## 4 Current Approaches in 3D-Printed Hand Orthotics
### Dynamic Orthoses
Bowden cables can also create actuation in both flexion and extension, while restricting other directional movements less than springs. However, they create undesired axial force on the joint. [PERSON] et al. [80] developed a hand exoskeleton, with 3D-printed segments on a textile and cables sliding through them to perform finger movements. The Bowden cable connects to the fingertips, resulting in a compact, lightweight design, though its robustness and safety in a medical context are questionable. [PERSON] et al. [81] combined Bowden cables and metal straps to create a joint that does not transfer axial force to the finger joints. This is achieved with a concertina-like mechanism fixing flexible segments on the strap, creating a rotation point for the joint, while the cable controls its position by adjusting the segment proximity. The joint mechanically limits the maximal finger extension by locking the segments, enhancing safety, and it is easily mounted on the hand using magnets. However, it can only be mounted on the hand's side, activating either one or a group of fingers together. [PERSON] et al. [82] integrated Bowden cables into a glove, connecting them to elastic rings easily placed on the fingers. Similarly, in [83], the cables also ran through the dorsal and palmar sides to support both finger flexion and extension. However, these designs cover the palmar side, making it difficult to grip, and they compress the finger joints. Moreover, glove-based designs present donning challenges. Another group of joints use elastic cords or strings connected to springs or motors. These strings support longitudinal movement, while allowing freedom in other directions, which is useful against misalignment. The next four orthoses use a similar concept to the one in the sixth row of Table 1, but with some variations. [PERSON] et al. [84] used the same design concept, but they used 3D printing only for customization of static parts. [PERSON] and [PERSON] [85] added sensors to such an orthosis, incorporating a 3D-printed linear encoder to read finger positions. [PERSON] et al. [86] designed an orthosis where strings limit the maximal finger flexion, while allowing full extension, used to stretch cramped spastic fingers. The position can be adjusted over time by rotating a pulley on the forearm's dorsal part. [PERSON] et al. [87] iteratively improved dynamic orthoses by implementing purchased sensors and a wrist support. Initially, they identified inefficiencies in force transmission for high finger flexion and conceptualized devices with a bendable outrigger or direct torque transmission via a bar mechanism. However, for a higher sensor accuracy, they compromised these designs into a stiff rotational outrigger with a short string connected to thimbles. The thumb joint has an additional unactuated degree of freedom, allowing thumb articulations. Additionally, they implemented a four-bar linkage mechanism for wrist movement, supporting flexion and extension, while blocking abduction and adduction. [PERSON] et al. [88; 89] proposed designs using a twisting cable connected to a bendable beam to move all fingers together. In the first design, it is placed on the palmar side; in the second, on the dorsal side. These designs allow bi-directional actuation with a single actuator. In the second orthosis, by adjusting the cable path, the movement is sequential, starting with the MCP joint, and followed by the proximal interphalangeal (PIP) joint. There have also been attempts to place actuators directly on the joint. [PERSON] et al. [90; 91] connected hydraulic cylinders over the MCP and PIP joints, using a compliant structure on the finger sides to absorb potential shear forces. However, this approach increases the overall bulk of the device and shifts its weight to a more external point on the hand. The use of MEX printing in dynamic hand orthotics has been limited. The literature features various designs using other AM technologies, including shape memory polymers [5] and complex four-bar linkage mechanisms for finger movement [92].