Adhesives In Medical Industry: Current Solutions & Future Developments

Adhesives employed in medical practices are commonly used for surgical procedures and appliance bonding, they are typically composed of synthetic or biological formulations. Medical grade adhesives may be categorised as one or two-part component epoxy systems, each featuring different curing reactions. Adhesives come in many different forms, structural, non-structural and pressure sensitive adhesive, to meet different needs.

The right adhesive should bond well to the substrates and provide a strong enough bond for the application requirements. Some examples of common applications that require medical adhesives include surgical instruments, biosensors, electrodes labeling, catheters, and implantable devices.

Why Separate Discussion Needed On Adhesives For Medical Application?
Medical adhesives applications vary widely, for example adhesives are ideal for bonding a wide variety of substrate surfaces, including metals, plastics and rubbers, while others are suitable for skin and suture applications but typically involve the bonding of plastics and metals. Though bond strength requirements are typically within reach for many medical adhesives, medical devices present a variety of unique challenges beyond strength.
In addition to bonding parts together successfully, medical adhesives must also be compatible with direct or indirect contact with the body, and may be required to withstand sterilization through a variety of potential methods. Furthermore, certain substrates may necessitate the use of primers or other surface preparations in order to achieve satisfactory adhesion.

Thoughts While Selecting The Right Adhesive Solution For Medical Application..
When selecting an medical adhesive solution, the most important factor to consider is the specific composition of the materials being bonded. We can simply say that surface energy is the primary determinant of a substrate’s affinity for bonding—the higher the surface energy, the easier it is to bond the substrate. It is also critical to understand the design and application of the device, the materials being used in its construction and application, the environment it will be exposed to from manufacturing to end use, the stresses it will see during the same period, the desired durability and shelf life, to name a few.

NOTE: If the adhesive is to be applied to skin, which is a nonstandard surface with properties varying from person to person and over time for the same person, special considerations have to be taken into account to allow for a successful attachment of the device.

Let's discuss different types of adhesives based on categories...

Structural adhesives: It can be one or two part epoxies, acrylics or urethanes, that are used for load bearing applications. These are chemically curing compounds and typically offer the highest strength. They can be used to bond metals, plastics, rubbers and other difficult to bond materials, sometimes without much surface preparation.
Common types of structural adhesives include Epoxy, Acrylic, Urethane, Cyanoacrylate.
They can be used to bond low and high surface energy materials metals, plastics, rubbers, PTFE, silicone, TPE (Thermoplastic Elastomers) Skin etc.
Applications that can be considered: Blood bag assembly, hospital equipment, IV tube set, respirators, skin sealant and walking cane and many more.

Non-structural adhesives:  This is a broad category that includes hot melts, contact, aerosol and rubber & gasket adhesives, amongst others. These adhesives do not cure chemically but instead attach physically, such as hot melt adhesives that solidify on cooling solvent or water based contact adhesives that bond as they dry. These adhesives are generally used to join surfaces that do not see an excessive load under normal use, and create effective bonds as they maintain flexibility after bonding.
Common types of non-structural adhesives include Hot melt, Aerosol, Solvent/water based liquid. They can be considered to bond rubbers, plastics, fabric, foam, leather, paper, metal, glass, etc.
Applications to consider: Decorative laminates, respirators, surgical drapes construction, wheelchair cushion assembly and many more.

Pressure sensitive adhesives (PSA): Now commonly used in the assembly of medical devices. PSA's are composed of a number of different materials, including rubbers, acrylate and silicone formulations. Whereas other adhesives may require heat or water to activate or “cure” a surface, this type of adhesive bonds to a surface by being pressed on to the substrate. PSAs are used in bonding any number of flexible and non-flexible materials and the bond strength can be moderated by selecting the appropriate adhesive family, such as acrylic, synthetic rubber, silicone, hydrogel, etc. PSAs offer a lot of flexibility, in that the adhesive properties can be formulated from being repositionable to ultra high bonds.
PSAs can further be divided into Skin use & Non-skin use. PSAs are also commonly used for attaching products to skin, such as a surgical drape, a first aid dressing for cuts and bruises or an insulin pump.

Medical Adhesive Compositions...

Synthetic Adhesives
Acrylics, or cyanocrylates (CA), are a common type of adhesive, used for suturing wounds, that reduce infection and scarring. Many acrylic adhesives, such as pressure sensitive tapes, are used for bandages and disposable applications. Various acrylic adhesives are solvent based, though hot melt acrylics are also used for an assortment of applications. In medical applications, curing often occurs by atmospheric moisture or UV light.

Polyurethanes are commonly produced in bandage adhesives and are available in various strengths and are efficient in processes and applications due to their water resistance.

Silicone is another common material used for medical bonding. Typically, this material is popular because it is biocompatible, and there are several curing formulations.

Biological adhesives
They are often used in surgical applications, such as skin grafting and for suturing wounds. The composition of these adhesives includes variations of proteins. Fibrin is a commonly used natural “clotting” protein material that is used to help mend damaged vessels and for wound applications.

What are current developments in Adhesive solutions for the medical industry?
As certain chemicals were flagged as carcinogenic or posing other serious risks, adhesive materials producers sought less-toxic alternatives. Many significant progress came with several major innovations, including:
- Water-based adhesives
- Ultraviolet (UV) and electron-beam cured adhesives
- Hot-melt adhesives
Industry has turned its attention to leachability risks of common compounds such as polyvinyl chloride (PVC), polyvinylidene chloride (PVdC), and plasticizers such as phthalates used to soften PVC. Medical barrier films, foams, and other materials free of these substances are becoming more important. Now, adhesive material suppliers and their customers are evaluating how products will tolerate different sterilization methods, such as X-ray irradiation and vapor phase hydrogen peroxide processes.

New adhesives are designed to work in concert with next-generation fabrics, films, and other carrier materials, such as:
- Soft, stretchable nonwovens
- Materials with advanced wicking properties
- Substrates enhanced with nanofibers
- Novel composites and polymer blends
- Special air-purifying filter media

Conclusion: It is important to know and understand the properties of the substrates involved, define the bond requirements well, identify all the environmental conditions the bond will be exposed to, select the most appropriate adhesives and then test and narrow the choices to the optimal product for the application.