The Application of PEEK to the Packaging of Implantable Electronic Devices

摘要

PEEK is a polymer that is used in many orthopaedic implants because of its favourable mechanical properties and its biocompatibility. High-reliability electronic implants such as pacemakers have ‘hermetic’ enclosures with the electronic components in dry gas. This type of package, generally made of metal or ceramic, guarantees a very long lifetime, but is also expensive. PEEK can be easily machined or injection-moulded, it is an attractive material for implant manufacturers and it may be that by novel design based on established material properties, PEEK may be used in some applications. This thesis examines the case of PEEK as a packaging material for applications which only require a more limited lifetime (less than three years). The process of moisture ingress through polymers is analysed, and a novel calculation method to quantify it is developed. A telemetry system is designed, which allows measuring the relative humidity inside a PEEK capsule. Diagrams, PCB layouts, microcontroller program and component lists are provided, making this design easily reproducible. The lifetime of PEEK packages is investigated, as well as the contribution of the adhesive seal vs. the package walls. In order to prolong this lifetime, the use of desiccant and thin film coating is suggested. The calculation method we developed is extended to the case of the use of desiccant. This model is found accurate in predicting the time constant when silica gel is used (Type IV isotherm), but not in the case of molecular sieve (Type I isotherm), because of the type of isotherm characterising the desiccant behaviour. Thin film coating of PEEK is also investigated as a means to reduce permeability. It is found that most PVD techniques do not provide a significant improvement, due to high residual stress and the porous morphology of the films. Nevertheless, applying a coating prior to deposition proves effective in mitigating these, and improves the time constant of the package by a factor of 2.3. Atomic Layer Deposition also shows an improvement in time constant of similar magnitude. Finally, guideline graphs and tables combining calculations and experimental results are presented, providing a quick way for the implant designer to evaluate which size package is required for their application. It is found that time constants in excess of 2.5 years are achievable, using the right combination of coating and desiccant