# Development of a Lightweight Prosthetic Hand for Patients with Amputated Fingers
## 2 Design of the Prosthetic Hand  
### Design Concepts and 3D Models  
To develop a prosthetic hand for amputees who have retained their thumb and partial metacarpal, we considered the following design concepts. First, the mounting part was designed with the minimum thickness (11 mm) to include the required electrical components, for example, motor drives, a microcontroller unit (MCU), and a sensor. Then, the actuators were installed vertically between the finger modules and the mounting part, as shown in Figure 2a. This configuration allowed for the installation of actuators despite the limited available space caused by the presence of the residual metacarpal. Moreover, the arrangement between the patient's palm and the fingers of the prosthetic hand can be adjusted by modifying the length (\(L\)) of the vertical frame shown in Figure 2a. Second, the product weight was reduced by manufacturing all frames with nylon by using Multi-Jet-Fusion (MJF) 3D printing, except for the bearings, shafts, springs, and tendon-fixing frame. Small-sized parts fabricated using the MJF method offer superior strength and durability than those fabricated using other methods [14; 15]. Consequently, this 3D printing technology is suitable for fabricating parts of the prosthetic hand. Third, each finger was designed as a module. Therefore, finger modules can be attached and detached easily during the assembly process, depending on the configuration of the amputated fingers. In each finger module, driving power is produced using a direct current (DC) motor and transferred to the joint frames in the distal, middle, and proximal phalanxes through tendons, which are depicted with red and green lines in Figure 2a. The tendons are connected to the actuator by means of a tendon-fixing frame. Fourth, the prosthetic hand can be fixed on the back of the user's hand by using two dials (Figure 1b). Finally, if the mounting part is designed as a single frame, it will cause user discomfort when objects are grasped because the frame cannot produce movements similar to those of the palm. Consequently, the mounting part of the proposed hand was designed with 2-DOFs and moved passively by using leaf springs (Figure 2b). The second to the fifth concepts are focused on practicality.  
The desired weight of the prosthetic hand was set to be \(<\)160 g, which is 40% of the suitable weight of the prosthetic hand suggested in [16]. The target length and width were determined to be 173 and 79 mm, respectively, based on anthropometric dimensional data for American female hands [17]. In extension and flexion motions (natural motion), the desired velocity of the distal phalanx was \(>\)180\({}^{\circ}\)/s. Moreover, the payload at the fingertip of a finger module was computed to be \(>\)250 g, which is adequate to lift an object weighing 1 kg by using four fingers, and the product cost was \(<\)250 USD. The ranges of each phalanx in natural motion, of self-adaptive grasping by the middle and proximal phalanxes, of the adduction/abduction motions, and of each joint for palm motion are 0\({}^{\circ}\)-90\({}^{\circ}\), 0\({}^{\circ}\)-25\({}^{\circ}\)  
Figure 1: (a) Developed prosthetic hand and (b) hand with prosthesis.  
\(0^{\circ}\)-\(15^{\circ}\), and \(0^{\circ}\)-\(15^{\circ}\), respectively. Furthermore, the target grasping motions include holding objects such as paper cups, pens, balls of various sizes, tools, and smartphones. To realize grasping motion, the figure module was designed as an under-actuated mechanism with 1-DOF active and 3-DOFs passive motions. In the proposed finger module, the DIP joint is driven directly. In the case of the PIP and MCP joints, although the joint frames in Figure 2a are directly connected to the tendon-fixing frame, the motions of the PIP (1-DOF) and MCP (2-DOFs) joints are realized passively by using linear and torsional springs. The prosthetic hand based on these design concepts and mechanisms can achieve self-adaptive grasping according to object shapes, extension/flexion and adduction/abduction motions.