Development of Patient-Specific Adaptive Assistive Devices for Brachial Plexus Injury

Injury to the brachial plexus prevents the arm, wrist, and hand from communicating with the spinal cord in whole or in part. The 'patient's upper arm limb appears to be completely incapable of performing any type of independent movement. The aim of this project is to design and develop a customized adaptive assistive device for patients with brachial plexus injury and to fabricate the prototype using 3D printing technology. The development of the device involved adapting the mechanical engineering design process, including conceptual design and finite element analysis, to predict the performance of the design and to select the best printing materials. The patient's left arm was 3D scanned to create a customized part that perfectly fit the patient. The 3D model of the prototype was developed using Autodesk Fusion 360 and Autodesk TinkerCAD. Two different materials, namely Polylactic Acid (PLA) and Acrylonitrile Butadiene Styrene (ABS), were considered in the computational analysis. Results show that the maximum von Misses stress of PLA is observed at 2.464 MPa, slightly higher than the ABS material (2.451 MPa), indicating a greater stress tolerance imposed on the material's strength. However, PLA has a smaller maximum displacement than ABS, at 0.019 mm and 0.030 mm, respectively. The PLA material was chosen for 3D printing based on several considerations, including mechanical qualities, cost, printing time, durability, and data evaluation. The adaptive device for brachial plexus injury was successfully delivered to the patient and demonstrated the capability to assist in arm movement.

Adaptive assistive device; Brachial plexus injury; Finite element analysis; Patient-specific; 3D printing

        The project starts with the problem identification from the patient consultation. The concepts can be filtered by using the Pugh method. The final selection of engineering design was modelled using CAD software. A static finite element analysis was performed to predict the product performance before 3D printing fabrication.

2.1. Patient Consultation

In this stage, meeting with the patient was essential to identify and understand the problems before proceeding to generate ideas and concepts to develop a suitable device. The literature review and benchmarking were extensively explored to better understand. Thus, various ideas and concepts were successfully interpreted and generated based on different types of assistive devices (Ahmed and Al-Shammari, 2023; Scherb et al., 2023; Rashid et al., 2012). Figure 1 shows the process flow in this project. The project was initiated by patient consultation with the rehabilitation specialist to identify the problems. The process is followed by a 3D scanning process to capture digitization data of the patient for patient-specific device development. The idea and several concepts were then generated before coming out with a preliminary design. The detailed design was created and analysed computationally before proceeding to the fabrication and testing of the product. In the analysis stage, stress concentration and safety factors were part of the criteria to be considered for the pass-and-fail statement.

Figure 1 Project flow in developing the patient-specific adaptive assistive device

2.2. Conceptual Design

Several ideas to solve the problem were sketched to get better pictures of the solution. Figure 2 illustrates the proposed solution, which consists of (a) a harness and (b) an upper and lower arm holder. The purpose of the holder is to fit the patient's arm securely, preventing any movement or loosening. Additionally, a harness utilizing a customized strap has been proposed to support the fixation to the patient's body. 

2.3. CAD Design and Modeling

3D scanning technology was adapted to capture the shape of the 'patient's arm. The scanning process was conducted at the Hospital Al-Sultan Abdullah, UiTM, under observation by a rehabilitation specialist using a handheld 3D scanner machine (Shining 3D, China). Figure 3(a) shows the 3D scanning process on the left arm (affected arm) of the patient, while Figure 3(b) indicates the raw images of the scanned data. The arm model was reconstructed and smoothened using Meshmixer software to obtain a precise and identical shape of the patient's left arm. To ensure that the design seems proportionate, the arm holder must resemble the geometry of the left arm. CAD software is used to design all parts of the device. Figure 4 illustrates the final design of the arm orthosis, which consists of two parts: (a) the upper arm holder and (b) the lower arm holder. After designing the arm using Autodesk Fusion 360, the file is converted to .stl files and then imported to the AutoDesk TinkerCAD software, as shown in Figure 5. This is for designing the slot for the resistance band and straps for the harness to be tied together.

Figure 2 Sketches of the proposed assistive device, which consists of the (a) harness and (b) upper and lower holder

Figure 3 (a) 3D scanning process on the left arm of the patient with the observation by the rehabilitation specialist, and (b) raw images of 3D scanned data 

2.4. Finite Element Analysis

Computational analysis is the fundamental approach to predicting a 'product's strength, material selection, and optimized design, which is widely used in medical science and engineering design (Hamza et al., 2023; Faadhila et al., 2022; Ahmad et al., 2020, Nor-Izmin et al., 2020; Abdullah et al., 2012). A static finite element analysis was performed to predict the 'product's performance before proceeding to 3D printing fabrication. This provided optimum parameter setting and design. The Autodesk Fusion 360 software was used for the static stress analysis on both upper and lower arm holders to observe the force-loading effects in the resistance band slot area of the holder. Two different materials, namely Polylactic Acid (PLA) and Acrylonitrile Butadiene Styrene (ABS), were evaluated to predict the suitability of these materials for the device.

Figure 4 Designing the (a) upper arm holder and (b) lower arm holder using AutoDesk Fusion 360

Figure 5 Designing the slot for the resistance band and strap harness for (a) the upper arm and (b) the lower arm holder using AutoDesk TinkerCAD

Figure 6 illustrates the loading and boundary conditions conducted in this study. The boundary condition (constraints) was set on the four-hole slots on the corners for the upper arm holder and two-hole slots on the corners (labeled in red line) for the lower arm holder where the strap of the harness is tied. Constraints are applied to a model to prevent it from moving in response to applied loads. A 10N structural load/force (labeled blue arrow) was applied onto the slot of the resistance band where the weight of the upper arm and lower arm react toward each other. The blue labels indicate the loading condition, while the red labels show the constraints. The resulting von Mises stress, displacement, and safety factors were the main focus of this simulation to assess the design's validity in real-world conditions (Farah, Anderson, and Langer, 2016).

Figure 6 Loading and boundary conditions setting in the analysis

2.5. Fabrication of the Assistive Device

In the fabrication procedure, both the upper and lower arm holders were created using a 3D printer capable of working with PLA and ABS materials. The selection of these materials was based on their widespread availability and common use in the development of adaptive devices and prototypes, as highlighted in previous studies (Wahid et al., 2022; Mazlan et al., 2021). The design model was generated using Autodesk Fusion360 CAD software and then exported in stereolithography (.stl) format to the slicing software Ultimaker Cura. Subsequently, the file was converted into a g-code file for the Creality Ender-3 V2 after the slicing process. For both arm orthosis components, a thickness of 5mm and an infill density of 20% were selected for 3D printing manufacturing. These parameter settings were informed by prior research (Wahid et al., 2022; Mazlan et al., 2021; Hamzah et al., 2019) while considering printing cost and orthosis weight. Safety factors and the weight-to-performance ratio were crucial in achieving a promising product. It's worth noting that increasing infill values will raise the young modulus and product weight (Mazlan et al., 2023). Additionally, PLA filament, chosen for its ease of printing and higher stiffness compared to ABS, exhibits a tensile strength greater than 37 MPa. The upper arm holder required 12 hours for printing, while the lower arm holder took 14 hours.

3.1. Comparison between PLA and ABS Materials

The performance of both materials was compared on the resulting von Mises stress, displacement, and safety factor, as shown in Table 1 (a), (b), and (c), respectively. The maximum von Misses stress of PLA is observed to be higher in comparison to the ABS material, indicating a greater tolerance of stress imposed on the material's strength. However, PLA has a smaller maximum displacement than ABS, at 0.019 mm and 0.030 mm, respectively. The reaction forces and reaction moments are often produced by applied force actions. Structure failure can occur when reaction forces exceed action forces, resulting in fracture and corrosion. When comparing the two materials in Table 1(a), PLA had lower maximum reaction forces than ABS, indicating that PLA opposes less force from the applied force from the detachable arm orthosis, which is also influenced by gravity force. The most common approach to express a safety factor is as a ratio between a measurement of the maximum load that will not cause the stated type of failure and a comparable measurement of the maximum load that is expected to be applied. The factor of safety is the most straightforward and extensively utilized strategy in handling variability and uncertainty in engineering design (Nigro and Arch, 2023; Moreland, 2009). The application of safety considerations to ensure that a building can fulfill its intended function reduces the chance of failure to an acceptable level (Amitrano et al., 2023; Sarma et al., 2020). Both materials have the same safety factor of 15, which is relatively high and considerably above one, indicating that neither material will fail under the current conditions. As a result, PLA is superior to ABS as a suitable material for producing this arm orthosis. One of the reasons for choosing PLA is to save cost, and the failure difference between PLA and ABS does not affect the design of arm orthosis and the 'patient's comfort when using the arm orthosis.

3.2. Prototype of the Assistive Device

The prototype of the patient-specific assistive device was successfully developed and printed. Although the finite element analysis had been conducted earlier, issues of fit- to-patient and comfort are subjective and need to be tested on the patient. Therefore, the prototype of the device was essential to observe the highlighted issues. Design changes, revisions, and improvements were made to each 3D-printed prototype. The patient's feedback and observations are critical for obtaining the greatest quality 3D-printed arm orthosis. Table 2 illustrates the changes implemented as a result of patient feedback and observations during multiple attempts. Furthermore, Figure 7 depicts the fitting process with the patient, which directly contributes to the enhancement and modification of the device.

Table 1 Variation of (a) von Mises Stress, (b) Displacement, and (c) Safety factor for the assistive device at different material properties, namely PLA (left) and ABS (right)