# Overview of 3D Printed Exoskeleton Materials and Opportunities for Their AI-Based Optimization
## 4 Control of Exoskeleton  
So far, it has not been clearly decided whether 3D-printed exoskeletons require a specific type of control. It is believed that the construction of this exoskeleton can be as close as possible to the normal dimensions and weight of individual limb elements, and the control method should not be too different from the natural one (here: based on BCI). 3D printing has enabled the design and manufacture of exoskeletons as intelligent and multi-functional tools, combining additive manufacturing from various materials with intelligent control systems. Developed, e.g., a multifunctional biomimetic soft actuator with a pneumatic movement system imitating the movement of a human finger with bending from \(-180^{\circ}\) to \(180^{\circ}\) (but without reaching extremes). The proportional-integral-derivative controller allows for monitoring the position of the exoskeleton in real time by a single-axis soft deflection sensor built into the actuator [42]. Controlling the exoskeleton must take into account several aspects in the area of symmetrical and asymmetric geometry and the behavior of the biological hand. 3D printing allows for the production and prototyping of both the mechanical design itself and the entire exoskeleton, along with the control system. As part of the tests, multiple sources of feedback can be used to determine symmetric and asymmetric activities described by torque, position, trajectory, and the laws of motion [43]. An integral part of controlling the exoskeleton is the detection, interpretation, and support of the intention of human movement, within the so-called feedforward control. Most of these types of solutions are based on surface electromyography (sEMG), which requires precise placement of electrodes on the muscles. This leads to attempts to build physical interfaces for exoskeletons with integrated 3D-printed sEMG and pressure sensors. Multimodal information from flexible, conductive sensors is fed to a classifier (usually K-Nearest Neighbors--kNN), calibrated against an EMG-based unimodal classifier. This gives a very good prediction performance even with a minimal number of sEMG electrodes and without their precise placement [44]. 3D-printed exoskeletons can also be used as scaffolds to induce soft tissue and bone growth. In addition, 3D printing with bioink allows for the production of complex geometries, including blood vessels, bones, cartilage, fabrics such as skin, and in the future also whole organs and simple devices built into the patient's body, e.g., for drug delivery or controlled stimulation [45]. In exoskeleton control, not only movement control but also the reception and processing of sensory stimuli are important. A combination of 3D printing techniques using soft materials and haptic fingertip stimulation has already been demonstrated. 3D printing from soft materials (PLA) was used to create soft pneumatic haptic feedback actuators (6.8 \(\pm\) 0.23 g each with PLA attachment and Velcro), as the tactile stimulation of the fingertips is synchronized with the movement of the entire hand exoskeleton. The aforementioned haptic stimulation increases the engagement level of individuals during exoskeleton-assisted glass grasping exercises [46]. In order to combine the advantages of rigid and soft exoskeletons, hybrid exoskeletons consisting of both rigid and soft components are being built where needed. However, these require expensive multi-material 3D printers and multi-step machining processes (including casting or machining) with a limited material selection. Modified technologies such as fused deposition modeling (FDM) have recently made it possible to deposit the filament onto a heated thermoplastic base layer, which improves the bonding strength between the stiffer material and the flexible (but non-stretchable) base layer. This enables the previously unattainable advantages of locally stiff and locally soft exoskeletons with the improved fatigue strength of 3D-printed components [47].