Dr. Pen Ultima M8S Replacement Cartridges - (10 Pack) 18-pin Disposable Replacement Parts Looking for replacement cartridges for your Dr. Pen M8S microneedling device? Look no further than these authentic 10 pack of 18-pin (0.25mm) bayonet slot cartridges. Remember safety comes first! Important safety features: Dr. Pen M8S Derma Stamp Pen Replacement Microneedle Cartridges are designed for single-use only. After each use, dispose of the cartridge properly. It is important to use the cartridges for personal use only and not share them with others. Avoid using the cartridges on open wounds, acne, or irritated skin.