Various Robots Made from Piezoelectric Materials and Electroactive Polymers: A Review <s> Page 7 of 11 <s> This paper, the first in a series of four review papers, presents a brief summary of the fundamental properties and characteristics of ionic polymeric-metal composites (IPMCs) as biomimetic sensors, actuators and artificial muscles. The forthcoming three review papers, to follow this paper, will address in detail such fundamentals and, in particular, manufacturing techniques and the electronic and electromechanical characteristics of IPMCs (part II), the phenomenological modelling of the underlying sensing and actuation mechanisms in IPMCs (part III) and the potential application areas for IPMCs (part IV). This paper is a summary of all recent findings and current state-of-the art manufacturing techniques, phenomenological laws and mechanical and electrical characteristics. A number of methodologies in developing high-force-density IPMCs are also reported. <s> BIB001 </s> Various Robots Made from Piezoelectric Materials and Electroactive Polymers: A Review <s> Page 7 of 11 <s> The development of a wireless undulatory tadpole robot using ionic polymer–metal composite (IPMC) actuators is presented. In order to improve the thrust of the tadpole robot, a biomimetic undulatory motion of the fin tail is implemented. The overall size of the underwater microrobot prototype, shaped as a tadpole, is 96 mm in length, 24 mm in width, and 25 mm in thickness. It has one polymer fin tail driven by the cast IPMC actuator, an internal (wireless) power source, and an embedded controller. The motion of the tadpole microrobot is controlled by changing the frequency and duty ratio of the input voltage. Experimental results show that this technique can accurately control the steering and swimming speed of the proposed underwater tadpole robot. <s> BIB002 </s> Various Robots Made from Piezoelectric Materials and Electroactive Polymers: A Review <s> Page 7 of 11 <s> In this paper, biomimetic underwater robots built using smart actuators, e.g., a shape memory alloy (SMA), an ionic polymer metal composite (IPMC), lead zirconate titanate (PZT), or a hybrid SMA and IPMC actuator, are reviewed. The effects of underwater environment were also considered because smart actuators are often affected by their external environment. The characteristics of smart actuators are described based on their actuating conditions and motion types. Underwater robots are classified based on different swimming modes. We expanded our classification to non-fish creatures based on their swimming modes. The five swimming modes are body/caudal actuation oscillatory (BCA-O), body/caudal actuation undulatory (BCA-U), median/paired actuation oscillatory (MPA-O), median/paired actuation undulatory (MPA-U), and jet propulsion (JET). The trends of biomimetic underwater robots were analyzed based on robot speed (body length per second, BL/s). For speed per body length, robots using an SMA as an actuator are faster than robots using an IPMC when considering a similar length or weight. Robots using a DC motor are longer while their speeds per body length are similar, which means that robots using smart actuators have an advantage of compactness. Finally, robots (using smart actuators or a motor) were compared with underwater animals according to their speed and different swimming modes. This review will help in setting guidelines for the development of future biomimetic underwater robots, especially those that use smart actuators. <s> BIB003 </s> Various Robots Made from Piezoelectric Materials and Electroactive Polymers: A Review <s> Page 7 of 11 <s> A kind of dielectric elastomer (DE) material, called ‘synthetic elastomer’, has been developed based on acrylonitrile butadiene rubber (NBR) to be used as a dielectric elastomer actuator (DEA). By stacking single layers of synthetic elastomer, a linear actuator, called a multistacked actuator, is produced, and used by mechatronic and robotic systems to generate linear motion. In this paper, we demonstrate the application of the multistacked dielectric elastomer actuator in a biomimetic legged robot. A miniature robot driven by a biomimetic actuation system with four 2-DOF (two-degree-of-freedom) legged mechanisms is realized. Based on the experimental results, we evaluate the performance of the proposed robot and validate the feasibility of the multistacked actuator in a locomotion system as a replacement for conventional actuators. <s> BIB004
Among the ionic EAPs, IPMC is an excellent material for use as a smart actuator, which can be used for robot applications. When an external electric input is applied, a mechanical bending deformation occurs because of a change in the chemical structure. This makes the IPMC material to behave as a biological muscle, and thus, IPMC can be used as an actuator. IPMC consists of a polymer and electrodes and its structural configuration is presented in Figure 14 . When an electric voltage is applied to the electrodes, an electrostatic attraction between the cationic counter ions and the cathode of the applied Figure 13 : Demonstration of the walking motion of the legged robot BIB004 . electric field occurs and the internal volume of water molecules will be unevenly distributed, resulting in a bending effect BIB001 BIB003 . The advantages of the IPMC material are its large strain at low driving voltages, lightweight structure, bio-compatibility, flexibility, and silent actuation. The disadvantages of the IPMC material are its small generated force and the loss of water molecules while working in air condition, which limits the application environment if the loss of water molecules inside the material is limited. Bar-Cohen et al. designed and manufactured a human hand-like robotic arm with four fingers made of IPMC  . To increase the grasping force, hooks are designed at the end of the fingers, representing the concept of human finger nails. The manufactured robotic arm is presented in Figure 15 The most common application of IPMC is in underwater swimming robots. The underwater swimming robots can be classified into three groups: Fish-like, ray-like, and jellyfish robots. The recent research for underwater robots using IPMC is summarized in Table 2 . Kim et al. developed a wireless undulatory tadpole robot using IPMC actuators BIB002 . A biomimetic undulatory motion of a fin tail is implemented to improve the thrust of the tadpole robot. By changing the frequency and duty ratio of the input voltage, the motion of the tadpole robot is effectively controlled. Its structural configurations are presented in Figure 16 By mimicking the jellyfish motion, many kinds of jellyfish type swimming robots were developed using IPMC actuators. Guo et al. developed a new jellyfish t-like swimming robot that imitated the motion of jellyfish [39] . The process of floating and sinking was controlled by the applied input voltage. Four legs with IPMC actuators were designed to replace the antennae of the jellyfish. The moving speed was controlled by changing the frequency of the input voltage. Yeom et al. developed a biomimetic jellyfish robot based on IPMC actuators [40] . The fabricated structure is presented in Figure 17(a) . The curved shape of the IPMC actuator through thermal treatments was successfully applied to mimic the real biomimetic robots with smooth curves. In order to obtain large vertical floating displacement and high Thrust force of the biomimetic jellyfish robot, a bio-inspired electrical input signal with pulse-recovery process was adopted. The maximum experimental frequency of the actuator was 16. Figure 18 . " In this study, the pectoral fin of a ray was mimicked and 12 IPMC were installed. By adopting a sequential control of the IPMCs, the maximum speed reached 0.005 mm/s at a wave frequency of 0.4 Hz. The Chen et al. developed a robotic cownose ray by using an IPMC actuator and PDMS [43] . The size of the robot was 210 mm, 330 mm, and 50 mm of length, width, and thickness, respectively. Its maximum speed was 7 mm/s at a frequency of 0.157 Hz. Chen et al. also developed a pectoral-fin-mimicking a manta-ray, which consisted of a PDMS membrane using four IPMCs [44] . Through their experiments, the maximum tip deflection of the actuator was 17.5 mm and the maximum speed was 4.4 mm/s at 0.4 Hz.