I see it this way: There are two requirements for making a DF
1- Manufacturing a fruit with powers. so far only fake Zoans have been created.
2- Applying a "Spirit" (Devil) or essence into the fruit in order to make it function.
ll
ll
ll====> Fake DF is created
–--------------------------------------------
After posting my Spirit theory, I remembered that I made a post about the Wavelengths of DF some months ago on another forum. Here it is:
_the idea of DF being effected by the Grandline and its crazy magnetic field is possible. Remember when Chopper discusses the Rumble Balls? He says that after some years of study he was able to create a pill (Rumble) that effects the "wavelengths" of the DF. The word wavelength is key…. if its something like energy (DF wavelengths) than its possible.
The basic wavelength is described as a wave of light, or to be more precised a "photon". This is then attributed energy based on the length of the wavelength.
short equals high energy (gamma rays and ultra violet)
long wavelengths equal low energy (Radio waves and Microwaves)
By inventing and developing the Rumble Ball Chopper has been able to artificially alter the wavelengths of his DF.
!
This would also explain the properties of the ocean and its effect on DF users. It was stated in the manga that the DF are rejected by the sea and are rendered powerless. From there it was explained that Sea-stone has properties similar to the sea…. in the manga it was stated that the Sea-stone (Kairoseki) emits a "similar energy" like the sea.
Similar Energy = Wavelength
If its possible to recognize the wavelength that is emit by the sea (possibly Vegapunk creating a method of making Sea-stones, since it was mentioned that it was developed by GL scientists iirc) then one could nullify the properties of the DF at will aka Kairouseki.
Conversely, if there is a way to eliminate the wavelength to nullify the properties of the DFs then the opposite (expanding the properties) is also possible. This is where Chopper is shown to be a true genius, he inadvertently created a similar concept of DF wavelength manipulation when he invented the Rumble Ball.
In short, if the DFs properties are based on wavelengths.... then the Grandline and its magnetic properties (thus by extension and negatively related, the sea as well) are irrevocably linked to the properties of the DFs._
I believe the Wave portion may be the criteria to make the DF "shell"….. and the "Spirit" may be the second requirement in order to make the Fake DF function as a true (still Fake) DF.