The working principle of TENG driven ionic patch is based on the coupling effect of contact electrification and electrostatic induction, which can be traced back to the Maxwell equations [
27]. Here, the mechanism of TENG during walking was schematically described (
Fig. 4A). In the beginning, the leather layer and PDMS layer carry equal charges with opposite polarity at contacted stage. While walking, the leather layer will separate from the PDMS layer, the balance will be broken. Since the PDMS layer will induce ionic movement. Thus, some positive charges will be produced on the surface of GelNCs and their amount will depend on the separating distance. When the gap is further widened, more positive charges will be generated on the surface of GelNCs. Contrarily, when the gap becomes narrow, the positive charges will become less and less until electrical equilibrium. The vertical contact–separation mode dominates the electric field as TENG, which enhances keratinocyte–fibroblast proliferation and thus accelerates wound healing [
23,
28]. Besides, the TENG field can regulate metal ions release rate and penetration depth from the conducting GelNCs, which was conducive to display the cocktail effect. Further, the simulated electric potential distribution is displayed in
Fig. 4B. When the separation distance increases, the hydrogel always carries the positive charges, which will drive the positively charged ions to the impaired skin and guide ions movement. To further test the devices' bendable performances, electrical output of the device with different angles was determined. As shown in
Fig. 4C (left), it is found that the output voltage has tended to reduce with the increase of bending angles. However, it keeps a stable electrical output at the same angle with different time, which indicated that the TENG patch can work well. Moreover, the influence of different Eggshell@CuFe
2O
4 NCs concentrations on the output voltage of TENG patch is also analyzed. As shown in
Fig. 4C (right), the average output voltage value of TENG patch without nanocomposites is a little smaller than that of adding different concentrations Eggshell@CuFe
2O
4 NCs, and the curves of the output voltage for different nanocomposites concentrations almost keep rather stable with different time, which imply that the nanocomposites concentrations do not significantly influence the performance of the device. In clinics, patients with diabetic foot often undergo intermittent claudication due to ulcers, pain,
etc. So, the output voltage will be reduced along the decrease of contact area between foot and TENG patch when walking. Therefore, it is necessary to simulate the output of open circuit voltage with different degrees of intermittent claudication. As shown in V-time curve (
Fig. 4D), curve Ⅰ simulates the voltage output during the walking process of early diabetic foot patients. In about 4 s, the output voltage of the TENG patch ranges from 44.87 mV to 67.19 mV, with an average value about 54.11 mV. Further simulation of the symptoms of diabetic foot patients aggravated to a certain extent is shown in curve Ⅱ. Compared with curve Ⅰ, only 4 pulse voltages are generated within 4 s due to the slower walking speed of the patient. It takes about 6 s to generate voltage of the same frequency (6 pulses), and the period time is extended. And, the voltage of the TENG patch ranges from 53.59 mV to 69.51 mV, with an average value about 60.87 mV. As shown in curve Ⅲ, when the symptoms of diabetic foot are further aggravated, the walking speed of patients is further decreased, and the duration of a single pulse is prolonged in curve Ⅲ. And the foot contact area was greatly decreased due to the aggravation of ulcer pain. Within 6 s, the voltage of TENG patch is only about 30 mV. Compared with curve Ⅱ, the voltage of TENG patch in curve Ⅲ decreases by half. The above data show that the TENG patch has a certain level of voltage generation, which can drive therapeutic ions to promote wound healing.