## 4 Discussion  
This study reports on the development of a new custom PD-AFO for active patients with light to mild drop-foot conditions. The PD-AFO is manufactured via selective laser sintering using a composite material that, thus far, has never been used for biomedical and orthopedic applications. The present manuscript describes the full workflow, from the patient's scanning to the 3D printing, characterization of mechanical properties, and functional evaluation. An exemplary application of the full procedure for a drop-foot patient is reported. Designing a new PD-AFO should comprise the analysis and critical selection of a combination of optimal ergonomic and biomechanical parameters that can be addressed with a variety of shapes and materials. With this PD-AFO, the authors were willing to provide less severe drop-foot patients, who are not satisfied with standard solutions, with an aesthetically appealing, light, and comfortable device that can be worn within their shoes. The aesthetic aspect is also becoming increasingly important when choosing an orthotic device. The current design comprises three parts: a footplate with arch support matching the weight-bearing foot plantar surface of the subject, calf support shaped around the leg, and a posterior hole to allow easy donning and improve the comfort at the calcaneus. In order to improve the aesthetics, the AFO design aimed to be very simple to minimize the encumbrance and enhance the wearability of any shoe and clothing. While the current setup requires some steps to be climbed to reach the scanning plate, which could be difficult for patients with low mobility, scanning the patient's feet and leg proved to be easy to perform. The quality of the 3D reconstruction of the foot plantar surface in weight-bearing was previously validated using a laser scanner device as the gold standard [18]. To further improve the setup, the rotating platform supporting the Kinect sensor could be driven by an electric motor to increase the scanning repeatability and thus minimize operator variability in the scanning process. Figure 5: For the patient recruited in the study, sagittal-plane rotations (deg) of the hip, knee, and ankle joints during normalized gait cycle duration in the custom AFO, Codivilla (COD), and shed conditions. Joint angles are shown as mean bands (\(\pm\)1 SD) across five walking trials. The cross-section of the calf shell, i.e., the flexible part of the PD-AFO, was dimensioned to obtain a bending stiffness consistent with the average values reported in the literature [23]. Larger sections are associated with a more rigid calf shell, as well as with larger strains and stresses in the more external regions (under the same deformation). In order to assess the actual stiffness of the calf shell of the printed AFO, this was determined by the slope of the torque/angle curve acquired with the experimental testing setup across five flexion/extension trials. While a larger sample size is required to assess the repeatability of the present experimental setup in measuring the PD-AFO stiffness, this was a highly consistent intertrial (CV < 5%). This stiffness value is rather consistent with what reported was by [23]. While the AFO stiffness was customized based on the patient's weight, future developments should investigate the criteria for the identification of the optimal mechanical properties considering other patient-specific constraints and requirements, such as the degree of functional impairment and the level of activity. Ultimately, any orthotic device such as the current custom PD-AFO must be capable of addressing the functional deficit it was designed for. Moreover, since AFOs are usually worn several hours per day, comfort is another critical parameter that contributes to the overall evaluation. In order to objectively measure the expected improvement in walking, the present design was assessed via gait analysis in controlled laboratory conditions using a validated and widely used skin-marker-based lower limb protocol.