S. B. Dunkerton and S. M. Tavakoli
TWI Ltd, Cambridge, UK
Paper presented at ASM 2004 Materials & Processes for Medical Devices Conference, St Paul's Minnesota, August 25-27, 2004
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. Medical devices, whether used outside the body in the form of instrumentation and control systems or surgical tools or used inside the body for diagnostic monitoring or therapeutic purposes (e.g. sensors, catheters, pacemakers or prostheses) usually consist of many materials which may be joined in some way.
This paper will overview a series of developments in laser, adhesive and general microjoining technologies.
Polymeric materials may be processed in many ways using lasers, for melting, vaporisation or ablation. Recent advances have been made utilising laser techniques for welding of plastics. Lasers are now being considered as an alternative to vibration, ultrasonic, dielectric, hot plate or hot bar welding and adhesive bonding, for medical devices, tubular systems, films and synthetic fabrics.
Transmission laser welding is used to carry out rapid welding of plastics by transmission of laser radiation through to an absorbing interface, where very localised, precise heating takes place, minimising any thermal damage. The process lends itself to automation, either by robotically manipulating the laser head to work over large components, or by scanning the laser beam via mirrors to weld small complex parts. The main limitation of the laser welding process is that the material on the upper side of the joint must transmit at least 10% of the laser energy. This excludes plastics with high contents of some pigments such as TiO 2 or carbon black and metallic additions or coatings.
A laser source for transmission laser welding must deliver a radiation wavelength in a range where the polymer transmits. Absorbers can then be chosen to be applied where the beam needs to be absorbed to create a weld. Typically apolymer will transmit visible and near-infrared (NIR) radiation. Readily available and moderately priced laser sources that fit this requirement include the high power diode and Nd:YAG laser sources. They both emit in the NIR range,808-980nm for diode sources and 1064nm for Nd:YAG sources. The diode or Nd:YAG laser beam can be transmitted down a fibre optic enabling easy flexible operation with gantry or robot manipulation. Additionally the whole diode lasersource is small and light enough to be manipulated by a robot arm or on a gantry. The beams from both laser types may also be scanned using galvanometer controlled mirrors to allow following of complex weld line shapes.
Recently developed fibre lasers operate at wavelengths of 100nm and above. The lower end of this range is suitable for transmission laser welding, which has been successfully demonstrated at TWI.
Clearweld ® is a variation of the transmission laser welding process that provides designers with a tool to laser weld clear, coloured or opaque thermoplastics. It is well suited to medical applications, because inaddition to the benefits of conventional laser welding, both substrates can be clear or the same colour.
Clearweld ® uses materials capable of strong absorption in the NIR spectrum which impart minimal visible colour. Material systems incorporating these absorbers have been developed to facilitate laser welding in a widerange of thermoplastic assembly operations, including compatibility with a variety of NIR lasers. Clearweld ® can be used in a number of ways, including, but not limited to, application as a thin layer at the joint (from a solvent-based ink), or inclusion within the lower substrate. Compared with previous solutions, it offers complete freedom in the choice of colour, reflectivity and transparency of the work pieces at visible wavelengths.
TWI invented the process, which was developed in partnership with the Gentex Corporation. Clearweld ® consumables are now commercially available from Gentex ( www.clearweld.com).
Fig. 1. Laser overlap weld in clear PMMA made with Clearweld ® absorber at the interface ( Courtesy of the Gentex Corporation)
There is a significant interest in using polymeric adhesives and coatings in medical and implantable devices as well as in dentistry, pharmacy and surgical operations. The main advantages are seen in the ability to join a wide rangeof materials, effective manufacturing and the availability of medical grade materials for implantation.
Adhesives may be identified in many different ways as follows:
- By chemical types (epoxy, silicone)
- By origin (natural, synthetic)
- By physical form (films, tapes, pastes, one or multi-part components)
- By curing methods (e.g. heat curing, moisture curing, radiation curing)
- By functional types (structural, pressure-sensitive, hot-melts)
- By end use (sensors, catheters, tissue or bond bonding).
Thermosets, thermoplastics and elastomers
Polymeric based adhesives can be mainly divided in three major classes of thermosets, thermoplastics and elastomers.
A thermosetting adhesive, as the name suggests, 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 (e.g. epoxies, polyesters and phenolics) are the basis of many structural adhesives for load bearing medical and engineering applications, as well as in precision joining in electronic applications.
In contrast thermoplastic adhesives (e.g. polyamides) may be defined as materials which soften, melt and flow on the application of heat and solidify on cooling.
Adhesives may also be based on natural (e.g. natural rubber) or synthetic (styrene-isoprene-styrene block, SIS copolymers) elastomers. Elastomers (e.g. polyisobutylene, PIB) are the main polymers used in many pressure sensitiveadhesives for producing medical tapes. Adhesives based on other natural origins (e.g. proteins, cellulose, starch etc) are also available and are very important for many medical and pharmaceutical applications.
Successful utilisation of adhesives in joining materials normally requires suitable surface treatment of the adherends prior to bonding. Selection and application of an appropriate surface treatment is one of the major factors in achieving good wett ability and improved long-term durability of adhesively bonded joints. Inadequate or unsuitable surface treatment is one of the most common causes of premature degradation and failure. The function of surface treatment includes the removal of contaminants or weak boundary layers and alteration of surface chemistry, topography and morphology in order to enhance adhesion and durability. Surface preparation techniques are generally divided into mechanical or chemical methods:
- Grit blasting
- Shot blasting
In many applications simple degreasing and abrasion is often sufficient to provide good adhesion. However, many medical polymers with low surface energy and bondability (e.g. polyolefins) often require a more specialised treatment(e.g. plasma treatments) in order to provide better adhesion and joint durability. Some adhesion promoters can also enhance bondability of certain polymers. Recently new grades of adhesives with the ability to bond polyolefins without the need for pretreatment (e.g. polyethylene) have become commercially available.
Pretreatments based on excimer lasers have been successfully developed at TWI to modify surfaces of a range of polymers to enhance adhesion.
Compliance with USP and ISO standards
Tests to determine the biological reactivity of polymeric materials and medical devices are described in the USP (United States Pharmacopedia) and ISO (EN ISO 10993-1) standards.
According to the injection and implantation testing requirements specified under Biological Reactivity Tests, in vivo polymers are scaled on a Class of I to VI. Polymers not requiring implantation are graded Class I, II, III or Vand those polymers requiring implantation testing are graded Class IV or VI.
The ISO Standard 10993 consists of 16 parts and each part describes specific tests which also include a variety of toxicity tests as identification and quantification of degradation products from polymers (Part 10).
Many polymeric adhesives are available which can be qualified as USP Class IV and VI materials. These materials can pass the relevant incutaneous toxicity (in vivo), acute systemic toxicity (in vivo) and implantation (in vivo)testing requirements. Passing USP Class VI standards does not guarantee that an adhesive will meet the FDA requirements in a particular application. However, passing the test is a strong indication of non-toxicity of an adhesive.
Certain types of medical grade epoxy adhesives are also capable of being sterilised by autoclave, ETO and chemical methods. These types of epoxy resins could be used in medical devices which require sterilisation prior to use.
In joining and coating, the condition of a surface is critical to enabling a successful process. This is particularly critical for adhesive bonding as described earlier and in ensuring biocompatibility.
Power beam processes offer good scope for consistent surface treatments and can provide thermal, chemical and physical alteration of the surface. A new technology has been developed by TWI taking advantage of the precise control of an electron beam.
Electron beams are extensively used in a wide range of materials processing, but this current development extends work on high power beam processing of metals to enable unique surface features to be introduced. In an earlier configuration, texturing of a surface has been undertaken by discrete heating and melting of individual points on a surface, Fig.2. Large numbers of heating sites are possible using rapid deflection of the electron beam. The recent development is repeated heating and melting on the same sites, to manipulate material movement and give rise to unique profile with both upstands (protrusions) and slots, Fig.3. Such features are already showing promise for dissimilar material joining (metal to polymer), and may give characteristics which are exploitable for adhesion of coatings and other materials and to give rise to specific functionality (e.g. thermal management, biocompatibility).
Fig. 2. Electron beam manipulation of metal surfaces 2a) surface texturing
Excimer laser ablation enables the precise removal of material by breaking down the chemical bonds of the polymer and expulsion of molecules from the surface. The biocompatibility of such surfaces has been examined with goodindication of cellular activity, Fig.3
Fig. 3. Cellular response on an excimer micromachining polycarbonate surface
The rapid development of micro-electronics and micro-machining technology is allowing the manufacture of increasingly smaller and more sophisticated medical products. These devices, often incorporating combinations of electronic, sensor and micro-mechanical elements, are being used for both short and long term implantable medical devices.
Typical examples include: muscle and spinal cord stimulators, catheters (scanning and surgical), stents, heart pacemakers, hearing aids, drug delivery systems and surgical tools for minimally invasive microsurgery and lab-on-chipbiosensors.
These developments have only been possible through advances that have been made in miniature electronics, micro-machining, materials and joining technology. It is now possible to mount sensors and signal processes on the tips of wires <1mm diameter and produce motors of only 120µm diameter. This opens up the prospect of a whole range of new and 'smarter' medical devices.
Fundamental to these developments is packaging, which influences product reliability, cost, size and performance.
Packaging technologies that can be adapted for medical devices include Ball Grid Arrays, Micro Lead Frame and Chip Scale Packages. The stacking of multiple dies allows mixing of technologies and this concept, linked with the potential 3D structures addressed by direct write technology, could lead to singnificant miniaturisation and greater scope for implantable devices.
Various connection methods exist for fine scale connection, with the main stay still being wire bonding. Wires down to 10µm diameter can be bonded, although the minimum used for volume applications is typically at 40-50µm,Fig.4.
Fig. 4. High dennsity wire bonding
Increasingly flip chip bonding is being the new interconnect method where fine scale features are required.
Flip chip technology
Flip chip bonding involves the use of perimeter or area array of very short, micro metallic or polymer bump connections, Fig.5, between a device that is inverted in a flip chip configuration over a mirror image pad array on a substrate. Flip chip die accounted for some 5% of the 60 billion Inegrated Circuit (IC) devices produced each year worldwide in 2002, and is predicted to climb to 10% by 2005. The vast majority of applications lie in electronics, photonics and sensors where high interconnection densities are needed, but medical applications include hearing aids, pacemkers, defibtillators, DNA analysers and smart pills. The main drivers for using this technology in this case are volume and size.
Fig. 5. Solder bump connections on device prior to flip chip bonding
Direct write technology
As the name implies, direct write technology is a means of directly depositing ciruitry onto substrates and devices, with the aim of reducing processes and enabling finer scale interconnection. The technologies available for this are extensive giving a series of options where selection is based on materials, dimensional requirements, performance and cost. Typical classifications include:
- Lasers - for laser machining, rapid prototyping, laser assisted CVD, laser assisted etching.
- Ink jet technology - for depositing inks, adhesives and solder to create circuits.
- Thermal spraying - for depositing metals, ceramics and polymers
- E-beam lithography.
- Dip pen nanolithography TM (trademark of NanoInk Inc)
Depositing methods allow for the build-up of strcutures downto the nanoscale, whilst other techniques rely on maching down of larger (micro-scale) structures.
Joining, coating and surface modification are key enabling technologies with particular relevance to microsystem technology, where is the need to develop multi-material structures with advantages taken of each materialcharacteristic rather than requiring compromise at the system integration level. As demand continues for miniaturised systems which extend into newer markets such as implantable medical devices, technology readiness is essential to ensure new robust products can be commercialised to meet market need. This paper has briefly introduced some of the upcoming capabilities, with laser technology, long term implantable adhesives, fine scale interconnect andbiocompatible packaging forming an essential foundation to many forthcoming products.
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