Indeed “Artos” was used in the septuigint to indicate unleavened bread. In fact, we do not know what bread Jesus used at the last supper, leavened or unleavened. But in my perspective it doesn’t really matter. Are we merely repeating what Jesus did at the last supper or are we participating in the eternal supper of the Lord every Sunday?
Unleavened bread symbolises the sinlessness of Jesus (purged of leaven). But what does such a symbolism say of our salvation? Pretty much nothing. And such a symbolism is not exactly correct, after all Jesus likened the Kingdom of Heaven to leaven - if we take the above analogy to its logical conclusion we should conclude that Heaven is sinful.
Leavened bread has a much deeper symbolism which echos and preserves theology throughout the ages. Work is done to produce bread- just as we work for the Kingdom of Heaven. Leaven is invisible when in bread, yet still visible to the eyes, just like the Holy Spirit who works in us. Jesus was Crucified, went down to Hades and rose from the dead in a glorified body - Just as the leavened bread is beaten, cooked in the oven (Hades) and rises into a new form (its glorified form).
There is so much more symbolism, which I can not utter at the moment. But, merely using leavened bread says much about our salvation.
"Regarding the usage of the primitive Church, our knowledge is so scant, and the testimonies so apparently contradictory, that many theologians have pronounced the problem incapable of solution.
Certain it is that in the ninth century the use of unleavened bread had become universal and obligatory in the West, while the Greeks, desirous of emphasizing the distinction between the Jewish and the Christian Pasch, offered up leavened bread."
taken from
newadvent.org/cathen/02172a.htm
It is pretty much widely accepted that the Western Church used leavened bread up until the 9th century - regardless of whether this was widely used or not.
But regardless, I can not say the Catholic Eucharist is invalid because they use unleavened bread. I do not know whether their Eucharist is valid or not - God alone knows - because they have seperated themselves from the Orthodox Church.
God bless.