## 1 Introduction
The restoration of thumb function is essential for a high quality of life and return to work. It is estimated that the absence of the thumb will cause a 40% loss of hand function ([PERSON] et al., 2016) and result in a 23% WPI (2018). Therefore, a functional hand rehabilitation or assistive robotic glove must be able to provide enough support for thumb motions. Unlike the other four-finger digits, the thumb has a large RoM requirement in both flexions (\(88^{\circ}\) of interphalangeal (IP) joint; \(60^{\circ}\) of MCP joint ([PERSON] et al., 2016)) and palmar abduction (\(49.4^{\circ}\)([PERSON] et al., 2016)). The thumb abduction RoM determines the largest object a hand can hold. Moreover, the primary contributor for effective handling and manipulation of small objects is thumb opposition, which involves flexion, abduction, and rotation at the carpometacarpal (CMC) joint. The rotation of the CMC joint can be omitted in a robotic glove, since it is not a voluntary movement ([PERSON] et al., 2016). Accordingly, soft thumb actuators must be capable of supporting at least independent flexion, independent abduction, and opposition (a combination of flexion and abduction). Without an effective thumb assist, it is difficult to apply soft-pneumatic actuators to hand rehabilitation exercises despite their advantages over motor-driven robotic devices. The whole-finger type in (2010) (fiber-reinforced), (2012) (fiber-reinforced), and (2013) (Pneu-net) have a specific unit for the thumb. These studies simplified thumb opposition as a coupled movement of flexion and twisting. As a result, the root part of the Pneu-net actuator was designed with oblique chamber networks, and the root part of the fiber-reinforced actuator (or the entire actuator (2012)) was coiled with fiber in a single-helix pattern. These actuators can make the thumb contacts the fingertip of the small finger, but they lack independent support in the thumb abduction direction. In [16] (Pneu-net), [18] (Pneu-net), and [23] (bellow-shaped), which designed modular/segmented type of actuators, the thumb opposition was simplified as a combination of flexion and abduction. The authors placed a mini version of their actuator transversely between the thumb and index finger's MCP joint to compel the thumb to abduct. These actuators can assist a hand to grip a large object; however, the initial length of the extra module prevents the thumb from reaching the lateral side of the index finger, restricting the thumb's abduction RoM. We previously developed a whole-finger thumb actuator [27] and a modular finger actuator (M-ACT) [28], both of which are fiber-reinforced soft actuators. The whole-finger thumb actuator has three tapering inner chambers designed to compensate for the hand's space limitation and reduce interference between adjacent chambers. The air inflation of the middle chamber supports thumb flexion, the two side chambers support abduction and adduction, respectively, and the simultaneous pressurization of the middle and left-side chambers supports thumb opposition. This three-chamber actuator (3C-ACT) can assist the fingertip of the thumb in touching nearly all the positions recommended in the Kapandji test [29], which is used to access the thumb's opposition. However, it showed chamber interference manifested as undesired flexion during left-side chamber pressurization. The M-ACT was not specifically designed for the thumb, but we can arrange multiple modules while taking space constraint and inter-chamber interference into account to achieve support in different degrees of freedom (DoFs) of the thumb's CMC joint, e.g., by attaching two modules vertically side by side. Air inflation of one of the modules would produce thumb abduction or adduction, and their simultaneous air inflation could cause joint flexion. There have been few studies that developed effective soft thumb actuators; moreover, the advantages and disadvantages of whole-finger types and modular types in thumb motion assist (multi-DoF assist) remain unclear.