (Δλ = 300 nm) by controlling the concentration of colloidal particles to produce large particle spacing. However, due to the extremely small Δn (≈0.01), the reflectivity < 40%, which limits its practical application. In summary, the preparation of force-induced photochromic photonic crystals with good stability, high reflectivity and sensitivity remains a huge challenge.
Inspired by chameleon structure, artificial chameleon skins with greater than 100 reversibility are prepared
Inspired by the chameleon structure, the research group started from the construction of a non-dense stacked ordered structure, based on the non-dense accumulation and assembly of SiO₂ colloidal particles in the polymer, and successfully prepared artificial chameleon skin with high sensitivity, bright structural color and good stability by changing the strategy of regulating Δn and D by changing the refractive index of the polymer and the concentration of SiO₂, respectively.
The prepared artificial chameleon skin has excellent force-induced color-changing properties, such as large wavelength modulation range (Δλ = 205 nm), high sensitivity (3.7 nm/%), fast response (2.2 nm/ms), good stability (>1 years) and good reversibility (>100 times). Due to their high sensitivity, the prepared force-colored colloidal crystals can report changes in skin strain during earthworm peristalsis by outputting different structural colors.
Figure | Biomimetic principle and synthesis diagram of artificial chameleon skin (Source: ACS Applied Materials & Interfaces)
Recently, the paper was published in ACS Applied under the title chameleon-Inspired Brilliant and Sensitive Mechano-Chromic Photonic Skins for Self-Reporting the Strains of Earthworms on Materials & Interfaces.[1]
Figure | Related papers (Source: ACS Applied Materials & Interfaces)
The reviewers found the group's work very interesting, and the prepared artificial bionic chameleon skin (force-induced photochromic photon crystals) can maintain a bright structural color at large viewing angles and during deformation due to its high reflectivity. In addition, it also has excellent force-induced color-changing properties, such as large wavelength modulation range (Δλ=205 nm), high sensitivity (3.7 nm/%), fast response (2.2 nm/ms), good stability (>1 years) and good reversibility (>100 times). The simple and efficient preparation of artificial bionic chameleon skin and its characteristics will facilitate its application in bio-coating, visual sensing and other fields. Another reviewer said that the team's visual sensing of the artificial bionic chameleon skin for the skin strain of the earthworm skin fits well with its sensitive characteristics and very intuitively demonstrates the discoloration performance of the artificial bionic chameleon skin.
Figure | Professor Huang Shaoming (left) and Associate Professor Yang Dongpeng (right) of the School of Materials and Energy, Guangdong University of Technology (Source: This research group)
The specific research steps are as follows:
The first is the selection of topics, including the research value of the topic and literature research. Force-induced photochromic photonic crystals have broad application prospects in the fields of display, printing, anti-counterfeiting, sensors, biological and optical equipment. The study found that the force-changing properties and structural color saturation of chameleon skin depended on its large particle spacing (D) and refractive index contrast (Δn). However, it has been reported that force-induced photochromic photonic crystals have problems such as low sensitivity, weak structural color, and poor stability.
The second is to select the study object, including the selection of structural primitives and the determination of preparation strategies. Based on the previous experience of the research group, a large particle spacing (D) can be obtained by controlling the assembly concentration of SiO₂ particles in diethylene glycol ether acrylate (DEGEEA). So they chose commercial sio₂ as the structural matrix to prepare force-induced photonic crystals by SiO₂ self-assembling in acrylate.
Figure | Group photo of Professor Huang Shaoming's team (Source: Huang Shaoming)
Finally, there is the experimental design, which includes the preparation of materials, optimization variables, and performance characterization. Force-chromogenic photonic crystals prepared in the SiO₂-DEGEEA system have low reflectivity due to the small refractive index contrast. Therefore, the team hopes to improve the refractive index contrast of the system by introducing polyethylene glycol phenyl ether acrylate (PEGPEA, n=1.52) with a higher refractive index.
However, SiO₂ is assembled at a higher concentration in PEGPEA (24%), so the amount of PEGPEA needs to be optimized to find the best value that balances large particle spacing (D) and refractive index contrast (Δn). Due to the high reflectivity (R>70%) of the force-oriented photonic crystals, it exhibits a bright structural color at both large viewing angles and during force-induced color change. Thanks to the large particle spacing (77 nm), force-induced photonic crystals have excellent force-induced color-changing properties.
It can be used in biomonitoring, fluorescence enhancement and other fields
In terms of application prospects, there are mainly the following categories.
The first category: biological monitoring, e.g. covering the skin of the bionic chameleon on the outside of the elastic bandage, can determine whether the elastic chameleon skin is appropriate by the structural color change of the bionic chameleon skin and the degree of tightness of the bandage winding and the joint activity during exercise. In addition, the bionic chameleon skin is capable of producing structural color changes in response to weak cellular forces, which, in combination with microfluidic techniques, can be prepared with a heart chip to test the response of cardiomyocytes to different types of drugs.
The second category: display, for example: when the bionic chameleon skin is pressed against a patterned template, the part of the structural color that is squeezed changes, resulting in a pattern of color contrast. This display is color-adjustable, responsive, and reversible.
The third category: fluorescence enhancement, using the characteristics of the tunable photon band gap of the bionic chameleon skin can achieve dynamic adjustment of fluorescence emission from different fluorophores.
The fourth category: anti-counterfeiting, for example: the bionic chameleon skin made of anti-counterfeiting label, it not only has the same color-changing characteristics as the color-changing ink with the observation angle, but also produces structural color changes when deforming, and has a double anti-counterfeiting effect when used as an anti-counterfeiting label.
After preparing the artificial bionic chameleon skin, they did not immediately think of a self-report of its use for earthworm skin strain. Because it is uncertain whether the creeping of earthworms can cause the skin of artificial bionic chameleon to change color, and there is a body cavity fluid on the surface of the skin of earthworms, the skin of artificial bionic chameleons may not be fixed.
With the mentality of giving it a try, the team purchased half a kilogram of earthworms from the Internet, wiped the surface of the earthworm skin clean, and then attached the artificial bionic chameleon skin directly to the surface of the earthworm skin. Surprisingly, it may be due to the friction of the bristles on the surface of the earthworm skin The artificial bionic chameleon skin is well fixed on the surface of the earthworm body. As the skin strain changes during earthworm peristalsis, the skin of the artificial bionic chameleon outputs different structural colors. This application demonstrates the high sensitivity of the skin of artificial bionic chameleons and the application potential in the fields of biomonitoring, wearable color change sensors.
Figure | Artificial chameleon skin for self-reporting earthworm skin strain (Credit: ACS Applied Materials & Interfaces)
In the future, the research group will conduct further research from two aspects, one is to prepare a flexible conductive substrate, and finally realize the composite bionic chameleon skin of optical signal and electrical signal combined with this study; the other is to further explore the application of artificial bionic chameleon skin in the field of biological monitoring and wearable color change sensors. In addition, they will seek partners in bio-tissue engineering-related directions to optimize and enhance the adhesion, biocompatibility and antimicrobiality of the skin of artificial bionic chameleons.
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reference:
1、Hu, Y., Wei, B., Yang, D., Ma, D., & Huang, S. (2022). Chameleon-Inspired Brilliant and Sensitive Mechano-Chromic Photonic Skins for Self-Reporting the Strains of Earthworms. ACS Applied Materials & Interfaces.