Sticking to the healthy option - using adhesive bonding in medical and implantable devices - Pt II

TWI Bulletin, July/August 2001

Mehdi Tavakoli received a BSc in Chemistry and an MSc and PhD in Polymer Science and Technology at Aston University in Birmingham. He joined TWI in 1989 and has been working on new joining and associated technologies with particular interest in joining and coating of medical and implantable devices. He has over 20 years research, industrial problem solving and product development experience on polymeric materials and has published more than 40 papers and patents and is currently working as Technology Manager on polymers/adhesives at TWI.

There is considerable interest in using polymeric adhesives and coatings in medical and implantable devices as well as in dentistry, pharmacy and surgical operations. As Mehdi Tavakoli reports in the second part of this materials, processes and applications review, medical devices used externally or inside the body are becoming more complex and sophisticated both in terms of their performance specification and structural complexity.

Part I of this feature looked at adhesive types and some of their applications. Part II looks at surface preparation, dispensing and curing options as well as examples of the use of adhesives in medicine.

Surface pretreatments

Successful use of adhesives in joining materials normally requires suitable surface treatment of the adherends prior to bonding. ^[7] Selection and application of an appropriate surface treatment is one of the major factors in achieving good wettability 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:

• Mechanical:

  - Abrasion

  - Grit blasting

  - Shot blasting

  
  
  
• Chemical:

  - Degreasing

  - Etching

  - Anodising

  - Adhesion promoters

  - Flame treatments

  - Corona treatments

  - Plasma treatments

In many applications simple degreasing and abrasion is often sufficient to provide good adhesion. However, many medical polymers with low surface energy and bondability ( eg polyolefins) often require a more specialised treatment ( eg 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 ( eg 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. ^[10-11]

Dispensing techniques

Mixing

Manual mixing and application should be avoided, if possible, as this can introduce voids, bubbles and regions of incomplete mixing. Pre-packaged cartridges, using hand held dispensing guns for single or multi-part paste adhesives, are recommended. For high volume production, semi-automated or automated pump dispensers should be used.

Component parts of the adhesive are mixed during the dispensing process by being forced through a static mixing nozzle. When a new nozzle is fitted, or when at the beginning of a new production run, it is recommended that a 50-75mm bead of adhesive is dispensed on to scrap material to ensure good mixing is achieved and aged adhesive is removed.

Dispensing

Basic dispensing principles

• Supply of energy to the bulk adhesive to move it through feed-pipes to a valve or pump, then to a dispensing nozzle.
• Control of the valve or pump, to vary volume to be applied each cycle.
• Control of the speed of movement between dispensing nozzle and component - either the component or the dispensing nozzle can be moved, depending on the application.

There are many methods of dispensing adhesives. Due to cost restraints and processing considerations ( ie speed and volume of adhesive to be dispensed) the techniques considered most suitable are:

• Cartridge
• Pressure/time systems
• Pumps
• Microjet printing

These important methods are described in the following sections.

Cartridge dispensing

These may be single or double cylinders, with mechanical or air movement of the piston(s). Figure 6 shows how applying pressure to the plungers allows equal measures of part A (adhesive) and part B (hardener) to be mixed automatically in the mixing nozzle. The mixed adhesive is then applied to the component as a single shot. Sizes typically range from 5ml to one litre. Cartridge dispensing is generally a manual process suited to batch production runs.

Pressure/time system

Adhesives with viscosities up to approximately 10000 poise can be dispensed. Bulk containers vary from 50ml to 100 litres. Pressure/time dispense systems are suitable for high volume production rates, in a semi-automated or automated process. Pinch valves or diaphragm valves are used to control dispense volumes and prevent adhesive stringing between components.

Pumps

Dispensing adhesives by pumping techniques can be achieved in a number of ways, depending on whether a one or two part adhesive is being dispensed.

One part adhesives are dispensed using direct metering extrusion pumps, shown in Fig.8. An electric motor pushes a follower plate into a drum of adhesive, which is extruded through a hose to the dispensing valve. This technique has been used for medium and high volume production rates.

Two part adhesives are generally dispensed by volumetric pumps for semi-automated and automated medium-high volume assemblies. A typical pump is shown in Fig.9, which is similar in principle to the cartridge system described above. These metered dispense systems are capable of processing adhesives with mix ratios from 1:1 to 15:1. A bulk dispensing system is shown in Fig.10.

Microjet printing

Two main types of approach are usually used in ink-jet printing: ^[12]

• Drop-on-demand (DOD)
  This can produce smaller droplets (10-100µm) at lower frequency (to 10Hz). ^[13]
• Continuous, charge and deflect
  This can provide larger droplets (up to 0.5mm in diameter) at rates up to 1MHz and is particularly suitable for high speed, large area coverage. ^[14]

The two main material requirements for dispensing by the DOD method are:

• The polymeric resin has to be stable and reducible in viscosity to the 0.4 poise level, typically by heating or diluting with a suitable solvent.
• The printed polymer must have the required properties for the intended application after solidification on the substrate.

100% solid polymeric materials may be jet printed provided that they do not degrade at the elevated temperature at which the required viscosity can be reached. For diluted versions of printable polymeric resin it is essential to select a solvent which does not evaporate too rapidly and block the ink jet head. The polymeric resins containing fillers ( eg filled adhesives) can also be printed providing they satisfy the viscosity and chemical stability criteria and filler particle size of <10µm.

Microjet printing is the subject of one of the current co-operative research programmes at TWI. It has been shown that suitable grades of a range of commercially available adhesives can be successfully printed in pico litre volumes with consistency and good printing quality. The microjet printing technique is emerging as one of the most attractive dispensing techniques for many new micro and optoelectronic, as well as medical device manufacturing applications. An example of microjet dispensing of a radiation curable microelectonic adhesive is shown in Fig.11.

Requirements of dispensing equipment

The choice of dispensing system will depend on:

• Adhesive type
  - One part so no mixing
  - Two part with mixing in the nozzle or on the part
  
• Viscosity (determines whether pumping or cartridges will be used)
• Curing mechanism (prevents adhesive hardening within dispenser)
• Curing time (prevents hardening in dispenser)
• Cycle time (to fit the production schedule)
• Factory environment
• Volume used per cycle and per day (manual, semi or automated process)
• Accuracy required per component
• Overall cost.

Curing techniques

Curing of polymeric adhesives or encapsulants can be achieved using moisture or catalysts in the presence or absence of air ( eg for anaerobics in the absence of air) at room temperature, thermally at elevated temperature or photochemically using irradiation ( eg UV or visible light). The heat activated curing could be achieved using:

• Local heat application at the joint(s)
• Overall heat application
• Conventional/conduction ovens
• Hot plate heating
• Infrared heating
• Vapour phase heating
• Liquid phase heating
• Laser heating (Nd:YAG and CO ₂ lasers)
• Microwave, particularly variable frequency microwave (VFM).

There are a range of materials available which can be cured using radiation sources such as UV and visible light. Acrylated resins (acrylated epoxies, polyesters, polyurethanes and silicones) can be cured using radiation energy. Radiation curable adhesives or encapsulants generally consist of low or medium molecular weight resins (called oligomers), monofunctional or multifunctional monomers, additives, pigments, photoinitiators or photosensitisers.

A typical UV energy of 80-120 mW/cm ² produced from a UV source (wavelength 300-400nm) is usually sufficient to cure a UV curing adhesive within 10-60sec. An alternative radiation curing technique is to use visible light (wavelength 470nm) curing ( Fig.12). Many dentists currently use this technique for dental curing materials. Radiation curing adhesives have also been used for joining many clear polymers in disposable and non-disposable medical devices. In general both UV and visible light curing can be achieved using light boxes or focused beams and light guides. In some cases heat is also used to encourage the curing process, for completion of cure, or to cure areas that cannot be reached by the radiation energy.

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. ^[15]

According to the injection and implantation testing requirements specified under Biological Reactivity Tests, in vivo polymers are scaled from Class I to VI. ^[16] To grade a polymer, extracts of the test material are generated in various media and injected systematically and intracutaneously into rabbits or mice to evaluate their biocompatibility. Polymers not requiring implantation are graded Class I, II, III or V and those polymers requiring implantation testing are graded Class IV or VI.

The ISO Standard 10993 consists of 16 parts with each part describing 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 epoxy resins could be used in medical devices which require sterilisation prior to use.

Typical examples of the use of adhesives in medical and implantable devices

Catheters

Cyanoacrylates have been used for joining latex balloons on to PVC, urethane and multi lumen tubes for balloon catheters. An example of the use of cyanoacrylates in balloon catheters used in angioplasty is shown in Fig.14.

Pacemaker

Needles

Polycarbonate devices

Masks

Tubesets

Concluding remarks

Acknowledgements

References