# A State-of-the-Art Review on Robots and Medical Devices Using Smart Fluids and Shape Memory Alloys
## 1 Introduction  
Historically, material technologies have had a profound influence on human civilization, and hence historians have defined distinct time periods by the dominant materials used during those eras. The term'smart materials' first appeared in the late 1980s, and since then, a multitude of research activities on smart materials has been, or is being, conducted in diverse industry areas. Among the many inherent characteristics of smart materials, the crucial common denominators to achieve high performances in application systems are their capabilities of sensing, actuating, and controlling under external stimuli that govern the responses of the systems. To date, more than 100 types of smart materials with these capabilities have been proposed, and their adaptability as'smart' has been validated. These include the identification of material characteristics showing sensitivity as sensors, generating forces as actuators, and performing control responses as a controller. In the last two decades, several classes of smart materials have received significant attention, over a broad range of engineering applications, because of their unique and useful actuator properties. Potential smart materials for robotic and medical applications include electro-rheological (ER) fluids (ERFs), magneto-rheological (MR) fluids (MRFs), and shape memory alloys (SMAs). It is noted here that ERFs also include homogeneous types such as liquid crystal in the sense of the electric field response smart material. However, in this review article, ERFs which can be expressed by Bingham model, in which the field-dependent yield stress is the dominant behavior, are considered only.  
In the robotic research society, many researchers have developed various types of robots using traditional actuators (e.g., electric motors, hydraulic fluidic actuators) to achieve some specificrequirements for desired movements. Although some robots with conventional actuators have demonstrated excellent performance, alternative actuators need to be explored when the capability to perform flexible and complex movements is required. Smart material-based actuators are one of these alternative solutions, because of their relatively small weight and volume, compared to those fabricated using conventional actuating methods. For example, SMAs have been widely used in a diverse range of humanoid-robotic applications since the 1980s, especially as artificial muscles, because SMA wire actuators can mimic the human muscle. The use of smart materials as actuators can be categorized in terms of their application areas: automotive, aerospace, robotics, and medical, amongst others. Despite numerous research works on smart materials applications, a comprehensive overview on the specific application areas of diverse robots, medical devices and rehabilitation systems is not readily available. Therefore, it is worthwhile to timely review one of attractive applications of smart materials, namely robot and medical applications.  
Consequently, the main objective of this review article is to present a broad perspective of the research efforts during the last two decades in relation to robot and medical devices, and their prototypes, based on ERFs, MRFs, and SMAs. This review describes the attributes of specific smart materials that make them ideal for actuating robotic and medical applications, and discusses their associated technical capabilities and limitations in order to emphasize the design challenges. Therefore, this article provides an updated review of recent smart material research of robot and medical applications, with 100 state-of-the-art references categorized into three types of smart materials. It is remarked here that this review article is focused on the operating principle and design concept of each application for easy understanding of the potential readers of this attractive research field, instead of the specific dynamics and control strategies of the adopted devices and systems.