Functional microfluidics: theory, microfabrication, and applications

Microfluidic devices are composed of microchannels with a diameter ranging from ten to a few hundred micrometers. Thus, quite a small (10−9–10−18 l) amount of liquid can be manipulated by such a precise system. In the past three decades, significant progress in materials science, microfabrication, and various applications has boosted the development of promising functional microfluidic devices. In this review, the recent progress on novel microfluidic devices with various functions and applications is presented. First, the theory and numerical methods for studying the performance of microfluidic devices are briefly introduced. Then, materials and fabrication methods of functional microfluidic devices are summarized. Next, the recent significant advances in applications of microfluidic devices are highlighted, including heat sinks, clean water production, chemical reactions, sensors, biomedicine, capillaric circuits, wearable electronic devices, and microrobotics. Finally, perspectives on the challenges and future developments of functional microfluidic devices are presented. This review aims to inspire researchers from various fields—engineering, materials, chemistry, mathematics, physics, and more—to collaborate and drive forward the development and applications of functional microfluidic devices, specifically for achieving carbon neutrality.

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Introduction
Microfluidic phenomena with micro/nanoscale features based on the capillary force were first discovered by da Vinci [1].In 1718, Jurin [2] proposed the famous Jurin criterion, demonstrating the inversely proportional relationship between the height of the liquid and the diameter of the microtubes.In 1751, Segner [3] discovered the surface tension of fluids.In 1806, Thomas Young and Pierre Simon Laplace [4] independently deduced the relationship between the additional pressure of the curved liquid surface, surface tension, and the radius of curvature of the liquid based on the wettability theory for the liquid/solid interface.Subsequently, Gauss [5] modified the capillary theory based on an energy analysis in 1830.Gibbs [6] considered free energy and chemical potential within the framework of thermodynamics (figure 1).However, those famous theories on capillary phenomena can only be applied to the ideal circular microchannels, and numerous simulation methods were thus applied to the theoretical analysis of microfluidics with the fast development of computers, such as the computational fluid dynamics (CFD) method [7], the molecular dynamics (MD) method [8], as well as several mesoscopic methods such as the lattice Boltzmann method (LBM) [9] and the direct simulation Monte Carlo method [10].
Meanwhile, the great demand for microfluidic devices is severely limited by the fabrication methods.Blowing glass tubes and parallel plates were commonly used to validate these proposed theories before Terry et al [11] performing a gas chromatographic air analyzer on a silicon substrate in 1979, ushering in the era of fabrication for microchannels based on lithography of silicon.In 1990, Manz et al [12] achieved the miniaturized total chemical analysis systems based on a silicon-based microfluidic chip, which is regarded as the prototype of functional microfluidic devices.However, the shortcomings of expensive and limited applications of silicon-based microchannels arouse the fast development of materials and fabrication methods for microfluidic devices afterward.Microfluidic systems based on polydimethylsiloxane (PDMS) were first developed by Mcdonald et al [13] in 2000.Then, various new materials were introduced to the fabrication of microchannels, such as polymethylmethacrylate (PMMA) [14], epoxy resins [15], hydrogels [16][17][18], and other plastics [19].However, these materials and traditional fabrication methods are limited to two-dimensional (2D) microchannels, including the popular patterned paper [20].In 2018, Yuan et al [21] fabricated structured multimaterial fibers with complex cross-shapes by using a thermal drawing process.With the fast development of three-dimensional (3D) printing techniques [22][23][24], more and more microfluidic devices are fabricated with such an advanced technique.In 2023, Wang et al [25] fabricated a bionic microfluidic device with arbitrarily shaped and sized microchannels based on projection microstereolithography (SLA) based 3D printing technique.
The fast development of the theory and fabrication methods for microchannels drastically drives their applications.In 2006, Whitesides [26] predicted the future development of microfluidics, pointing out that microfluidics devices can be used in many fields, including screen conditions, manipulation of multiphase flows, chemical synthesis, cell biology, and microanalytical devices.He also pointed out that the future development of microfluidics would be in the design and manufacturing systems for microfluidic devices.Due to the merits of microchannels, such as compactness, microscale, and ease of integration discovered in the past few decades [26], the applications of microfluidic devices have been also boosted in various fields, such as heat sinks [27], heat transport [28], diagnostics [29], detection [30], biomedical engineering [31], production of materials [32], and reactors [33].Recently, more and more complex microfluidic devices have been proposed [34], including capillaric circuits [35], wearable electronic devices [36], microrobotics [37,38], and triboelectric generators [39,40].
Over time, functional microfluidics referring to the integration of various components and functionalities to perform specific tasks and functions has achieved a lot in the past century.They are designed to not only manipulate fluid flow but also incorporate additional features such as sensing, mixing, separation, reaction, and detection capabilities.Functional microfluidics enables more complex and versatile operations, allowing for a wide range of applications in various fields like chemistry, biology, medicine, engineering, and so on.However, there are still many deficiencies in microfluidics which severely limit their further development, especially in materials, fabrication of scale-up microfluidic devices, programmability of functional microfluidic devices, and so on.Herein, in this review, we summarize the recent progress in functional microfluidics from theory, fabrication methods to various promising applications.First, we briefly introduce the theories and numerical methods for studying transport phenomena in microfluidics.Then, we focus on the manufacturing methods of functional microfluidic devices simultaneously considering categories and materials.Also, the recent significant advances in its applications for microfluidic devices in various fields are highlighted.Finally, we provide personal perspectives on the challenges and future trends of functional microfluidic devices, especially in their applications on carbon neutrality.

Interface and theory for the wettability
Wetting of liquids on solid surfaces is a common phenomenon in nature [41][42][43].The main factors affecting the wettability of a solid surface are its chemical composition and roughness [44][45][46].With the development of technology over the past decades, the wettability of a solid surface has been studied extensively.As shown in figure 2, there are superhydrophilic, superhydrophobic, superoleophilic, and superoleophobic phenomena in air, superoleophobicity, superoleophilicity, superaerophobicity, superaerophilicity, and superhydrophilic phenomena underwater, as well as superhydrophobic phenomena in oil [47].To classify the wettability of different kinds of surfaces, a series of theories from Young's equation to the Wenzel and Cassie-Baxter models have been established.For smooth surfaces under equilibrium at the interface of solid and liquid (figure 3(a)), Young's equation describes the relationship between the solid-gas (γ SG ), solid-liquid (γ SL ), and liquid-gas (γ LG ) interfacial tensions to the static contact angle (θ) as follows [48]: ( The static contact angle is a quantitative characterization of the surface wettability [49].It is expressed as the angle between the liquid/vapor interface to the solid surface when the liquid contacts with a solid surface.However, the static contact angle is not enough because the contact angle of an actual surface fluctuates under the action of external forces [50].For a moving droplet, the maximum and minimum contact angles are called advancing (figure 3(b)) and receding (figure 3(c)) contact angles, respectively.Moreover, a smooth surface does not exist in reality, thus the effects of solid surface's roughness on its contact angle must be considered.For rough surfaces with uniformly distributed microstructures, the commonly used models for illustrating their contact angles are the Wenzel and Cassie-Baxter models.In the Wenzel model [51], the gaps between microstructures are assumed to be uniformly distributed, who are filled with liquid (figure 3(d)).Thus, the surface roughness (R 0 ) is introduced to characterize its effects on the static contact angle of rough surfaces.The modified contact angle can be expressed as where θ * and θ are the contact angles of rough (after correction) and smooth surfaces, respectively.R 0 is the roughness of a surface (for rough surfaces, R 0 is invariably greater than 1).Generally speaking, the rougher the surface is, the greater the hydrophilicity or hydrophobicity.However, for some surfaces with extremely-high roughness, the absolute value of the righthand side of the foregoing equation ( 2  In the Cassie-Baxter model [52], the assumption is that the microstructures on a solid surface are not penetrated by the liquid.Instead, it is treated as a composite surface (figure 3(e)).Hence, the states are divided into two cases, solid-liquid contact and gas-liquid contact.The expression for such a phenomenon is where f s and f v represent the ratios of the areas in the solidliquid contact and gas-liquid contact to the total contact area, respectively.In this case, assuming that the contact angle at the gas-liquid interface is The superhydrophobicity of a rough solid surface is well explained by the Cassie-Baxter model.That is, the superhydrophobicity appears when the ratio of the solid-liquid contact area to the total contact area is extremely small.Moreover, an intermediate state in which the liquid is partially submerged on a rough surface is shown in figure 3(f) [53].However, the foregoing model is derived under ideal conditions, and many problems are encountered in practical applications.In subsequent, the Wenzel and Cassie models are found capable of transforming and coexisting under certain conditions [54].With the development of these surface wettability theories, the wettability of many biologically inspired surfaces with special microstructures has been fully investigated [55].These unique characteristics include the fluid harvesting properties of caninervis (figure 4(a)) [56], cactus spines (figure 4(b)) [57], and desert beetles (figure 4(c)) [58], the superhydrophobic properties of Salvinia molesta leaf (figure 4(d)) [59], butterfly wings (figure 4(e)) [60], and water strider legs (figure 4(f)) [61], as well as the fluid self-transport properties of Nepenthes alata (figure 4(g)) [62], spider silk (figure 4(h)) [63], and shorebirds (figure 4(i)) [64].All the foregoing unique surfaces in nature are inextricably linked to surface wettability theories.

Driving force and hydrodynamics inside microchannels
The capillary circulation in the early renaissance and the modern capillary action in the micro/nano-field was first discovered by Leonardo da Vinci, which is commonly found in all aspects of life (figure 1).Typically, the capillary circulation phenomenon can be simplified as a fine microchannel being vertically inserted into liquid.Based on the theory of surface wettability, the liquid wets the wall of the microchannels and rises inside of them, while the liquid that cannot wet the wall of the microchannels drops inside.Such a fluid transport behavior enabled by the capillary force is called the capillary phenomenon [65].In 1718, Jurin [2] proved that the height of the liquid in microchannels is inversely proportional to their diameters, which is the famous Jurin criterion: where h is the height of capillary rise or fall, γ is the surface tension of liquid, r is the radius of the capillary tube, ρ is the liquid density, g is the gravitational acceleration.
In addition, the Gaussian capillary equation related to the perimeter and area can be applied to calculate the curvature of the end concave liquid surface to obtain the capillary rise height [66].Furthermore, Kelvin [67] theoretically described the change in vapor pressure caused by the curved liquid-gas interface in microchannels, where R g , T represent the gas constant and the thermodynamic temperature, respectively.P r , P 0 are the saturated vapor pressure of the droplet and the plane liquid, respectively.γ is the surface tension of the liquid, M is the molar mass and R d is the radius of a droplet.The equation ( 6) explains various microfluidic phenomena, such as capillary adsorption, coalescence, and meniscus radius.Previous studies on the capillary phenomena are mostly  [57].Reproduced from [57], with permission from Springer Nature.(c) Desert beetles [58].Reproduced from [58], with permission from Springer Nature.Superhydrophobic properties of (d) S. molesta leaves [59].Reproduced from [59].Used with permission of The Royal Society (U.K.), from [59]; permission conveyed through Copyright Clearance Center, Inc. (e) Butterfly wings [60].[60] John Wiley & Sons.© 2023 Wiley-VCH GmbH.(f) Legs of water striders [61].Reproduced from [61], with permission from Springer Nature.Fluid transport properties of (g) N. alata [62].[62] John Wiley & Sons.[© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim].(h) Spider silk [63].Reproduced from [63].CC BY 4.0.(i) Shorebirds [64].From [64].Reprinted with permission from AAAS.
discussed from the perspective of hydrostatics.In contrast, the steady-state flow of the liquid in microchannels can be approximated as Hagen-Poiseuille flow, 1 r d dr ( r dv dr where v is the flow velocity along the tube length (z-axis), and η is the viscosity of the fluid.In 1921, Washburn derived the relationship between the horizontal transport distance of liquid and the time in the initial stage of capillary rise based on Poiseuille's law for porous media [68], where c is the capillary shape factor, and t is the duration of the capillary rise process.The whole process will be greatly affected by capillary force, gravity, inertia force, and viscous force at different stages, where the capillary force is the driving force while the rest forces are the resistance terms.Capillary force enables the long-distance transport of liquids (figure 5(a)) [69].Recently, the involvement of external forces in microfluidics is attracting more and more attention [70,71].For example, electric field forces have been added to the microfluidic devices, including electrically induced wetting, liquid flow, and droplet motion enabled by charges [72,73] (figure 5(b)).In addition, liquid motion can also be achieved by the momentum carried by photons transferred to the surface of the liquid, which is also known as radiation pressure (figure 5(c)) [74].Moreover, light-induced capillary forces can be used to generate wettability gradients and Marangoni effects to drive the fluids.Also, a moving magnet can be used to control the movement of droplets containing magnetic particles (figure 5(d)) [75].The advantages of magnetic driving force over electric driving force are that they can be operated manually without an additional external power supply.Furthermore, surface acoustic waves are also used in microfluidic devices for the manipulation of cells and particles owing to their simplicity, low cost, fast fluid drive, and broad compatibility (figure 5(e)) [76].

Simulation methods for functional microfluidic phenomena
In general, capillary theories with analytical solutions can only be applied to a few special situations.In addition, the experimental study of capillary phenomena strongly depends on the highly precise processing and fabrication of micro/nanochannels, leading to a fact that the results obtained from theory and experiments are inconsistent with each other, though the trends are similar to each other [77].Therefore, numerous simulation methods (table 1) have been proposed to reveal the underlying mechanisms for those phenomena obtained from experiments in the past century [78][79][80].These numerical methods have been widely used for simulating multiphase flow in microchannels, capillary filling, surface wettability, solid-liquid interface slip, liquid flow in porous media, and so on [81].
The flow states of the fluid are typically gaseous, liquid, and multiphase.The most popular numerical calculation methods used from the macroscopic point of view are CFD [7], such as the finite element, finite volume, and finite difference methods.These methods are based on the continuous medium model of the fluid, ignoring microscopic interactions.Then, the Navier-Stokes equation is solved to obtain the state of motion of the  [74].Reproduced from [74].CC BY 4.0.(d) Schematic of droplet manipulation by magnetic force [75].Reproduced from [75] with permission from the Royal Society of Chemistry.(e) Manipulation of cells in fluids by varying sound field frequency [76].Reprinted from [76], Crown Copyright © 2016 Published by Elsevier B.V. All rights reserved.fluid.However, these macroscopic approaches never consider surface wettability effects.
The numerical calculation technique used from the microscopic perspective of the fluids which can consider surface wettability effects is the MD method [8] and the LBM [9].The MD method is based on the Newtonian classical mechanics for modeling the physical trajectories and states of atoms/molecules [79].Thus, the results obtained from MD simulation method can provide a more detailed description of the conformational space, equilibrium properties, dynamics of molecules, and statistical properties.Unfortunately, even a simple question calculated by MD method requiring an overly long simulation time is a critical drawback for such a method.Moreover, because the chemical bonds among molecules or atoms are established in MD simulations, they cannot be broken or created during chemisorption, chemical reactions.Also, the time and scale of the MD simulations are relatively microscopic, typically below 10 ns at scales below 10 nm [84].Based on MD method, Martic et al [85] investigated capillary flow processes applying Washburn equations to characterize the properties of porous media and obtained a more satisfactory fit.Koplik et al [86] studied the low-Reynolds-number Poiseuille flow and kinematic contact lines using MD method.
The LBM is a CFD method at the mesoscopic scale [9], which is now widely used to describe multiphase flow and phase-change heat transfer problems in which wettability effects play a key role [87].The LBM is simple and efficient in describing fluid interaction.It can be easily used to set boundary conditions of complex geometries and different wetting conditions, and is well adapted to multiphase flow and multicomponent problems.It is also highly parallelizable.However, the key drawback of this method is instability under certain conditions [88].In 1996, Chen et al [89] proposed simple extrapolation to simulate the capillary fluid flow and 2D Poiseuille flow with LB method.In 1997, Spaid and Phelan [90] improved the traditional LBM to solve the microscale flow in fibrous porous media.In 2006, van der Graaf et al [91] investigated the process of droplet formation in individual pores of T-shaped microchannels.
The DSMC is also used for the numerical study of microfluidics, which is a direct particle simulation method based on kinetic theory [10].The DSMC is highly applicable to the solution of flow problems with high Knudsen number [92], which is quite suitable for the commonly high Knudsen number flow of gases in microchannels [93].Compared with those other methods, the runtime and storage requirements are the two major limitations of DSMC simulation method [94].
In addition, there are numerical and statistical errors for the DSMC method because its computational results are determined by the sample size and averaging process, and the solution must be quantified to verify the accuracy of results [95].
In the past, the DSMC method was mainly applied to thinatmosphere dynamic problems [96].In 2003, Wang and Li [97] used DSMC to study the non-ideal gas flow and heat transfer in micro/nanochannels.In 2004, Wang and Li [98] simulated microgas flow in micro-electro-mechanical system (MEMS) devices using a modified DSMC method, and the geometry of the microchannels significantly affected the flow performance inside them.In 2016, Kawagoe et al [99] used the DSMC method to simulate the pressure-driven gas flow through porous media.The results show that Darcy's law can be applied to porous media with micro/nanopores, which open the gate for theoretical study of such kind of structures.

Materials and fabrication methods for functional microfluidic devices
The applications of functional microfluidic devices are severely limited by the fabrication methods [100,101].State-of-the-art fabrication methods used in the semiconductor industry decrease the minimum characteristic sizes of microchannels down to several micrometers.However, the enormous initial investment and extreme-high operational fee are unacceptable.Moreover, ensuring that the facility can provide patterns with feature sizes matching the critical dimension required by microfluidic devices is critical [102].Therefore, adopting other available fabrication methods with various resolutions to manufacture functional microfluidic devices [103,104] is necessary.After the first transistor being invented by the team of Shockley in 1947, many advanced fabrication techniques, including CO 2 lasers [105,106], femtosecond lasers [107][108][109], imprinting methods [14], molding [110,111], sintering [24,112], etching [113][114][115], lithographic fabrication [116,117], 3D printing techniques [41,[118][119][120][121][122][123][124], etc. have been well developed and applied to the manufacture of functional microfluidic devices (figure 6 and table 2).
Another primarily used material for biomedical microfluidic devices is PMMA [126].It is an amorphous thermoplastic [142] owning attractive mechanical and chemical properties, considerable toughness, excellent insulativity, outstanding weather resistance, satisfactory processing characteristics, and excellent compatibility [128,133,151].In addition, the transparent PMMA enables light of various wavelengths to pass through [14] with unique aging resistance [14].Thus, PMMA has been extensively applied to microfluidic devices, including pressure-driven 3D microfluidic chips with multiple logic Boolean functions [143], PMMA optical detection chips [144], pneumatic microvalves and micropumps [145].However, PMMA is easily scratched because of its extremely low elastic modulus [126].
Resins are also utilized to manufacture microfluidic devices [41,69,152,153] because of their excellent mechanical properties, chemical stability, high/low-temperature resistance, low shrinkage rate, low cost, etc [15].The resins are mainly composed of polymer monomers and prepolymer into which a light initiator is added.Under the action of a certain ultraviolet (UV) light (250-405 nm), polymerization reaction inside resins occurs, conversing them from liquid to solid, forming the designed complex 3D structures [154].Functional microfluidic devices made of resins have been applied to underwater anaerobic chemical reactions [155], bionic open microchannels for transpiration [156], cellular fluidics [69], bending tubes with peristome-mimetic structures for controlling water elevation [157], and so on [158,159].
Hydrogels are hydrophilic 3D network structured gels that rapidly expand in water and absorb a lot of water without dissolving in the swelling state [133,160].Reproduced from [178].CC BY 4.0.(e) Schematic of functional microfluidic devices fabricated by using imprinting methods.(f) Functional microfluidic devices with more complicated structures manufactured by imprinting thermoplastic substrate with silicon wafer [179].Reproduced from [179] with permission from the Royal Society of Chemistry.

Traditional fabrication methods for microfluidic devices
3.2.1.Laser methods.The process of microfluidic devices fabrication by laser is a complicated combination of photothermal and photochemical actions [166,167].Some chemical bonds in the workpiece burst directly during photon absorption process, while the others will be thermally burst due to the heat released by excited molecules [168][169][170].Such a process is dominated by the frequency and wavelength of laser beam [171].Infrared radiation with a wavelength of 10.6 µm is commonly employed for such a method [105].The temperature of workpiece rapidly increases wherever the continuous CO 2 laser beam is focused on it.Consequently, the material melts and disintegrates to form microchannels (figure 7(a)).The strength of chemical bonds and structures of materials are crucial to the decomposition process.For example, as one of the most used materials for fabricating microfluidic devices, PMMA [172] vaporizes in the form of monomers producing microchannels when the temperature considerably exceeds its melting point.In contrast, femtosecond lasers [108,173,174] typically own a wavelength of 1030 nm, pulse width of 250 fs, frequency of 100 kHz, and focal point of 5 µm.Such a method has an advantage over traditional lasers in fabricating transparent capillary tubes with micro/nanopatterns on internal surfaces [128].In particular, the micro/nano-grooves in the axial direction have an active influence on capillary rise [175,176], such as a Janus membrane for bubble unidirectional transportation underwater (figure 7(b)) [177].

Molding methods.
As a traditional method, the molding method is one of the most convenient and cheap approaches to manufacture functional microfluidic devices [111], which fundamentally leverages the viscoelasticity, optical transparency, biological compatibility, durability [180,181], and biodegradability [182,183].First, a gel wire is used to determine the geometric parameters of the microfluidic devices, which have been prepared and cured using an aqueous solution of glycerol and agarose in a glass microchannel.Then, the gel wire is arranged in the desired pattern on a film of precured PDMS in a dish covered by another layer of PDMS, which will be cured below the melting point temperature of agarose.Finally, the PDMS microchannels are formed after removing the original gel wires using boiling water and hydrophobic treatment (figure 7(c)).The two major problems during the fabrication of the negative replica are the formation of small pores induced by spontaneous de-wetting and discontinuous seam (figure 7(d)) [178].
3.2.3.Imprinting methods.Imprinting methods are simple, convenient, and low-cost for fabrication of functional microfluidic devices [184,185].They obviate the indispensability of the thermoplastic substrate during the stamping process and improve the production efficiency to more than 100 samples per template.The repeatability of imprinting methods is extremely high that the difference of the imprinted microchannels is less than 1% before and after dozens of imprints [14].A small wire is impressed on a plastic substrate softened by heating for the first-generation imprinting technique [186].In contrast, the second-generation imprinting technique involves the manufacture of functional microfluidic devices with more complicated structures by imprinting a thermoplastic substrate with a silicon wafer (figure 7(e)) [14].In addition, other silicon etching process and materials can be used to fabricate microchannels with high aspect ratios [14].They can overcome problems caused by the anisotropic etching angle, which makes a big difference on the feature size of functional microfluidic devices [126].Finally, an open microchannel can not only attach to other microchannels but also serve as a flexible and adhesive polyfilm to seal the microchannels [14].More importantly, the length and inner diameter of microchannels can be conveniently regulated to obtain the best microfluidic performance (figure 7(f)) [179].

Lithography and etching
With the rapid development of functional microfluidic systems, the methods to fabricate microfluidic devices with more complex 3D geometries, smaller feature sizes, and larger sample widths are in urgent need [187].Thus, lithography and etching techniques have been proposed and developed by researchers for manufacturing complex functional microfluidic devices even at the submicron-scale [188].

Lithography.
Lithography can be used to manufacture functional microfluidic devices with extremely high precision (figure 8(a)) [189].First, a layer of uniform photoresist is deposited on a silicon dioxide (SiO 2 ) wafer.Then, a mask is used to expose and cure the photoresist and form a cured geometric figure completely corresponding to the mask on the photoresist layer.A pattern corresponding to the mask on the photoresist is developed, which causes the SiO 2 wafer to resist the etching process.A wafer shape corresponding to the photoresist pattern is etched into the SiO 2 using various light sources.Finally, the photoresist layer is peeled off, and an upper cladding layer is deposited onto the already formed wafer layer pattern.The feature sizes of the microchannels depend on the wavelength of the light sources in the exposure system.
To date, many light sources, such as UV, deep UV, x-rays, silicon-oxide-nitride-silicon [116], are available.One of the advantages of lithography is that it can precisely control the shape and size of the patterns [190].In addition, it can generate contours on the entire chip's surface.However, as the main disadvantage of lithography, it must be used on flat surfaces under extremely clean conditions.Also, it is less effective when used on uneven surfaces.Currently, an inexpensive and convenient approach to manufacture microchannels is combining soft lithography and molding methods [148].First, a 30 µm-thick initial mold of the SU-8 layer determining the shape of the final microchannels is created by photolithography.Then, the liquid PDMS prepolymer is poured onto the mold to obtain the same structure as the initial SU-8 layer after complete curing at 65 • C for 2 h.The bottom plane of the microchannels, manufactured by the spin costing method with the support of oxygen plasma activation, is bonded to the PDMS microchannel layer (figure 8(a)).Owing to the excellent elasticity of the materials, the PDMS functional microfluidic devices exhibit a considerable capability of flexibility.Moreover, such a straightforward and low-cost fabrication process can create functional microfluidic devices with complex topologies and diameters ranging in 25-150 µm for various biomedical applications (figure 8(b)) [149].

Etching method.
Etching, a technology of pattern processing associated with lithography, is a process of removing undesired materials from a silicon wafer using chemical/physical methods [191].Wet etching and dry etching are two simple and typically used etching techniques.Wet etching uses solvents/solutions for etching [192], which is a purely chemical reaction process [193].Accordingly, a solution and pre-etching materials are used to remove the parts that are not masked by the film materials.It has the advantages of satisfactory selectivity, excellent repeatability, high production efficiency, simplicity in terms of equipment and low cost [114].Its disadvantages are the necessity for considerable drilling, inadequate control over graphics, and waste of considerable amounts of chemicals [194,195].In contrast, there are various dry etching methods, including light volatilization, vaporphase etching, plasma etching, metal etching, dielectric etching, silicon etching [196], and so on.There are several advantages of dry etching methods for fabrication of microfluidic devices, such as repeatability, acceptable anisotropy, controllability, high selection ratio, flexibility, and easy automation [136].Moreover, they are clean without producing chemical waste or introducing pollution during the treatment process [136].However, dry etching is costly and requires complicated equipment [197].
An efficient, low-cost, and distinct method for manufacturing functional microfluidic devices involves combination of etching and thermal blowing [114].Square microchannels with a depth of 30 µm can be patterned on a silicon wafer with photoresist using deep reactive ion etching.Applying anodic bonding, a 500 µm-thick glass is irreversibly bonded with a silicon wafer at 410 • C and 1 atm in a nitrogen environment.Because the softening point of glass is around  [113].(d) Functional microfluidic devices manufactured by etching methods [113].Reproduced from [113] with permission from the Royal Society of Chemistry.820 • C, the sample must be annealed at a temperature around 820 • C-950 • C for 2 h.The glass behaves as a Newtonian fluid above the softening point and is deformed by trapped air (like glass blowing), forming cylindrical microchannels during bonding.Finally, the samples with different shapes and feature sizes (fabricated without any chemical foaming agents or pollutants) must be cooled gradually to keep them transparent and free from crazing (figure 8(c)).Depending on the annealing temperature, circular microchannels with a diameter ranging from 20 µm to 75 µm have been fabricated by such a method [113] (figure 8(d)).Furthermore, a method for nanochannels' fabrication via the galvanic corrosion of coupled metals has also been previously reported [198].

Fused deposition modeling (FDM).
As one of the most widely used 3D printing technology, FDM is an additive manufacturing process belonging to the material extrusion family (figure 9(a)) [209].The material used for such a method is thermoplastic polymer in the form of filaments, which are fused at temperatures ranging from 190 • C to 230 • C. The filaments are selectively squeezed out through the nozzle with a circular hole of 200 µm on the platform to build objects layer by layer in a predetermined path.The distance between the nozzle and platform, extraction speed of the material, and relative moving speeds of the nozzle and platform are the primary parameters influencing the feature sizes and shape of printed objects.The FDM technology is typically used to fabricate functional microfluidic devices with molding methods [150].A sacrificial microchannel mold is firstly manufactured with a polyvinyl alcohol (PVA) filament and then removed from the printing platform, which determines the shapes of the microchannels.The microchannel mold is placed on a thin cured polymer layer and covered with a fully degassed uncured Photopolymer is jetted through linearly arranged nozzles in a continuous or drop-on-demand mode, sprayed onto the platform in a form of microdroplets, and finally cured with an even UV light source to form functional microfluidic devices [210,211].(d) Spiral microchannels [199].[199] John Wiley & Sons.© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(e) Single-photon polymerization (SPP) process repeatedly occurs on the surface of photosensitive resin where the UV light irradiates to fabricate microfluidic devices [212].(f) 3D capillary ratchet-induced liquid directional steering [213].Reproduced from [213].CC BY 4.0.(g) Focal point moves over the photosensitive area to solidify the material with the two-photon absorption effect [214].(h) A printed complex microscale check valve [215].Reproduced from [215], with permission from Springer Nature.
polymer.An intact microchannel is obtained after ultrasonic cleaning in a water bath and eliminating the printed PVA mold.Furthermore, sacrificial molding based on FDM for fabrication of microchannels within biomimetic matrix can be potentially applied in tissue engineering (figure 9(b)) [201].

Polyjet/multijet modeling (MJM)
. MJM exhibits high resolution and incomparable compatibility with various materials having a wide range of features (e.g.hardness, flexibility, long-range elasticity, and multiple colors [210]) numbering more than 100 [216].However, the materials are expensive and exclusive.The photopolymer is jetted through linearly arranged nozzles in continuous or drop-on-demand mode, sprayed onto the platform in the form of microdroplets, and finally cured with an even UV light source.The water-soluble holder, which is completely removed after finishing the printing, is added under a high cantilever structure during processing.Depending on the feature sizes of the functional microfluidic devices, the water-soluble holder must be soaked in water for a period ranging from 1 h to 6 h, which is followed by ultrasonic cleaning in 2% NaOH solution for 1-3 d (figure 9(c)).
For a roller texture direction similar to that of the microchannels, the 3D printer orientation must be aligned with the nozzles [199].The smallest feature size of the printed microchannels is approximately 200 µm.Based on polyjet modeling, the same coaxial nozzle method was reused by Gao et al [161].Once the sodium alginate solution and calcium chloride solution come into contact, Ca 2+ diffuses into the sodium alginate solution and begins to crosslink, producing a calcium alginate filament with hollow microchannels.In addition, hollow microchannels with inner gelled alginate and outer ungelled alginate can be manufactured by regulating the concentration and flow rate.Finally, by printing on the previous layer via the vertical movement of the z platform, the desired microchannels will be obtained [217].After confirming the printability and stabilization of the hydrogels, spiral microchannels can be prepared to mimic the blood vessels (figure 9(d)).

Digital light processing (DLP).
As one of the most popular 3D printing techniques, DLP printers project the image signal after digital processing [158,218], which is based on the digital micromirror devices (DMDs) for achieving the Bionic structures [69,155], seawater desalination [158] Two-photon polymerization (TPP) Fabrication of nanostructures, high-speed Extremely expensive, few material options Microrobots [237], biomedical engineering [238] visual digital information display [212,219].The precision of the functional microfluidic devices is determined by the projected DMD pixel size and the optical system, which is currently as small as 500 nm.Every 2D slice from the 3D functional microfluidic devices is projected onto the platform through a release liner in the form of UV light (405 nm).The light curing process occurs on the top of the photosensitive resin where UV light irradiates.This operation process is repeated after the longitudinal shift of the platform driven by a high precise stepper motor (figure 9(e)).The functional microfluidic devices are removed from the platform and cleaned in isopropanol using an ultrasonic machine.Finally, the uncured resin and isopropanol are removed from the functional microfluidic devices using compressed N 2 air [41].Currently, the spectral response features of liquid resin, DMD feature size, and z-axis resolution limit the precision of DLP technology [158].As a recently reported bionic microfluidic system, a dual-bionic superwetting gears system achieves high separation efficiency of oil and water by utilizing surface wettability and precisely printed topological microstructures (figure 9(f)) [213].

Two-photon polymerization (2PP).
Consequently, the 3D printing of low-roughness and high-precision devices (such as optical components and nano-structural devices [214]) is difficult to implement.Two-photon polymerization (2PP) is expected to solve the problems of limited fabrication precision.Normally, owing to the linear relationship between matter and light, the transmittance and absorption rates for a specific wavelength of matter are definite and do not vary with the change in light intensity.However, two-photon absorption is a third-order nonlinear effect [118].That is, the effect increases as the optical energy density increases.Compared with SPP, which utilizes one photon as the base unit for light curing (used by SLA, DLP, and MJM), the 2PP utilizes two photons as the base unit for light curing under extremely harsh conditions that require specific substances and extremely high energy density.Sufficient irradiance for ensuring the simultaneous absorption of two photons is observed only at the center of the highly focused laser.The two-photon absorption effect only occurs when the light intensity reaches a certain threshold value.If the laser is focused, the polymerization reaction area can be limited to a small area near the focal point.The focal point moves in the photosensitive material using a nanoscale precision stepper motor.The photosensitive material solidifies at the position where the focal point passes (figure 9(g)) [220].A complex shape readily assembled microscale check valve can be fabricated based on such an advancing method, which exhibited good dimensional accuracy (figure 9(h)) [215].
In summary, researchers have extensively studied various fabrication techniques with certain materials to precisely manufacture functional microfluidic devices (table 3).Different fabrication methods own unique advantages, disadvantages and processing properties, so it is particularly important to choose a processing method that best meets their own needs and application prospects.

Design and applications of functional microfluidic devices
From the foregoing discussion, there are numerous merits of functional microfluidic devices, such as compactness, microscale, and ease of integration [26].These advantages of functional microfluidic devices have played an influential role in many promising applications, such as heat sinks, mass transfer, chemical reactions, detection, and biomedical engineering.

Heat sink
Microfluidic heat sinks own small size with ultra-high heat transfer coefficients [239], thus, various microfluidic heat sinks with marvelous heat transfer performance have been developed in the past 40 years [27].Moreover, they demonstrate excellent pressure resistance, which can be widely utilized in aerospace, chemical engineering, industrial manufacturing, and many other fields [240].Recently, van Erp et al [241] proposed co-designing electronics with sustainable microfluidic cooling capability (figure 10(a)), which integrates electronics and microfluidics for efficient cooling of extreme heat flux extraction.Wang et al [242] investigated the effect of inlet/outlet configuration on flow boiling instabilities in parallel microchannels (figure 10(b)), and the flow boiling instabilities in parallel microchannels with three types of connections are compared.Quan et al [243] further experimentally studied the injection flow during condensation in microchannels (figure 10(c)).The flow pattern maps in terms of heat transfer rate versus mass flux for the annular flow regime and slug-bubbly flow regime in microchannels are proposed.Besides, Qu et al [244] also reported the thermal performance of an oscillating heat pipe with water based Al 2 O 3 nanofluids under different mass fractions and filling ratios (figure 10(d)), which decreased the thermal resistance with a minimum of 0.14 • C•W −1 (or 32.5%) when compared with that of the pure water.
Subsequently, Drummond et al [245] proposed a 3 × 3 array of microfluidic heat sinks (figure 10(e)), which can remove high heat flux (910 W•cm −2 ) with a pressure drop and a temperature of 162 kPa and 47 • C, respectively.Zhou et al [246] demonstrated a novel manifold microfluidic heat sink with a stacked configuration (figure 10(f)), which can mitigate flow maldistribution by optimizing the flow path.The overall thermal resistance is reduced by 7%-13% with a volumetric flow rate of 0.12-1.17l•min −1 .Wang et al [247] proposed a porous medium as a microfluidic heat sink (figure 10(g)).The performance of such a microfluidic heat sink can be effectively enhanced by a 3D fluid-solid multiobjective and multi-parameter genetic algorithm optimization method.Recently, Zeng and Lee [248] fabricated a liquidcooled microfluidic heat sink being optimized by using a 3D CFD simulation to achieve a reliable and energy-efficient cooling process (figure 10(h)).Besides, Wang et al [249] proposed a solar-powered cooling coating that exhibits remarkable cooling capabilities utilizing atmospheric water.

Passive mass transfer
The liquid-gas and liquid-liquid two-phase transfer as well as the reaction processes in functional microfluidic devices are also well developed in the past decades.Owing to their excellent hydrodynamic and mass transfer performance, microfluidic systems consisting of programmed microchannels are widely applied, such as droplet transportation, liquid/gas separation, and two-phase reaction.As shown in figure 11(a), Mertaniemi et al [105] demonstrated a microfluidic system for transporting droplets based on superhydrophobic technology.Droplets are transported at a high speed in tracks made of hydrophobic microchannels with low friction, enabling the programming of complex trajectories for droplets without any external energy input.In addition, inspired by the beak of shorebirds, Li et al [62] proposed a mimetic surface with narrow microchannels realizing directional liquid transportation, demonstrating that unidirectional and bidirectional transport of droplets can be facilely controlled with the change in surface wettability (figure 11(b)).Wang et al [153] proposed a bionic functional membrane which can be penetrated by a water droplet within 20 ms from hydrophilic surface to superhydrophilic surface, but the droplets will be blocked in the opposite direction.Significantly, the time it takes for a water droplet to penetrate through the bionic functional surface is much shorter than the time it freezes, even at temperatures as low as −90 • C (figure 11(c)).
Yin et al [255] proposed a type of functional microfluidic device to achieve the gas-liquid transfer during CO 2 absorption by embedding baffles in the microchannel.The flow falls into a broken Taylor regime with the increase of the gas flow rate, leading to an ultra-high fluid disturbance and bubbles breakup.Moreover, Xie et al [155] proposed a 3D printed bionic cell (figure 11(d)) with superhydrophilic outside surfaces, enabling ultrafast unidirectional water transportation underwater.Chen et al [250] also proposed a microfluidic system with 3D splitting structures (figure 11(e)).Owing to the 3D symmetrical microchannels, the microfluidic system can easily split both single and double emulsions into multiple portions.Moreover, Xie et al [152] proposed water engine boats with peristome-mimetic structures directionally driven by droplets (figure 11(f)).Such a water fuel boat equipped with five water engines passing through a meter-scale labyrinth as short as 217 s.
The manipulation of droplets using functional microfluidic devices is also attracting more and more attention.Inspired by the heterogeneous wettability of the back of the dessert beetle, directional-dependent architecture of the butterfly wing, and ultraslippery configuration of the N. alata, Yang et al [251] reported a multi-bioinspired SLIPSpatterned superamphiphobic surface control droplet sliding resistance with precise pattern arrangement, enabling handling of multiple droplets and precise droplet friction control (figure 11(g)).Caggioni et al [256] presented the control of droplet shapes through a single microfluidic device only based on the changes in operating conditions.Xie et al [69] proposed 3D printed bionic Janus porous matrices which achieve the successful implementation of programmable liquid flow in a desired direction within them working as a precisely printed liquid displayer (figure 11(h)).Zhang et al [252] proposed a self-supported monolayered porous polymembrane with special micropores and superhydrophilic-hydrophilic wettability on opposite surfaces (figure 11(i)), achieving the unidirectional transport of droplets.Also, the anti-gravity unidirectional ascent of such a porous membrane in a wide range of pH values can be utilized as a 'liquid diode' for moisture wicking.Moreover, Hu et al [253] designed a pneumatic programmable superrepellent surface to tailor conventional wetting materials (such as PDMS) with embedded flexible chambers connected to a microfluidic system (figure 11(j)).Yang et al [251] proposed a bionic surface diode with amazing performance of directional droplet sliding and precise control of droplet friction.Zhan et al [254] developed a bionic surface with inclined micro-mushrooms for programmable droplet bouncing, offering applications in water transportation, self-cleaning, antigravity bouncing, and clean energy generation (figure 11(k)).

Clean water production
The excellent mass transportation capability enables functional microfluidic devices utilization in the field of solar water evaporation [32].For such an application, functional microfluidic devices not only transport water but also act as heat insulators after the photothermal conversion with nanostructures [169,[257][258][259][260][261][262].Generally speaking, clean water production can be achieved by water absorption from the air and solar vapor generation.Due to the extremely strong water absorption capability of some porous media made of organic materials, the structures can effectively absorb water from the air.As shown in figure 12(a), Fan et al [263] developed a 3D MXene-based solar absorber with metalorganic framework-derived carbon nanoplates, achieving efficient solar-driven desalination with high vapor conversion efficiency and stable performance over time.The solar-vapor conversion efficiency of the device can reach around 93.4% and maintain over 91% for 100 h to produce clean vapor for stable  [155].Reprinted with permission from [155].Copyright (2022) American Chemical Society.(e) 3D splitting of droplets by the glass functional microfluidic devices [250].Reproduced from [250] with permission from the Royal Society of Chemistry.(f) Self-driven water boats enabled by Janus membranes [152].Reprinted from [152], © 2023 Elsevier Ltd.All rights reserved.(g) Multi-bioinspired slippery lubricant-infused porous surface (SLIPS) [251].Reproduced from [251].CC BY 4.0.(h) A precisely printed liquid displayer [69].[69] John Wiley & Sons.[© 2023 Wiley-VCH GmbH].(i) Unidirectional penetration of liquid enabled by a flexible monolayered porous membrane with superhydrophilic-hydrophilic surfaces [252].Reprinted with permission from [252].Copyright (2020) American Chemical Society.(j) A pneumatic programmable superrepellent surface [253].Reproduced from [253].CC BY 4.0.(k) A bioinspired functional surface with inclined micromushrooms for programmable and patterned droplet bouncing [254].and continuous water desalination.Besides, inspired by the leaf vein, Lin et al [264] proposed four-level wedge tracks for directional water collection (figure 12(b)).Superhydrophilic Cu(OH) 2 nanowires are utilized to provide abundant microfluidic paths for promoting droplet absorption and forming water film tracks.[264].Reprinted with permission from [264].Copyright (2018) American Chemical Society.(c) Solar thermal desalination by loofah sponge with internal microchannels as water pumps [265].Reproduced from [265] with permission from the Royal Society of Chemistry.(d) Tree-like functional microfluidic devices for solar water evaporation [266].Reproduced from [266], with permission from Springer Nature.(e) Bioinspired hierarchical evaporator for solar desalination [267].Reproduced from [267].CC BY 4.0.(f) Freshwater collection with a solar evaporator and a passive condenser [268].Reproduced from [268] with permission from the Royal Society of Chemistry.(g) Light-permeable solar evaporator with 3D functional microfluidic devices for water purification [269].Reprinted with permission from [269].Copyright (2022) American Chemical Society.(h) 3D opening functional microfluidic devices for mimicked transpiration [156].Reprinted with permission from [156].Copyright (2022) American Chemical Society.
As shown in figure 12(c), a multitude of microchannels are arranged in coarse fibers for rapid vapor transportation during the solar evaporation process [265].Dudukovic et al [266] proposed a microfluidic device based on 3D unit cell structures (figure 12(d)), which achieves deterministic control of multiphase flow as well as reaction processes with a tree-like structure consisting of numerous tetrahedral cells.Inspired by the architecture of leaves in pristine plants, Zhang et al [267] presented a bionic photothermal aerogel with numerous microchannels serving as stems (figure 12(e)).Cheng et al [268] demonstrated a solar water evaporator with hierarchical carbonized microchannels (figure 12(f)), enabling high water transportation performance and ultralow thermal conductivity.Furthermore, Ma et al [269] proposed a 3D volatile organic compound-based solar water evaporator (figure 12(g)).The reactive interface of such a solar evaporator is increased by tens of times compared with a 2D membrane, resulting in a high solar water evaporation efficiency.Inspired by the keratinized membrane at the tip of the hummingbird's tongue, Wang et al [156] provided a new strategy for opening functional microfluidic devices working for solar vapor generation (figure 12(h)), which keeps the liquid inside but squeezes out the gas with unique printed openings.

Microfluidic chips for chemical reactions
Functional microfluidic devices have also been utilized as chemical reaction vessels due to the great mass transfer capability of the microchannels with unique structures and chemical coatings.Guo et al [33] proposed a novel 3D serpentine microfluidic reactor (figure 13(a)), which achieves efficient mixing of three different kinds of liquids.Besides, inspired by porous structures by rocks, Ge et al [270] proposed a special microfluidic chip with a series of converging-diverging geometries (figure 13(b)), whose mass transfer coefficient is approximately four times to traditional microchannel by increasing the interfacial area.As presented in figure 13(c), Xie et al [155] proposed a 3D-printed microfluidic reactor based on unidirectional cellular fluidics, which achieves anaerobic chemical reactions underwater, enabling potential applications in chemical reactions underwater.Moreover, Wang et al [156] also reported a bionic open microfluidic device with openings Functional microfluidic devices for chemical reactions.(a) A 3D serpentine microfluidic device with periodic vortex-inducing structure for mixing [33].Reprinted with permission from [33].Copyright (2019) American Chemical Society.(b) Solvent extraction enhancement with a series of converging-diverging microfluidic devices [270].Reprinted from [270], © 2022 Elsevier Ltd.All rights reserved.(c) Complex chemical reactions underwater [155].Reprinted with permission from [155].Copyright (2022) American Chemical Society.(d) 3D open microfluidic devices for precise control of chemical reactions [156].Reprinted with permission from [156].Copyright (2022) American Chemical Society.never affected its fluidic performance but discharged gas from the microchannel via the openings (figure 13(d)), solving the severe problem of gaslock in functional microfluidic devices, which can be utilized for precisely controlled chemical reactions, oil-water separation, and controllable drug delivery.

Microfluidic chip for sensors
With the rapid development of analytical chemistry and MEMS technology, microfluidic chips with complex microchannels are boosting [30].Owing to the controllable liquid flow inside the microchannels, microfluidic chips achieve fast detection and separation of fluid with low cost.They can also be used to test multiple samples simultaneously, showing considerable application potentials as real-time sensors and chemical detection.As presented in figure 14(a), Zhu et al [271] proposed a self-priming fractal-branching microfluidic chip for digital polymerase chain reaction.The fractal tree-like microchannel network structures are inspired by mammalian circulatory and respiratory systems, which achieves sequential reagent loading and isolation for point-ofcare detection.In addition, Olanrewaju et al [272] developed an advanced capillary microfluidic device that can achieve the autonomous delivery of eight liquids (figure 14(b)).The liquid moves in a pre-programmed drainage order with different hydrostatic pressures to ensure effective real-time detection.
Farmehini et al [273] presented a novel circuit implementation for on-chip real-time measurement of resonance frequency and feedback control based on integrated functional microfluidic devices (figure 14(c)).A piezoelectric transducer generates acoustic waves to selectively trap and position target particles in microchannels.Hu et al [274] reported a fully integrated and self-contained microfluidic sensor for the automated and quantitative detection of biological hormones (figure 14(d)).The microfluidic device can simultaneously detect 16 samples with a high speed.Ghosh and Ahn [29] reported a microfluidic system using microchannels to form spiral reaction chambers for detecting specific proteins (figure 14(e)).Moreover, Song et al [275] proposed a label-free detection method for droplets using microfluidic chips (figure 14(f)).The microscopic device visualizes droplets full-field with high spatiotemporal resolution, which can be used for the observation of the dynamic formation of droplets.

Biomedical field
Functional microfluidic devices also play a crucial role in biomedical fields, such as regenerative medicine and cell/tissue engineering, which can be employed for cell behavior investigation, drug delivery, tissue regeneration, etc [31].Compared with other traditional medical testing methods, the use of  [271].Reproduced from [271] with permission from the Royal Society of Chemistry.(b) Schematic of microfluidic capillary circuits [272].Reproduced from [272] with permission from the Royal Society of Chemistry.(c) Microfluidic chips for real-time detection in acoustic trapping systems [273].Reprinted with permission from [273].Copyright (2021) American Chemical Society.(d) Quantitative detection of biomarkers enabled by microfluidic chemiluminescence immunoassay [274].Reproduced from [274] with permission from the Royal Society of Chemistry.(e) Highly sensitive point-of-care diagnostic system with microchannel-based lateral flow assay [29].Reproduced from [29] with permission from the Royal Society of Chemistry.(f) 3D label-free detection of droplets in microchannels [275].Reproduced from [275] with permission from the Royal Society of Chemistry.microfluidic devices can work fast and efficient.For example, Pinho et al [276] proposed a novel microfluidic device for the partial extraction of red blood cells in 2013 (figure 15(a)), which achieves red blood cell separation and deformability assessment in a single step, demonstrating a potential diagnostic technique for separating healthy and diseased cells.In addition, Rodrigues et al [277] presented a type of microfluidic devices to separate a certain amount of red blood cells without any clogging or jamming (figure 15(b)).Blood cells from initial blood samples can be continuously and simultaneously separated using both cross-flow microfilters and hyperbolic microchannels.
Mane et al [278] proposed a T-shaped sealed microchannel to purify motile sperm cells (figure 15(c)).The motile sperm is separated progressively from the fluid flow at the zone near the 'T' junction.Jiang et al [279] presented a chitosan scaffold with controllable geometric microchannels using the freezedrying method (figure 15(d)), demonstrating that functional microfluidic devices can promote cell infiltration and distribution as well as tissue ingrowth.Vu-dinh et al [280] proposed a novel microfluidic device specifically for isolating human lung carcinoma in microchannels (figure 15(e)).By exploiting magnetic nanoparticles, the target A549 cells immobilized by magnetic beads can be manipulated by an external magnetic field and trapped in cavities, demonstrating excellent cell separation performance.Tee et al [281] adopted a dynamic microcarrier culture platform for chondrocyte expansion (figure 15(f)).Microcarriers with microchannels promote homogenous culture environment, and facilitate oxygen and nutrient transfer, promising a stratified zonal repair of articular cartilage.

Capillaric circuits
Programmable capillary circuits for the self-powered delivery of liquids are attracting more and more attention in recent years.Based on the capillary force inside functional microfluidic devices, capillaric circuits are widely utilized for the detection, analysis, and viscosity measurement of different fluids.In 2017, Oh et al [35] presented a novel 3D-printed microfluidic circuit to analyze the viscosity of the blood.A smart pipette generates controlled shear rate conditions to operate the 3D-printed microfluidic circuit for whole blood analysis (figure 16(a)).Subsequently, Oh and Choi [282] measured the calibration-free viscosity of both Newtonian and non-Newtonian fluids (figure 16(b)).In addition, Yafia et al [283] presents a microfluidic chain reaction (MCR) system that allows for the automated control of sequential fluids release for various applications.The MCR system uses capillary domino valves to encode and control the release of fluids from reservoirs in a predetermined order (figure 16(c)).Safavieh and Juncker [284] presents the concept of 'capillarics', which are pre-programmed, self-powered microfluidic circuits built from capillary elements.The authors introduce two novel capillary elements, retention burst valves and low aspect ratio trigger valves, and combine them with other components  [276].Reproduced from [276], with permission from Springer Nature.(b) Red blood cell deformability assessment in continuous flow with microfluidic devices [277].Reproduced from [277], with permission from Springer Nature.(c) T-shaped sealed microchannels for the separation of motile human sperms [278].Reproduced from [278], with permission from Springer Nature.(d) Chitosan scaffolds construction with microchannels for tissue engineering [279].Reprinted from [279], © 2021 Elsevier B.V. All rights reserved.(e) Lung adenocarcinoma cells selection by cavity-added serpentine microchannels [280].Reproduced from [280], with permission from Springer Nature.(f) Inertial spiral microchannels for clinical applications [281].Reprinted from [281], © 2019 Elsevier Ltd.All rights reserved.Reproduced from [282].CC BY 4.0.(c) A microfluidic chain reaction system with a microfluidic capillaric circuit [283].Reproduced from [283], with permission from Springer Nature.(d) The pre-programmed, self-powered microfluidic circuits which are built from capillary elements [284].Reproduced from [284] with permission from the Royal Society of Chemistry.Microfluidic flexible and wearable electronic (a) E-skin sensing with a highly stretchable and conformable microfluidic network inspired by skin [36].Reproduced from [36].CC BY 4.0.(b) Ultra-sensitive pressure detection with flexible organic thin-film transistors [285].Reproduced from [285].CC BY 4.0.(c) Self-powered flexible dual-parameter sensor for stimuli detection of temperature and pressure [286].Reproduced from [286].CC BY 4.0.(d) Electroencephalogram (EEG) signal caption with flexible printed microfluidic electrodes [287].Reproduced from [287].CC BY 4.0.
to build a capillary circuit that can autonomously deliver a sequence of multiple chemicals (figure 16(d)).

Flexible and wearable electronic devices
Microfluidic flexible and wearable electronic devices are also becoming increasingly popular.Microfluidic 'electronic skin' can measure various body parameters, such as heartbeat, blood pressure, concentration of ions, and body temperature.For example, Hua et al [36] achieved e-skin sensing of humidity, temperature, magnetic field, in-plane strain, light, proximity, and pressure, with a highly stretchable and conformable matrix microfluidic network inspired by skin (figure 17(a)).Based on such a microfluidic wearable device, various categories of sensing can be achieved with different types of sensor units.
Zang et al [285] proposed flexible organic thin-film transistors that enable ultrasensitive pressure detection.As shown in figure 17(b), the electronic devices achieve ultrahigh sensitivity (192 kPa), short response time (less than 10 ms) and low power consumption (less than 100 nW) for real-time sensing of acoustic vibrations and radial artery pulse.Similarly, Zhang et al [286] reported a type of flexible sensor for stimuli dual-parameter (temperature and pressure) detection based on microstructures made of organic thermoelectric materials (figure 17(c)).The resolution of temperature detection is less than 0.1 K and the sensitivity for high-pressure-sensing is up to 28.9 kPa for such a flexible sensor.In addition, Debener et al [287] proposed microfluidic flexible screen-printed electrodes.As presented in figure 17(d), comfortable and extremely lightweight electrode arrays manufactured with low-cost printed flexible screen technology can detect electroencephalogram (EEG) signals.

Microrobotics
Advances in soft materials are prompting the development of flexible sensors and actuators for soft robots.For example, Barbot et al [37] developed a mobile microfluidic microrobot with 3D motions (figure 18(a)) based on selective selfintegration of 3D helical micro-swimmers.In 2021, Ahmed et al [288] described a mechanism that used externally triggered acoustic and magnetic fields to transport swarms of microparticles along the boundaries of a microchannel against imposed flow (figure 18(b)).Additionally, Milana et al [289] proposed an artificial cilium that imitates the motion of biological cilia with two actuators for independent control, allowing for different asymmetrical motions (figure 18(c)).Based on electrohydrodynamics, Cacucciolo et al [40] proposed a type of soft bidirectional pump with charge injection (figure 18(d)).A self-contained fluidic muscle with such an embedded pump can be potentially applied as wearable devices, thermally activated clothing, microfluidic sensors, and autonomous soft robots.

Fuel cell
Fuel cell is a revolutionary alternative to traditional fuels [290].Microfluidic devices can increase the reaction area, improve the mass transfer efficiency, reduce the mass transfer resistance, and increase the stability and safety of fuel cells [291].Braff et al [292] reported a microfluidic membranefree hydrogen bromide laminar flow battery as a potential high power density solution (figure 19(a)).The membraneless design achieved a power density of 0.795 W•cm −2 at room temperature and atmospheric pressure with a round-trip voltage efficiency of 25% at 92% of peak power.In addition, proton exchange membranes composed of perfluorinated polyelectrolytes are costly, mechanical strength shortage and dimensional instable.Hence, Yameen et al [293] proposed a silicon based multichannels modified with sulfonated polymer brushes, which enabled proton-conducting channels with tailor-made, finely tuned physicochemical characteristics.Regardless of humidity, the highly proton-conducting selfhumidifying microchannels generated by copolymer brushes on a scaffold display a high conductivity (ca. 10 −2 S•cm −1 ), which was much more than that of nafion.Furthermore, Wang et al [294] proposed bionic Janus membranes with distinct water-repellent properties on one side, allowing for efficient and rapid transportation of bubbles in one direction underwater.The microholes in the membrane are superhydrophobic, preventing water from entering but allowing for the passage of gas through numerous microchannels.They have successfully utilized these bionic Janus membranes to quickly remove hydrogen bubbles that adhere to the copper cathode during the hydrogen evolution reaction (figure 19(b)).
As shown in figure 19(c), Bombelli et al [295] fabricated a microbial fuel cell-inspired photovoltaic device with multiple microchannels, which enabled a straightforward comparison between sub-cellular photosynthetic organelles and entire cells, as well as a quantitative assessment of the factors that affect power generation.Yang et al [296] proposed a solid oxide fuel cell (SOFC) anode that consisted of a microfluidic reactor and highly active catalysts (Ni-Y 2 O 3 -Ce 0.5 Zr 0.5 O 2 ) (figure 19(d)).With the help of SOFCs, the conversion rate of methane was boosted from 23.59% to 43.22% at 750 • C, while the electrochemical output was increased from 905 mW•cm −2 to 1208 mW•cm −2 .Additionally, there was a significant improvement for the ability to resist coking of the fuel cells.Zhao et al [297] designed a direct methanol fuel cell (DMFC) with microchannels (figure 19(e)).It is revealed that bubbles formation and elimination occurred periodically due to transient capillary blockage in small flow microchannels.The reduction in microchannel width led to an increase in the length of gas slugs and the duration of gas slug blockage in the flow channels.Zhao et al [298] devised a fuel-delivery system comprising a fuel reservoir (figure 19(f)).Methanol is conveyed from the fuel reservoir through the porous plate and the openings of the current-collector to reach the anode.At the same time, the CO 2 generated at the anode electrode is transported through the same components, but in the opposite direction.The DMFC system enabled a relatively high performance with nearly pure methanol (22.0 M) as the fuel source.

Microfluidic reactors for CO 2 reduction and H 2 production
Microfluidic devices provide efficient gas-liquid interfaces that facilitate the mixing and transfer of reactants while controlling the temperature and pressure of the reactions [299].Chai et al [300] designed a novel micromixer with 3D helical, threaded microchannels (figure 20(a)) for CO 2 conversion to formic acid via enzymatic cascade reaction.Optimum performance was observed with a liquid flow rate of 1 ml•min −1 .Brooks et al [301] proposed a microfluidic Sabatier reactor for the methanation of CO 2 by hydrogen reduction to produce H 2 O and CH 4 (figure 20(b)), which offers efficient heat and mass transfer between the reactive-gas flow and the channel walls, resulting in precise temperature control within a compact reactor system for CO 2 reduction.
Similarly, microfluidic devices can also provide highly controlled reaction conditions for H 2 production, including parameters such as temperature, pressure and flow rate for efficient hydrolysis reactions that split water into hydrogen and oxygen while controlling the purity and flow of reaction products [304].Khani et al [302] synthesized a new composite consisting of cerium, yttrium, and ruthenium as a support in the methanol steam reforming (MSR) process for copper metal (figure 20(c)), resulting in exceptional methanol conversion rates and product selectivity.Gribovskiy et al [303] manufactured a thermally autonomous microfluidic reactor with a microchannel catalytic unit (MCU) containing a catalyst for hydrogen production in the steam reforming of methanol (figure 20(d)).The high thermal conductivity of the MCU along with the narrow cross-sectional area of its channels facilitates efficient and mass transfer, resulting in isothermal conditions.

Conclusion and perspective
This paper reviews the functional microfluidic devices mainly from the viewpoints of the driving force, manufacturing methods and various promising applications.First, capillary theory has been built based on the wettability of a surface in the past four centuries.Second, the fabrication of microchannels has been greatly developed in the past half century, including the materials, fabrication methods, post-treatment, and so on.Last and the most significant, various applications based on functional microfluidic devices have been proposed in the past three decades, such as heat sinks, diagnostics, detection, biomedical engineering, production of materials, capillaric circuits, flexible and wearable electronic devices, and microrobotics.All the pieces of evidence have strongly demonstrated that functional microfluidic devices promise new capabilities for the future because much technology needs to be developed to manipulate liquids and gas in various functional devices.
significant advances have been achieved in such a promising field, much effort is still needed to address big challenges in functional microfluidic devices ahead.First, a analytical solution for capillary force theory to various microchannels needs to be obtained because the present ones are only suitable for smooth microchannels, while complex microchannels with inner micro/nanostructures are the future.Second, materials and related fabrication methods for functional microfluidic devices are still in their fancy.For example, there is only few functional microfluidic devices with inner micro/nanostructures proposed in the past, and the large-scale fabrication methods with extremely high precision for complex microfluidic devices are almost impossible except for the newly developed 3D printing techniques.Finally, limited by the shortcomings mentioned above, the applications of functional microfluidic devices are far from meeting expectations.For example, the gaslock inside microchannels severely limits all applications related to two phase flow with gas, it is far from satisfactory, though our group at Hunan University is trying to solve such a problem now.Also, functional microfluidic devices should be developed interdisciplinarily in corporations with artificial intelligence, bionic engineering, etc.
Regarding those various promising applications for functional microfluidic devices now and in the future, their applications in the field of carbon neutrality should be emphasized.For example, special pesticides are needed to enhance their utilization efficiency and lower the pollution of the land by enhancing their attachment to the leaves enabled by wettability.In addition, flow cells strongly depend on the microfluidic performance of the microfluidic systems, and special micro-matrices suppress lithium dendrite growth.Furthermore, almost all the electrodes for hydrogen evolution and CO 2 reduction are porous media, no matter carbon paper electrodes, foam nickel, or others, and large-scale functional microfluidic electrodes are one of the promising candidates for those kinds of carbon neutral catalytic electrodes.Most significantly, the exploration of space is attracting more and more attention all over the world, and most of those planned processes related to liquid and gas are supposed to depend on functional microfluidic devices under microgravity conditions.We prospect the rapid development of novel integrated functional microfluidic devices, no matter in theory, materials, manufacturing methods, applications, which are so important to reshape our understanding of numerous physical and chemical processes, which will benefit our carbon neutral daily life soon.

Figure 1 .
Figure 1.The history of capillary phenomena.

Figure 2 .
Figure 2. Wettability, including superhydrophilic, superhydrophobic, superoleophilic, and superoleophobic phenomena when the surfaces are in the air.Derived superoleophobicity, superoleophilicity, superaerophobicity, and superaerophilicity when the surfaces are underwater.Achieved wettability cases, such as superhydrophilic and superhydrophobic surfaces are in oil.
) exceeds 1.Under such circumstances, the Wenzel model fails, and the Cassie-Baxter model is developed with a series of corrections based on the Wenzel model.

Figure 3 .
Figure 3. Wettability of solid surfaces.(a) The mechanical force balance on three-phase contact line in Young's equation.Schematic of (b) advancing contact angle and (c) receding contact angle.(d) Wenzel model.(e) Cassie-Baxter model.(f) Intermediate states of Wenzel and Cassie-Baxter models.

Figure 6 .
Figure 6.Significant benchmarks in the progress of functional microfluidic device fabrication methods.Based on information from the references [125-127].

Figure 7 .
Figure 7. Traditional fabrication methods for functional microfluidic devices.(a) Schematic of functional microfluidic devices fabricated by using lasers.(b) Janus microhole-arrayed polydimethylsiloxane fabricated by a one-step femtosecond laser [177].[177] John Wiley & Sons.[© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim].(c) Schematic of functional microfluidic devices fabricated by using molding methods.(d) Cylindrical superhydrophobic microchannels fabricated by replicating lotus leaf structures on internal walls [178].Reproduced from[178].CC BY 4.0.(e) Schematic of functional microfluidic devices fabricated by using imprinting methods.(f) Functional microfluidic devices with more complicated structures manufactured by imprinting thermoplastic substrate with silicon wafer[179].Reproduced from[179] with permission from the Royal Society of Chemistry.

Figure 8 .
Figure 8. Lithography and etching methods for the fabrication of functional microfluidic devices.(a) Schematic of functional microfluidic devices fabricated based on lithography methods.(b) Lithography methods can be easily applied to manufacture circular microchannels with a wide range of diameters from 25 µm to 150 µm [149].Reprinted from [149], © 2016 Elsevier B.V. All rights reserved.(c) Schematic of functional microfluidic devices fabricated based on etching methods[113].(d) Functional microfluidic devices manufactured by etching methods[113].Reproduced from[113] with permission from the Royal Society of Chemistry.

Figure 9 .
Figure 9. 3D printing methods for fabrication of functional microfluidic devices.(a) FDM technology is typically used to fabricate functional microfluidic devices with thermoplastic polymer [209].(b) Sacrificial molding method based on FDM for fabrication of functional microfluidic devices [201].[201] John Wiley & Sons.© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(c)Photopolymer is jetted through linearly arranged nozzles in a continuous or drop-on-demand mode, sprayed onto the platform in a form of microdroplets, and finally cured with an even UV light source to form functional microfluidic devices[210,211].(d) Spiral microchannels[199].[199]John Wiley & Sons.© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(e) Single-photon polymerization (SPP) process repeatedly occurs on the surface of photosensitive resin where the UV light irradiates to fabricate microfluidic devices[212].(f) 3D capillary ratchet-induced liquid directional steering[213].Reproduced from[213].CC BY 4.0.(g) Focal point moves over the photosensitive area to solidify the material with the two-photon absorption effect[214].(h) A printed complex microscale check valve[215].Reproduced from[215], with permission from Springer Nature.

Figure 14 .
Figure 14.Microfluidic detectors.(a) Large-scale microfluidic net chips with self-priming fractal branching[271].Reproduced from[271] with permission from the Royal Society of Chemistry.(b) Schematic of microfluidic capillary circuits[272].Reproduced from[272] with permission from the Royal Society of Chemistry.(c) Microfluidic chips for real-time detection in acoustic trapping systems[273].Reprinted with permission from[273].Copyright (2021) American Chemical Society.(d) Quantitative detection of biomarkers enabled by microfluidic chemiluminescence immunoassay[274].Reproduced from[274] with permission from the Royal Society of Chemistry.(e) Highly sensitive point-of-care diagnostic system with microchannel-based lateral flow assay[29].Reproduced from[29] with permission from the Royal Society of Chemistry.(f) 3D label-free detection of droplets in microchannels[275].Reproduced from[275] with permission from the Royal Society of Chemistry.

Figure 16 .
Figure16.Capillaric circuits.The 3D-printed microfluidic circuits to (a) analyze the blood viscosity[35], Reprinted from[35], © 2017 Elsevier B.V. All rights reserved.As well as (b) measure the calibration-free viscosity of both Newtonian and non-Newtonian fluids[282].Reproduced from[282].CC BY 4.0.(c) A microfluidic chain reaction system with a microfluidic capillaric circuit[283].Reproduced from[283], with permission from Springer Nature.(d) The pre-programmed, self-powered microfluidic circuits which are built from capillary elements[284].Reproduced from[284] with permission from the Royal Society of Chemistry.

Figure 20 .
Figure 20.Microfluidic reactors for CO 2 reduction and H 2 production.(a) Schematic diagram of biocatalytic micromixer setups used for the CO 2 reduction [300].Reprinted from [300], © 2021 Elsevier B.V. All rights reserved.(b) Schematic diagram of a microfluidic reactor, where the reactive gases are directed to flow through the central rectangular microchannels while the outer oval channels are reserved for the flow of oil [301].Reprinted from [301], Copyright © 2006 Elsevier Ltd.All rights reserved.(c) Schematic of a lab set-up for testing the MSR process [302].Reprinted from [302], © 2021 Hydrogen Energy Publications LLC.Published by Elsevier Ltd.All rights reserved.(d) A thermally autonomous microfluidic reactor consists of microchannel catalytic units [303].Reprinted from [303], Copyright © 2015 Elsevier B.V. All rights reserved.

Table 1 .
The comparison of advantages and disadvantages of different simulation methods for microfluidics.

Table 2 .
The characteristics of various fabrication methods.

Table 3 .
Summary of advantages and disadvantages for functional microfluidic devices fabrication methods.