# Development of a Lightweight Prosthetic Hand for Patients with Amputated Fingers
## 1 Introduction  
For amputees who have lost their fingers or hands in accidents or because of complications, various dexterous prosthetic hands (or fingers) have been designed. Such prosthetic hands can be classified into three types according to the amputated patterns and levels of patients: Type 1 (with middle or proximal phalanx), Type 2 (with only a partial carpal), and Type 3 (with thumb and partial metacarpal) [1; 2; 3]. Prosthetic hands for patients of Type 1 have been designed as finger modules that consist of a linkage mechanism driven by the patient's body because the proximal interphalangeal (PIP) or metacarpophalangeal (MCP) joints can be moved by the patients. The linkage mechanisms used inthese prosthetic fingers (by [PERSON] et al. and by [PERSON] et al.) can achieve flexion and extension motions [4; 5]. In particular, the self-adaptive finger designed by [PERSON] et al. consists of a linkage mechanism that can be driven using body power by patients with partial proximal phalanx motion. In Type 2, actuators should be installed in prosthetic hands to produce finger motions because there are no sources of body power. Most actuators are installed in the palm of the prosthetic hand. An under-actuated five-finger prosthetic hand was developed (by [PERSON]) for grasping, and fifteen compliant joints were driven using only one ultrasonic motor [6]. [PERSON] developed a biomimetic prosthetic hand with the aim of reducing weight and cost by using two actuators: one to produce flexion/extension of the five digits and the other to generate adduction/abduction of the thumb [7]. In addition, the under-actuated tendon-driven prosthetic hand and the Bebionic hand (by [PERSON]) have been developed, and the specifications are summarized in Table 1[8; 9]. In comparison with the above two types, the development of prosthetic hands for patients of Type 3 is difficult because the available space for installing actuators is limited, and sources of body power do not exist. I-limb digits (by Touch Bionics) is a commercialized prosthetic hand for patients with a partial metacarpal [10]. The device consists of two joints in each finger and is driven by an actuator. Although it is compact and includes electrical components, it cannot perform self-adaptive grasping and adduction/abduction motions. In addition, a prosthetic finger with an actuator embedded in the proximal phalanx has been developed to eliminate the mounting part [11]. However, the finger does not have the space to install the electrical components. Additionally, diverse prosthetic hands have been developed [12; 13]. In this study, a prosthetic hand was developed to realize self-adaptive grasping, adduction/ abduction, and extension/flexion motions, as shown in Figure 1. The design concepts were focused on the development of a lightweight, practical hand for amputees of Type 3. The finger module was designed using an under-actuated mechanism with four-degree-of-freedom (4-DOFs). The three revolute joints of the finger corresponding to the distal interphalangeal (DIP), PIP, and an axis of MCP joints are coupled and driven by tendons, which transfer power produced by an actuator to each joint. Moreover, by using three-dimensional (3D) printing technology, a lightweight prosthetic hand was developed. This paper is organized as follows: The design concepts and 3D models of the suggested prosthetic hand are described and illustrated in Section 2. In Section 3, the kinematic, static, and finite element method (FEM) analyses of the proposed prosthetic hand are explained. Finally, in Section 4, the prototype hand is represented, and experiments are performed. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Developer [Ref. ]** & **Type** & **Weight (g)** & \begin{tabular}{c} **Overall Size (mm)** \\ **[Length, Width, Thickness]** \\ \end{tabular}  & \begin{tabular}{c} **Number** \\ **of joints** \\ \end{tabular}  & \begin{tabular}{c} **Number** \\ **of joints** \\ \end{tabular}  & \begin{tabular}{c} **Joint** \\ **of joints** \\ \end{tabular}  &