Neither FHIR Core nor US Core clearly define "implantable device" without ambiguity. All existing definitions assume the reader/user already understands the meaning of both "implantable" device, "patient-use" device, what might (by exclusion) be considered a non-patient-use and non-implantable device.  To assist with clarity perhaps these three references may add more specification for users to understand how to use various resources:

Implantable device:

• a device that is intended to be placed in a surgically or naturally formed cavity of the human body…for a continuous period of 30 days or more ----- US Food and Drug Administration Center for Devices and Radiological Health. Home Use Devices. https://www.fda.gov/medical-devices/home-health-and-consumer-devices/home-use-devices. Accessed 13 March 2020.
• any active medical device which is intended to be totally or partially introduced, surgically or medically, into the human body or by medical intervention into a natural orifice, and which is intended to remain after the procedure ---- BSI Group. Active implantable medical devices: https://www.bsigroup.com/en-US/medical-devices/Technologies/Active-Implantable-Medical-Devices/. Accessed 13 March 2020.

Patient-use device:

• Patient-use, or home use devices, intended for users in any environment outside of a professional healthcare facility. This includes devices intended for use in both professional healthcare facilities and homes — US Food and Drug Administration Center for Devices and Radiological Health. Home Use Devices. https://www.fda.gov/medical-devices/home-health-and-consumer-devices/home-use-devices. Accessed 13 March 2020.

Greater clarity in definitions is essential.  Trying to navigate through the various FHIR device-related resources is highly problematic without clear definitions (Device, DeviceRequest, DeviceUseStatement, DeviceDefinition, and when to differentiate between DeviceRequest and ServiceRequest). AND US Core inherits the definition of implantable device from FHIR Core leading the user into endless circular loops to find there really is no clear definition.