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The most common implants are RFID implants, or Near Field Communication implants. They are tiny bio-glass capsules with a metallic coil and memory inside. They usually have a programmable space of around 800 bytes, up to 8kB (growing).
From Wikipedia: "A human microchip implant is typically an identifying integrated circuit device or RFID transponder encased in silicate glass and implanted in the body of a human being. This type of subdermal implant usually contains a unique ID number that can be linked to information contained in an external database, such as personal identification, law enforcement, medical history, medications, allergies, and contact information."