Silicone Encapsulation and the Effect of Primer, Dye and
Width of Gap between Conductors, on performance.
By Shixin Bickerton
(Orthopaedic Science iBSc)
2
ABSTRACT
Background and Aims: An instrumented tibial nailed could allow the earlier detection of
complications, such as delayed unions. It could also allow a well tailored rehabilitation
regime, which could decrease the likelihood of poor outcomes and improve the quality and
speed of recovery. The aim was to test 4 possible silicone rubber candidates for the soft
encapsulation of strain gauges on an instrumented nail. Attempts were made to assess the
effect of rubber type, primer, black pigment, and width of gap between conductors, on
leakage currents.
Methods: Eight thin film strain gauges and similar thin film structures with different width
gaps between conductors, were encapsulated in 4 types of silicone, some of which were
primed or included a black pigment. These were tested under electrical bias for leakage
currents, over 6 days in water.
Results: The use of primer decreased the ability of the MED-6015 silicone encapsulation to
protect against leakage currents (p = 0.007). The effect of the black dye, width of gap and
rubber type, were not statistically significant, but corroborated with observed macroscopic
deterioration.
Summary and Conclusions: MED6-161 Primer appears to be detrimental to the encapsulant
MED-6015 silicone rubber. Increasing the width of gap between conductors may increase the
resilience against current leakage.
Tests for the effect of dye and rubber type were
inconclusive, but all the rubbers showed promise and more research is needed.
Shixin Bickerton SN: 707229
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CONTENTS
Page
Abstract
2
Abbreviations
4
Introduction
5
Method
15
Results
25
Discussion
34
Conclusions
45
Appendix
46
References
47
Shixin Bickerton SN: 707229
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ABBREVIATIONS
ADC =
Analogue to Digital Converted
CB1
=
First Test Interface Circuit Board
CB2
=
Second Test Interface Circuit Board
Ch
=
Channel for Data Collection
FDA
=
Food and Drug Administration
IPA
=
Isopropyl Alcohol (Isopropanol)
IM
=
Intramedullary
PGA =
Programmable Gain Amplifier
PTFE =
polytetrafluoroethylene
RS232 =
Recommended Standard 232
SEM =
Scanning Electron Microscope
SMD =
Strain Measurement Devices Ltd.
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INTRODUCTION
Context
An instrumented tibial nail has been proposed with the ability to monitor fracture healing, by
using strain gauges to assess loading on the nail. A silicone encapsulant is intended for use to
protect the gauges from the destructive effects of body fluids. The encapsulation must be
durable enough to survive for up to 9 months after implantation. The strain gauges will be
powered periodically, for up to 20 minutes of activity at a time, with the total duration of data
collection unlikely to exceed 90 hours over the 9 months.
Primer is often used to improve adhesion between silicone rubber and other materials. It may
also be advantageous to add black dye to the soft encapsulation. The dye would prevent
daylight from temporarily stopping the instrumented implant from working, thus allowing
surgeons to check that the implant is functional before it is implanted.
The nail will employ thin film strain gauges. Unlike foil gauges, these are not subject to
creep, due to the molecular bonding of the gauge to the backing material which eliminates the
need for an adhesive layer. They are also capable of higher resistances. It is possible for the
strain gauges on the instrumented nail to have different width gaps between adjacent terminal
conducting parts.
A larger gap may improve the performance of the encapsulation by
providing a larger area to which the insulation can adhere, as well as reducing the electric
field which the encapsulant must insulate to prevent currents leakage.
Tibial Fracture Epidemiology
40% of all long bone open fractures occur in the lower limbs, with the tibial diaphysis being
most common (Howard M & Court Brown C M 1997). These tend to be more severe than
upper limb fractures, due to increased soft tissue damage and more frequently associated
musculoskeletal injuries. For these reasons, the fixation of tibial fractures is both a common
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and prominent concern in orthopaedics. Although most tibial fractures heal within 24 weeks
(Ferguson M et al. 2008), they have the highest non-union rate of all the long bones and a
high rate of delayed union or infection. This predisposition is due to the tibia’s anatomy, with
little soft tissue coverage to protect against contamination and infection (den Outer A J et al.
1990).
Different Fracture Fixation Methods
Fixation methods include plates and screws, reamed and unreamed intramedullary (IM) nails
and external fixation. Plates are considered by some to be contraindicated in severe open
fractures, due to extensive devitalisation caused by disruption of the periosteal blood supply
(Gregory P & Sanders R 1995). Compared to external fixation, reamed nailing requires fewer
re-operations (Bhandari M et al. 2001). IM nailing is an effective treatment of diaphyseal and
metaphyseal fractures and may also be used when a simple fracture extends into the
articulation (Nork S E et al. 2005). Reaming increases disruption to the endosteal blood
supply, but evidence suggests it reduces the risk of non-union and implant failure (Bhandari
M et al. 2000). It also reduced the risk of screws breaking (Blachut P A et al. 1997).
IM Nail History and Rationale
Many modern nailing techniques owe their origins to Gerhardt Küntscher, who established
that “elastic nailing” using a long, metal, tight-fitting intramedullary nail, was a reproducible
method of fixating long bone fractures (Küntscher G 1940; Küntscher G & Maatz R 1945).
The principles of its use were reported (Allen W C et al. 1968), helping to make it a well
recognised and widely accepted method of fracture fixation.
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IM nailing re-aligns the limb and fracture fragments, yet still allows load transfer across the
fracture site during functional activities. It prevents gross movements of the fracture, thus
preventing further soft tissue damage and helps decrease the spread of contamination. In
comparison to conventional external treatments, such as casting, it permits earlier limb
mobilisation, enabling patients to return to normal activities sooner. IM nailing is recognised
as a standard treatment for lower limb, diaphyseal long-bone fractures(Zenios H, Malik M H
A, & Al-Mesri A R 2004).
Different Methods of Monitoring Fracture Healing
The complex biomechanics of the musculoskeletal system make it difficult for analytical
models to accurately predict forces in bones and joints. Many direct in vivo measurements,
have shown previous analytical models to produce too high a value (Brand R et al. 1994).
Models are often based on estimations and thus need validation, whereas direct measurements
from implants are essentially valid for the site at which they are taken.
Radiographs and three or four point bending tests are commonly used to assess fracture
healing. Many other alternative methods of monitoring fracture healing exist, including:
strain gauges on instrumented implants or external fixators; the measurement of vibrations
modes along a fractured long bone (which is compared to the unfractured bone on the
contralateral side); and assessing the velocity of ultrasound across the fracture (again in
comparison to the contralateral side) (Lowet G et al. 1993; Nishimura N 1984).
Why use Stain Gauges to Monitor Fracture Healing?
The use of strain gauges on implants can provide instant and ongoing monitoring. It allows
earlier detection and therefore earlier treatment of delayed unions, mal-unions or non-unions,
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in comparison to commonly used alternatives, such as radiographs, which are usually taken
periodically with long periods between each reading. Strain gauge telemetry systems can use
conveniently portable equipment and do not carry the risks associated with X-radiation
exposure. Unlike assessing vibrations or the velocity of ultrasound through bones and soft
tissues, strain gauges do not require a corresponding healthy bone for comparison.
Data from the instrumented implant can also be used to improve post-operative management
of the fracture, by tailoring the load-bearing rehabilitation therapy as healing progresses. This
helps to maximise the effectiveness and minimise the duration of the recovery.
Instrumented Implants in Orthopaedics
The first instrumented implant reported, was a hip hemi-arthroplasty (Rydell N W 1966).
Strain gauges on the inner surface of a hollowed neck, were used to ascertain the axial force
and moments. The gauges were wired to a subcutaneous connector, later exposed by incision.
The first reported clinical use of an implant with telemetric data transmission was a hip
prosthesis with a battery powered transmitter buried in the patient’s subcutaneous fat, used
for 42 days (English T A & Kilvington M 1979). In vivo loads telemetered from nail plates at
the hip (Brown R H, Burstein A H, & Frankel V H 1982) produced clinically significant
results in three of five implants.
Attempts were also made to measure strain along the stem of implants (Barlow J W et al.
1984). Here, the strain gauges were protected only with a layer of silicone rubber. Unsure of
the encapsulation’s efficacy, these were not used in vivo. More commonly, studies used
gauges placed in a cavity, either welded shut or using a combination of silicone, Teflon™ and
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epoxy to create a hermetic seal. Longer term instrumentation was developed, with several
studies on the force and temperature of hip implants in 7 patients, for a duration of up to 10
years (Bergmann G et al. 1988).
Two femoral prostheses were implanted to study the
relationship between stress distribution and implant fixation over time (Taylor S J G et al.
1997; Taylor S J G & Walker P S 2001).
Recent Progress in Instrumented Implants
The progression of electronics has enabled higher performance and more complex circuits to
be made with increased efficiency (low power consumption), in an ever decreasing volume of
space.
Improved integrated circuit technology was one factor at the forefront of
developments in modern telemetric implants. However, there has not been a corresponding
increase in the longevity of implants encapsulated in soft materials where very low leakage is
a necessity, and most implants designed for long term monitoring in vivo are welded. This
was not an option here, principally due to cost, as the current study is a precursor for a
commercial system.
A telemetrised femoral nail was used successfully in 1 human subject, to provide data on load
changes during fracture healing, from 2 to 26 weeks post-operatively (Schneider E et al.
2001). A hermetically sealed cavity protected the electronic circuits from body fluids. The
loading in the implant, was found to decrease by about 50% as the fracture healed. In
addition, the medial–lateral force, anterior–posterior force, and moments about all three of
these decreased progressively, as the fracture healed.
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However, a study in sheep found no clear correlation between implant strain and fracture
healing (Wilson D J et al. 2009). Telemetrised femoral nails were used to treat midshaft
osteotomies and static strain measurements were taken during leg stance in 11 sheep, for 12
weeks.
Although 8 sheep showed evidence of fracture healing (by plain radiographs,
microcomputed tomography and histology), only 2 showed decreased loading on the implant.
However, a correlation may have been masked by other variables, such as the sheep putting
less weight on the injured leg during the early stages of healing.
Encapsulation of Electronic devices for Surgical Implantation
The primary issue of any implantable (hermetic or nonhermetic) encapsulation is to prevent
body fluid ingress damaging the device. In 1973, it was first shown that hermetically sealed
cavities may be unnecessary, if soft encapsulation alone could provide sufficient protection
(Donaldson P E K 1973). Two mechanisms for protecting devices against moisture were
established; an “impermeable envelope,” such as a hermetic packages or a silicon nitride
passivating layer, or a “conformal layer,” such as silicone rubber, resin, or tacky wax
encapsulation (Donaldson P E K 1976). With implanted devices, the packaging has to be
small, biocompatible, protect the electronics from water and ions (largely sodium) and often
provide mechanical protection.
Current packaging often uses one of three techniques: soft encapsulation with a polymeric
material; an outer case made of glass or ceramic with epoxy or metal solder sealing; or
metallic flat-packs sealed with resistive electron or laser beam welding, or solder sealing
(GülerN F & Übeyl E D 2002).
Why Use Soft Encapsulation?
Soft encapsulation can provide a viable alternative in implants that lack the space to enclose a
cavity, or cannot be hollowed without compromising structural integrity. Since the 1070’s,
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Nonhermetic conformal encapsulants have been applied in many different fields and have
shown that they can be effective (Lin A W & Wong C P 1992). However, where small
leakage currents are to be avoided, the encapsulant must be reliable and predictable.
Silicone Rubber
Silicone rubbers are synthetic polymers with alternating atoms of oxygen and silicon (with or
without organic groups attached). They are convenient to apply as 1 or 2 part adhesives, and
have a range of material and mechanical properties which make them very suitable for
encapsulating electronic assemblies in vivo. There is a growing range of FDA (Food and
Drug Administration) unrestricted grade rubbers for long term implantation. They also have
advantages in cost, weight and volume, in comparison to welded cavities. The use of silicone
resins to encapsulate electronic devices was described by C. P. Wong in two papers. Its
electrical performance and correlation to the cure temperature and time were shown (Wong C
P 1989) and further explanation was given with regards to understanding its use (Wong C P,
Segelken J M, & Balde J W 1989). The properties of silicones make them well suited for the
packaging of electronic devices and can improve their reliability, performance and longevity
(Vanlathem E & Oellers E 2001).
They can encapsulate electronic devices such as integrated circuit devices and hybrid
integrated circuits, due to their thermal stability, as well as their dielectric, mechanical and
chemical properties (Lin A W & Wong C P 1992). Good compliance of the cured rubber
prevents failure due to large interfacial stresses tearing the rubber from the component parts.
Silicones have a low order of toxicity to human health and the environment(Joint Assessment
of Commodity Chemicals 1994) and do not start to peel, crack, harden or become brittle over
time. They are relatively easy to use, containing no flammable or toxic solvents and not
requiring any special handling precautions (Mollie J P 1999).
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Most silicones are very permeable to water vapour (Robb W L 1968), but can adhere well to
hydrophilic surfaces, thus preventing water from adsorbing onto the surface and condensing
to form a conduction path for leakage (Ko W H & Spear T M 1983).
Only highly purified polymers function reliably as encapsulants, because mobile ionic
contaminants affect the electrical reliability of encapsulated devices(Smith A L 1974; Wong
C P 1988; Wong C P & Looby-Calvert M M 1993).
With electric fields and ionic
contamination, moisture can cause galvanic and electrolytic corrosion, associated with
electro-oxidation and metal migration (Wong C P 1988). Studies have shown that some
silicones are viable encapsulants, with the ability to resist moisture ingress and mobile ion
permeation (Wu J et al. 2000).
Silicones have a low Young’s modulus which can help absorb stresses caused by different
coefficients of thermal expansion (such as with silicone on a titanium implant). However, this
means the encapsulation may not provide sufficient mechanical protection. If necessary, a
harder material can be placed over the silicone to provide additional mechanical stability to
withstand, for example, the forceful insertion of an implant.
Implantable soft encapsulations have been studied for a number of purposes, such as the
implantation of neural microelectrodes. Here, microelectrodes extending from the edge of a
MEMS chip into the brain, prevent hermetic sealing. Flexible encapsulations using silicone
and parylene is acknowledged by some to be a key component for future of neural prostheses
(Stieglitz T, Schuettler M, & Koch K P 2005). In other studies, a composite of nylon and
silicone-gel meshes were considered to most successfully meet the requirements for
nonhermetic encapsulation (Jackson N et al. 2009).
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Silicones have been used effectively for short term potting, encapsulation, and sealing of
microelectronics (Ko W H & Spear T M 1983). Good results with long term use have also
been produced, enabling an implanted radio receiver to work effectively after 11 years, with
predictions of efficacy for up to 30 years (Donaldson P E K 1989; Donaldson P E K 1991).
Recent studies have added additional evidence in support of these early findings.
Investigation Introduction
Aim
This investigation intends to establish the potential of the following candidate silicone
rubbers: MED-6015, MED4-4220, MED3-4013 and EPM-2420, to effectively encapsulate
strain gauges under water for extended periods. The first two were chosen for their design as
low viscosity, biocompatible, encapsulant materials, MED3-4013 was chosen for its
biocompatibility and superior adhesion, and although EPM-2420 was not medically approved,
it was investigated for purely scientific purposes, as its properties such as very low viscosity
made it an otherwise promising candidate. To be effective, all encapsulants had to adhere
sufficiently without voids or deterioration, to protect against water ingress which could cause
leakage currents and corrosion. The effects of dye, primer and the width of the gap between
adjacent conductors should also be established.
My investigation will be comprised of 2 parts; first, preliminary testing of the data collection
system using Med-6015 encapsulated strain gauges on titanium beams (Substrates1), and
secondly (the primary investigation) testing 4 silicone rubbers (MED6015, MED4-4220,
MED3-4014 and EPM-2420) and the effect of primer (MED6-161), dye (MED-4900-2
Pigment) and the width between conductors, on the effectiveness of encapsulation. The
primary investigation will use purpose made samples supplied by Strain Measurement
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Devices Ltd (SMD) (Substrates2). Follow up investigations will be used to assess any voids
or failures of adhesion between the rubber and the substrates.
Hypotheses for Preliminary Work
1) The data collection system will remain stable and not drift over time. This will be shown
by the strain gauges which have not failed and the controls (fixed resistors), maintaining a
relatively constant level of current over 7 days under water.
2) Some of the thin film gauges will exhibit leakage currents when immersed in water for 7
days, due to loss of adhesion between the silicone rubber and the gauge. Loss of adhesion
will be shown using an Ink test.
Hypotheses for Primary Investigation
1) The level of current leakage will be inversely proportional to the size of the gap between
conductors. Gaps of 10, 20, 40, 80 and 160µm on similar thin film structures (substrates2)
will exhibit progressively better immunity to leakage currents, when tested for 6 days under
bias, immersed in water. This will be due to the greater path required for moisture ingress.
2) The use of MED6-161 primer will decrease current leakage, when tested by the same
method, due to improved adhesion between the silicone and the surface of the substrate.
3) The addition of 2% MED-4900-2 pigment (black dye) to MED-6015 silicone rubber, will
not significantly affect the level of leakage currents, when tested by the same method.
4) Different types of silicone rubbers will perform differently, with different levels of leakage
currents, again when tested by the same method.
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METHOD
Preliminary Work: Equipment Stability Tests
The purpose of this was to test for drift in the measuring equipment over time, to ensure that
any observed drift was due to the test samples and not the instrumentation. It also enabled the
encapsulation method to be practised, in order to assess its weaknesses and make amendments
for the primary investigation. The stability tests were carried out on 8 thin film strain gauges
with 10µm gaps between the adjacent terminal conductor parts. These gauges were situated
on two titanium beams (4 on each) and encapsulated in MED-6015 silicone rubber.
Primary Investigation: The Effect of Silicone Type, Primer, Dye and Gap-width, on leakage
The encapsulation may deteriorate with or without bias, but to be suitable for the intended
purpose, any leakage must not be significant enough to cause failure of the strain gauge
within 9 months after implantation. However, time constraints prevent intermittent testing of
the samples over 9 months, to simulate clinical usage. To test the encapsulation, the current
was measured every 5 seconds for 6 days (144 hours), which exceeds the total time the
instrumented implant will experience under electrical bias. The current levels were assessed
across different width gaps between conductors.
Substrates2 consisted of 25x25x1mm polished titanium squares, made by SMD, using the
same materials and methods employed to make the thin film strain gauges to be used on the
instrumented nail. A layer of glass ceramic (electrical insulator), then nichrome (conductor)
was sputtered onto the titanium. On each square, a laser cut the outlines of a 5 rectangles and
a strain gauge through the nichrome, exposing the ceramic beneath. The rectangles were cut
using 1, 2, 4, 8 and 16 passes of the laser, to create cuts of 10, 20, 40, 80 and 160µm.
Exposed triple wire bonds connect the nichrome to soldered pads (lead-free solder 430-20S),
which were connected to coloured external wires (500mm 32awg PTFE wires).
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From left to right, the blue wire was connected to the ‘background’ nichrome (to be
grounded), the red, orange, yellow, pink and black wires were connected to the centre of each
a.
1cm
b.
rectangle; and two white wires
(one joined to ground) were
connected to either end of a
strain gauge (Figure 1),
Figure 1 – a) Photograph of Substrate2, b) diagram of Substrate2.
Section 1: Method For Preliminary work on Substrates1
1.1) Making the Mould
A mould was constructed from 2 pieces of Teflon® (PTFE (polytetrafluoroethylene)), to
ensure the silicone encased the substrates completely. Two parallel 80x15mm holes were cut
through the upper piece, and 4mm into the lower piece using a milling machine (Figure2). A
120x12mm indent was then cut over each of the 2 existing holes, on the upper surface of the
lower piece, to a depth of 1.1mm, to accommodate
the ends of the beam. The substrates were placed in
the mould, with strain gauges facing superiorly and
wires exiting through the hole. The mould was held
together by 12 screws.
Figure2 – Mould for Substrates1
1.2) Cleaning Process
The mould (containing the substrates) was immersed in a solution of TEEPOL (½ %) and Na3
PO4 (2 ½ %) for 2 minutes, rinsed under running water for a further 2 minutes, then
transferred to a tank with de-ionised water (15MΩ resistance) running through it. The mould
was removed when the lyometer testing the water leaving the tank had fallen back down to a
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normal current level, which indicated the ions from the detergent (initially causing the current
to rise) had been largely removed. The mould was then dried in an 80°C oven for 30 minutes.
1.3) Encapsulation Process (Potting)
Each beam was encapsulated in 10ml of MED-6015. This 2 part silicone was mixed (10:1)
using a DAC150 FVZ-K Speedmixer™, at 2000rpm for 2 minutes, before being injected
down one side of each beam, to chase out any air from under it. The rubber was cured in a
compression chamber at 2bar on a 45°C hotplate for 24hours, before the substrates were
removed from the mould.
1.4) Data Collection System
12 channels for data collection on circuit board 1 (CB1) were connected to multiplexers
which sampled each channel in turn.
The multiplexers were connected to an AD621
instrumentation amplifier with a fixed gain of 100. The amplifier was connected to a Semtech
XE8801A Microcontroller, which had an inbuilt ADC (Analogue to Digital Converter) and
PGA (Programmable Gain Amplifier) with a gain set to 10, which brought the total gain to
1000. The PGA also allowed the offsets associated with each signal to be nulled in software,
thus allowing each strain gauge signal to occupy the full available range of the amplifier.
A second test interface board (CB2), connected CB1 to a computer via an RS232
(Recommended Standard 232) cable. The computer read the information via MK4 (C++
Programme), allowing it to be imported into a LabVIEW programme. CB2 also acted as a
voltage reference circuit, to stabilise the voltage at 5V to CB1, as well as containing an
oscillator to drive the microcontroller, and acting as a driver for the output RS232 telemetry
signal (to convert to standard RS232 voltage levels) (Figure 3).
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Figure 3 – Photograph of Circuit Boards 1 and 2 (CB1 and CB2)
CB2
CB1
10cm
The strain gauges were wired into 8 of the 12 channels available for data collection, arranged
as quarter bridges, with 20kΩ fixed precision resistors. The 3 remaining channels used two
fixed precision resistors each, to function as controls.
1.5) Shielding and Noise reduction
Both circuit boards were housed in separate metal boxes to shield against electrical noise. The
box for CB1 was grounded to CB1 and the box for CB2 was grounded to the power supply.
Aluminium foil was used extensively to shield the surface of the table, the wires extending
from CB1 to the substrates, the power supply cable and the water bath. All of these were
connected to ground via the metal box of CB1, along with the metal lid of the water bath.
1.6) Other Adjustments
Foam attached to the edge of the water bath and its lid, prevented damage to the thin wires
exiting the bath. The soldered connections between the strain gauges and the circuit board
were covered by heat-shrink wrap and remained outside the water bath, to reduce exposure to
humidity, and thus reduce the risk of corrosion.
A temperature sensor was attached to CB1. When the substrates were immersed in water, the
sensor was placed in a glass test tube full of oil, taped to the inside of the water bath. Oil was
used due to its high thermal conductivity and the glass provided good insulation from the oil.
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1.7) Reprogramming of the Microcontroller Chip
The chip was re-wired to power all channels continuously, so that the full number of hours
under bias could be achieved in the time available and better electrical shielding was possible.
Permanently powering the circuit, more closely reflected the use of the instrumented nail,
which would be powered continuously for 20 minutes per session of activity.
The ADC output of the Microcontroller had a range of 0 to 215 (32760) counts. Its offset was
re-programmed to match the offset of the strain gauges. Firstly, a chip with the PGA set to a
gain of 1 ensured that any voltage offset present was within the linear range of the amplifier
so that it could be recorded for subsequent nulling. The counts for each channel were
recorded and the change required to make them all approximately 14000 counts (roughly the
centre of the range) with a gain of 10, was calculated and expressed as a hexadecimal number,
using an Excel PGA Offset Calculator Programme.
1.8) LabVIEW Set Up
A LabVIEW programme was set up to record various columns of data: the day, hours,
minutes and seconds of each reading; the counts and standard deviation for each channel; the
temperature (in °C); and the voltage. Readings were set to be taken every 5 seconds and
appended to an Excel spreadsheet. 16 points were automatically averaged when taking a
reading, to filter out noise. After the data was gathered, every 12 readings were averaged
(making each row of data an average of 1 minute or 192 points), to further increase filtering.
1.9) Initial readings
The counts (LabVIEW units) were initially recorded when a 33MΩ resistor was joined in
parallel to one of the 20kΩ fixed precision resistor on CB1, and the results were used to
determine the relationship between counts and microstrain (Appendix 1).
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1.10) Record Data
i) Run the data collection for 3 days in air, to establish the relationship between temperature
and counts (Results Figure 6a, Discussion 10).
ii) Run the data collection for 6 days under water to gather results and observe for leakage
(Results Figure 6b, Discussion 11).
1.11) Ink test
The substrates were submersed in a bath of ink to assess adhesion (Discussion 11).
Section 2: Method For Primary Investigation on Substrates2
Changes to the encapsulation method for Substrates2 were made, based on the lessons learned
from the preliminary work on Substrates1.
2.1) Making the Mould
A mould was made with the ability to encapsulate four substrates at once. Two 15cm long L
shaped pieces of metal were screwed together to create the base, back and top of the mould.
Four 3.2x3cm plastic trays were used to house the
substrates, and four longer screws with Teflon® boots
would hold the substrates by a corner in the bottom of the
trays and packing pieces were placed beneath the trays
(Figure 4).
Figure 4 – Mould for Substrates2.
2.2) Cleaning Process
First, the mould and trays were cleaned with Isopropyl Alcohol (IPA). The substrates were
washed in 3M solvent (HFE-72DA), to remove both organic and ionic contaminants, and left
vertically for the solvent to run off onto paper. They were then transferred to a tank of deionised water (see Discussion 4), before being dried in an oven, as described for Substrates1,
but without the mould. The substrates were then secured into the bottom of the trays.
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2.3) Encapsulation Process (Potting)
One square substrate was encapsulated by a 2mm thick layer of each of the following:
- MED-6015 Silicone rubber
- MED-6015 Silicone rubber with MED6-161 Primer
- MED-6015 Silicone rubber with 2% MED-4900-2 Pigment (Black Dye)
- MED-6015 Silicone rubber with MED6-161 Primer and 2% Black Dye
- MED4-4220 Silicone rubber (designed as an encapsulants)
- MED4-4220 Silicone rubber with MED6-161 Primer
- MED3-4013 Silicone rubber (designed as an adhesive)
- EPM-2420 Silicone Rubber (Not medical, but chosen for its promising properties)
The four MED-6015 encapsulations were completed together, whereas the others were
completed at a later date, due to the moulds ability to encapsulate only four substrates at once.
Prior to encapsulation, the appropriate substrates were primed with a thin layer of MED6-616
primer, applied with a small clean paintbrush and left to dry in air for 45 minutes.
4cm3 of silicone rubber were mixed for each tray. All rubbers were mixed from 2 parts.
MED-6015 was mixed in a ratio of 10:1, EPM-2420 was mixed 1:1, as were MED4-4220 and
MED3-4013. The former two were mixed as described for Substrates1, whereas the latter two
and could be applied using a 2-cartridge dispensing gun, with a static mixing nozzle attached.
The MED-6015 was mixed in 2 syringes (8cm3 in one and 7.84cm3 in the other), then 2%
MED-4900-2 Pigment (0.18cm3) was added to the second syringe, before mixing for a further
1 minute in the Speedmixer™, at 2000rpm.
Once all the encapsulants had been applied,
the mould was placed into a compression
chamber at 2bar, heated with heating tape to
70°C, for 24 hours (Figure 5).
Figure 5 – Compression chamber with heating tape.
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Once the rubber had cured, the screws were removed from the top of the mould, allowing the
trays to be removed. The encapsulated substrates were not removed from the trays.
2.4) Data Collection System
The 12 channels available for data collection were able to accommodate two substrates at a
time (each substrate having 5 different width cuts and a strain gauge). A second 20kΩ fixed
precision resistor was added as a half-bridge completion resistor to all the channels, with the
exception of the channels to be connected to the strain gauges on Substrates2. The resistance
of these strain gauges were all different, so the existing resistors were removed from these
channels and replaced with resistors made to the specific resistance of the first two substrates.
2.5) 2.6) Shielding and Noise Reduction Measures, and Other Adjustments – No changes.
2.7) Reprogramming of the Microcontroller Chip
Each time the substrates were changed over, the microcontroller chip was reprogrammed (as
in the method for Substrates1), because the strain gauges on Substrates2 all had slightly
different levels of resistance.
2.8) LabVIEW Set Up - No changes.
2.9) Initial readings
Prior to each test, the new substrates were run in air for 1 hour (Discussion 9).
2.10) Record Data
Tests were run under water for 6 days, for all encapsulated samples. (Results Figures 7(a-d),
8(a-d) and 9, Discussion 13).
2.11) Ink Tests
No ink tests were performed with Substrates2, but SEM (Scanning Electron Microscope)
Images were taken of the damaged areas (Discussion 11 and 13).
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2.12) Statistical Analyses
Once all the data was gathered, graphs were drawn to enable quick comparisons between the
encapsulants. Each of the channels were ranked by their range of the data (rank 1 being
smallest and rank30 being the largest) over the 6 days, in order to create a more sensitive
measure of outcome than “success” or “failure”. The average level of noise for each channel
after filtering was deducted from the range of data prior to statistical analysis (Discussion 13).
For the statistical analyses, the strain gauges were not comparable to the other channels, and
could not be included. Because there were only 8 strain gauges in total, each with a different
encapsulant, they could not produce statistically significant results if analysed for differences
caused by any of the factorial comparisons below.
Factorial comparisons were made to determine the effect (if any) of primer, dye, width of the
gap between conductors, or rubber type on the outcome. Different groups were compared for
the effect of primer (Table1), and the effect of dye (Table2). The results from all
encapsulations could be used for the comparison between gap widths, because each gap width
was covered by every encapsulant.
Table1 – The primed substrates (left) were compared to unprimed substrates (right).
MED-6015 with primer
MED-6015
MED-6015 with primer and dye MED-6015 with dye
MED4-4220 with primer
MED4-4220
Table2 – The substrates with dye (left) were compared to substrates without dye (right).
MED-6015 with dye
MED-6015
MED-6015 with primer and dye MED-6015 with primer
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Because there cannot be a normal distribution with so few results, non-parametric tests were
used to determine the p-value. The Mann–Whitney U Test was used for the effect of primer
or dye (comparing 2 independent samples) and the Kruskal-Wallis Test was used for the
effect of gap-width or rubber type (comparing 5 and 4 independent samples respectively).
This test showed whether or not there was a significant difference between each of the groups
(it did not test for correlation between increasing gap width and current leakage). Because
MED-6015 and MED4-4220 encapsulants were both tested with and without primer, a
separate comparison between these two rubbers could be made, with more statistical
significance than the comparison between all the different rubber types, where dyed or primed
samples were excluded.
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RESULTS
During the investigation, all substrates were placed in the bottom half of the bridge, so an
increase in currents (as caused by leakage currents) was presented as a decrease in counts on
the graphs. A change of 16.51 counts equated to a change of 1 microstrain (Appendix 1).
Preliminary Work Results
Figure 6 – Graphs for preliminary work, showing Substrates1, encapsulated in MED-6015, a)
run in air and b) run in water.
6a) Graph to show Substrates1, Encapsulated in MED- 6015, in Air
6b) Graph to show Substrates1, Encapsulated in MED- 6015, in Water
Water Bath Topped up
Strain Gauge Counts
Temperature (°C)
18000
38
16000
37
14000
36
12000
10000
35
8000
34
6000
33
4000
32
2000
0
0
12
24
36
48
60
72
84
96
108
120
132
31
144
Time (hours)
Ch = Channel for Data Collection
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Ch 8 Gauge
Ch 7 Gauge
Ch 6 Gauge
Ch 5 Gauge
Ch 4 Gauge
Ch 3 Gauge
Ch 2 Gauge
Ch 1 Gauge
Temperature (°C)
26
Preliminary work Results (Method 1.10.i and ii)
In air, the strain gauge counts correlated (inversely) with temperature. A 1°C increase caused
a decrease of 100 counts, which was equivalent to a change in 6.06 microstrain (Figure 6a).
The water-bath maintained the temperature at 37-37.5°C, except when it was topped up at 117
hours. It was decided the water bath would not be topped up during the primary investigation,
because it would cause large errors in the statistical analyses based on the range of counts.
The strain gauge on the first channel for data collection (Ch1, blue), began to fail at 50 hours,
from which point, it exhibited increasing leakage currents until it fell below the observable
range, at 130 hours. Channel 9 (Ch9, pale green) appeared to be severely unstable between
the periods 0-3hours, and 46-51hours, yet it remained stable at an appropriate count after this
point, suggesting a poor connection, as opposed to a fault with the sample or encapsulant.
The remaining 6 strain gauges remained relatively constant.
After Substrates1 had been tested, it was revealed that the strain gauges were covered by
SMD’s conformal silicone coating, prior to the encapsulation with MED-6015, and therefore
it was probably the conformal coating that was being tested. However, these results still
showed that silicone could be an effective encapsulant, and that the measuring equipment was
not subject to drift.
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Primary Investigation Results
Figure 7 - Graphs showing the current across the different width gaps,
on Substrates2, encapsulated in:
a) MED-6015 alone,
b) MED-6015 with primer,
c) MED-6015 with 2% MED-4900-2 Pigment (Black Dye), and
d) MED-6015 with primer and Black Dye.
Key
The count for all lines have been adjusted to start at zero, to aid
comparisons between the different width gaps.
Strain Gauges are not shown.
7a) MED-6015
7b) MED-6015 with Primer
Counts
Temperature (°C)
Counts
Time (hours)
Time (hours)
7c) MED-6015 with Dye
Counts
7d) MED-6015 with Primer and Dye
Temperature (°C)
Time (hours)
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Temperature (°C)
Counts
Temperature (°C)
Time (hours)
28
Figure 8 - Graphs showing the current across different width gaps,
on Substrates2, encapsulated in:
a) MED4-4220 alone,
b) MED4-4220 with primer,
c) MED3-4013, and
d) EPM-2420.
Key
The count for all lines have been adjusted to start at zero, to aid
comparisons between the different width gaps.
Strain Gauges are not shown.
8a) MED4-4220
8b) MED4-4220 with Primer
Counts
Temperature (°C)
Counts
Time (hours)
Temperature (°C)
Time (hours)
8c) MED3-4013
8d) EPM-2420
Counts
Temperature (°C)
Time (hours)
Shixin Bickerton SN: 707229
Counts
Temperature (°C)
Time (hours)
29
Figure 9 – Graph showing the strain gauges on Substrates2, in all the different encapsulants.
All Strain Gauges on Substrates2, in Water
Strain Gauge Counts
31000
26000
EPM-2420
MED3-4013
MED4-4220 with Primer
MED4-4220
MED-6015 with Primer and Dye
MED-6015 with Dye
MED-6015 with Primer
MED-6015
21000
16000
11000
Results of Statistical Analyses
The channels were ranked from 1 to 30, based on their range of data over the 6 days (Method
2.21). A lower rank number indicates a more successful encapsulation.
Table 1a: Effect of Primer – Mean and sum of ranked data for primed (1) and unprimed (0)
substrates (encapsulated in MED-6015 and MED4-4220),
Mann-Whitney U Test
P-value = 0.101
Table 1b: Effect of Primer on MED-6015 – Mean and sum of ranked data for primed (1) and
unprimed (0) substrates (encapsulated with MED-6015_
Mann-Whitney U Test
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P-value = 0.007
30
Table 2: Effect of Black Dye – Mean and sum of ranked data for the comparison of substrates
with dye (1) and without dye (0).
Mann-Whitney U Test
P-value = 0.112
Table 3: Effect of Width of Gap – Mean of ranked data for the comparison of gap width.
(in µm)
Kruskal-Wallis Test
P value = 0.115
Table 4: Comparison between all 4 Different Rubber Types
– Mean and sum of ranked data for the comparison of MED-6015 (Rubber1), MED4-4220
(Rubber2), MED3-4013 (Rubber3) and EPM-2420 (Rubber4). Only samples without dye and
without primer used.
Kruskal-Wallis Test
P value = 0.395
Table 5: Comparison between MED-6015 and MED4-4220
– Mean and sum of ranked data for the comparison of MED-6015 (Rubber1) withMED4-4220
(Rubber2).
Mann-Whitney U Test
Shixin Bickerton SN: 707229
P-value = 0.070
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Primary Investigation Results
(Method 2.10, Results Figures 7(a-d), 8(a-d) and 9, Discussion 14)
For clinical use, the encapsulant on the instrumented nail would be considered successful if a
change of 1-2 microstrain (16.5 – 33 counts) was not compromised by leakage, corrosion or
drift. The length of the perimeter of each rectangle on Substrates2 (5cm) was approximately
50 times the distance between the pads (1mm) of the nail’s thin film strain gauges (where
leakage is most important). The strain gauges on Substrates2 were also at least 4 times larger
than those on the instrumented nail. This was done in order to amplify any differences in
performance between the encapsulants, by simulating a worse-case-scenario in vivo. When
examining the graphs, it should be noted that the level of leakage was amplified by the
increased length over which leakage could occur.
Parameter Specific Investigations
1) Primer
When comparing the graphs for the encapsulations in MED-6015 (Figure 7a-d), the most
prominent feature was the negative effect of primer. Without primer (Figure 7a), the counts
remained relatively constant across all the different width gaps, with an average range of 34.6
counts. In contrast, the substrate encapsulated with primer (Figure 7b) showed much greater
current leakage, with an average range of 1965 counts.
However, the MED4-4220
encapsulation did not show this effect when primed. The statistical analysis of the effect of
primer using both rubbers (Table 1a) produced statistically insignificant results with a p-value
of 0.101. A second analysis was carried out comparing the effect of primer on the MED-6015
encapsulation alone. These result showed that the primer caused significant deterioration of
the encapsulant (p-value = 0.007) and it corroborated with the appearance of the graphs and
the appearance of the encapsulation. With MED-6015 and primer, voids were seen prior to
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encapsulation (Discussion 7, Figure 12) and brown patches of corrosion were observed after
testing (Discussion 12, Figure 17).
2) Width of Gap between Conductors
On the primed MED-6015 encapsulation graph (Figure 7b), the level of leakage decreased as
the width of the gap between conductors increased. The statistical analysis comparing the
different width gaps appeared to show differences which corroborated with this hypothesis,
with a smaller mean rank for each larger width (with the exception of the 80µm gap) (Table
3). However, these results was not statistically significant (p = 0.115) and no obvious
explanation was found for the slight anomaly of the 80µm gap.
3) Black Dye
The analysis of the effect of 2% MED-4900-2 black dye (Table 2) appeared to suggest a
possible detrimental effect, but once again, this result was not significant (p = 0.112).
4) Comparison between Silicone Rubber Types
The comparison between all 4 rubber types showed that MED3-4013 had a mean rank about
half that of the others (Table 4), but these results were very dubious with a p-value of 0.395.
The comparison between MED-6015 and MED4-4220 suggested the former was not as
effective as the latter and due to the higher number of samples used, this result was closer to
being significant (p = 0.070) (Discussion 14.d).
5) Dendrite Formation
Some of the graphs with discernible current leakage (in particular Figure 7d), had a particular
shape dissimilar to that of the strain gauge failure observed in the preliminary work (Figure
6b). The characteristic of a sudden, yet short lived increase in current (decrease in counts),
may have represented a dendritic growth across the gaps between adjacent conducting parts.
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The electric field could have caused metal ions on one side of the gap (anode) to move to the
opposite side (cathode) where the field strength was highest. The metal ions could then have
been deposited, forming a thin strand of metal (dendrite). When a dendrite spanned the full
width of the gap, it could have caused a momentary short circuit, before it broke, causing the
distinctive graph shape.
6) Strain Gauges
Only the strain gauge encapsulated in MED-6015 with primer failed completely. However,
this was not due to leakage currents, because the counts increased until they fell above of the
observable range. This meant that the current was decreasing with failure, indicating an
increase in resistance. The failure was caused by corrosion of the thin film gauge, which had
an appearance similar to that seen in Figure 17d (Discussion 12). All the others strain gauges
showed relatively little current leakage or drift with time.
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DISCUSSION
1) Gas bubbles during the Encapsulation of Substrates1
Initially, the silicone was placed in a vacuum-jar after mixing in the Speedmixer™ for
deaeration (to extract excess gas). However, the bubbles which formed would not burst and
were reabsorbed once the vacuum was removed. The silicone was cured in a pressure
chamber to eliminate bubbles as the rubber cured.
On the first attempt to remove the substrates1 from their mould, bubbles appeared at the
interface between the polished titanium and the silicone, at the ends of the beams. To ensure
the rubber had cured completely, the mould was placed in a 70°C oven for a further 2 hours.
Afterwards, the bubbles still appeared, suggesting insufficient adhesion between the rubber
and the titanium. For the purpose of rubber adhesion, it may be more desirable to have a matt
or roughened titanium surface, however, the instrumented nails must have a polished surface
for strain gauges to be sputtered on.
To avoid this problem, Substrates2 were placed in individual trays and not removed. To
ensure adequate curing, a thinner (2mm) layer of silicone was used and the substrates were
placed in a compression chamber heated with heating tape to 70°C to accelerate the process.
2) Excess Noise on Particular Channels of Data Collection
The shielding described in the method successfully reduced noise to an acceptable level
(below 20 counts) on all except the last three channels (Ch 10, 11 and 12), which remained
high (70-80 counts). All shielding and ground connections were checked, and the external
wires and substrates for the noisy channels were switched with those of low noise. This did
not affect the outcome, therefore the noise was not due to a component outside CB1. The
circuit board was cleaned with IPA (Isopropyl Alcohol), its joints were re-soldered, old
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unused temperature sensors were removed from the board, and the fixed resistors,
multiplexers and microcontroller chip were replaced.
When these all proved ineffective, an Oscilloscope showed noise on the last three channels at
the amplifier, which identified a problem between there and where the channels exited the
board. Examining the circuit diagram revealed that capacitive coupling may have been
occurring, where two digital traces for unused temperature sensors ran close to a resistors and
analogue trace for the affected channels. These traces were cut near the microcontroller and
grounded to increase shielding between the last three channels and the rest of the board.
3) Scanning Electron Micrograph (SEM) images of Substrates2 Prior to Encapsulation
The widths of the gaps cut by the laser may not have been exact if subsequent passes of the
laser overlapped the adjacent cuts. SEM (Scanning Electron Micrograph) images were taken
to assess the accuracy of the gap widths and examine their appearance (Figure 10).
Figure 10 – SEM Images of a) a 40µm gap, b) a 80µm gap, and c) a 160µm and 80µm gap.
This substrate was not encapsulated.
a)
b)
c)
Although the accuracy of the widths was acceptable, some unexpected features were
observed. Firstly, a significant amount of debris (probably nichrome left behind by the laser),
remained within the gaps. Secondly, the edges of the gaps appeared scalloped. Some of these
‘defects’ extended more than 10µm into the gap, so increased failures of the 10µm and 20µm
gaps might have been expected. However, the scanning process affected the substrates. The
laser cuts initially appeared dark, but became lighter under the SEM scan. The SEM worked
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well on the metallic surfaces, but the electrons disrupted the ceramic insulation exposed. The
scalloped edges may have been an artefact produced by the scanning process and not an
accurate representation of the edge of the gap. Due to the permanent change to the surface of
the gap, the substrate used to take these images was not tested.
4) Cleaning of Substrates2
Prior to the encapsulation of Substrates2 with MED-6015, the water deioniser broke. Due to
time constraints, store-bought deionised water was fed into the water tank, via a peristaltic
pump. The pump ran for 20 minutes before the substrates were immersed, but the lyometer
continued to read high levels of ionic contamination, probably <1% of ultimate purity
(Appendix 2). The deioniser was functional for all other encapsulations, but the impurity of
the water used for cleaning substrates encapsulated with MED-6015 may have biased these
results (Discussion 14d).
5) Encapsulation of Substrates2
A mould with 4 trays (to hold 4 substrates at once) was made to enable an efficient, yet
manageable encapsulation process. The Teflon® boots held the substrates down by a corner,
within the 6mm blank space below the upper edge (the wires were attached 8mm below this
edge), so the encapsulation of the strain gauges and rectangular cuts were not obstructed.
6) Silicone Rubber Types
Differences between the silicones became apparent with encapsulation. MED-6015 was
designed as an encapsulant and is essentially the same (materially and electrically) as the
biocompatible, clinical grade MED-6215 silicone proposed for use in the instrumented nail.
Its viscosity was low enough to enable easy application and although many bubbles appeared
as it was applied, these were eliminated by the compression chamber (Figure 11a).
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MED4-4220 cured very quickly before it even entered the compression chamber. It formed
fewer bubbles than MED-6015 during its application, but those which were formed were
never removed, due to its rapid curing. Fortunately, none of the bubbles disrupted adhesion
between the rubber and the substrate, so should not have affected the results (Figure 11b).
MED3-4013 was designed as an adhesive. Its high viscosity made it difficult to force through
the end of the syringe, and it formed a snake-like mass as opposed to flowing onto the surface
of the substrate.
It had to be spread out manually once dispensed, to ensure adequate
coverage of the substrate. Prior to curing, it appeared to have several bubbles, although
observations were hindered by its milky appearance and uneven surface. However, it cured
less rapidly than MED4-4220, allowing the bubbles to be eliminated by the compression
chamber (Figure 11c). Although this method of application was troublesome, better results
may be possible using high pressure injection into a mould; the method by which the
instrumented nails will be encapsulated.
EPM-2420 is not a medically approved silicone, so its testing was for scientific purposes only.
However, its properties made it a promising candidate for soft encapsulation and the mixing
and encapsulation process was very successful. Its low viscosity enabled it to flow into the
tray with ease and it did not form any bubbles (Figure 11d).
Figure 11 – Photograph of Substrates2 (all without primer or dye), encapsulated in:
a) MED-6015, b) MED4-4220, c) MED3-4013, and d) EPM-2420.
a.
b.
c.
d.
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7) Encapsulations with Primer
The primer caused large voids at the interface between the silicone and the substrate, for
encapsulations with MED-6015 (Figure 11).
Figure 12 (right) – Photograph of Substrates2
encapsulated in MED-6015, a) alone, and b)
primed.
a.
b.
8) Low Miscibility of MED-6015 and MED-4900-2 Pigment (Black Dye)
When 2% black dye was mixed into MED-6015, the mixture initially appeared homogenous,
but separated upon injection over the substrate (Figure 13a). A 2mm thick sample of MED6015 with 5% black dye showed similar separations (Figure 13b). With 10% dye, the rubber
had the desired dark homogenous appearance.
Figure 13– Photograph of MED-6015 mixed with black dye, a) at 2% (over Substrates2), and
b) at 5% in an aluminium tray.
a)
b)
Although MED4-4220 and MED3-4013 encapsulations were not tested with dye, samples
with 2% and 5% dye all appeared black and homogenous with good miscibility (Figure 14).
Figure 14–Photograph of MED4-4220 with dye at 2% and 5%, and MED3-4013 with dye at
2% and 5% (from left to right)
2%
MED4-4220
5%
MED4-4220
Shixin Bickerton SN: 707229
2%
MED3-4013
5%
MED3-4013
5cm
39
These tests also showed that if MED4-4220 was placed into the pressure oven before it cured,
the bubbles would be removed and a perfect finish could be achieved on its lower surface,
where it would contact the substrate (Figure 15).
Figure 15–Photograph of a) the upper surface and b) the lower surface, MED4-4220 with dye
at 2%. The small indents in the upper surface remained after the bubbles had burst, but no
defects were present on the lower surface.
a.
b.
9) Initial Readings in Air for Substrates2 (Method 2.9)
In every case, the readings taken for one hour before the substrates were immersed in water,
did not drift and were the same as the initial starting values of the tests under water.
Interestingly, when substrates were dried at room temperature and humidity after testing, the
values of some channels returned to the values observed at the start of the tests under water.
This showed that some of the disruptions caused by water ingress were reversible.
10) Preliminary Work (Substrates1) in Air (Method 1.10.i)
The initial intention was for LabVIEW to record a reading every 5 seconds and start a new
file at midnight every night. However, on the first data-gathering attempt, the system ran
until midnight, when a LabVIEW error prevented the creation of a new file. LabVIEW was
then set to run continuously. With one reading every 5 seconds, 17280 rows of data were
collected every 24 hours. Excel could hold 65000 rows of data, so as long as a new file was
started manually every 3½ days, no issues arose.
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Before the test was re-started, different correlations were observed between some of the
channels and temperature. The gauges fluctuated by up to 200 counts, 5 of which correlated
directly with temperature (current decreased with decreasing temperature) and 3 of which
correlated inversely with temperature (current decreased with increasing temperature).
Possible explanations included: variation due to the coefficient of thermal expansion of the
titanium beams; variation due to the temperature coefficient of the fixed precision resistors; or
variation due the temperature coefficient of the strain gauges themselves.
A change due to the expansion coefficient of titanium would be expected to cause the
longitudinal strain gauges to change in the opposite direction to the perpendicular gauges.
However, no correlation to the strain gauge orientation was found. In addition, the expansion
coefficient was < 9ppm, which was insufficient to account for the changes observed.
The temperature coefficient of the strain gauges was < 20ppm (nichrome). This corresponded
to a change of 10microstrain/°C (20 ÷ Gauge factor of 2) or 165.1counts/°C, which was large
enough to account for the changes observed. The 20kΩ fixed precision resistors had a
temperature coefficient of <15ppm and could also have accounted for these changes. To test
if it was due to the fixed precision resistors, a resistor known to correlate directly with
temperature, was switched with one known to correlate inversely. The resultant graphs
switched in response, identifying the resistors as the primary cause of change.
To reduce the fluctuations with temperature, the <15ppm 20kΩ fixed precision resistors were
replaced with <5ppm resistors. Before the test was re-started, two of these resistors were also
wired into each of the spare channels (10, 11, 12) to function as controls. Variations seen in
these channels, could not be influenced by the substrates.
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11) Ink Test (Method 1.11)
The ink test showed a failure of adhesion primarily at the end nearest to the channel that had
failed. However, the defect did
not extend over the strain gauge
and was therefore unlikely to
affect the results (Figure 16).
1cm
Figure 16 – Substrates1 (titanium beams) encapsulated in
MED-6015, then immersed in blue ink. Failure of adhesion
occurred primarily on the right hand side of the top beam.
This test was only sensitive to large failures of adhesion that extended inwards from the
rubber’s edge, making it unsuitable for Substrates 2, which were not removed from their trays
and had no such damage. The failures in Substrates2 occurred in the central regions where
ink would not penetrate. Brown areas appeared, clearly identifying regions of failure which
were further investigated using SEM imaging (Discussion 12), thus negating the need for ink
tests.
12) SEM Images of Damage to Substrates2
SEM imaging was used to gain insight into the failures on Substrates2 (Figure 17), which
could be compared to the normal appearance of the gaps (Figure 10). Images were taken of
the substrate encapsulated in MED-6015 with primer and dye, after it was tested and had had
the encapsulant removed. Brown patches were visible extending into the rectangles and the
strain gauge appeared darker on its far left (Figure 17a). The SEM images showed an
apparent increase in gap-width where damage had occurred (Figure 17(b-d)). This may have
been caused during dendrite formation, when metal was removed from one side, before being
deposited on the other (Results: Primary Investigation Results, 5).
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Figure 17 – a) Photograph of a damaged substrate with brown marks visible.
SEM Images of b) damage of 10µm gap, b) damage of 20µm gap, and b) damage of strain
gauge.
a)
b)
c)
d)
13) Effect of Noise on Statistical Analysis
The level of noise varied slightly between channels. Although not apparent on the graphs,
this would have caused errors in the statistical analyses, where the noise could significantly
increase the range of data for particular channels, affecting their rank. This would have
caused substantial bias when testing for the difference caused by gap width, because the
channels with higher noise levels were always wired to the same width gap.
14) Discussion of Results, with respect to Hypotheses and Relevance to Clinical Use
Although many of the results were not significant (p>0.05), they appeared to be supported by
the appearance of the substrates and graphs. The lack of statistical significance is probably
due to the limited number of samples available and not a true lack of effect.
a) The Effect of Gap Width (Introduction: Hypothesis for Primary Investigation 1)
Most of the graphs showed the 10µm gap to have less instability than wider gaps. Where
primer had decreased the effectiveness of the encapsulation, the larger widths of the gaps
exhibited lower levels of leakage. Although the results were not statistically significant, gap
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width is likely to affect current leakage. I would recommend a gap >10µm to be used on the
instrumented nail, due to its likely benefits and no suggestion of detrimental effects.
b) The Effect of Primer (Introduction: Hypothesis for Primary Investigation 2)
In contrary to the hypothesis, the results have shown the primer MED6-161 to be detrimental
to MED-6015 silicone encapsulations. However, it did not significantly affect the MED44220 encapsulation. On an electrical basis, I would not recommend the use of a primer for
the instrumented nail, but if one was to be used (possibly for mechanical advantages),
investigations should be carried out first to ensure that it would not be detrimental to the
encapsulant.
c) The Effect of Dye (Introduction: Hypothesis for Primary Investigation 3)
Where the results suggested the dye may be detrimental, it should be noted that the mixing
process was ineffective, which may have led to high concentrations in certain areas affecting
the adhesion. A darker, homogenous mixture may be attainable if the dye is first mixed alone
(to negate any settling of the bottles contents), then mixed with part A of the MED-6015 and
left to cool, before the 2 parts of the rubber are combined. This method of mixing may prove
to be more effective and it should be investigated in the future.
d) The Effect of Rubber Type (Introduction: Hypothesis for Primary Investigation 4)
Despite finding some very different properties (different viscosities, miscibility with the black
dye and reaction to primer) the effects of the different rubbers on current leakage was not
statistically significant. It was suggested that MED-6015 may not be as effective as MED44220. However, significant bias may have been introduced by the difference between the
cleaning methods used for those two encapsulants (Discussion 4).
The increased ionic
contamination of the deionised water used to clean MED-6015 encapsulated substrates, may
have been responsible for difference in performance. With few complete failures and graphs
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44
that showed promise for all the rubber types tested, further future investigations into different
rubber types is recommended.
15) In the Context of Existing Literature
Without primer, almost all of the thin film structures were successfully protected against
leakage currents, using the 4 types of silicone. The results of this investigation have added
more evidence to the existing literature, in favour of using silicone rubbers as an effectively
encapsulant under water. More specifically, it has added additional data with the specific
consideration of requiring very low leakage currents; a condition on which far fewer studies
have been conducted. Although the lack of statistical significance limits these results, much
has been learnt from the methodology, forming a good basis for further research.
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CONCLUSION
The most striking conclusion drawn from this investigation, was the detrimental effect of
MED6-161 primer on the ability of MED-6015 silicone to protect the thin film structures
against leakage currents. The analyses of the effect of the black dye was inconclusive. No
statistically significant results were found for the effect of the width of gap or rubber type.
However, the graphs and results do suggest that small gaps (10 and 20µm) may be subject to
more fluctuations and leakage currents, than larger ones.
Again from observation of the graphs, all the silicones investigated have shown promising
results, but further research to better differentiate between the choices of silicone and
refinement of techniques (such as the use of MED-4900-2 Pigment) could prove highly
beneficial.
Despite numerous minor adjustments to the methodology, the main point for improvement of
this investigation, would be to use a larger number of samples, allowing more statistically
significant results to be gained, in order to better answer the hypotheses initially proposed.
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APPENDIX
1) Relationship between Counts and Microstrain (Method 1.9, Discussion 2)
Initial readings were taken to establish the relationship between “counts” in LabVIEW and
Microstrain, in order to determine how many counts constituted an acceptable level of noise
(equivalent to 2 microstrain). A much larger (33MΩ) fixed precision resistor was used to
simulate a known change in strain. This resistor was joined in parallel to one of the 20kΩ
fixed precision resistor on the circuit board, which produced an increase of 5000 counts.
Both Resistors in Parallel
=33000 x 20 = 19.9878861296…
33000 + 20
Change in Resistance
= Resistance in parallel - Original Resistance
=
19.98…
20kΩ
= 0.01211387037… kΩ
Change in resistance
Original Resistance
= 0.012…
20
= 0.000605693519…
Divide by Gauge Factor (2) = 0.00030284675… strain
= 302.846759 microstrain (known change in strain simulated).
Counts per microstrain
= 5000
302.8…
= 16.51 (to 4s.f.)
A noise level of about 33 counts equates to a change in 2 microstrain, which shows that all the
channels after shielding (with a standard deviation <20 counts) are acceptable.
2) The Purity of Store-bought Deionised water (Discussion 4)
With the store-bought deionised water, the lyometer showed a current >10mA and probably
closer to 20mA. With 5MΩ.cm equating to 500µA, the water was <250kΩ.cm and probably
<18kΩ.cm, which meant it was probably <1% of the ultimate purity of water (18MΩ.cm).
The standard water deioniser produced water at 15MΩ.cm (83.3% of ultimate purity).
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