The evolution of relative humidity and corrosion inside active implants
Project reference: SIE_11_22
First supervisor: Anne Vanhoestenberghe
Second supervisor: Francesco Restuccia
Start date: October 2022
Active implants bring hopes of new treatment for many chronic conditions. To ensure long implanted lifetimes (years to decades), the implanted electronics must not come into direct contact with the body fluids, as that would lead to corrosion. Using hermetic implant packages ensures that the electronics operates in a dry environment.
The theory that describes the evolution of relative humidity inside a hermetic or near-hermetic package is well established. However, as we create increasingly miniaturised implants, experimental data suggests we are reaching the limits of the assumptions that underly the theory, and new models are needed for the next generations of active implants.
The PhD candidate will design instrumented implants packages, manufactured in our new implant manufacture facility (MAISI), for long-term immersion studies. The data analysis will further our understanding of the evolution of relative humidity and corrosion in micro-implants, leading to a new theoretical model to support the next generation of active implants.
The project includes the design and validation of a long-term accelerated ageing setup to test implant samples, as well as the design of 8 types of implant packages with embedded sensors (relative humidity, temperature and corrosion). The latter will be done in collaboration with the engineers at MAISI (where the samples will be produced), and following good design for manufacture practices.
The accelerated-ageing setup will hold the samples, immersed in Phosphate Buffer Saline at a controlled, elevated temperature (the acceleration factor increases with the temperature). The temperature must be stable to a couple of degrees for a period of 12 months, the intended duration of the experiment. The setup must permit the automated recording of the sensor data at regular intervals.
This dedicated accelerated ageing setup, and associated test protocol, will be produced during the PhD, and remain available for future studies, increasing the School’s capabilities. The test equipment and protocol will be designed, manufactured and validated during the first half of the PhD, ensuring enough time remains for the long-term study itself.
For the implant samples, a minimum of eight instrumented implant packages will be designed, using either of two package material (titanium and ceramic) and two internal volumes ( >2 cm 3 and <10 mm3), each achieved with two different geometries, to have two form factors for each volume. These packages will enable comparison of the impact of the free volume, the internal surface area, the length of the seal, and the material, on the variables of interest (the internal relative humidity and temperature, as well as surface corrosion). For the large packages, two drying methods will be used to create two different sets of initial conditions. In total, the long-term accelerated ageing study will include 12 sets of 15 samples. The samples will be manufactured at MAISI, and every sample will be tested for hermeticity as manufactured, before starting the long-term study. The initial condition of every sample will be recorded, as it is crucial to understand the evolution of the internal relative humidity and surface corrosion over time after immersion.
Analysing the data from such a large number of samples to derive a new model for the evolution of the relative humidity inside the packages, based on the geometry and initial conditions, requires dedicated expertise, which is provided jointly by the two supervisors.
By altering the acceleration factors, without affecting the degradation mechanisms, and careful planning of the main study, it is possible to schedule both the validation and the ageing studies within the duration of the PhD.
The outcomes will be:
New equipment for long-term accelerated ageing studies of Active Implants
An understanding of the limits of the assumptions that support the Howl and Mann equation
A new model to predict the long-term survival of next generation micro-implants in situ
Representation of the types of active implant packages that will be tested during the project.