Fluid manipulation is an important foundation of microfluidic technology. Various methods and devices have been developed for fluid control, such as electrowetting-on-dielectric-based digital microfluidic platforms, microfluidic pumps, and pneumatic valves. These devices enable precise manipulation of small volumes of fluids. However, their complexity and high cost limit the commercialization and widespread adoption of microfluidic technology. Shape memory polymers as smart materials can adjust their shape in response to external stimuli. By integrating shape memory polymers into microfluidic chips, new possibilities for expanding the application areas of microfluidic technology emerge. These shape memory polymers can serve as actuators or regulators to drive or control fluid flow in microfluidic systems, offering innovative approaches for fluid manipulation. Due to their unique properties, shape memory polymers provide a new solution for the construction of intelligent and automated microfluidic systems. Shape memory microfluidic chips are expected to be one of the future directions in the development of microfluidic technology. This article offers a summary of recent research achievements in the field of shape memory microfluidic chips for fluid and droplet manipulation and provides insights into the future development direction of shape memory microfluidic devices.

Shape memory polymers (SMPs) are capable of sensing changes in the surrounding environment and undergoing corresponding shape changes.1,2 Some SMPs also exhibit transparency and biodegradability.1 Consequently, deformable structures constructed from these intelligent materials present potential applications in diverse fields such as biomedicine, aerospace, and soft robotics. Thermoresponsive SMPs consist of a stable phase and a reversible phase. The stable phase is the molecular chain formed by physical or chemical cross-linking, which can memorize the initial shape of the SMPs. The reversible phase can be reversibly softened and hardened with temperature changes, which can maintain the temporary shape of the SMPs.3 Therefore, thermoresponsive SMPs exhibit an initial shape at room temperature. Upon elevating the temperature to the transition point, these SMPs can be reshaped. Following a cooling phase, the SMPs retain their transient form. A reheating process to the transition temperature reverts the SMPs back to their original shape. This reshaping procedure can be repeated multiple times, thereby enabling the SMPs to assume multiple temporary forms but maintain only one initial shape. Depending on the stimulus mechanism, SMPs can be classified into various types, including temperature-responsive,4–6 electrically responsive,7,8 light-responsive,9–11 and magnetically responsive.12,13 Each stimulus mechanism has its own characteristics: direct heating is simple but slow in response; electrical and magnetic stimuli shorten the response time of SMPs; light-responsive SMPs enable precise and remote control of shape changes. Researchers can choose the appropriate stimulus mechanism according to the specific application environment.

Microfluidic technology has undergone more than 30 years of development and has obtained numerous successful applications in biochemical analysis,14–16 tissue engineering,17,18 material synthesis,19,20 and so on. It also holds great promise for emerging areas like flexible electronics21 and wearable devices.22–24 The driving and control of fluids are fundamental to the development of microfluidic technology. In traditional microfluidic systems, fluids are driven by syringe pumps, and fluid control relies on microvalves such as pneumatic valves.25–27 These technologies can achieve accurate control of fluids but are complex and costly in manufacturing, requiring specialized training for the operators. With the advancement of microfluidic technology, traditional fluid driving and control strategies are no longer suitable for emerging fields. The integration of shape memory materials into microfluidic systems endows them with shape memory capability. The commonly used shape memory materials include shape memory alloys (SMAs) and SMPs, both of which can be integrated into microfluidic systems. SMAs can be processed into wires,28 films,29 and springs,30 and they can convert temperature changes into mechanical energy, enabling their use as actuators in microfluidic systems. Compared to SMAs, SMPs possess several advantages, including low cost, lightweight, high deformability, ease of processing, various response modes, good chemical stability, and biocompatibility.31 Under external environmental stimuli, the local or overall shape of the microfluidic chip can change.32 Thus, the microfluidic chip serves as both a carrier and an actuator. Therefore, it is possible to manipulate microfluids by stimulating shape changes in microfluidic chips. Currently, research on shape memory microfluidic chips mainly focuses on SMPs functional surfaces for droplet manipulation and the design of microfluidic chips for fluid flow control (Fig. 1). Although research in this area is still at an early stage, it represents an important direction for the development of microfluidic technology, with the potential to bring new breakthroughs and applications to the field of microfluidics. In this paper, we summarize the recent progress of shape memory microfluidic chips in the field of fluid and droplet manipulation and look forward to the future development direction.

FIG. 1.

Shape memory polymers utilized in microfluidic systems.

FIG. 1.

Shape memory polymers utilized in microfluidic systems.

Close modal

SMPs can change its shape spontaneously after receiving external stimulation. Integrating SMPs into microfluidic chip or preparing microfluidic chip with SMP can realize partial or whole deformation of chip through external stimulation. Initially, the researchers integrated SMPs film into the outlet of microfluidic chip, and stimulated SMPs to generate negative pressure to provide power for fluid.4,9,33,34 Lee and Hong34 developed a vacuum module utilizing SMPs, which was installed at the outlet of a microfluidic chip [Fig. 2(a)]. The deformation of the SMP was thermally induced, subsequently generating negative pressure that facilitated fluid flow and led to the production of oil-in-water droplets. The vacuum module with a diameter of 15 mm provided a maximum driving pressure of approximately 9653 Pa. Wang et al.9 incorporated photothermal effect-active AuNPs into polyvinyl alcohol, subsequently preprogramming the shape memory composite materials for integration at the outlet of a microfluidic chip. Upon exposure to light radiation, the SMP returned to its original shape, generating negative pressure to draw liquid samples into microfluidic channels. Furthermore, the membranes of SMPs can be fabricated into liquid capsules, which facilitate the transportation of liquid into the channel by controlling their contraction. Robertson et al.4 pasted reversible deformed cross-linked poly(cyclooctene) membrane on the inlet of microfluidic chip to realize fluid pumping [Fig. 2(b)]. They fixed the temporary shape of cross-linked poly(cyclooctene) film into a bubble-like structure and stored liquid within it. Upon heating to 60 °C, the SMP film contracted, squeezing the fluid into the channels. The fluid flow direction could be reversed when the temperature returned to 8 °C. These fluid drive strategies do not require syringe pumps, simplify the complexity of microfluidic devices, and realize small-scale fluid drive under limited conditions.

FIG. 2.

(a) Schematic diagram of negative pressure driving fluid generated by temperature triggering SMP module deformation. Reproduced with permission from Lee et al., Microfluid. Nanofluid. 20(12), 158–167 (2016). Copyright 2016 Springer Link.34 (b) The fluid is driven by controlling the expansion and contraction of SMP liquid capsule (xPCO film). Reproduced with permission from Robertson et al., Smart Mater. Struct. 25(8), 085043 (2016). Copyright 2016 IOP Publishing.4 (c) The temperature triggers the restoration of the concave channel on the SMP surface to its original shape, consequently interrupting fluid flow. Reproduced with permission from Takehare et al., Appl. Phys. Express 6(3), 037201 (2013). Copyright 2013 IOP Publishing.35 (d) The shape memory microfluidic channel is programmed into a temporary shape. The non-contact shape programming of channels is achieved by mixing magnetic particles in PDMS layer. Reproduced with permission from Wang et al., ACS Appl. Mater. Interfaces 14(13), 15599–15607 (2022). Copyright 2022 American Chemical Society.36 (e) Liquid transport is controlled through the deformation of shape memory microcolumns in an open-channel microfluidic chip, enabling controllable liquid transportation. Reproduced with permission from Ye et al., Lab Chip 23(8), 2068–2074 (2023). Copyright 2023 Royal Society of Chemistry.37 

FIG. 2.

(a) Schematic diagram of negative pressure driving fluid generated by temperature triggering SMP module deformation. Reproduced with permission from Lee et al., Microfluid. Nanofluid. 20(12), 158–167 (2016). Copyright 2016 Springer Link.34 (b) The fluid is driven by controlling the expansion and contraction of SMP liquid capsule (xPCO film). Reproduced with permission from Robertson et al., Smart Mater. Struct. 25(8), 085043 (2016). Copyright 2016 IOP Publishing.4 (c) The temperature triggers the restoration of the concave channel on the SMP surface to its original shape, consequently interrupting fluid flow. Reproduced with permission from Takehare et al., Appl. Phys. Express 6(3), 037201 (2013). Copyright 2013 IOP Publishing.35 (d) The shape memory microfluidic channel is programmed into a temporary shape. The non-contact shape programming of channels is achieved by mixing magnetic particles in PDMS layer. Reproduced with permission from Wang et al., ACS Appl. Mater. Interfaces 14(13), 15599–15607 (2022). Copyright 2022 American Chemical Society.36 (e) Liquid transport is controlled through the deformation of shape memory microcolumns in an open-channel microfluidic chip, enabling controllable liquid transportation. Reproduced with permission from Ye et al., Lab Chip 23(8), 2068–2074 (2023). Copyright 2023 Royal Society of Chemistry.37 

Close modal

By leveraging the shape transformation of SMPs, microchannels can be closed/opened to control fluid flow. Takehara et al.35 presented a temperature-responsive shape memory microvalve constructed from polycaprolactone. The microvalve had an original flat shape and a temporary concave shape, allowing fluid to flow through the concave channel. Upon exposure to heat, the microvalve reverted to its original flat state, thereby ceasing fluid flow [Fig. 2(c)]. Aksoy et al.5 reported locked microfluidic valve arrays, which were constructed using SMP membranes. They further demonstrated the practical applications of these valves in reagent mixing and fluid propulsion. The valves were controlled by an air pathway to maintain different shapes, enabling the opening or closing of fluid channels. The valves exhibited a single locking time of more than 15 h and a cycle count of more than 3000. Fu et al.38 reported a paper-based microfluidic chip integrated with shape memory valves that can be controlled to open and close by a heater. The device delivered reagents in a programmed manner, and multi-step immunoassays were completed on the chip. Yang et al.39 utilized acrylic resin to fabricate shape memory microfluidic chips. The microchannels in the chip could be deformed and retained in a temporary shape under external forces after heating and could be restored to their initial shape through heating. Local deformations in the branching channels altered the pressure distribution inside the channels, causing particles to flow out through another branching channel, representing a logic circuit-like behavior. Although the shape memory valves are capable of regulating fluid flow, their response speed is slower than that of pneumatic valves. Shape memory valves are more suitable for microfluidic systems where integrating precision valves is difficult, such as wearable microfluidic devices.

Conventional microfluidic chips, such as glass chips, PDMS chips, and PMMA chips, are typically fabricated with planar channels. However, these conventional designs fall short in replicating intricate fluidic models such as tissue development or vascular network simulation, which require a more realistic representation of the actual scenario. The integration of SMPs into microfluidic systems provides a solution to this limitation by enabling programmable control over the shape of the microchannel. Wang et al.36 developed a shape-programmable three-dimensional microfluidic chip by sealing a PDMS channel layer onto an SMP membrane [Fig. 2(d)]. The chip can be programmed into a temporary shape through heating and then returned to the initial shape upon reheating. This approach leverages the shape-fixing properties of SMPs, thereby eliminating the necessity for mechanical assistance to preserve the three-dimensional structure of the channels. Additionally, by incorporating magnetic particles into the PDMS layer, a magnetically responsive 3D channel structure was obtained. This enables rapid and controllable programming of the channels using a portable magnet. This study offers novel perspectives on the design of three-dimensional microfluidic chips and research in artificial blood vessels and other areas.

Open microfluidic chips have gained significant attention due to their flexibility and simplicity. However, the inability to integrate valves and pumps in open channels is a common drawback, resulting in limited control over fluid flow. Xu’s team37 has developed a shape memory open-channel microfluidic chip utilizing ethylene-vinyl acetate copolymer, where the microcolumns in the channels exhibit photothermal and magnetic responsiveness. With magnetic assistance, the bending angle and direction of the microcolumns can be adjusted through near-infrared light irradiation. Control over fluid flow rate and flow path planning was achieved by adjusting the wetting property of the channel wall surface and the bending angle of the microcolumns. Furthermore, by incorporating local hydrophobic modification and reversible deformation of the microcolumns, “micro-bridges” can be formed or opened to control the initiation and cessation of fluid flow.

Droplets can serve as independent reaction units and can be manipulated on functional surfaces through external stimuli, such as driving, fusion, and separation.40 They have significant application value in biochemical analysis41 and cell culture.42,43 Unlike the macroscopic shape changes of shape memory microfluidic chips, the functional surface of SMPs undergoes shape changes at the micro/nanoscale on its surface structures in response to external environmental triggers, thereby altering the surface wettability.44–47 Yang et al.48 fabricated high aspect ratio microcolumn arrays on an epoxy resin surface through molding. In the upright state, the contact angle of the droplets on the microcolumn array was greater than 150°. After heating and pressing the microcolumn array, the microcolumns tilted, and the contact angle of the droplets on the shaped surface became 127°. The droplets transitioned from the Cassie state to the Wenzel state. Creating patterns on SMP functional surfaces can achieve differentiated wettability. Zhang et al.49 created patterns on the surface of SMP microcolumn arrays to change their surface characteristics. For example, dot patterns can enhance surface adhesion, and line patterns can provide the surface with anisotropic wetting properties. Shao et al.50 reported the incorporation of carbonyl iron powder into shape memory polyurethane to confer photothermal responsiveness. Temporary shape microcolumn arrays can be used to adjust the microstructure of the SMP surface, thereby influencing the adhesion and sliding paths of droplets on the SMP surface through near-infrared light irradiation. However, the effect of wettability regulation solely based on the deformation of microstructures on the SMP surface is limited. Cheng et al.51 reported a SMP surface containing ZnO, under UV light irradiation, ZnO can regulate the chemical properties of the SMP surface, and under temperature stimulation, SMP can regulate the microstructure of the surface [Fig. 3(a)]. Compared to merely adjusting the surface topography, the synergistic regulation of surface chemistry and microstructure can significantly improve the controllability of the wetting state.

FIG. 3.

(a) Surface chemistry and microstructure coordinate to regulate the wetting state of the SMP functional surface. Reproduced with permission from Wang et al., Chem. Res. Chin. Univ. 39(1), 151–158 (2022). Copyright 2022 Springer Link.51 (b) Mechanism diagram of switchable droplet adhesion on the bionic SMP membrane surface. The liquid–solid contact area on the curved film is small, and the film surface shows low adhesion, while the expanded film increases the liquid–solid contact area of droplets, and the film shows high adhesion. Reproduced with permission from Wang et al., Nanoscale 11(18), 8984–8993 (2019). Copyright 2019 Royal Society of Chemistry.52 (c) The manipulation of water-in-oil droplets on a light/magnetic dual-responsive shape memory micropillar array chip. Reproduced with permission from Ye et al., Chin. Chem. Lett. 35(1), 108494 (2024). Copyright 2024 Elsevier.53 (d) Schematic diagram of droplet transportation in magnetic response SMP lubrication tube. Reproduced with permission from Wang et al., Adv. Funct. Mater. 33, 2305766 (2023). Copyright 2023 Wiley.54 

FIG. 3.

(a) Surface chemistry and microstructure coordinate to regulate the wetting state of the SMP functional surface. Reproduced with permission from Wang et al., Chem. Res. Chin. Univ. 39(1), 151–158 (2022). Copyright 2022 Springer Link.51 (b) Mechanism diagram of switchable droplet adhesion on the bionic SMP membrane surface. The liquid–solid contact area on the curved film is small, and the film surface shows low adhesion, while the expanded film increases the liquid–solid contact area of droplets, and the film shows high adhesion. Reproduced with permission from Wang et al., Nanoscale 11(18), 8984–8993 (2019). Copyright 2019 Royal Society of Chemistry.52 (c) The manipulation of water-in-oil droplets on a light/magnetic dual-responsive shape memory micropillar array chip. Reproduced with permission from Ye et al., Chin. Chem. Lett. 35(1), 108494 (2024). Copyright 2024 Elsevier.53 (d) Schematic diagram of droplet transportation in magnetic response SMP lubrication tube. Reproduced with permission from Wang et al., Adv. Funct. Mater. 33, 2305766 (2023). Copyright 2023 Wiley.54 

Close modal

The triggering methods for SMP surfaces are also diverse. Drawing inspiration from the gecko foot pads, Wang et al.52 created functional films by attaching s-PU microcolumn arrays onto water-responsive shape memory substrates, enabling the adjustment of surface adhesion through bending/unfolding changes. By controlling the degree of bending, droplets can be captured or released in situ [Fig. 3(b)]. Wang et al. explored the application of electric-responsive SMP surfaces in de-icing/anti-icing. Ma et al.55 integrated SMP functional surfaces into wearable electronic devices, demonstrating the potential applications in areas such as hydrophobic electronic skin and wearable droplet manipulators.

Continuous manipulation of droplets can be achieved by dynamically changing the microstructure of a functional surface through external stimuli. Park et al.6 developed a droplet manipulation platform based on an SMP functional surface. This strategy utilized an indium tin oxide heater to continuously change the surface microstructure of the SMP, thereby altering the surface wettability and controlling the droplet movement. However, this heating method undoubtedly extended the response time of the SMP, resulting in slow droplet movement. Additionally, exposing the droplets to an open environment increased the evaporation rate during heating. In order to address these challenges, Xu’s team10 developed a near-infrared (NIR) light-responsive shape memory microcolumn array microfluidic chip platform. This platform facilitated programmable manipulation of droplets through the continuous deformation of the microcolumns induced by NIR light. The microcolumn array chip was made of cross-linked ethylene-vinyl acetate copolymer, which was doped with modified gold nanorods, allowing the shape memory effect to be triggered by NIR laser. Prior to the initiation of droplet movement, the microcolumn array was temporarily tilted via heating and pressure application. Under NIR light irradiation, the microcolumn array restored to its original shape from the temporary shape. The Marangoni force induced by NIR light irradiation and the imbalance of surface tension caused by microcolumn deformation collectively drove droplet movement. Both the microcolumn array chip and the droplets were immersed in hexadecane to prevent droplet evaporation. The platform could manipulate droplets within a volume range of 0.1–10 μl. This method effectively reduced the response time of the SMP through NIR light heating. However, the shaping of the microcolumns still required manual operation, and immersing the microcolumn array chip in hexadecane increased the complexity of the device. To overcome these limitations, they designed a shape-reconfigurable shape memory microcolumn array chip to manipulate water-in-oil droplets.53 The microcolumn array possessed both magnetic and photothermal responsiveness, allowing the shape of the microcolumns to be changed under magnetic force and laser irradiation without manual intervention. The water-in-oil droplets provided a closed environment, avoiding evaporation in an open environment [Fig. 3(c)].

In addition to manipulating droplets on open surfaces, there are also techniques for droplet transport in shape memory tubes. Cheng et al.56 reported a shape memory tube with a smooth inner surface for droplet transport. The shape memory tube can be programmed into different asymmetric states, and these asymmetric states can change the curvature radius at both ends of the droplet, inducing a Laplace pressure difference to drive droplet motion. This strategy enables control over the speed and direction of droplet transport. Moreover, Cheng’s team54 presented a magnetic-responsive shape memory tube. In this configuration, the internal microstructure of the tube was regulated by a magnetic field. This allows for on/off control of droplet transport, and by controlling the shape of the tube, more precise control over the speed and direction of droplet motion can be achieved [Fig. 3(d)].

In summary, researchers have developed a variety of shape memory microfluidic chips. The response time of these shape memory microfluidic chips typically ranges from 5 to 30 s,4,9,37,54 with the shortest response time being as fast as 100 ms.35 Increasing the temperature or improving the thermal conductivity of polymers can effectively shorten the response time of temperature-responsive SMPs. In addition, alternative triggering methods like near-infrared light triggering and electrical triggering can also be employed for more efficient activation. In Table I, we summarized the important indicators, materials, external stimulation methods, and response temperatures of some typical shape memory microfluidic chips.

TABLE I.

Comparison of important indicators, materials, external stimulation modes, and response temperatures for different types of shape memory microfluidic chips.

Shape memory microfluidic chipsImportant indicatorsMaterialsExternal stimulusResponse temperature (°C)Reference
Fluid drive 0.22–0.40 mm3 s−1 (flow rate) Poly(cyclooctene) Hot water 60 4  
24 μl/min (flow rate) Butyl ethacrylate/methyl methacrylate Hotplate 40 33  
 Poly (vinyl alcohol) Light 82 9  
2.4 μl/min (flow rate) Methyl thacrylate/butyl methacrylate/TEGDMA Hotplate 40 34  
Fluid regulation (1.1 ± 0.3) × 102 kPa (withstand pressure) Poly(ɛ-caprolactone) Microheater 52.3 35  
 SMP MM4520 CB/PDMS heater 45 5  
Fluid manipulation 0.05–0.35 mm/s (flow rate) Ethylene-vinyl acetate Light 52 37  
Switchable wetting 160–700 μN (adhesion force range) Polyurethane Light  47  
 Polyurethane-cellulose nanofiber Water  51  
Droplet manipulation 0.13 mm/s (flow rate) Ethylene-vinyl acetate Light 52 10  
 Epoxy mixture Heater 62 53  
Shape memory microfluidic chipsImportant indicatorsMaterialsExternal stimulusResponse temperature (°C)Reference
Fluid drive 0.22–0.40 mm3 s−1 (flow rate) Poly(cyclooctene) Hot water 60 4  
24 μl/min (flow rate) Butyl ethacrylate/methyl methacrylate Hotplate 40 33  
 Poly (vinyl alcohol) Light 82 9  
2.4 μl/min (flow rate) Methyl thacrylate/butyl methacrylate/TEGDMA Hotplate 40 34  
Fluid regulation (1.1 ± 0.3) × 102 kPa (withstand pressure) Poly(ɛ-caprolactone) Microheater 52.3 35  
 SMP MM4520 CB/PDMS heater 45 5  
Fluid manipulation 0.05–0.35 mm/s (flow rate) Ethylene-vinyl acetate Light 52 37  
Switchable wetting 160–700 μN (adhesion force range) Polyurethane Light  47  
 Polyurethane-cellulose nanofiber Water  51  
Droplet manipulation 0.13 mm/s (flow rate) Ethylene-vinyl acetate Light 52 10  
 Epoxy mixture Heater 62 53  

Shape memory microfluidic chips provide new approaches for fluid and droplet manipulation. Compared to traditional microfluidic devices, shape memory microfluidic chips require simpler and smaller auxiliary devices, offering great potential for improving the multifunctionality and integration of microfluidic chips. They are expected to serve as the materials for the next generation of microfluidic chips. The fluid manipulation strategies based on SMPs are particularly well-suited for situations where precise fluid control is not necessary, such as wearable devices, self-driven microfluidic chips, and on-site detection devices. In addition, in biomedical research (e.g., organ-on-chip), shape memory microfluidic chips can construct more complex and three-dimensional in vitro models, and the local or overall shape of the microfluidic chip can be regulated, which is not possible with traditional microfluidic chips. Wearable devices based on shape memory microfluidic systems can change their morphology according to the user actions or environmental changes. Shape memory microfluidic devices are also suitable for use on spacecraft with limited space, effectively reducing the complexity and weight of the device. With the availability of the various triggering methods of shape memory polymers, suitable SMPs can be chosen and integrated into microfluidic chips according to specific requirements. So far, the most common shape memory microfluidic chips are fabricated based on temperature-responsive SMPs, but these SMPs generally have low thermal conductivity, leading to uneven temperature distribution when direct heating is applied. Therefore, it is becoming more common to incorporate conductive materials or photothermal materials into SMPs to trigger the shape memory effect of microfluidic chips through electric fields or NIR laser.

The slow response speed of SMPs is a major challenge limiting the further development of shape memory microfluidic devices. Currently, the response time of SMPs generally ranges from a few seconds to tens of seconds, which fall short of achieving instantaneous response. Developing SMPs with faster response time is crucial to overcome this limitation, and there are already literature reports on SMPs with response time in the millisecond range. In addition, some shape memory microfluidic devices still require manual shaping to change their initial shape, which greatly undermines the automation level of the devices. Currently, the solution to this problem involves incorporating magnetic materials into SMPs and utilizing magnetic force to change the shape of the SMPs. However, the inclusion of magnetic particles in SMPs may potentially interfere with the chemical reaction on the chips. In the future, two-way SMPs or triple SMPs can be introduced into the microfluidic systems so that the shape memory microfluidic chips can realize the control of different shapes solely by changing the external environment. In summary, the current shape memory microfluidic devices can serve as powerful complements to traditional fluid manipulation techniques, expanding the application areas of microfluidic technology.

The authors acknowledge the Fundamental Research Funds for the Central Universities (Nos. N232410019 and N2005024), Natural Science Foundation of Chongqing China (Grant No. cstc2021jcyj-jqX0031), and Interdiscipline Team Project under auspices of “Light of West” Program in Chinese Academy of Sciences (Grant No. xbzg-zdsys-202106).

The authors have no conflicts to disclose.

Wen-Qi Ye: Conceptualization (equal); Investigation (equal); Writing – original draft (equal). Wei Zhang: Supervision (equal); Writing – review & editing (equal). Zhang-Run Xu: Conceptualization (equal); Supervision (equal); Writing – review & editing (equal).

Data sharing is not applicable to this article as no new data were created or analyzed in this study.

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