Guest Post by Xuemin Du. Views expressed in this article do not represent the opinion of Synced Review or its editors.
What inspire the team proposes novel interactive sensors and actuators?
Interactive sensors and actuators, with the “intelligence” of sensing the environment and responding accordingly, are essential for human-machine interaction. However, it remains a critical challenge for a long time.
Interestingly, creatures in nature such as chameleons show various environmentally adaptive and interactive behaviors, such as altering their body postures, moving, and changing their skin colors in response to environmental changes. If we can mimic the environmentally adaptive and interactive behaviors of chameleons and prepare the chameleon-inspired interactive sensors and actuators, it will open new avenues for human-machine interaction.
The coloration of chameleons is based on the optical diffraction and reflection by the ordered packed micro/nanostructures in their skins, which is called structural color. The change of colors results from the adjusting spacing among the microstructures through the contraction/relaxation of muscles.
Learning from the principle to chameleons, we have successfully developed novel chameleon-inspired interactive sensors and actuators with adaptive colors based on “smart” polymers embedded with ordered microstructures – inverse opal structures.
The expansion and contraction of polymers change the spacings among the microstructures, thus changing the color. With the color-shifting and further integrating actuating capabilities, these chameleon-inspired interactive sensors and actuators have shown great promise in biomedicine, information storage, and adaptive soft robotics.
The team designs biocompatible contact lens sensors based on “smart” polymers
Early diagnosis is important for avoiding severe eye problems. However, conventional methods for monitoring physiological signs relevant to eye problems usually require complex equipment and professional operators. To address this issue, we have developed a chameleon-inspired interactive contact lens sensor for real-time monitoring the eye-related physiological signs.
Upon the changes of the physiological signs (eg. moisture, pressure), the color of the contact lens will change simultaneously, according to the research published in the Journal of Materials Chemistry B.
https://pubs.rsc.org/en/content/articlelanding/2020/TB/C9TB02389E#!divAbstract
This contact lens sensor is made solely from a biocompatible and soft hydrogel with inverse opal structures, without the addition of any pigment, thus exhibiting superior biosafety and wearing comfort.
When the hydrogel is shrunk or compressed, the inverse opal structures are changed accordingly. The contact lens sensor can therefore show different colors in response to the changes of moisture and pressure.
Based on these properties, the contact lens sensor has been investigated for monitoring the amounts of tears and intraocular pressure, which are of significant pathologic relevance to xerophthalmia and glaucoma, respectively.
We observe that their colors do not change in normal eye-simulation conditions over time but change from red to blue in the xerophthalmia-simulating conditions in about 25 minutes. Also, we observe decreases in the wavelength of the reflectance peak for the “smart” contact lens sensors when intraocular pressure reaches pathological values.
This study implies a smart wearable device for early diagnosis of diseases in eye. It also offers the inspiration for the design of new-generation wearable biodevices for real-time colorimetric sensing various human body signs and diseases.
Further researches realize its optical interactivity
In nature, chameleons usually sense environmental light and change the color accordingly. However, it is challenging to mimic the light-responsive color-shifting behaviors of chameleons by bio-inspired sensors. Recently, we have prepared a chameleon-inspired sensor with the capability of interacting with light, according to the research published in the Materials Horizons.
https://pubs.rsc.org/en/content/articlelanding/2020/mh/d0mh00150c#!divAbstract
The light-interactive chameleon-inspired sensors are formed by using a shape memory polymer with inverse opal structures. Their inverse opal structures can be programmed among various temporary states and a permanent state by compression and laser irradiation. The different states of the inverse opal structures correspond to diverse colors. Through locally regulating the specific states of the shape memory polymers by tuning different light irradiation time, the sensors can display assigned multi-color patterns.
With excellent light interactivity, the chameleon-inspired sensors have shown promise in information storage, such as employed as rewritable papers enabling inkless multi-color laser-direct writing and laser-scanning copying. Such capabilities make them as an excellent alternative for office papers. They may revolutionize the paper and printing industries that cause too much consumption of forests and too heavy burden to ecosystems now.
Functional upgrade: Shape-transformation, color-shifting and movable actuators at the same time
Going forward, we are eager to further develop interactive sensors and actuators that can not only sense the environments and report by changing colors but also actuate and even move like a real chameleon, which is an unmet task by previous studies.
As the first step, we have successfully developed intelligent actuators that can transform, move, and change their colors, responding to the environmental changes, according to the research published in the Matter, a sister journal of Cell.
https://www.sciencedirect.com/science/article/pii/S2590238519300372
The main bodies of the actuators are composed of the “smart” polymers that can sense chemical vapors. Once exposure to specific chemical vapors, the polymer matrix can absorb the chemicals and expand, subsequently leading to changes in both reflection index and the spacing of the inverse opal structures.
On basis of that, the chameleon-inspired actuators realize not only shape transformations such as the rotation of pinwheel and the blooming of flower but also changing colors at the acetone atmosphere.
Moreover, we achieve the worm-like walking after designing “legs” for generating asymmetrical forces, showing the abilities of reporting the environment they arrive by changing colors in a real-time manner.
It could even change the color and form simultaneously
The above-mentioned actuators have demonstrated excellent interactive capabilities of soft robotics with human-machine interaction. However, their locomotion capabilities are still at relatively initial levels which cannot accommodate complicated terrains and landforms. To achieve this goal, a hydrogel-based millirobot with the capabilities of multimodal locomotion, shape adaptation and color changing has been developed, according to the research published in the Advanced Functional Materials.
https://onlinelibrary.wiley.com/doi/10.1002/adfm.201909202
The millirobot consists of a composite head of hydrogel with embedded magnetic particles and a hydrogel tail with an inverse opal structure. Through magnetic-guided vibrations of its head in different modes, the millirobot shows multimodal locomotion behaviors including helical propelling over obstacles, crawling within a low tunnel, and swinging through a narrow channel.
Based on the thermoresponsive characteristics of the hydrogel body, a laser or changes in temperature can alter its volume. We can therefore achieve the robot to squeeze the body size irradiated by a laser and change the color by changing the environmental temperature. Such capabilities bring the robot with enhanced environmental adaptability and interactivity.
The possibilities for future applications are endless
In general, the environmentally adaptive and interactive behaviors of chameleons have inspired the developments of diverse sensors and actuators with enhanced human-machine interaction. Many application areas ranging from biomedicine to robotics have been benefited from them. And we can still envision that, with the assistance of the chameleon-inspired interactive sensors and actuators, some imaginations in science fictions such as humans and robots with color-changing skins for active camouflage and information interaction can be realized soon.
About Prof. Xuemin Du
Xuemin Du is a full Professor at Research Center for Nanobiomechanics, Shenzhen Institutes of Advanced Technology (SIAT) of the Chinese Academy of Sciences (CAS). He received dual Ph.D. degrees (2012) from University of Science and Technology of China and City University of Hong Kong. He was awarded with “Member of Youth Innovation Promotion Association, CAS”, “Guangdong High-level Personnel of Special Support Program-Outstanding Young Scholar in Science and Technology Innovation”, “Oversea High-Caliber Personnel”, and “High-Level Professional Talent”. His research interests cover mainly bio-inspired smart materials and adaptable biodevices, such as smart polymers, photonic crystals, soft actuators, flexible electronics and tissue engineering.
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The smart polymers market is expected to be driven by the surging use in the textile industry, for 3D printing applications, increasing healthcare and biomedical expenditure, the need for efficient delivery systems, and the development of thermoresponsive polymers.
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