Flexible and stretchable electronics, also known as e-skin, have been a technology to create diversified sensors and wearable devices. Implantable bioelectronics have recently been recognized as a promising research field to modulate biological signals and treat many diseases and pathological conditions. The marriage of two technologies gives us a new cutting-edge research area, i.e., implantable flexible electronics. While strain sensors, ECG sensors, pH sensors, temperature sensors and LED chips have been integrated together as a novel platform for measuring physiological signals, one of critical challenges for long-term use of such devices is a reliable power source with sound output power. To support operation of the implantable bioelectronics, triboelectric nanogenerators (TENGs) have recently been explored, as a promising technology to harvest energy, as the concept of scavenging human body energy into useful electrical power. In this work, we investigate stacked TENGs with output voltage of 160 Vp-p and a short circuit current of 6.7µA as a potential power source for neural stimulation using flexible and adjustable neural interfaces. To advance a generic design of flexible neural interfaces which is good at sciatic nerve recording and stimulation, we optimize a new flexible sling electrode and successfully achieve neural signal recording with different amplitudes and latencies. More importantly, successful selective stimulation achieved in this work proves that the flexible sling electrode is a good generic neural interface. We demonstrate direct stimulation of a sciatic nerve and a common peroneal nerve in rats by the TENGs connected with the suggested interface and a pair of Pt/Ir wires, respectively, while monitoring muscle signals. The muscle contraction can be controlled by the operation of the TENGs. This prove-concept result indicates that this technology could be the way of realizing battery-free wearable neuromodulators in the future.