## 2 Materials and Methods
### Prototype Development
#### 2.1.4 Manufacturing of the Device  
In Figure 9, the 3D-printed finger is shown. The subassembly contains 6 individual parts manufactured using PLA filament (Norditech Machinery S.R.I., Buteasa, Romania, EU) at a working temperature of \(t_{n}=210^{\circ}\) for the nozzle and \(t_{bp}=60^{\circ}\) for the build plate. The individual parts have an average manufacturing time of half an hour each. The nozzle used is \(\mathcal{O}=0.4\) mm in diameter with the printing speed set at \(\mathrm{v}=50\) mm/s. Although AM technology has a growing material variety, not all of them meet the criteria for use in open-source upper limb prosthetics. The material selection was carried out using a decision matrix. The contents of the matrix can be found in the Supplementary Materials. The proposed design was manufactured using a desktop 3D printer Creality Ender 3 (Creality 3D Technology Co., Ltd., Shenzhen, China). After the analysis with the multicriterial decision matrix, PLA was chosen as the manufacturing material for the proposed design. Figure 8: Overall dimensions of the prototype. Figure 7: Pretension system subassembly with view of inside geometry. To analyze the manufacturing potential of the prosthetic device, a list of the following materials was considered. The material characteristics were extracted from the datasheet of PLA [9], ASA [10], PETG [11], PETG + FC [12], PETG tungsten [13], PC + FC [14], PVB [15], F3 PA pure [16], F3 PA FC pro 15% FC [17], F3 PA FS pro 15% FS [18], F3 PA FS 30 pro 30% FS [19], PA11 FC [20], FilaFlexible 40 [21], TPU [22], Fiberflex 40D [23], Flexfill 98A [24], XT-CF 20 [25], ABS Carbon [26], ABS Kevlar [27], FKU wood [28], PVA+ [29], Bronzefill [30], Copperfill [31], Corkfill [32], Steelfill [33], BVOH [34], CPE HG 100 [35], PP FS 30% [36], PEEK [37], and HIPS [38]. The decision matrix was compiled to decide the best material for manufacturing the device. To build the matrix, data were gathered and 2 tables were created. Firstly, the list of materials of which 3D printing filaments are made of was compiled in Table S1. Secondly, properties of the materials were extracted from their datasheets, as seen in Table S1. Thirdly, weights were attached to each of the properties. Lastly, based on the highest score in the decision matrix, the material was chosen. The scores attained by each material were compared. A value of 1 to 7 is given to each material property in Table S2. The allocated score reflects the considered level of compatibility between the considered material property and the conditions set to reach the goal of manufacturing the parts of the proposed assembly design. Each property is weighted from 1 to 7, as seen in Table S2. The sum of the weights is 28. To compute the score of each material, the value is multiplied by the weight, and the sum total is given in the last column of Table S2. The finger subassembly is made up of multiple other parts besides the 3D-printed ones. The 2 adjacent parts of each phalanx assemble through a snap-fit method. At the ends of each phalanx a housing holds a miniature MR72 zz deep groove ball bearing with a \(\mathcal{O}_{out}=7\) mm outside diameter, \(\mathcal{O}_{in}=3\) mm inside diameter, and 3 mm width. The joint housing also holds a generic torsion spring responsible for keeping the fingers open. As illustrated in Figure 10, the phalanx has fasteners to prevent disassembly. There are 2 fastener systems: (1) a bolt-nut-washer arrangement and (2) a bolt-threaded insert assembly type. The rigid cable is a cable with limited elasticity present in Figure 10. This is so that it deforms minimally during the actuation. Usually, transitional prosthetics use 1 type of cable for actuation and another for the return function. The former should keep its dimensions during operation while the latter should constantly deform to accommodate the actuation and fulfil its role. Figure 9: Three-dimensionally printed finger subassembly and bearing placement. Traditionally, low-cost hand prosthetics use a rigid cable for actuation.