## 3 General Classification of Modern Haptic Interfaces
### Nerve Stimulation-Based Haptic Interfaces  
Electrical nerve stimulation, sometimes known as \"electrotactile feedback,\" is one of the most promising techniques for delivering efficient haptic feedback, along with mechanical forces and heat transfer. The direct application of conductive electrodes to the skin causes an electric current to flow through that body area, particularly the hands or fingertips, stimulating the local nerves under the skin to create a haptic sensation. Since it requires no energy conversion and is simple, the term \"electrotactile\" is frequently used in haptic feedback research. Direct electrical stimulation is the most successful and frequently reported method of using electricity for stimulation. This topic will be the main focus of this section, even though there are other ways to use electricity for stimulation (such as electro-vibration and electrostatic tactile display). Electrotactile feedback is typically in a frequency range from 1 to 800 Hz. The particular mechanoreceptors would be stimulated to a great extent by the electrical input frequency. For instance, [PERSON] et al. developed a stimulation technique that used a 200 Hz pulsed current to activate Merkel receptors and induce pressure feeling. The Meissner corpuscles, which have a sensitivity range of 20-70 Hz, were stimulated by a current with a frequency below 100 Hz, leading to a perception of low-frequency vibration [43]. The solicitation frequency is not the unique parameter in stimulating specific mechanoreceptors; indeed, the direction of the current flow plays an important role. A tingling, itch, vibration, buzz, touch, pressure, pinch, or pinch can all be qualitatively defined as the result of electrostimulation. At skin-electrode interfaces, frequency has an impact on impedance as well. Increasing the current intensity, the resistive component rapidly drops, producing erratic feedback strength. Current tuning is an approach that has been extensively used to address this issue [44]. In 2018, [PERSON] et al. presented a new model that captures the relation between electrical characteristics and the strength of electrotactile sensation. Additionally, they presented an automated controller that regulated parameters following the values above, thereby maintaining the same feeling intensity even during exercise or when electrodes peeled off (Figure 7) [45]. The main parameters are peak resistance (R\({}_{\text{p}}\)), peak pulse energy (E\({}_{\text{p}}\)), and phase charge (Q). It was possible to calculate R\({}_{\text{p}}\) from the measured peak voltage V\({}_{\text{p}}\) supposing known current intensity I:  
\[\text{R}_{\text{p}}=\frac{\text{V}_{\text{p}}}{\text{I}} \tag{1}\]  
Additionally, the Q charge for a monophasic input waveform with current I and duration T is as follows:  
\[\text{Q}=\int_{0}^{\text{T}}\text{Idt}=\text{IT} \tag{2}\]  
E\({}_{\text{p}}\) at the same condition:  
\[\text{E}_{\text{p}}\text{=R}_{\text{p}}\int_{0}^{\text{T}}\text{I}^{2}\text{ dt}=\text{I}^{2}\text{R}_{\text{p}}\text{T} \tag{3}\]  
In their testing, participants had to alter I and T to achieve the same feeling intensity under various electrode peeling-off circumstances (impedance changes). The results suggest that E\({}_{\text{p}}\) vs. R\({}_{\text{p}}\) exhibits an almost perfect linear proportional trend, considering various slopes for the different feeling intensities of each subject. Figure 7: Operating principle of neural stimulation-based tactile interfacing system. Schematic depicting the mode in which electric current excites the nerve to produce electro-tactile sensations (**a**) using pulsed signals (**b**); an example of prosthesis based on neural stimulation tactile where feedback intensity is steadily controlled when an amputated user pounds a nail (**c**) [45]. To accomplish electrotactile stimulation in the human body, a pair of electrodes is required since a current flow demands a closed circuit. Nano-sized metallic contacts of gold, silver, platinum, or stainless steel are the most popular.