Electrical stimulation using non-periodic biomimetic stimulation pattern has been shown to be effective in various critical biomedical applications. However, the existing programmable stimulators that support this protocol are non-portable and have architectures that are not translatable to wearable or implantable applications. In this work, we present a 32-channel neural stimulator system based on an implantable System-On-Chip (SoC) that addresses these technological challenges. The system is designed to be portable, powered by a single battery, wirelessly controlled, and versatile to perform concurrent multi-channel stimulation with independent arbitrary waveforms. The experimental results demonstrate multi-channel stimulation mimicking electromyography (EMG) waveforms and randomly-spaced stimulation pulses mimicking neuronal firing patterns. This compact and highly flexible prototype can support various neuromodulation researches and animal studies and serves as a precursor for the development of the next generation implantable biomimetic stimulator.