## 4 Current Approaches in 3D-Printed Hand Orthotics
### Dynamic Orthoses  
Dynamic orthoses employ various actuation types and joint mechanisms to facilitate hand movement [63]. Their complex mechanical structure makes them difficult to customize and don [64]. Nonetheless, several approaches have utilized 3D printing for dynamic orthoses. [PERSON] et al. [65] developed a device for the entire arm, excluding fingers, covering two degrees of freedom of the wrist joint. Motors are directly placed on the rotary joints of the orthosis, aligned with biological joints. Such an approach has several drawbacks, such as the bulkiness and hard alignment of the device. On the other hand, it applies pure torque to move a joint, making it effective and comfortable. However, in such solutions, the point of force application must be carefully chosen to not cause discomfort. A commonly used joint mechanism is the four-bar linkage [3; 66; 67; 68], which provides joint stability and allows for complex trajectories, while utilizing a single actuator, keeping the device lightweight. This mechanism is often employed for thumb opposition in grasp performance (Figure 2C). [PERSON] et al. [69] introduced 8- and 10-bar linkage mechanisms to guide the three phalanges of each finger. [PERSON] et al. [70] achieved a similar movement with their bar linkage mechanism. [PERSON] et al. [71] designed an exoskeleton using two four-bar linkage mechanisms on two fingers, coupled to a single actuator via a gear-based differential mechanism. This 3D-printed device allows each finger to move independently when encountering obstacles, reducing the weight, while effectively supporting object grasping. [PERSON] et al. [72] introduced a modular design with an exchangeable thumb mechanism. This design allows the four-bar linkage mechanism supporting opposition pinch to be replaced with a rotational bar supporting spherical grasp, by inserting the component into a slot. Both joints are actuated by a gear-adjustable spring. [PERSON] et al. [73] proposedanother modular idea: a four-bar linkage mechanism that can be coupled with a 3D-printed Pneu-Net (pneumatic actuator) (Figure 2D) to support wrist extension. This actuator uses compressed air for bi-directional bending movement and is applicable in hand orthotics. This combination achieves a complementary effect, as the soft material actuator lacks the necessary stability, which is effectively provided by the rigid four-bar mechanism. [PERSON] and [PERSON] [74; 75] developed finger exoskeletons almost entirely from these actuators, creating lightweight, soft devices that are safe and comfortable. Despite their widespread use, four-bar mechanisms and their variations have drawbacks: they are bulky, complex to design and fit, and use rigid materials that decrease comfort. Moreover, they have one degree of freedom, which, while allowing for single-point actuation, often hinders natural joint movement. Although, this feature can be desirable in some cases. Another type of joint in dynamic orthoses is based on elastic elements, such as Bowden cables or metal straps, also simply referred to as springs. These elements are not 3D-printed but are embedded into 3D-printed structures that guide their movement, such as a three-layered sliding spring mechanism (Figure 2E), which contains metal (or other elastic, yet robust) straps connected to 3D-printed segments [76; 77; 78]. Some springs are fixed to the segments, while others slide, resulting in translating linear movement into joint bending and enabling rotational motions of the finger joints. This compact design transfers a high level of force, without harming the user due to misalignment and without causing discomfort from finger joint compression. It supports bidirectional movement and is only placed on the dorsal side of the hand. [PERSON] et al. [79] introduced a similar design, but reduced the number of springs to one by implementing a multi-segment mechanism attached to a fabric glove with Velcro, mechanically limiting the maximal extension and enhancing the safety.