# Upper Limb Bionic Orthoses: General Overview and Forecasting Changes
## 3 Mechanical Construction  
Progenitors of the modern bionic orthosis were used to improve human muscle force. They had a simple and non-autonomous control system, while the majority of current robotic technology use brain-computer interfaces [42; 43; 44; 45; 46; 47; 48]. Primitive exoskeletons were unnecessarily heavy, with their weight reaching over 700 kg [49]. It was caused by the weight of materials and drives used in their first prototypes. The basic construction material used to build bionic orthoses in the middle of the 20 th century was steel (with breaking tensile stress-to-density ratio of 75), which was because of its strength properties [50]. For the same reason, in combination with old type massive actuators, these devices had enormous sizes. Therefore considerations of mobile solutions had to be abandoned [7]. In the last applications we can distinguish several types of motion assistance: active devices, passive device, haptic device, coaching device, active exercise and passive exercise [51]. The first construction is recently one of the most developed due to its multipurpose, light weight and price competitive potential [52]. The main features of mechanical design of such orthosis are the actuating and transmission system as well as applied materials (Figure 2). Same as in prosthetics, weight, mobility and comfort play a significant role [53; 54]. However, prosthetic devices offer an additional inner space which allows placing actuators and transmission mechanisms in device contours [55; 56; 57]. Thus, the choice of the actuating unit may affect the entire apparatus. Most frequently, the elbow joint is the one which is driven directly with a motor installed on lateral side of the arm. This solution, as well as rigid transmission (e.g., n-bar linkage, gear) requires a special attention to compatibility of joint rotation axes and bionic orthosis axes. Currently, several types of mechanisms are used for this purpose, including: direct matching of joint centres, linkage for remote centre of rotation, redundant linkage structure, and serial linkage attached to distal segment [58]. Incorrect matching of rotation axes can lead to exoskeleton damage or limb injury [15; 59; 60]. In opposition to these solutions that are very bulky or actuators cannot be placed directly in joint axis [61; 62]. It is possible to use flexible transmission. The best example of such appliance is a tendon driven system. In the simplest way, appropriate parts of the body are connected to motors via a Bowden cable, thus making them mostly unnoticeable and providing frictionless movement [63; 64]. On the other hand, the mechanism imposes an antagonistic control thereby redoubles the number of actuators. It is possible to avoid this phenomenon with pulley mechanism application, but simultaneously, as with rigid solutions, construction gets more complex and enlarged [65; 66]. Moreover, underactuation must be taken into account. Aspirations to build lighter active orthosis, which would be useful in everyday life turned engineers' attention, in the aspect of applied materials, to alloys. Similar strength properties,  
Figure 2: Components of a bionic orthosis - adapted with permission from [PERSON], 2020 [67]. significantly lower weight, and higher breaking tensile stress-to-density ratio (162) than steel were the reason why aluminium is often used in modern bionic orthoses [68; 69]. In general, materials used to create an exoskeleton are: bakelite, surgical stainless steel, aluminium alloys (copper, magnesium, or manganese), acrylonitrile butadiene styrene (ABS), polylactide (PLA), or shape memory alloys (SMA) [44; 68; 70; 71; 72; 73; 74; 75]. A compilation and comparison of properties of the aforementioned materials can be found in Table 1.  
It is also common to produce some parts using carbon fibre to make the orthosis lighter and more durable. Only a couple of years ago there were no rehabilitation devices fully manufactured of carbon fibre because of its poor ability to resist transverse loads [77].