An implant unit, comprising: a substrate； an implantable circuit arranged on the substrate； and an encapsulation structure disposed over at least a portion of the substrate and at least a portion of the implantable circuit, the encapsulation structure including: a first polymer layer having a first density； a second polymer layer disposed over the first polymer layer and having a density which is less than the first density； and at least one pair of modulation electrodes connected to the implantable circuit.
