## 1 Introduction  
The last several years have shown an increasing interest in the applications and clinical assessments of Additive Manufacturing's (AM) capability to produce user-customized orthotic products such as upper limb braces and casts [1; 2], prostheses [3], neck and back braces [4; 5], and foot orthoses, colloquially known as insoles [6; 7; 8]. All these medical devices are used to alleviate discomfort and pain, to support or improve the functions of limbs or other body parts, to prevent injuries, and to assist the healing process. Various foot pathologies need insoles for improvement in daily living activities; pain and functional impairment during walking, flexible and rigid flat foot, cavus foot, diabetic foot, heel spur, and plantar fascitis are among them. The main purpose of orthotic interventions is to address the too-rigid foot by using specific materials that enhance foot flexibility and shock absorption through custom biomechanical foot orthoses [9]. There is room for improvement in insole fabrication in order to obtain a product that is comfortable for the patient, easy to be worn in all kinds of shoes, as well as durable. Only a comfortable, easy-to-wear, and durable insole will be helpful in achieving patients' compliance with orthotic therapy. This is why customized insoles are expected to gradually replace conventional technologies, an evolution seen before in the orthosis field [10]. In this sense, the main benefit offered by AM technology is the ability to adapt the design of these products to fit an individual's specific body shape, size, and medical condition, in contrast to the prefabricated and off-the-shelf products [11]. In the case of 3D-printed insoles, design customization refers not only to their shape and dimensions but also to the design of various stiffness zones that can accommodate various plantar pressures. This feature proves highly beneficial for individuals with conditions like diabetes [7], where the nominal pressure and area of contact are changing continuously because of weak foot stability and progressive deterioration in midfoot joints and bones [12]. The variable stiffness is commonly obtained by infill density and pattern settings [13], but new filaments, such as varioShore TPU--thermoplastic polyurethane (Colorfabb, Bremweg, The Netherlands), also enable hardness variation through printing temperature settings [14]. During use, the insoles typically undergo a series of movements that alternate with periods of rest, the cyclic flexion movements in the forefoot area representing the primary mechanical stress. Hence, the footwear testing related to fatigue resistance is focused on this zone by using dedicated equipment [15; 16]. As far as the authors are aware, there have been no published investigations into the similar testing of 3D-printed insoles. Conducting initial tests on these medical devices will facilitate the examination of various 3D printing configurations with the goal of reducing failure rates during real-world usage and improving their reliability. A limited number of studies are available in the scientific literature that offer data on the impact of various types of elastomers or 3D printing settings on the mechanical durability of insoles, whether through real-world usage or laboratory testing. The effect of the infill pattern on insoles' hardness was investigated by [PERSON] et al. [17], but tests were performed on specimens subjected to tensile and flexural loads, rather than cyclic flexion testing. [PERSON] et al. [18] conducted a study on the compressive strength of PolyFlex TPU specimens, investigating four infill patterns and three infill densities to customize different areas of insoles based on experimental outcomes. Similarly, [PERSON] et al. [19] aimed to modify the infill gradient and contact stress through infill design in diabetic insoles to achieve the same goal. All the aforementioned studies examined the short-term use and the peak plantar pressure reduction achieved by 3D-printed insoles.