The design of a fully 3D printed easy-to-assemble and highly functional body-powered hand prosthesis for low-resource countries

Abstract

Background: The majority of the amputated population is living in low-resource countries, but only a small minority has access to rehabilitating services. The accessibility of upper limb prostheses in these countries is poor due to limited technical and medical resources. 3D printing is regarded as a promising manufacturing alternative in contrast to the expensive and labour-intense conventional processes. However, the majority of the developed 3D-printed prostheses show weak functionality characteristics and do not meet basic user requirements. The goal of this research is, therefore, to design and evaluate a new body-powered prosthetic device which is fabricated with a 3D printer and easy to assemble, capable of performing high force-transmissions with an appropriate pinch force output, and meets the basic user demands. Methods: An analysis of the widely used conventional prostheses and available 3D printed prostheses has been performed, and as a result, design requirements have been set up. Based on these requirements a prototype has been designed using SOLIDWORKS, and is printed with a FDM printer. Mechanical characteristics have been measured using a test bench, and the functional performance has been evaluated by ten able-bodied subjects using the SHAP and the BBT. These results were finally compared to results of conventional prostheses, as found in the literature. Results: The final prototype of the 3D-Sanhand is presented; the 3D printed Simple to Assemble Natural-looking hand prosthesis. This prosthetic device has a one-degree-of-freedom rotating thumb and is, except for a pressure spring and an elastic rubber band, completely printed with a FDM 3D
printer. The device is capable of performing a 15 N pinch with an actuation force of 24 N, which is lower than the required actuation for conventional prostheses and within the boundaries of fatigue-free use. It has a low mass of 142 gram which is a reduction of over 50% in comparison to the commercially available designs. The opening width of 75 mm is in line with conventional prostheses. The sliding mechanism in the rotation point enables the users to change the initial position of the thumb and to reduce the opening width, to allow all users to pick up objects from different sizes. User tests showed that the prosthesis was capable of picking up a variety of different object shapes and did not encounter difficulties with heavy weights. The main problems were subjected to the smoothness of the finger tips and the impossibility to pick up thin objects from the surface. Conclusion: This thesis describes the design of the first 3D printed hand that can be assembled without any technical and medical knowledge or complex tools and machinery. The hand performs a pinch force high enough to perform activities of daily living under comfortable operational forces, and that is comparable to conventional prostheses on the market. Future research should further investigate what the wear effects of 3D printed materials are on the long-term use, and should propose directions to increase the durability and efficiency of the design.