## 1 Introduction  
The sensorimotor system plays a central role in childhood development. A growing body of research highlights how the acquisition of motor skills influences a broad range of developmental domains, extending well beyond motor function alone. Among the various motor abilities that emerge in early life, object manipulation is considered a key milestone, as it significantly enhances infants' interaction with--and understanding of--the external environment [1]. However, the development of these skills can be substantially hindered by several pathological conditions that affect hand mobility [2]. These include neurological disorders, such as cerebral palsy (affecting approximately 1 in 500 live births), pediatric stroke (1.2 to 13 cases per 100,000 children annually), and Duchenne Muscular Dystrophy (approximately 1 in 5000 live male births), as well as traumatic injuries, which affect an estimated 691 per 100,000 children each year [3]. Such impairments have been consistently associated with reduced autonomy in Activities of Daily Living (ADLs), limiting independence and quality of life [2, 3, 4, 5]. Recently, pediatric exoskeletons have gained attention for their potential to assist children with physical impairments, improve rehabilitation outcomes, and promote independence in daily activities [6]. A significant challenge in the design and fabrication of these devices lies in the widespread adoption of the one-size-fits-all approach, which often neglects the anatomical variability of individual users [7]. Even if this strategy can reduce manufacturing complexity and costs, it severely limits the applicability and effectiveness of the devices across the heterogeneous target population [6; 8]. This limitation is particularly critical in the pediatric context, where hand morphology evolves significantly throughout growth. During the first 18 years of life, the hand--like the rest of the musculoskeletal system--undergoes progressive changes in bone structure, tendon alignment, muscular development, and joint configuration. An atlas of radiographs documenting this evolution in both male and female subjects from birth to adulthood is provided in [9]. Although individual variability exists, the most notable developmental changes occur in the phalanges rather than in the carpal bones [10]. In early childhood (0-7 years), carpal ossification progresses and phalangeal epiphyses begin to appear. Between ages 8 and 18, these epiphyses gradually fuse with their respective diaphyes, leading to the completion of skeletal maturation.  
This study addresses the challenge of the custom design of a pediatric hand exoskeleton. It focuses on the design, development, and preliminary testing of a finger module tailored to the anatomical features of the final user. To test the proposed procedure, we used data collected from a 12-year-old volunteer and developed a module for the index finger. This choice allowed us to test the module that demands the greatest customization effort in terms of geometry, structural properties, and material selection. The primary goal of the device under development is to assist patients with mild to moderate spasticity (Modified Ashworth Scale \(\leq\) 2) in performing fingers flexion movements during ADLs. Our work aims to offer insights on the interaction between design strategies, material properties, and additive manufacturing (AM) techniques, thereby contributing to the advancement of more effective, personalized, and accessible pediatric assistive technologies.