# Hand Exoskeleton--Development of Own Concept
## 2 State-of-the-Art in the Area of Hand Exoskeletons
### Specificity of Hand Exoskeletons Manufactured Using 3D Printing  
Parametric modelling and the production approach [18; 19] ensure that the dimensions and features of the hand exoskeleton are matched to the dimensions of a given hand, the type and degree of deficit, and the individual goals of the rehabilitation process. Real-time control combined with 3D printing technology translates into a better fit of the exoskeleton to the hand while providing a methodology for the universal use of an active exoskeleton to support hand movements [17]. Moreover, 3D printing provides cheaper and faster iterations for the development of exoskeleton prototypes of consistent quality. For the reasons mentioned above, more and more 3D-printed hand exoskeletons appear in the literature and support hand function or improve the effectiveness of the rehabilitation of people after a stroke or who are affected by other causes of hand motor deficits. They are limited by the earliness of the prototype and the small number of users tested (less than 10). In such a situation, the effectiveness of the hand exoskeleton may be jeopardised by a non-optimal combination of printing parameters and material properties or by the lack of refinement in the methodology of selecting the exoskeleton for the patient's needs and therapeutic goals. The 3D-printed soft exoskeleton of the hand HEXOES (hand exoskeleton with embedded synergies) has ensured maximum flexion angles in the metacarpophalangeal and proximal interphalangeal joints and the maximum angular velocity [20]. In a study by [PERSON] et al., a 3D-printed hand exoskeleton improved Fugl-Meyer and Box and Block tests, and increased electromyographic (EMG) activation of the extensor muscles was observed while wearing the exoskeleton. The 3D printing design uses new antibacterial polymers that can prevent skin infections during rehabilitation [21]. [PERSON] et al. designed a 3D-printed hand exoskeleton controlled by EMG signals to aid the gripping function in patients with cervical spinal cord injury (including quadriplegia). The subjects' hand function improved, as reflected in the Toronto Rehabilitation Institute Hand Function Test (TRI-HFT); the participants gained immediate functionality when eating after wearing the braces, and most participants were satisfied with the exoskeleton, indicating that it is intuitive and easy to use. No side effects of using the exoskeleton were observed [22]. The computational optimisation of the 3D printing process, as regards the characteristics and selection of the material in terms of the maximum tensile strength of the hand exoskeleton element, can be implemented based on the optimisation of the artificial neural network (ANN) supported by genetic algorithms (GAs) [23]. This can play a key role in personalising and enhancing the performance and safety of a medical device, such as the hand exoskeleton, and meeting the limitations of patient-specific solutions [23].In order to increase the effectiveness of the rehabilitation process, it is possible to combine the tactile stimulation of the fingertips with hand rehabilitation supported by an exoskeleton. An exercise with a glass handle (150 g, 300 g) showed a higher level of attention and involvement of users with additional tactile stimulation compared to the use of an exoskeleton alone, as well as with a greater weight of the glass [24]. The rehabilitative hand exoskeleton (HERO) combines 3D printing (including actuators) and textiles, as well as electroencephalographic (EEG) signal control, to recover finger extension and flexion movements. The torque of the DC motors has been converted into a linear force transmitted by the Bowden tendons for passive finger movement. With an exoskeleton weight of approx. 100 g, the participant was able to control the exoskeleton with a classification accuracy of 91.5% [25].