In sensor nodes, there arises a need to store the master secret key safely. Which is the most perfect location to store the key when the sensor nodes (such as implanted body sensor node) are highly resource-constrained ones?
Generally the "master key" is different from derived device keys. You do not store the master key in the devices. Where to store the device key is dependent, but generally you'd store it in flash / EEPROM (or whatever persistent but writeable storage there is).
If possible the storage is protected and / or encrypted. This is perfectly possible, it's commonly used with smart cards. However, it does come at a cost and it depends on the nodes if such cost is economic. When it comes to implants, I guess you don't expect an adversary to remove it from the body first, so I presume some attacks are not applicable.