Regulatable interfacial adhesion between stamp and ink for transfer printing

As an emerging processing technology, transfer printing enables the assembly of functional material arrays (called inks) on various substrates with micro/nanoscale resolution and has been widely used in the fabrication of flexible electronics and display systems. The critical steps in transfer printing are the ink pick‐up and printing processes governed by the switching of adhesion states at the stamp/ink interface. In this review, we first introduce the history of transfer printing in terms of the transfer methods, transferred materials, and applications. Then, the fundamental characteristics of the transfer printing system and typical strategies for regulating the stamp/ink interfacial adhesion strength are summarized and exemplified. Finally, future challenges and opportunities for developing the novel stamps, inks, and substrates with intelligent adhesion capability are discussed, aiming to inspire the innovation in the design of transfer printing systems.


| INTRODUCTION
Printing technologies play important roles in many fields related to human life, such as inkjet printing in the publishing industry, screening printing in the textile industry, and novel three-dimensional (3D) printing in the engineering industry.However, the pattern resolution and the classes of printed materials are restricted in these methods.For example, the resolution of inkjet printing patterns is limited by the nozzle diameter (typically 20-70 µm), only reaching the micron level. [1][4][5][6] On the other hand, transfer printing is an emerging manufacturing technology from which the material fabrication and printing processes are separated.A variety of ink materials can be first fabricated on a donor substrate through suitable micro-nano processing methods, such as photolithography, [7] electron-beam evaporation, [8] ion sputtering, [9] molecular beam epitaxy, [10] and chemical vapor deposition (CVD), [11] which ensure high manufacturing accuracy and nanoscale resolution.Subsequently, the functional ink can be transferred from the fabrication substrate to the arbitrary-shaped application substrate without material performance deterioration.Therefore, the categories of inks in transfer printing have been greatly expanded from traditional dyes to functional materials with micro/nanostructures. [12][13][14][15][16] Benefiting from the diversity of ink materials, transfer printing shows potential applications in flexible electronics, [17] sensors, [18] solar cells, [19] display systems, [20] and biomedicine. [21]he major milestones in transfer printing technology are summarized in Figure 1 in chronological order.The history of transfer printing can be traced back to the 1750s.This technique was first used in Italy and England to print designed patterns on porcelain, offering high quality and lower cost than hand painting.Afterward, in 1930, Dreyfus and Wight proposed an apparatus in a US patent that used a transfer sheet for printing designed dyestuffs on fabrics. [22]During the printing process, fabrics were moistened by solvent to dissolve the dye on the transfer sheet, and the dye was permanently fixed on fabrics after evaporating the solvent.This invention marked the expansion of transfer printing applications from porcelain printing to textile printing.Later, another transfer printing technique, known as the heat, sublimation, or vapor-phase transfer printing, was commercialized in the 1970s for textile printing. [23]Briefly, the disperse dyes on transfer papers would sublimate from a solid to a gaseous state under heat and pressure and adsorb on the textile substrate to facilitate dyes transfer.Thereafter, the range of transfer printing substrates was extended from flat textiles to the curved surface of objects.In 1977, Nakanishi and Kabushiki first proposed a printing apparatus to transfer a pattern film floated on a water surface onto the surface of a molded resin or a metallic product.This can be considered as the prototype of water transfer printing. [24]In 1986, a new technique called laser-induced forward transfer printing was reported by Bohandy et al. for depositing metal film onto arbitrary target substrates using a highenergy excimer laser. [25][28][29] This led to the emergence of a novel technique called nanotransfer printing, which allowed the fabrication of nanoscale metal patterns on flexible polymer substrates through interfacial chemical reactions between the ink material and the receiver substrate. [27]This advancement showcased the promising potential application in flexible electronics. [30]In 2006, Meitl et al. proposed the kinetically controlled transfer printing using an elastomeric polydimethylsiloxane (PDMS) stamp whose adhesion strength depends on the speed of delamination. [31]urthermore, Ahn et al. repeatedly used this technique to transfer different semiconductor nanomaterials onto a separate substrate for constructing layered heterogeneous integrated electronic devices. [32]Subsequently, transfer printing technology employing elastomeric stamps was applied to different fields, such as the assemble of silicon solar cells, [33] the fabrication of inorganic light-emitting diodes (LEDs) and quantum dots (QDs) display F I G U R E 1 Milestones in the development of transfer printing.The blue, purple, and green blocks represent the typical transfer printing technologies, the application fields associated with transfer printing, and the transferred materials, respectively.systems, [20,34] and the preparation of implantable biointegrated electronics. [35]In the 2010s, stimuli-responsive transfer printing was proposed. [36][43] However, printing nanomaterials over a large area on an arbitrarily curved surface is still of great challenge.Recently, Zabow proposed a novel technology that utilizes reflowable sugar mixtures as stamps to achieve 3D microprinting on surfaces with high curvature. [44]ome excellent reviews have summarized transfer printing from the aspects of transferred materials, [45] stamp microstructures, [46] and specific application fields, such as thin film solar cells, [47] optoelectronic devices, [48] flexible and stretchable electronics, [49] micro-LED displays, [50] and so forth.The regulation of interfacial adhesion is a critical aspect of transfer printing.However, few manuscripts systematically take insight into the methods for controlling the decisive stamp/ink interface strength during the transfer printing process.In this review, we first briefly introduce the system, process, and mechanism of transfer printing, and then focus on the critical interfacial adhesion issue involved in ink pick-up and release steps.Typical strategies for regulating the stamp/ink interfacial adhesion between strong and weak states are discussed and exemplified in detail.At the end of the review, we propose our perspectives on the challenges and future opportunities in designing the transfer printing system with rapid, high interfacial adhesion switchability.

| FUNDAMENTAL FEATURES OF TRANSFER PRINTING SYSTEMS
In Section 2, elements of the transfer printing system, steps, and the mechanism of transfer printing are discussed.A basic transfer printing system includes ink, donor substrate, stamp, and receiver substrate.Among these elements, the ink encompasses a variety of functional materials or integrated devices toward different application scenarios, which can be categorized as metals (nanoparticles, [51] nanowires, [52] and liquid metals [53] ), inorganic materials (metal oxide semiconductors, [54] III-V compound semiconductors, [55] silicon, [56] and QDs [20] ), organic materials (conducting polymers [57] ), carbon-based materials (CNTs [11] and graphene [58] ), integrated units (micro-LED pixels, [59] sensor arrays, [60] and photovoltaic cells [61] ).The ink is prepared on the donor substrate and latter patterned through various micro-nano processing methods.Therefore, the donor substrate needs to remain stable under harsh manufacturing conditions, such as high temperature, high pressure, and chemical etching.Silicon wafer, [62] semiconductor wafer, [63] glass, [64] and other rigid substrates are widely selected as donor substrates.Stamps typically consist of adhesive tapes and polymer elastomers with micro/ nanostructures, which are utilized to transfer ink materials from the donor substrate to the receiver substrate.PDMSbased stamps are the most commonly used for transfer printing. [37,65,66]Traditional receiver substrates for transfer printing include stiff SiO 2 /Si and glass. [67,68]Recently, soft polymers, such as polyethylene terephthalate and polyimide, [69,70] have been selected as receiver substrates to meet the demands of flexible electronics, implantable biosensors, wearable devices, and other application fields.
After ink is constructed on the donor substrate, the transfer printing process can be divided into four steps including adhesion, pick-up, alignment, and printing step, as illustrated in Figure 2.During the adhesion step, a stamp establishes intimate contact with the ink surface under suitable pressure and temperature, promoting the formation of a strong adhesive state through various interactions including van der Waals force, [31] electrostatic force, [39] covalent chemical bond, [27] negative air pressure, [43] surface tension and capillary force, [71] and so forth.Subsequently, the stamp is detached from the donor substrate, temporarily transferring ink to its surface during the pick-up step.In the alignment step, the ink-containing stamp should be precisely aligned with the receiver substrate and forms conformal contact to ensure accurate ink placement.Finally, in the printing step, the adhesion between the stamp and ink is weakened under the stimuli of the external environment, leading to the release of the ink from the stamp and printing onto the receiver substrate.In an ideal transfer printing process, the stamp can be reused to achieve cost-effective mass transfer, which enables a transfer printing cycle.The separation of ink material processing and assembly in transfer printing effectively avoids damage to the delicate receiver substrate during ink manufacturing and resolves the incompatibility issue between the micro-nano processing technology and flexible substrates.
In a typical transfer printing system, three interfaces are involved: the ink/donor substrate interface, the stamp/ink interface, and the ink/receiver substrate interface.During the adhesion and pick-up step, the adhesion strength between the stamp/ink interface exceeds the ink/donor substrate interfacial strength, leading to the fracture of the ink/donor substrate interface and allowing the stamp to retrieve the ink.In the printing step, the adhesion strength between the stamp/ink interface is reduced to a weak adhesion state, which is lower than the ink/receiver substrate interfacial strength, allowing the ink to release from the stamp to the receiver substrate (center of Figure 2).It is generally considered that the ink/donor substrate interfacial strength is determined by the ink fabrication process, while the ink/ receiver substrate interfacial strength is impacted by the surface conditions of the receiver substrate.These properties are difficult to alter during the transfer printing process. [49,72]herefore, the critical point of the transfer printing process is to regulate the stamp/ink interface adhesion strength to switch between strong and weak states.

| STRATEGIES FOR REGULATING STAMP/INK INTERFACIAL STRENGTH IN TRANSFER PRINTING
Typical strategies regulating stamp/ink interfacial strength are summarized in Figure 3.To achieve a versatile transfer printing technique compatible with different functional inks, stamps are often designed with switchable adhesion capabilities.][88] Besides endowing the stamp with adhesion conversion ability, another regulation strategy is to utilize the solid-liquid transition occurring on the stamp or ink to remove a certain phase at the interface, [44,54,89,90] which results in a sharp decline in the interfacial strength.The above regulation strategies will be systematically discussed in this section.

| Controllable adhesion based on intrinsic properties of stamps
Various materials with intrinsic switchable adhesion properties, including viscoelastic polymers, [91] stimuliresponsive tapes, [92] swellable hydrogels, [93] and shape F I G U R E 2 Schematic illustration of the general process and mechanism of transfer printing.A transfer printing system comprises a donor substrate, transferred inks, a stamp, and a receiver substrate.The transfer process is regulated by the adhesion strength between the stamp/ink/substrate interfaces.During the adhesion and pick-up step (1 → 2), inks are retrieved from the rigid donor substrate by the stamp, facilitated by the stronger adhesion strength of the stamp/ink interface compared with the ink/donor substrate interface.Afterward, the stamp-carrying inks are aligned with the flexible receiver substrate and form conformal contact (2 → 3).Finally, in the printing step, the strength of the stamp/ink interface is switched to the weak adhesion state under external stimuli to print ink on the flexible receiver substrate (3 → 4).The stamp can be reused for subsequent transfer printing process (4 → 5).
F I G U R E 3 Typical strategies for regulating stamp/ink interfacial adhesion strength in transfer printing.CNT, carbon nanotube; LCST, lower critical solution temperature; PNIPAAm, poly-N-isopropylacrylamide; UV, ultraviolet.
memory polymers (SMPs), [94] have been employed in stamp fabrication.The adhesion states between these stamps and inks can be simply modulated by mechanical control and external stimuli to carry out transfer printing in a massively parallel manner, without requiring additional adhesive coating or chemical bonding layer modifications.

| Controlled by mechanical manipulation
Elastomer stamps with viscoelastic behavior can interact with inks through van der Waals force to form an adhesion state.The strength of the elastomer stamp/ink interfacial adhesion has been successfully modulated for ink pick-up and release by mechanical manipulation, such as adjusting the delamination speed, shear strain, and bending radius of the stamp.Meitl et al. first utilized the rate-dependent adhesion behavior of a viscoelastic PDMS stamp to regulate the stamp/ink interfacial adhesion strength. [31]As shown in Figure 4A, the adhesion strength between the PDMS stamp and ink was strong enough to grab inks at high peel-back speed (typically ∼10 cm/s).After the PDMS stamp conformally contacts with the receiver substrate, the PDMS stamp/ inks interfacial strength was weakened and inks were printed on the receiver substrate at low delamination speed (∼1 mm/s).This rate-dependent interfacial adhesion transition can be explained by the Griffith criterion in fracture mechanics, that is, the interfacial fracture occurs when the interfacial energy release rate G exceeds the critical energy release rate G c . [96]According to the rolling experiment, the critical energy release rate of the PDMS stamp/ink interface G c PDMS/Ink increases with the separation speed v (Figure 4B), expressed by Equation (1): where ϕ v ( ) is an increasing function of v.The critical energy release rate of the ink/substrate interface G c Ink/Substrate determined by the elastic nature of the rigid ink and substrate remains consistent under different stamp separation speed. [97] ) onto a variety of receiver substrates. [31,73]plying shear strain to elastomer stamps is an alternative method for switching stamp/ink adhesion states, known as shear-enhanced adhesiveless transfer printing. [74]he pick-up step is similar to the kinetically controlled transfer printing through the strong adhesion originated from viscoelastic effect of stamp; while in the printing step, the receiver substrate is latterly displaced to apply shear strain on the stamp, which causes asymmetric stress distribution and reduces delamination force (Figure 4C).The mechanics underlying shear-enhanced adhesiveless transfer printing were analyzed from a fracture mechanics model by Cheng et al. [95] The normalized peel-off force F decreases linearly with increasing shear strain γ, which can be approximated by Equation ( 2): where Γ 0 is the interface fracture toughness, E is Young's modulus of the stamp, and L and h are the size parameters of the stamp.The theoretical analysis result was consistent with the experiment data by Carlson et al. (Figure 4D). [74]herefore, the adhesion strength of the stamp/ink interface as indicated by F can be modulated by applying shear strain to the stamp, endowing a 10-fold improvement in transfer yield compared with the kinetically controlled transfer printing. [74]he switchable interfacial adhesion can also be regulated by controlling the bending radius of an elastomeric stamp. [75]As shown in Figure 4E, Si plates are picked up by a flat PDMS stamp with a large bending radius and subsequently printed on various receiver substrates by peeling the stamp with a relatively smaller bending radius.The schematic illustration of the stamp with different bending radii reveals the existence of a flat region and a curved region (Figure 4F).The flat region and curved region contribute to the pressure on the substrate and the adhesion to the elastomeric stamp, respectively.For a larger bending radius, the curved region of the stamp is wider, providing higher adhesion and lower pressure for retrieving ink from the donor substrate.Conversely, the stamp with a smaller bending radius possesses relatively narrow curved regions, resulting in lower adhesion and higher pressure between the ink and stamp during the printing process.The rolling test in Figure 4G illustrates that rods with large radii yield a higher critical energy release rate at the same rolling speed, indicating that the adjustability of the PDMS/ink interfacial adhesion strength by altering the bending radius of the PDMS stamp.
Modulating the elastomer stamp/ink interfacial adhesion strength through mechanical manipulation is a simple and versatile method for the large-scale integration of heterogeneous functional materials onto the target substrate.This method has been extensively used in the assembly of electronic devices, [32] the pattern of full-color QD LED arrays, [98] and the fabrication organic photovoltaic cells. [99]However, the effectiveness of this modulation method is limited by the unchangeable ink/substrate interfacial strength.An excessively strong ink/donor substrate interface can lead to pick-up failure, while a weak ink/receiver substrate interface results in printing failure.Additionally, physical parameters such as peel-off speed, shear strain, and bending radius of the stamp should also be controlled precisely by additional instruments.Therefore, it is necessary to develop new methods for regulating the adhesion state across a wide range to meet the evolving demand.

| Controlled by external stimuli
Several responsive materials, capable of altering their adhesion state under external stimuli, have been selected to create stamps for transfer printing, such as thermal release tape (TRT), [76] photosensitive tape, [38] wetresponsive hydrogel, [77] and SMP. [78]These responsive materials usually possess adhesive properties under ambient conditions, regarded as a strong adhesion state.However, upon exposure to external stimuli (e.g., heating, light illumination, and hydration), their adhesion strength decreases to a lower level, corresponding to a weak adhesion state.Stamps composed of these responsive materials are defined as intrinsic responsive stamps in this review, which could offer a higher strongto-weak adhesion ratio than the flat elastomer stamp discussed in Section 3.1.1.
TRT is the most commonly used intrinsic responsive stamp. [76,100]TRT is a thin and flexible tape that exhibits a high adhesion switching capability under heating.The process of thermal release transfer printing using the TRT as the stamp is depicted in Figure 5A. [76]Initially, TRT establishes conformal contact with the functional membrane at room temperature for the detachment of the membrane from the donor substrate.Thereafter, TRT is heated to a temperature exceeding its transition temperature (T r ) (approximately 100°C) to release the functional membrane onto the receiver substrate.The Griffith criterion can also be applied to elucidate this thermal release process.Similar to an elastomer stamp, TRT demonstrates the rate-dependent behavior.Therefore, the critical energy release rate of the TRT/ membrane interface (G TRT/membrane ) is controlled jointly by temperature and velocity (Figure 5B).At room temperature and high peel-off velocity, G TRT/membrane surpasses G membrane/substrate , indicating the strong adhesion state between the TRT and membrane during the pick-up process.As the temperature exceeds T r and the peel-off velocity decreases, G TRT/membrane gradually reduces to zero, representing the weak adhesion state between the TRT and membrane during the printing process.The adjustable range of G TRT/membrane spans from 0 to 951 J/m 2 , enabling a high switching ratio between the strong and weak adhesion states to ensure a high transfer yield.Photosensitive tape with lightmediated adhesion reduction property through the photochemical crosslinking reaction induced by ultraviolet (UV) irradiation has also achieved advanced electronics programmed assembly through a similar transfer printing process under UV irradiation. [38]lthough the above responsive tapes have demonstrated great potential in high-performance transfer printing, they suffer from a drawback of being disposable due to the irreversible transition of adhesion states.To address this issue, Yi et al. proposed a reusable, wetresponsive, and reconfigurable hydrogel adhesive film for transfer printing (Figure 5C). [77]The hydrogel adhesive film was composed of poly(ethylene glycol) dimethacrylate (PEGDMA) and possessed wet-responsive capability, allowing it to absorb significant amounts of water and reconfigure the shape of the hydrogel. [101]The cylindrical stem arrays with protruding tips on the PEGDMA film surface enabled the intrinsic strong adhesion with the nanomembrane on the donor substrate by van der Waals force.Once the PEGDMA film attached with the F I G U R E 4 Regulation of stamp intrinsic properties by mechanical manipulation for transfer printing.(A) Schematic illustration of the kinetically controlled transfer printing process exploiting the rate-dependent viscoelasticity of the elastomer stamp.(B) Relationship between the energy release rate of the PDMS stamp/ink interface and the stamp separation speed measured by rolling experiments.Reproduced with permission. [31]Copyright 2006, Springer Nature.(C) Schematic illustration of the shear-enhanced transfer printing steps controlled by shear strain on the PDMS stamp.Reproduced with permission. [74]Copyright 2011, AIP Publishing.(D) The normalized pulloff force of PDMS stamps with different sizes as a function of the shear strain γ.The solid line and dotted line represent the experimental data and the theoretical result, respectively.L denotes the width of the PDMS stamp.Reproduced with permission. [95]Copyright 2012, Elsevier.(E) Schematic illustration of the adhesiveless transfer printing regulated by the bending radius of stamp and (F) stamp models with a large and a small bending radius.(G) The energy release rate of glass rods with different radii measured by a rolling test on an angled flat PDMS slab.Reproduced with permission. [75]Copyright 2016, American Chemical Society.PDMS, polydimethylsiloxane. Reproduced with permission. [77]Copyright 2018, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(E) Schematic illustration of thermaldependent transfer printing employing a shape memory polymer (SMP) stamp.(F) Influence of temperature on the grip force of the SMP stamp and the adhesion strength at the SMP/objects interface.Reproduced with permission. [78]Copyright 2020, American Association for the Advancement of Science.PEGDMA, poly(ethylene glycol) dimethacrylate; TRT, thermal release tape.
nanomembrane was picked up and placed on the receiver substrate, the adhesive film was swelled by dropping water causing the volume expansion of the stem arrays and the macroscopic bending of the PEGDMA film for the release of the nanomembrane.Figure 5D illustrates the repeated wet-responsive transition of adhesion states of the PEGDMA films with various stem diameters (Ds).In the dry state, the introduction of cylindrical microstructure arrays reinforced the adhesion strength of the PEGDMA adhesives (≈191 kPa); while in the hydration state, the adhesion strength dropped to almost zero (0.19-0.34 kPa).Besides the exceptional adhesion switch ability (>640), the PEGDMA film is biocompatible to transfer functional materials onto biological substrates without the need for toxic solvents or external energy inputs.For example, using the PEGDMA adhesive film, a strain sensor was transfer printed onto a bovine eye to monitor intraocular pressure, thereby indicating the risk of glaucoma.The aforementioned intrinsic responsive stamps are quite suitable for the transfer printing of planar inks, such as nanomembranes and two-dimensional (2D) materials.However, they are inadequate for transferring nonplanar 3D objects because of the difficulty in establishing conformal contact with their curved surfaces, resulting in weak van der Waals interaction between the stamp and 3D objects.SMP is another class of thermal/light responsive material as the candidate for transfer printing stamp, [86] which can deform to a temporary shape and then recover to its original geometry under external stimuli. [102]Linghu et al. developed a thermally triggered epoxy SMP stamp for gripping and placing multiscale arbitrary 3D-shaped objects (Figure 5E). [78]To grip objects, a SMP stamp was heated above its glass transition temperature (T g = 45°C) to soften and achieve conformal contact with the objects under pressure, and a temporary shape was fixed to lock the embedded objects after cooling the SMP stamp to a stiff state below T g .Afterward, the stiff SMP stamp locked with objects was retrieved from the original substrate and approached to the target substrate.Finally, the SMP stamp was reheated above T g to restore its original geometry and accurately placed the objects onto the target substrate.The above transfer process is controlled by the grip force attributed to the friction, suction, and interlocking effects of the stiff SMP stamp and the adhesion force determined by the interfacial contact area between the SMP stamp and objects.Figure 5F demonstrates that both the grip force and the adhesion strength are temperature-dependent, and the maximum grip force at 30°C (26.64 N) is 11.7 times greater than the maximum adhesion force at 60°C (2.28 N), which implies that the grip force originating from the interlocking effect dominates the strong adhesion state at a temperature below T g .Conversely, at temperatures above T g , 3D objects are released during the SMP-shape recovery process due to the diminishment of the interlocking effect caused by the softening of the stamp and the weakening of the adhesion strength caused by the reduction in contact area.SMP-based stamps have demonstrated promising potential in the selective transfer printing of flexible electronics, [103] micro-LED chips, [104] and multilayered organic-inorganic thin films. [79]1.3| Controlled by liquid volume Solid-state stamps utilized in the above transfer printing techniques should form intimate contact with inks to generate van der Waals force or interlocking effect for inks pick-up.However, the elevated contact pressure caused by solid-state stamps induces shear stress on the ink surface due to the Poisson effect, which may lead to device breaking or curling during the transfer process, particularly for flexible ultrathin films.Recently, liquid droplet stamps have been investigated to transfer print flexible devices without causing damage through precisely controlling the liquid volume to regulate interfacial adhesion.[71,81] The typical process of liquid droplet stamp transfer printing is shown in Figure 6A.A rubber capillary tube is employed to handle the volume of liquid droplets.For ink (microstructured film) pick-up, a microdroplet on the tip of the capillary tube contacts with the ink surface, and a liquid bridge is established to detach the ink from the donor substrate (silicon wafer) by lifting the capillary tube.For ink printing, the volume of the droplet is increased to break the liquid bridge and the ink is placed on the soft target substrate through van der Waals interaction after the liquid droplet evaporates.The adhesion force of the liquid stamp is composed of the Laplace pressure F lp and surface tension F st , as repre- sented by Equation (3): Where r c is the cross-section radius of the droplet, r r is the vertical-section radius of the droplet, γ is the surface tension coefficient, and θ 1 is the angle between surface tension and horizontal direction (Figure 6B).Equation ( 3) is subjected to dimensionless treatment to provide the relationship between the dimensionless adhesion force f ̅ and the dimensionless volume V ̅ (Figure 6C), where f F πr γ ¯= / c and V V r ¯= / c 3 , V is the volume of the droplet.f ̅ decreases with the increasing of V ̅ and the experimental results (red dots) match well with the theoretical curve (black line).Therefore, the adhesion state between the liquid stamp and ink can be easily modulated by controlling the liquid volume.Liquid droplet stamp transfer printing has been successfully used in the micropatterning of organic materials, [105] the fabrication of 3D helical antennas, [81] and the assemble of epidermal hybrid optoelectronics. [71]Nevertheless, the radius of liquid droplet should be carefully selected to adapt the ink size to avoid the planer ink curving caused by liquid surface tension which is known as the elastocapillarity effect. [106]

| Controllable adhesion based on responsive surfaces on stamps
Besides exploiting the intrinsic properties of stamps materials, the stamp/ink interfacial adhesion strength can also be regulated by modifying responsive materials on stamp surfaces.Poly-N-isopropylacrylamide (PNIPAAm) is a well-known thermal-sensitive polymer that exhibits reversible changes in surface wettability due to the molecular conformation transition (Figure 7A). [107]Below the lower critical solution temperature (LCST ≈ 32-35°C for PNIPAAm), the carbonyl and amide groups in PNIPAAm are exposed toward the exterior to form intermolecular hydrogen bonds with water molecules, resulting in a hydrophilic swollen state of the PNIPAAm surface.In contrast, above the LCST, these intermolecular hydrogen bonds reversibly transform to intramolecular hydrogen bonds, and the isopropyl groups in PNIPAAm are exposed to the exterior, leading to a hydrophobic surface.Khabbaz Abkenar et al. utilized initiated CVD to grow a PNIPAAm coating on PDMS surfaces to construct stamp with thermally switchable adhesion ability (Figure 7B). [83]The hydrophilic nature of the PNIPAAm coating enabled the strong adhesion with gold nanoparticles (GNPs) arrays at 5°C during the pick-up process.Subsequently, the collapse of the PNIPAAm coating structure and the decrease in its hydrophilicity at 50°C induced the release of the GNPs arrays onto the receiver substrate.The adhesion conversion ability of PNIPAAm at room temperature has also endowed its broad application in tissue engineering. [108,109]For example, Ryu et al. grafted PNIPAAm onto a nanothin, highly porous (NTHP) membrane to achieve thermalresponsive transfer printing of stem cell-derived cardiac sheets. [82]The hydrophobic interaction between PNIPAAm and adhesive proteins (e.g., fibronectin) promoted the adhesion of stem cells on the PNIPAAm-coated NTHP membrane at temperature above the LCST, while the entropic repulsion of the protein adsorbed on the hydrophilic PNIPAAm surface initiated the detachment of the cell sheets at temperatures below the LCST.
In addition to the thermal-responsive PNIPAAm, magnetic-sensitive materials are incorporated onto the elastomer stamp to gradually control the stamp/ink interfacial adhesion strength. [40,110] 3) describing the adhesion force.(C) Correlation between the dimensionless interfacial force and the dimensionless liquid volume.Red dots and black solid lines represent the experimental results and the theoretical curve, respectively.Reproduced with permission. [81]Copyright 2021, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
F I G U R E 7 Controllable adhesion in transfer printing based on responsive surfaces on stamps.(A) Mechanism of the molecular conformation transition of PNIPAAm chains induced by temperature variations.Reproduced with permission. [107]Copyright 2023, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(B) Transfer printing process using a PDMS stamp coated with PNIPAAm for the transfer of gold nanoparticles (GNPs) arrays.Reproduced with permission. [83]Copyright 2017, The Royal Society of Chemistry.(C) Schematic illustration of the adhesion on/off states of a magnetic-actuated stamp filled with magnetic particles.(D) Comparisons between analytical predictions and experimental results of the adhesion strength of the magnetic-actuated stamp under different magnetic pressures.Reproduced with permission. [84]Copyright 2019, The Royal Society of Chemistry.(E) Schematic illustration of the electrical-responsive micro/nanotransfer printing utilizing a soft nanocomposite electroadhesive (SNE) stamp decorated with Al 2 O 3 -CNT.(F) Pull-off force of the Al 2 O 3 -CNT SNE at different external voltages.Reproduced with permission. [85]Copyright 2020, American Association for the Advancement of Science.CNT, carbon nanotube; LCST, lower critical solution temperature; PDMS, polydimethylsiloxane; PNIPAAm, poly-Nisopropylacrylamide.
particles (pure iron, 250 µm). [84]In this design, the flat PDMS membrane exhibited strong adhesion for ink attachment through van der Waals force at high retraction speed, referred to the adhesion "on" state.After applying a magnetic force, the magnetic particles were magnetized to protrude the PDMS membrane outward, thereby resulting in around stamp surface bulge and decreasing the contact area with the ink to reduce adhesion, which corresponding to the adhesion "off" state (Figure 7C).An analytical model was proposed to elucidate the mechanics behind the magnetic-controlled transition of adhesion states.The analytical predictions demonstrated an inverse relationship between the adhesion strength of the stamp and the applied magnetic pressure at different retraction speeds (Figure 7D), which aligned with the experimental results.The adhesion strength could be even reduced close to zero under sufficient high magnetic pressure, indicating that this magnetically actuated stamp possesses extremely high adhesion switching ratio.Apart from the stamp decorated with magnetic particles, a buckled iron film was installed on the PDMS surface to realize noncontact magnetic-driven transfer printing of a wide range of macroscopic objects (e.g., silicon platelets, pearl cottons, glass slides, and porous acrylic plates) using the same principle. [111]lectrostatic attraction force generated by applying voltage on a conductive graphite stamp has been used to print few-layer graphene nanopatterns on a silicon substrate. [36,39]Recently, utilizing the voltage-tunable electrostatic interactions similarly, Kim et al. designed a soft nanocomposite electroadhesive (SNE) with dielectric (Al 2 O 3 )-coated CNT arrays grown on a conductive electrode (TiN) for micro/nanotransfer printing of various objects, such as Ag nanowires, polystyrene microparticles, and micro-LED chips. [85]he electrostatic charge accumulated at the Al 2 O 3coated CNT/objects interface yielded an attraction force for picking up objects under an operation voltage of 30 V, and the weak van der Waals force between the SNE and objects owing to the low CNT density and high surface roughness of the SNE allowed the objects to be placed on the receiver substrate after turning off the voltage (Figure 7E).The adhesion strength of the SNE stamp was quantified by measuring the pull-off force of a Pt-coated spherical tip against the SNE using atomic force microscopy (AFM).The pull-off force of the AFM tip is proportional to the quadratic of the applied voltage on SNE (Figure 7F).Compared with the reference surface ((flat Al 2 O 3 -coated TiN), the SNE exhibited approximately a 40-fold lower adhesion strength at the voltage off state, and approximately a threefold stronger adhesion strength at the voltage on state (30 V), resulting in a high adhesion switching ratio of over 100.
Modifying responsive materials on the stamp surface is an effective strategy for enhancing the adhesion switching ability of the stamp.However, compared with the raw stamp with intrinsic adhesion transition capability, the inclusion of additional steps to modify the stamp will concurrently elevate the complexity and cost of stamp fabrication.Furthermore, the robustness of the modification layer in transfer printing cycles also needs to be further considered.

| Controllable adhesion based on bioinspired structures on stamps
Some creatures have exhibited remarkable abilities to adhere firmly on diverse surfaces, and the adhesion states can be switched rapidly and repeatedly to adapt to their surrounding environment.These intelligent adhesion phenomena originate from the unique structures on the adhesion organ of the creature, such as retractable pulvilli on aphid legs, [112] setae on gecko feet, [113] suckers on octopus wrists, [114] hexagonal cells separated by deep channel on tree frog toe pads. [115]Inspired by these creatures, some distinctive structures have been constructed on the surface of elastomer stamps to enhance their adhesion conversion performance.
Aphid can hold on a smooth leaf by everting the pulvilli through increased blood pressure, and the pulvilli are retracted by the contraction of tibial muscles to diminish adhesion when the aphids need to move (Figure 8A). [84,112]Inspired by the adhesion process of aphids, Kim et al. designed a microscale PDMS stamp with pyramidal reliefs on four corner regions for pressure-controlled transfer printing. [41]The pyramidal reliefs were compressed under high preload to maximize the contact area, providing a high van der Waals force for ink pick-up, which simulates the congestive state of the pulvilli.After removing the high preload, the elastic restoring force in the PDMS stamp was released, allowing it to recover to its original pyramidal shape and the decrease in contact area weakened adhesion strength for ink printing, which simulates the retractable state of pulvilli (Figure 8B).The transfer printing process utilizing this microstructured PDMS stamp is shown schematically in Figure 8C.In addition to regulating the contact area through the preload on the stamp, the viscoelastic effects of PDMS discussed in Section 3.1.1also contribute to the transition in adhesion states through picking up ink rapidly from the donor substrate and retracting the stamp slowly after placing ink on the receiver substrate.This microstructured stamp featuring a high adhesion switching ratio (more than three orders of magnitude) was employed for the deterministic assembly of silicon platelets with nearly 100% yield.
Gecko is another creature possessing remarkable strong adhesion ability.Millions of hierarchical setae on gecko feet provide strong van der Waals force to support them climbing on vertical surface. [113]Notably, the adhesion strength of gecko feet is directional-dependent and can be adjusted by the direction angle of the setae through the movement of the gecko toes (Figure 8D). [116,118,119]A series of gecko-inspired dry adhesives have been proposed for gripping macroscale objects. [42,88,120]For example, Song et al. fabricated a mushroom-shaped microfibrillar F I G U R E 8 Controllable adhesion in transfer printing based on bioinspired structures on stamps.(A) Adhesion mechanism of aphid pulvillus controlled by blood pressure and tibial muscles.Reproduced with permission. [84]Copyright 2019, The Royal Society of Chemistry.(B) Scanning electron microscopy (SEM) images of the compressed and relaxed states of aphid-inspired pyramidal reliefs on a PDMS stamp.(C) Schematic illustration of the transfer printing process for deterministic assembly using a stamp with pyramidal microtips on the surface.Reproduced with permission. [41]Copyright 2010, The National Academy of Sciences.(D) The uncurling and curling of gecko toes control adhesion states through alternating the direction of hierarchical setae.Reproduced with permission. [116]Copyright 2006, The Company of Biologists.(E) Photographic image of gecko-inspired fibrillar adhesives on a membrane (FAM) for macroscopic objects transfer and SEM image of a cross-section of the FAM.(F) Adhesion force of the FAM and the flat membrane under different preloads.Reproduced with permission. [87]Copyright 2014, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(G) Photograph and adhesion mechanism of suckers on octopus wrists.C, R, and M represent circular muscles, radial muscles, and meridional muscles, respectively, which jointly control sucker adhesion.(H) Schematic illustration of the octopus-inspired smart adhesive pad with temperature-controlled switchable adhesion.Reproduced with permission. [43]Copyright 2016, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(I) Schematic illustration of the elastic energy storage enabled magnetically actuated, octopus-inspired smart adhesive.W 0 , W 1 , and W 2 are PDMS membrane deflections at different states.Reproduced with permission. [117]Copyright 2021, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.PDMS, polydimethylsiloxane; PNIPAAm, poly-N-isopropylacrylamide.
elastomer adhesive membrane (FAM) gripper capable of transfer printing of 3D objects (steel ball with different diameters) with a high switching ratio (≈204) (Figure 8E). [87]The objects could fully conform with the FAM to be retrieved up through the strong van der Waals force enhanced by microfiber arrays.To release the objects, a syringe pump applied air pressure to inflate the FAM, inducing mechanical bending of the microfibers on the FAM to reduce the adhesion strength (Figure 8F).However, none of these gecko-inspired adhesives can perfectly match the adhesion performance of natural Gecko feet.Jeong et al. cut setae arrays from living geckos serving as the natural setae stamp for transferring microelectronics on irregular surfaces. [119]The natural setae stamp exhibited excellent controllable adhesive characteristic and high durability for manipulating microscale functional devices.Each seta on the stamp could be repeatedly used for more than 30,000 cycles without performance deterioration.Nevertheless, the process of preparing this stamp is cruel and unsustainable as it kept causing injure to animals, which limits its practical application.
Octopus has demonstrated outstanding switchable underwater adhesion capacity through suckers on wrists.The adhesion strength of the suckers is controlled by the cavity pressure driven by the actuation of a 3D muscle array surrounding the suckers (Figure 8G). [114]By mimicking octopus suckers, Lee et al. designed a smart PDMS pad with microcavities for the transfer printing of semiconductor nanomembranes. [43]A thermal-sensitive PNIPAAm hydrogel layer was deposited on the microcavities to imitate the muscle array.Below the LCST of PNIPAAm, the hydrogel is hydrophilic and swelled by water to expand volume causing an increase in microcavity pressure (P caivity = P out ), simulating the muscle relaxation at a low adhesion state.Above the LCST of PNIPAAm, the hydrogel becomes hydrophobic and PNIPAAm molecule chains collapse causing volume shrinkage to form negative pressure in the microcavity (P caivity < P out ), simulating the muscle tension at a high adhesion state (Figure 8H).In a typical thermal-sensitive transfer printing process employing the smart pad as the stamp, InGaAs nanomembranes array was picked up at high temperature (T H = 35°C), and then transferred to a SiO 2 /Si substrate by immersing the smart pad into water at low temperature (T L = 22°C).Although the PNIPAAm-coated smart pad has shown a high switching ratio (≈293), the long switching time of 1 h needs to be addressed.Additionally, octopus can prestrain its muscle to mantain a long period surface attachment without contracting muscle constantly. [114]Recently, inspired by this elastic energy storage strategy, Wang et al. proposed a magnetic-actuated smart adhesive that enables the rapid transfer of common objects under dry/wet conditions with high-energy efficiency (Figure 8I). [117]agnetic particles (carbonyl iron) were embedded in a chamber sealed with a PDMS membrane, which serves as a muscle to control the cavity pressure under different magnetic fields.Initially, a relatively low magnetic pressure (P m0 ) was preloaded on the smart adhesive for elastic energy storage.After removing P m0 , the stored elastic energy was released to retract the PDMS membrane upward, which caused a negative pressure in the cavity to arise the strong adhesion state.To switch to the weak adhesion state, a higher magnetic pressure (P m2 > P m0 ) was applied to induce large deflection on the PDMS membrane and elevate the cavity pressure.This adhesion state conversion process could be completed in 0.5 s, which significantly shortens the time required for transfer printing.Besides introducing an actuation layer on the surface of the stamp structures to imitate the muscle contraction and relaxation, Baik et al. presented an adhesive patch featuring micrometer-scale dome-like protuberance architectures, which is similar to the structure of octopus suction cups. [121]This octopusinspired adhesive patch was fabricated simply by molding a polymeric master without complicated surface modifications and exhibited strong and reversible adhesion for transferring large-area wafers in wet conditions without energy consumption.
Learning from nature, a range of bioinspired structured stamps with fast and robust adhesion conversion ability have been developed for the transfer printing of various functional objects.The shape of macro/microstructures on stamp can be altered under different stimuli such as preload, air pressure, and external fields, which converts the adhesion states of the stamp by altering the intensity of interactions (e.g., van der Waals force and negative air pressure) between the stamp and ink.However, the fabrication of structured stamps usually involves complicated manufacturing techniques with increased processing costs and time, such as photolithography [41] and dry etching. [43]Furthermore, the adhesive performance and switchability of these human-made structured stamps still fall short of natural creatures.

| Controllable adhesion based on solid-liquid transition
Except for changing the properties and structures of the stamp to switch between strong and weak adhesion states, the interfacial strength between two phases can be sharply decreased by removing one phase through solid-liquid transition.][124] However, the application of a sacrificial layer on the donor substrate only assists the pick-up process by decreasing the ink/donor substrate interfacial strength.As previously mentioned, regulating the stamp/ink interfacial strength is important in transfer printing.Therefore, selecting materials with solid-liquid transition properties to build stamps or inks represents a novel strategy for controlling the corresponding interfacial strength.

| Controlled by solid-liquid transition of stamps
[127] For example, Liu et al. demonstrated a graphene-assisted method for transfer printing wafer-scale metal electrode arrays by a water-soluble polyvinyl alcohol (PVA) stamp (Figure 9A). [89] single-crystal monolayer graphene was prepared on germanium surface by CVD serving as a donor substrate (Gr/Ge substrate), and metal arrays were deposited on the Gr/Ge substrate through photolithography and electronbeam evaporation.Due to the absence of dangling bonds, the monolayer graphene surface resulted in a weak interfacial strength between the metal and Gr/Ge substrate, [128,129] ensuring an almost 100% transfer yield during the pick-up process.After applying pressure and temperature (80°C) on the PVA stamp to conform with the receiver SiO 2 substrate, the PVA stamp was dissolved by adding deionized water to release metal arrays entirely.This method has enabled the successful transfer of various metal electrode arrays including weak adhering Cu, Ag, Au, and strong adhering Pt, Ni for the integration of MoS 2 field-effect transistors (Figure 9B).Besides the water-soluble PVA stamp, Zabow recently Reproduced with permission. [44]Copyright 2022, American Association for the Advancement of Science.3D, three-dimensional; PVA, polyvinyl alcohol.
demonstrated that thermal reflowable sugar mixtures can function as stamps for 3D transfer printing on arbitrary high curvature surfaces (Figure 9C). [44]Corn syrup was added to the sugar solution to prevent crystallization and offer a low T g (near room temperature).After caramelizing the sugar mixture and pouring over it onto the donor substrate, remaining water was evaporated under controlled heating to solidify the sugar layer which formed intimate contact with the structures on the flat donor substrate to provide adhesion force for delamination.Afterward, the dried sugar stamp with transferred microstructures was gently heated (at 35-40°C) to reflow as a viscous and creeping fluid for ultraconformal contact with the curved receiver substrate.Finally, the sugar stamp was removed by dissolving to fully release the microstructures on the receiver substrate.The reflowed flexible, stretchable sugar fluid with high viscosity enables 3D transfer printing of microstructures (e.g., Au disks, Pt ellipses, and Ni rings) on complex-shaped surfaces (e.g., pin tip, cube with sharp edge, and red blood cell) with micrometer-level accuracy (Figure 9D).Stamps capable of being removed through solid-liquid transition theoretically offer an infinite adhesion switching ratio superior to other regulation methods, thereby guaranteeing a 100% transfer yield.In particular, water-  [90] Copyright 2017, Springer Nature.(C) Schematic illustration of the confined laser transfer printing process employing EGaln nanoparticles as ink for the fabrication of 2D metal oxide films with different thicknesses.Reproduced with permission. [54]Copyright 2023, Elsevier.2D, two-dimensional; EGaIn, eutectic gallium-indium; NPs, nanoparticles.soluble sugar stamps exhibit potential for 3D transfer printing on arbitrary surfaces.Nevertheless, owing to the relatively weak adhesion strength between the solid-liquid transitional stamp and ink materials, additional sacrificial layer or assisted 2D graphene layer sometimes need to be coated on the donor substrate to reduce interfacial strength between the ink and substrate to ensure the success of the pick-up step.Moreover, the irreversible solid-liquid transition of the stamp necessitates the reconstruction of the stamp for each transfer cycle and causes challenges in massive materials transfer.

| Controlled by solid-liquid transition of inks
Certain functional materials with the solid-liquid transition property have been selected as transferred inks.For example, azobenzene compounds possess the photoswitchable reversible solid-liquid transition induced by isomerization, [130] which have shown applications in photoactuators, [131] gas separation, [132] and solar thermal fuels. [133]hou et al. synthesized azobenzene-containing polymers (azopolymers) with photoswitchable T g through reversible addition-fragmentation chain-transfer (Figure 10A). [90]hese azopolymers can transition from solid-to-liquid state under UV irradiation due to the decrease of T g induced by trans-cis isomerization.While exposed to visible light or heated, the azopolymers experience cis-trans isomerization leading to a transition from liquid-to-solid state.On the basis of this reversible phase transition property, the azopolymers can be selectively transferred onto a tape.As shown in Figure 10B, only the left spot of the azopolymer liquefies under UV irradiation and can be picked up by the tape.Besides azobenzene compounds, liquid metals with low melting temperatures also undergo a phase transition between solid and liquid states mediated by heat energy at room temperature, [134] which are frequently used for fabricating flexible electronics through transfer printing. [135,136]Eutectic gallium-indium (EGaIn) is an important member of liquid metals that can spontaneously form a self-limiting solid oxide shell with a thickness of 0.7-3 nm in air. [137]Recently, An et al. proposed a laser-energy modulated transfer printing technique for the construction of 2D metal oxide film patterns, which utilized the liquidto-solid oxidation reaction of self-limiting oxide skin encapsulated EGaIn nanoparticles (Figure 10C). [54]EGaIn nanoparticles were spray coated onto the SiO 2 /Si wafer substrate and confined by a sapphire receiver substrate.Laser-energy injection beyond the critical value causes the entire rupture of the oxide film on EGaIn nanoparticles and the explosive fusion of adjacent particles to form sintered liquid metal.This process generates high temperatures, enabling the preferential oxidation of gallium in EGaIn, as well as a high recoil force to push the thin gallium oxide film onto the sapphire substrate.The existence of a strong van der Waals force between the gallium oxide film and the sapphire substrate enables successful transfer after lifting up the sapphire substrate.The electrical properties such as the resistance and band gap of the transferred gallium oxide film are controlled by film thickness, which increases with laser fluence.
Efficient and selective transfer printing can be achieved by exploiting the solid-liquid transition property of ink materials to regulate interface strength.However, the most commonly used functional ink materials (e.g., metal patterns, inorganic metal oxide semiconductor films, and organic conductive polymer membranes) lack this phase transition property at room temperature, thus limiting the potential applications of this method.Transfer printing is a versatile technology used to assemble diverse multiscale functional inks on flexible and curved substrates.In this review, we focus on the critical issue of adhesion at the stamp/ink interface in transfer printing.Recent advances in strategies for regulating interfacial strength have been summarized (Table 1) and typical examples for each approach are highlighted.The strong and weak adhesion states at the stamp/ink interface can be converted efficiently under various external stimuli, including mechanical force, light, electric, magnetic fields, solvent addition, and energy input.This stimuli-responsive transition of adhesion states ensures a high success rate in transfer printing, which relies on variations in intrinsic adhesion properties of the bulk stamp (e.g., rate-dependent viscoelasticity of elastomer stamps), transitions of smart surface coatings on the stamp (e.g., thermal-induced conformation transition of PNIPAAm coating), shape changes of surface structures on the stamp, and the solid-liquid transition of the stamp or ink phase.Despite significant progress in advanced transfer printing technology, several challenges in the design of stamp, ink, and substrate should be considered in the future as outlined below: (1) For stamp design: Currently used stamps have achieved a reversible transition of different adhesion states with a high switching ratio.However, constructing stamps with fast-tunable adhesion for massive transfer printing still remains a challenge.Some attempts have been made to rapidly actuate adhesives using voltage and magnetic fields instead of relying on temperature and light fields.For example, the adhesion states of magnetic-actuated smart adhesives can be switched within 0.5 s, allowing for the fast manipulation of macroscale objects. [117,138]Another challenge is to construct a stamp suitable for the precise assembly of nanoscale objects.Scaling down the size of smart adhesives to function as nanotransfer printing stamps may meet the demand for the rapid fabrication of highresolution devices.Furthermore, transferred inks possess different natures of modules and shapes.The adhesion strength of a stamp is hard to simultaneously accommodate to diverse inks.In contrast, natural adhesive organs in creatures can adapt to a wide range of flat, curved, rigid, and soft surfaces by instantly altering muscle stiffness. [139]Recently, a bioinspired sensing-triggered stiffness-tunable smart adhesive controlled by the magnetorheological effect has shown promise in gripping various nonflat objects. [140]itating biological muscle and nervous system to create a universal stamp with dynamically self-adaptive adhesion strength regulated by real-time sensing may provide a path suitable for transferring various inks.Therefore, the development of next-generation functional stamps should prioritize fast responsibility, nanoscale dimension, and dynamically self-adaptivity for the demand of large-scale, high-resolution, and universal transfer printing.(2) For ink design: Regulating the adhesion of ink to control interfacial strength is a complementary approach to stamp design strategies.Ink design strategies at present are restricted to utilizing the solid-liquid transition of a few types of responsive inks (e.g., azobenzene compounds and liquid metals) to reduce interfacial adhesion strength.Nevertheless, the limited classes of responsive inks are insufficient for the broad application scenarios.Developing specialized functional ink materials with gradient adhesive properties to realize different stickiness on upper and lower surfaces may enable high-performance transfer printing without relying solely on well-designed stamps.(3) For substrate design: A common problem in transfer printing arises from the strong interactions formed between the ink and the donor substrate during ink manufacturing, which decreases the success rate in the subsequent pick-up step.Conversely, the weak van der Waals force between the ink and the receiver substrate makes the printed ink pattern unstable and prone to failure.Marine mussels exhibit rapid and robust wet adhesion on solid surfaces in seawater, attributed to proteins containing catecholic amino acid, 3,4-dihydroxyphenylalanine (Dopa) on the mussel byssus. [141]The Dopa in proteins can form various supramolecular interactions with different substrates to enhance interfacial adhesion, including bidentate hydrogen bond, metal-catechol coordination bond, π-π/π-cation interactions, and electrostatic interactions. [142,143]Drawing inspiration by mussels, a Dopa-based hydrogel has been employed as the substrate for fabricating hydrogel-based multielectrode arrays via microtransfer printing. [144]herefore, introducing strength-adjustable supramolecular interactions at the substrate surface may address the challenge of inappropriate interfacial strength between the substrate and the ink.
The design of transfer printing systems involves interdisciplinary collaboration.This requires chemists, material scientists, and mechanics to jointly explore new materials synthesis, structural design, mechanism analysis, and potential applications to further promote the development of this technology.

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I G U R E 5 Regulation of stamp intrinsic properties by external stimuli for transfer printing.(A) Schematic illustration of temperaturecontrolled transfer printing utilizing a TRT as the stamp.(B) The contour map of the energy release rate at the TRT/membrane interface under different temperatures and peeling velocities.Reproduced with permission.[76]Copyright 2017, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(C) Schematic illustration of wet-responsive transfer printing using reconfigurable PEGDMA hydrogel adhesives.(D) Reversible adhesion strength of PEGDMA adhesives with different stem diameters (Ds) during swelling and deswelling circles.

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I G U R E 6 Regulation of stamp intrinsic properties by liquid volume for transfer printing.(A) The process of liquid droplet stamp transfer printing.(B) Schematic illustration of the adhesion force of the liquid stamp on the film, where parameters r c , r r , γ, and θ 1 are present in Equation (

9
Controllable adhesion in transfer printing based on the solid-liquid transition of stamps.(A) Schematic illustration of the graphene-assisted transfer printing process using a water-soluble PVA stamp for both weakly and strongly adhering metals.(B) Structures and images of MoS 2 back-gated transistors with different transferred metal electrode patterns.Reproduced with permission.[89]Copyright 2022, Springer Nature.(C) Schematic illustration of the steps of 3D microtransfer printing utilizing a reflowable and water-soluble sugar stamp.(D) Laser confocal optical micrograph of a 1-μm Au disk array transferred onto the irregular tip of a pin by the sugar stamp.

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I G U R E 10 Controllable adhesion in transfer printing based on the solid-liquid transition of inks.(A) Mechanism of the azopolymer reversible photoisomerization.(B) Schematic illustration (top) and real photos (bottom) of the selective transfer process of azopolymer inks with photo-induced solid-liquid transition characteristics.Reproduced with permission.
Comparison of different interfacial adhesion regulation strategies in transfer printing.
T A B L E 1