The application of PEEK to implanted electronic
devices: Lifetime of PEEK packages with silica gel desiccant
Nathaniel Dahan1, Prof. Nick Donaldson1, Dr Stephen Taylor2, Dr Nuno Sereno3
1Implanted
Devices Group, Department of Medical Physics and Bioengineering, UCL
2Biomedical Engineering, Stanmore, UCL
3Invibio Ltd.
1. Introduction
4. Discussion
O
Implanted devices that must be reliable for many years, such as
pacemakers, have packages made of glass, ceramic or metal [1]. For
short term implantation however, using these types of packages is
expensive and not necessary. It can be acceptable to have a package
made of a permeable material such as a polymer, provided calculations
and tests show that low internal relative humidity is guaranteed for the
device’s lifetime.
Let us then evaluate the type of lifetime achievable for packages
made of PEEK, a biocompatible polymer with low permeability [2],
and to what extent this lifetime may be prolonged using an
adequate type of desiccant.
Our package, which has 1mm thick top
and bottom walls, and 2mm thick side
walls, shows a lifetime in excess of 2
months. Fig. 7 and 8 show the
influence of the wall thickness and the
use of desiccant on the time constant,
for a cylindrical package of uniform
thickness. They can be used as
guidelines for achievable lifetimes.
Fig. 2. Electrical analogy
2. Materials and methods
Take the example of an ‘implant size’ cylindrical PEEK package (cavity
volume 1.6 cm3) with desiccant, as shown on Fig. 1. Water vapour can
permeate this enclosure either through the PEEK, or through the
adhesive seal. The progression of the relative humidity (RH) level
inside the cavity can be evaluated by considering each element of the
package as having a resistive and a capacitive element, corresponding
to their ability to resist the flow of water vapour and store water
respectively (see Fig. 1).
RpTOP
CpTOP
RpSEAL
CpSEAL
Air in cavity
Desiccant
CV
CD
O
Fig. 7. Influence of wall thickness and use
of desiccant (10%) on the time constant
RpSIDE
CpSIDE
Fig. 3. Humidity sensing
circuit
Fig. 1. Moisture ingress into polymer package
Using an electrical analogy (see Fig. 2), a standard model for water
diffusion through a porous wall [3] can be adapted to more complex
packages [4]. The equations for the relative humidity and the
associated time constant are:
t


(1) RH t  RH i  RH a  RH i   1  e   and (2)   RP CP  CV  CD 


The time constant τ for this enclosure were calculated using this
method. τ=390 hrs for the capsules without desiccant, and 81 days
with 6% of the cavity volume filled with silica gel desiccant. Silica gel
was selected as the best suited desiccant for this application because
of its large water adsorption capacity at high humidities (up to 33% of
its dry mass at 60% RH and 40% at 100% RH).
C
O
Fig. 4. Experimental set up
3. Experimental results
Fig. 8. Influence of wall thickness and
amount of desiccant on the time constant
Whereas the cavity volume normally
has very little influence over the lifetime
(when V increases, so does the surface
area available for moisture ingress), it
shows a much more important part
when desiccant is used. This is
because a fixed fraction of this volume
is occupied by desiccant. When filling
10% of the volume with silica gel,
the time constant can reach 7.6
months for a 3 mm thick cylindrical
enclosure with 1.5 cm3 cavity
volume. This value goes up to 13
months when using 20% desiccant,
and 18.5 months with 30%, which is
sufficient for many applications.
The PEEK packages were placed in water at 37oC (n=3) and the
n
relative humidity in the enclosure cavity was recorded over time.
Humidity sensors measure the relative humidity inside the enclosures
(see Fig. 3 & 4).
The results are shown on Fig. 5. for a capsule with and without
[1] G. Schneider, “Non-metal hermetic encapsulation of a hybrid circuit,” Microelectronics Reliability, vol. 28, no. 1,1988.
desiccant. Experimentally, we find τe=381 hrs when no desiccant is
[2] S. M. Kurtz and J. N. Devine, “PEEK biomaterials in trauma, orthopaedic, and spinal implants,” Biomaterials, vol. 28, no.
used, and 75 days with silica gel (see Fig. 5). Fig. 6. shows the water
32, pp. 4845-69, Nov. 2007.
[3] M. Tencer, “Moisture ingress into non hermetic enclosures and packages. A quasi-steady state model for diffusion and
mass gain by these capsules. When no desiccant is used, 8mg of water
attenuation of ambient humidity variations,” Electronic Components and Technology, pp. 196-209, 1994.
is absorbed. When using a desiccant, this value reaches 27mg. The
[4] N. Dahan, A. Vanhoestenberghe and N. Donaldson, “Moisture ingress into packages with walls of varying thickness
and/or properties: a simple calculation method”, currently under review
amount of water in the cavity itself is negligible (0.05mg) compared to
these values. Nevertheless, it rises much more slowly than in the PEEK
or the desiccant.
This is because the humidity level in the cavity (as measured on Fig.5.) will only rise
when it is inferior to the level in the PEEK and the desiccant. As long as these
elements can absorb and adsorb water respectively, they keep the RH level in the
cavity low. We can also understand from here that the effect of a desiccant in this
type of ‘porous’ package is less than in traditional packages, which are much less
permeable but which have walls that cannot store water.
5. References
Fig. 5. Influence of
desiccant on RH level
Fig. 6. Mass gain of capsule with and
without silica gel desiccant
Contact: ndahan@medphys.ucl.ac.uk
Acknowledgment: This work is funded by Invibio Ltd
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