Sure it can. You can definitely apply it directly using a toothbrush, your hand, or something else. However we designed the system so that you wouldn't have to, to make it easier. The important thing is that once it is in the mouth, the dog generates some friction by chewing on something, either a toy, a bone, or a chewy treat. That friction is what helps activate the enzymes in the gel so that they are more effective than just leaving them on the teeth.