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Stimuli-Responsive Blue Phase Liquid Crystalline Photonic Crystal

Meng Wang , He Song , Yifei Zhu

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (12) : 2588 -2603.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (12) : 2588-2603 DOI: 10.7536/PC220528
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Stimuli-Responsive Blue Phase Liquid Crystalline Photonic Crystal
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Abstract

Intelligent materials with stimulus responsiveness become the research hotspots in recent years. Liquid crystal (LC) materials with supramolecular self-assembly nanostructure and stimulus responsive properties exhibit inherent advantages in the development of the novel intelligent functional materials. Blue phases (BPs), as the LC phases with exotic fluid self-assembled 3D periodic supernanostructures and the characteristic of selective reflection of circularly polarized light in visible light range, have been regarded as one kind of the most promising candidates for smart photonic crystals. The crystallographic parameters or phase states of BPs are susceptible to various external stimulation such as temperature, light irradiation, electric field or humidity. This causes a change in the photonic band gap of BPs, which exhibits visually reflection color variance. Therefore, BPs have recently drawn vast and increasing attentions due to their external-field responsive properties and great potential applications in intelligent materials. Herein, we provide the frontier research advancements in the stimuli-responsive blue phase liquid crystalline photonic crystals. The important research results obtained on the optical, magnetic, electrical, mechanical and humidity responsiveness of BPLC photonic crystals were introduced in detail. At the end of this review, the challenges and possible development direction of this novel soft matter intelligent material are prospected briefly.

Contents

1 Introduction

2 Supramolecular self-assembled structures and three-dimensional photonic band gap of blue phases

3 External-field responsiveness of blue phase liquid crystalline photonic crystals

3.1 Light responsiveness

3.2 Magnetic responsiveness

3.3 Electric field responsiveness

3.4 Mechanical responsiveness

3.5 Humidity responsiveness

4 Conclusion and outlook

Graphical abstract

Keywords

intelligent materials / blue phase liquid crystal / photonic crystal / stimulation response

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Meng Wang, He Song, Yifei Zhu. Stimuli-Responsive Blue Phase Liquid Crystalline Photonic Crystal. Progress in Chemistry, 2022, 34(12): 2588-2603 DOI:10.7536/PC220528

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1 Introduction

With the rapid development of science and technology, stimuli-responsive smart actuation materials have gradually become the focus of research. Intelligent response materials refer to special molecules with "intelligent" behavior, which can respond to external stimuli, such as temperature, light, electric field, magnetic field, pressure, humidity, etc., such as color changes, changes in physical and chemical properties[1⇓⇓⇓~5]. At present, according to the physical and chemical characteristics of materials, it can be divided into metal, inorganic non-metal and polymer intelligent materials. Among them, intelligent polymer materials have many kinds, simple preparation process and wide application range, and have great application potential in the fields of soft robots, artificial muscles, drug delivery, intelligent sensing and optoelectronics[6].
Responsive photonic crystals are considered to be a new type of smart materials and have broad application prospects in the next generation of nanophotonic technology because of their controllable periodic structure and effective control of specific frequency light waves. However, the fine microstructure and complex preparation process requirements have brought significant challenges to its development. Among many organic and inorganic materials, Liquid crystal (LC) materials have both the microscopic molecular arrangement structure of crystals and the macroscopic fluidity of liquids, which can self-assemble to form a certain periodic structure and maintain a highly sensitive response to external stimuli, so they are ideal materials for the preparation of responsive photonic crystals. Among many liquid crystal phases, the Blue phase (BP), which is located in a narrow temperature range between the cholesteric phase (N*) and the isotropic phase (I), is considered to be one of the most potential photonic crystal materials because of its unique self-assembled periodic cubic lattice structure, three-dimensional photonic band gap in the visible light band and excellent external field response[7][8][9⇓~11].
In order to realize the application of BPLC materials, researchers have done a lot of work in the temperature range stabilization, such as polymer network stabilization, hydrogen bond stabilization, nanoparticle doping, new biaxial molecular design and other methods, to achieve a significant improvement in the thermodynamic stability of BP. In 2002, Kikuchi et al. Added a small amount of acrylate and photoinitiator to BP small molecule system, and obtained polymer stabilized blue phase liquid crystal (PSBP) by in situ photopolymerization, which broadened the temperature range of BP to above 60 ℃[12]. PSBP has the advantages of wide temperature range, fast response and no orientation, which is one of the best preparation methods for BPLC optoelectronics applications[13,14]. In addition, wide temperature range BPLC can also be prepared by doping small molecular compounds with specific structures or nanomaterials with certain sizes. In 2009, Professor Yang Huai's research team of Peking University added LC molecules capable of forming hydrogen bonds to BPLC for the first time, which successfully broadened the temperature range of BP from 1 ℃ to 23 ℃[15]. In 2009, Yoshida et al. Widened the temperature range from 0.5 ° C to 5 ° C by incorporating gold nanoparticles into small molecule BPLC[16]. In addition, many researchers have used nanoparticles to stabilize the crystal structure of BP[17⇓⇓⇓~21]. In 2005, Yoshizawa et al. Found a wide temperature range of BP (about 13 ℃) in the T-type biaxial molecule, and then introduced a flexible molecular chain into the T-type structure, thus broadening the temperature range of BP to about 25 ℃[22]. Similarly, bent-type molecules with oxadiazole and thiophene as the central core have also been used to extend the temperature range of BP[23⇓⇓⇓~27]. In 2021, Professor Yang Huai's research team of Peking University made rod-like LC molecules fill in the disclination line position of BP self-assembled lattice structure through the cooperative self-assembly of dimeric liquid crystal and fluorine-containing liquid crystal monomer, thus broadening the temperature range of small molecule BPLC system to 130 ℃[28].
In recent years, due to its unique self-assembly structure and optical properties, BPLC materials have attracted the attention of many researchers in the fields of photonic crystal structure, phase transition process, orientation control and external field response. At present, a variety of stimuli have been used to trigger BPLC photonic crystal materials to respond, such as light, temperature, magnetic field, electric field, external force, humidity and so on. For small molecular liquid crystal systems, the photonic band gap can be shifted by inducing the change of crystal structure parameters or phase transition through temperature, photoisomerization molecules or doping of photoresponsive nanoparticles. For the liquid crystal/polymer composite system, the small molecule BPLC is combined with the polymer network or other stimuli-responsive molecules by covalent crosslinking or physical doping, so that the composite system has both the anisotropy of the liquid crystal material and the excellent mechanical properties of the polymer material. Through the stimulation of external force, electric field, humidity and so on, the crystal structure parameters can be changed to realize the movement of photonic band gap, so as to prepare intelligent BPLC photonic crystal films with different response characteristics. These unique stimuli-responsive behaviors are derived from the soft matter properties of BPLC and the tunable three-dimensional periodic self-assembly structure, and the related functional materials have the advantages of reprogrammability, repeatability and visualization, which lay the foundation for the development of BPLC in the field of stimuli-responsive smart materials. In this paper, the recent research results on the response performance of BPLC photonic crystals are summarized, the related response mechanism is analyzed, and the future development trend of smart response BPLC photonic crystals is prospected, in order to provide new ideas for researchers to design stimuli-responsive smart materials and devices with advanced performance and function.

2 Supramolecular self-assembly structure and three-dimensional photonic band gap of blue phase

Chiral liquid crystal materials have photonic crystal-like structures due to their spontaneous formation of periodic structures. For chiral smectic and N*, the PBGs are partially one-dimensional and one-dimensional, respectively, while for BP, the three-dimensional periodic structure in space determines that it has an incomplete PBG in three-dimensional space[11]. Among these liquid crystal phases, the photonic band gap and properties of N* liquid crystal have been studied most widely, and the existence of photonic band gap has been proved theoretically by solving the eigenvalues of Maxwell's equations. In application research, N* liquid crystal is considered to be an optical soft material with one-dimensional self-assembled spiral microstructure, which has excellent stimulus response performance.It can be driven by electric field, temperature, light or mechanical force to change the reflection color or shape of the material, and has important application prospects in liquid crystal lasers, optically encoded liquid crystal superstructures, multiple intelligent anti-counterfeiting and other fields[29⇓~31]. Compared with the N*, BP has a three-dimensional photonic band gap in the visible light band, and has better reflection band monochromaticity and narrow photonic band gap, which is more conducive to the control of ultra-long wavelength incident light in the three-dimensional direction. The theoretical calculation of BP photonic band gap has also made some breakthroughs, but due to the complexity of its structure, the theoretical research is difficult and the progress is slow. The early research is limited by the narrow temperature range, and the research and application of BP in the field of photonic crystals are relatively few. With the development of various methods to broaden the temperature range and the research and development of new materials, the self-assembly behavior of BPLC and the device preparation and application of stimulus-responsive BPLC materials have attracted wide attention in recent years[32⇓⇓⇓~36].
Microstructurally, BP is a supramolecular multilevel self-assembly structure. Compared with the helically twisted arrangement of N* molecules along one direction, the BPLC molecules are twisted along two different orientation axes, thus showing a double-twisted columnar arrangement in space, forming a double-twisted cylinder. These double-twisted cylinders are periodically arranged in space through supramolecular self-assembly, thus forming a certain crystal structure. At the same time, disclination lines are formed in the three-dimensional long-range space because the molecules at the intersection of the double twisted cylinders can not be arranged continuously. Therefore, BP is a thermodynamically stable liquid crystal phase coexisting with defects. Compared with other liquid crystal phases, BP is easy to dehelix and form N*, and its stability is poor, so its stable temperature range is narrow, usually only 0.5 ℃. According to the crystal structure formed by self-assembly, BP can be divided into three sub-phases, which are BPI, BPII and BPIII in turn with the increase of helical twisting force or temperature. Among them, BPIII is in an amorphous state, with a random arrangement of double twisted spiral cylinders, similar to the isotropic state, and usually appears as a dark blue or purple haze under a polarizing microscope[37]. BPI and BPII have body-centered cubic structure and simple cubic structure, respectively. Reflection wavelength and photonic band gap of the two According to the lattice reflection theory, when the lattice parameter is equivalent to the wavelength, Bragg reflection will occur, satisfying the following relationship:
λ= 2 n a h 2 + k 2 + l 2
In the formula, λ, n and a are respectively the wavelength of incident light, the average refractive index of liquid crystal and the lattice constant of BP; H, K, l are the Miller indices of the crystal plane. In BPI, H + K + l is an even number, and the usual diffraction plane families are { 110 }, { 200 }, { 211 }, { 220 }; In BPII, the usual diffractive crystal plane families are { 100 }, { 110 }, { 111 }. Since the lattice constant of BPII is smaller than that of BPI, the reflection peak of the (100) plane of BPII generally appears at the wavelength between the (110) peak and the (200) peak of BPI[38]. For BPII and BPI, the three-dimensional periodic structure in space determines that they have an incomplete photonic band gap in three-dimensional space. The crystal structure of BP can be observed by Kossel diffraction technique, transmission electron microscopy (TEM), small angle X-ray diffraction technique, etc[11][39][40][41]. Nondestructive observation of BPI structure was also achieved by confocal laser scanning microscopy (CLSM)[42]. BPI and BPII are widely considered as one of the most promising liquid crystal photonic crystal materials due to their three-dimensional self-assembled photonic band gap in the visible light band[43][44,45].

3 External Field Response of Blue Phase Liquid Crystal Photonic Crystal

Due to the soft matter properties of BPLC photonic crystals, the photonic band gap can be changed under external field stimulation. At present, a variety of stimuli, such as light, magnetism, force, electricity and humidity, are used to dynamically regulate its photonic band gap in the visible range. The study of external field response characteristics and the fabrication of functional photonic crystal devices have become the focus of current research in this field. The following is a brief review of the relevant research work in recent years according to the different ways of stimulation.

3.1 Photoresponse

In the aspect of photoresponse of BPLC, the method of doping photosensitive molecules with different shapes and characteristics into BPLC is mainly used to control the photonic band gap or refractive index of BPLC. The optical properties of liquid crystals can be changed by indirectly inducing the phase transition or changing the lattice constant through the photoisomerization of photosensitive molecules. Photoisomerized molecules contain photochromic units such as azobenzene, spiropyran, furan ether, etc[46⇓⇓~49][50⇓⇓⇓~54][55]. Among them, azobenzene compounds have attracted much attention due to their fully reversible photoisomerization process and good miscibility in liquid crystals. Photoresponse has the advantages of safety, cleanness, rapidity and accuracy, and has broad prospects in tunable lasers, optical modulators, tunable photonic crystals and optically addressed blue phase displays[56⇓~58].
The photoresponse performance of BPLC is mainly reflected by the range of band gap shift, and the continuous shift of band gap mainly changes the double twist structure of BPLC by means of the change of photosensitive molecular structure, resulting in the change of lattice constant, and then controls its Bragg reflection characteristics. In 2013, Professor Yang Huai's research team at Peking University synthesized a series of azobenzene dimers, which achieved a band gap blue shift of more than 80 nm under ultraviolet light while broadening the BP temperature range[59]. The researchers doped the synthesized azobenzene dimer into small molecule BPLC, and found that the sample containing 1 wt% azobenzene dimer changed from red (650 nm) to yellow (600 nm) after 15 s irradiation with ultraviolet light (365 nm) at 38. 5 ℃, and changed to green (570 nm) after 20 s irradiation, and the peak disappeared within 25 s, achieving a blue shift of the band gap of more than 80 nm. After that, with the increase of irradiation time, the photonic band gap no longer changes. Under visible light (450 nm), the sample gradually returns to its original color (Figure 1A). As shown in Figure 1b, the researchers believe that under UV irradiation, the rigid part of the azobenzene dimer bends, causing the azobenzene dimer to surround the rod-like molecules of LC, making the LC molecules more compact, causing the double-twisted cylindrical structure of BPI to shrink, resulting in a decrease in the lattice constant and a blue shift in the reflected light. Under visible light irradiation, the azobenzene dimer changes from the bent cis-isomer back to the linear trans-isomer, the double-twist structure shrinks back, and the lattice constant returns to the initial state. In addition, the researchers found that when the doping concentration of azobenzene dimer was increased to 5 wt%, the sample changed from BPI to N* under UV irradiation, which may be due to the large content of azobenzene cis-isomer, which caused the double twist arrangement structure of liquid crystal molecules to be disturbed too much and then dechirped to single twist arrangement.
In 2018, the research team of Professor Zheng Zhigang of East China University of Science and Technology grafted photosensitive azobenzene on the eight vertices of oligosiloxane (POSS) through thiol-ene click reaction to synthesize photosensitive molecular switch functionalized nanocage (azo-POSS), as shown in the left panel of Figure 2A[60]. A dual-phase transition-based (BPII and N*) patterning function was achieved by doping azo-POSS in BPLC. As shown in the right image of Figure 2a, the sample changed from blue-green to bright green, yellow, orange and red after 24 s of irradiation with 365 nm ultraviolet light source of 1.0 mW/cm2. The reflection spectrum is gradually shifted from 508 nm to 610 nm, and a red shift in the wavelength range of 102 nm is achieved. The UV irradiation was continued, and the sample began to transform to N* from 28 s, and completely transformed to N* at 35 s. The sample could not recover to BPII after standing in the dark for more than 48 H. After irradiation with visible light (10.0 mW/cm2) at 532 nm for 50 s, the N* phase began to transform into BPII and completely transformed into BPI after irradiation for 120 s. This is due to the fact that the transition between the trans and cis isomers of azobenzene grafted on the eight vertices of POSS occurs under photoexcitation, which affects the arrangement of LC molecules, resulting in the increase of lattice constant and the phase transition between BPII and N*. This reversible transition between the BPII and N* phases using alternating UV and visible light irradiation can be repeated at least 30 times.
In 2015, Professor Guo Jinbao et al. Of Beijing University of Chemical Technology developed two polyoxometalate organic-inorganic hybrids with different shapes containing azobenzene mesogenic ligands to stabilize BP[61]. Bent and dendritic polyoxometalate hybrids can stabilize BPI and BPII respectively. Due to the trans-cis isomerization of the four azobenzene modification groups of the dendritic polyoxometalate hybrid under UV irradiation, the rigid part of the azobenzene in the hybrid is bent, the BPII double-twisted cylindrical structure of the sample is expanded, the lattice constant is increased, and the Bragg reflection of the sample is red-shifted. Therefore, the reflection wavelength of BPII doped with the dendritic polyoxometalate hybrid shown in the left panel of Figure 2b can be red-shifted from 539 nm to 617 nm, ranging from 78 nm, under UV illumination. After 15 s of irradiation, the reflection band of BPII does not move any more and remains at 617 nm, as shown in the right panel of Figure 2B. In addition, when exposed to visible light (450 nm,15 mW/cm2), the sample returned to its original state.
In addition to the band gap shift due to the change of lattice constant, photoinduced phase transition is also a major source of band gap shift. In 2018, the research team of this project synthesized azobenzene-functionalized bent molecules with V-type or W-type molecular configurations showing different mesogenic behaviors as shown in Figure 2C I, and found that the molecular configurations had a significant impact on the stability and photoresponse behavior of BP[62]. For the sample doped with 5 wt% V-type molecules, as shown in Figure 2C ii, under the irradiation of 53.0 ℃ and 1.0 mW/cm2 ultraviolet light, the reflected light shifts to blue, and the sample continuously changes from red to green within 10 s and to blue at 20 s, and then there is no change in the texture color with the increase of irradiation time, that is, the phase transition from BPI to BPII occurs. The sample can be restored to the initial state under the visible light irradiation of 5.0 mW/cm2. For the sample doped with 10 wt% W-type molecules, as shown in Fig. 2C iii, when BPII is irradiated at 46.0 ℃ with the same intensity of ultraviolet light, the reflected light is red-shifted, and the sample changes from the initial blue to red, that is, the phase transition from BPII to BPI occurs. In these two BP material systems, the corresponding reflected light can be shifted between red and blue, respectively, due to the phase transition.
Because liquid crystal molecules themselves have no response to light, the main research field is to dope photosensitive polymers into BPLC matrix, but there are many kinds of dopants with different mechanisms, which make it difficult to form empirical rules and high trial and error costs. Therefore, the importance of exploring other factors affecting photoresponse besides photosensitive molecules can not be ignored. In 2020, Chia-Rong Lee et al. Of National Cheng Kung University in Taiwan studied the effect of surface orientation layer and chiral azobenzene dopant (CA) on the combined effect of BPLC texture and optical properties[63]. The experimental results confirm that the anchoring force on the inner surface of the liquid crystal cell is helpful to stabilize the various phases of BP. Reversible optical control between BPLC triphasic States is possible for samples with a surface orientation layer by reversible trans-cis, cis-trans isomerization of CA dopants. The researchers used 405 and 450 nm light sources to induce the trans-cis and cis-trans photoisomerization of CA dopants at different temperatures (51.0, 47.5, 46.5 ℃), respectively, resulting in the isothermal phase transition and the shift of the reflection wavelength of the samples. As shown in Figure 3A, when the sample with the surface-oriented layer was held at BPII (51.0 ° C). Illuminated with a 405 nm source, BPII gradually transforms into the isotropic phase. The reflection peak wavelength is slightly red-shifted (< 7 nm). When illuminated by 450 nm light source, the sample gradually transforms from isotropic phase to BPII, and after several cycles of optical control, the isothermal phase transformation process has good repeatability. As shown in Fig. 3 B, when the sample with the surface orientation layer is kept at 47.5 ° C and illuminated with a 405 nm light source, the sample undergoes a phase transition from BPI to BPII, changing from green to blue. The central reflection wavelength is blue-shifted from 560 nm to 470 nm. The sample can be converted from BPII to BPI by illumination with a 450 nm light source. As shown in fig. 3C, when the sample with the surface alignment layer is held at 46.5 ° C and irradiated with a 405 nm laser, the sample changes from yellow to green, blue, and black in turn. For comparison, the researchers conducted an experiment on the sample with a non-oriented layer. As shown in Figure 3D, the BPI reflection peak of the sample with a non-oriented layer continuously blue-shifted from 640 nm to 540 nm after being irradiated by a 405 nm light source at 45.8 ℃ for 45 s, that is, only the lattice constant of the sample changed under ultraviolet light irradiation, and there was no isothermal phase transition.
In the study of the response performance of BPLC, the combined effect of two or more external fields is also a major research hotspot. Because of the dielectric anisotropy of liquid crystal, it can respond to the external electric field without doping. This enables the joint response of the electric and optical fields. In 2020, Geetha et al. Studied the opposite effects of optical field and electric field on BP lattice, and realized the continuous tuning range of selective reflection wavelength with a wide spectrum of 220 nm[64]. As shown in Figure 4A, after the sample was subjected to an electric field of 6 V/μm at 30.5 ° C, the reflection wavelength was red-shifted by about 20 nm, and the reflection color changed from purple to blue. When the electric field is further increased to 6.3 V/μm, the reflection wavelength is red-shifted by 220 nm, and the sample turns red. The red-shift of the reflection wavelength due to the electric field is caused by the lattice reorientation and the electrostriction effect. The sample was illuminated with ultraviolet light (365 nm;0.05 mW/cm2), keeping the electric field at 6.3 V/μm, as shown in Fig. 4 B. After irradiation for 168 s, the reflection wavelength is blue-shifted by 102 nm, and the sample changes from red (664 nm) to green (562 nm). After irradiation for 228 s, the reflection wavelength of the sample was totally blue-shifted by 220 nm, and the sample turned purple (444 nm). This is due to the fact that as the duration of UV irradiation increases, more and more trans isomers are converted to cis isomers, resulting in an increase in the ratio of chiral compounds to trans isomers. An increase in the concentration of the chiral compound leads to a decrease in the interhelical distance, which results in a contraction of the BP lattice and a blue shift of the reflection wavelength. Fig. 4C shows the red shift and blue shift of the reflection wavelength with the increase of the electric field and the irradiation time of ultraviolet light, respectively.
In addition to the use of visible light and ultraviolet light regulation, there is no lack of infrared light regulation in light regulation. In 2015, Li Quan et al., Kent State University, USA, fabricated a near-infrared photoresponsive self-organized 3D photonic superstructure by incorporating a novel hydrophobic polyarylate-functionalized gold nanorod (M-GNR) as shown in Figure 5A into the BP matrix[65]. For the first time, the BP phase transition was observed by applying a near-infrared light source, and the dynamic near-infrared directional red, green, and blue reflection of an optically driven three-dimensional soft photonic crystal was realized. When the sample is irradiated with 808 nm near-infrared light in the BPI state at 5 ℃ below the clearing point, the reflection wavelength shows an obvious blue shift, and the reflection center wavelength shifts to 560 nm (green) within 10 s. At about 15 s, the sample undergoes a phase transition from BPI to BPII, and the reflection center wavelength suddenly decreases to 491 nm (blue). With the increase of illumination time, the sample gradually changed to the isotropic state, resulting in a continuous decrease in the reflection intensity in the BPII state, and the reflection peak disappeared completely after the sample completely changed to the isotropic phase within 60 s. However, a reversible recovery process occurred after the near-infrared irradiation was stopped. In the control experiment, the BPI sample without M-GNR was irradiated for 5 min under the same near-infrared radiation conditions, and the sample still did not transform into the isotropic phase. This significant difference is caused by the local heating induced by the photothermal effect of M-GNRs under near-infrared laser irradiation. The response time from BPI to isotropic state decreases with the increase of temperature and the doping amount of gold nanorods. This study lays a foundation for the use of infrared light to control the dynamic optical properties of BPLC.
On the basis of the above research, Professor Wang Ling's team of Tianjin University combined core-shell upconversion nanoparticles (UCNP) with a new axial chiral azobenzene molecule, and doped the synthesized new upconversion nanomaterials as shown in Figure 5B into the BPLC matrix.The reflection wavelength of BPLC is red-shifted under the irradiation of near-infrared light (808 nm) with high power density and blue-shifted under the irradiation of near-infrared light with low power density[66]. The possible reason for the red shift of the reflection wavelength from 510 nm (green) to 596 nm (red) in 120 s at high power density (7 W/cm2) is that the NIR light induces a decrease in the helical twisting force value of chiral azobenzene molecule 2, as shown in the lower panel of Figure 5 B. At low power density (0.5 W/cm2), the reflection wavelength was blue-shifted from 590 nm (red) to 550 nm (green) in 130 s. The above two processes are reversible and can be repeated many times without significant degradation. Centimeter-thick BPLC films can still maintain the effect of photonic band gap shift controlled by near-infrared light. This study is expected to provide new ideas for the study of molecular switches driven by near-infrared light.

3.2 Magnetic response

In the study of BPLC magnetic response performance, in 2016, He Wanli et al., University of Science and Technology Beijing, prepared BPLC doped with magnetic Fe3O4 nanoparticles and tested the magnetic drive display performance[67]. The BPLC was endowed with magnetic response properties due to the ability of the Fe3O4 magnetic nanoparticles to move in the BPLC under a magnetic field. The researchers doped BPLC with Fe3O4 magnetic nanoparticles modified by oleic acid to prepare a magnetic response sample, as shown in Figure 6a. By applying a magnetic field on one side of the liquid crystal cell, the magnetic nanoparticles inside the sample moved to the corresponding inner surface, and the writing operation of the pattern was realized macroscopically. A magnetic field is applied to the other side of the liquid crystal cell, so that the magnetic nanoparticles aggregated on the inner surface of the sample move back into the BPLC matrix, and the pattern erasure operation is realized macroscopically. In terms of the lifetime of the sample, when there is no nylon network in the liquid crystal cell of the sample as shown in Figure 6B, the particles near the area where the magnetic field is applied will be affected by the magnetic field during writing and erasing, resulting in uneven distribution of magnetic particles in the sample, which can not be displayed normally after multiple writing and erasing. When there is nylon network in the sample, the writing traces can not disappear after several months, and after dozens of times of writing and erasing, they can still be displayed normally.

3.3 Electric response

In the early study, it was found that the application of an external electric field to BPLC would cause the shift of the selective reflection wavelength, and thus BP was found to have electrical response[68]. After a series of subsequent studies, it is concluded that BP usually has a variety of response effects under the action of electric field, including Kerr effect, electric-field-induced crystal orientation, electrostriction, and electric-field-induced phase transition[69,70][71][72][73]. When a small electric field is applied to the BPLC material, the internal lattice of the BP will be reoriented, and the birefringence phenomenon will be induced macroscopically, that is, the Kerr effect. The lattice constant of BP is changed by the external strengthening electric field, which leads to the change of selective reflection wavelength, that is, electrostriction. The lattice reorientation of BP is induced by an external electric field, and the phase change of BP under a stronger external electric field is induced by an electric field[38,74,75].
The above electrical response properties of BPLC are based on AC electric field. Using the electrical response properties of BPLC, scientists have made many recent research progress, including adjusting the photonic bandgap wavelength and developing the application of BPLC materials in tunable lasers. In 2019, Zhu Jiliang et al., Hebei University of Technology, studied BPLC photonic crystals with continuously tunable photonic band gap by applying a low AC electric field[76]. By changing the field strength and frequency of the electric field, the perfect balance between dielectric heating and electrostriction is achieved, and the BP reflection wavelength can be continuously, reversibly and rapidly adjusted in the whole visible range. At the same time, its reflectivity is increased by four times, and the reflection color is more bright. In addition, the dye was doped with the above BPLC, and the laser emission with continuously tunable wavelength was obtained.
However, small molecule BPLC materials have a narrow temperature range and are difficult to achieve practical application. Introducing polymer network stabilized blue phase (PSBP) is one of the important methods to obtain BPLC materials with wide temperature range. However, because the polymer network almost freezes the lattice structure of BP, PSBP generally only has Kerr effect and requires a high driving voltage, and the electrostrictive effect directly caused by the AC field requires a higher driving voltage. It is believed that the existence of polymer network structure may hinder the movement of BPLC molecules under the action of electric field[12].
In 2010, Shin-Ying Lu et al. First prepared PSBP samples with a wide reflection wavelength tuning range by applying a DC electric field to cause the movement of the polymer network[77]. With the increase of the voltage, the reflection color of the sample gradually changes from the initial blue to green and red. However, due to the low polymer content, the BP temperature range of the system is only 6 ℃, which is difficult to meet the practical application requirements.
In 2015, Professor Lin Tsung-hsien's research team at Sun Yat-sen University in Taiwan also used a DC electric field to drive the PSBP to achieve both the shift and widening of the photonic band gap[75]. When an AC electric field is applied to the BPLC prepolymer in the BPI temperature range, the BPI lattice is distorted under the action of the electric field to induce the BPX phase. This state was stabilized by UV irradiation in situ to obtain PSBPX. When a DC electric field is applied to a sample with a liquid crystal cell thickness of 12 μm, the BP reflection wavelength is red-shifted with the increase of the electric field intensity, and the maximum shift range is up to 200 nm. In the sample with a cell thickness of 27 μm, the researchers observed the phenomenon of symmetrical widening of the reflection band induced by the electric field, and the reflection band width was widened from 30 nm to nearly 310 nm. Researchers believe that the electric field-induced photonic band gap shift or widening in this system is caused by the lattice distortion caused by the movement of the charged polymer network under the action of the electric field. The polymer monomer captures the positively charged cation during polymerization to make the polymer network positively charged. Because the upper and lower liquid crystal cell substrates are fixed, when an electric field is applied, the polymer network with positive charges moves to the conductive negative electrode side of the liquid crystal cell, causing the network to be compressed near the negative electrode,It is stretched near the positive electrode position, so that the lattice at different positions is distorted to different degrees, resulting in the red shift or broadening of the photonic band gap.
In 2017, the research team of this project prepared PSBPI samples with asymmetric bidirectional electric field induced photonic band gap shift by controlling the polymerization conditions[78]. The photonic band gap of the prepared sample shows excellent thermodynamic stability in the range of -30 ℃ ~ 60 ℃, and the position of the photonic band gap is controlled by the direction and strength of the electric field, and the moving range can cover the visible light band. It is found that the existence of the polymer network plays a vital role in the electric field response characteristics of the sample. The schematic diagram of the mechanism is shown in Figure 7A. The polymer monomer adsorbs positively charged ions during the polymerization reaction, and the polymer network density of the sample gradually decreases from the surface to the bottom of the sample by controlling the polymerization conditions. The polymer network at the bottom surface is sparse, and the anchoring force with the lower substrate of the liquid crystal cell is small, so the binding effect of the lower substrate can be ignored. Under the action of a direct current electric field, the polymer network is moved by the force of a directional electric field, and the movement direction is controlled by the direction of the electric field. As a result, the BPI lattice structure is stretched and the photonic band gap is red-shifted under the positive electric field. Under a negative electric field, the BPI lattice structure is compressed and the photonic band gap is blue-shifted. Based on this material, an electrically tunable BPLC laser with low excitation threshold can be prepared, and the laser wavelength can be controlled up to 22 nm[78]. On the basis of this study, the BPLC template was successfully obtained by the "wash-out and infusion" preparation method. This template retains the property that the direction of the field strength controls the bidirectional shift of the photonic band gap (Fig. 7 B). The laser wavelength tuning range is extended to 48 nm by infusing fluorescent dye-doped small molecule liquid crystal[58]. As shown in fig. 7C, the scanning electron microscopy characterization confirms that the polymer network density decreases from top to bottom. In the following study, a chiral polymer network was constructed by using chiral liquid crystalline polymerizable monomers, which made the photonic band gap of BPI shift from 450 nm to 750 nm in a wide range under the action of a unidirectional DC electric field[79]. The analysis shows that the morphology and microstructure of the polymer network in the BP three-dimensional periodic structure can be qualitatively controlled by controlling the experimental conditions such as the type and proportion of liquid crystalline polymerizable monomers, the thickness of the liquid crystalline cell, the polymerization temperature, and the intensity of ultraviolet light, and then the BPLC photonic crystal with a variety of electric field response characteristics can be prepared. The electric field response type three-dimensional flexible liquid crystal photonic crystal film material has fast response time and good repeatability and stability, and is expected to be widely applied to the fields of reflective liquid crystal display, adjustable grating, adjustable laser and the like.
In 2017, Geetha G. Nair et al. Prepared a BP gel composite by adding 15 wt% bending compound and 1 wt% low molecular organic gelling agent to the parent BPLC[74]. In this system, the flexible gel fiber is integrated into the BPLC, and the selective reflection color switching can be generated by changing the applied electric field, thus realizing the reversible tuning of the photonic band gap. And attribute the overall blue shift (95 nm) to the combined effect of lattice reorientation and lattice contraction shown in Fig. 8 a. The change of the reflection color of the sample from red to orange is due to the reorientation of the lattice. Compared with the (110) plane, the (200) plane with four axes along the direction of the electric field is more stable after the electric field is applied, so the sample tends to orient along the (200) plane, and the Miller index changes, resulting in a shorter reflection wavelength. As shown in Figure 8 B, polarizing microscope photographs corresponding to electric fields of 2.16 and 2.34 V/μm show a color change of the sample platelet domains from orange-yellow to green, confirming the reorientation of the crystal lattice. As shown in Figure 8 a, when the electric field intensity continues to increase beyond 2.34 ~ 2.7 V/μm, the reflection color of the sample changes to green, which is due to the contraction of the lattice and the decrease of the lattice constant after applying a higher electric field, resulting in a shorter reflection wavelength. At the same time, the gelated BPI can also produce a nematic state under a high electric field, and the switching between the dark state and the transparent state is realized by using a crossed polarizer, which provides a possibility for manufacturing devices such as tunable mirrors by using BP composites.
Single-domain BPLC has higher monochromaticity and good electro-optical performance, but it still has the problems of short band gap tuning range, slow response time and hysteresis. In 2020, MinSu Kim et al. prepared a polymer-stabilized single-domain BPLC based on a non-liquid crystalline monomer and a chiral liquid crystalline monomer by using a frictional antiparallel oriented liquid crystal cell. The polarization photo of the single-domain texture is shown in Figure 8C, and the photonic band gap of the single-domain polymer-stabilized BPLC was dynamically controlled by electrical stimulation[80]. Even after the strong lattice deformation caused by the electrostrictive effect, it can well maintain the internal director and chirality of the system, so that the internal structure of the system remains stable. The initial color of the BPLC photonic crystal is controlled by controlling the concentration of the chiral monomer in the BPLC and the ratio of the liquid crystal to the chiral dopant, so that the spectral range of the BPLC photonic crystal is controlled within 216 nm.By adjusting the electric field intensity, reversible tuning can be achieved in the range of 241 nm, which improves the tuning range of photonic band gap and reduces the electric field response time, and the hysteresis problem is also improved.
In 2019, Professor Lin Zongxian's research team at Sun Yat-sen University realized the electrostrictive reconstruction of BPLC photonic crystal by repeatedly applying electric field pulses (RAF)[81]. Unlike existing methods of continuously applying electric fields, this technique allows the time interval between each field to be optimized to continue to establish the band gap shift caused by the cubic to non-cubic lattice transition,At the same time, the heat generated in the process of applying the electric field is allowed to dissipate and the orientation axis relaxes to a metastable intermediate state, which will be used as a new initial state for the next field. This technique suppresses electric field heat production, so electrostriction becomes the only mechanism of lattice distortion. As shown in Fig. 8 (d), the body-centered cubic lattice of BPI is transformed into orthorhombic lattice and tetragonal lattice under the action of electric field by using the above technique of repeatedly applying electric field pulses. After removing all the applied electric fields, the resulting non-cubic BP crystal is very stable, as can be seen from the optical characterization of its photonic band gap and Kossel diagram. In addition, after removing the applied electric field, the crystal structure treated by repeated electric field pulses was stabilized by ultraviolet light to form a 1 cm size polymer-stabilized BPX single-domain sample with a tetragonal lattice structure. The thermodynamic stable temperature range of the sample is also increased from 3 ℃ to 30 ℃, and the photonic band gap is red-shifted after electrification. It provides the feasibility of preparing stable non-cubic BPLC and making BPLC into large size 3D single crystal.

3.4 Force response

As a new type of polymer intelligent material, liquid crystal elastomer (LCE) has been favored by many researchers because of its excellent mechanical response. By applying external stress, the BPLC photonic crystal film shows good tuning performance. In 2014, Coles et al. Prepared a stretchable free-standing BPLC photonic crystal film by increasing the monomer ratio of polymer and in situ photopolymerization, and the optical properties of the film can be controlled by applied strain[82]. Using polarized light microscopy, spectral analysis and Kossel diffraction techniques, the researchers found that by applying surface transverse strain to the film, the BPI lattice structure was stretched along the strain direction and compressed along the perpendicular strain direction, and the lattice constants in the two directions increased and decreased, respectively. When viewed along the vertical direction of strain, as shown in Figure 9a, the reflection color of the film moves from red through green to blue with the increase of tension, that is, it has the characteristics of mechanochromism in the visible band.
In 2020, Chia-Rong Lee et al. Prepared a stretch-adjustable free-standing blue phase liquid crystalline elastomer (BPLCE) film by all-photopolymerization[83]. The mechanical and optical properties of BPLCE films were measured at 30 ° C by applying a stretching force perpendicular to the film normal direction. By gradually increasing the stretching force on the film, the corresponding deformation along the stretching direction is measured, and the stress-strain relationship curve of the film can be determined. The film exhibits linear elastic mechanical properties before the final fracture, and the Young's modulus is about 1.1 MPa. Through tests, the researchers found that the stretching-relaxation process was completely reversible under an applied stress of 0.6 MPa and without an applied stress. When stretched, the reflection color of the film changes continuously from red (622 nm) to blue (487 nm) at the same time, and the wavelength change Δλ is about 135 nm. This is due to the fact that after elastic deformation with uniaxial strain applied to the plane perpendicular to the [110] direction, the effective three-dimensional photonic crystal lattice period decreases with decreasing film thickness due to volume conservation. With the increase of axial stress, the degree of length deformation and the reflection peak wavelength increase gradually, which shows the tuning performance of mechanical stress on BPLCE. In addition, the application of BPLCE thin film as a broadband tunable free-standing laser device is demonstrated.
In 2021, Timothy J. White et al. Prepared a BPLCE sample by adding a chiral dopant R811 and a dithiol BDMT to a mixture of a liquid crystalline diacrylate monomer C6M and a chiral liquid crystalline diacrylate SLO4151, followed by UV curing initiated by a photoinitiator 819, and studied the effects of mechanical stress on the lattice constant and related optical properties of the BPLCE[84]. As shown in Figure 9 B, the sample was observed at a tilt of 45 °, and a significant red shift in the reflected color was found. This is due to the fact that uniaxial deformation elongates the length of the material on the loading axis, while reducing the width and thickness of the film, the total number of unit cells is unchanged in the crosslinked polymer network. Therefore, the redistribution of the volume of the liquid crystal elastomer is regulated by the asymmetry within the unit cell, which affects the lattice spacing within BPI and BPII. The lattice constant of the unit cell becomes longer in the direction of the strain axis lattice plane and shorter in the direction of the non-strain axis lattice plane, so that the reflection spectrum is blue-shifted when viewed perpendicular to the sample and red-shifted when tilted by 45 °. This study lays a foundation for the application of BP in the fields of smart wear, sensors, soft robots and so on. However, the biaxial strain of BPLCE leads to the alternation of yellow and blue patterns, and the unique physical reasons for this phenomenon need to be further studied.
The stretchable self-supporting BPLC photonic crystal film realizes the tuning of the photonic band gap by applying an external force, but does not have programmability and shape memory (SM). In 2019, Professor Yang Zhou's team at University of Science and Technology Beijing prepared a BPLC photonic crystal film with shape memory function[40]. The shape is temporarily stored by heating the original reflective red BP film above its glass transition temperature and applying different forces for shape memory programming, which locally deforms the polymer network. The lattice length of the film in the deformation region changes, and the reflection wavelength shifts in the visible range, resulting in color changes, and a variety of blue-shifted colors can be obtained. Pressure of 5, 10 and 15 N was applied to the BPLC film, respectively, and all the samples showed color changes. As shown in Fig. 10, the red BP film changes from red (619 nm) to yellow-green (577 nm). It deforms into a temporary state, and cooling the sample below the glass transition temperature fixes the temporary shape, which can be preserved for a long period of time. With the change of the field of view, the reflection color of the deformation area changes from red to blue. The BP film shows reversible deformation through the shape memory process, that is, when heated above the glass transition temperature, the deformed BP film can return to its original shape and color. This study provides a simple and sensitive optical technique for exploring the shape memory effect of self-assembled structures.

3.5 Humidity response

Molecular self-assembly through the use of weak bonds and non-covalent bonds (such as hydrogen bonds) as carriers has been developed as an effective way to prepare new multifunctional supramolecular structures. The active carboxyl group in the monomer and the hydrogen bond formed by it are the source of the humidity responsiveness of the material.
In 2020, Professor Yang Huai's research team at Peking University prepared a self-supporting BPLC film with humidity-responsive behavior by using a variety of carboxyl-containing acrylate polymerization monomers[85]. As shown in Figure 11 a, the POM image of the film after KOH etching treatment gradually changes from green to red, and the photonic band gap in the spectrum correspondingly shifts from 518 nm to 635 nm. After the water in the film evaporates, the POM image of the film gradually changes from red to blue, and the reflection wavelength changes from 635 nm to 459 nm. With the increase of relative humidity, the film swells back to red after absorbing water, and the reflection color of the film changes from red to green rapidly within about 1 min after evaporating water, which can be repeated for multiple cycles. The mechanism of this phenomenon is that the film contains a large number of carboxylic acid derivatives, and a large number of hydrogen bonds formed between the bimolecular carboxyl groups are retained in the original BPLC film. After corrosion treatment, the carboxylic acid group is converted into carboxylate, and the hydrogen bond is broken. It is found by measurement that the contact angle of the membrane in the K salt state is less than 90 degrees, and the membrane has hydrophilicity, so that the membrane absorbs water and expands; In the acidified state, the contact angle is greater than 90 degrees, which is hydrophobic, thus causing the membrane to shrink and deform. Different degrees of contraction and expansion deformation lead to different changes in the lattice, so the film shows different colors at different humidities. The self-supporting humidity-responsive BPLC film has reusable write and erase functions. As shown in Figure 11 B, the bold font "BP" was written using water in a blank K-salt film, and spontaneous erasure occurred after water evaporation. Subsequently, the fine font "LC" was obtained by a second writing. Therefore, a colorful reflective writing board with an automatic erasing function can be prepared through the functionalized film. In addition, the film can also realize a variety of functions such as a sensing actuator with dual shape and color responses, an acid gas or liquid naked eye detection device, and a visual anti-counterfeiting model without device dependence, and has application potential in the fields of sensors, displays, and anti-counterfeiting.
Free-standing BPLC film with humidity-responsive behavior gets rid of the limitation of liquid crystal cell. Although it has good humidity-responsive performance, the stability of the film is poor. In 2021, Professor Wang Ling's research team of Tianjin University used a chemical method to anchor the hydrogen-bonded BPLC network on the substrate surface modified by functional silane methacrylate-3- (trimethoxysilyl) propyl methacrylate (TMSPMA), which significantly improved the stability and adhesion of the BPLC photonic crystal coating to the substrate, as shown in Figure 12A[86]. The expansion or contraction of this polymer film occurs only in the direction perpendicular to the substrate surface. The area proportion of the polymer coating does not change due to expansion and contraction caused by changes in relative humidity while maintaining humidity response. After the polymer film is treated by alkali, the hydrogen bond is broken, and the existence of carboxylic acid group leads to the formation of carboxylic acid potassium salt in the BP network. The synthesized polymer salt film is highly sensitive to the change of humidity. The color of the BPLC polymer photonic coating gradually changes from green to red when the relative humidity changes from 0% to 80%, and the relative humidity has an almost linear relationship with the reflection color. The average thickness of BP polymer salt film becomes thicker with the increase of humidity, and the change trend is similar to that of the reflection band. As shown in Figure 12B, the polymer film can be used to make a colorimetric humidity sensor for fruit growth, transportation and preservation, as well as real-time monitoring of other organic analytes. Humidity-driven, color-changing BPLC polymer coatings enable advanced applications of information encryption. Combining the dolphin sticker with the humidity-driven color-changing BPLC polymer film, the film appears green in the low relative humidity state, and the dolphin pattern has low visibility. In the high relative humidity state, the film appears red, as shown in Figure 12c, and the encrypted dolphin pattern has high visibility. As shown in Figure 12d, the long-horned beetle can change different colors at low relative humidity and high relative humidity. Based on this BPLC polymer film, bionic applications similar to the long-horned beetle can be realized to help camouflage or hide targets, which has broad prospects in many civil and military fields.
In addition, the combination of BPLC photonic crystal and 3D printing technology can be used to produce color-changing materials for multiple applications. In 2022, the research team of Academician Jiang Lei of the Institute of Physics and Chemistry of the Chinese Academy of Sciences combined BPLC photonic crystals with inkjet printing technology to prepare a high-precision "live" pattern[87]. By controlling the dispersion of liquid crystal ink (5CB) on the hydrophobic BPLC film, the shape and color of the printed pattern can be controlled over time. In the preparation process, the polymer network (PT-BPLC) was obtained by washing away the LC molecules in PSBP, and then PT-BPLC with different surface properties was obtained by Fas modification or plasma treatment with 1H, 1H, 2H, 2H-perfluorodecyltrimethyl-silane oxide. Plasma-PT-BPLC was obtained by Plasma treatment of PT-BPLC. For the Fas-PT-BPLC sample, as shown in Figure 13A, the diffusion of the liquid crystal ink from the surface to the internal structure causes the BP structure in the printing area to expand outward, which is a supersaturated state, the lattice becomes larger, and the reflection color of the sample is red-shifted. Then the liquid crystal ink diffuses from the printed region to the non-printed region, which is in a low saturation state, and the concentration of 5CB in the structure is diluted, resulting in the contraction of the previously formed printed region and the expansion of the newly formed printed region, resulting in the contraction, the decrease of the lattice, and the blue shift of the reflection color of the sample. The researchers focused on the need for hydrophobic treatment to improve pattern resolution by comparing Plasma-PT-BPLC with PT-BPLC and Fas-PT-BPL. Hydrophobic treatment can effectively control the spreading of ink on the substrate surface and the aggregation and fusion of ink droplets, and improve the printing resolution. The Plasma-PT-BPLC film has the characteristic of repeatable erasing and writing, and different high-resolution images can be obtained through multiple inkjet printing and cleaning. According to the Bragg formula, when the amount of ink penetration increases, the average refractive index n increases, resulting in an increase in the reflection wavelength λ, so the red shift phenomenon occurs. As the ink continues to penetrate, the polymer network expands, resulting in a growth in a, an increase in λ, and a further red shift. The researchers also controlled the amount of penetration by adjusting the number of printing layers, as shown in Figure 13b, using only one ink to achieve multi-color pattern preparation. The study combines BPLC photonic crystals with inkjet printing, as shown in Figure 13C, to prepare high-resolution color-changeable patterns, which can promote the development of optical applications such as anti-counterfeiting.

4 Conclusion and prospect

Based on the current frontier trends in the field of BPLC photonic crystal materials, this paper summarizes a series of important research results in the field of optical, magnetic, electrical, mechanical, humidity and other external field response properties of BPLC photonic crystal in recent years from the perspective of self-assembled structure and three-dimensional photonic band gap. The dynamic regulation of the photonic band gap of the small molecule BPLC system can be realized by doping photoisomerization molecules or photoresponse nanoparticles; Through the preparation and functional design of the liquid crystal/polymer composite system, the thermodynamic stability of the BPLC system can be significantly broadened, and the BPLC system has excellent external field tunability and good mechanical properties, so that intelligent BPLC photonic crystal films with different response characteristics can be prepared. BPLC-related functional materials have the advantages of reprogrammability, repeatability and visualization, which lay the foundation for the development of BPLC in the field of stimuli-responsive smart materials.
Through the unremitting efforts of researchers in this field in recent years, stimuli-responsive BPLC photonic crystals have been rapidly developed in the functional design of materials and the preparation of smart devices, but its research is still in its infancy. In terms of improving the optical performance of the device, it is still difficult to obtain a BP single-domain structure with a large size and a high quality factor. The traditional methods of surface orientation control, temperature heat treatment control or electrical orientation control still have the problems of complex process and long time consumption, and the related preparation methods need to be further improved and optimized to improve the efficiency of device preparation. In addition, the response mode of the intelligent response BP photonic crystal studied at present is relatively single, and the response sensitivity or response speed performance still needs to be improved. The development and design of BPLC optoelectronic devices with advanced performance and customized functions will be the focus of current research. Looking forward to the future, through the more full and innovative combination of BPLC with polymer, nano and other fields, composites with higher sensitivity and better response can be prepared. For example, in the field of photoresponse, the photonic band gap response effect or functional anti-counterfeiting effect of BPLC materials can be further improved by combining with azobenzene polymerizable monomers, fluorescent polymers or aggregation-induced luminescent materials. In terms of electric response characteristics, the electric field response performance can be improved through the molecular design of liquid crystalline polymerizable monomers and the structured customization of polymer network morphology. In terms of force response characteristics, new BPLC elastomers with mechanochromism, shape programmability and self-healing function can be developed through the design and preparation of specific functional polymerization monomer molecules. In the field of humidity response, humidity-driven bending, distortion and color synergistic change can be realized through the synergistic effect of deformation color, which can further realize the multi-functional driving of BPLC devices. In addition, functional composites with various electrical response characteristics can be prepared by combining BPLC with functional nanomaterials. Combining BPLC with two-dimensional materials such as Mxene and graphene, it is expected to prepare multi-response actuators with photothermal response characteristics and improve response speed. With the unremitting exploration of the basic research and application fields of BP by researchers, it is believed that BPLC can blossom and bear fruit in the application fields.

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Funding

National Natural Science Foundation of China(52003293)

National Natural Science Foundation of China(51927806)

Fundamental Research Funds for the Central Universities(2022YQJD07)

Opening Subject Projects for University Students of China University of Mining and Technology-Beijing

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