# Artificial-Hand Technology--Current State of Knowledge in Designing and Forecasting Changes
## 4 Actuating System
### Actuator Types
For a better understanding, human-muscle parameters are included [151; 152; 153; 154]. Another actuator powered by heat is the twisted and coiled polymer (TCP). Also called the fishing-line muscle since the same material is used. TCP muscles are fabricated by twisting fibers under a particular load and then coiling them together. Such a composite must then be annealed and trained to keep the form of the shape [117; 160]. Parameters such as a high power-to-weight ratio, elasticity, and cheap and easy fabrication make them a prospective solution in robotic applications [145]. [PERSON] and [PERSON] in [113] presented electrically powered muscles that actuated both a finger and a locking mechanism. More sophisticated control is realised by liquid flow. In [31; 161], a device was presented with TCP muscles enclosed in a silicone tube (equipped with a spring for prestrain). Cold water, 21 \({}^{\circ}\)C, and hot, 60-99 \({}^{\circ}\)C, went through the silicone channel and cooled and heated the muscle. As high as 0.5 Hz contraction frequency was available, but the problem of hydraulic inertia was exhibited, as it took 1.5 s for water to flow from source to actuator. To prevent this phenomenon, the authors in [117] increased muscle length by a pulley system, and bigger and faster displacement was possible. They also investigated coil-number impact on dynamic properties. As a result, it was observed that a muscle with three more fiber contracts has higher force, while two coils are faster; contraction time of 1-1.15 s was achieved. Finally, there are dielectric elastomers based on polymers with a high elasticity coefficient and powder electrodes on each side. Due to Maxwell stress, large deformation is generated, but it must be noted that 100-400 MV/m electric fields are used [144; 158]. The most studied material is a 3 M Company product (VHB, also used in tapes) that allows a working frequency in the range of 5-20 Hz [157]. Instead of longitudinal expansion that causes considerable strain, even up to 380% [145], lateral utility was also considered [32]. So, this type was used in [162] where a multilayer, trapezoidal DEA actuator was placed directly onto the joint. With an average 2 kV supply, a maximal force of 3.3 N was reached as well as 8% strain. To clarify, the above-described types of actuators, both classical and those from the group of artificial muscles, were described due to their direct use in artificial hands, which was the main requirement in all solutions compared in this article. Nevertheless, it should be noted that huge interest exists in the field of new actuation methods, e.g., carbon nanotubes, electroactive polymers, and stimuli-reponsive gels [144; 145; 163]. Moreover, hybrid solutions were also considered, for example, in [164], where the McKibben muscle was combined with SMA wire; actuators with two movement sources, e.g., in [34], of which the gripper is based on a DEA with a special architecture of active layers, \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Actuator** & **Stress [MPa]** & **Strain [\%]** & **Power Density [W/kg]** & **Efficiency [\%]** & **Life Time [cycles]** & **Activation Type (Value)** & **Speed [Hz]** & **Cost [5/kg]** \\ \hline human & & & & & & ATP & & \\
[144; 145; 151; 152; 153; 154] & 0.1 & 20–40 & \(>\)100 & 30 & 10\({}^{0}\) & (70–140 mM/min) & 4–20 & – \\ SMA & & & & & & & & \\
[33; 144; 145; 155; 156] & 100–400 & 2–3 & 1000 & \(<\)10 & 10\({}^{5}\) & heat (30–130 \({}^{\circ}\)C) & \(<\)3 & 1.4k–13.7k \\ DEA (VHB) & & & & & & & & \\
[32; 144; 145; 157; 158] & \(<\)7 & \(<\)380 & 400 & 60–90 & 10\({}^{6}\) & electric field & & \\ TCP & & & & & & & \\
[31; 145; 159; 160] & 19 & \(<\)49 & 27,100 & \(<\)1.32 & 1.2 \(\times\) 10\({}^{6}\) & heat (50–250 \({}^{\circ}\)C) & \(<\)7.5 & 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of artificial and human muscles. so that electroadhesion occurred and stated the main gripping force of the device.