There is not a single unique internal and external structure common to all 3-D printed products. The point is that each product can have its own structure, distinct from that of any other product printed by the same technique or even on the same machine. For example, a heart valve printed for you need not have the same structure as a heart valve printed for me.
I would say that Van Gogh's paintings have a unique style -- that's one style, specific to Van Gogh but shared by his paintings. Since a single style applies to several paintings (but only one artist) the indefinite article is appropriate.
In the context of your report, the indefinite article is not appropriate. You want to use the word "structure" in its uncountable sense to show that the uniqueness applies to each product, and not merely to the technology which produces them.
With the indefinite article, it seems that all 3-D printed products share a common structure which is distinct from the structures produced by any other technology, in much the same way as Van Gogh's paintings share a common style which is distinct from the styles used by any other artist.
Both structures are grammatical, but only one carries the meaning that you intend.