For best performance the septa should be inert, low off-gassing of silicone oligomers, soft enough to avoid bending the needle and reseal after injection, and resistant to coring by the syringe. Pre-slit PTFE / Silicone Septum. In order to pierce easily. Diameter: 8mm to 22mm. Thickness: 1mm, 1.5mm, 3mm.