## 4 Current Approaches in 3D-Printed Hand Orthotics
### Static Orthoses  
Many MEX-printed splints take advantage of the ability to print complex shapes, integrating cut-outs and lattice structures for ventilation, weight reduction, and increased aesthetic appeal [46; 47]. Typically, these orthoses are printed vertically, for time efficiency and a better surface quality compared to horizontal printing, as well as being self-supporting during the process [48]. However, this comes at the cost of robustness, due to the weaker build direction [48]. The MEX process is relatively slow, and the surface exhibits visible layer lines that may require post-processing to meet standards for skin-contact applications [49]. The time efficiency of the process is especially compromised in simple and small orthoses, such as the one in the third row of Table 1, used for finger immobilization [50]. However, for more complex orthoses, even if the manufacturing process takes several hours [46], it can still offer advantages [3; 49]. One approach to overcoming some of MEX technology's limitations, first proposed by [PERSON] et al. [50] and further investigated by [PERSON] et al. [51], involves printing the orthosis flat using low-temperature thermoplastics, similarly to materials used in commercial orthoses, and then thermoforming it on the user's hand (Figure 2A). [PERSON] et al. [52] further developed this idea by proposing a method to produce an origami-shaped orthosis that can be printed flat in its unfolded form and then folded, gaining robustness. Figure 2: Different approaches used in the design of 3D-printed hand orthoses: (**A**) printing orthosis flat with ventilation patter, heating it up and shaping on the hand; (**B**) spiral design of the orthosis that immobilizes joints; (**C**) four-bar linkage mechanism utilized to support object grasping using one actuator; (**D**) Pneu-net mechanism presented in two positions—before and after inflating the compressed air; (**E**) a three-layered sliding spring mechanism presented in two positions. Three-dimensional-printed orthoses are not only limited to rigid structures, commonly utilizing materials like polylactic acid (PLA) or acrylonitrile butadiene styrene (ABS) [2], but can also utilize soft and flexible materials, such as thermoplastic polyurethane (TPU) [2], to omit fasteners and increase the comfort of the device, without the need for very precise modeling and providing sufficient support to the limb [53]. For higher comfort, [PERSON] et al. [54] introduced dual-material orthoses, consisting of a rigid and robust outer shell, with soft padding. However, this approach requires a dual extrusion printer and good bonding between materials, as well as longer printing times, due to material switching, [PERSON] et al. [55] used the multi-material concept to produce a more comfortable design, easing the donning and doffing of the orthosis and maintaining cleanliness by eliminating Velcro straps and replacing them with a flexible section on the side of the splint, similarly to in the comparable solution introduced by [PERSON] et al. [56]. However, this orthosis was produced using two different methods of 3D printing, and the parts were glued together, hindering its potential implementation. To overcome some of the challenges of multi-material printing, [PERSON] et al. [57] proposed using a flexible material with variable mechanical properties to deliver an orthosis that offers enough immobilization, while being comfortable; with the whole process using a simple printer with one nozzle, thereby saving time and simplifying the process. Research has also focused on adding versatile functionalities to 3D-printed orthoses. [PERSON] et al. [58] proposed an orthosis with changeable elements, fixed by a magnet, to support different ADL. [PERSON] et al. [59] integrated sixteen multi-axial soft load-sensing nodes into a thumb splint to measure the forces between the hand and the splint in ADL. To measure these forces, [PERSON] et al. [60] developed a 3D-printed soft sensor with a fish-scale structure, which they placed under the orthosis.