A Review of Bioinspired Vibration Control Technology

Due to huge demand in engineering, vibration control technology and related studies have always been at the frontiers of research. Although traditional vibration control methods are stable and reliable, they have obvious shortcomings. Through evolution and natural selection, certain body-parts of animals in the natural world have been cleverly constructed and well designed. This provides a steady stream of inspiration for the design of vibration control equipment. The prime objective of this review is to highlight recent advances in the bionic design of vibration control devices. Current bionic vibration control devices were classified, and their bionic principles were briefly described. One kind was the bionic device based on the brain structure of the woodpecker, which is mostly used to reduce vibration at high frequencies. Another kind of bionic device was based on animal leg structure and showed outstanding performance in low frequency vibration reduction. Finally, we briefly listed the problems that need to be solved in current bionic vibration control technology and gave recommendations for future research direction.


Introduction
Vibration is a common physical phenomenon in industrial production. On the one hand, vibration can play unique roles such as screening and cleaning; On the other hand, vibration also brings a lot of serious harm. Strong vibrations can cause serious damage to bones, muscles, joints, and ligaments [1][2][3]. When the vibration frequency is close to the natural frequency of the organs, it will cause resonance and damage to the viscera such as accelerated breathing, altered blood pressure, accelerated heart rate, or a reduction in the blood output of myocardial contractions [4][5][6]. In machining, vibrations increase the roughness of the workpiece surface, and even produce grain and destroy the machined surface [7,8]. Vibrations also increase the heat of cutting tools in machine work, thus reducing their service life [9,10]. Besides this, for precision instruments, vibration can cause the misoperation of sensitive switches such as relays, which can then cause the main circuit to misoperate, causing major safety accidents [11,12]. Given the many hazards caused by vibration, vibration control has become an urgent research task for scholars and engineering technicians [13].
Vibration control usually means the control of the dynamic response or dynamic instability of a system so that vibration is limited to the minimum allowable degree. Classical vibration control mainly uses vibration damping [14,15], vibration isolation [16,17], and vibration absorption methods [18,19]. Although they have the advantages of simple structure, easy maintenance, and low cost, they all have their own disadvantages. To achieve vibration reduction under wide frequency resonance excitation, vibration damping technology must increase the weight of the system and the cost. In addition, the shear modulus and damping loss factor of viscoelastic damping materials in the vibration damper are sensitive 2 of 14 to temperature and vibration frequency, which limits the damping effect. The quasi-zero stiffness (QZS) vibration isolation structure of common nonlinear vibration isolation systems is not only able to realize effective vibration isolation in a wide range of frequency bands, but also has good static bearing capacity [20][21][22]. However, the development and application of the QZS vibration isolation structure are limited because of its disadvantages such as instability, bifurcation, and strong non-linearity. Common vibration absorbers include damped dynamic vibration absorbers [23,24], dynamic vibration absorbers using nonlinear springs [25], and discrete distributed dynamic vibration absorbers [26] and so on [27][28][29]. Although the passive dynamic vibration absorber has a good absorbing effect on the narrow-band response, it is still not ideal for the structure with multiple resonance peaks in the excitation frequency band and other broadband vibrations.
Through long-term evolution, various creatures have evolved unique structures that are highly suited to their natural environment [30,31]. For example, woodpeckers need to hit tree trunks at high speed to catch insects inside the trees [32,33]. In this process, acceleration can reach 1000 times the acceleration of gravity, and velocity can reach six times the speed of sound [34]. However, the vibration caused by the strong impact does not damage or destroy the woodpecker's brain [35][36][37]. Therefore, it is one of the important development directions of current vibration control systems [38][39][40][41], which can simulate biological structures and functions in order to improve the performance of traditional vibration control devices. Based on revealing different creatures' mechanisms for vibration reduction and buffering, various vibration control systems are designed with the idea of bionics in mind.
In this article, a comprehensive and profound review of bioinspired vibration control mechanisms has been executed, based on a survey of real-world deployments and published papers over the past 15 years in the Web of Science. Various bionic vibration control mechanisms, devices, or systems in the field of mechanical engineering are classified and summarized, and their bionic basic principles are briefly analyzed. On this basis, limitations and future development trends are analyzed.

Bionic Devices Based on the Woodpecker's Head
Because of the outstanding vibration reduction performance of their heads, woodpeckers have become one of the biological objects that various vibration devices are eager to imitate. The head structure of a woodpecker is shown in Figure 1a [42]. Firstly, the hard skull of a woodpecker's head can withstand the impact of hitting a tree trunk. Secondly, its head has many developed muscles and dense soft tissue, which can effectively absorb vibrations caused by the impact [43]. Finally, there is a very narrow interspace between the outer meninges and the brain, which can weaken the vibration to a certain extent. Based on the above vibration reduction principle, Jiang reported an active vibration isolation system [42]. As shown in Figure 1b, the system was mainly composed of a rubber layer, an air spring, and an actuator made of magnetostrictive material, which were used to imitate the cartilage and muscle in the outer meninges, the interspace between outer meninges and the brain, and nerves and muscle, respectively. The dynamic model of the system is displayed in Figure 2. The bionic system used a neural network algorithm to control the actuator, and the results showed that the system can effectively reduce the impact of vibration caused by ground disturbance or direct disturbance on the target platform.
After that, a similar bionic vibration control system with viscoelastic materials was proposed [44]. As shown in Figure 3, the difference between this system and the above system was that an air spring and a damper composed of viscoelastic materials were used to simulate the interspace between the outer meninges and the brain in woodpeckers. The experimental results proved that the system can effectively suppress middle-frequency and high-frequency vibrations.  After that, a similar bionic vibration control system with viscoelastic materials was proposed [44]. As shown in Figure 3, the difference between this system and the above system was that an air spring and a damper composed of viscoelastic materials were used to simulate the interspace between the outer meninges and the brain in woodpeckers. The experimental results proved that the system can effectively suppress middle-frequency and high-frequency vibrations. Further exploring the vibration reduction principle of the woodpecker, Yoon found that spongy bone, skull bone with cerebrospinal fluid, hyoid, and beak make up the unique endoskeletal structure of the woodpecker's head [45], as displayed in Figure 4. Some studies have shown that their beaks are not only large, but can also be self-  After that, a similar bionic vibration control system with viscoelastic materials was proposed [44]. As shown in Figure 3, the difference between this system and the above system was that an air spring and a damper composed of viscoelastic materials were used to simulate the interspace between the outer meninges and the brain in woodpeckers. The experimental results proved that the system can effectively suppress middle-frequency and high-frequency vibrations. Further exploring the vibration reduction principle of the woodpecker, Yoon found that spongy bone, skull bone with cerebrospinal fluid, hyoid, and beak make up the unique endoskeletal structure of the woodpecker's head [45], as displayed in Figure 4. Some studies have shown that their beaks are not only large, but can also be self-   After that, a similar bionic vibration control system with viscoelastic materials was proposed [44]. As shown in Figure 3, the difference between this system and the above system was that an air spring and a damper composed of viscoelastic materials were used to simulate the interspace between the outer meninges and the brain in woodpeckers. The experimental results proved that the system can effectively suppress middle-frequency and high-frequency vibrations. Further exploring the vibration reduction principle of the woodpecker, Yoon found that spongy bone, skull bone with cerebrospinal fluid, hyoid, and beak make up the unique endoskeletal structure of the woodpecker's head [45], as displayed in Figure 4. Some studies have shown that their beaks are not only large, but can also be self- Further exploring the vibration reduction principle of the woodpecker, Yoon found that spongy bone, skull bone with cerebrospinal fluid, hyoid, and beak make up the unique endoskeletal structure of the woodpecker's head [45], as displayed in Figure 4. Some studies have shown that their beaks are not only large, but can also be self-sharpening, effectively reducing the damage to the brain caused by mechanical impacts [46,47]. Besides, their unique hyoid bone structure is able not only to effectively protect the brain but can also evenly disperse vibrations caused by knocking on the tree trunk [33,46]. As shown in the head section view ( Figure 5), the dense and evenly distributed spongy bone closely connected with the hyoid bone also greatly reduces the damage of intense vibration to the woodpecker. Subsequently, a four-degree-of-freedom mass-damper-spring damping model was proposed to represent the brain damping system in the woodpecker (Figure 6), where m, c, and k are, respectively, the mass, damping coefficient, and stiffness, and the subscripts of 1, 2, 3, 4, and 5 represent a tree, the beak, the hyoid, the skull bone, and the brain of the woodpecker. According to the Lagrange equation of motion, the system was expressed as ..
(1) [33,46]. As shown in the head section view ( Figure 5), the dense and evenly distributed spongy bone closely connected with the hyoid bone also greatly reduces the damage of intense vibration to the woodpecker. Subsequently, a four-degree-of-freedom massdamper-spring damping model was proposed to represent the brain damping system in the woodpecker (Figure 6), where m, c, and k are, respectively, the mass, damping coefficient, and stiffness, and the subscripts of 1, 2, 3, 4, and 5 represent a tree, the beak, the hyoid, the skull bone, and the brain of the woodpecker. According to the Lagrange equation of motion, the system was expressed as = (1)    [45]. [33,46]. As shown in the head section view ( Figure 5), the dense and evenly distributed spongy bone closely connected with the hyoid bone also greatly reduces the damage of intense vibration to the woodpecker. Subsequently, a four-degree-of-freedom massdamper-spring damping model was proposed to represent the brain damping system in the woodpecker (Figure 6), where m, c, and k are, respectively, the mass, damping coefficient, and stiffness, and the subscripts of 1, 2, 3, 4, and 5 represent a tree, the beak, the hyoid, the skull bone, and the brain of the woodpecker. According to the Lagrange equation of motion, the system was expressed as = (1)   Based on the above analysis, Yoon et al. proposed a new bionic vibration absorption system [45,48]. The system consisted of two layers of metal material, one layer of viscoelastic material, and close-packed micro-glass, as shown in Figure 7. Metal layers made of steel and aluminum mimicked, respectively, the beak and skull of the woodpecker. The viscoelastic layer corresponded to the hyoid bone. The porous structure formed by the Figure 6. Simplified vibration reduction model of woodpeckers' brains as they hit tree trunks [45].
Based on the above analysis, Yoon et al. proposed a new bionic vibration absorption system [45,48]. The system consisted of two layers of metal material, one layer of viscoelastic material, and close-packed micro-glass, as shown in Figure 7. Metal layers made of steel and aluminum mimicked, respectively, the beak and skull of the woodpecker. The viscoelastic layer corresponded to the hyoid bone. The porous structure formed by the micro-glass spheres served as spongy bones to absorb the mechanical vibrations generated by the micromechanical devices for a short time. The close-packed micro-glass, with a large number of air gaps, was tightly packed around the micromachined devices, thereby absorbing short-duration mechanical excitations in a kinetic way. Metal enclosure II was filled with the close-packed micro-glass under mallet tapping, in order to achieve a particlefilling ratio of about 62.5%. The experimental results displayed that the system can control high-frequency vibrations and protect the precision instrument effectively under external shock. Figure 6. Simplified vibration reduction model of woodpeckers' brains as they hit tree trunks [45].
Based on the above analysis, Yoon et al. proposed a new bionic vibration absorption system [45,48]. The system consisted of two layers of metal material, one layer of viscoelastic material, and close-packed micro-glass, as shown in Figure 7. Metal layers made of steel and aluminum mimicked, respectively, the beak and skull of the woodpecker. The viscoelastic layer corresponded to the hyoid bone. The porous structure formed by the micro-glass spheres served as spongy bones to absorb the mechanical vibrations generated by the micromechanical devices for a short time. The close-packed micro-glass, with a large number of air gaps, was tightly packed around the micromachined devices, thereby absorbing short-duration mechanical excitations in a kinetic way. Metal enclosure II was filled with the close-packed micro-glass under mallet tapping, in order to achieve a particle-filling ratio of about 62.5%. The experimental results displayed that the system can control high-frequency vibrations and protect the precision instrument effectively under external shock.

Bionic Devices Based on Animal Leg
The ostrich, horse, cheetah, and other animals can run quickly in complex terrain environments without being harmed by the vibrations caused by the impact of the ground. Their unique leg structure plays an indispensable role in vibration reduction [40]. As shown in Figure 8a, there are usually X-shaped and Z-shaped structures in the legs of birds. Additionally, an X-shaped sponge structure is also found in the skull (Figure 8b).

Bionic Devices Based on Animal Leg
The ostrich, horse, cheetah, and other animals can run quickly in complex terrain environments without being harmed by the vibrations caused by the impact of the ground. Their unique leg structure plays an indispensable role in vibration reduction [40]. As shown in Figure 8a, there are usually X-shaped and Z-shaped structures in the legs of birds. Additionally, an X-shaped sponge structure is also found in the skull (Figure 8b). Figure 6. Simplified vibration reduction model of woodpeckers' brains as they hit tree trunks [45].
Based on the above analysis, Yoon et al. proposed a new bionic vibration absorption system [45,48]. The system consisted of two layers of metal material, one layer of viscoelastic material, and close-packed micro-glass, as shown in Figure 7. Metal layers made of steel and aluminum mimicked, respectively, the beak and skull of the woodpecker. The viscoelastic layer corresponded to the hyoid bone. The porous structure formed by the micro-glass spheres served as spongy bones to absorb the mechanical vibrations generated by the micromechanical devices for a short time. The close-packed micro-glass, with a large number of air gaps, was tightly packed around the micromachined devices, thereby absorbing short-duration mechanical excitations in a kinetic way. Metal enclosure II was filled with the close-packed micro-glass under mallet tapping, in order to achieve a particle-filling ratio of about 62.5%. The experimental results displayed that the system can control high-frequency vibrations and protect the precision instrument effectively under external shock.

Bionic Devices Based on Animal Leg
The ostrich, horse, cheetah, and other animals can run quickly in complex terrain environments without being harmed by the vibrations caused by the impact of the ground. Their unique leg structure plays an indispensable role in vibration reduction [40]. As shown in Figure 8a, there are usually X-shaped and Z-shaped structures in the legs of birds. Additionally, an X-shaped sponge structure is also found in the skull (Figure 8b). Inspired by the above-mentioned animal leg structures, Jing et al. proposed an X-shaped bionic vibration control device [49]. In order to better achieve the vibration reduction effect, the animal body structure was simplified. The part of the body above the leg was considered a mass. The leg bones were simplified into rods and retained their characteristic of varying lengths. Springs with different degrees of stiffness and different directions were used to imitate the developed muscles of the legs (Figure 9). Then, the influence of each structural parameter on the system stiffness, loading capacity, and vibration isolation effect was systematically analyzed. Through mathematical modeling and experimental analysis, the bionic device was shown to have the same nonlinear characteristics as the animal leg, and its nonlinear stiffness and damping coefficient were able to be adjusted by adjusting the number of layers, the length of the connecting rod, and the installation angle [41,50]. Compared with the quasi-zero stiffness vibration isolation devices, the bionic device not only had low resonance frequency, but also had excellent loading capacity. After this, Jing et al. also proposed two novel X structures and analyzed their static stiffness, the vibration isolation effect, and the nonlinear law of vibration frequency under three different constraint conditions [39]. In recent years, they have also developed a six-free vibration isolation platform based on the X structure, which has been successfully applied in many fields such as aerospace and machinery [51][52][53][54].
tion effect, the animal body structure was simplified. The part of the body above the leg was considered a mass. The leg bones were simplified into rods and retained their characteristic of varying lengths. Springs with different degrees of stiffness and different directions were used to imitate the developed muscles of the legs (Figure 9). Then, the influence of each structural parameter on the system stiffness, loading capacity, and vibration isolation effect was systematically analyzed. Through mathematical modeling and experimental analysis, the bionic device was shown to have the same nonlinear characteristics as the animal leg, and its nonlinear stiffness and damping coefficient were able to be adjusted by adjusting the number of layers, the length of the connecting rod, and the installation angle [41,50]. Compared with the quasi-zero stiffness vibration isolation devices, the bionic device not only had low resonance frequency, but also had excellent loading capacity. After this, Jing et al. also proposed two novel X structures and analyzed their static stiffness, the vibration isolation effect, and the nonlinear law of vibration frequency under three different constraint conditions [39]. In recent years, they have also developed a six-free vibration isolation platform based on the X structure, which has been successfully applied in many fields such as aerospace and machinery [51][52][53][54]. When animals or people move, the angles and shapes of the joints of the legs change ( Figure 10a). These changes may be closely related to the damping effect of the entire leg [55]. Inspired by the above phenomenon, Jing et al. designed a multi-joint bionic vibration control device with different stiffness and joint parameters [56]. In contrast to the Xshaped device described above, small parallelograms with springs of different stiffness imitated the functions of each joint of the leg (Figure 10b). Compared with the traditional QZS system, this system had better static load capacity and lower resonance frequency. When animals or people move, the angles and shapes of the joints of the legs change ( Figure 10a). These changes may be closely related to the damping effect of the entire leg [55]. Inspired by the above phenomenon, Jing et al. designed a multi-joint bionic vibration control device with different stiffness and joint parameters [56]. In contrast to the X-shaped device described above, small parallelograms with springs of different stiffness imitated the functions of each joint of the leg (Figure 10b). Compared with the traditional QZS system, this system had better static load capacity and lower resonance frequency.
Cats are known to be able to fall tens of meters through the air without being hurt [57,58]. As shown in Figure 11a, cats have three leg bones that allow them to adjust their posture flexibly during a fall to minimize damage. Based on the above analysis, Yan et al. developed a bionic vibration isolation device with a polygonal structure [37]. Based on the overall skeleton structure of the cat, a symmetrical quadrilateral structure with four springs was designed ( Figure 11b). As shown in Figure 11c, a three-rod structure mimicked the three long bones of the cat's legs. The bearing platform was designed at the top and had a chute to adjust the distance between the legs. The bionic vibration isolation platform displayed excellent suppression effects on the three forms of vibration (periodic, scanning, and random excitation). Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 15 Figure 10. (a) Schematic of human leg (b) multi-joint bionic vibration control device [56].
Cats are known to be able to fall tens of meters through the air without being hurt [57,58]. As shown in Figure 11a, cats have three leg bones that allow them to adjust their posture flexibly during a fall to minimize damage. Based on the above analysis, Yan et al. developed a bionic vibration isolation device with a polygonal structure [37]. Based on the overall skeleton structure of the cat, a symmetrical quadrilateral structure with four springs was designed ( Figure 11b). As shown in Figure 11c, a three-rod structure mimicked the three long bones of the cat's legs. The bearing platform was designed at the top and had a chute to adjust the distance between the legs. The bionic vibration isolation platform displayed excellent suppression effects on the three forms of vibration (periodic, scanning, and random excitation). In order to protect itself, the frog can effectively suppress vibration during jump. Inspired by the M-shaped limb structure of the frog, Zeng proposed a bionic QZS structure. The bionic QZS structure was mainly composed of plates, torsion springs, and connecting rods (Figure 12b). The negative stiffness and positive stiffness of the isolator were generated by the torsion springs in the first layer (a6) and second layer (a3), respectively. Compared with traditional QZS devices, this device had a wider vibration isolation range and a shorter decay time for vibrations. In order to protect itself, the frog can effectively suppress vibration during jump. Inspired by the M-shaped limb structure of the frog, Zeng proposed a bionic QZS structure. The bionic QZS structure was mainly composed of plates, torsion springs, and connecting rods (Figure 12b). The negative stiffness and positive stiffness of the isolator were generated by the torsion springs in the first layer (a6) and second layer (a3), respectively. Compared with traditional QZS devices, this device had a wider vibration isolation range and a shorter decay time for vibrations. spired by the M-shaped limb structure of the frog, Zeng proposed a bionic QZS structure. The bionic QZS structure was mainly composed of plates, torsion springs, and connecting rods (Figure 12b). The negative stiffness and positive stiffness of the isolator were generated by the torsion springs in the first layer (a6) and second layer (a3), respectively. Compared with traditional QZS devices, this device had a wider vibration isolation range and a shorter decay time for vibrations.

Other Bionic Vibration Control Devices
Some studies have shown that the heads of pigeons, chickens, and other birds are able to maintain a stationary state in the hold phase when moved forward or back, and that this is not affected by movement [61,62]. The necks of birds, which usually consist of 8-25 vertebrae (Figure 13), play an important role in this process. Deng et al. combined the function of the bird's neck with a quasi-zero stiffness vibration isolation system to propose a new bionic vibration isolation device ( Figure 14) [63]. They equivalented the whole body of the bird to the rigid support of the bionic system. The head was simplified as a mass block at the top of the system. Multiple quasi-zero stiffness damping mechanisms with springs and dampers were used to simulate the necks of birds. The system had an excellent vibration isolation effect under low and ultra-low frequency vibration, which provided a new solution for the nonlinear bionic vibration control system.

Other Bionic Vibration Control Devices
Some studies have shown that the heads of pigeons, chickens, and other birds are able to maintain a stationary state in the hold phase when moved forward or back, and that this is not affected by movement [61,62]. The necks of birds, which usually consist of 8-25 vertebrae (Figure 13), play an important role in this process. Deng et al. combined the function of the bird's neck with a quasi-zero stiffness vibration isolation system to propose a new bionic vibration isolation device ( Figure 14) [63]. They equivalented the whole body of the bird to the rigid support of the bionic system. The head was simplified as a mass block at the top of the system. Multiple quasi-zero stiffness damping mechanisms with springs and dampers were used to simulate the necks of birds. The system had an excellent vibration isolation effect under low and ultra-low frequency vibration, which provided a new solution for the nonlinear bionic vibration control system.   In daily life, people encounter various types of vibration caused by a variety of situations such as running, jumping, and walking on rough roads [55,65,66]. The human body acts as a shock absorber to prevent brain injury caused by the vibration and impact. On the one hand, the swing of both arms can maintain the stability of the human body and weaken the harm of vibration to the human body [67,68]. The arm swing can significantly reduce the vertical reaction moment of the ground against the person and reduce the vertical displacement of the body's center of mass. On the other hand, the leg plays an excellent role in low-frequency vibration isolation [69], preventing a large amount of vibration In daily life, people encounter various types of vibration caused by a variety of situations such as running, jumping, and walking on rough roads [55,65,66]. The human body acts as a shock absorber to prevent brain injury caused by the vibration and impact. On the one hand, the swing of both arms can maintain the stability of the human body and weaken the harm of vibration to the human body [67,68]. The arm swing can significantly reduce the vertical reaction moment of the ground against the person and reduce the vertical displacement of the body's center of mass. On the other hand, the leg plays an excellent role in low-frequency vibration isolation [69], preventing a large amount of vibration damage to important organs of the human body [66,70]. Elastic muscles and tendons can act as nonlinear energy-conserving springs and shock absorbers during locomotion. Vibration energy is absorbed to prevent brain injury caused by vibration and impact during lengthening contractions of the muscles and tendons.
The motion state and simplified model of the human arm and leg when walking is displayed in Figure 15. According to the above analysis and a simplified human motion model, a new bionic vibration isolation device based on the structure of the human body was reported [69]. The device consists of two parts: a swing rod and a rotating plate, which imitate the swing of human arms, and an X-shaped supporting structure with spring and damper, which imitate legs ( Figure 16). The experimental results showed that the device not only had the nonlinear performance of X-type structures, but also had ultra-low tunable frequency and excellent anti-vibration characteristics.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 15 damage to important organs of the human body [66,70]. Elastic muscles and tendons can act as nonlinear energy-conserving springs and shock absorbers during locomotion. Vibration energy is absorbed to prevent brain injury caused by vibration and impact during lengthening contractions of the muscles and tendons. The motion state and simplified model of the human arm and leg when walking is displayed in Figure 15. According to the above analysis and a simplified human motion model, a new bionic vibration isolation device based on the structure of the human body was reported [69]. The device consists of two parts: a swing rod and a rotating plate, which imitate the swing of human arms, and an X-shaped supporting structure with spring and damper, which imitate legs ( Figure 16). The experimental results showed that the device not only had the nonlinear performance of X-type structures, but also had ultra-low tunable frequency and excellent anti-vibration characteristics.    Later, Feng et al. proposed another nonlinear anti-vibration device [71]. As shown in Figure 17, the difference between this device and the previous one was that it adopted the structure of up and down distribution and used two small X structures with a spring or damper to replace the large X structure. The results demonstrated that the bionic vibration isolation device had a lower resonance frequency, which was conducive to its Later, Feng et al. proposed another nonlinear anti-vibration device [71]. As shown in Figure 17, the difference between this device and the previous one was that it adopted the structure of up and down distribution and used two small X structures with a spring or damper to replace the large X structure. The results demonstrated that the bionic vibration isolation device had a lower resonance frequency, which was conducive to its performance in low frequency vibration control. The device had good nonlinear coupling characteristics, and these nonlinear parameters could be adjusted according to actual needs.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 11 of 15 performance in low frequency vibration control. The device had good nonlinear coupling characteristics, and these nonlinear parameters could be adjusted according to actual needs.

Challenges and Future Directions
Although bionic vibration control technology has achieved good results, there are still many problems that need to be studied deeply and carefully. For example, the existing bionic vibration reduction equipment needs to further broaden its vibration reduction frequency band, reduce the resonance peak, and further improve its high static and low dynamic stiffness in large displacement ranges. At present, the biological prototype used for bionic control is solely limited to the typical parts of several typical creatures. In the

Challenges and Future Directions
Although bionic vibration control technology has achieved good results, there are still many problems that need to be studied deeply and carefully. For example, the existing bionic vibration reduction equipment needs to further broaden its vibration reduction frequency band, reduce the resonance peak, and further improve its high static and low dynamic stiffness in large displacement ranges. At present, the biological prototype used for bionic control is solely limited to the typical parts of several typical creatures. In the future, the research scope of bionic prototype creatures should be expanded, so as to find the common principle of vibration control in nature and reveal its biological mechanism. The vibration reduction mechanism of typical biological vibration control organs and the influence of morphological parameters on the vibration control effect should be quantitatively explored and qualitatively analyzed. The mechanism and law of vibration control should be qualitatively and quantitatively analyzed in relation to surface morphology, material science, kinematics, dynamics, etc., and the influence of relevant parameters on the vibration control effect should be investigated in the future.
Bionic vibration control technology is based on revealing biological functions and mechanisms. It is especially important to accurately model the biological prototype. At present, although many researchers have carried out experimental and theoretical research, there are still many problems in the biological mechanisms of vibration control such as a lack of depth in the research results and simplistic models, which mean that there is still a gap between vibration control equipment and the actual function of biology. Future research should focus on multi-disciplinary, long-term cooperation and the acquisition of multi-faceted biological data, so as to systematically and comprehensively explain the biological vibration control mechanism.
Most tissues and organs have nonlinear elastic or plastic properties. The introduction of new nanomaterials and intelligent control algorithms can help the existing bionic controllers improve the vibration control effect and enrich their functions. The original need for complex mechanical design mechanisms can be replaced by intelligent nanomaterials. Abo Sabah et al. has proposed a kind of bionic sandwich beam composed of a carbon fiber and aluminum honeycomb composite, which is not only able to withstand high impact force, but also has the characteristics of low deformation and low damage [72]. In recent years, the emergence of some intelligent materials, such as variable stiffness and variable damping materials, has brought new possibilities for the development of vibration control. On the one hand, these intelligent materials can avoid the wear and deviation of the original mechanical processing and assembly; on the other hand, they also hope to realize the function of muscle, ligament, and other tissues in the organism. In addition, the emergence of artificial intelligence technology also facilitates new ideas for active vibration control methods.

Conclusions
Creatures in nature have shown perfect biological rationality and high adaptability to their natural environment over billions of years of natural selection, which provides new ideas and new methods for the research and development of vibration control technology [73][74][75]. Herein, various kinds of bionic vibration control technology in mechanical engineering, reported in recent years, were reviewed. These bionic devices were divided into three types according to their bionic principle. The multilayer vibration absorber, based on the brain of the woodpecker, displayed its unique advantages in high frequency vibration excitation. Based on the animal leg, the X-shaped vibration isolation device could effectively suppress low frequency vibrations, and their parameters could be adjusted according to actual working conditions. Other bionic vibration control devices also achieved a good vibration reduction effect. In future development, using multidisciplinary advantages and starting from biological performance, bionic control technology will develop towards an intelligent system, integrating structure and biological materials, thus playing an important role in production and life [76,77].