...the chances of your ball becoming impacted from the substrate is slim. Their digestive "juices" can handle bones with no problems...why not a little cellulose? (Cellulose is the stuff many plant fibers are made of)
Generally, most animals cannot digest cellulose (which is why things like corn come out whole and why a cow needs to regurgitate it's cud to be further chewed). Just because it can be broken down does not mean it cannot pass through the digestive system. Fiber works this way for constipation...your body cannot break down the fiber so it essentially "pushes" out the blockage (the process is more complex than that but you get the drift). This does not mean that if your snake is compacted or constipated that you should be doling out the aspen bark at meal time!
But, usually, when your snake is feeding most of the larger pieces of aspen will fall off the prey. If you are worried about them consuming any of the substrate a simple "fix" (it's not 100% effective, but it helps) is to lay down a piece of newspaper or something similar over the substrate before feeding.
BTW, bark "chips" are generally larger and coarser than shredded aspen and can lead to problems. Many use them with no problems, but laying down something for them to feed on prior to offering the prey is not a bad idea. Some people move their snake to a feeding tank but this can cause stress for the snake and can lead to an unintentional fast. (There have been debates about this in the past so I won't get into it now).
Personally, I use Craft or Packing paper. It's the same stuff that "cage liners" are made from but I buy it on a roll and cut it myself. Look in the office supply section of Kmart or Walmart. Don't bother going to the post office for it because you'll be charged an arm and a leg for much less. I think I pay about $3 for a 30 foot roll.
Okay, I've rambled enough.
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