Introduction to joining methods in medical applications
Abstract:
The joining of materials has been used in manufacturing since the beginning of mankind and has become one of the key technologies in many manufacturing industries. This chapter begins with an introduction where general information is given about the joining techniques that are commonly used, followed by a description of some of the joining processes used in medical devices. The last section presents a few photographs of medical devices where welding has been employed.
2.1 Introduction
The joining of materials has been used in manufacturing since the beginning of mankind and has become one of the key technologies in many manufacturing industries. With few exceptions, all products, machines or structures are assembled and fastened from parts. Joining is achieved through rivets, seaming, clamping, soldering, brazing, welding and the use of adhesives.
The medical-device industry is no exception. Medical devices are becoming more complicated, both in performance specifications and structural complexity. Medical devices typically consist of components and materials that must be joined in some way, whether used outside the body (in the form of instrumentation or surgical tools) or inside the body (for diagnostic monitoring or therapeutic purposes). Moreover, recently other joining methods besides stitching are also being applied for tissue bonding and skin closure.1,2
The ability to choose which joining process is appropriate at every step is critical in producing highly reliable devices, and the alternatives are diverse though all have advantages and limitations which ever application is targeted. The choice depends upon production economics as well as the mechanical properties desired in the final assembly. These properties go beyond just strength, covering issues such as vibration damping and durability, corrosion or erosion resistance, as well as the ability to correct a defective final assembly.
Medical devices which are manufactured in a variety of materials such as metals, polymers and ceramics can be joined by mechanical fastening, welding and adhesive bonding. The complexity of the applications, the bio compatibility and the health-related legal requirements necessitate giving serious consideration to the joining processes used so that all requirements are met.
In this chapter a general overview of the joining technologies available is given, followed by a short description of the techniques more commonly used nowadays for medical devices. Joining processes are usually divided into three categories:
1. mechanical joining, where parts are joined through clamping and riveting, etc.;
2. welding, where parts are joined by fusion welding, brazing and soldering, and solid-state welding;
3. adhesive bonding, where parts are joint by applying an adhesive to the surfaces.
All three methods are used in medical devices though the material, size of the parts and application of the device imposes stringent limitations on the scope of processes with potential use.
Joining the parts of surgical instruments which are not for invasive use has different requirements than joining prosthesis which are for invasive use and thus have special requirements in, for example, durability and corrosion resistance to cope with use inside the human body (see Chapter 14).
Mechanical joining is a process for joining parts through mechanical methods, which clamp or fasten the parts of the assembly together (e.g. nails, screws, bolts, rivets). Joining parts using screws or nuts and bolts are common examples of mechanical joining and can be found in many applications from simple chairs to sophisticated airplanes.
The advantages of mechanical joining include versatility, easiness to use and possible dismantling of the product, making it ideal for cases where periodic maintenance requires disassembly. The possibility of the joining of dissimilar materials is also an advantage. It allows movement of components relative to one another, for example by the use of hinges and bearings.
Mechanical joints also have disadvantages: the joint is achieved through discrete points thus not creating a continuous connection between the parts, while the holes employed for mechanical joining are vulnerable to fractures and galvanic corrosion.
Welding technology comprises three types of processes (Fig. 2.1):1,3,4
1. fusion welding, where there is melting and solidification of the zone being joined;
2. brazing and soldering, where joining is achieved by adding a melted filler material between the surfaces to be joined;
3. solid-state welding, where plastic deformation and diffusion are the main phenomena involved and there is no melting of base or filer materials.
Welding includes the joining process for metals and plastics where both the work pieces to be joined as well as the filler material used experience some melting. Examples include laser, electron beam, arc and resistance welding.
A common method for welding metals is arc welding, where an electric arc between an electrode and the work pieces to be welded together generates enough heat to melt the material under the arc. The joint is completed by solidification of the melted material from the parts being welded.
Arc-welding processes can use a consumable electrode from which the arc is generated, so that molten metal from the electrode and molten base metal from the work pieces all get mixed together, solidifying to form a strong joint upon cooling. In order to protect the molten material from contamination or the surrounding atmosphere a flux or a shielding gas are used.5
Most arc-welding processes are not used in medical applications since the related heat input, associated distortions and metallurgical transformations are inappropriate for the welding of small devices and, even in the cases where applicable, it would create defective welds. The required accuracy and quality are difficult to achieve.
High temperatures generated by the welding process alters microstructure in the welded areas, which creates a fusion zone associated with the molten metal and a heat affected zone (HAZ) that undergoes metallurgical transformations. This can change the mechanical behaviour of the material. The process of fusion and solidification also generates residual stresses that can lead to distortion. For these reasons the welding process must be optimized (heat input, metal composition and cooling rate) with a view to minimizing microstructural changes and residual stresses in welded joints. Post-weld heat treatments are often performed to relieve residual stresses and also for the purpose of changing the welded microstructure.
Resistance welding includes the fusion processes where heat is generated by the resistance of the work to the passage of electric current. Force is always applied during and after the application of current to create the weld.5 The contacting surfaces in the region of current concentration are heated by a short time pulse of low-voltage, high-amperage current to form a fused weld nuget. When the flow of current ceases, the electrode force is maintained until full solidification of the weld metal.1 Further information on these processes is given in Chapters 5 and 6.
Brazing is a process for joining two work pieces by the introduction of a filler metal that is melted between the two surfaces of the base material which is kept in a solid state.5 The temperature at which brazing is done must be high enough to melt the filler material but not the base material. Materials used as fillers for brazing are those that melt above 450 °C. Flux is also used during brazing for the purpose of eliminating oxide films from work-piece surfaces and avoiding oxidation. This ensures a good metallurgical bond between the work pieces and the filler. Filler alloys used for brazing include copper–silver, zinc–copper and copper–nickel. The brazing material, once melted, fills up the spaces between the surfaces being joined, and flows into tight spaces by capillary action. A strong joint is thereby achieved after the brazing material has cooled down.
Soldering is a similar process to brazing, but performed at much lower temperatures. In soldering two work pieces are joined, so that only the filler metal undergoes melting; that is work pieces do not experience melting. Typically soldering materials melt at less than 450 °C while brazing materials melt above this temperature. Common soldering materials include alloys in tin–lead, tin–zinc, lead–silver and cadmium–silver alloys. Like welding and brazing processes, soldering also employs materials (i.e. flux) to clean the surfaces being bonded. In the case of soldering flux, residues must be removed to reduce the risk of corrosion.
Solid-state welding includes the joining processes where pressure or temperature, or both, are used to achieve the joint.5 No melting occurs in the base or filler material. In some cases a relative motion of the parts being joined is used to achieve the generation of heat. The application of pressure assures the plastic deformation or diffusion needed to accomplish joining. A filler metal may be inserted between the surfaces being welded.
Solid-state welding has the advantages of allowing to join similar and dissimilar metals and to obtain a continuous joint between the faying surfaces with minimal distortion. The disadvantages of these processes are that they cannot be adaptable to high production rates, surface preparation needs greater care than in fusion welding or brazing and soldering and the need of applying simultaneously heat and pressure leads to high equipment costs.
Adhesive bonding is a process for joining parts using bonding chemicals (materials known as adhesives).6 This process is employed to join polymers and polymer–matrix composites, as well as polymers to metals, metals to metals and ceramics to metals. Adhesive-bonded joints have the characteristic that they can withstand shear, tensile and compressive stresses, but they do not exhibit good resistance against peeling. This last weakness of adhesive bonding requires a good design (i.e. the adhesion area must be maximized and mechanical interlocking is employed).
Maximum strength in adhesive bonding requires thoroughly cleaned surfaces. This, in essence, minimizes the interfacial gap between the adhesive and the adherent (material to be joined). Surface roughness improves mechanical interlocking and thus bond strength.
Medical devices which are manufactured in a variety of materials such as metals, polymers and ceramics can be joined by mechanical fastening, welding and adhesive bonding. The complexity of the applications, the bio compatibility and the health-related legal requirements necessitate giving serious consideration to the joining processes used so that all requirements are assured. Not all the processes presented in Fig. 2.1 are used in medical devices.
Welding and adhesive bonding are the processes most widely used in medical devices. For metals the fusion welding process most often referred in the literature is laser welding while solid-state welding processes examples are ultrasonic, diffusion and friction welding. When considering polymers, hot-plate and hot-bar welding, ultrasonic and impulse welding are some of the techniques used, while adhesive bonding is possibly the most common joining method for this type of materials.
In the following chapters a description of the characteristics of the joining processes most often used in medical devices is presented. The list of processes addressed in not meant to include all joining processes that can be used in medical devices. It refers to joining processes frequently referred in the literature as giving good results in medical devices. Many more processes exist, some of which are variants of the referred ones and some based on different principles.
2.2 Welding processes
The welding process most commonly used in assembling metallic medical devices is laser welding. Other processes such as resistance welding,7,8 and gas tungsten arc welding,9 are described in the literature as achieving good results in the welding of NiTi parts or in particular applications like catheters and pacemakers. More recently the application of friction-stir welding (FSW) is increasing. For welding of polymers laser welding is also used and other processes include radio-frequency (RF) welding, ultrasonic welding (USW), diffusion welding, hot-plate and hot-bar welding.
Selecting the best process for the welding of medical products involves a number of considerations which include the type of base material, the size of the part, the number of parts to be welded, the process capability and the cost.
The following chapters of this book detail the use of the different joining techniques for specific applications, while this chapter gives a brief explanation of the characteristics of the welding processes most often used in medical applications.
2.2.1 Laser welding
Laser-beam welding is a fusion welding process where radiant energy is used to produce the heat required to melt the materials being joined.5,10–13 A concentrated beam of coherent, monochromatic light is directed by optical devices and focused to a small spot, for higher power density, on the abutting surfaces of the parts being joined (Fig. 2.2). Gas shielding is generally used to prevent oxidation of the melted material. It provides consistent joining and high flexibility. Different parts and even different metals can be joined in a non-contact process. The required accessibility to the work piece being from one side only and the opportunity to abandon filling material completely are the main advantages of laser-beam welding. Laser joining can be performed using either pulsed or continuous lasers. A pulsed laser can be used to create weld seams by means of overlapping pulses.
Lasers have played an important role in the joining of materials since the invention of high-power solid-state and gas lasers in 1964, especially for resistance trimming of electronic circuits, until the introduction of a reliable high-performance laser in the late 1970s, which allowed its application for welding of sheet-metal parts.
Several types of lasers can be used in materials processing though the most common have wavelengths of 10.6 µm – CO2 lasers (gas lasers), or 1.06 µm – Nd-YAG lasers (solid-state lasers). The last decades have seen the rise of diode lasers and diode-pumped solid-state lasers. More recently high-power diode-pumped fibre lasers were developed. Fibre lasers are a serious alternative to solid-state and carbon-dioxide lasers for different materials-processing applications.
The CO2 laser is a well-established materials-processing tool, available in power output up to 50 kW, and most commonly used for metal cutting.14 CO2 laser radiation (wavelength 10.6 µm) is readily absorbed by the surface layers of most metals and plastics. The CO2 laser beam cannot be transmitted down a silica fibre optic, but its path can be determined using mirrors, lenses in optical systems and wither gantry or robotic movement.
The Nd:YAG laser (Neodymium Yttrium Aluminum Garnet) is also well established for materials processing; it has a wave length of 1.06 µm and is a smaller laser available in powers up to 10 kW. The Nd:YAG laser beam can be transmitted down a silica fibre optic enabling easy, flexible operation with gantry or robot manipulation. However, the light from Nd:YAG lasers is absorbed far less readily in plastics than that of CO2 lasers.
Small focus spots for CO2 and Nd-YAG lasers are achieved through optical systems mounted in the arm of the robot or gantry systems.
High-power diode lasers (>100 W) have been available since the late nineties.14 Recent developments have made available diode lasers up to 5 kW and competitively priced compared with CO2 and Nd:YAG lasers. Diode lasers are energy efficient compared with other solid-state lasers (i.e. power input versus power output is 30 per cent, while for Nd:YAG laser it is 15 per cent). With diode lasers a rectangular beam shape is produced, which, while being preferential for some applications, limits the minimum spot size and maximum power density available. The diode laser source is small and light enough to be mounted on a gantry or robot for complex processing.
Excimer lasers are gas lasers which were first operated in 1975, some years after the CO2 and Nd:YAG lasers.14 The gases used to produce the laser beam are inert gas (e.g. Ar, Xe, Ne). They are available with average powers up to about 1 kW and focusable to very high-power densities. There is a family of wavelengths available by exciting different phases within the laser. These are all in the ultraviolet (0.15–0.35 µm), and lie in a photon energy range capable of breaking chemical bonds and splitting molecules. Excimer laser light is absorbed by molecules in the surface of plastics (<10 µm depth) and rapidly breaks the molecular bonds within the polymer structure. Consequently, this leads to a rapid increase in pressure and expulsion of material over a very precise region defined by the laser-beam size. Excimer laser beam is transmitted by mirrors and often focused through masks to give the required features on the material surface.
Fibre lasers were created in the early sixties and used widely at low-power levels throughout the 1980s and the 1990s as optical amplifiers. In 2000 the first 100 W fibre laser was launched. The first reports on the potential of these lasers in materials processing are very recent.15,16 The multi-kilowatt range for materials processing with fibre laser is now possible with the recent commercialization of 7–10 kW power lasers.
These new lasers have multiple advantages: high efficiency, compared with the other types of lasers; a compact design, which simplifies its installation; a good beam quality, due to the use of thin fibres and thus a small beam focus diameter; and a robust set-up for mobile applications. The scaling of the laser power is achieved by a modular design comprising several single-mode lasers. The lifetime of the pumping diodes exceeds the expected lifetime of other diode-pumped lasers, which leads to low costs of ownership.
The high-power lasers can be used for deep penetration welding in a diversity of materials and constructions as the low wavelength, similar to the one from the Nd-YAG lasers, that characterizes these lasers allows its absorption by almost all metals and alloys and the fibre delivery system provides the necessary flexibility on the positioning of the beam. Also high-speed welding of sheet-metal joints can surpass the productivities achieved with high-power CO2 lasers. Due to the low wavelength its use on the welding of plastics presents the same limitations as Nd:YAG lasers.16
In general, the use of lasers to deliver the energy necessary for welding has the following advantages:
Laser techniques are also used for the welding of plastics in medical application as an alternative to vibration, ultrasonic, dielectric, hot-plate or hot-bar welding and adhesive bonding.14
Lasers are an attractive tool for processing of metals and also polymers as it allows a precise delivery of a controlled amount of energy precisely on the point where it is required (e.g starting point of a weld, spot welding). In addition, lasers are available with outputs covering a range of wavelengths, which has a large bearing on the interaction of the light with the plastic materials. An understanding of these absorption characteristics has led to the development of other novel applications in plastics processing. Common applications include end effector tools to tube bodies and hermetically sealing implantable devices.
2.2.2 Friction welding
The heat necessary to induce plastic deformation in metals and to soften and melt some plastics can be achieved by simple friction when the two parts being joined are moved rapidly while in mutual contact (Fig. 2.3). This technique is most commonly used when two circular parts are welded, a circular part is being welded to a flat plate or a circular rod is being welded inside a hole. For maximum efficiency, the surfaces being friction welded should be relatively smooth and free of contamination. Very large surfaces are difficult to weld by this method because of the large amount of energy that is required to melt or soften the entire surface simultaneously.
Friction-stir welding (FSW) is a solid-state welding process in which a spinning non-consumable tool is forced along the joint line, heating the abutting components by friction, and producing a weld joint by extrusion (around the tool pin), forging (under the tool shoulder and the base of the tool pin) and large-scale plastic flow mixing (stirring) of the material from the two components, in the vicinity of the tool (Fig. 2.4).17
Friction-stir welding has been reported to be possible to use for joining parts in Nitinol, a shape-memory alloy, which is difficult to fusion weld to itself and to other metals.18,19 One of the most common uses of Nitinol is in the production of biomedical devices such as arterial stents. The Nitinol retains its shape memory and superelastic properties following processing.
2.2.3 Ultrasonic welding
Ultrasonic welding (USW) is a solid-state welding process that produces a weld by local application of high-frequency vibratory energy while the work pieces are held together under pressure.5 A sound metallurgical bond is produced without melting the base material.
Typical components of an ultrasonic-welding system are illustrated in Fig. 2.5; these include sonotrode, transducer and anvil. The ultrasonic vibration is generated in a transducer and the vibration is transmitted through a coupling system or sonotrode. The sonotrode tip makes direct contact with one of the work pieces there by transmitting the vibratory energy into it. The clamping force is applied axially through the sonotrode. The anvil supports the welding and opposes the clamping force.
Ultrasonic welding is used for applications involving both similar and dissimilar joints as well as the welding of polymers. The process is used to produce lap joints in metals, plastic sheets and plastic films in varied shapes as wires (crossed or parallel), ribbons and flat surfaces. For joining other types of assembles that can be supported on an anvil, set-ups are designed for the material and the part shape required.
Next to fusion, the most important non-adhesive and non-mechanical method of joining plastics is ultrasonic welding.
2.2.4 Diffusion welding and brazing
Diffusion welding (DFW) is a solid-state welding process that produces a weld by the application of pressure at elevated temperature with no macroscopic deformation or relative motion of the work pieces.20 A filler metal may be inserted between the faying surfaces.
Several kinds of metal combinations can be joined by diffusion welding:18
1. Similar metals may be joined directly to form a solid-state weld. The required pressures, temperatures and times are dependent only upon the characteristics of the base metals and their surface preparation.
2. Similar metals can be joined with a filler metal where this takes the form of a thin layer of a different metal which is situated between the metals to be joined. In this case one of the roles of the filler metal is to promote a more rapid diffusion and permit increased microdeformation at the joint to provide more complete contact between the surfaces. The filler metal may then be diffused into the base material by suitable heat treatment.
3. Two dissimilar metals may be joined directly where diffusion-controlled phenomena occur to form a metallic bond. The mechanisms are similar to those in category 1 above. In addition dissimilar metals may create added effects such as improved initial contact.
4. Dissimilar metals may be joined with a third metal; that is, a filler metal, between the faying surfaces to enhance weld formation either by accelerating diffusion or permitting more complete initial contact in a manner similar to category 2 above.
Diffusion brazing (DFB) is a process that forms liquid braze metal by diffusion between dissimilar base metals or between base metal and filler metal pre-placed at the faying surfaces. The process is used with the application of pressure. The filler metal may be diffused into the base metal to the extent that a distinct layer or brazing filler metal does not exist in the joint after the diffusion-brazing cycle is completed. The joint properties approach those of the base metal. The process is sometimes called liquid-phase diffusion-bonding eutectic bonding, or activated diffusion bonding.
2.2.5 Radio-frequency welding
Radio-frequency welding (RFW) has similarities to USW.14,18 The main differences are that the frequency is higher and the power is lower. This process is mainly used for the welding of plastics. Some plastics are highly susceptible to induced vibrations in the radio-frequency range which produce an oscillation in the plastic molecules as the alternating current switches polarity. Some molecules attempt to align with these alternating signals and produce heat when radio-frequency energy is applied.
2.2.6 Induction welding
Induction welding relies on the heat generated by utilizing the property of some materials to vibrate in a magnetic field.14,18 This process is mainly used for the welding of plastics. Induction welding is produced by generating electromagnetic heating in metal particles located inside a plastic matrix. That is, heat is generated by utilizing the property of metals to vibrate in a magnetic field inducing current. Heating in the plastic results from conduction of heat generated in the metallic particles.
One inductive welding technique is to place a thin metal shim between two plastic surfaces and apply an external electromagnetic field. The result is that heat generated in the shim wall causes local melting of the plastic which will lead to fusion. Another induction-welding technique is simply to place the plastic materials to be joined around a metal part (such as a rod) and then pass the rod and material through a magnetic field. Sufficient heat can be generated to fuse the plastic materials.
2.2.7 Hot-plate and hot-bar welding
Hot-plate welding is performed by pressing the parts being joined against a heated plate, and removing the parts when these are sufficiently melted and pressing them until the joint cools down.14,18
The plates are usually heated using resistance heaters and for most of the applications flat plates are used, though more complex shapes can be manufactured. The welding parameters are the temperature of the plates, the heating time, the welding pressure and the welding time. Thermoplastics can be welded using this technique and provided a good welding procedure is used the tensile strength of the joint equals the base material.
Hot-bar welding is a technique mainly used to join thermoplastic films, that is, materials having a thickness of less than 0.5 mm. 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. Some times two heated bars are employed, on either side of the films, and this has the effect of reducing the welding time.11
2.3 Adhesive bonding
Adhesive bonding is used to join metallic and non-metallic materials and is a technique used for medical devices both in plastic and in metals. The surfaces being joined are not melted although they may be heated.6,18,21–24
When two materials are joined together using adhesive bonding, those two materials are called the adherents, or substrates. The material that forms the bond between them is called the adhesive.
The first step in the joining process is the preparation of the adherent, by cleaning, etching or surface treatment to create the surface on which the adhesive is applied.18 An adhesive is then applied in various ways where these include a liquid, a paste or a tacky solid each of which is to be placed between the faying surfaces of the joint. Liquid adhesives are commonly applied by brush, spatula, roller, flow gun, spray and dip coating. Some of these methods such as dip coating and spraying are especially useful for adherents that have complex shapes. Rolling and spatulas are better for flat surfaces. Film adhesives work best on flat panels but can be shaped into fairly complex forms by cutting and applying in strips (i.e. pressure is required to make sure that the adhesive flows to fill in gaps between the strips).
After application of the adhesive, the coated adherents are placed together and held until the adhesive becomes solid. A press can be used to hold the pieces together but jigs and fixtures that keep the alignment of the part and apply moderate pressure are also common. Most adhesives need to be pressurized during cure. If a press is not used, mechanical pressure devices like clamps or springs work well. Another way to exert pressure is with a vacuum bag.
When under pressure or in the jig/fixture, the adhesive solidifies. That step can be done by simply allowing the adhesive to cool (if it is a hot-melt adhesive) or by curing the adhesive (if it is a thermosetting material). During cure the important parameters are temperature and time. Ideally, the bond between the adherent and the adhesive will be so strong that if fracture occurs, it will be either within the adhesive itself or within the adherent itself rather than at the junction between them.
Typical examples of medical-grade adhesives are given below:
• Silicone adhesives: Organo-polysiloxanes, generally known as 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. Depending on the nature of side groups and the inter-chain cross-linking, silicones are available in the form of gels, liquids or elastomers.
• Epoxies: The epoxy resins are thermoset adhesives based on the epoxide group, a three-membered carbon, carbon and oxygen ring structure which is also known as oxirane group. The ability of this group to undergo a large variety of polymerization and cross-linking reactions leads to many different types of epoxy resins with a wide range of chemical and physical properties, molecular weight and molecular structures. Epoxies are one of the most widely used adhesives for both structural and non-structural applications. Epoxies have been used in a number of medical devices for bonding and sealing applications.
• Acrylics: There is a wide range of acrylic-based adhesives that have been 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 or tapes, or as monomer–polymer mixtures (one- or two-part components), with liquid or powder curing agents. Acrylic-based adhesives may be polymerized or cured using moisture, catalysts, heat, ultraviolet (UV), visible light or other sources of radiation. These adhesives are of particular interest in the medical industry both for joining medical devices and as tissue bonding agents.
• 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 alcohol and isocyanate results in the formation of urethane.
Table 2.1 presents a summary of the benefits and limitations of these adhesives and gives examples of applications.
2.4 Examples of applications
Many examples of applications of joining techniques for medical devices can be found in the literature. Figures 2.6–2.9 depict a few of these applications where the welds are signalled. Many more will be found in the later chapters of this book.
2.5 References
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8. Fukumoto, S., Fujiwara, K., Toji, S., Yamamoto, A. Small-scale resistance spot welding of austenitic stainless steels. Materials Science and Engineering: A. 2008; 492(1–2):243–249. [Sept].
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14. Dunkerton, S.B., Tavakoli, S.M. Process developments enabling more effective joining of medical devices. ASM 2004 Materials & Processes for Medical Devices Conference, St. Paul, MN, 2004 www.twi.co.uk/content/spsbdaug2004.html [Available from].
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16. Quintino, L., Costa, A., Miranda, R., Yapp, D., Kumar, V., Kong, C.J., Welding with high power fiber lasers – a preliminary study. Materials & Design, 2007;28(4):1231–1237, doi: 10.1016/j.matdes.2006.01.009. Included in Science Direct Top 25 Hottest Articles, October
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18. Chipperfield, F.F., Dunkerton, S.B., Welding and joining techniques for polymeric medical devices. Medical Device Technology. 2001 http://www.twi.co.uk/content/spfamay2001.html [Available from].
19. London, B., Fino, J., Pelton, A., Fuller, C., Mahoney, M.Friction stir processing of nitinol. The Minerals, Metals and Materials Society, 2005.
20. Schwartz, M.M., Gerken, J.M. Diffusion welding and brazing. In: Welding Handbook. American Welding Society (AWS); 1991:814–837. [chapter 26].
21. Shah, T. Polyurethane thin-film welding for medical device applications. Medical Device and Diagnostic Industry. 2002. [September].
22. Salerni, C. Selecting engineering adhesives for medical device assembly. Medical Device and Diagnostic Industry. 2000. [June].
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24. Quintino, L., Pires, I. Overview of the technology of adhesive bonding in medical applications. Business Briefing: Medical Devices Manufacturing and Technology, Materials/Biomaterials. 2004; 55–56.