## 1 Introduction  
Passive Dynamic Ankle-Foot Orthoses (PD-AFOs) are assistive medical devices prescribed for individuals affected by neuro-muscular deficits of the ankle dorsiflexor muscles, usually associated with neurological conditions like cerebral palsy [1, 2, 3], stroke [4, 5, 6], and hemiplegia [7, 8]. These orthoses aim at restoring physiological ankle kinematics and dynamics by supporting the foot of the affected limb, especially in the swing phase of walking. While standard AFOs are mainly designed to support the foot and ankle in the swing phase of walking, PD-AFOs can also address impairments and deficits of the ankle plantar muscles by incorporating energy-storing and -returning mechanisms into their design. For this purpose, variable-stiffness mechanical springs or the high elastic properties of composite materials have been used to store energy and to mimic the physiological gait pattern [1; 3; 7; 9; 10; 11]. This ability to store and return energy effectively reduces the reliance on muscle activation, leading to lower energy expenditure and improved gait efficiency. According to [PERSON] et al. [12], around a 10% decrease in energy cost can be achieved by walking with a PD-AFO. [PERSON] et al. [13] reported a significant 9% lower net non-dimensional energy cost of walking in a population of children walking with either a posterior leaf spring or a solid ankle-foot orthosis. In the systematic review by [PERSON] et al. [14], four studies reported a significant reduction in total and net energy cost using different types of AFOs in people with stroke. [PERSON] et al. [15] reported an almost significant reduction in energy cost with either a posterior leaf spring or the articulated Chignon(r) orthosis during treadmill walking. A significant reduction in energy cost was also reported with a carbon-fiber AFO [16]. The mechanism behind these energy savings is attributed to the ability of PD-AFOs to optimize ankle kinematics and dynamics. PD-AFOs can effectively control ankle plantarflexion and dorsiflexion, reducing excessive ankle movements and promoting more natural gait patterns [17; 18]. PD-AFOs can also enhance balance, reduce pain, and improve overall quality of life for individuals with gait impairments. Efficient and comfortable walking result in increased participation in activities of daily living and improved social engagement [19]. It should, though, be highlighted that outcome measures of energy expenditure often differ between studies and AFOs' performance can be difficult to compare. Normalized oxygen consumption, either in mL/kg/m or mL/kg/min, normalized energy cost in J/kg/m, heart rate, and physiological cost index have all been used to quantify the energetic contribution of dynamic AFOs in different motor tasks. While the net energetic effect of dynamic AFOs is mostly calculated as the difference in energy consumption in gait between AFO and shed (no AFO) conditions, the actual energy stored and released due to the elastic deformation of the orthosis is seldom measured or reported [3]. The adoption of 3D printing technologies in the production of AFOs offers substantial advantages over traditional plaster casting and manual molding manufacturing methods. While the latter are inherently variable and time-consuming, 3D printing enables the creation of highly personalized orthoses through 3D scanning and 3D modeling tools. This digital workflow allows for precise replication of patient anatomy, leading to devices with improved fit and comfort and which can be tailored to individual patient needs [20]. Three-dimensionally printed AFOs have shown comparable or superior biomechanical and kinematic effects to conventional AFOs, including improvements in gait speed and foot plantar pressure [21; 22]. The technology offers advantages such as design optimization, reduced labor and production costs, and weight savings [23].