Have a look at panels on devices like ECG. Usually these don't even have real buttons exposed. A common approach here is to make the whole device as smooth as possible so there are no not-cleanable holes / ridges. For example displays will be actually behind transparent part of the case, not sticking out / in a hole. Same with buttons - all of the actual switches will be behind pillow-like buttons built into the case.
Edit: also when considering industry / medical devices, keep in mind that most touchscreen are not meant for the conditions these devices are in. You should be able to splash water / gel / oil / ... over it and just wipe it afterwards. They should also survive people who think they just need to press it harder if it responds slowly. Simple buttons (with extra protective cover) are more likely to survive this. Next time you're around in the hospital, check out how many of the button labels are completely worn down.
It should also be noted that a common practice in medical environments is to cover any control panels with a static-cling plastic sheet (usually light blue color, I imagine everyone is using the same brand) that can be quickly replaced between patients. So make your legends large and high-contrast.
See http://g02.a.alicdn.com/kf/HTB1fNYoHVXXXXbkXFXXq6xXFXXXt/Fre...
Edit: also when considering industry / medical devices, keep in mind that most touchscreen are not meant for the conditions these devices are in. You should be able to splash water / gel / oil / ... over it and just wipe it afterwards. They should also survive people who think they just need to press it harder if it responds slowly. Simple buttons (with extra protective cover) are more likely to survive this. Next time you're around in the hospital, check out how many of the button labels are completely worn down.