@cholash My thoughts to share until Sue has a chance to answer. Unless you are making your own brush (Ha!) I would imagine the different types of brushes bought new will do...It's just a matter of finding the right size brush for the job. I use a bottle brush I found at Walmart for bottles and if I was scrubbing my 'medical tools" for Bronchiectasis I would think a toothbrush would possibly be helpful???? Of course there are the vegetable scrubbing brushes also. Hope this helps with the thoughts of brushes for the job at hand.
Barbara

@cholash I bought a set of heat resistant silicone bottle and nipple/straw brushes on Amazon. I don't see the exact ones anymore.

"Scrubbing" is not like using a scrub brush on a dirty floor, it is more like brushing over all accessible surfaces to disrupt any film that might be trying to form, so it rinses away easily.

If you want to know why it's important, put some boiled or spring water in your reusable water bottle and drink out of it for a day, then let it sit at room temperature full of water for a couple days. Pour out the water and run you finger over the inside - the chemicals in the water and the germs in your saliva will have created slime you can feel. It needs friction to wash it away.

The brushes can be sanitized in the dishwasher - I do my brush, glass or stainless steel water bottles, soaking bowl - on the sanitize cycle which is over 180F for about 30 minutes.