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

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.