A Flexible Escape Skin Bioinspired by the Defensive Behavior of Shedding Scales

Abstract

Artificial skins with functions such as sensing, variable stiffness, actuation, self-healing, display, adhesion, and camouflage have been developed and widely used, but artificial skins with escape function are still a research gap. In nature, every species of animal can use its innate skills and functions to escape capture. Inspired by the behavior of fish-scale geckoes escaping predation by shedding scales when grasped or touched, we propose a flexible escape skin by attaching artificial scales to a flexible film. Experiments demonstrate that the escape skin has significant effects in reducing escape force, escaping from harmful force environments, and resisting mechanical damage. Furthermore, we enabled active control of escape force and skin hardness by changing temperature, increasing the adaptability of the escape skin to the surrounding. Our study helps lay the foundation for engineering systems that depend on escape skin to improve robustness.

Introduction

Artificial functional skins are receiving considerable attention in applications such as soft robots and human-machine interfaces. Electronic skins are the most popular artificial skins,^1–4 which are mainly used as soft sensors,^5–9 soft driving systems,¹⁰ and flexible display screens.¹¹ In addition, artificial skins with functions such as stiffness control,¹² adhesion,^13,14 actuation,^15–18 and color change^19–21 have also been developed. To improve the performance of artificial skins in harsh environments, researchers have attempted to integrate functions such as self-healing,^22,23 antifreezing,²⁴ energy harvesting,²⁵ water retaining, and ultraviolet filtering²⁶ into artificial skins. In general, artificial functional skins are developing in the direction of diverse configurations, comprehensive functions, and high versatility.

However, the existing artificial functional skins do not involve soft skins with escape function. Enabling escape by causing a body part to undergo autotomy is a defensive behavior of many animals,^27,28 such as lizards,²⁹ leaf-footed bugs,³⁰ crickets,³¹ crabs,³² sea slugs,³³ feather stars,³⁴ and sea cucumbers.³⁵ From the perspective of engineering, the combination of escape function and flexible skin is very conducive to the application of escape technology to hosts that have different shapes, sizes, and materials. The mentioned escape skin integrates biological or artificial escape principles into skin-like soft materials to help robots, humans, and animals get out of danger safely in environments with mechanical damage.

How to make soft robotic systems as robust in their ability to resist damage as their biological counterparts has always been a challenge. To tackle this robustness challenge, researchers mainly developed three methods:^36–38 damage isolation, material elasticity, and self-healing mechanism. The damage isolation^39,40 and material elasticity^41–44 focus on improving the tolerance of soft robots to mechanical damage, while the main function of self-healing is to self-repair the damaged part of the soft structure.^45–49 However, there is an important technical link missing in the existing damage defense methods, that is, how to get out of the harmful force environment safely and efficiently. The escape skin studied in this work can fill this technical gap.

Scherz et al.⁵⁰ presented a species of fish-scale gecko that has the ability to autotomize a large portion of their scales when grasped or touched, most likely to escape predation. They provide a detailed description of the skeleton of the fish-scale gecko based on micro-computed tomography (micro-CT) analysis. Inspired by this escape behavior, we propose a design method of bioinspired escape skin, which is to attach rigid artificial scales on a thin soft substrate, as shown in Figure 1A and B.

FIG. 1.

Principle and application field of flexible escape skin. (A) The fish-scale geckoes have the ability to autotomize a large portion of their scales when caught or touched to escape predation. (B) Flexible escape skin is mainly composed of artificial scales and a soft substrate. Escape principle (C) and fabrication method (D) of flexible escape skin. (E) The flexible escape skin provides an escape function by attaching to the surface of the host. It can be highly integrated with various soft robots such as soft locomotion, soft gripper, and soft manipulator, and can combine with wearable devices of humans or animals to provide escape protection, and can enhance the ability of underwater vehicles and flying robots to deal with harmful environments.

The basic principle of the escape skin is that the external force is directly applied to the artificial scale to cause the scale to separate from the soft substrate, thus weakening the restraint effect and mechanical damage of the external force on the host with escape skin (Fig. 1C). The proposed escape skin is a soft-rigid coupling structure, and we use multimaterial three-dimensional (3D) printing technology to fabricate escape skins with different specifications (Fig. 1D).

Compared with other biological escape phenomena such as autotomy, discharging internal organs, and secreting mucus, escape skin has two advantages. On the one hand, the escape skin is installed on the surface of the host, and shedding scales does not cause serious damage to the host like losing organs. On the other hand, the host with escape skin can resist the damage of sharp objects, but the method of secreting mucus cannot do this.

The flexible escape skin has good versatility and application prospects and can provide escape functions for soft robots, rigid robots, wearable devices, and so on, only by attaching to the surface of the host (Fig. 1E). It can be highly integrated with soft locomotion, soft gripper, and soft manipulator to improve the damage tolerance of these soft robots in unstructured environments and help these robots escape from harmful surroundings in time. Although rigid robots have strong damage resistance, the escape skin still has application potential. For example, covering the escape skin on the surface of a long-running underwater vehicle is very helpful to get rid of parasitic animals such as barnacles. In addition, the application of escape skin on wearable devices is expected to produce new escape suits, which can greatly improve the escape ability of people and animals performing search and rescue tasks in the ruins.

This article mainly presents the escape skin from three aspects. First, we mainly demonstrate the application of the escape skin on soft robots and wearable devices. Second, we show that the escape skin can change its own hardness and scale adhesion by controlling temperature. Third, we experimentally studied the effects of pressure, velocity, stiffness, shape, and escape direction on the scale-shedding ratio and escape force of the escape skin.

Materials and Methods

Design and fabrication

As shown in Figure 1B, the escape skin is mainly composed of scales and a soft substrate. One side (functional surface) of the soft substrate is designed in a stepped shape to arrange the scales, and the other side (installation surface) is designed in a planar shape to adhere to the surface of the host. The scales are uniformly arranged on the functional surface in an array. The scale shape and dimensions are shown in Supplementary Figure S1. The scale angle with the installation surface is 20°.

The main reasons why we chose the scale angle of 20° to carry out the research are as follows. First, the escape skin should be thin enough to avoid affecting the performance and structural compactness of the host. We made escape skins with different scale angles through 3D printing and found that when the angle is 20°, it can not only ensure that the printed scale does not adhere but also reduce the damage rate of escape skins when removing the support material. Second, the increase in scale angle will lead to an increase in skin thickness, which increases the 3D printing time and wastes the supporting materials. Third, the increase in scale angle leads to an increase in the movement resistance of the host.

The escape skin is fabricated by a 3D printing machine (Stratasys J750) with a multimaterial hybrid printing function. To avoid the failure of the design and manufacturing software caused by the excessive number of scales, we usually combine hundreds of scales into one part when developing large-size escape skin, which can greatly improve the design and manufacturing efficiency and ensure a high success rate.

TangoPlus FLX930 is mainly used for the soft substrate, and VeroCyan RGD841 and VeroMagenta RGD851 are mainly used for scales. The maximum size of the escape skin we have fabricated is 400 × 358 mm, and it takes about 3 h to print one piece. When removing support materials with a high-pressure water gun, ensure that the water gun is tilted to the left (refer to Fig. 1B for the direction of escape skin), because flushing in other directions will easily cause scales to fall off in advance.

The soft escape skin fabricated by multimaterial 3D printing is shown in Figure 2A–C. Theoretically, the maximum size of the escape skin depends on the workspace of the 3D printer. The good elasticity of the escape skin benefits from the rubber-like material (elongation at break is 170–220%) used in the soft substrate, which also makes the escape skin suitable for hosts of various shapes.

FIG. 2.

3D printed flexible escape skins and their application examples. Two sizes of flexible escape skins: (A) 58.5 × 50 mm, (B) 358 × 400 mm. The envelope size of a scale is 5.4 × 4.8 × 0.4 mm. The number of scales per square millimeter is about 0.078. The thickness (1.5 mm here) of the soft substrate shall not be <0.8 mm to avoid insufficient strength. The weight of the printed skin is 0.0036 g/mm². (C) A flexible escape skin in a stretched state. (D) A flexible escape skin can withstand being run over by a car. (E) Flexible escape skins coated with fluorescent paint and their scale-shedding image. (F–H) Photos of the scale shedding of the escape skin when a sharp object moves in V_R, V_F, and V_L directions. Application examples of flexible escape skin: (I) Crab, (J) simulated gecko, (K) robotic crawler, (L) wearable device, (M) soft robotic tentacle, (N) soft enclosed gripper. 3D, three dimensional.

Although the scale is a thin structure made of rigid material, the size of a single scale is small and can transfer external force to the soft substrate, which can ensure that the escape skin can withstand large normal pressure, as shown in Figure 2D and Supplementary Figure S2. As shown in Figure 2E, the key to the escape function of the flexible escape skin lies in the peelable scales, which means that the escape function will cease to exist when all the scales fall off. There is no specific requirement for the escape direction because the scales can be peeled in multiple directions, such as reverse, forward, and lateral (Fig. 2F–H).

There are two methods for assembling the escape skin on the host: one is to fabricate the skin and the host integrally, and the other is to paste the finished skin onto the surface of the host. In consideration of universality and manufacturing efficiency, the second method is mainly applied in this work. The escape skin can be cut into any shape or directly printed into a preset shape, which can basically fit various structural surfaces.

In the experiment of this work, the escape skin and the host were bonded together by industrial adhesive (Kafuter K5705T; Guangdong Hengda New Materials Technology Co. Ltd.). In addition to the shape, the escape skin can also be compatible with the movement and deformation of the host (Fig. 2L, M), which makes it widely applicable to various rigid, flexible, or soft structures. This work mainly demonstrates the application of the escape skin through robots and wearable devices (Fig. 2I–N).

Adhesion and hardness

Both scale adhesion and soft substrate hardness can be adjusted by setting the structural size and material parameters. We assume that the adhesion force between the scale and the soft substrate is uniform. Therefore, we only need to set the size of the scale to control the initial adhesion force. The 3D printer we use can control the hardness (Shore A 26–90) of rubber-like materials by adjusting the ratio of Tango materials and Vero materials, which can meet the requirements of most hosts for skin hardness. The adhesion reduction during the use of escape skin is enabled by heating films (voltage 12–24 V and thickness 1.5 mm).

Results and Discussion

Application of flexible escape skin

In Figure 3 and Supplementary Movie S1, we show some demonstrations of the escape function of the escape skin. In addition to the escape function, the most outstanding advantage of the flexible escape skin is to protect vulnerable structures from damage (Fig. 3A). The escape skin has an escape mechanism similar to that of a fish-scale gecko, making captures ineffective against the robot with escape skin (Fig. 3B). The combination of the escape skin and wearable devices, as shown in Figure 3C–E, can give full play to the compliance, protection ability, and escape function of the escape skin, and can assist people or animals working in dangerous environments such as lifeguards and police dogs to escape from the harmful force environment in time. The wearable devices with the escape skin have advantages in weakening mechanical damage, passing through narrow spaces, and preventing adhesion.

FIG. 3.

Demonstration of escape function of flexible escape skin. (A) The tightly arranged artificial scales enable the balloon, pasted with flexible escape skin, to resist the collision of sharp objects. (B) When a crawling robot with escape skin encounters an external clamping force, its scales will fall off first to break away from this external force constraint. (C) Gloves with escape skin can resist the touch of nails by using hard scales, and at the same time, they can help the hand pull out of the board with nails by shedding scales. Such gloves can also help hands easily separate from strong adhesions (D), and a similar function can be applied to shoes to avoid high-viscosity surfaces that prevent people or robots from walking normally (E). (F) A soft robotic tentacle with escape skin is placed in liquid silicone rubber. After the silicone rubber is cured, the soft robotic tentacle can be easily pulled out of the silicone rubber. Demonstration of the escape process of soft robotic tentacles with escape skin when they are pressed by a wheel (G), heavy object (H), or clip (I). (J) Experiments of a soft robotic tentacle with escape skin against various mechanical damages. (K) A soft robotic tentacle with escape skin can be used to grasp destructive objects. When the objects break free, the escape skin can protect the soft tentacle from damage. (L) The flexible escape skin can help a soft enclosed gripper to break away from strong adhesion. (M) A soft enclosed gripper with escape skin operates in an environment with mechanical damage.

As described in the Introduction section, current soft robots lack escape function when dealing with mechanical damage, and the solution of this article is to attach the escape skin to the surface of soft robots. Taking a soft robotic tentacle⁵¹ as an example, the escape skin has three beneficial effects: (1) Helping the soft robotic tentacle escape from the burying, heavy pressure, nipping, and other force constraints (Fig. 3F–I); (2) isolating mechanical damage such as cutting, clamping, and shearing (Fig. 3J); and (3) preventing noncooperative targets damage the soft robotic tentacle (Fig. 3K). Especially as shown in Figure 3F, without the assistance of the escape skin, it is difficult for the soft robotic tentacle to be separated from the burying of silicone rubber, and the escape skin will also produce the same beneficial effect in other burying such as concrete and highly viscous materials.

Soft robots are mainly made of soft materials with Young's modulus of 10⁴–10⁹ Pa,⁵² which determine that they cannot bear large stresses. Through the loss of scales, the escape skin prevents irrecoverable damage to the soft robotic tentacle caused by large stress contact such as cutting and shearing (Fig. 3J).

The integration method and application effect of the escape skin on other soft robots such as a soft enclosed gripper^53,54 are similar, as shown in Figure 3L and M. The escape skin has no obvious interference on the soft enclosed gripper because the soft enclosed gripper in this study grasps objects through the inner cavity, and the escape skin is installed on its outer wall. However, the escape skin has some interference on the soft tentacle, such as increasing bending resistance and reducing friction coefficient. In summary, the escape skin can improve the structural robustness of soft robots, enabling them to work in harsher environments and manipulate more dangerous targets.

The host that can install the escape skin must meet the following requirements: (1) The surface of the host has no special functions such as sensing and display, so as to avoid conflicts between different functions; (2) the host has a sufficient payload to overcome the weight of the escape skin; (3) there is no special cleanliness requirement for the working environment, allowing the sloughed scales to be discarded in the environment. The main limitation of the escape skin is that the escape ability will be weakened with the reduction of scales.

Escape effect comparison

To quantitatively and intuitively demonstrate the beneficial effect of the escape skin, we conducted comparative experiments, as shown in Figure 4. In this study, we define the escape force of a rigid piece without escape skin as F_iR, the escape force of a rigid piece with escape skin as F_iRE, the escape force of a soft piece without escape skin as F_iS, and the escape force of a soft piece with escape skin as F_iSE. i (i = 1, 2, … 6) is the serial number of the comparison experiment.

FIG. 4.

Comparison of escape effects for flexible escape skin. (A) Comparison of escape forces of pieces with and without escape skin under different constraint settings. The rigid piece and the soft piece are made of resin and silicone rubber, respectively. The size of the piece used is 65 × 30 × 6 mm (length × width × height). Each escape force value is the average of test results of three pieces. The error bar is the standard deviation of the measurement result. (B) A comparative photograph of the silicone rubber cylinder with and without escape skin after being cut many times. (C) The soft piece without escape skin is instantly entangled in the rotating gear, while the one with escape skin can effectively prevent being entangled by the gear. (D) A finger with escape skin can be easily separated from the cable tie, while a finger without escape skin can hardly be pulled out of the cable tie, and the cable tie must be cut to release. In addition, the ability of escape skin to detach adhesions and protect the surface of the host is also prominent. (E) Compared to the skinless flexible rod, the one with the escape skin drastically reduces the risk of being caught by the pliers.

The test results of the rigid piece show that the escape force can be reduced by escape skin by a minimum of ∼36.8% [(F_5R − F_5RE)/F_5R] and a maximum of ∼91.3% [(F_3R − F_3RE)/F_3R], and the average percentage is ∼65.6%. The test results of the soft piece show that the escape force can be reduced by escape skin by a minimum of ∼50% [(F_3S − F_3SE)/F_3S] and a maximum of ∼88.9% [(F_1S − F_1SE)/F_1S], and the average percentage is ∼68%. The contrast effect is most obvious in the case of adhesion (comparison 3) because no matter how firm the adhesion is, the escape force only depends on the connection force between the scales and the soft substrate.

Whether the effect of the escape skin is significant is closely related to the magnitude of the applied binding force. For example, in the experiment of comparison 5, the tip force of the trap was ∼15 N, and the escape forces of the rigid piece and soft piece were only reduced by ∼36.8% [(F_5R − F_5RE)/F_5R] and ∼55.3% [(F_5S − F_5SE)/F_5S], respectively. In contrast, in comparison 6, the tip force of the pliers was ∼200 N, and the escape forces of the rigid piece and soft piece were only reduced by ∼71.7% [(F_6R − F_6RE)/F_6R] and ∼66.4% [(F_6S − F_6SE)/F_6S], respectively. It can be seen from the above comparison that the escape skin can reduce the escape force for both rigid and soft hosts, and the average level of reducing escape force is similar.

We show several typical comparative cases, as shown in Figure 4B–E and Supplementary Movie S2. In the case shown in Figure 4B, the escape skin plays a significant role in preventing the host from being destroyed. There are two main reasons why the piece with the escape skin is not easy to be entangled by the rotating gear (Fig. 4C).

On the one hand, the artificial scales are flexibly connected with the soft substrate and can automatically comply with the contact force between the gear and the skin; on the other hand, the shedding of scales weakens the entanglement force of the gear on the piece. It can be concluded from Figures 3 and 4 that the harmful force constraints that the escape skin is adept at coping with include adhesion (Fig. 3D–F, L, comparison 3 of Fig. 4A, D) and mechanical damage with power such as cutting, shearing, and clamping (Fig. 3B, J, comparisons 1, 4 and 6 of Fig. 4A–E).

Active escape control

How to control the adhesion force of scales on the soft substrate is the key to determining the escape force, escape success rate, and active escape ability. Although we can fabricate the escape skin with different adhesion forces by changing the contact area between the scale and the substrate, the adhesion force controlled by this method is invariable. In this work, we control the adhesion force of scales by controlling the temperature of the escape skin. From Figure 5A and B, it can be seen that the escape skin is more sensitive to temperature. When the temperature is increased to 40°C, the pull-off force of scales is only 29.9% of the initial value (at 26°C). When the temperature >120°C, this proportion becomes 5.2%, and at this time, just one feather is enough to sweep the scales off (Fig. 5A).

FIG. 5.

The escape force and hardness control of flexible escape skin. (A) Flexible escape skin is heated by a heating plate and its temperature imaging. The temperature displayed in the upper left corner of the temperature image is the temperature at the cross mark. (B) The relationship between the pull-off force of a scale and the surface temperature of the escape skin. The soft enclosed gripper (C) and soft robotic tentacle (D) with heating film and escape skin can control the escape force, and when the skin temperature >60°C, the screws attached to the escape skin can be shaken off only by slight shaking. (E) Stiffness comparison of flexible escape skin at room temperature (24.5°C) and low temperature (−4°C). (F) The relationship between the hardness of the soft substrate and the surface temperature of the escape skin. Each pull-off force and hardness value is the average of three measurement results. The error bar is the standard deviation of the measurement result.

Harmful force constraints can be divided into static and dynamic types. Dynamic ones tend to stimulate the escape skin to exert greater efficiency (Fig. 3B, J, comparisons 1, 4, and 6 of Fig. 4A–E), while static ones often require the host to implement active escape movements (Fig. 3E–I, 3L, M, and comparisons 2, 3, and 5 of Fig. 4A). To reduce the escape burden of the host and improve the escape success rate, we made the escape skin with controllable escape forces by attaching a heating film to a soft substrate and applied it to a soft enclosed gripper and a soft robotic tentacle (Fig. 5C, D and Supplementary Movie S3).

In addition to controlling the escape force, changing the temperature can also control the hardness of the escape skin, which can also cause stiffness changes, as shown in Figure 5E. It can be seen from Figure 5F that the hardness when the temperature drops to −5°C is 2.9 times that at room temperature (24.5°C). Controlling the hardness of the escape skin can not only ensure sufficient flexibility but also enhance the resistance of the skin to large stress by increasing the hardness.

The escape behavior in animals is generally triggered by rapid and urgent external stimuli, but the response speed of the temperature control mechanism is relatively slow. Therefore, the temperature control mechanism is often used to reduce the escape force in the case of difficult escape and maintain the skin's softness in a low-temperature environment. Improving the heating efficiency and reducing the skin temperature control range are the main methods to promote the application of temperature control mechanisms.

Properties of flexible escape skin

The main behavior of the escape skin is shedding scales, and therefore the shedding regularity of scales is crucial for the application of the escape skin. The results of the scale-shedding test are shown in Figure 6. As shown in Figure 6A, the general trend is that the pull-off force of a scale decreases with an increase of the angle between the scale and the soft substrate. For example, the pull-off force at 180° is only 0.22 N, which is 22.4% of that at 0°. Based on this conclusion, when dealing with adhesion constraints, we can reduce the escape resistance by controlling the motion direction of the host.

FIG. 6.

Scale shedding test of flexible escape skin. The size of the escape skin used in the scale shedding test by linear movement is 50 × 30 mm (length × width). The diameter of the escape skin used in the scale shedding test by rotation is 30 mm. The β angles of the three shedding tools are 135°, 90°, and 45°, respectively. The contact area between the shedding tool and the escape skin is ∼12 mm². (A) The relationship between the pull-off force of a scale and the angle α between the scale and the soft substrate. When the escape skin is pasted on the rigid base, the curves of scale shedding ratio on normal force and tool inclination β: (B) forward movement along scales (V_F), (C) reverse movement along scales (V_R), (D) lateral movement along scales (V_L), (E) clockwise rotation and tool tip faces V_R, and (F) clockwise rotation and tool tip faces V_F. When the escape skin is pasted on the soft base, the curves of scale shedding ratio on the normal force in two cases of movement (G) and rotation (H). When the escape skin is pasted on the rigid base (I, J) and the soft base (K, L), respectively, the curves of scale shedding ratio on motion speed. Each pull-off force value and shedding ratio value is the average of test results of three samples. The error bar is the standard deviation of the measurement result.

As shown in Figure 6B–H, the shedding ratio of scales is positively correlated with the pressure applied to the escaping skin. This positive correlation phenomenon indicates that the lifespan of escape skin is closely related to contact pressure.

Comparing Figure 6B–D or 6E and F, it can be concluded that it is easier to shed scales along the V_R direction, but as long as the contact pressure is large enough, the shedding ratio is almost the same regardless of the direction. Comparing the two cases of movement (Fig. 6B–D) and rotation (Fig. 6E, F), it is found that rotation can effectively reduce the shedding ratio of scales. In addition, by comparing the shedding ratio when using different shedding tools (Fig. 6B–D), it is found that the tools with small tip angles (β) are more likely to shed scales, but the tool with the smallest tip angle shows the opposite trend in the case of rotation (Fig. 6E, F), which once again illustrates that escaping through rotation can prolong the lifespan of the escape skin.

As shown in Figure 6G, when the escape skin is pasted on the soft base, only 20–30 N normal force is required to approach the maximum shedding ratio, which is significantly easier to shed scales than the escape skin pasted on the rigid base. Comparing Figure 6E, F, and H, it was found that for the rotation case, there was less difference in the shedding ratio between the skins pasted on the rigid and soft bases. The motion speed also affects the shedding ratio of scales, as shown in Figure 6I–L.

The general trend is that the shedding ratio of scales decreases with the increase in movement speed. In addition, when a rigid base is used, the speed has a great influence on the shedding ratio of the rotation case (Fig. 6J), and the shedding ratio between the maximum and the minimum rotational speed differs by more than 0.26. And when a soft base is used, the speed has a great influence on the movement case (Fig. 6K), and the difference can reach more than 0.41. Therefore, when controlling the escape motion, appropriately increasing the movement speed can effectively reduce the scale loss.

The situation where the escape skin is difficult to escape is when it is pressed by stationary heavy objects, as shown in Figure 3G and H. This is mainly because the host needs to overcome the large resistance between the escape skin and the heavy object, and the escape skin faces the risk of tearing. As shown in Figure 7A–C, the normal force applied to the escape skin is positively related to the pullout force, and the pullout force is the smallest when the escape skin moves along the V_R direction. When the escape skin escapes along the V_F (Fig. 7B) and V_L (Fig. 7C) directions, the shedding tool with a small tip angle (β) has a greater binding force.

FIG. 7.

Pullout force test of flexible escape skin. When the escape skin is pasted on the rigid base, the bar charts of pullout force on normal force and tool inclination β: (A) reverse movement (V_R), (B) forward movement (V_F), (C) lateral movement (V_L). When the escape skin is pasted on the rigid base, the bar charts of pullout force on scale coverage ratio and tool inclination: (D) reverse movement (V_R), (E) forward movement (V_F), (F) lateral movement (V_L). When the escape skin is pasted on the soft base, the bar chart of pullout force on normal force and tool inclination (G), and the bar chart of pullout force on scale coverage ratio and tool inclination (H). Each pullout force value is the average of test results of three samples. The error bar is the standard deviation of the measurement result.

As shown in Figure 7D–F, changes in scale coverage ratio can cause fluctuations in pullout force, and the fluctuations are more pronounced when the escape skin moves in the V_F and V_L directions. It can be seen from Figure 6C and D that when the normal force is 25 N, the shedding ratio of scales is more than 70%, which means that the pullout force mainly comes from the shedding force of scales and the resistance of the soft substrate, and for the case of low shedding ratio, the pullout force mainly comes from the friction between the shedding tool and the scales.

When the escape skin is pasted on the soft base, the pullout force is obviously larger than that on the rigid base, especially when the normal force is 45 N, the pullout force increases by 46.2, 8.5, and 44.1 N in the V_R, V_F, and V_L directions, respectively (Fig. 7A–C, G). The reason for this phenomenon is that the escape skin pasted on the soft base is more prone to wrinkle, which increases the pullout resistance. However, the soft base also increases the compliance of the escape skin, making the pullout force of the escape skin with different scale coverage ratios almost equal (Fig. 7H).

Conclusion

As one of the most common behaviors in the animal kingdom, the escape function also shows unique technical advantages in getting out of the harmful environment and damage resistance. Inspired by the behavior of a species of fish-scale gecko escaping predation by shedding scales, we developed a flexible escape skin enabled by multimaterial 3D printing and applied it to robots and wearable devices. Although not able to regenerate scales like geckos, it could improve the robustness of hosts in unstructured environments. Experiments demonstrate that the proposed escape skin can adapt to different shapes and large deformations, and has significant effects in reducing escape force, escaping from harmful force environments, and resisting mechanical damage. Furthermore, we enabled active control of escape force and skin hardness by controlling temperature, increasing the adaptability of escape skin to the surrounding.

However, the escape skin based on the shedding scale principle also has the disadvantage that it cannot be reused and the fallen scales pollute the environment. In the future, we will try to use a more biodegradable and less harmful material to make scales, to solve the pollution problem. Also, we will study the potential application of escape skin in robot locomotion, because the scales of animals not only have a protective function but also play a role in locomotion.

Footnotes

Authors' Contributions

H.L. contributed to the conception and experimental design of the research. H.L. and P.Z. wrote the article with support from all authors. H.L. and X.L. collected the data. H.L., X.Z., and C.W. designed and fabricated the escape skin. J.Y. provided funding for the project and was responsible for project administration.

Author Disclosure Statement

The authors declare no conflict of interest.

Funding Information

This research is supported by Ningbo Natural Science Foundation of China (No. 2022J134) and National Natural Science Foundation of China (No. 51975505).

Supplementary Material

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