Bioinspired Adhesive Manufactured by Projection Microstereolithography 3D Printing Technology and Its Application

Many methods are used to manufacture bioinspired adhesive arrays such as photolithography and nanoimprinting methods, which are very high cost and time‐consuming. In this manuscript, projection microstereolithography 3D printing method is firstly adopted to prepare the bioinspired dry adhesive in several micrometers, which is low cost, time saving, convenient and can realize rapid, large‐scale, high‐precision, and controllable complex structure manufacture. The morphology and adhesive properties of four kinds of adhesive with flat punch, mushroom‐shaped, suction cup‐shaped, and titled micropillar structures are investigated. Scanning electron microscope (SEM) observation shows that the very regular pattern of the adhesive is generated and no collapse phenomenon is observed. The adhesion test results show the mushroom‐shaped adhesive reach the maximum adhesion of 15 KPa, which can be maintained after repeated uses. The contact angle of the adhesive is 143.7°, showing the good self‐cleaning ability. The mushroom‐shaped adhesive exhibits excellent adhesion property, high repeatability, and good self‐cleaning ability, which is further applied in grasping and transferring various surfaces such as solar panels, printed circuit board, flanges, and so on.

DOI: 10.1002/admi.202202465 mold rolling, and chemical vapor deposition (CVD) methods [4][5][6][7][8][9][10][11][12][13][14][15][16] . For injection molding, Sitti et al., [8] Liu et al., [13] and Shao et al. [17] present a method for the fabrication of biomimetic dry adhesives by conventional photolithography and molding. For mold rolling, Jan [18] invents a process for creating adhesion elements on a substrate material using at least one plastics material which is introduced into at least one shaping element. By producing adhesion elements with flared ends which adhere primarily as result of van der Waals forces. However, the above two kinds of molds are prepared by photolithography or precision finishing, which is difficult to demold and very expensive. For CVD methods, Fe as catalyst layer and Al 2 O 3 film as buffer layer plated on silicon substrate by magnetron sputtering and a following complex growth process are needed. [5][6][7] Although good adhesion properties have been achieved by the above methods, they are also very high cost and time consuming. Projection microstereolithography (PµSL) 3D printing method is a layer-by-layer additive micromanufacturing process capable of fabricating arbitrary 3D microscale structures, which is ideal for 3D articles with high structural complexity and with feature sizes ranging from tens of micrometers to centimeters. [19][20][21] Grigoryan et al. [22] adopted PµSL technology to successfully fabricate multivascular networks and functional intravascular Many methods are used to manufacture bioinspired adhesive arrays such as photolithography and nanoimprinting methods, which are very high cost and time-consuming. In this manuscript, projection microstereolithography 3D printing method is firstly adopted to prepare the bioinspired dry adhesive in several micrometers, which is low cost, time saving, convenient and can realize rapid, large-scale, high-precision, and controllable complex structure manufacture. The morphology and adhesive properties of four kinds of adhesive with flat punch, mushroom-shaped, suction cup-shaped, and titled micropillar structures are investigated. Scanning electron microscope (SEM) observation shows that the very regular pattern of the adhesive is generated and no collapse phenomenon is observed. The adhesion test results show the mushroom-shaped adhesive reach the maximum adhesion of 15 KPa, which can be maintained after repeated uses. The contact angle of the adhesive is 143.7°, showing the good self-cleaning ability. The mushroom-shaped adhesive exhibits excellent adhesion property, high repeatability, and good selfcleaning ability, which is further applied in grasping and transferring various surfaces such as solar panels, printed circuit board, flanges, and so on.

Introduction
Geckos can climb and run on horizontal or vertical and smooth or rough surfaces, which is attributed to the high aspect ratio beta-keratin consisted of mesoscale lamellae, microscale setae, and nanoscale spatulae hairs on their toe-pads. [1][2][3] Many methods have been carried out to fabricate the bioinspired microfibrillar structures, such as injection molding, topologies. Zheng et al. [19,23] fabricated a group of ultralight mechanical metamaterials that maintain a nearly constant stiffness per unit mass density by PµSL. Shea et al. [24] used PµSL to fabricate 3D imprinted microstructures. Ge et al. [25] created high resolution, multimaterials shape memory polymers architectures, based on the new PµSL 3D printing approach. Chen et al. [26,27] and Tian et al. [28] manufactured rectangle, cylindrical micropillars, and trumpet-shaped adhesive by PµSL.
Mushroom-shaped structure universally exists in insects such as beetles, plays a significant role in biological adhesion properties. [29,30] As indexed, there is no mushroom-shaped bioinspired dry adhesive in several micrometers manufactured by PµSL 3D printing method. PµSL 3D printing can realize the rapid and large-scale manufacture, have the advantage of high-precision, strong controllability, further promoting the practical development of bioinspired adhesive. In this manuscript, PµSL 3D printing method is adopted to firstly manufacture the microscale bioinspired adhesive positive mold, which is low cost, time saving, and convenient. In the process, two easy replication steps including polyvinyl chloride (PVC) gel negative molding and the last molding were necessary to obtain the bioinspired adhesive. It is worthy to note that PVC gel negative mold can easily demold from the 3D printed positive mold due to the slightly shrinking property of casted PVC. By this method, mold damage was successfully avoided. The morphology and adhesive properties of this new adhesive are investigated using a friction test machine. Furthermore, the mushroom shaped adhesive was successfully integrated with mechanical arm and further applied in grasping and transferring various surfaces such as solar panels, printed circuit board, flanges, and so on.

Fabrication of Bioinspired Adhesive
The microscale positive mold was firstly manufactured by the 3D printer with the 2 µm resolution (P130, BMF Material, Shenzhen). The acrylic based photosensitive resin fabricated by BMF material was used as the printing material. The platform surface was always kept 10 µm height from the liquid surface by auto focus. The liquid resin was cured and fixed on the platform by a 21.6 mW cm −2 ultraviolet (UV) light, by using the Digital Micromirror Device (DMD, Texas Instruments) as a dynamic mask, the mask patterns are dynamically generated as bitmap images on a computer-programmable array of digital micromirrors on the DMD chip. The light illuminated on the DMD chip is shaped according to the defined mask pattern, and then, the modulated light is transferred through a reduction lens. [21] The micropatterns projecting on the platform surface, forming a first layer. Then, the platform moves down to make a second auto focus, keeping 10 µm height from the liquid surface. By this kind of layer and layer curing, the bioinspired adhesive positive mold with flat punch, mushroom-shaped, suction cup-shaped, and tilted shaped structure can be manufactured. Dibutyl adipate (DBA) and tetrahydrofuran (THF) were added in the Erlenmeyer flask according to the mass ratio of PVC:DBA:THF = 1:1.75:15, and placed on the magnetic stirrer to stir at 500 r min −1 for 5 min PVC powder was then added to the Erlenmeyer flask and placed on the magnetic stirrer at 1500 r min −1 for 24 h under room temperature, obtaining PVC gel solution. 3D printed bioinspired adhesive positive mold was placed in a beaker, into which the stirred PVC gel solution was poured, and bubbles in the PVC gel solution were removed in vacuum state. Then, the beaker was placed in the ventilated drying box, dried at 60 °C for 24 h, a PVC gel negative replica mold can be peeled off. It is worthy to note that PVC gel negative mold can easily demold from the 3D printed positive mold due to the slightly shrinking property of casted PVC. [31] Polydimethylsiloxane (PDMS) was prepared according to the mass ratio of substrate and crosslinker components 10:1, and stirred evenly. The PVC gel negative replica mold was placed in the beaker, into which the above PDMS was poured, and air bubbles were removed in vacuum state. The beaker was placed in a dry box and kept at 80 °C for 1 h. Then, the PVC gel negative replica mold was peeled off to obtain a PDMS bioinspired adhesive material with the same structure as the 3D printed mold (Figure 1).

Contact Angle Test
Contact angle measurements were performed using a contact angle meter (JY-82B Kruss DSA, Dataphysics OCA20, Germany) with a sample size of 2 × 2 cm sheet sample (considering droplet rolling), and the static contact angle of the sample with water was tested.

Adhesion Test
The adhesion forces of four samples were tested by UMT mechanical tester (UMT TriboLab, BRUKER, USA). The sample size was 5 × 5 mm. The tester consists of a highprecision mobile platform, a microforce sensor, a silicon wafer, and a substrate (as shown in Figure S1, Supporting Information). The PDMS sample is fixed on a high precision mobile platform above, which is connected with a micro force sensor. Beneath the platform are smooth silicon wafers and a substrate. The testing process of adhesion was as follows: Firstly, the high-precision mobile platform was lowered along the normal direction at a speed of 80 µ s −1 , and the preload varied between 0 and 7 N. After reaching the normal preload, the platform was maintained for 5 s. Then, the platform was raised at the same speed to measure the normal adhesion of the sample. www.advmatinterfaces.de

FEA Simulation
We used 3D printed positive mold to prepare four different microstructures of bionic adhesive in flat punch, mushroomshaped, suction cup-shaped, and tilted shaped, respectively. We used finite element analysis to simulate the mechanical behavior of four types of microstructures when subjected to compression and tension.
In the compression state (Figure 2ai,ii), the flat punch micropillar is difficult to withstand large preloads, causing micropillar to bend to one side, which also causes them to fail to form contact surface with the object. As shown in Figure 2aiii,iv, when the flat punch micropillar is under tension, the stress concentration at the root of the micropillar is more prominent, and there is also a larger stress in every micropillar.
In the compression state of mushroom-shaped micropillars (Figure 2bi,ii), the compressive stress inside the micropillars is evenly distributed and there is no obvious stress concentration. The mushroom-shaped micropillars can transfer the compressive stress to the substrate so that the micropillar can withstand large preload without deformation. When the micropillar is separated from the object, it is subjected to tensile stress. When it is subjected to tensile stress (Figure 2biii), the maximum stress region is at the junction of the micropillar base and near the top of the micropillar (Figure 2biv). The tensile stress is evenly distributed in the micropillar, and the micropillar neck is elongated without deformation at the top, effectively avoiding stress concentration.
Compared with the mushroom-shaped micropillar, the suction cup-shaped structure at the top of the suction cup-shaped micropillar is more likely to produce stress concentration. In the finite element analysis of tension and compression (Figure 2c), the micropillar produces stress concentration and large deformation at the edge of the suction cup-shaped structure. When suction cup-shaped micropillar is compressed (Figure 2ci,ii), due to the side is thinner, compressive stress concentration area is located in edge of the suction cup-shaped structure, lead to "suction cup" outer, stress can't transfer to the base and main stress areas are located in the suction cupshaped structure, which lead to large deformation of suction cup-shaped structure, and is difficult to maximize the effective contact. When the suction cup-shaped micropillar is under tension (Figure 2ciii,iv), the tensile stress also concentrates on the edge of the suction cup-shaped structure, causing the suction cup-shaped structure to produce adduction deformation and reducing the contact area with the object.
In compression state, it is easier for tilted micropillar (Figure 2ci,ii) without the main supporting structure to collapse. There is serious stress concentration on the side of the micropillar, and the upper surface of the tilted micropillar cannot form stable contact with the object. When the tilted micropillar is under tension (Figure 2ciii,iv), a certain angle is generated between the tip of the tilted micropillar and the contact surface, so it cannot form a stable contact with the contact surface.
The finite element analysis shows that the four kinds of micropillars have different stress states under compression and tension. The flat punch micropillar will bend to one side under large preloads, and has a large stress concentration in tension, which was not suitable for creating stable adhesion states. The mushroom-shaped micropillar keeps stable contact with the contact surface under compression, and reduces stress concentration while increasing the contact area, effectively avoiding the collapse of the micropillar, preventing the decrease of the adhesion area, which can possess larger adhesion strength. Compared with the mushroom-shaped micropillar, the diameter of the suction-cup shaped micropillars increases and deforms due to the evagination and adduction of the top, and the suction cup-shaped structure cannot maintain stable contact with the contact surface, which reduces the contact area and leads to partial reduction of the adhesion strength. Tilted micropillar is prone to collapse and cannot maintain stable contact with the contact surface when normal loads are applied, resulting in very low adhesion. Through the FEA simulation of four types www.advmatinterfaces.de of structures, it is well explained why the mushroom-shaped microstructures has the best performance.

SEM Images
The morphology of flat punch, mushroom-shaped, suction cupshaped, and tilted micropillars bioinspired adhesive with various dimensions were observed by SEM images (Figure 3a-d).
Very regular patterns and no collapse phenomenon occurred, which show that the PµSL 3D printing method is exactly a good choice to manufacture the bioinspired adhesive. As shown in Figure 3, we found that the dimension of obtained adhesive microstructure are 43%-60% in height and 81%-90% in diameter compared to the 3D printed positive mold (as illustrated in Figures S2 and S3, Supporting Information), which is attributed to the non-uniform shrinking property of PVC casting process and is very easier to demold.

Contact Angle
Contact angle measurements were performed on flat punch micropillars, mushroom-shaped micropillars, suction cupshaped micropillars, and tilted micropillars using a contact angle measuring instrument. The test fluid was water. The test results are shown in Figure 3, and the contact angle of flat punch micropillars is 116.6° (Figure 4a). The contact angles of PDMS with mushroom-shaped, suction cup-shaped and tilted micropillars were 143.7°, 138°, and 128.6°, respectively (Figure 4b-d). Compared with the unstructured PDMS (106.2°, Figure S4, Supporting Information), all four samples prepared by using additive manufacturing molds have better hydrophobicity. Compared with the other samples, the mushroomshaped structure obtained highest hydrophobicity through the surface structure, suction cup-shaped structure obtained second higher hydrophobicity. Utilizing this method, the contact angle of the adhesive is increased and the surface energy of the material is reduced, thus enhancing the self-cleaning ability of the material surface.

Adhesion Test and Discussion
To test the performance of adhesives prepared with additive manufacturing molds, we tested the adhesion strength of flat punch micropillars, mushroom-shaped micropillars, suction cup-shaped micropillars, and tilted micropillars.
The UMT tester was used to test the normal adhesion of all PDMS samples, and the results are presented in Figure 5a,b. During the test, the adhesive is fixed on high-precision mobile platform and then pressed down. After reaching set preload value the sample is lifted up, obtaining the maximum pullingoff force, named the adhesive force. When the preload strength is lower than 3 KPa, the adhesion strength of the four samples increases gradually with the increase of the preload strength. However, when the preload strength is greater than 3 KPa, the normal adhesion strength of the tilted structure gradually decreases to 0, while the normal adhesion strength of flat punch, mushroom-shaped, and suction cup-shaped adhesives continue to increase and gradually stabilize with the increase of preload.
We conducted repeatability tests on mushroom-shaped and suction-shaped micropillars, as showed in Figure 5c,d. The repeatability tests were carried out under the same conditions under preload strength of 20 KPa. In the repeatability tests, the samples had good repeatability in 50 cycles. The adhesion of mushroom-shaped micropillars was stable at ≈14 KPa (Figure 5c), and that of suction cup-shaped micropillars was stable at ≈10 KPa (Figure 5d).
Among them, the mushroom-shaped micropillars have the highest adhesion strength, up to 15 KPa. The largest adhesion strength of the flat punch micropillars and suction cup-shaped micropillars is 7 and 10 KPa, respectively. The adhesion strength of the tilted structure is only 1 KPa. According to Purtov et al. [33] the mushroom-shaped micropillars produces collapse of the micropillars when the preload is too high during the test. This leads to a certain decrease in adhesion. Flat punch, mushroomshaped, and suction cup-shaped micropillars in this work are able to withstand greater preload because they have a smaller aspect ratio of ≈1, allowing the adhesion to first increase and then smooth out, maintain the stabilization value, nearly no reduction. This result was further verified in the simulation analysis.
Gorb and Wang et al. [34,35] modeled the gecko-inspired dry adhesive material, and the adhesion force F ad of the micropillar was related to the preload F N applied above it. When the preload F N was applied, the adhesion force F ad was: where, F pull-off is the detachment force when all micropillars are separated from the contact surface. According to the studies of Gorb and Baik et al. [36,37] on the adhesion force of mushroom micropillars, F pull-off is: Where α is the coverage factor (≈40%), A is the area of the surface with mushroom-shaped micropillars, R is the initial radius below the micropillars, R E is the tip radius of micropillars, λ is the diameter of defect size for detachment, Δγ is the surface energy of the material, E is the elastic modulus and μ is Poisson's ratio. It can be seen that (ΔγE/(1 − μ 2 )) (1/2) is determined by the properties of the material from the Equation (2). The adhesion depends on R E , R, and λ for the same material. From the Equation (1), when F N is less than F pull-off , F ad increases nonlinearly with the increase of F N . When F N is greater than or equal to F pull-off , F ad equals F pull-off . Since , F pull-off depends on R and λ for the same material. For flat punch micropillars, during the molding process, the top radius of the micropillar is greatly reduced due to nonuniform shrinking of PVC, resulting in a small contact area A and a significant decrease in adhesive strength. For mushroomshaped micropillars, the diameter of defect size for detachment λ is very low, close to zero, leading to an increase in adhesion. The suction-cup shaped structure in the middle of the suction cup-shaped micropillars reduces the effective contact area compared to the mushroom-shaped micropillars. This leads to an increase in the defect coefficient λ, which results in a decrease in the adhesion force F ad . For the tilted micropillars, when F N is too large, the effective contact radius decreases from 67.48 µm to 0 due to the tilt angle of 60° and the lack of rigid support, resulting in the reduction of contact surface. Despite the large initial radius, the surface area A is significantly reduced due to the inability of the tilted micropillars to withstand larger preload, decreasing the effective contact radius. When the micropillars is subjected to a larger preload, the micropillars collapses, leading to a gradual decrease of the adhesion force to 0.

Application
We use mushroom-shaped bioinspired adhesive to make a grasping device, as shown in Figure 6a. This device can simulate the adhesion behavior of gecko's toes and switch between grasping and releasing behaviors. Using grasping device, we carried out the grasping of objects of various scales (Figure 6b), such as rulers, flanges, PCB, plexiglass panels, mobile phones, and solar panels. These objects weigh between a few grams to 380 g ( Figure S5, Supporting Information). The grasping device can realize grasping objects with different surface and different materials. In addition, only ≈12 cm 2 mushroomshaped bioinspired-adhesive can be used to grasp and release objects with a maximum weight of ≈380 g. Taking solar panels as an example, the single grasping and releasing process of the grasping device is shown in Figure 6C. The grasping device first approached the solar panel, and in the process of falling, the two side panels recovered from the bending state to the flat state and contacted the solar panel ( Figure 6cii). When preload is applied, the grasping device picks up the solar panel, moves it to another location and places it on the platform (Figure 6ciii,iv). At this time, the two side panels become bent, the mushroom-shaped bioinspired-adhesive is separated from the solar panel, and the grasping device returns to the initial position to complete the grasping and releasing process (Figure 6cv). Compared with the traditional pneumatic grasping device, grasping device can grasp the object surface with defects and adapt to a variety of surfaces, reducing the requirement of the integrity of object surface, expanding the scope of application of grasping device.

Conclusions
In this work, bioinspired dry adhesives including flat punch, mushroom-shaped, suction cup-shaped, and titled micropillar structures in large scale are successfully and rapidly manufactured by PµSL 3D printing method, which have the advantages of high precision and strong controllability. Due to the slightly shrinking property, PVC gel adopted in the process easily demold from the 3D printed positive mold, which provides an exactly convenient way for demolding. The mushroom-shaped adhesive exhibits excellent adhesion property (15 KPa), high repeatability (>50 times), good self-cleaning ability (contact angle of 143.7°), which is attributed to the lower diameter of defect size for detachment, reduced stress concentration, and high tolerance. The bioinspired-adhesive is further applied in grasping and transferring solar panels, flanges, and printed circuit board. Especially the object surface with defects, which expands the scope of application of grasping device, improves the grasping surface adaptability.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.