Praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
In principle, washing the wudu` organs (the face, the arms, the head and the feet) is an obligation because Allah the Almighty says {What means}: "O ye who believe! when ye prepare for prayer, wash your faces, and your hands (and arms) to the elbows; Rub your heads (with water); and (wash) your feet to the ankles." {Al-Ma`idah, 6}. In addition, it is lawful for a Muslim who has a broken or a wounded organ to wipe on the cast during wudu` or Ghusl (Ritual bath). Narrated Jabir bin 'Abdullah (RAA): concerning the man who had a head injury, and then he made Ghusl and died: Allah's Messenger Peace be upon him said, "It would have been enough for him to perform Tayammum and wrap it with something and wipe over the wrapping and wash the rest of his body." [Reported by Abu Da'ud].
As for the diabetes skin patch that gives speed-reading of glucose level, it takes the same ruling of the cast. In other words, it is lawful to wipe on it especially if the person with diabetes is likely to have a fall in blood sugar (Hypoglycemia) causing, God forbid, sudden fainting and endangering their life. In such case, these patches are a necessity because preserving life is among the higher objectives of Sharia. In this regard, the Sharia maxim states, "Anything fulfilling an obligation is an obligation." Thus, speed-reading of the glucose level enables the diabetic person to take the right course of action while being allowed to wipe on the patch during obligatory Ghusl.
Scholars have permitted using the cast or bandage in non-deadly health conditions. Not only this, but Maliki jurists even permitted placing a piece of paper-which has writings (Verses of Quran or supplications) on it-on one`s head to relieve headache, and it is permissible to wipe over such paper while making ablution or Ghusl. For further details, kindly check the book entitled {Al-Shareh al-Kabeer, V.1:163} by Imam Dardeer.
As regards the conditions that don`t require immediate care to treat fall or increase in glucose levels, having this skin patch isn`t a valid excuse for wiping over it, so it should be removed when making wudu` or Ghusl. And Allah the Almighty knows best.