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Article

Bioinspired light-driven photonic crystalactuator with MXene-hydrogel muscle

Mingzhu Li, Lei Yuan, Yifan Liu,

Florian Vogelbacher, Xiaoyu

Hou, Yanlin Song, Qunfeng

Cheng

mingzhu@iccas.ac.cn

Highlights

Responsive photonic crystal (PhC)

films mimicking the flapping of

butterflies

PhC pixels with rapid and

programmable color change over

the whole visible light

Durable and sensitive PhC

actuator, exhibiting stable

performance after 500 cycles

Responsive structural color devices with stable structure and good repeatability

have attracted the attention of many scientists. Li et al. report a display using

photothermal-response hydrogel muscles to actuate photonic crystal pixels, which

are capable of robust and precise color changing.

Li et al., Cell Reports Physical Science 3, 100915

June 15, 2022 ª 2022 The Author(s).

https://doi.org/10.1016/j.xcrp.2022.100915

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Article

Bioinspired light-drivenphotonic crystal actuatorwith MXene-hydrogel muscle

Mingzhu Li,1,2,4,5,* Lei Yuan,1,2,4 Yifan Liu,1,2,4 Florian Vogelbacher,1 Xiaoyu Hou,1,2 Yanlin Song,1,2

and Qunfeng Cheng3

1Key Laboratory of Green Printing, Institute ofChemistry, Chinese Academy of Sciences, Beijing100190, China

2School of Chemistry and Chemical Engineering,University of Chinese Academy of Sciences,Beijing 100049, China

3School of Chemistry, Key Laboratory ofBio-inspired Smart Interfacial Science andTechnology of Ministry of Education, BeijingAdvanced Innovation Center for BiomedicalEngineering, Beihang University, Beijing 100191,China

4These authors contributed equally

5Lead contact

*Correspondence: mingzhu@iccas.ac.cn

https://doi.org/10.1016/j.xcrp.2022.100915

SUMMARY

The butterfly Apatura ilia’s wings change colors during flapping,which plays a major role in conveying information and avoidingpredators. Inspired by the muscle-driven flapping mechanism,here, we fabricate a highly efficient photothermally responsiveMXene-hydrogel muscle and achieve a dynamic structural color im-aging system based on the MXene-hydrogel muscle manipulatedphotonic crystal actuator arrays. Our optimized MXene-hydrogel-muscle artificial muscle has a stable and quick photothermalresponse owing to the high photothermal transformation efficiencyof MXene (�100%). Consequently, our photonic crystal (PhC) actu-ator exhibits robust structural stability and fatigue resistance(more than 500 cycles), and its response time is �5 s. The PhC actu-ator can give real-time visual feedback in response to heating andnear-infrared irradiation. They offer exciting possibilities for appli-cations in sensors, displays, camouflage coatings, cryptography,and many other fields. Our experiments and theoretical analysisreveal the quantitative structure-activity relationship of the respon-sive PhC actuator.

INTRODUCTION

Our world provides us with an astonishing diversity in coloration. Nature’s color

occurs in two forms: pigments and structural colors. The brilliant colors of

chameleons,1 peacocks,2,3 and butterflies4 are due to delicate micro-/nanostruc-

tures that have evolved for the purposes of intimidating predators, communication,

courtship, and self protection. These colors are named ‘‘structural color,’’ a kind of

coloration caused by the interaction of visible light with micro-/nanostructures. Tak-

ing advantage of excellent durability, non-photobleaching characteristics, and envi-

ronmental friendliness, structural coloration has long interested scientists in devel-

oping adaptive camouflage,5 smart coatings,6 solar arrays,7 biomimetic tissues,8

and many other fields. Many photonic nanomaterials with vivid structural colors

have been developed, especially for responsive photonic crystals (PhCs), which

can be exploited for applications in chemical and biological sensors, and intelligent

displays.9–12 Responsive PhCs are materials that can display optical changes in

response to external stimuli, such as light,13 heat,14 electricity,15 magnetism,16,17

humidity,18,19 strain,3 and pH.20 The principles of the stimuli-responsive PhCs re-

ported so far to generate visual optical signal variations are based on the change

of the lattice constant of PhCs or the refractive index of the materials under external

stimuli. In most cases, the delicate micro/nano optical structures in the film are

readily destroyed, affecting the optical properties and structural stability of the

Cell Reports Physical Science 3, 100915, June 15, 2022 ª 2022 The Author(s).This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1

Figure 1. MXene-hydrogel actuating PhC film inspired by the wings of the Apatura ilia

(A) Optical photographs of Apatura ilia wings flattened (left) and lifted (right). The wing’s color is changed with the wings flapping through the relaxation

or contraction of the muscles.

(B and C) The SEM images of the scale of the wing. The color of the wings is assigned to the period nano-photonic structure. Scale bars: 100 mm (B), 1 mm

(C), and 200 nm (C, inset).

(D) Snapshots of the artificial flapping butterfly viewed from side and top. Scale bar: 5 mm.

(E) (i) Optical photos and (ii) normalized scattering spectra of PC-24K at different view angles from 15� to 55�and a fixed incident angle of 25�. (iii) Thetop-view scanning electron microscope (SEM) image of the PhC film deposed on the surface of a polished, thin stainless-steel sheet. Scale bar: 1 mm.

(F) Schematic diagram of the lifting process of the PhC film. The PhC film lifted up and down driven by NIR irradiation and moisture due to the shrinkage

and swelling of the printed MXene-pDADMAC.

(G) The mechanism of the color changing as the wing flapped.

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PhCs. Thus, these approaches are limited by poor repeatability. In addition to the

dependence of the lattice constant or the refractive index, most of the structural

colors of PhC films are angle dependent due to their pseudo band gap.21,22 If we

could manipulate the flow of light and the direction/orientation of PhC films, this

would enable us to obtain responsive structural color systems.23,24 In nature,

many species are capable of adaptive shape changes, diverse geometric morphing,

and highly efficient locomotion through muscles.25,26 For example, the butterfly

Apatura ilia can change colors from tawny to bright purple through muscle-driven

wing flapping (Figures 1A–1C).

Inspired by muscle-driven wing flapping, we fabricated PhC pixel arrays manipu-

lated by soft actuators and achieved photothermally triggered structural color imag-

ing. The soft actuator is made from MXene-doped polydiallyldimethylammonium

chloride (pDADMAC) (MXene-pDADMAC), serving as the artificial muscle to actuate

the PhC film (Figure 1D).27 We demonstrated that MXene can maximize the

2 Cell Reports Physical Science 3, 100915, June 15, 2022

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efficiency, speed, and sensitivity of the photothermal response of the pDADMAC

due to its internal light-to-heat conversion efficiency approaching 100%.28–32

Without changing the fine nanostructure and taking advantage of the good

robustness and quick response of the optimized MXene-pDADMAC, our PhC film

exhibits robust structural stability and fatigue resistance (more than 500 cycles),

and its response time is �5 s. Moreover, the flexible PhC film we fabricated has

high reflectance (�28 times higher than that of the PhC on glass substrate) due to

the high reflectance of the substrate, a polished, thin stainless-steel sheet with

high gloss (�90% in reflectance, 10 mm in thickness).33 The colors of the PhC pixel

arrays change over the whole visible range from blue to red (420–670 nm) through

near-infrared (NIR) ray irradiation and can be fixed at any certain color precisely by

controlling the irradiation time. The brilliant colorful imaging can be easily scribed

using an NIR light and erased by moisture absorption. The color imaging is highly

controllable, durable, reversible, and repeatable. Moreover, taking advantage of

the patterning capability of the printing technique, we fabricated programmable

and designable self-feedback intelligent PhC actuators by adjusting the position

and orientation of the muscle pattern. These versatile and powerful deformable

PhC actuators have advanced motion controls and real-time color feedback. Addi-

tionally, hydrogel actuators, smart responsive polymers, and other components

that can convert external energy (such as pH, light, heat, magnetic field, and ion

strength) into mechanical motion all can serve as man-made soft actuators.34–40

Thus, we believe that the actuator-manipulated PhC film can shed new light on

the construction of responsive structural colors. This is expected to exhibit great

application potential in smart sensing, visual displays, and environmental

monitoring.

RESULTS AND DISCUSSION

An artificial PhC butterfly integrating with MXene-pDADMAC soft actuator

We made an artificial butterfly integrating with MXene-pDADMAC soft actuator at

the joints between the wing and the body (Figures 1D–1F and S1). The wing of the

butterfly is made from a close-packed monolayer of polymer spheres deposited

on a high-brightness stainless-steel sheet (�10 mm in thickness) through the interfa-

cial self-assembly method (Figure S2). An aqueous solution of MXene-pDADMAC

was printed at the joint to form a muscle structure. The scanning electron micro-

scope (SEM) images clearly show that the close-packed polystyrene (PS) micro-

spheres are in long-range-ordered arrangement and give rise to an angle-depen-

dent structural color, exhibiting different colors with the change of the viewing

angle (Figures 1E and S2; Video S1). The two-dimensional PhC (2D PhC) material ex-

hibits red-shifting scattering peaks from �420 (purple) to �670 nm (red), with

observed angles ranging from 26� to 60� (Figures 1Ei and 1Eii). Figure 1F shows a

schematic diagram of how the MXene-pDADMAC manipulates the butterfly’s

wing to flap. pDADMAC is a cationic polyelectrolyte, which is responsive to heat

and humidity. The shrinkage and expansion of the pDADMAC leads to the reversible

up-down lifting of the PhC sheet similar to a muscle (Figure 1Fii). MXene has high

photothermal transformation efficiency close to 100%.19 The introduction of MXene

into pDADMAC can boost the shrinkage of the muscle and induce quick lifting of the

PhC sheet. The color of the artificial butterfly’s wing changes from purple to red as

the wing lifts up from 15.1� to 50.8� under NIR light irradiation. After removing the

NIR source and humidifying the sample, the wing’s PhC film returns to its initial state

(Video S2). The change of the observed color is a result from the angle dependent

diffraction of incoming light at the PhC sheet. Here, the irradiation light source

and the camera are both fixed during taking the optical photos and movies of the

Cell Reports Physical Science 3, 100915, June 15, 2022 3

Figure 2. High-brightness diffraction and simulation of 2D PhC

(A) Mechanism giving rise to the high-efficiency diffraction.

(B) Reflectance of the 2D PhC assembled on glass and steel sheets with different gloss levels named

2B, 8K, 12K, 16K, and 24K.

(C) Numerically (left) and experimentally (right) determined dependence of diffraction and

reflection spectrum on the emission angle for an unpolarized plane wave excitation of the 2D PhC

high-reflectivity substrate. The input angle is 25�.(D) Actuation performance of experiment and FEA of PhC film with the MXene-pDADMAC soft

actuator (1:50) and the corresponding color.

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wing flapping. When the artificial wing lifted up, the incident angle and the view

angle from the PhC film both change, resulting in the observed color change (Fig-

ure 1G). In Figure 1G, the reflected white light results from the specular reflection

of the incident light mainly from the high-brightness stainless-steel substrate and

partially from Fresnel reflection at the PhC film. The input angle qin refers to the angle

between the incident light and the normal of the PhC film, and the view angle qout

refers to the angle between the observation orientation and the normal of the PhC

film.

When a beam of light is incident on a 2D PhC film at an angle qin, the relationship

between wavelength and diffraction angle follows the grating equation

ml = dðsin qout-sin qinÞ; (Equation 1)

wherem is the diffraction order, l is the wavelength, d is the grating period, qin is the

incident angle, and qout is the diffraction angle. When the artificial butterfly wings are

lifted, the normal direction of the PhC changes and, subsequently, the incident angle

qin changes, so the wavelength l of the diffraction light at a specific observation di-

rection will change, thus leading to a new observed color.

Highly bright angle-dependent color of 2D PhC

We theoretically and experimentally demonstrated that the 2D PhC deposited on a

highly reflective steel substrate presents bright angle-dependent color. Figures 2A

and S3 show the mechanism of creating bright colors induced by our designed

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structure. The high-efficiency diffraction results from the combination of Bragg scat-

tering of the 2D PhC and good feedback of the highly reflective substrate. The sur-

faces of the polished stainless-steel sheets had a mirror gloss. When the 2D PhC is

irradiated, the incident light is diffracted back to the observer on the incident

side, thereby generating a weak diffracted beam (D1). The transmitted first-order

diffracted beam is reflected by the surface of the stainless-steel substrate back to

the observer, forming a strong beam (D2). The remaining incident light passing

through the 2D PhC is reflected by the substrate surface and diffracts from the 2D

PhC. Part of this beam is diffracted forward forming beam (D3), and the other is re-

flected by the substrate (D4). Therefore, the intensity of the diffracted light is the sum

of the intensities of the beams diffracted by 2D PhC from the top (D1) and the feed-

back beams tuned by reflection from the high-gloss steel sheet surface and trans-

mitted through the 2D PhC (D2, D3, and D4). This intensity is much higher than

that of the 2D PhC on glass (D1).

The polished stainless-steel sheets exhibit a broadband optical reflection in the

visible range arising from the free electrons present in the metal and the resulting

resonant suppression of the forward radiation. According to their surface gloss,

the steel sheets are named 2B, 8K, 12K, 16K, and 24K (Figure 2B). Their surface

roughness values are 65.4, 1.26, 1.08, 0.675, and 0.624 nm, respectively (Figure S4).

It is clearly seen that the reflectance of the steel sheet is greatly enhanced with the

increasing level of smoothness (Figure S5). The reflectance of the 24K-steel sheet

is more than 90% over the whole visible wavelength range from 400 to 800 nm.

From the spectral characteristics shown in Figure 2B, the enhancement factors at a

wavelength of 555 nm increase with the surface quality from h2B = 8 to h24K = 28,

where the substrate enhancement factor h = R/R0 is introduced, defined as the ratio

between the spectral reflectance R from a sample with a stainless-steel substrate and

the reflectance R0 of the reference sample with glass substrate. Additionally, the full

width at half maximum (FWHM) is reduced from 44 nm for the 2B stainless steel to

33 nm for the 24K stainless steel, which can be attributed to the increased reflection

efficiency at the metal interface and the improved quality of the PhC from the

reduced surface roughness. The narrow FWHM of these sharp scattering peaks gua-

rantees better saturated structural colors. In summary, the structural color of the

PhC-steel sheet is brighter and spectrally narrower than that of the PhC-glass sample

due to the synergy between the high reflection of the steel sheet and the diffraction

at the 2D PhC.

To further study the optical coupling of the specular stainless-steel sheet and the

2D PhC, the propagation of light inside the composite structure was numerically

analyzed by the finite-difference time-domain (FDTD) method. Figures 2C and

S6 show the simulated and experimental results of angle-dependent diffraction

spectra of the 2D PhC structure assembled on a perfect electric conductor (PEC)

layer for an unpolarized plane-wave excitation at an input angle qin = 25� with

respect to the surface normal. The FDTD simulation shows that the optical intensity

of both the diffracted light and the reflected light of a 2D PhC assembled on a

mirror substrate is significantly higher than that of a 2D PhC on a glass substrate,

which is consistent with the experimental results. The enhancement hPEC for the

diffraction efficiency improvements when a highly reflective substrate is employed

can be derived from the reflectance data shown in Figure S6, which results in an

enhancement factor between �20 to �65 depending on the wavelength. For

example, at a wavelength of 560 nm, the numerically determined reflection of a

PhC on a PEC substrate is enhanced by hPEC = 25 compared with a glass substrate.

Cell Reports Physical Science 3, 100915, June 15, 2022 5

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This is in general agreement with the experimentally observed enhancement of up

to h24K = 28.

The output angle qout of the reflected light is at qout= 25�, which is consistent with theinput angle. In contrast to the reflection, the diffraction at the periodic structure

leads to a dispersion of the light into angles ranging from approximately -60� to

-25�, which creates the vivid colors observed in the experiments. The wavelength-

dependent diffraction angle qout is determined from the Bragg diffraction

2p

lsinðqinÞ + m

2p

L=2p

lsinðqoutÞ; (Equation 2)

where L = ðO3 aÞ=ð2Þ denotes the Bragg period for a hexagonal lattice with lattice

constant a, wavelength l, diffraction order m, and the input angle qin and output

angle qout. For the studied PhC and a wavelength between 400 and 700 nm, only

the diffraction order m = 0, i.e., reflection, and m = -1 satisfy the Bragg equation.

This leads to an efficient diffraction into the backward direction with suppressed

higher-order diffraction.

The dispersion of the incoming light into different angles derived from Equation (2) is

given by

v

vlqout =

m

L

1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1-�sin qin + lm

L

�2q (Equation 3)

and results in -0.131 deg nm-1 at 500 nm, which is consistent with the slope from Fig-

ure S6 of approximately -0.13 deg nm-1. Both the diffracted light and reflected light

show complementary spectral characteristics from coupling to PhC modes.

Programmable color changing through muscle-driven flapping

The PhC film exhibits sensitive morphing behavior in response to temperature,

which is attributed to the difference in thermal strains between the MXene-

pDADMAC and the stainless-steel sheet layer. In order to understand the activa-

tion behavior of the MXene-pDADMAC muscle toward the flapping of the PhC

film, we established a mechanical model for force analysis and performed finite

element analysis (FEA) to analyze the microscopic stress of the interaction between

the MXene-pDADMAC layer and the substrate (Figures S7–S12; Videos S3 and S4).

The detailed parameters of the model are shown in Tables S1–S3. In our model,

the MXene-pDADMAC layer and the stainless-steel sheet layer are set as linear

elastic materials, and thermal strain is used as the activation force for model defor-

mation during calculations. The left end of the PhC film is fixed to the substrate.

The contraction force at the interface between the MXene-pDADMAC layer and

the substrate is the reason for the deformation of the PhC film. The stainless-steel

sheet does not shrink, and its compressive stress comes from the external force

generated when MXene-pDADMAC shrinks. Under the illumination of NIR, the

pDADMAC shrinks due to water vaporization (the relative humidity of the environ-

ment is about 40%–60%). Since MXene-pDADMAC adheres to the substrate, the

resulting intrinsic stress on the interface caused by asymmetric thermal expansion

produces a bending movement, making the PhC sheet curl. The relationship be-

tween the coefficient of thermal expansion of MXene-pDADMAC and temperature

is given as

a = lnðL = L0Þ=ðT-T0Þ; (Equation 4)

in which L is the length of the hinge, L0 is the initial length, T is the temperature of the

hinge during the NIR irradiation, and T0 is the initial temperature (25�C). Figure 2D

6 Cell Reports Physical Science 3, 100915, June 15, 2022

Figure 3. The actuation mechanism of PhC film

(A) Schematic diagram of the actuation of PhC film. The MXene-pDADMAC shrinks under the NIR

irradiation, leading to bending of the steel sheet.

(B) AFM image and cross-sectional profile of MXene. The thickness of the MXene is around 1.5 nm.

(C) Color switching of the PhC film under stimulation of NIR and moisture. The PhC film lifts and the

color changes from gray to purple, blue, yellow, and finally red under NIR irradiation. Then, the

colors recovers when the lifting angle returns to the same degree after moisture absorption. Scale

bar: 5 mm.

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shows the relationship between the temperature of MXene-pDADMAC and the lift

angle of the PhC film during NIR irradiation. The result shows that the simulation

data are consistent with the experimental data, demonstrating that the movement

of the PhC film can be accurately predicted and showing great potential for a

controllable structural display.

Figure 3 presents the activation fundamentals of the MXene-pDADMAC muscle un-

der NIR light. pDADMAC has thermomechanical responsivity, and MXene has excel-

lent photothermal conversion efficiency under NIR irradiation. MXene can quickly

convert light energy into heat when exposed, while the enriched NH4+ groups in

pDADMAC provide hydrophilic sites for ultrafast water diffusion and reversible hy-

dration/dehydration upon thermally provoked relative-humidity changes. The

desorption of water under thermal conditions causes the MXene-pDADMAC to

collapse vertically and contract laterally, resulting in a macroscopic thermomechan-

ical conversion. Figure 3B shows the atomic force microscopy (AFM) image and the

cross-sectional profile of MXene. The size of MXene is about 5 mm in length (also

seen in SEM image; Figure S15A), and the thickness is about 1.5 nm. We explored

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the influence of different MXene concentrations on the actuation performance of the

MXene-pDADMAC muscle, as shown in Figure S14. The photothermal conversion

rate of the muscle increases with the increase of MXene concentration, leading to

a rapid enhancement in actuation speed. The actuation speed of the muscle

(MXene:pDADMAC = 1:50) is 2.33�/s, which is 2.2 times faster than that of pDAD-

MAC (0.72�/s). The cross-sectional SEM image shows that MXene is well distributed

in pDADMAC (Figure S15B). However, when the MXene concentration is over 2%,

the actuation performance of the muscle is reduced. This is because a large amount

of MXene aggregation is formed (Figures S15C and S15D), harming the photother-

mal conversion efficiency of MXene-pDADMAC, and the excessive proportion of

MXene reduces the driving component content of pDADMAC as well as the ratio

of the volume changing.

Figure 3C exhibits the reversible deformation and color cycle process of the PhC film

in an actuation cycle exposed to NIR and humidification (Video S5). Under the stim-

ulation of NIR, the PhC film gradually lifts from the initial flat state. The color of the

PhC film changes from the gray of the substrate to purple (16.4�), cyan (21.6�), yellow(25.7�), and red (33.3�), covering the entire visible-light spectrum. It is noteworthy

that the lift angle increases linearly with theNIR irradiation time (Figure S14B). There-

fore, the pDADMAC layer can achieve precise control of the deformation and color

of the PhC film by controlling NIR irradiation time (Figure S16).

When the NIR light is turned off and the hinge is humidified, the PhC film can return

to its original flat state with a reverse color change. Thanks to the direct relationship

between color and angle, by changing the irradiation power and irradiation time of

the NIR light to the sample, precise control of the PhC-film deformation can be

achieved. By repeating theNIR on-off exposure process, the wavelength of the sheet

color could vary stably between 420 and 670 nm. The results demonstrate that the

PC film’s color is reversible and can be precisely controlled.

Photothermally triggered PhCs display versatile colorful actuators

Based on the precise control of the PhC film, we fabricated a photothermally trig-

gered PhC display from the PhC pixel arrays integrated with an MXene-hydrogel

soft actuator (Figure 4). The simple approach to fabricate the PhC device is shown

in Figure 4A. First, the stainless-steel sheet substrate is cut into a 5 3 7 array by a

laser-cutting machine, and each small unit represents a rectangular flap gate. Sec-

ond, a close-packed 2D PhC is prepared by self-assembly of spreading a monolayer

of particles on the surface of the substrate. Then,MXene-pDADMAC is printed at the

junction of each small unit and the substrate. The original array shows a purple color

with a typical reflection peak around 430 nm. The unit lifts up and shows a different

color from the substrate under NIR. After removing the illumination, the lift angle of

the illuminated unit is fixed. The elevated unit can return to its original flat state after

humidification. An NIR laser beam scans the hinges along defined paths on the PhC-

film array sequentially for the same amount of time, and a pattern related to the scan

path can be shown. By setting up an appropriate scan pathway, different information

appears on the pixelated PhC array, which is erasable and rewriteable. Figures 4B

and S17 show the top and side view of the letters ‘‘ICCAS’’ in the array. It should

be mentioned that the pixels in the pattern are actually formed one by one, so mul-

tiple NIR irradiations are required. This PhC-array display device realizes precise

control of the structural color actuator unit on deformation and color display and

further provides the possibility of manufacturing visual intelligent display devices.

Even after 500 cycles, no deterioration is observed in the activation performance

of the PhC array, as presented in Figure 4C.

8 Cell Reports Physical Science 3, 100915, June 15, 2022

Figure 4. Patterned display of PhC film array

(A) Schematic illustration of the preparation of PhC array. The NIR laser was used to write the

pattern on the PhC-film array.

(B) Top view of the letters ‘‘ICCAS’’ written by NIR scan on a 5 3 7 PhC-film array. Scale bar: 1 cm.

(C) Plots of the reversible color change over 500 cycles.

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Moreover, our strategy can be exploited to fabricate versatile and powerful deform-

able structures with advanced motion control and real-time color feedback. Taking

advantage of the patterning capability of the printing technique, we built a program-

mable and designable self-feedback intelligent PhC actuator by adjusting the posi-

tion and orientation of the muscle pattern. For example, we printed MXene-

pDADMAC onto the surface of the prepared PhC sheets in three different directions

along the length direction (parallel [0�], perpendicular [90�], and 45� arranged) to

exhibit different driving behaviors. Figure 5A shows that the prepared PhC actuators

deform continuously and predictably with a vivid color change. Under NIR irradia-

tion, the structural color actuators oriented at 0�, 45�, and 90� bend into tube, helix,

and ring shapes, respectively (Video S6). The bending direction is perpendicular to

the muscle orientation direction. After removing the NIR irradiation, the structural

color actuator can undergo a reversible shape change to restore its original shape

by humidifying. Based on the same mechanism, we also made a blooming artificial

PhC flower and self-folding PhC cube actuated by pure pDADMAC (Figures 5B and

5C; Videos S7, and S8). Our strategy holds great potential for fabricating a soft robot

with visible self feedback. As a proof of concept, we have also prepared a crawling

robot that can move spontaneously when exposed to the illumination and humidity

alternately (Figure 5D). Many reptiles, such as worms and snakes, move forward

through asymmetric friction. Inspired by this, we curled the head of the actuator

slightly and designed the tail to be zigzagged to generate asymmetric friction

Cell Reports Physical Science 3, 100915, June 15, 2022 9

Figure 5. Controllable deformation guided by preprogrammed hinge orientations

(A) NIR-driven deformation of the structural color actuator with different hinge orientations (0�, 45�, and 90�).(B and C) The self-folding cube and the closing flower under NIR irradiation.

(D) The structural color robot capable of crawling forward, driven by NIR irradiation and moisture. Scale bar: 1 cm.

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between the fore and hind parts. MXene-pDADMAC is printed on the back of the

substrate in a 90� orientation. Upon exposure to the illumination, the front and

back of the crawling robot move to the middle, forming an arch. After removing

the illumination, the arched crawling robot stretches to its original shape by humid-

ifying, resulting in a forward slipping of the fore part with the hind part pinned. Due

to the asymmetrical friction, the robot can crawl forward under the cyclic stimulation

of illumination and humidity, accompanied by a dynamic change of color (Video S9).

Additionally, we prepared a portable temperature sensor that can monitor water

temperature (Figure S18; Video S10). The color of this sensor is red when the water

temperature is high (>40�C), while it displays green when the temperature drops to a

level suitable for drinking. With the ability to interact with the environment, these

structural color actuators provide a method for the preparation of various intelligent

actuators and soft robots that can sense and respond to the environment.

In conclusion, we developed a dynamic structural color device using a PhC film acti-

vated by a soft actuator. We created a durable and sensitive actuator using photo-

thermal hydrogel and MXene exhibiting a rapid dynamic photothermal response

and made an artificial muscle to activate bending of the PhC films by directly print-

ing. The PhC actuator shows robust structural stability and fatigue resistance. The

PhC actuator integrating the soft actuator and PhC film presented here offers a

pluripotent platform for new display and sensor technologies. For example, soft

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actuators can easily be fabricated from smart-response materials that have different

chemical, electronic, and mechanical properties, which creates opportunities for

further development of self-actuated PhC functional devices that respond to light,

electronic or magnetic fields, changes in temperature, or chemical signals. The addi-

tion of elements such as bimorphs or chemical tags to self-actuated PhC devices

could create environmentally responsive metamaterials. This design is expected

to have a wide range of applications in environmental monitoring, smart sensing,

and visual display.

EXPERIMENTAL PROCEDURES

Resource availability

Lead contact

Further information and requests for resources and materials should be directed to

and will be fulfilled by the lead contact, Professor Mingzhu Li (mingzhu@iccas.ac.cn).

Materials availability

This study did not generate new unique reagents.

Data and code availability

All data supporting this study are available in this paper and in the supplemental

information.

The data supporting the plots in this paper and the other findings of this study are

available from the corresponding author upon reasonable request.

Materials

Polystyrene (PS) nanoparticles with a size of 600 nm were purchased from HugeBio

(Wuhan, China). pDADMAC (20 wt % in water) was used as received from Aladdin

Chemistry. Anhydrous ethanol was obtained from Commercial Alcohols (Concord,

Tianjin).

Numerical simulations

The FDTDmethod was employed to numerically calculate the response of the PhC to

a plane-wave excitation at an input angle of -25� to unpolarized light. The 3D-simu-

lation domain was based on one unit cell of the hexagonal lattice with Bloch bound-

ary conditions for the periodic dimensions. The orientation of the lattice to the plane

of incidence was in the G�M direction. A perfectly matched layer (PML) was set as

the top boundary. Depending on the type of substrate, either a PML boundary inside

a glass substrate (nglass= 1.52) or a PEC boundary acting as a perfect mirror replacing

the substrate were employed. The refractive index of the PS spheres was set to h =

1.6 + 0.05i. The small complex loss term in the refractive index ensured stable

convergence and reduced artificial resonances.

Characterization

The photographs and videos were taken with a digital camera (Canon, EOS 700D,

Tokyo, Japan). SEM images were recorded using a field-emission SEM (JEOL,

JSM-7500F, Tokyo, Japan) to determine themorphologies of the 2D PhC. Scattering

spectra of the 2D PhC materials were measured by a spectrometer (Ideaoptics, R1,

Shanghai, China). The stainless-steel sheets (10 mm in thickness) were cut by a laser-

cutting machine (Leapion, LC-1390, Jinan, China). NIR was obtained by 808 nm

adjustable focal length NIR point laser (FU, FU63511L5-BD10, Shenzhen, China).

The temperature change data during the NIR irradiation process were obtained

by a thermal imager (FLIR, A655sc, Wilsonville, OR, USA).

Cell Reports Physical Science 3, 100915, June 15, 2022 11

llOPEN ACCESS Article

SUPPLEMENTAL INFORMATION

Supplemental information can be found online at https://doi.org/10.1016/j.xcrp.

2022.100915.

ACKNOWLEDGMENTS

This work was financially supported by National Natural Science Foundation of China

(22073107, 52125302, 22075009, and 21522308), the National Key R&D Program of

China (grant no. 2018YFA0208500 and 2021YFA0715703), the Bureau of Interna-

tional Cooperation, and the Chinese Academy of Sciences (GJHZ201948,

121111KYSB20150022).

AUTHOR CONTRIBUTIONS

M.L. conceived the project and designed the experiments; L.Y. and Y.L. carried out

the experimental setup, fabrication, and measurements; L.Y. and F.V. worked on the

simulations for the sample. M.L., L.Y., Y.L., F.V., X.H., Y.S., and Q.C. wrote the text.

DECLARATION OF INTERESTS

M.L. is an advisory board member at Cell Reports Physical Science.

Received: March 14, 2022

Revised: April 12, 2022

Accepted: May 5, 2022

Published: June 8, 2022

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