Question: Can we use plasters (band aids) on Shabbos? Do they need to be prepared beforehand?
Answer: The Shulchan Aruch (OC 328:1) writes that one mustn’t perform any act of healing for one who is healthy but slightly uncomfortable (mechush) on Shabbos. Nonetheless, as plasters primarily serve to protect the wound from becoming infected, one may apply plasters to cuts on Shabbos (See Shemiras Shabbos Kehilchasa 34:3).
The Shulchan Aruch (OC 340:14) writes that sticking is a tolda of tofer, sewing. Thus, R’ Eliezer Waldenberg (Tzitz Eliezer 8:15:14:6) writes that ideally one should stick the plaster onto one’s body, rather than stick the ends over each other, thereby creating a weaker attachment.
The Rema (OC 317:3) cites a machlokes as to whether one may undo temporary stitching on Shabbos or not (See Mishna Berura 317:21). Accordingly, there is a machlokes as to whether one may remove the plastic tabs from plasters on Shabbos.
R’ Yitzchak Yaakov Weiss (Minchas Yitzchak 5:39:2; 9:41) writes that one would only be allowed to undo threads or removing something sticky that was tied or stuck on for very temporary use. As tabs on plasters aren’t so temporary, one mustn’t remove them on Shabbos (See Baer Moshe 1:36).
Nonetheless, R’ Shlomo Zalman Auerbach (Shulchan Shlomo 328:45; Shemiras Shabbos Kehilchasa 9:n55) held that the halacha follows the lenient opinion, and one may undo or unstick something that is not supposed to be tied or stuck long-term. Thus, the tabs may be removed on Shabbos. Likewise, R’ Benzion Abba Shaul (2:36:15), R’ Ovadia Yosef (Yechave Daas 6:24) and R’ Pesach Eliyahu Falk (Machazeh Eliyahu 1:70:1) explain that although the tabs were placed on the plasters during production and may have been there for a while, that does not preclude them from being considered temporary, and one may remove them (See Az Nidberu 7:34; 35).
In conclusion, R’ Eliezer Waldenberg (Tzitz Eliezer 16:6:5) writes that it is ideal to remove the tabs and replace them before Shabbos. Either way, most poskim allow one to open and apply a plaster on Shabbos.