Encapsulation

ENCAPSULATION Oxford,

12 November,

1982

When the BES Transducers Group decided to hold a joint meeting with the Materials Group on Encapsulation, it seemed that inviting speakers from various technological sciences might be fruitful since encapsulation shares problems with techniques like paint-protection, enamelling and others. We heard speakers on polymer permeability, glasses, polymer adhesion and metallic corrosion. There were also six shorter contributions concerned with the encapsulation of actual implanted devices or tests on implant materials. P.E.K. Donaldson (MRC Neurological Prostheses Unit, London) opened the meeting with a short talk called ‘Pathways to Mortality in Implanted Devices’ in which the cause and effect of various supposed types of implant failure were related to the specialist subjects to follow. As regards encapsulation, the simplest type of implanted device would be one with no cables passing through the encapsulant; e.g. a telemetering thermometer. The electrical components in such a device could be in an envelope, either gas-filled or moulded round the components, which was sufficiently impermeable to keep the contents dry and uncontaminated during the required life. Permeability is therefore a convenient place to start and this was the subject of a paper by Dr. G. Murray (Department of Materials Engineering and Design, Loughborough University). Starting with a brief summary of metals, glasses, and pore-free ceramics, which are all impermeable to water-vapour and ions, a conflict arises because permeability is inversely related to melting point and so the need to encapsulate at low temperature tends to restrict one to the remaining class of polymers. A possible exceptfon might be glassy films laid down on cold substrates by Chemical Vapour Deposition. Generally, polymers are easy to use though moreor-less permeable to water vapour and gases, if they contain polar groups, they also pass ions slowly. In paint technology, these flows are not prevented but the part is thoroughly cleaned before painting and inhibitors may be added to the coat to prevent degradation by the diffusing species at the interface. The diffusion rate may be reduced either by loading the polymer, preferably with a flakey filler or by partially crystallizing. However both these changes reduce the compliance and Dr. Murray pointed out that where highly flexible implant cables (c.f. pacemaker cable) emerge for

the encapsulant, one must guard against future fatigue failure due to the surrounding polymer too stiff. He suggested that a possible approach would be to place the electrical part in a labyrinth of impermeable baffles, filled with stiff lowpermeability polymer and surrounded by an outer layer of flexible polymer. The author’s contribution was also partly concerned with material stiffness. Most polymeric encapsulants shrink during their cure and this would-be strain has to be translated into stress if the electrical parts are themselves rigid and adhesion is maintained. Encapsulants may be ranked by dividing their adhesive strength by the product of linear cure shrinkage and Young’s Modulus. On this basis a silicone rubber is better than an epoxy and an acrylic because they are over one thousand times stiffer. This is the case when shrinkage is prevented in one, two and probably three perpendicular directions. Adhesion is important in all implants which, unlike the example above, do have emerging wires, for loss of adhesion here will allow liquids to be drawn by capillarity into the interface between the part and its encapsulant. It appears that the requirement for low permeation favours stiffer polymers which tend to cause loss of adhesion; also it will be desirable to have thick encapsulations which will lead to larger implants. An alternative approach is to use encapsulants which are not impermeable over the projected implant life, which may therefore be thinner, more flexible and have lower residual stresses but which remain adherent to the encapsulated part. Will electrical failure follow? Experience in the MRC Neurological Prostheses Unit is that it does not if (and it is a big if) adhesion remains. Dr. K.W. Allen (Adhesion Science Group, City University) spoke on the subject of Polymer Adhesion. Adhesion theory was developed from the theory of gas absorbtion on solids; bonding forces being either Van der Waal’s or chemical by nature. Unfortunately practical adhesive joints are invariably weaker than predicted, due to surface contamination. The long-term stability of the adhesion depends on the bonding energy compared to the sum of the surface energies between encapsulant and an invading liquid and between liquid and solid. Work at The City University has shown that, for silicone rubber

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adhesives, the adhesion may just be thermodynamically stable. Mr. Allen said that this might be improved either by making the rubber more reactive (but this would effect biocompatibility and might cause corrosion of some metal parts) or by using a primer (almost certainly a silane material) to couple the surface hydroxyl groups to the polymer. However, as Dr. Griffin pointed out in questioning, it may be difficult to achieve the required monomolecular layer of primer in practice.

manner. Also it is imaginable used as an encapsulant if the withstand the required flow Drake (Standard Telephone gave a paper about glass.

that glass might be component used could temperature. C.F. Laboratories, Harlow)

Glasses are highly impermeable but because of brittleness, they must not have to endure tension. This means careful selection of thermal expansion coefficient to match the other parts. Since expansion coefficient and flow temperature depend on bond strengths, the low coefficient required to match (say) a silicon semiconductor chip would generally lead to impractically high flow temperatures. Fortunately though, glass can be made from a wide range of oxides; based on other glass formers besides the silicates and borates which are usually used in electronic glasses. Also, properties may be further extended by loading the glass with ceramic or allowing the glass to devitrify to a glass-ceramic (or sittel). Mr. Drake was particularly hopeful about sodium or calcium phosphate glasses which he claimed could be less soluble than silica while flowing at under 4OO’C; some he had tested were biocompatible.

This discussion of adhesion assumes that the solid surface remains unaffected. Experience has shown that, where the surface is metallic, corrosion may occur and so we invited Dr. J.B. Johnson (Corrosion 8c Protection Centre, University of Manchester, Institute of Science SCTechnology) to address us on this topic. Aqueous corrosion is electrochemical, currents flowing electronically in the metal and ionically in the electrolyte. The encapsulant protects the surface partly by preventing the ionic current and Dr. Johnson said that attack could only occur after a void had formed at the interface so that liquid was present. The potentials that drive the corrosion may be calculated from Thermodynamics: for a given solution and metal, a Pourbaix diagram may be drawn which also incorporates other properties (e.g. solubility of corrosion products). This will show at what potential and pH the surface is corroded, passive or immune from attack. However, it says nothing about rate but the reaction kinetics may be measured with fairly simple apparatus; and there are techniques for measuring very low corrosion rates. Although a perfectly adherent encapsulant would prevent corrosion, the question is whether a small, perhaps minute, flaw at the interface will be stable or whether it will cause progressive failure (corrosion causing adhesion loss causing corrosion, etc). The flaw allows ionic currents in the (unspecified) electrolyte. Since polymers eventually allow oxygen and moisture to be present for a cathode reaction, the surface can only be protected at anodes, possibly by plating with a noble metal or using a metal with a passivating oxide layer. Unfortunately, both of these protective films may themselves be flawed in which case two sorts of progressive local damage can follow - pitting or crevice attack. Chloride ions are a particular case of pitting in passivated metals.

One way to fabricate a high-density circuit in which the conductors are embedded in glass is by printing a thick-film multilayer circuit whose top layer is glass, except for vias for the input and output wires. The author tested some such available silicate glasses and found that one was more durable than 96% silica under the most unfavourable alkali conditions (its flow temperature was 900°C). Test circuits were made, protected by this glass, having wires soldered to the exposed metalisation, these wires protected by a silicone rubber over the glass. However, first in z&-o test results suggest that glass hydrolysis rates at flaws between the rubber and the glass may be much higher than expected from these weight-loss measurements. (This result was not given at the meeting due to shortage of time.) When glass is confined with a small volume of liquid, dissolution of the glass in an increasingly concentrated liquid may cause accelerating attack (auto-catalysis) or the reverse (auto-inhibition). In this respect, study of the stability of solid surfaces at flaws in the encapsulation seem to be similar for glasses and metallic surfaces, perhaps because both are dependent on oxide films.

The surface of an electrical device which is encapsulated will have both insulators and conductors at different voltages. After the water vapour has suffused the encapsulant, electrolysis between conductors is prevented so long as adhesion remains. The adhesion depends on in-built stresses, the remaining adhesive strength and also on the stress due to mechanical forces and impacts to which the implant is subjected. For a given encapsulant, this will put a lower limit of conductor spacing, which will restrict circuit density. This limitation is usually avoided by using hermetic packages for the electronics so that only the lead-outs are subject to the spacing restriction. The seals of these lead-outs usually require the use of glass in some

Dr. A.T. Woakes (Department of Zoology & Comparative Physiology, Birmingham University) outlined his work with implanted transmitters which allow him to monitor heart rate and airflow in the trachea of wildfowl while they are flying or diving. The transmitter was made by soldering fine copper wires between the leads of miniature components under a microscope, and encapsulating in epoxy. Pacemaker cable was used and the implant coated with wax and a silicone rubber. These devices lasted four weeks implanted, which was sufficient for the experiment. Then failure occurred due to moisture arriving at the soldered joints which were near the encapsulant surface; adding glass plates to either side increased

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the life to two years by increasing the diffusion path length. Dr. Woakes said that simple empirical methods like this were quite adequate for work on animals though they might not be suitable for chronically implanted medical devices. Dr. J.A. Hodgson (Department of Physiology, Bristol University) described the encapsulation of E-shaped stainless-steel buckles bearing strain gauges which are used in studies of an extensor muscle in the cat, routing the tendon through the buckle. The encapsulation of this implant was made less stringent due to the low impedance of the gauge, more so because the adhesive had to be very stiff. Early devices were protected by a conformal layer of Parylene C (Union Carbide) which may be polymerised onto a cold substrate, this worked well but was very expensive. Without the Parylene, the devices failed after only 3 days. Dr. Hodgson then changed his adhesive/encapsulant from ‘ordinary AraIdite’ epoxy to Araldite 2004 (Ciba Ltd.) which is good enough, although water is seen to be present, condensed into bubbles in the epoxy. Great care had been taken with cleanliness before encapsulation; trouble due to highly active fluxes which were trapped inside the Teflon covering of the stainless steel wires was usually prevented by sucking an insulating barrier material into the wire sleeves before fluxing. Two of the speakers were concerned with the production of sensors for acute medical use. Dr. J.M.L. Engels (Honeywell and Philips Medical Electronics B.V., The Netherlands) had developed sensors for intravascular measurement and demonstrated the advantage of using silicon chip transducers in what he called a ‘Micro Transducer Catheter’. His chief concern seemed to be quality control in devices intended for sale. He emphasised the need to have gentle curvature round the lumen and catheter tip to prevent blood clotting. The sensors described by Dr. A. Sibbald (Department of Physical Chemistry, Newcastle University) were to be mounted in measuring cells and not in catheters. Nevertheless, small size is desirable and semiconductors were used. Because these sensors are readily replaceable, the encapsulation had to be adequate but cheap. Adequate encapsulation for ISFETS is not straightforward because the sensitive chip areas have to be left exposed while the bonding wires are protected. However, this had been achieved by using a bis-GMA acrylic dental sealant which was kept off sensitive areas by a gas jet while the polymer was cured with ultraviolet light from a light-pipe. The required life was 30 days but the sensors survived for two to three months. Dr. Sibbald had also made a amperometric ~0, sensor by the ingenious method of bonding up an operational amplifier chip on a header but leaving one gold and one silver wire extending upwards. The header was dipped in the acrylic, encapsulating the chip but leaving the wire ends exposed. This made a satisfactory sensor for about El-50.

Mr. J.J. Heyson (Cardiology Department, Groby Road Hospital, Leicester) described the history of pacemaker encapsulation. Pacemakers have been implanted since 1958, the early ones invariably were encapsulated in epoxy resin, some using an ordinary printed circuit board which was very carefully cleaned. Others used hermetic packages though not including the battery. Another design used a polypropylene enclosure which was closed by welding, this was successful until a desiccant was included. When lithium batteries were introduced, the implant life had to be increased to keep up with the batteries and modern pacemakers use multiple hermetic packages so that all parts including the battery are sealed. The outer case is usually pure titanium having one metal-in-glass seal. The case is welded by laser, argon arc or electron beam to try and prevent subsequent corrosion at the weld. Cables are multistart with stainless steel or Elgiloy wires in silicone rubber or polyurethane. The plug on the cable and the socket on the pacemaker are of the same alloy. The socket is connected by a wire to the package feedthrough and this is encapsulated in epoxy or silicone rubber: the most common problem that still occurs is due to a liquid film allowing ionic conduction across the glass of the feedthrough after liquid has tracked up this wire, despite the fact that the wire is helical to increase the path length. About a quarter of a million pacemakers are now made per annum. The meeting ended with some discussion but no attempt to draw any conclusions. Some outstanding points are these:

( i)

Parts to be encapsulated cleaned.

must be thoroughly

( ii)

The possibility exists of using a glass film deposited by Chemical Vapour Deposition (perhaps of silicone nitride) to protect components.

(iii)

Semiconductor technology allows miniature sensors to be made often at costs which make them ‘throwaway’ devices. For these, the best encapsulant may not give very long submerged lives.

(iv)

Fortunately for the biologists, there is an enormous difference in the desired reliability of implants for short-term animal experiments and chronic medical applications.

( v)

It is important to determine whether an encapsulation is intended as a barrier to diffusion, a barrier to liquid seepage along radial wires, or both.

(vi)

There is much Materials Science to be done on the polymers, metals and glasses used in implants. One criterion for success will be the stability of adhesion flaws at the encapsulant interface.

Our thanks to John Crowe, Madeleine Radburn and Peter Rolfe of the Bioengineering Unit at the John Radcliffe Hospital, Oxford, for organizing this meeting. N. de N. Donaldson

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