The growth of high performance plastics has significantly widened the options for designing and bonding disposable and reusable medical devices. Assembling with adhesives can offer big cost and performance advantages over traditional mechanical fasteners, such as screws and snap fits, as well as other bonding methods, including welding, brazing, and soldering. The increase of microelectronics in devices calls for greater use of adhesive bonding of assemblies that are smaller and lighter than previous designs. What's more, modern adhesives that perform well are now available and have posted excellent safety records along with minimal environmental impacts.

The problem with traditional mechanical fasteners is that they become increasingly difficult to apply when the joined components are small and thin, such as sheet metal less than 0.01-in. thick. Also, skilled labor is usually needed for joining small-diameter wires in devices such as fibrillators, especially when they call for high reliability.

Joining without fasteners

Adhesives are available for a range of materials to resolve manufacturing problems. They have the additional benefit of spreading mechanical stresses over wide areas instead of point-to-point contact typical of mechanical fasteners. Adhesives can also greatly reduce or eliminate corrosion problems in the often hostile environments of human bodies and hospitals. Modern, well designed, structural-medical adhesive compounds are used safely for either disposable, or even reusable medical devices that must be sterilized, especially when it involves contact with skin and other body parts.

These adhesives can be described as large-molecular weight polymeric materials, commonly called plastics. They conveniently divide into thermoplastic and thermosetting classifications. Thermoplastic adhesives soften, melt, and flow when heated. They are readily processed by injection molding, extruding, and calendaring as well as various casting techniques. Other methods include solubility in carefully selected solvents, and dispersion in water and other solvents called emulsions. Thermosetting adhesives harden when heated. They are generally processed as liquids or low-melt temperature solids.

Today's medical devices involve assembling and joining many different components made from plastics along with a wide variety of metals such as aluminum, titanium, stainless steel, and copper as well as ceramics, optically transparent materials, and semiconductors.

Successfully bonding medical components involves a thorough understanding of design options for joining similar and dissimilar materials, performance characteristics of the adhesive and substrate properties, assembly method, including curing procedures, performance testing of the bonded joints for assembled devices, and if required, appropriate sterilization procedures.

The test data for substrates bonded with selected adhesive compounds come from carefully machined test specimens made in accordance with ASTM test procedures and other recognized testing organizations. Physical test machines do the pulling and peeling. Two widely used test procedures are ASTM D-1002, which measures tensile-lap-shear strength, and ASM D1878 for measuring peel strength.

Tensile shear strength determines the degree of adhesion to a substrate and the bond rigidity. A peel-strength test finds the flexibility of the joint when pulled apart in a tensile tester.

Many other tests are required to sufficiently describe an adhesive's performance and characteristics designers should be aware of. They include compressive strength, elongation, flexibility, hardness, dimensional stability, thermal expansion, thermal conductivity, insulation resistance, resistance to repetitive vibration, shock and temperature cycling as well as resistance to a wide range of environmental conditions including exposure to water and various solvents.

Also, resistance to sterilization is a primary consideration particularly for reusable devices. Many medical applications are required to conform to USP Class 6 (for biocompatibility) and its ISO equivalents for device safety with minimal decrease in performance.

One or two components

Adhesives come as one or two-component systems. One-component adhesives are easy to use because they need no mixing. On their downside, they have a generally more limited application range than two-component adhesives. While some single-component adhesives are applied at ambient temperatures, most require heat cures to optimizing their performance. The structural strength of one-component adhesives, such as epoxy-resin-based formulations, is generally realized only after a cure at elevated temperatures. Some can be stored at ambient temperatures for 12 months, and longer with dry ice.

Two-component adhesives generally have longer pot life, the period after mixing during which the material can be applied. They cure at ambient and more quickly at elevated temperatures. Two-component adhesives can be frozen and stored in dry ice for extended periods before use.

Viscosity

An important property of adhesive selection is its viscosity before it cures. A simple definition of viscosity is resistance to flow. Commercially available adhesive systems have viscosities that range from water-like liquids to peanut butter. The characteristic is generally measured in centipoises (cP) or millipascal-sec. (mPa-s).

Viscosity also governs the method that will apply the adhesive. In general, a low-viscosity adhesive is readily applied at ambient temperature with minimal pressure but may require containment or fixturing to limit unwanted flow. Measurements are usually conducted at a constant shear rate. Not surprising, viscosity greatly depends on temperature. As a rule of thumb, for each 10°C increase in temperature, viscosity decreases by roughly 40 to 50%. Also, adding solid fillers such as silicas, and aluminas (used to change the adhesive's properties) greatly increases viscosity. Highly filled adhesives are called thixotropic. Flow is shear dependent, meaning viscosity decreases with a high shear rate and is usually highest at rest. Additives are available to adjust viscosity (flow) as may be necessary for some applications.

Probably the most widely used flow-control agents are finely divided high purity silicas, some of which are coated with hydrophobic silicones to make them easier to apply. However, most medical adhesives do not contain inorganic fillers other than flow-control agents.

How fillers add function

Adhesives are formulated to be electrically conductive by adding large amounts of silver, platinum, nickel, copper, or graphite. They can cure at ambient for heat sensitive substrates or more quickly at elevated temperatures.

Also available are one-component electrically-conductive systems which cure between 200 to 250F. Thermally conductive yet electrically insulating formulations which contain thermally-conductive fillers (mainly aluminum oxide or aluminum nitride) have been developed for specific medical uses. Radiopaque medical adhesives are likewise available. They contain ceramic fillers such as barium sulfate or bismuth salts.

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Temperatures

Adhesives for bonding components of medical devices cover a wide range of temperature services from cryogenic to over 660F for brief periods, such as, when wave soldering or brazing. Cure schedules depend on the adhesive formulation, the substrate temperature characteristics, time of exposure, amount of compound used, and environmental conditions. Preferred high-performance-type structural adhesives can be cured from ambient to as high as 400 to 500F. The preferred range is about 250 to 357F. Here are a few pertinent temperatures and characteristics for a widely used range of adhesives.

  • Epoxy structural adhesives are recognized for providing the highest physical-strength properties and outstanding versatility in widely different applications from cryogenic to +500F service.

  • Polyamides and so-called liquid-crystal polymers may be considered specialties particularly for high-temperature service.

  • Polyurethanes are tough and flexible but are generally limited to 212 to 257F service.

  • Silicones feature unmatched flexibility and color stability in service to 500F, but are considered to have limited strength, little abrasion resistance at elevated temperatures.

  • Cyanoacrylates offer unmatched cure speed and high strength for many substrates, but lack toughness and flexibility. Usage is limited to 212F.

  • Thermoplastic adhesives are presently limited to joining plastics to each other, or to metals and ceramics. Many exhibit limited service performance.

  • Fluoropolymers offer the best chemical-resistance properties but exhibit relatively low adhesive strength with service capabilities to 500F.

  • UV and light-curing adhesives are currently limited to applications in which such light contacts the initially liquid composition. Then it cures in a few seconds. A fast cure enhances productivity.

Surface prep

It's not surprising that the best adhesion or bond requires properly prepared, cleaned, and roughened substrates. Physical abrasive treatments and appropriate chemical cleaning, or both, are essential manufacturing steps for achieving desired performance characteristics with most metallic substrates. Specially formulated primer coats are also useful in some applications. Be aware, however, that mold releases and surface treatments all adversely affect adhesive performance and are considered contaminants. A great deal of R&D has been carried out to develop suitable pretreatments for metallic as well as nonmetallic substrates. An accompanying table details a few recommended pretreatments.

Sterilization

Biocompatibility and sterilization are among the most important factors for bonding medical components to assure the safety of patients and hospital personnel. Medical devices conveniently divided into disposable and reusable products. Most disposable plastic materials for medical devices are thermoplastics such as polyethylene, polypropylene, ABS, and polyvinylchloride. Reusable devices from plastics are possible thanks to particular thermosetting epoxy compounds.

Test and performance criteria include USP VI and more importantly ISO VI protocols as well as ISO10993. These and other regulations describe systemic and intracutanacious in vivo and in vitro testing, in vivo implant tests, and certain animal tests.

But passing these tests is not sufficient for obtaining FDA approval. The agency's requirements depend on specific applications and may well require additional testing. Conformance to the USP VI test protocol and biocompatibility tests are a good indication of the suitability and toxicological safety of a proposed adhesive component for a medical device. Depending on the medical device, sterilization may be required before and after applying the adhesive.

Materials and adhesives common to medical devices
Thermoplastics Thermosetting Plastics Metals Ceramics Semiconductors
Polystyrene Epoxies Aluminum Aluminum oxide Silicon
ABS Thermoset polyesters Copper Magnesium oxide Germanium
ABS Phenolics Stainless steel Silica Aluminum gallium arsenide
Polycarbonates Polyimides Titanium Quartz gallium arsenide
Polyethylene, polypropylene and copolymers Polyurethanes Nickel Aluminum oxide Silicon carbide
Polyamides (Nylons) Silicones Cobalt Magnesium salts III-V semiconductors e.g. aluminum, gallium arsenide phosphate
Polyacetols Cyanoacrylates Silver Titanium oxide diamond
Polyvinylchloride and copolymers Polysulfide Gold Graphite Tin sulfide
Polyesters (PET, PBT, etc.) UV and Light curing compounds Platinum Boron nitride Lead telluride
Polyurethanes
Tungsten Zirconium silicate Zinc oxide
Polysulfones Vapor phase applied polymers Boron Aluminum oxide Cuprous chloride
Polyether ether ketones Epoxies Magnesium Barium sulfate
Fluoropolymers
Lithium talc
Viscosity of typical liquids and solids (cP) at 75F
Liquid Viscosity
Water 1.0
Milk 6.0
Ethylene glycol (antifreeze) 15
SAE 30 motor oil 150 to 200
Castor oil 1000
Maple syrup 4000
Honey 10,000
Chocolate syrup 25,000
Ketchup 50,000
Mustard 50,000 to 60,000
Sour cream 100,000
Peanut butter 250,000
Shortening 360,000
Recommended substrate pretreatments for bonding and assembling medical components
Substrate Recommended pretreatments
Aluminum and its alloys Solvent degreasing, detergent scrubbing, acid etching, sulfuric acid, sodium dichromate immersion, water rinse, physical abrasion.
Beryllium or Beryllium-copper Solvent degrease, detergent scrubbing, physical abrasion, sodium hydroxide, hydroxide/water solution at 80°C, water rinsing and dry.
Chromium Solvent degreasing, physical abrasion, nickel electroplating or hydrochloric acid etching; water rinsing, dry.
Copper and its alloys Solvent degreasing, physical abrasion, fast concentrated hydrochloric acid etch, acid etching (nitric acid or ammonium persulfate solution), rinse and dry.
Magnesium and alloys Solvent cleaning, dry abrasion, acid etching, rinse and dry.
Nickel and its alloys Vapor degreasing, acid etching, water rinsing, drying.
Stainless steel Solvent degreasing, detergent scrubbing, acid etching (sulfuric acid or chromic acid). Water rinsing, drying.
Silicon semiconductors Solvent cleaning or abrasion.
Titanium Solvent degreasing, physical abrasion, acid etching, strong mensoral acids, neutralizing rinse, air drying.
Zinc Solvent degreasing, dry or wet abrasion, water rinsing, drying.
Acrylic plastics Solvent cleaning, abrasion, acid etching epoxy/solvent adhesives, UV/light curing adhesives.
ABS plastics Solvent cleaning, abrasion, selective etching, rinsing, drying.
Acetal resins and copolymers Solvent cleaning, wiping, abrasion, acid etching, rinse and dry.
Celluloric homo and coploymers Solvent treatment, abrasion, solvent bonding, heating,
Diallylphthalates Solvent cleaning, abrasion, bonding with thermoset polymers such as epoxies
Epoxy resins and epoxy phenolics Solvent treatment, abrasion, epoxy resin adhesives.
Polyamides (nylons) Solvent abrasion treatments, polyurethanes and polyamide resins, cyanoacrylates
Polyesters, thermoplastics (PET, PBT) Solvent treatments, abrasion, primers, caustic solutions, UV or light curing adhesives
Polycarbonates Solvent cleaning abrasion, acrylic or epoxy adhesives, UV/light curing adhesives.
Polysulfones and Polyether elastomers Solvent cleaning, abrasion, epoxy or epoxy phenic adhesives.
Polyolefins (Polyethylene, Polypropylene) Solvent cleaning, abrasion, oxidizing flame treatment, acid etching.
Thermo plastic elastomers such as EPDM Corona discharge, plasma treatment, hot melts, primers.
Ceramics Solvent cleaning, abrasion, epoxy or epoxy phenic bonding
Carbon and carbon fibers Solvent cleaning, abrasion.
Fluoropolymers (e.g. Teflon, Viton) Solvent cleaning, followed by specialty primer, epoxy type adhesives.
Silicones Solvent cleaning, specialty primers, silicone adhesives.
Sterilization methods for bonded medical devices
Sterilization How it's done Example adhesives
High energy radiation Isotopes, electron beam accelerators, x-rays EP62-1MED, EP30MED
Autoclaving Steam usually 6 to 12 min at 130 to 140C, and a specified number of times. EP42HT-2, EP3HTMED
Ethylene oxide Exposure at ambient or near ambient temperatures for specified times, such as, over 10 hrs. EP21LV, EP41SMED
Liquid sterilants Glutaraldehyde EP41SMED, EP21LV
Plasma Hydrogen peroxide solutions EP42HT-2, EP30MED
Corona discharge
All medically approved adhesives
Peroxide acid solutions
All medically approved compounds
The transparent MB297 cyanoacrylate adhesive has a refractive index of 1.48, a Shore A hardness of 85, and excellent resistance to most environmental conditions such as moderate heat, aging, and many chemicals. It has a volume resistivity of 8.6 x1012 ohm-cm and a dielectric constant of 3.5 at 1,000Hz.

The transparent MB297 cyanoacrylate adhesive has a refractive index of 1.48, a Shore A hardness of 85, and excellent resistance to most environmental conditions such as moderate heat, aging, and many chemicals. It has a volume resistivity of 8.6 x1012 ohm-cm and a dielectric constant of 3.5 at 1,000Hz.

A closer look at a recent medical adhesive

Developments in medical adhesives are coming at a steady pace. A recent introduction, for example, includes an ultra-fast, room-temperature curing ethyl cyanoacrylate that bonds well to glass, ceramics, metals, rubbers and most plastics. MB297 Medical is a high strength bonding compound in a one-component system. UV-curable cyanoacrylates are also available.

Many medical-grade adhesives features a low viscosity of 2,000 to 2,400 cP making it easy to apply. Bond periods generally range from a few seconds to less than 60 depending on atmospheric humidity and the substrates being bonded. Some compounds need only contact pressure after applying them.