In light of the observation that e-beam does not penetrate as deeply as gamma, is e-beam also limited by directional exposure? And can the lower penetration depth be overcome with multiple runs or higher doses?
The main factors governing the penetration depth of e-beam are the mass and charge of an electron. Gamma rays emitted by a 60Co source in a gamma sterilization process are not charged and do not have mass. As an electron interacts with the product being sterilized, it interacts with the material and transfers its energy rather quickly and is therefore not able to penetrate the product to the same degree as a gamma ray. Therefore, the depth of penetration of e-beam for a given material can only be governed by the accelerating voltage used in the e-beam process, much like the penetration of a bullet in increased by increasing its velocity.
Directional exposure concerns are typically only encountered in sterilization processes in which surface sterilization is required. This is true, for instance, in UV sterilization. Only the surfaces of the part that are in a direct line with the UV source will be sterilized. This can be mitigated with multiple sources or with product rotation. If an e-beam process has been designed and is being used to sterilize the surface of a product only (i.e. a low accelerating voltage is being used) then consideration will need to be given to ensuring that all product surfaces are exposed. However, if the e-beam process has been designed and is being used to sterilize the entire product (for instance a sterile package containing a 3D part with complex geometry and internal cavities) the directional exposure concerns are minimized, although the process may include product rotation to ensure dose uniformity. The sterilization process will still need to be validated to ensure that appropriate bio-burden reduction levels are achieved.