## 1 Introduction  
Responses to natural or human-made disasters--such as chemical, biological, radiological, and nuclear (CBRN) incidents--are always a race against time. Extracting casualties from a hazardous scene is such an example of an emergency case in which a significant amount of pressure and risk is placed on the people working as the first responders. Efficient and timely action is crucial since it is known that the mortality rate increases and reaches a peak after 48 h, implying that the chance of survival drops significantly after this period [1, 2, 3, 4, 5]. While it is crucial to act fast, ensuring the rescue operation is safely performed is also critical. Apart from ensuring the safety of the first responders, minimising the possibility of creating further damage or additional risk to the casualties is a high priority [5].  
Currently, research and development in robotics and its applications is an actively growing field, including the development of robotics applications in assisting first responders in rescue missions, in which the main objective is to improve the search and rescue (SAR) operation to become faster and safer. A wide range of robotics design has been introduced and deployed for specifically performing SAR missions [4; 5; 6; 7; 8; 9; 10; 11]. Most of these robots are designed to assist with some specific tasks that can be grouped into four general categories according to their purpose in the SAR response process, namely search, extraction, evacuation, and treatment [5].  
The field of search robotics has received the most attention so far, particularly unmanned aerial vehicles (UAVs), which have been used in many rescue operations [12; 13; 14; 15; 16; 17; 18], collecting evidence about the position of a missing person, but not interacting with the casualty. However, the field of casualty extraction using rescue robots is significantly less mature, with fewer applicatFions and proposed designs. The main reason for this is that these robots are often significantly larger and more complex due to the medical casualty handling requirements [9]. Other than academic institutions, most of the research and many technical implementation proposals were carried out by military organisations [6].  
This paper presents a novel design of a mobile rescue robot, called ResQbot 2.0, capable of safely rescuing a casualty lying on the ground (i.e., casualty extraction procedure). This robot design is a significantly improved version of the initial proof-of-concept prototype--ResQbot (retrospectively referred to as ResQbot 1.0)--that has been developed in our previous work, and has been presented in [19; 20; 21; 22; 23]. The main contributions of this work can be summarised as follows:  
1. The novel ResQbot 2.0 design comprises seven main novel components: A differential-drive mobile base; a stretcher bed tilting mechanism; a stretcher bed sliding mechanism; a motorised stretcher bed conveyor module, a pair of motorised stretcher strap modules; and a neck securing device module.
2. The proposed methodology of casualty extraction procedure using ResQbot 2.0 for safely loading a full-body casualty onto the robot's'stretcher bed'.
3. Validation of the proposed design and the casualty extraction procedure via simulation experiments in the Gazebo physics engine simulator.  
The remaining of this paper is organised as follows: In Section 2, we review the state of the art on the major work related to the existing mobile rescue robots designed for casualty extraction. In Section 3, we describe the design specification and development of the robot, as well as the medical and safety considerations. The proposed robot design and method of casualty extraction procedure using the proposed robot design are described in Section 4. Furthermore, the results and discussion of the proposed design are presented in Section 5. Finally, we conclude our findings in Section 6.