Felicity A Chipperfield and Sue B Dunkerton
Paper published in Medical Device Technology, May 2001
There is significant interest in the development of medical devices capable of performing satisfactorily at their intended use conditions and complying with the stringent health care rules and regulations. Medical devices whether temporary or permanent, used externally or inside the body, are becoming more complex and more sophisticated both in terms of their performance specification and structural complexity. As a consequence, many devices in current use are multi-component and require methods of assembly in production. Joining is one of the key issues in many manufacturing industries and the medical industry is no exception.
2. Joining requirements for plastics in medical products
Plastics' joining is used widely in medical applications for products of ever increasing diversity; ranging from tablet packs and sterile dressing packages to intravenous fluid bags and device packages. The wide range of applications; including joining of films, laminates, sheet and moulded plastics call for a variety of joining and welding methods. The joining processes currently used in medical products include:
For flexible products
For rigid products
- Hot bar or heat sealing
- Impulse welding
- Dielectric welding
- Induction welding
- Infra-red welding
- Adhesive bonding
- Ultrasonic welding
- Hot plate welding
3. Joining methods for flexible products
3.1 Hot bar welding (also known as heat sealing)
This is a technique, which is mainly used to join thermoplastic films, i.e. materials having a thickness of less than 0.5mm. It is based on the principle that if two thermoplastic films are pressed against a heated metal bar, they will soften and a joint can be made between them. Since the technique relies on the conduction of heat through one of the films, this limits the thickness of material that can be welded. Sometimes two heated bars are employed, one either side of the films, and this has the effect of reducing the welding time.
Hot bar welding can be a rapid process with typical weld times, for thermoplastics of approximately 100µm thickness, in the order of 1-3 seconds. Many medical devices are heat-sealed in packages of thermoplastic and plastic coated paper. The heated region can be textured to reduce and control the joint strength. This offers the opportunity to supply a sterile component in an easy-to-open package, see Fig.1. Outer packets for intravenous bags, laminated packs for powders, ostomy bags and some tablet blister packs are also heat-sealed.
Fig. 1. Hot bar welded package for tubing
3.2 Impulse welding
Fig. 2. Impulse welded package for surgical gloves
This is a variation of hot bar welding where one or both bars are heated and then allowed to cool rapidly. In this way the parts being welded experience a well-controlled heating and cooling regime, while still being held under pressure. This is a fast welding process, which creates minimal flash and can be used manually or can be automated.
Surgical gloves (usually made from polyethylene) are impulse sealed to maintain hygiene standards, see Fig.2.
3.3 Dielectric (high frequency) welding
Dielectric (high frequency or radio frequency (RF)) welding relies on certain properties of the material in the parts being welded to cause the generation of heat in a rapidly alternating electric field. This means that only certain materials can be dielectrically welded, mainly PVC. The process involves subjecting the parts to be joined to a high frequency electric field that is normally applied between two metal bars. These bars also act as pressure applicators during heating and cooling. The dynamic electric field causes the molecules in some thermoplastics to oscillate. Depending on their geometry and dipole moment, these molecules may translate some of this oscillatory motion into thermal energy and cause heating of the material.
Fluid (e.g. blood) and ostomy bags are essential, disposable hospital products, which are joined using the dielectric welding process, see Fig.3. These products are used for collection, containment and storage. This process is selected as it is low cost, has a high production rate and has great design flexibility. Dielectric welding is also highly suited to cutting at a tool edge and for the production of tear/seal areas in film products allowing easy opening or user-adaptation of the product.
Pneumatic support cells used in the care of pressure sores are dielectric welded from PVC sheet. Other applications include gloves (both industrial and medical), condoms, organ bags for endoscopic surgical procedures, inflatable catheter balloon cuffs, and barrier structures to isolate articles from environmental contamination, infectious material and the like.
Fig. 3. Sachet containing antiseptic fluid, sealed by radio frequency welding
3.4 Induction welding
This is a technique where heating is generated by an induced electric current in a conductor built into the product. The two most commonly encountered mechanisms by which heat can be generated by an induction field, are eddy current heating and heating due to hysteresis losses.
Induction welding by eddy current generation is similar to resistive implant welding in that an implant is generally required. Since most unfilled thermoplastics can be described as good insulators, an electrically conducting implant must be present at the joint line. A work coil, which is connected to a high frequency power supply, is then placed in close proximity to the joint. As electric current at high frequency passes through the work coil, a dynamicmagnetic field is generated whose flux links the implant. Electric currents are induced in the implant and when these have sufficiently heated the conducting material, the surrounding thermoplastic parts melt and soften. If some pressure is applied to the joint, this aids wetting of the molten thermoplastics and a weld forms as the joint cools.
Glass and plastic bottles, which are used in the medical and consumer-health industry, have their plastic coated aluminium tops sealed by this process. A metallic-coated plastic tab (e.g. polyethylene coated aluminium foil) is attached to the top by an induction field. The process is rapid, facilitates seal removal and provides tamper evidence, while retaining product quality. Some toothpaste tubes are also sealed in this way, see Fig.4, as are the majority of tablet blister packs.
Fig. 4. Induction welding of toothpaste tubes
3.5 Infra-red welding
For the welding of thermoplastics, two different approaches to infrared welding have emerged, both based around the principle of hot plate welding. One is to use tungsten filament line heaters as the heat source; the other, is to use an electrically heated ceramic plate. Both systems involve bringing the two plastic parts to be joined in close proximity to the infrared source for sufficient time for the parts to become molten, withdrawing the source, and then pushing the parts together to form a weld.
Infrared welding has a number of advantages over hot plate welding: weld times are reduced, the joints are free from contamination (since it is a non-contact process), and low modulus materials can be welded (since there is little or no shear of the parts during heating).
This technique is likely to find applications in many areas where hot plate welding is currently used and also in the medical industry, where high production rates of welded seals are often required in flexible materials.
4. Joining methods for rigid products
4.1 Ultrasonic welding
Fig. 5. Blood filter for autologous transfusion, joined by ultrasonic welding
Ultrasonic welding involves the use of high frequency mechanical sound energy to soften or melt the thermoplastic at the joint line. Parts to be joined are held together under pressure and are then subjected to ultrasonic vibrations usually at a frequency of 20 or 40kHz. The ability to weld a component successfully is governed by the design of the equipment, the mechanical properties of the material to be welded and the design of the components. Ultrasonic welding is a fast process (weld times are typically less than one second), can easily be automated and can produce hermetic seals. It is therefore ideally suited to welding components in mass production.
A medical device that uses this technique is a blood filter, which recycles the patient's own blood (autologous transfusion), see Fig.5. Collection flasks, ostomy bag flanges and anaesthesia filters are also ultrasonically welded.
Ultrasonic sewing can be used to join textiles together, replacing the use of thread and staples. Typical applications include; disposable garments, hair and shoe covers, hook and loop straps, and leg and arm covers.
4.2 Hot plate welding
Hot plate welding, is probably the simplest welding technique for thermoplastics. The parts to be welded are held in fixtures, which press them against a heated tool. The heating takes place in two stages; the heated tool melts the surfaces and material is displaced so that a smooth surface is obtained; mechanical stops in the equipment or a reduction in pressure prevent further displacement, but the parts continue to be heated by the tool until they are softened some distance away from it. The fixtures then open, the heated tool is withdrawn and the fixtures then force the parts together.
Medical products that are joined by hot plate welding include: filters (e.g. anaesthesia, blood, IV fluid, oxygenator), manifolds and pumps, tubes and catheters (e.g. angioplasty, renal and suction catheters), analytical devices(e.g. chest drainage units, home testing kits and testing trays), and other speciality devices (e.g. artificial joints, braces and splints, dialysis systems).
This article has summarised the wide range of existing welding techniques available for medical devices in plastics. For most applications there is a welding solution available. As certain products meet more stringent requirements or require more flexible processes to aid product design or reduce cost, further improvements or totally new processes may be developed.
Quality control aspects are also becoming more important to meet existing and potentially new regulations or legislation. Monitoring and control of welding processes and the standardised testing of joints will meet some if not all of these requirements.