You can clean them like any other cart. I use the Syringe method. I got one of
these at Walmart for $2. The little cone they come with has an opening that (fairly) tightly holds a cart. I just draw clean, hot water
through it a bunch of times, then pump air through it to dry. Let it sit for a day to completely dry and it's good as new.
Works better than boiling for me as it cuts down the drying time and I don't have to deal with strong flavored juices contaminating all my carts causing me to have to boil them over and over. Just grab fresh water, nuke it for a minute, pump clean the cart, then change the water and repeat.