This article presents a novel impedance matching layer (IML) placed on skin tissue to enhance the transmission ability for implantable antenna. The IML is designed with a particular dielectric constant to mitigate the reflection occurred at skin-air interface at 2.45 GHz Industrial Scientific and Medical (ISM) band. To obtain this targeted effective dielectric constant, periodic posts are inserted into a 3-D printed substrate to increase its shunt capacitance, forming an IML with a height of 17 mm for enhancement of transmission ability. Furthermore, a slow wave structure is adopted to lower the profile of IML further to 7 mm, making it more comfortable for routine wearing. To testify the property of the proposed IML, a slot implantable antenna is designed, and its gains with and without IML are comparatively studied. The simulated results exhibit a gain enhancement of over 5 dB with IML loaded, providing a promising solution for a typical communication link for implantable application.