Sticking to the healthy option - using adhesive bonding in medical and implantable devices - Pt I

TWI Bulletin, May/June 2001

Mehdi Tavakoli received a BSc in Chemistry and an MSc and PhD on Polymer Science and Technology at Aston University in Birmingham. He joined TWI in 1989 and has been working on new joining and associated technologies with particular interest in joining and coating of medical and implantable devices. He has over 20 years research, industrial problem solving and product development experience on polymeric materials and has published more than 40 papers and patents and is currently working as Technology Manager on polymers/adhesives at TWI.

There is considerable interest in using polymeric adhesives and coatings in medical and implantable devices as well as in dentistry, pharmacy and surgical operations. As Mehdi Tavakoli reports in the first part of this materials, processes and applications review, 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.

This article provides an introduction to the use of adhesives for joining medical and implantable devices. The main types of adhesives used in medical applications, as well as issues such as surface pre-treatment, curing and dispensing techniques and typical applications, are reviewed.

Adhesives

General classification

• Chemical types (epoxy, silicone)
• Origin (natural, synthetic)
• Physical form (films, tapes, pastes, one or multipart components)
• Curing methods ( eg heat curing, moisture curing, radiation curing)
• Functional types (structural, pressure-sensitive, hot-melts)
• End use (sensors, catheters, tissue or bond bonding).

Thermosets, thermoplastics and elastomers

A thermosetting adhesive becomes set into a given network, normally through the action of a catalyst, heat, radiation or combination of these factors, during the process of crosslinking. As a result of this thermosets become infusible and insoluble. Thermosetting resins ( eg epoxies, polyesters and phenolics) are the basis of many structural adhesives for load-bearing medical and engineering applications, as well as for use in precision joining in electronic applications.

In contrast thermoplastic adhesives ( eg polyamides) may be defined as materials which soften, melt and flow on the application of heat and solidify on cooling. A simple illustration of thermoplastic and thermosetting polymers is shown in Fig.1.

Adhesives may also be based on natural ( eg natural rubber) or synthetic (styrene-isoprene-styrene (SIS) block, SIS copolymers) elastomers. Elastomers ( eg polyisobutylene, PIB) are the main polymers used in many pressure sensitive adhesives for producing medical tapes. Adhesives based on other natural origins ( eg proteins, cellulose, starch) are also available and are very important for many medical and pharmaceutical applications.

Adhesive types

Acrylics

There is a wide range of acrylic-based adhesives used to join a variety of similar and dissimilar materials. The main types of acrylic-based adhesives are cyanoacrylates, anaerobics and modified acrylics.

These adhesives are usually available as solvent-based liquids, emulsions, tapes, or as monomer-polymer mixtures (one or two part components), with liquid or powder curing agents. Acrylic-based adhesives may be polymerised or cured using moisture, catalysts, heat, ultraviolet (UV), visible light or other sources of radiation. These adhesives are of particular interest to the medical industry both for joining medical devices and as tissue bonding agents. ^[4-6] There are a number of medical grade tapes and films available ( eg single- or double-side coated, woven and non-woven, elastic and absorbent materials). They are used in a variety of applications including allergy patch testing, nicotine patches, ostomy devices and general dressings. Many of these tapes and films can withstand ethylene oxide (ETO) and gamma sterilisation.

One of the important types of acrylic adhesives are cyanoacrylates, commonly known as 'superglue'. Polymerisation or hardening of cyanoacrylates can be initiated by the presence of moisture or weak bases in the atmosphere or on the substrate. Considerable improvements have been made in using cyanoacrylates as surgical tissue adhesives, but there are limitations in terms of toxicity, strength, durability and safety. ^[3]

Epoxies

Epoxy resins are thermoset adhesives based on the epoxide group, a three membered carbon, carbon, oxygen ring structure which is also known as oxirane group:

The ability of this group to undergo a large variety of polymerisation and crosslinking reactions leads to many different types of epoxy resins with a wide range of chemical and physical properties, molecular weight and structures. Epoxies are one of the most widely used adhesives for both structural and non-structural applications.

Epoxies have been used in medical devices for bonding and sealing applications. A clear, medical grade low viscosity epoxy adhesive has found application in fabrication of access ports that are implanted beneath the skin of patients requiring multiple injections. ^[7] 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.2, 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. This particular adhesive passed all USP (United States Pharmaceutical) Class VI biological and toxicity tests ( ie acute systemic and intracutaneous toxicity, implantation tests and cytotoxicity tests).

Polyurethanes

Medical grade polyurethanes are being used as adhesives, encapsulants or coatings in many medical devices. Most commercially available polyurethane systems are based on polyethers or polyesters with terminating hydroxyl functional groups. The reaction of an alcohol and an isocyanate results in the formation of a urethane as follows:

Materials used in most polyurethane systems usually consist of di- or polyfunctional alcohols or polyhydroxy compounds (known as polyols) and di- or polyfunctional isocyanates as well as low molecular weight alcohols or amines. A general scheme for the reactions involved in the formation of a polyurethane system is as follows:

A novel echogenic coating recently developed at TWI ^[8] for coating needles for biopsy procedures is mainly based on the use of a hydrophobic polyurethane ( Fig.3).

Silicones

Organo-polysiloxanes, or as generally known, silicones, are semi-inorganic based polymers with a molecular structure made up of alternating silicon and oxygen atoms with organic side or end groups attached to some or all of the silicon atoms. The repeating units of these polymers can be shown as follows: 

Depending on the nature of side groups and the interchain crosslinking, silicones are available in the form of gels, liquids or elastomers.

The most widely used side groups (R and R') are based on methyl (-CH ₃), phenyl (-C ₆H ₅), vinyl (-CH=CH ₂) or hydroxyl (-OH). Vinyl and hydroxyl groups are chemically reactive and in the presence of a suitable curing agent can be crosslinked. A simple general classification of silicone based adhesives may be shown as follows:

Silicones

Organo-polysiloxanes, or as generally known, silicones, are semi-inorganic based polymers with a molecular structure made up of alternating silicon and oxygen atoms with organic side or end groups attached to some or all of the silicon atoms. The repeating units of these polymers can be shown as follows: 

References

Part II of this feature to be published next issue, looks at surface pre-treatments, dispensing, curing and more medical applications.