Medical devices - the today and tomorrow of joining and manufacturing technology.

TWI Bulletin, January - February 1998

Kalpana Mistry joined the Advanced Materials & Processes dept at TWI in 1995 as Project Engineer. Her research focuses on the welding and joining of plastics and composites for the aerospace, defence and medical industry.

The current status of medical device materials, products and manufacturing technology is highly specialised, although it has been suggested that the medical industry is a 'second target market' ^[1] because the newer materials and technologies are developed primarily for the aerospace and defence markets. The future for the medical industry lies with the device designers and engineers who are responsible for keeping abreast of new developments and applying this knowledge to their existing and new products. Kalpana Mistry identifies some of the newer areas of materials and joining development which can be exploited for future medical device manufacture.

Background to the UK medical device industry

The UK medical device industry's total sales in 1993 were estimated at £5 billion. Of this, approximately £2.7 billion were sales within the UK ^[2] . A breakdown of total sales by sector is shown in Fig.1. The medical industry is diverse and it comprises predominantly small companies. It is often characterised as innovative, secretive and fiercely competitive in nature. This reputation exists partly because medical device manufacturers devote substantially large investments to in-house research and development, which includes rigorous regimes of testing to ensure compliance with European legislation and FDA regulations and standards. An average of 4% of annual turnover is spent on research and development.

Initial stages of device development

Manufacturers will develop a new device when there is consumer demand and a market need. The results of a recently published DTI sponsored study, following the development of a new medical device from an idea to final product ^[3] , revealed some vital statistics:

• 15% of the total cost* is required to define a product
• A working prototype can be built and tested by spending only 8% of the total cost
• But, the design thinking amounts to almost 85% of the total cost of product

*Total cost refers to the total investment that is committed to the product over its lifetime.

The main conclusion to be drawn from these figures is that product development needs to be a continuous process in order to enable:

• Promotion of new ideas for product design as demanded by consumer market
• Creation of new products which provides the life-blood of the company
• Upgrade and evolution of old products as they become less competitive
• Generation of high returns on investment once the total initial development costs have been expended

The methods used in current device development use engineering materials such as steel and many established joining techniques such as welding. However, in order to be innovative and generate new products, designers are urged to think again about new lighter, stronger, cheaper and easier to fabricate materials to take the device industry forward.

Moving current device development technology into the future

The 12 sectors highlighted in Fig.1 can be categorised in five distinct areas of medical devices; packaging, electronics, surgery, prosthetics and hospital equipment. The current methods used for device development in these areas can be examined and ideas can be put forward where new materials and joining technologies can be effectively introduced ^[4] .

Packaging

The manufacture of many medical products and devices requires the ultimate in cleanliness. Some devices often go through a sterilisation process prior to use. In this case, it is the packaging of the product which is critical to ensure the shelf life of the product is achieved. The materials need to be sealed in such a way to protect from the external environment. However, the user needs easy access when the product is required in an emergency. Plastic film materials are often used for packaging purposes.

Examples of current uses of plastics packaging include the sealing of bags containing hearts and other transplant organs. Typically, polyethylene film is used and joined with heat sealing or impulse welding equipment. Problems can occur when the non-stick fabric coating on the heat seal or impulse electrode, which is resistively heated, wears out and needs replacing. This can introduce contamination in the joint and prevent a hermetic seal forming. To overcome this hazard, a novel method of film sealing such as infrared welding could be used. The method uses infrared radiation to soften the plastic and produce a weld. Potential benefits of this process include non-contact heating (thus eliminating contamination in the joint) sealing speeds of 5m/min and the equipment is portable.

Other packaging products include colostomy bags. These devices are currently produced using dielectric welding. Dielectric welding only creates heat in polar plastics materials which contain molecules which oscillate when subjected to a high frequency field. Hence, the process dictates the materials used for these bags. Typical materials include PVC and EVA/PVDC laminates. An alternative fabrication technique can be found in laser welding. This process uses a high density light beam of discrete wavelength, depending on the type of laser, to fuse plastic film materials together. New developments include lower cost diode lasers for joining which are now on the market. Potential benefits of the process include non-contact heating and hence reduced contamination, joining speeds of 500m/min for polyethylene film using a CO₂ laser and the use of lasers which deliver the beam via fibre optics for increased flexibility.

Electronics

A vast majority of smart electronic systems involve joining on a microscopic level. A widely used microjoining process includes wire bonding. This provides a means of delivering an electrical connection between a semiconductor chip and circuit. The importance of these joints is apparent in their application, for example, heart pacemakers. Wire bonding uses ultrasonic energy or heat and pressure (thermocompression bonding) or a combination of both (thermosonic bonding). The chip and circuit are generally encapsulated within a hermetic package, typical materials being Kovar™ (iron-nickel low expansion alloy), stainless steel and titanium.

Electronic packages such as heart pacemakers are sealed using resistance seam welding or laser welding/brazing. The advantage of brazing involves the use of a metallic material which melts at a lower temperature than the package material, thus minimising heat input and accommodating small fit-up variations.

One promising newcomer to joining of electronic devices is adhesive bonding. Examples include the attachment of piezoelectric sensors to substrates for ultrasonic scanning catheters using electrically conductive adhesives. Potential benefits include rapid cure times, accurate placement of minute components and reliability. Some adhesives have the added advantage of curing under UV light, which can speed up the assembly process.

Surgery

Surgical tools such as scalpels have traditionally been made from stainless steel. In order to reduce post-operative patient trauma, the surgeon takes care to make small and neat incisions. This task can prove difficult if the scalpel blade has suffered wear from previous cuts. A novel approach to surgical tools is the use of ceramic materials, such as zirconia. A ceramic blade would stay sharp as zirconia is a very hard and wear-resistant material; consequently leading to reduced patient trauma.

Prosthetics

Surgical implants such as hip and knee prostheses are required to provide support and manipulation to the human body. As implants, it is essential that materials do not wear, producing wear debris, which can be harmful within the body. Typical implant materials include titanium, stainless steel and cobalt-chrome. To make these materials biocompatible and bioactive (encourage coupling of the implant within the body) coatings can be applied. Hydroxyapatite, which is a form of calcium phosphate, can be sprayed onto the metallic implant to give it a bioactive surface. Bioactive coatings could also be considered for other non-metallic and composite materials.

Areas of new orthotic materials include the use of carbon fibre composite materials for artificial limbs. Potential benefits of composites include lightweight, strong and durable structures for the limb. Other applications include walking aids which combine traditional materials such as leather with modern carbon fibre composites. Joining techniques used include mechanical fastening and adhesive bonding.

Hospital equipment

Hospital beds, operating trolleys and invalid aids are currently fabricated from steel and steel alloys using the MIG (metal inert gas) and TIG (tungsten inert gas) welding processes. New activated fluxes have been developed to be used with TIG welding. Potential benefits include improving weld quality by increased penetration and reducing sensitivity to material composition. ^[5] Hospital trolleys for X-ray purposes can now be made with carbon fibre reinforced composites. The advantage is that carbon fibre is X-ray transparent.

Aiding future device development

Industrial Members of TWI in the medical industry can take advantage of the newer materials and their potential benefits by using TWI as part of the preliminary stage to aid future device development. An engineer can partake and supplement brainstorming discussion in the following areas:

• Predesign
• Concept design
• Detail design
• Preproduction
• Production troubleshooting
• Product development

The process of brainstorming can help identify the constraints raised, provide design advice, materials selection, joining/production technology, structural integrity and preliminary prototypes. The benefits of brainstorming are:

• Defines opportunities
• Solves problems
• Generates options for action
• Helps plan future strategies
• Stimulates enthusiasm and teamwork
• Evaluates ideas with respect to time and cost

It is part of TWI policy to maintain strict confidentiality of client projects, an essential requirement in this rapidly moving and innovative market.

Promoting partnership in innovative research

A recent NHS report ^[6] stated that in order to remain innovative, competitive and maintain a leading edge in the international market for well designed and proven medical devices and equipment, it is necessary for the medical industry to 'establish improved channels of communications and collaboration in R&D'. This view is endorsed by government-supported advisory group documents with a recommendation to 'explore the development and funding of additional communication networks and information databases to improve the opportunities for technological transfer and the formation of collaborative partnerships'. Cost to an individual company is an important factor. If there is a national/global problem which needs addressing and is shared by the medical industry, a potential solution is to form a collaborative consortium. If project partners are selected appropriately, added value will be achieved. This has proven a successful way forward for the automotive,

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