From: Rabbi Akiva Eiger
To: Unknown recipient
Rabbi Akiva Eiger affirms the ruling of the Avodas HaGershuni that parchment designated for a Sefer Torah may not be used for tefillin without a prior stipulation, since in matters of guf ha-kedushah hazmanah milsa hi, and this prohibition extends not only to lowering a sacred item to secular use but even to lowering it to a lesser sanctity. He resolves an apparent contradiction between Menachos 34b and Sanhedrin by demonstrating that combining R. Yishmael's view on idolatry with the position that arifah creates the prohibition yields a scenario where even in guf ha-kedushah the dispute over hazmanah remains genuine.
Source: Shu"t Rabbi Akiva Eiger §3