Attachment ability of combined biomimetic adhesive micro-textures of different shapes

There are various potential applications of biomimetic adhesive solutions including climbing robotic systems, mobile sensor platforms, and biomedical applications such as patches for external use. Achieving resistance to both normal and tangential loads, however, is a critical issue that still needs to be addressed. Some animals have developed exceptional attachment mechanisms based on combined fibrillar elements of different shapes and functions. Experimental investigation of combined biomimetic adhesive micro-textures on tribological performances such as adhesion, friction, and peeling resistance is needed to apply this idea to the design of an artificial texture having similar “biomimetic” properties. In the present study, we demonstrate that combinations of different shapes of biomimetic adhesive micro-textures show increased efficiency under different contact environments and enable long-term adhesive solutions. Our work sheds light on combinations of different element shapes inspired by nature and their adhesive efficiency as a function of the ratio of each biomimetic element, as well as their spatial repartition.


Introduction
Natural evolution helps species promote survival in their ecological niche or habitat by developing diverse biological mechanisms. For this reason, observing and studying the biological, physical, and chemical principles behind the unique adaptation and fantastic abilities of some species offer engineers an unlimited source of inspiration and imagination to solve engineering problems, in which classical engineering design has failed or has reached its limits. Biomimetics has been growing rapidly in recent decades and has interested more researchers and engineers from diverse disciplines, including architecture, aviation, medicine, optics, and other engineering fields [1].
Today reversible and glue-free attachment solutions for dry and wet surfaces are required in diverse engineering fields such as robotic and biomedicine.
This has led to the development of a newly emerging field of adhesive science-based research regarding the observation and mimicking of the biological attachment mechanisms that have evolved in some species.
The ability of some animals to run on walls and ceilings of uneven surfaces is critical to their survival and the continued existence of their species. Different attachment mechanisms have evolved depending on an animal's habitat, size, and even breeding needs [2]. Examples are various insects [3,4], beetles in particular [5,6], and other animals such as spiders and geckos [7]. These species can produce remarkable attachment capabilities steming from an increase in van der Waals forces. It was first believed that the division into sub-surfaces was the main factor for the increase in van der Waals forces [8,9]. It became clear, however, that division into sub-areas without a practical www.Springer.com/journal/40544 | Friction increase of the contact area increases the ability of the elements to adapt (match) to uneven surfaces [10]. The contact perimeter is the dominant factor in increasing adhesion force [11]. It has also been proven that contact sub-division increases resistance to contamination [10].
The varied environment found globally does not allow animals to live on unique types of completely homogeneous counter-surface (substrates on which they adhere and walk). Thus, in many cases, animals must adapt their attachment properties to different types of surfaces with various properties such as surface roughness, stiffness, and directionality. Therefore, animals have developed several types of hairy attachment elements on the same foot, and even on the same adhesive pad. Recently, biologists have begun studying the combination of the different mechanisms. Stick insects (Carausius morosus) use two attachment mechanisms simultaneously to optimize the combination of adhesive and frictional forces [31]. Another example is the Nezara viridula L. (Heteroptera: Pentatomidae), whose combination of both smooth and hairy pads allows the ability to produce friction and adhesion on surfaces of different roughnesses [32]. The Gastrophysa viridula (Coleoptera: Chrysomelidae) beetle, whose adhesive pads on the male legs contain three different types of micro-textures: (1) micromushroom hairs located at the middle of the pad, (2) spatulae hairs arranged circumferentially around the micro-mushrooms, and (3) pointed tip hairs located in the pad's outermost parts [6,33]. Due to this unique combination, beetles achieve resistance to both normal and shear forces, and they allow even homogenous load distribution across their adhesive pads. This ability allows better resistance to peeling detachment [33].
It is essential to generate both normal and tangential forces simultaneously to maximize the applicability of reversible and glue-free attachment. Thus, some research reports the synergetic effect of combining claws and micro-texture in both biological animals [34] as well as artificial models [35]. However, the specific combination of micro-mushroom and spatula-like adhesive elements has yet to be studied experimentally with artificial models, although the unique synergetic effect has already been proven in biological models [6,33]. In light of the above, the present work bridges the gap by studying experimentally the synergetic effect of combining biomimetic micro-mushroom and micro-spatula on tribological performances such as adhesion, friction, and peeling resistance.

Micro-textured samples
This research investigates experimentally the synergetic effect of the combination of different biomimetic micro-textures, as shown in Fig. 1, i.e., micro-mushroom and micro-spatula to evaluate their tribological performances in adhesion, friction, and peeling resistance. Two biomimetic adhesive micro-textures are used in the present research: (1) micro-mushroom and (2) micro-spatula. The micro-mushroom (Klettband Technik, Germany) has caps with 40 μm diameter and 40 μm high, as described in Ref. [17] (Art. 70140-Gecko ® Nanoplast ® ). The micro-spatula was cast from polyvinyl siloxane (PVS) according to the process presented in Refs. [12,25]. The microspatula tip is 5 μm thick, 50 μm base, and about 150 μm high.
The micro-textures are arranged to have the micro-mushroom in the center of the combined sample to simulate the tarsal micro-textures of the Gastrophysa viridula beetle as closely as possible [6], while the circumferential ring is covered with micro-spatula, i.e., the spatulae surround the mushrooms.

Biomimetic combined sample
To combine both micro-mushroom and micro-spatula textures into a single sample, hole punches with different diameters were created to achieve different ratios between the area of the sample covered with mushrooms and spatulae. In addition, to unify the textures together in the combined sample, a specific template was created to fit the dimensions of the samples. Both the punches and the template were created from aluminum in a C.N.C process.
The punches (Fig. 2) were manufactured with internal diameters of 5 (for sample outer diameter), 3.4, and 3.15 mm. These dimensions allow 50:50 and 60:40 ratios between the area covered with mushrooms and spatulae. The total sample diameter (5 mm) was chosen to allow the comparison with Ref. [30]. Figure  2 illustrates several steps of the sample's preparation schematically: (i) The textures were cut with the specific punch. For a ratio of 50:50, the spatula texture was cut with the punch of 5 mm diameter, and then perforated in the center with the punch of 3.15 mm to form a type of ring. From the mushroom texture film, a 3.15 mm diameter disk was cut out. For a 60:40 ratio, a 5 mm punch was similarly used, and then perforated in the center with the punch of 3.4 mm. (ii) To locate the samples inside the specific assembling mold when it is laid on smooth and clean glass, the samples were positioned so that the mushroom disc is inserted into the hole of the spatula ring. (iii) After the textures were placed inside the mold, a small quantity of PVS was gently poured over their backside, which once solidified fixes their position together. A cover glass was also used to remove extra PVS and to unify the shape and thickness of the final combined sample. (iv) The combined sample in its final form was released from the mold and was ready to be used for testing. (CAD illustration of the combined sample in Fig. 3).

Counter surface
The tribological performances, adhesion, friction, and peeling resistance of the different samples were investigated using a flat rigid smooth glass as counterface. The glass (Paul Marienfeld GmbH & Co. KG, Germany) was 76 mm × 25 mm × 1 mm, and its average roughness R a was about 30 nm. To conduct tests in wet environments, a hard and smooth glass dish with a diameter of 100 mm was used to contain the liquid during the experiment (Fig. 4). The dish was attached directly to the tribometer and replaced for each new sample.

Experimental set-up
This study used a customized tribometer, which was built in Tribology Laboratory at Azrieli College of Engineering (Israel). This tribometer is specially designed to allow the measurement of the friction and adhesion forces generated by micro-textures and/or soft materials in dry, humid, and wet environments. A full description of the used tribometer, as well as its self-alignment system, can be found in Ref. [30].

Procedure velocities and loads
The micro-texture samples were made of soft elastomers with viscoelastic properties, and thus the behavior of the material depends upon the velocity, at which the loads are applied and released. For this reason, the experiments were conducted at different velocities (i.e., 0.1, 0.5, and 1.5 mm/s) and different loads (i.e., 200, 500, and 1,000 mN) to examine their possible effect on the tribological performances, adhesion, friction, and peeling resistance.

Contact environment
In addition to dry contact, the adhesion and friction performances were examined under distilled water and glycerin to examine the effect of fluid viscosity under a wet contact environment. The viscosity of the glycerin was 0.97 ± 0.18 Pa·s, and that of the distilled water was 0.98 ± 0.01 mPa·s. All experiments were conducted under a relative humidity of 40% ± 2% and a room temperature of 23±1 °C.

Experimental procedure
Before testing, each sample was cleaned with distilled water and soap and dried with nitrogen to remove | https://mc03.manuscriptcentral.com/friction dirt and residue from the surface. Then the opposite surface (glass) was placed according to the experimental conditions (dry or wet), and the combined textured sample was anchored to the self-alignment system of the tribometer. The tribometer control software allows for programming the applied load as well as the velocity of the counterface vis-a-vis the combined textured sample.
In this study, three types of measurements were made: 1) Adhesion tests consist of progressively bringing the counterface into contact with the micro-textured sample until the required normal load is reached. Then withdrawal in the normal opposite direction is achieved, while the generated pull-off force is measured and saved on a computer. The maximum adhesion force is obtained at the detachment point during the separation stage. In this section, we used the condition mentioned in Table 1 for all three environments (dry, wet, and glycerin).
2) Friction tests bring the counterface into contact with the micro-textured sample until the required normal load is obtained, and then move the stage holding the counterface in the tangential direction, while the normal load is kept constant. The friction force resisting the tangential motion of the sample is measured. To take into consideration of the effect of adhesion, the friction coefficient is computed from the slope of the friction curve as a function of normal load for each test, as previously presented in Ref. [19]. We used the condition mentioned in Table 2 for all three environments (dry, wet, and glycerin) individually. 3) Peeling tests bring the counterface into contact with the micro-textured sample until the desired normal load is obtained. Then a small pre-sliding of 0.05 mm at a sliding velocity of 0.05 mm/s is applied before the translation stage is moved simultaneously in both tangential and normal opposite directions. The peeling angle θ is determined by the velocity ratio of the translation stage in the normal and tangential directions. We used the condition mentioned in Table 3 for all three environments (dry, wet, and glycerin).
As mentioned above, all measurements were performed on a glass counterface. It is important to note that a new friction pair (combined textured sample and glass counterface) was used for each new experiment combination.

Adhesion
In adhesion experiments, the measured values correspond to the maximum pull-off force obtained at the detachment point between the textured sample and the counter surface (as explained in Ref. [30]). The average values of the maximal adhesion force, as well as the standard deviations, are then calculated from four test repetitions under the same experimental conditions. Figure 5 shows the results of the adhesion force measurements. Under a dry environment, the pull-off force (adhesion resistance) gradually increases with the normal load from 356 to 529 mN for the 50:50 ratio and from 375 to 661 mN for the 60:40 ratio (60% mushroom). As for the wet environment (distilled water), the values obtained for the pull-off force are notably lower compared to those under dry conditions; however, a slight upward trend with the increase of normal load can be found. For the viscous wet  , a similar behavior is found, however with pull-off (adhesion) force values ranging in-between those obtained for the dry and wet conditions. The 60:40 ratio textured sample generates higher adhesion force compared to the 50:50 sample, regardless of operational and environmental conditions ( Fig. 5(b)). It appears that the combined sample has a clear preference for dry contact compared to wet contact. A possible explanation for this behavior can be the presence of interfering layers that might form in the interface between the mating surfaces, leading to a reduction of the real contact area. For a viscous environment (glycerin), a higher adhesion is obtained compared to the wet environment (distilled water). Since there is no significant difference between the surface energy of the fluids (value of the water and glycerin), we assume that this is due to viscous shear forces developed between the micro-mushroom and spatula textures and the fluid. For a ratio of 60:40, the difference between the obtained adhesion force under a dry environment and a wet environment is reduced. A greater amount of mushroom texture produces a larger real contact area with the opposite surface and increases shear forces between the mushrooms and the fluid. Compared to the results obtained in Ref. [30], circumferential distribution seems to be more advantageous than random samples. It is important to recall that in the present study, all experiments were performed using a hard and smooth glass counterface. Consequently, parameters such as hardness and surface roughness of the counterface were not considered. Their effect can be established in the future work.

Friction
As for the friction experiment, we examined the effects of sliding velocity, initial normal load, and contact environment. Figures 6 and 7 show the results of the static friction coefficient of the 50:50 and 60:40 mushroom-spatulae samples. The friction coefficient is computed from the slope of the friction curve. The friction coefficient has a moderate dependence on both the applied load and the sliding velocity for the 50:50 sample, especially in a dry environment (Fig. 6). The slope value decreases with normal preload increasing, and the curve is shifted upward when the sliding velocity increases. The decrease of the friction due to the increase of the applied normal load can be attributed to the collapse of the micro-textures, which under high normal load, behave similarly to a smooth surface without texture, as already noted in Ref. [30]. The increase of the friction at higher sliding velocities under the different environments can be related to In contrast to the 50:50 sample, a less distinct dependence of the friction coefficient on the applied normal load was observed in the 60:40 sample (Fig. 7). However, the sliding velocity seems to affect the magnitude of the friction coefficient, and therefore emphasizes the importance of the viscoelasticity on the friction coefficient, which was around μ = 0.96 under dry contact at v = 1.5 mm/s, and decreases to μ = 0.73 at v = 0.1 mm/s, μ = 0.74 under glycerin, and μ = 0.59 under distilled water. These results trend to prove that the circumferential distribution of the biomimetic sample (spatulae around the mushroom in the middle, Fig. 3) improves the achieved friction coefficient compared to those of the samples with randomly distributed spatulae studied in Ref. [30].

Peeling
In peeling experiments, the resulting peeling force is obtained by vector addition of the maximum adhesion and friction forces recorded at the detachment point between the textured sample and the glass counterface. www.Springer.com/journal/40544 | Friction   Figures 8 and 9 show the forces measured by the two load sensors, when F n , F t , and F com represent the forces recorded by the normal sensor and the tangent sensor and the total peeling force, respectively. Figure 8 shows the average values of the resulting peeling force as well as the standard deviation as a function of the peeling angle . These results are obtained from four repetitions for the mushroomspatulae sample. Under dry contact conditions, the component of normal force gradually increases from 172 mN for a peeling angle of 15° to 403 mN for a peeling angle of 75°, while the tangential force component decreases from 427 mN for a peeling angle of 15° to around 228 mN for a peeling angle of 75°. Although both the normal and tangential components of the peeling resistance change with the increase of the peeling angle , the resulting peeling force (obtained via vector addition) is more or less constant with an average value of around 461 mN. The same behavior is also obtained under wet contact (distilled water) and viscous (glycerin) conditions, with lower values of the resulting peeling forces of 291 and 368 mN, respectively. Figure 9 shows the average values of the resulting peeling force as well as the standard deviation as a function of the peeling angle . These results are obtained from four repetitions for the 50:50 mushroomspatula sample. Under dry contact conditions, the component of normal force gradually increases from 159 mN for a peeling angle of 15° to 308 mN for a peeling angle of 75°, while the tangential force component decreases from 450 mN at a peeling angle of 15° to around 160 mN at a peeling angle of 75°. In contrast to the 60:40 sample, which receives a  Tests were performed under 500 mN preload for the 60:40 mushroom-spatula sample: (a) dry contact, (b) distilled water, and (c) glycerin. The words "n", "t", and "com" represent the forces recorded by normal sensor and tangent sensor and the total peeling force, respectively. contribution from the mushrooms at high angles, in the 50:50 sample, the resulting peeling force gradually decreases when increasing the peeling angle  due to fewer mushrooms, regardless of the contact environment condition.
The results of the peeling experiments show that the peeling force remains almost constant overall in tested peeling angles  for the 60:40 sample (60% mushroom and 40% spatula). These results were reproduced for all contact environments with different values. Figure 10 illustrates the peeling force of a Fig. 9 Maximum peeling force as a function of the peeling angle. Tests were performed under 500 mN preload for the 50:50 mushroomspatula sample: (a) dry contact, (b) distilled water, and (c) glycerin. The words "n", "t", and "com" represent the forces recorded by normal sensor and tangent sensor and the total peeling force, respectively.

Fig. 10
Resulting peeling force at different angles.
www.Springer.com/journal/40544 | Friction biomimetic integration sample (studied in the present work) as a function of different peeling angles compared to those of a sample with a randomly distributed combination and a sample only containing mushroom micro-textures [30]. A significant advantage can be seen for the biomimetic integrated sample compared to the two others in terms of achieving greater and more stable peeling force even under small peeling angles. Both other textures (randomly distributed combination and only mushrooms) behave similarly to the Kendall peeling model [36], showing a net decrease of the peeling force as the peeling angle increases from 0° to 15°, an angle beyond which the mushroom textures begin contributing to adhesion force. In contrast, the biomimetic integrated samples tested in this study allow for enhanced load capacity, resulting in greater peeling force at small angles. These findings agree well with the evolutionary development of different insects that have these two textures on the same pad [6,33], and take advantage of the benefit of their unique combination.
The obtained peeling force presented in Fig. 10 can be adapted for long-term adhesive solutions. Unlike the previous studies that focus only on the single mushroom micro-textures, of which most are conducted under dry contact environment [37-40], the novel combined concept proposed here will be suitable for a variety of engineering applications under diverse environments. The unique arrangement inspired by nature allows for the reduction of tangential loads from the mushroom samples at small angles. This essentially overcomes the range of angles, at which the mushrooms lose efficiency, as seen previously in a sample containing only mushroom texture or a random combination of mushrooms and spatulae [30].

Conclusions
The tribological performances of the bio-inspired combination of adhesive elements such as micromushroom and micro-spatula are investigated experimentally. The influence of the ratio, contact environment, and operational conditions are studied. The following conclusions are drawn: 1) Compared to random distribution investigated in Ref. [30], circumferential distribution of the spatula around the mushrooms, as appears in nature, provides advantages in producing adhesion strength and higher friction coefficient.
2) The increase of the friction coefficient with the increase of sliding velocity indicates that viscoelasticity plays a significant role in the frictional behavior of the combined biomimetic adhesive elements under different environmental conditions, especially under low normal preloads.
3) The unique circumferential arrangement inspired by nature allows for enhancing the peeling force resistance regardless of the peeling angle.
4) The optimization of the combination of the two textures (mushroom and spatula) and their tribological study will constitute a solid knowledge base and know-how for the development of diverse adhesive engineering solutions, especially in the field of biomedicine, in which good optimization of the resistance to both normal and tangential loads is required.
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