The use of adhesives for medical devices is growing at a near 10% annual rate, claims Skeist Inc. in their 2007 assessment of this market. Medical adhesives can be classified into the categories of implant, device and equipment, tissue bonding, pressure sensitive, dental, and wound closure.
Within the device and equipment category are several types of adhesives for the assembly of disposable and non-disposable devices, surgical tools, and medical equipment. These adhesives include cyanoacrylates or instant adhesives, light curing acrylics, epoxies, polyurethanes, and silicones, each with different properties that can be grouped into uncured, curing, and cured categories.
An adhesive’s flow characteristics play a critical role in its application and function. Viscosity, or a liquid material’s resistance to flow, is the typical value reported for most “flowable” adhesives. Its common unit of measure is the centipoise (cP).
Temperature can affect viscosity and other flow measurements. In many instances, as temperature increases, adhesives flow more or become lower in viscosity. This characteristic can greatly affect adhesive application in warm environments or on warm components.
Flow dictates an adhesive’s migration potential. Very low viscosity adhesives can be applied to assembled devices, for example on cannula to hub assemblies in medical needles where a close fit is present. The adhesive’s thin consistency allows it to migrate into designated areas of the preassembled components; a 100 cP adhesive can successfully flow into a 0.002-in. diametrical cannula-hub gap in less than 5 sec. It is important, however, to control low viscosity adhesive application on assemblies where migration could affect nearby components and overall device function.
Adhesive viscosity is also closely linked to gap filling. As a general rule, thick or high viscosity adhesives with minimal migration handle large gaps well. Low viscosity adhesives like cyanoacrylates fill only very limited gaps. When gap size is exceeded, the adhesive cures inadequately.
Because adhesives are often visible on medical devices, their cured and uncured appearances are critical. Most medical device manufacturers desire adhesives that are clear and colorless in their final cured state. But clarity can present challenges during and immediately following application. Therefore, adhesive suppliers add fluorescent dyes that are visible only when the adhesive is placed under a black inspection light. The adhesives remain invisible to the casual observer but can be inspected during and following assembly and cure.
Most adhesives begin as liquids and convert to solids during the curing cycle. Curing properties include fixture time, tack-free time, exothermic output, and cure depth.
Fixture time is the time required for an adhesive assembly to reach handling strength and is highly dependent on test conditions. Handling strength is unique to each device and its associated assembly process, so its value varies for each application.
A device that will bear a load within several minutes of adhesive application must develop sufficient strength to accommodate the load without sacrificing the integrity of the bond. For example, fluid reservoirs are pressure tested within seconds or minutes of assembly, so you must account for the applied load on the bond joint when calculating the fixture time.
In general, cyanoacrylates and light curing adhesives have the fastest fixture times. Epoxies, polyurethanes and room temperature curing silicones can take up to 15 minutes to develop handling strength. Cold temperatures generally lengthen cure time, as do large bond areas and adhesive volumes. The substrates undergoing bonding can also positively or negatively affect cure speed. For example, acidic substrates can slow the strength development of a typical cyanoacrylate adhesive.
The surface dryness or tack-free nature of an adhesive can be critical in medical device applications. Certain adhesives cure to a tack-free surface faster than others. For example, cyanoacrylates perform optimally when confined between close fitting parts with minimal squeeze out and should not be used in applications with large exposed surface areas of adhesive, as they will not readily cure dry-to-the-touch.
Conversely, light curing adhesives are often recommended as coatings or for exposed bondline applications. Cone to cushion assembly of medical masks, catheter balloon coating, and molding of actual hearing aid shells are examples where the ability to cure tack-free is critical to device performance and biocompatibility.
The majority of liquid adhesives emit heat (exotherm reaction) during their curing process. Significant amounts of exotherm can deform or permanently damage temperature sensitive parts. Large volumes of any adhesive can potentially generate heat that could be hazardous to parts and personnel.
Epoxy adhesives generate the greatest amounts of heat during cure. In small volumes, most epoxies will not generate enough heat to cause damage. However, fast curing versions and large volumes can generate surprisingly high temperatures. In general, silicones, polyurethanes, and light cure adhesives generate the lowest amounts of heat, while cyanoacrylates and epoxies can be on the higher end.
Cure-through depth (CTD) or volume is closely linked to exotherm. It refers to the amount of adhesive, typically measured in inches or millimeters, that can be fully cured. It varies widely between adhesive chemistries and formulations in an adhesive family. Epoxies and polyurethane adhesives are widely known for their virtually unlimited CTD. However, large volumes of adhesive can generate more heat from the exothermic reaction.
Cyanoacrylates exhibit the lowest CTD at a maximum of 0.010-in. for the highest viscosity formulations. Recently introduced light curing cyanoacrylates that cure with light as well as moisture offer cure depths of 0.125-in. and greater. A variety of other light curing adhesives, including acrylics and silicones, can cure to depths in excess of 0.25-in. with the proper light and intensity conditions.
CTD is critical for fluid device assembly applications. In some cases, it lets manufacturers reduce the part tolerances typically required for solvent welding. Manufacturers of polycarbonate-based blood and membrane oxygenators, cardiotomy reservoirs, heat exchangers, filters, and pressure transducers all rely on light curing adhesives to fill the gap between two components and provide a strong bond and tight hermetic seal.
Properties for an adhesive in the cured or solid state include bulk mechanical, such as tensile, elongation, and hardness; and performance, such as bond strength, environmental resistance, sterilization exposure and biocompatibility.
Bulk mechanical properties are tested by preparing cured films or sheets of adhesive. A tester, such as an Instron, then records values that indicate hardness and flexibility. The tensile value of an adhesive – typically reported in pounds per square inch (psi) – is defined as the force required to break a specimen divided by the area of the specimen. The higher the psi, the greater the force required. Typical tensile strength for a medical epoxy adhesive is 8,000 psi, for a light cure acrylic adhesive it is 2500 psi, and a medical silicone adhesive is 150 psi.
Elongation dictates adhesive flexibility; higher values indicate increasing flexibility. The flexibility of an adhesive joint is critical for devices that bend or move during use and where significantly different substrates are joined together (for example, plastic to metal, flexible plastic to rigid plastic, ceramic to plastic).
Catheters, tubing connections, and various respiratory devices commonly incorporate flexible adhesive joints. In general, cyanoacrylate and epoxy adhesives exhibit low percent elongation values, while silicones are generally the most flexible. Light curing acrylic adhesives vary widely in their elongation properties with formulations ranging from 5 to 300%.
The Condensed Chemical Dictionary, 10th Edition, defines hardness as “…resistance of a material to deformation of an indenter of specific size and shape under a known load….” The hardness of a film of cured adhesive is evaluated using the Shore indentation test method and scale. Although the A and D scales are most commonly used for adhesives, there are a total of 12 scales that can be used, all with a data range of 0 to 100. The higher the value recorded for an adhesive, the harder the material.
Medical device adhesive performance properties are tested on materials or substrates representative of the finished device. Bond strength, the most common criteria, is measured in several ways.
The lapshear method involves two specimens that are approximately 1-in. wide by 4-in. long by 1/8-in. thick. Adhesive is applied to the leading edge of one specimen covering an area about 1-in. wide by ½-in. long. The specimens are mated and a clamp load applied to ensure that the adhesive is compressed and spread evenly within the bondline. They are then tested using a mechanical properties tester that pulls the lap shears away from each other in tensile mode. The strength to break the adhesive bond or the substrate is recorded.
Developed more than 50 years ago, lapshear testing can cause commonly used plastics to undergo “differential straining.” The configuration of the bond and the inherent flexible nature of plastics can cause the test specimens to bend, exposing the bondline to more forces than just tensile.
Block shear is a more appropriate method to evaluate bond strength on plastics. It uses smaller, thicker specimens (1 in. by 1 in. by 0.25 in.) and exposes them to a compression test load. It eliminates the differential straining experienced with lapshear testing, and is believed to offer the truest assessment of bond strength.
Other bond strength tests are also used. For example, varying needle hubs and cannula gauges are often used to determine the pull strength of an adhesive.
Regardless of the test method, the goal is to achieve the highest strengths possible. The adhesive strength should exceed substrate strength, where the adhesive bond outlasts the substrates in withstanding stress.
When reviewing and comparing bond strength data, ensure that the most appropriate test method was used and that test methods are consistent among suppliers.
Environmental resistance is also a critical factor. Unlike other assemblies, medical devices are
exposed to sterilization and accelerated aging to estimate shelf life. Both activities involve exposure to elevated temperatures and chemicals.
Although most cured adhesives are thermoset polymers, many are still affected by elevated temperatures. As epoxies offer optimum temperature and chemical resistance, they are often selected for applications with high temperatures and aggressive environments, for example, repeated autoclaving at temperatures greater than 121°C.
Silicones are also temperature resistant and maintain their flexibility over a wide temperature variant. In their cured state, cyanoacrylates are thermoplastics and are quite susceptible to temperatures above approximately 90°C, although improved formulations can withstand temperatures as high as 121°C. Most grades of medical device adhesives can withstand common accelerated aging conditions for several weeks at 60°C. Carefully evaluate your choice of adhesive for applications with temperatures in excess of 60°C.
For sterilants and other chemicals, limited cycles of most gaseous sterilization processes such as gamma, ethylene oxide, and plasma-phase hydrogen peroxide do not typically have a dramatic impact on cured medical device adhesives. Excessive cycling in these environments, however, should be screened to ensure minimal impact on materials and adhesives.
The most challenging sterilization environment for adhesives is autoclaving, having a particular effect on cyanoacrylates. Limited cycles of autoclave with most other bonding materials yield minimal loss in strength, but testing should always be completed to ensure that no other interactions effect performance.
Biocompatibility is an additional unique requirement for adhesives used in medical device assembly. Adhesive suppliers test their products to biocompatibility standards similar to those that were originally developed and used for plastics. Test programs vary widely and should be reviewed to minimize risk.
Christine Salerni Marotta is a development chemist for Loctite (now Henkel), and was active in the development of cyanoacrylate adhesives and primers. She spent eight years as an applications chemist in the company’s North American Engineering Center, where she became the technical liaison for medical device manufacturers. She has a BS in Chemistry and a MS in Technical Management. She is an active member of the American Chemical Society, Society of Manufacturing Engineers, and, most recently, the Society of Plastic Engineers.
Selecting an adhesive supplier
When selecting an adhesive supplier, be sure to ask the following questions:
• What USP Class VI or ISO 10993 protocol does the supplier use?
• How often does the supplier re-test to validate continued conformance to this standard?
• Does the supplier have quality controls to ensure continuity of adhesive from batch to batch?
• Are certificates of compliance available and readily accessible?