A Designers Guide on Development of Advanced Medical Adhesives

Master Bond Inc. has developed a wide variety of adhesives with a range of physical strength requirements designed to best meet specific application needs. High strength structural adhesives are available for metal to metal bonding uses subjected to high stresses under hostile environmental conditions. Toughened structural adhesive systems are available for uses where thermal cycling and/or severe mechanical vibration stresses are encountered. Special formulations are offered for bonding materials with widely different thermal expansion coefficients such as glass to metal or plastic to ceramics. Compositions can be designed to optimize shear strength, peel strength, fatigue resistance etc. as desired. Many Master Bond formulations meet or exceed applicable MIL specifications for high performance structural adhesives.

Advanced Adhesives for Medical Devices

Advanced adhesives can improve the design of joints and the assembly of disposable and reusable medical devices. A few of the more widely used joint designs for adhesively bonding similar and dissimilar substrates. Lap joints work well in medical device assemblies.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.

Figure 1. A few of the more widely used joint designs for adhesively bonding similar and dissimilar substrates. Lap joints work well in medical device assemblies.

Benefits of Adhesives in Overcoming Manufacturing Challenges

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.

Thermoplastic and Thermosetting Adhesives

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.

Techniques for Testing Performance and Characteristics of Adhesives

Adhesive test procedures per ASTM D1002 and D1878. All dimensions are in inches. 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.

Figure 2. Adhesive test procedures per ASTM D1002 and D1878. All dimensions are in inches.

One-Component Adhesives

One and two-component adhesives are available in a variety of packaging including cans, bottle and syringes. 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

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.

Figure 3. One and two component adhesives are available in a variety of packaging including cans, bottle and syringes.

Viscosities of Adhesives

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).

A viscometer is used to measure the flowability of an adhesive.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.

Figure 4. A viscometer is used to measure the flowability of an adhesive.

Electrically-Conductive Adhesive Systems

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 250°F. 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.

Effect of Temperature on Adhesive Systems

Adhesives for bonding components of medical devices cover a wide range of temperature services from cryogenic to over 660°F 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 500°F. The preferred range is about 250 to 357°F. 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 +500°F 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 257°F service.
• Silicones feature unmatched flexibility and color stability in service to 500°F, 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 212°F.
• 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 500°F.
• 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 Preparation for Optimal Adhesion

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 of Medical Devices

Autoclaving is one of the many methods available for sterilizing medical devices. 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.

Figure 5. Autoclaving is one of the many methods available for sterilizing medical devices.

Table 1. Sterilization methods for bonded medical devices

Materials and Adhesives Common to Medical Devices

The materials and adhesives commonly used in medical devices are outlined in table 2.

Table 2. Materials and adhesives common to medical devices

Summary

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.

Figure 6. 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.

New Epoxy Systems for Medical Devices

New Master Bond two component epoxy system EP42HT-2ND-2MED BLACK cures at room temperature and is resistant to repeated sterilization. This compound is colored black and has a paste consistency. It meets USP Class VI requirements. Master Bond Inc. also recently introduced EP3HTSMED. This one component epoxy system offers high electrical conductivity, high shear strength and cures rapidly at elevated temperatures. It is ideally suited for demanding manufacturing specifications.

This information has been sourced, reviewed and adapted from materials provided by Master Bond Inc.

For more information on this source, please visit Master Bond Inc.