Medical device assemblies present unique challenges beyond those associated with manufacturing and assembly of products for consumer and industrial applications. While sharing the need for ensuring reliable, robust seals or bonds with these industrial applications, medical applications are subject to stringent requirements for biocompatibility and must withstand harsh treatment during sterilization using high temperatures, radiation, or chemicals.
Specialized silicone adhesives combine characteristics for meeting more conventional assembly challenges while also meeting the unique requirements of medical device assembly. By drawing on a variety of one- and two-part silicone medical adhesives, manufacturers of medical products can address the diverse requirements of medical applications without compromising the strength and integrity of the various bonds, seals, and coatings essential for safe and reliable medical products.
Silicones are elastomeric systems that offer a unique combination of flexibility and strength that makes them attractive for ensuring successful bonding in diverse applications and industries. Silicone adhesives, sealants, coatings, and potting compounds can withstand shock, vibration, impact, and aggressive thermal cycling, while maintaining elasticity, stress resistance, and long-term high-temperature resistance. Along with their thermal stability, silicone adhesives are available in optically clear formulations and a wide range of hardnesses while ensuring high bond strength even between dissimilar substrates.
At the same time, the biocompatible characteristics of silicone adhesives have earned them a leading role in medical applications. Whether used for adhesive purposes, as sealants, coatings, or as potting/encapsulation compounds, all medical-grade silicones need to be nontoxic and show compatibility with blood and body fluids. Additionally, they must be biologically inert and must satisfy ISO 10993-5/USP Class VI specifications for biocompatibility, nonirritating, and nonsensitizing behavior. At the same time, medical silicones are able to withstand the impact of different types of sterilization methods including gamma radiation, ethylene oxide (EtO) sterilization, and various other chemical sterilizing agents. Medical-grade silicones are designed to meet these requirements.
Applications for Medical Adhesives
With their unique characteristics, silicone adhesives offer cost-effective solutions for the assembly of reusable and disposable medical devices. Indeed, they find a long history of use in medical products. Easy to apply, silicones bond well to a wide variety of substrates, speed productivity, and cure at room and elevated temperatures. Consequently, these materials continue to find ready application in meeting diverse requirements for bonding, sealing, potting, or coating in a wide range of medical and surgical devices, instruments, and peripheral products.
While nonimplantable medical devices are the prime market for the use of silicone adhesives, the growing variety of medical-grade silicone formulations provides an effective solution to an expanding range of medical applications. These applications fall into three basic categories:
- Disposable medical devices
- Reusable medical devices
- External medical equipment and systems
- Disposable Medical Devices
Disposable medical devices have always been a prime area for the exceptional bonding characteristics of medical-grade silicones. Although these products are used only once, they must nevertheless first go through some type of sterilization or disinfecting process before they are packaged for one-time use. In turn, the silicone adhesive material must be able to stand up to this process. Because of their unique characteristics, medical-grade silicones find extensive use in disposable medical devices, including catheters, infusion pumps, IV kits, blood exchangers, and syringes and cannulae.
Reusable Medical Devices
Reusable devices and instruments are typically used on multiple patients. Reusable instruments include the following:
- Most surgical instruments, including surgical forceps and all types of cutting instruments
- Stethoscopes and endoscopes (which may be partially reusable/partially disposable)
- Robotic surgical instruments
- Laparoscopic instrumentation and related surgical accessory instrumentation
- External medical equipment and systems
All of these are subjected to multiple sterilization processes, depending upon the type of instrument. This sterilization process can combine a wide range of methods including heat/steam autoclave, EtO, vaporization, plasma, gamma radiation, and electron beam. Silicones typically stand up to these sterilization methods without degradation. Only repeated autoclaving environments can cause a loss in flexibility or lead to them turning brittle.
Although the working area of most external medical systems does not penetrate the human body, they all come into close contact with the patient. Consequently, all of these systems must be sterile, and all bonding components used anywhere in or on the systems must be both biocompatible and resistant to continual antimicrobial cleaning, disinfecting, and partial device sterilization. External medical equipment examples include MRI machines, PET and CT scanners, ultrasound machines, electrocardiograph devices, tonometers, x-ray machines, otoscopes, spirometers, and drug-delivery systems.
Matching Unique Requirements
All medical-grade silicone adhesives and sealants provide biocompatibility — verified by meeting the testing requirements for USP Class VI and ISO cytotoxicity specifications. While these attributes are extremely important, equally important are the materials’ benefits and performance properties themselves. Silicones are well known for their ability to form strong bonds with a wide variety of substrate materials including glass, plastics, metals, and rubbers, among other substrates. It is also important to note that only silicone adhesives can bond to silicone substrates.
The availability of silicone adhesives in one- or two-part formulations offers a range of fast cure schedules to match the unique requirements of each application. Both one- and two-part silicone systems provide high reliability and withstand difficult conditions encountered in use. Both cure at ambient or elevated temperatures, and select formulations provide fast tack-free times and withstand liquid sterilants, gamma radiation, and EtO very well. Both are also available in a wide service temperature range. Some of the formulations are even optically clear.
These two types of silicone materials add to the flexibility of choice when matching the material to the application. Each offers benefits and special properties as described below.
One-Part Medical Silicones
No-mix one-part medical silicones offer a unique combination of flexibility and high-temperature resistance. Whenever medical device bonding is required, one-part silicones excel. These one-part silicones bond exceptionally well to a wide variety of substrates including metals, composites, ceramics, and glass, as well as many rubbers and plastics. In fact, certain medical silicone adhesives bond particularly well to metals.
One-part silicones cure at room temperature via moisture in the air. Typical of one-part silicones, the curing rate is based on the humidity level: The higher the humidity, the faster the cure, and thinner sections cure more quickly than thicker ones. In many cases, it is essential to use a fast-curing system that offers a tack-free time ranging from 10 to 20 minutes, depending upon the application. For applications requiring a more extended tack-free time, a faster curing medical-grade silicone system with a tack-free time of 15–30 minutes has been developed.
These adhesives can resist the most aggressive types of thermal cycling and can endure mechanical and thermal shock. Furthermore, silicones offer a very broad service temperature range.
As a one-part material, these adhesives can be manually dispensed from their application container and can also be adapted to semi-automated or fully automated silicone-dispensing systems, if required. Their unique combination of fast curing, thermal stability, and wide range of viscosities make these materials ideally suited to a variety of bonding applications such as bonding cannulae to hubs in virtually all types and configurations of syringe-needle assembly applications.
Two-Part Medical Silicones
Two-part silicones are among the most versatile polymer compounds used in medical devices, particularly in the area of electronics. They adhere well to a variety of substrate materials and operate over a wide range of temperatures. Furthermore, they provide excellent protection from chemicals, moisture, and mechanical shock and vibration. These characteristics make two-part adhesives the first choice for use in a variety of functions in medical electronics, including bonding, sealing, potting, encapsulating, and coating electronic parts. Indeed, two-part silicone systems are commonly the choice for the assembly of medical, electrical, electronic, optical, and other devices.
Manufacturers can find two-part medical silicone adhesives that offer an exceptionally low viscosity, providing a particularly effective material for potting and encapsulation. As a result, these materials routinely find use in medical packaging applications including board coating, all types of potting, lid and housing seals, LED assembly, and different types of component attachment and sealing.
Manufacturers can also find two-part medical silicone adhesives designed to meet unique assembly requirements for bonding complex structures or even forming bonds at interfaces with little or no access to air.
Widely used in medical devices, such two-part systems offer high flexibility and excellent temperature resistance typically required for those applications. They meet USP Class VI testing and ISO 10993-5 for cytotoxicity and offer robust performance characteristics. Their exceptionally long working life aids exacting assembly operations, and they readily cure at room temperature or at elevated temperatures for faster cure times. Once cured, these two-part systems provide stress relief for the bonded assembly with their ability to withstand severe thermal cycling, vibration, and shock.
Primed for Certification
Adhesive manufacturers offer a range of specialty epoxies, primers for polyolefins, UV curable adhesives, and silicones that have been fully tested to meet USP Class VI requirements for the medical device industry. In fact, many of Master Bond's special-grade products have passed this rigorous test. Nevertheless, Class VI compliance only aids product certification; it does not automatically mean the adhesive system can be used a priori in a medical device.
Meticulous care and attention must be paid to all FDA regulations regarding medical devices and their constituents. In the case of Master Bond special-grade products, their Class VI certification can be considered a strong indication that its adhesive systems should not cause any problems when the device is submitted to the FDA for approval. Choosing the appropriate adhesive, sealing, coating, or potting compound that is certified for the USP Class VI testing or cytotoxicity thus provides a good starting point for medical device manufacturers.
Medical product assemblies are subject to stringent requirements for biocompatibility of adhesives needed to seal, bond, pot, or encapsulate components. At the same time, medical products can present a broad array of requirements for flexibility, strength, thermal stability, and more, which are encountered in any consumer or industrial product.
For manufacturers, the challenge lies in finding an adhesive system able to address assembly requirements while meeting the unique challenges associated with medical products and the medical environment in general. Medical-grade silicone adhesive systems such as those from Master Bond provide manufacturers with a broad selection of choices needed to match the unique requirements of each individual medical product.
Venkat Nandivada is manager of technical support at Master Bond. For more information, Click Here .