A bioactive substrate is any substrate that recreates an ecosystem, essentially. For our purposes, it is in a closed system. Many keepers are making the switch towards it due to the natural and enriching aspects of it, though it has been around since reptiles and amphibians have been kept. This requires a basic understanding of ecology of the species you would like to keep on it. Without this knowledge, you have nothing to base the substrate on as it will vary from species to species, and keeper to keeper.
For quite a while, it was restricted to only the more tropical species because most of the time, they needed or benefited from it. It was also a lot easier because due to the humidity, feces broke down quickly with the help of bacteria, fungi, and detritivorous insects, hexapods, etc.
With this being said, reptiles, like every organism on Earth, came from the wild. This essentially means that bioactive (or naturalistic) substrates or enclosures can actually be artificially created with them in mind. You create a "self-contained ecosystem with its own inhabitants, food chain, nitrogen cycles, and climates." Essentially, you create a living thing. This will invoke animals to use their natural behaviors.
NOW A FEW FEARS THAT MANY KEEPERS CAN THINK OF.
IMPACTION
Impaction is the collection of debris in the guts. The debris can really be anything at all, be it food, substrate, etc. Impaction is what is known as a secondary illness. This means that something in your husbandry must be wrong for it to happen. Of course, there are exceptions like feeding massive prey items, but generally speaking, it's a secondary illness. The ingestion of substrate while hunting WILL NOT be enough to impact a HEALTHY animal. The key word is HEALTHY. A lot of animals in captivity, believe it or not, are not very healthy.
The metabolism and digestion of the debris depends generally on temperature and hydration. If animals are not kept at the correct temperatures, they will be able to pass them easily. However, it would also depend on the hydration level of the animal. Around inanimate or indigestible objects, the body will create a mucus coat around it as to not tear the guts but rather let the object slide through easily without causing damage. This process is very water intensive, which means that your animal needs to stay consistently hydrated all the time.
To talk about a wild animals, s/he will likely have a large fluctuation in resources because they are constantly on the move to find food and shelter. Because they do not consume multivitamins, they will often be deficient in vitamins and/or minerals. This is where they may consume the substrate to refill this. It's a natural instinct to do this. This means that our captive animals, if deficient of a mineral or vitamin MAY eat the substrate openly, in which case, you should reconsider a loose substrate until the husbandry is fixed.
“Reptiles that are provided with varied diets, supplemented with calcium, multivitamins, and have access to UVB lighting and are kept at proper temperatures should not experience impaction.”
— ZooMed
