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Advanced Adhesives for Medical Applications

Mon, 08/15/2011 - 9:58am
Jeremy Cooler

Fueled by continued growth in the medical and surgical device market to meet the demands of an aging population, research in adhesives for medical applications is generating new products with enhanced functionality, higher purity, and unique properties. For design engineers focusing on wound care, ostomy appliances, surgical drapes, and medical diagnostics, it becomes increasingly important to work with materials experts who can access a broad range of adhesives and materials to meet unique application requirements.

 

Different materials and adhesives
Among materials that are used in the manufacture of medical devices, there is a growing interest in polyolefins. This category of materials includes polystyrene, polycarbonate, acrylics, silicone rubber, polyethylene, polypropylene, and synthetic rubbers. Each material has its own benefits and limitations, and must be carefully matched with the appropriate adhesive. For example, “non-stick” Low Surface Energy (LSE) plastics, such as polyethylene and polypropylene, require new adhesives specifically designed for strong bonding.

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Adhesives for medical device applications include acrylics, epoxies, and styrene block co-polymers.

 The primary types of adhesives used in medical device applications include:

  • Acrylics
  • Epoxies
  • Silicones
  • Styrene block co-polymers

Fast curing acrylics are available as one-part anaerobic adhesives that cure in the absence of oxygen, one-part light-cure adhesives that set up in seconds, and two-part formulations with improved viscosity and handling characteristics. Cyanoacrylates are also popular; they include one-part formulations that cure within seconds, and they are well-suited for joining LSE and other difficult-to-bond materials.

Epoxies are also used extensively, especially with film adhesives that can be die-cut to intricate custom shapes. These types of thin film bonding systems are a good choice when the bond line geometry presents a gap that must be filled. Blocking is crucial in ostomy applications, where a strong seal is required to prevent leakage.

Advanced Adhesive Formulations
Whether used in wound care products or for adhering medical electrodes to skin, advanced adhesives are being created to improve stretchability, conformability, breathability, absorbency, porosity, and durability. New classes of adhesives with custom formulations include hydrocolloid, hydrophilic, and conductive adhesives. These adhesives provide kinder, gentler adherence to skin with long lasting bonds for extended wear, and in some cases, the ability to withstand or absorb fluids. They have the following general properties:

  • Nontoxicity
  • Adhesion to organic and inorganic surfaces
  • 100% solid before and after curing
  • Optimized for wetting and gap filling
  • Working characteristics suitable for high-volume production
  • Compatibility with different forms of sterilization
  • Anti-microbial characteristics

Hydrocolloids
Challenges for medical devices that require skin contact can often be reduced to a wrestling match between adhesion and irritation. Important factors include skin type, age, ethnicity, how often the dressing is changed, exposure to fluids, physical activity, and so on.


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 Hydrocolloid adhesives provide new alternatives to traditional dressings with greater “skin-friendliness.”

Hydrocolloid adhesives are the most “skin-friendly” alternative and are body fluid resistant. They represent a special type of pressure-sensitive adhesive with both fast adhering characteristics, as well as fluid absorbency. These adhesives have become key components in ostomy applications and wound dressings. Hydrocolloids are available in different formulations for different applications depending on the need for skin-friendly and absorbent properties. They can also be formulated for extended wear.

Hydrocolloids are multi-phase systems with an elastomeric (or continuous) phase and a hydrocolloid (or discontinuous) phase. They consist of a rubber- or synthetic rubber-based adhesive, the continuous phase, into which is dispersed a particulate absorbent medium, the discontinuous phase. The continuous phase is primarily optimized and responsible for adhesion and cohesion strength, while the discontinuous phase is responsible for moisture handling and moisture interaction. Adhesion and cohesion require a balance of competing behaviors, and strong elastomers don’t necessarily yield the most effective adhesives. Moisture handling includes absorption rate, absorption capacity, and the properties of the hydrated material.

The elastomers used in hydrocolloid adhesives are typically modified with tackifiers that enable the elastomers to form bonds while retaining cohesiveness and contributing strength to the overall formulation. The elastomer is usually blended with a styrenic block copolymer for optimum cohesive strength.

In order to attain greater skin adhesion beyond one to two days, it is necessary for the adhesive to absorb fluid. Absorption effectively manages skin perspiration. Without its absorption ability, the adhesive would fail in a short period of time, and the skin would be irritated. However, absorption also can cause adhesive degradation. It is, therefore, important to limit absorption to the minimum necessary to manage perspiration.

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New adhesive formulations can provide strong bonding plus improved breathability.

Adhesive suppliers creating medical grade adhesives have developed high-tack, high-shear formulations with improved abilities to maintain adhesion after exposure to fluids. Hydrocolloid adhesives can be applied to many substrate materials, including polyethylene foam, PVC foam, and polyurethane foam or film. Hydrocolloid tapes are available as single- or double-coated tapes. Double-coated tapes can have a hydrocolloid adhesive on one side to stick to skin, and an industrial strength adhesive on the other to adhere to another device, such as in a surgical drape application.

Since hydrocolloid tapes are custom applications, working with an adhesive and material expert is critical to achieve the exact formulation and combination required. Many new formulations make the hydrocolloids more suitable for a diverse range of materials such as, low surface energy (LSE) films or plastics, surgical drapes, foams, wovens, and non-wovens. Biocompatibility can be dramatically improved and the adhesives can better withstand autoclave, gamma, and ethylene oxide sterilization.

Applications for hydrocolloids include:

  • Bordering and fenestration drapes
  • Fixation tapes
  • Incise films, including ophthalmic surgical incise films and drapes
  • IV and catheter placement and secondary dressings
  • Acute and chronic wound care
  • Wound care that requires MVTR (moisture vapor transmission) and extended wear
  • Medical device assemblies
  • Antimicrobial wound care
  • Ostomy applications
  • Disposable electrodes
  • Diagnostic equipment, including home healthcare equipment

Hydrophilics
In addition to hydrocolloids, new work is being done with hydrophilic and hydrophobic tapes and films. These adhesive systems are formulated to manage moisture vapor transmission and adhere effectively on wet or moist surfaces, and on surfaces that may become moist or wet during use. These adhesives are liquid stable to water or bodily fluids during skin attachment so that their adhesion properties are not significantly affected by fluids over the period of use. The adhesive composition is design to be moisture vapor permeable while also capable of providing a seal against liquid leakage.

Hydrophilic adhesives can also be formulated to allow for multiple reapplications without losing adhesive properties. Popular diagnostic applications for these tapes include:

  • Blood glucose testing
  • Blood coagulation monitoring
  • Lateral flow diagnostic devices
  • Microfluidic point-of-care testing

Electrically Conductive Adhesives
Adhesives that provide electrical conductivity are widely used in medical device applications that require high, sustained adhesion with no residue after removal. Applications include:

  • Disposable ECG electrodes
  • Grounding plates
  • Electrode labels

In addition to being skin friendly, these adhesives are gel resistant. Conductive adhesive tapes use a broad range of carriers, including films, foams, and non-wovens.

Cold Seal Adhesives
Cold seal adhesive technology offers new and exciting alternatives to traditional packaging techniques. Cold seal adhesives are based on mixtures of adhesive components and natural latex rubber. Since 1999, the FDA has required that cold seal adhesives that use natural latex rubber in medical packaging be labeled appropriately due to allergens attributed to natural latex rubber. To accommodate this ruling, synthetic rubber-based cold seal adhesives have also been developed to take advantage of the low temperature, low cost, and high bond benefits of cold seal technology.

Cold seal adhesives are widely used for wound dressing applications and are usually available in paper film, transparent plastic film, transparent carrier film, and paper combinations. Cold seals are also available with sealing forces ranging from low at 0.5N/15 mm to higher at 4.0 N/15 mm. Low peel forces are desirable mostly for bandages applied to children.

Cold Seal Advantages
Why use a cold seal product? Cold seal adhesives are a special type of pressure-sensitive adhesive (PSA) that forms a strong bond at room temperature with very slight pressure. Sometimes called cold seal, self-seal, or cohesive seals, they are applied to each of the substrates that will bond together. Cold seal adhesives bond only to themselves; and exhibit no tack to other substrates. As a result, they do not require a release liner. For some applications, this simplifies assembling materials within paper packaging, without damaging the contents.

The base polymer used in cold seal adhesive formulations must be able to bond to itself with only slight pressure and, at the same time, be hard enough to resist bonding to another substrate during storage and assembly. The polymers that provide the best autohesion characteristics include natural rubber, polyisoprene, butyl, and certain formulations of silicone rubber. Typical cold seal formulations combine a natural or synthetic rubber elastomer (usually a latex) with a tackifier and other compounds.

The Role Of The Converter
The importance of working with an experienced converter in the medical device industry, especially to take advantage of advanced adhesive capabilities, can not be overstated. Converters deliver a range of die-cut capabilities, advice in selecting the most appropriate materials, and the ability to identify the best adhesive for the application.

A converter can select from servo driven rotary die-cutting, CNC die-cutting, laser die-cutting, and water jet die-cutting to meet the complex specifications of medical components. For example, a servo driven rotary die-cutter can feature repeatable tight tolerances ranging from 0.015” to +/-0.005” at speeds up to 500 fpm. It is ideal for complex, multi-layer die-cutting and lamination. 

For complex foam tape die-cutting, water jet technology provides clean edges with no distortion. Laser die-cutting, kiss-cutting, slitting, and laminating can also be used in converting for medical applications.

In many instances, a converter can also provide label printing for tamper-evident and custom pressure-sensitive labeling of medical devices.

An experienced converter can suggest the appropriate adhesive alternatives for a specific application, such as single- or double-coated tapes, the best liner for the application, whether a foam tape is well-suited for the application, and what might be available in hydrocolloid tape formulations.

Jeremy Cooler is a technical specialist for Fabrico in the bonding, joining, and sealing business unit. He can be reached at 678-202-2700 or info@fabrico.com.


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