A review of adhesive bonding techniques for joining medical materials

S M Tavakoli, D A Pullen and S B Dunkerton

TWI Ltd

Paper published in Assembly Automation, Jun 2005 Volume: 25 Issue: 2 Page: 100 - 105.

Abstract

Medical products often require joints or seals in their construction or packaging. There are ranges of adhesive bonding and welding techniques available for joining materials for medical device applications. Adhesive bonding offersa number of advantages compared to welding and could be the only option when joining dissimilar materials. This paper will describe the adhesive bonding techniques available for joining a range of polymeric, metallic and ceramic materials. Examples will be given of typical medical products, and the ability of the bonding processes to provide high integrity and reliable joints. Knowledge of the processes will provide engineers with greater design freedom, and production engineers with more reliable, and sometimes, more cost effective assembly options.

Introduction

Medical devices, whether temporary or permanent, used externally or inside the body, are becoming more complex and sophisticated both in terms of their performance specification and structural complexity.

As a consequence many devices in current use are multi-component and require assembly in production. Joining is one of the key issues in many manufacturing industries and the medical industry is no exception. ^[1] Medical devices, whether used outside the body (e.g. instrumentation, control systems or surgical tools) or inside the body for diagnostic monitoring or therapeutic purposes (e.g. sensors, catheters, pacemakers or prostheses)usually consist of many materials which may be joined.

The use of adhesives for joining medical and implantable devices has significantly increased in recent years. Selection of suitable adhesives and the development of selective bonding procedures, including surface preparation, adhesive dispensing and curing are critical parameters in successful usage of this technology. Furthermore, most adhesives (e.g. radiation curing and pressure sensitive adhesives) offer the opportunity for semi or fully automated process assembly.

Adhesive materials

There is a wide range of medical grade adhesives ^[2-5] that are available for joining materials and components for short-term (<30 days) medial device applications. The most common types are based on acrylics, epoxies, polyurethanes and silicones.

Acrylic-based adhesives are used to join a variety of similar and dissimilar materials. The main types of acrylic-based adhesives are cyanoacrylates, anaerobics and modified acrylics. These may be polymerised or cured using moisture, catalysts, heat, UV, visible light or other sources of radiation. The UV acrylics and Cyanoacrylates are of particular interest to the medical industry both for joining medical devices and for tissue bonding agents. ^[6-7] Medical grade pressure sensitive tapes or film adhesives are also used in many medical applications (e.g. wound dressings, nicotine patches, etc).

Epoxies have also been used in medical devices for bonding and sealing applications. For load bearing applications and where exposure to more aggressive sterilisation exposure are required this type of adhesives provide better service performance assuming the adhesion to a specific substrate is also satisfactory. A clear, medical grade low viscosity epoxy has found application in fabrication of access ports that are implanted beneath the surface of the skin of patients requiring multiple injections. ^[8] By allowing access for subsequent treatments, the access ports enhance patient comfort and reduce complications.

In combination with a catheter assembly, access ports are used to deliver drugs to a particular area. As shown in Fig.1, they can be used for arterial, venous, peritoneal or interspinal access. The access port assembly is based on potting the stainless steel or titanium access port with an epoxy adhesive/encapsulant.

Medical grade polyurethanes are being used as adhesives, encapsulants or coatings or coatings in many medical devices. A novel echogenic coating recently developed at TWI ^[9] for coating medical devices, such as needles for biopsy procedures, as shown in Fig.2. The coating is based on medical grade polyurethane with blowing agents. The coating becomes reactive in a moist tissue environment and the coated area of the needle will be completely visible during ultrasound diagnostic procedures.

The most important types of silicone used in medical implants are fluids, gels and elastomers (rubbers). Silicones used in breast implants are based on silicone gels. These silicones are lightly cross-linked polysiloxane networks swollen with polydimethylsiloxane (PDMS). The PDMS fluid is not chemically bound to the crosslink network but is retained only by physical means (e.g. water in a sponge).

There are wide ranges of polymeric materials available, which are currently being used, as adhesives/encapsulants or as substrate for interconnection, encapsulation/protection of electronic components in medical devices. Medical electronic packages are similar to those for military electronics in that reliability is more important than cost, although in today's increasingly cost conscious environment there is always a limit in developing a product to market if the electronics are unfavourable.

Joining and micro-packaging of materials used in microsystems is one of the most active area of research and development in developing devices (e.g. sensors) in new generation of medical devices. In recent years the use of adhesives and encapsulants in electrical components as well as electronics in medical and implantable devices has increased considerably. This is due to the availability of a wide range of materials with different properties, better adhesion, improved durability, and ability for automated dispensing and rapid curing. The adhesive bonding assembly based on acrylic adhesives will particularly lend itself to automation which could include; robotic material handling, in line or automated surface preparation (plasma discharge use of adhesion promoters) of adherends, automated adhesive dispensing, curing and component assembly.

Surface Pre-treatment

Selection and application of an appropriate surface treatment is one of the major factors in achieving good wettability and improved long-term durability of adhesively bonded joints. Inadequate or unsuitable surface treatment is one of the most common causes of premature degradation and failure. The function of the surface treatment includes the removal of contaminants or weak boundary layers and alteration of surface chemistry, topography and morphology in order to enhance adhesion and durability. Surface preparation techniques are generally divided into mechanical or chemical methods:

1. Mechanical Methods
   • Abrasion
   • Grit blasting
   • Shot blasting
2. Chemical Methods
   • Degreasing
   • Etching
   • Anodising
   • Adhesion promoters
   • Flame treatments
   • Corona treatments
   • Plasma treatments

In most applications simple degreasing and abrasion is often sufficient to provide good adhesion. However, many medical polymers with low surface energy and bondability (e.g. polyolefins) often require a more specialised treatment(e.g. plasma treatments) in order to provide better adhesion and joint durability. Some adhesion promoters can also enhance bondability of certain polymers. Recently new grades of adhesives with the ability to bond polyolefins (e.g.polyethylene) without the need for pre-treatment have become commercially available. However it must be emphasised that any surface preparation selected for preparation of medical materials prior to assembly has to comply with the requirement of the relevant medical device. That is, no contamination or degradation of the adherend during service, as result of surface preparation, can be accepted. Therefore, whenever possible, simple and non-toxic surface preparation (e.g. grit blasting and plasma treatments) techniques, which are already used by some medical device manufacturers.

Dispensing Techniques

Adhesives should be applied immediately after surface preparation is complete, and in a manner that minimises the risk of air entrapment in the joints. Manual mixing and application should be avoided, if possible, as this can introduce voids, bubbles and regions of incomplete mixing. Pre-packaged cartridges, using hand held dispensing guns for single or multi-part adhesives, are recommended. For high volume production, semi-automated or automated pump dispensers should be used.

Dispensing adhesives by pumping techniques can be achieved in a number of ways, depending on whether a one or two part adhesive is being dispensed. One-part adhesives are dispensed using direct metering extrusion pumps, shown in Fig.3.

An electric motor pushes a follower plate into a drum of adhesive, which is extruded through a hose to the dispensing valve. This technique has been used for medium to high volume production rates.

Two part adhesives are generally dispensed by volumetric pumps for semi-automated medium-high volume assemblies. These systems are capable of processing adhesives with mix ratios from 1:1 to 15:1. A bulk dispensing system is shown in Fig.4.

One of the most attractive methods of precision dispensing polymeric resins and particle filled fluids is microjet/printing technology. This technology is normally based on piezoelectric demand-mode ink-jet printing which can produce droplets of polymeric resins of 25-125µm in diameter, at rates up to 1000 per second. The microjet printing technique is emerging as one of the attractive dispensing techniques for placing adhesives for some new micro and optoelectronic component assembly, as well as in medical device manufacturing applications.

Curing techniques

Curing of polymeric adhesives or encapsulants can be achieved using moisture or catalysts in the presence or absence of air (e.g. for anaerobics in the absence of air) at room temperature, thermally at elevated temperature or photochemically using irradiation (e.g. UV or visible light, electron beam, lasers, etc). In heat activated process the heat activated curing could be achieved using a variety of sources as follows:

• Local heat application at the joint(s)
• Overall heat application
• Conventional/conduction ovens
• Hot plate heating
• Infrared heating
• Vapour phase heating
• Liquid phase heating
• Laser heating (Nd:YAG and CO₂ lasers)
• Microwave, particularly variable frequency microwave (VFM).

There are a range of materials available, which can be cured using radiation sources ^[4] such as UV and visible light. Acrylated resins (acrylated epoxies, polyesters, polyurethanes and silicones) can be cured using radiation energy. Radiation curable adhesives or encapsulants ^[3] generally consist of low or medium molecular weight resins (called oligomers), monofunctional or multifunctional monomers, additives, pigments, photoinitiators or photosensitisers.

A typical UV energy of 80-120 mW/cm² produced from a UV source (wavelength 300-400nm) is usually sufficient to cure a UV curing adhesive within 10-60 seconds. An alternative radiation curing technique is to use visible light (wavelength 470nm)curing (see Fig.5) Many dentists currently use this technique for dental curing materials. Radiation curing adhesives have also been used for joining many clear polymers in disposable and non-disposable medical devices. In general both UV and visible light curing can be achieved using light boxes or focused beams and light guides (see Fig.5). In some cases heat is also employed to encourage the curing process, for completion of cure, or to cure areas that cannot be reached by the radiation energy.

Applications

Adhesives are increasingly being used in a range of medical and electronic device applications, ranging from the attachment of silicon die and sensing elements (piezo-electric sensors) to substrates to the sealing and encapsulation of packages and devices.

This has come about by the significant improvements in polymeric adhesives, their properties and durability, and dispensing and curing methods. In particular, rapid cure resins provide the opportunity for rapid placement and setting giving greater throughput and, in some cases, reliability.

Typical examples of the adhesives in medical devices are described in the following sections.

Catheters

A silver-loaded electrically conductive adhesive has been used to join a piezo electric transducer (PZT) ring to a tungsten carbide (WC) tube as two components of a cardiac catheter tip as shown in Fig.6. The catheter tip has been designed to function as part of an ultrasound-imaging device for quantitative and diagnostic analysis of coronary arteries. ^[10] After a series of investigations an optimised condition was found. A control insertion of conducting adhesive between the WC and PZT components and a curing process which resulted in a void free 80µm conducting layer with the desired acoustic properties were established. Ultrasound catheter tips fabricated with conducting adhesive have been taken through mechanical product testing to clinical evaluation trials.

Cyanoacrylates have been used for joining latex balloons onto PVC, urethane and multi lumen tubes for balloon catheters. An example of the use of cyanoacrylates in balloon catheters used in angioplasty is shown in Fig.7.

Needles

Lancets, syringes, injectors, hypodermics, blood collection sets and introducer catheters have been assembled using acrylic-based adhesives. The polypropylene mouldings of a drug administration gun were bonded together using a cyanoacrylate adhesive as shown in Fig.8.

Polycarbonate devices

Acrylic based adhesives are being used in bonding polycarbonate medical devices including filters, blood pressure transducers, arteriograph manifolds, carditomy reservoirs and blood oxygenators. Epoxies are also being used for joining filter components. Figure 9 illustrates the use of a two-part epoxy for bonding end caps to the main tube of a blood filter.

Masks

UV curing acrylic adhesives have been employed for bonding cushion (flexible PVC) to nose (rigid PVC) in anaesthesia and face masks (see Fig.10).

Tubesets

Blood and drug delivery sets, suction tubes and IV tubes, were assembled using acrylic based adhesives. An example of an acrylic and PVC components of a drug administration tube, bonded with an acrylic adhesive, is shown in Fig.11.

Concluding remarks

The use of adhesives for joining medical and implantable devices has significantly increased in recent years. Selection of suitable adhesives and the development of selective bonding procedures, including surface preparation, adhesive dispensing and curing are critical parameters in successful utilisation of this joining technology. Although there are many commercially available medical grade adhesives, their use for new applications require a detailed investigation. It is also important to remember that as well as the initial joint strength; durability of the bonded components in intended service environments (e.g. exposure to low and high temperatures, stress, fluids, sterilisations etc) has to be investigated. Design of accelerated ageing tests, which can simulate the service environments, is critical in providing realistic durability data. Interpretation of ageing data and lifetime prediction of the joint is essential in the assessment of performance of medical devices. Emergence of new types of adhesives as well as further development of precision dispensing and rapid curing technologies offer many exciting and commercially attractive opportunities for joining medical devices as well as in many other areas of medicine (e.g. surgery), dentistry and pharmacy.

Acknowledgements

The author wishes to thank material suppliers and users, particularly Henkel/Loctite UK, Epoxy Technology, Braun Medical, Emerson and Cuming for their support and supply of some of the photographs for this publication.

References

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