In an effort to select the best possible medical grade of polyether ether ketone (PEEK) for active medical implants application, it was necessary to inspect surface thermodynamic properties of different grades and correlate with self-bonding (autohesion) strength. Autohesion is the process of choice for encapsulating active medical implants made of PEEK. Temperature modulated DSC was used to investigate crystallinity and thermal events of range of medical grade PEEK (semi-crystalline, amorphous, semi-crystalline with amorphous surface, minerals and black masterbatch filled). Lab shear test was used to measure autohesive forces between PEEK samples pressed at moderate temperature (200 degrees C) and pressure. Samples with higher mobile amorphous region (MAF) and lower degree of crystallinity (W-c) resulted in higher autohesivce bonding strength. However, semi crystalline PEEK with surfaces that showed high MAF and low W-c resulted in higher autohesive bonding strength when compared with crystalline samples. The presence of fillers (minerals and black masterbatch) in the crystalline structure doubled the autohesive bonding strength values of the semi-crystalline sample. The presence of large crystalline region and a larger portion of the rigid amorphous region acted negatively on autohesion strength. Semi-crystalline PEEK with a special manufacturing procedure to produce amorphous surface produced the best choice among the PEEK grades. This grade have the desired thermodynamical properties in the bulk while exhibiting best surface adhesion strength.