# Upper Limb Bionic Orthoses: General Overview and Forecasting Changes
## 4 Actuators Overview
The new generation of actuators, based on phase transformation materials and activated by different stimuli, is constantly developed in order to obtain units that are more efficient and have smaller dimensions. \begin{table}
\begin{tabular}{l c c c c c} \hline  & **Power Density** & **Torque (Nim)** & **Dimensions** & **Weight (g)** & **References** \\  & **(W/kg)** & & **(mm)** & & \\ \hline Micro Servos & 110 & 0.109 & \(21.6\times 11.2\times 19.1\) & 9.1 & [105] \\ Expert & Electronics SL260 & & & & \\ Coreless motor & 675 & 0.728 & \(\diameter 22\times 51.3\) & 6 & [105] \\ MicroMo & & & & & \\
2224-012 SR & & & & & \\ Artificial muscle & 46 & - & \(260\times 30\times 30\) & 136 & [106] \\ Festo & & & & & \\ Brushless motor & 125 & 9.8 & \(60\times 59\times 56\) & 80 & [107] \\ Maxon Motors & & & & & \\ Pololu Micro & 914 & 0.89 & \(10\times 12\times 29.5\) & 10.5 & [108] \\ Metal Gearmotor & & & & & \\ Pololu Micro & 136 & 1.57 & \(\diameter 25\times 68\) & 106 & [108] \\ Metal Gearmotor & & & & & \\ \hline \end{tabular}
\end{table} 
Table 2: Characteristics of conventional bionic orthosis drives. Figure 3: Types of bionic orthosis drives. These will definitely revolutionize robotics, including orthotics; however, most of them are in early design stages. The most popular materials, which use a change in crystal lattice as the source of movement, are shape memory materials (SMA). They are activated with a temperature field, e.g., when SMA is formed to shape under an external load (martensite) and when it is heated, it will recover to original shape (austenite). Such a working principle offers silent work, small weight, high power density, long lifetime (Table 3) and the possibility to program a shape of the material, where it is commonly used in shape of a thin wire or a spring [54]. Nevertheless, shape memory alloys exhibit some serious problems, where relatively slow work and high bandwidth, associated with low thermal conductivity, are the main ones [71; 109]. Whilst it may be used for a slow movement rehabilitation, a high hysteresis makes it difficult to control. Polymeric plastics and natural elastomers with elastic deformation capabilities are also used as materials for bionic orthoses drives. These materials increase the performance of conventional drives, such as pneumatic and hydraulic drives [110]. Polymers of this type possess high electrostatic features (material elongation in one direction under the applied voltage) [71]. There are also dielectric elastomers, which change their shape in two directions upon applied voltage [111]. A feature that currently disqualifies this solution is high voltage, expressed in kV, required to activate actuator [45; 112; 113]. Another material for smart actuators is carbon nanotubes (CNT). The CNT find their application in many areas of technology and medicine [114]. In bionic orthoses, their use was proposed in the early 2000s as material for artificial muscles [71]. However, satisfactory parameters of CNT actuators were recorded only in the tests published by [PERSON] et al. in 2016. The study created carbon nanotube polymer actuators containing metal chalcogenides: boron nitride (\(BN\)), tungsten disulphide (\(WS_{2}\)), and molybdenum disulphide (\(MoS_{2}\)). These supplements have a positive effect on elasticity, and increased strain and blocking force. CNT have good strength properties, especially the tensile strength is extremely impressive. Intelligent fluids are also used in the production of bionic orthosis drives. [PERSON] et al. [115] used electrotherapeutic fluid as an actuator in an upper limb rehabilitation robot. This material changes its dynamic viscosity under the influence of an electric field. Magnetorheological fluid (MRF) is a substance composed of ferromagnetic particles, which change their dynamic viscosity under the influence of a magnetic field.