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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