We never want you to feel disappointed when using your iO2. Our products undergo many quality checks to ensure they reach you in perfect condition, providing the ultimate teeth-cleaning experience. The iO Series 2 is designed with openings on the brush heads to allow water to flush through the brush head tube and allow air inside. This venting ensures the brush head dries sufficiently after use and also reduces moisture inside the tube. What you have described can occur if the brush is stored in a location exposed to mold, bacteria, or if the plastic comes into contact with ingredients of a corrosive nature. Certain ingredients used in those types of products can make the plastic porous, allowing dirt and moisture to find places to settle. With proper cleaning, as you've mentioned, you've done, this isn't what we'd expect. Your satisfaction means the world to us, and we'd like to learn more about your experience with your iO2. When you have a chance, please give us a call at 1-800-566-7252, 9 am-6 pm EST, Monday-Friday.