Bonding exercise

Driven by increasing surgery and dental work required by an ageing population, the medical sector will increasingly accept adhesives and sealants in hospitals and the home. Mark Venables reports.

Joining two materials together is one of engineering’s most fundamental requirements, and new solutions are emerging all the time – driven by demands for cost reduction, environmental regulations and the desire for more aesthetically pleasing products. Mechanical fasteners are one method, but often more desirable is bonding, using adhesives.

The medical sector is making use of adhesives, both in manufacturing devices and frontline healthcare with products such as adhesive tape. In surgery, adhesives present an attractive alternative to stitches and other mechanical closures, as they reduce the risk of scarring. for this and other applications, the sector’s stringent demands mean that careful material selection is vital.

Judging the exact size of the medical adhesives market is difficult, but according to Cleveland-based market researcher The Freedonia Group, demand for medical adhesives and sealants in the US alone is set to rise seven per cent a year to $1.5bn in 2009.

Gains will be driven by the increasing surgery and dental hygiene required by an ageing population, continued new product development and increasing acceptance of adhesives and sealants in hospitals and the home.

The predominant material in medical products is plastic, which presents its own challenges. modern medical device manufacturers use engineered resins such as polycarbonate and thermoplastic elastomers, which have favourable cost-performance benefits. Polypropylene, polyethylene, polyurethane and polyvinyl chloride will retain leading positions in non-invasive medical products and standard medical packaging due to their low cost and amenability to radiation sterilisation.

The problem here is that it is difficult to surface bond on polyolefins. Polypropylenes, for instance, are crystalline and, although they have excellent thermal and chemical resistance properties and high moisture resistance, they are non-polar. As such, this makes them difficult substrates to bond since they feature no surface functional sites or inherent surface roughness to which an adhesive can secure itself. In addition, polypropylenes are linear or branched carbon chain polymers and have low surface energies and low porosity.

Hard-to-bond plastics like polyolefins are most often assembled using adhesives. While adhesives are the most versatile assembly method for plastics, only a few offer suitable bond strengths on hard-to-bond plastics. Cyanoacrylate, light-curing cyanoacrylate, hot-melt and light-curing acrylic adhesives have typically been used.

Loctite has a host of successful medical bonding applications including catheters, endoscopy accessories, dialysis machines, PVC tube bonding, syringes, prosthesis and anaesthetic masks.

A thermal dilution catheter manufacturer requested help in bonding a latex balloon to a rigid, multi-lumen PVC tube. The catheter is inserted into a major vein near the heart to take blood samples, administer fluids, and monitor blood pressure and body temperature.

The transducer must endure temperatures ranging from freezing to 43oC (110oF), while the balloon must withstand pressures up to 7.5 bar (110 psi). After bonding, the balloon is inflated 50 times to check durability. For this application, a cyanoacrylate adhesive was selected for its low viscosity, thermal resistance and controlled setting time, allowing time to assemble and place the balloon precisely. The customer reported that the adhesive works so well that the balloon ruptures before the adhesive fails.

A manufacturer of a device used in dialysis machines had a production line shut down. The problem was that one vendor had supplied out-of-tolerance parts, and the solvent used for bonding could not fill the excessive gap. Their PVC tubing supplier also made a substitution, creating additional problems. The process used solvent welding, a mixture of 90 per cent methylene chloride and 10 per cent cyclohexanone, to join a flexible PVC tube to a copolymer elastomer (TPE). The parts were joined by slip fit, with a .002in gap, using a dip-and-assemble technique. The assembled product had to meet a 1.3-1.7 bar (20-25 psi) burst test and a 15lb pull test.

Loctite 4011, a surface insensitive cyanoacrylate, was specified. It filled the gap and had sufficient strength to pass the burst and pull tests with ease. Production goals were met, inventory was used, product quality was assured, and a potentially troublesome toxic solvent was eliminated.

A syringe with pre-attached needles is designed to prevent accidental needlesticks in the dental industry. An anaesthesia cartridge is loaded into the K-Resin body assembly, which penetrates the back of the needle, allowing the anaesthesia to flow into the syringe. After the injection is made, a second guard slides down over the needle, fully enclosing it. Traditional dental syringes use ‘screw-on’ needles, presenting multiple chances to get stuck.

Loctite was in at the design stage to analyse the application, which involved bonding a stainless steel cannula to a K-Resin syringe body. Heat cure epoxies were evaluated, but this caused the viscosity of the epoxy to lower, resulting in occluded needles and misaligned parts. The customer required consistency, automation, and a minimum pull strength of 4.4lbs.

Loctite 3051 was evaluated and proved superior on the substrates. It was easily automated, and eliminated heat cures and migrating epoxy issues. Cure time was 4-5 seconds under a high-intensity light cure conveyor. Production and product quality increased significantly, allowing the customer to achieve 4-5 times greater pull strength than required.

It is not just in manufacturing medical devices that adhesives play an important role in healthcare. Correct attachment of electrodes to skin is critical for accurate body function monitoring and testing, and in the performance of procedures requiring electrical conductivity.

Adhesives Research, with a long history in medical adhesive tapes, uses vinyls, nonwovens, and foam substrates in its adhesive systems – each one conforming with the applicable skin test requirements.

A recent addition to the company’s range is a skin-friendly and electrically conductive pressure-sensitive adhesive (PSA) tape, MA-46. The non-sensitising tape – for use in wound care applications and to attach medical devices to the skin – facilitates immediate bond formation to a variety of skin types, yet also allows for easy removal.

Medical adhesives are now being used in operating theatres and emergency rooms to close wounds in a faster and less painful way. According to the Society of Plastics Engineers, the procedure is four times faster than administering sutures and does not require painful injections.

To apply the adhesive, medical staff squeeze a sterile ampule while holding the edges of the wound together. Studies also demonstrate that the adhesive has superior healing results.

‘This is particularly important for use in head or face surgery where scarring is an important consideration,’ said Dr Dean Toriumi, associate professor of facial plastic and reconstructive surgery at the University of Illinois at Chicago. A liquid adhesive bandage by Closure Medical Corporation is being developed for use in the home and will be available soon for treating minor cuts and abrasions.