The membranes are layers of thin skin that are sensitive to dryness.
How to make this sentence shorter?
How to avoid two "are" in the sentence?
Is it possible to rephrase it?
An adjective which is followed by a complement does not require a relative clause: it can be placed directly after the noun phrase it modifies:
layers of thin skin that are sensitive to dryness
layers of thin skin sensitive to dryness
Some linguists call this a reduced relative clause, as if it were a relative clause from which the relativizer (that/wh-) and BE have been deleted; but I think that is an unnecessarily complex analysis.