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In 3D printing, successive layers of material are applied under computer control in order to create a particular shape. This technology allows us to make products of a unique internal and external structure.
Should there be a here?
I wrote the text without the article, but a native speaker corrected me. Would both variations be grammatical?
"Unique" is essentially superlative. If you call something "unique", you state that nothing in the world is like the entity to which you just referred.
The indefinite article is a stand-in for "one", but the actual meaning is "one out of at least several", and they all belong to some category. In this case the category is "unique structure".
While a unique structure (or an object that is characterised by it) can have similar structures (or objects), thus allowing the use of 'a' when referring to one of them, there is an inherent conflict between admission of similarity and calling it "unique".
That is why I would omit the article altogether.
In 3D printing, successive layers of material are applied under computer control in order to create a particular shape. This technology allows us to make products of unique internal and external structure.
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.
Either way is correct grammatically, but they are subtly different. An indefinite article should be used if the referent is irrelevant, vague, or hypothetical; it is also used the first time mentioning something. In this case, the details of the "structure" don't seem well-defined or important to the sentence, and this is also presumably the first time mentioning the "structure". Therefore, I think the use of "a" is appropriate here, but not necessary.
Both are grammatical. The version without the indefinite article comes closer to your intent of one of a kind, but the form of ... [singular] may be more significant than the presence or absence of a.
The form "x of y" carries the connotation that x belongs to y or that x is a property, collection or part of y. E.g. tip of the tongue, set of cutlery, etc. The focus is on y.
In your sentence, the phrase "products of [a] unique ... structure" talks more about the structure than the product. Both forms allow the sense that every product is different, although the form with a also allows the literal sense that every product has the same structure.
While this is interesting from a linguistic and philosophical perspective, you seem to be aiming for a phrase that emphasises mass customisation over amazing artwork. In that case, you would be better off with a phrase like "products with unique ... structures". In your wider context of bioprinting, perhaps even change unique to customised or personalised.
Well you would use the "a" there defiantly, however, you could take it out if you were saying that sentence plural.
This technology allows us to make products of unique internal and external structure(s).
You could not say that sentence with an "a", because the "a" means literally just one. A and one correspond with each other. But you could say something like multiple or many.
This technology allows us to make products of multiple unique internal and external structure(s).
This technology allows us to make products of many unique internal and external structure(s).
But if you aren't using that sentence plural then defiantly use "a".
