## 1 Introduction  
Research activities in the field of wearable robots have expanded rapidly over the past 10 years, providing remarkable advancements for rehabilitative medicine, assistive living, and similar industries [1]. For people suffering paraplegia caused by spinal-cord injury (SCI), gait training using a wearable robot goes beyond simple walking rehabilitation. It has additional therapeutic effects, such as improving blood circulation, increasing bone density, and easing digestive function [2]. It is also known to significantly lower mortality and improve life expectancy [3]. In the field of rehabilitative medicine, studies have been actively conducted to evaluate the safety, efficacy, and benefits of wearable robot technology for such patients [4, 5, 6, 7]. Most research has focused on improvements to machine performance [8]. Although the wearability of such devices is an important factor, developments have not considered this aspect from the user perspective [9]. Moreover, the physical human-robot interface (pHRI) for the wearable robot appliance has been overlooked in related studies [10]. A wearable robot comprises a driving unit, an exoskeleton frame, and a pHRI that connects the human body and the robot. The force generated by the driving unit is transmitted to the pHRI, dispersed through the part contacting the human body, and transmitted to the body [11]. The gait-assistance performance of a wearable robot dependson how effectively the force generated by the robot is transmitted in this fashion [12]. A recent study reported that a force transmission loss of up to 50% can occur, owing to the relative movement between the wearable robot and the human body, which is exacerbated by improper customization [13]. From a biomechanical perspective, the lower extremities must have enough strength to support the body weight [14]. Accordingly, in addition to the local dysfunction of the parts in question, problems occurring in the lower extremities may lead to other problems that negatively affect the upper body (e.g., pelvis and spine). Considering the biomechanical mechanism of the lower extremities and the potential loss of force generated by the robot driving unit during transmission, the pHRI should be configured to ensure integrated connectivity between the human body and the wearable robot to ensure appropriate contact area for improved comfort and safety [15]. Because people with paraplegia are more susceptible to skin lesions and bedscores, this interaction is a crucial factor [16]. Excessive pressure from pHRI can lead to skin ulcers [17], and improper physical contact often causes pain and discomfort [18; 19]. Therefore, it is important to maintain a safe contact pressure applied to the human body via the pHRI so that damage and discomfort can be minimized [20]. Based on this information, it is clear that a wearable robot for people with paraplegia should be manufactured according to the shape of the body of the individual wearer. When examining extant pHRI applications, there have been cases where off-the-shelf orthopedic pHRIs were applied to reduce costs [21; 22; 23]. However, most were indeed custom-made [10]. The most traditional customization methods have been manually intensive, and include plaster casting and polyurethane foaming. Recently, with the development of three-dimensional (3D) printing manufacturing technology, a new method of customization based on body shape has been introduced. However, such approaches involve direct modeling, which is quite laborious. Direct-modeling customization requires remodeling the baseline template according to the shape of the 3D-scanned body surface, which exhibits low productivity and poor efficiency across multiple patients. In this respect, to increase the usefulness of customization to the body shape via 3D printing, a new method is needed to realize customization without template remodeling. Therefore, this study proposes a new 3 DP pHRI based on a sizing system to facilitate customization via 3D printing.