Medical Design Briefs - January 2025 - 11

S
terilization plays a vital role in the use of medical
devices. Prior to the 1980s, most medical products
were reusable and required sterilization or disinfection
between uses. The advance of contagious diseases
has raised some concerns over the risks of reusable
medical devices, spurring the medical device
manufacturing industry to develop disposable, single-use versions
of many medical instruments.
Although reusable medical devices are still the mainstay, disposable
instruments using engineered plastics have become
more widespread. And most significantly, adhesive products
have been increasingly employed to replace mechanical fasteners
for performance, cost, and compatibility with the sterilization
processes that are being used today. Part 1 of this article,
which appeared in the December 2024 issue, focused on the
factors and requirements for selecting adhesives for use in
medical devices. Part 2 delves more into the material properties
of plastics for medical device assembly.
This article briefly discusses some key properties of plastics
that relate to adhesive bonding. The polarity of the polymer
and its resulting surface energy are important factors when assessing
the ease with which adhesives will form a strong bond to
the polymer surface. Generally, a greater extent of polarity will
provide greater opportunity to form a strong bond between the
plastic and the adhesive system. The availability of active sites or
functional groups on the polymer surface such as alcohol,
amine, or carboxylic acid functional groups may provide for
stronger adhesion potential with certain adhesive systems. Surface
treatment of polymers to improve adhesion exploit this
phenomenon by chemically modified or derivatizing the surface
to promote greater adhesion.
Finally, it is important to consider that plastics are complex
formulations of both the polymeric material as well as additives,
stabilizers, release agents, fillers, and lubricants that assist the
manufacture of plastic articles as well as to promote their longterm
stability. For small, migratory components present in the
plastic article, it is important to consider the potential for these
components to migrate to the surface of the plastic and potentially
compromise adhesion by forming a weak boundary layer.
Migration of small components is enhanced at higher temperatures
due to more labile diffusion; as such, this is an important
factor to consider for medical devices that must undergo
repeated, high-temperature stabilization. For example,
common lubricants used in the extrusion of polymeric resins
such as polypropylene are magnesium stearate and calcium
stearate. Studies have shown that magnesium stearate offers
greater resistance to migration under accelerated aging.1
Surface
migration of these small molecules may both degrade the
quality of the thermoplastic material over time as well as potentially
compromise adhesion at the interface between the plastic
surface and the adhesive.
Theories of Adhesion
Several theories attempt to explain the complex phenomenon
of adhesion. Specifically, adhesion to plastic substrates is of
particular interest due to the challenging nature of forming a
strong bond with the nonporous and generally chemically inert
surface of plastic materials.2
Further, many plastics are very hydrophobic
and their low surface energy results in difficulty wetting
the surface when adhesive is applied. Wetting can be genMedical
Design Briefs, January 2025
EP42HT-2Med is a two-component, high-temperature-resistant epoxy for medical
device assembly. (Credit: Master Bond)
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11
erally understood in terms of contact angle - the greater the
contact angle, the lower the extent of wetting - a failure to
achieve appropriate wetting will result in poor adhesion at the
interface. Wetting can be improved by adding active agents
such as surfactants to the adhesive in order to lower the surface
tension of the adhesive, although this has limited effect.
Alternatively, some kind of surface treatment is often needed
when bonding plastics in order to increase the surface energy
of the plastic and thus the extent of wetting when forming the
bond. Common means of surface treatment include corona
treatment, plasma treatment, UV/ozone treatment, or chemical
surface modification. Generally, the goal of these treatments
is to increase the number of oxygen-containing moieties
or other polar functional groups on the surface of the polymer,
thus increasing the surface energy and the resulting strength of
adhesion at the interface.
Other means to improve adhesion include surface abrasion
to increase the surface area present at the interface thus increasing
the overall strength of adhesion. Generally, it is always
critical to make sure that the surface of the polymer is clean
and free of any small compounds that will interfere in the
bonding process and form a weak interfacial layer between the
bulk of the polymer and the adhesive. Many surface treatment
properties such as plasma, corona, and flame treatment have
the added benefit of ensuring that any small, organic contaminants
on the plastic surface are destroyed and ablated prior to
bonding.
Generally, three different models of adhesion can be used to
help engineers understand the dynamics of forming a lasting
bond with a plastic substrate, as well as providing insights when
troubleshooting bond failures. These include mechanical interlocking,
diffusional theories, and adsorption theories -
electrostatic theories, most applicable to metal and glass substrates,
are less applicable to plastic bonding and will not be
discussed.2
Fundamentally, sufficient wetting is critical to achieving sufficient
adhesion per adsorption theory because it is dependent
upon the intimate contact between the plastic surface and the
adhesive at the interface. The extent of wetting can be described
by the contact angle, and the theoretical thermodynamic
work of adhesion can be conceptualized. This theory
contributes to assessing the ability to bond plastics; for example,
it explains the difficulty of bonding polyolefins without
surface pretreatment.
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Medical Design Briefs - January 2025

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