Medical Design Briefs - December 2022 - 12

Plasma Treatment
inks to plastics, improving the adhesion of
plastics to dissimilar materials, and applying
protective coatings that repel or attract
fluids. Plasma today is being used to
treat various medical device products,
such as syringes.
Plastic parts manufacturers are always
looking for unique ways to gain a technology
edge to become a market leader. To
achieve this today, top tier products incorporate
some form of advanced coating to
functionalize the surface. When the medical
device industry uses plastics, more specialized
offerings can create a competitive
advantage and drive up the value of each
part or product. When you treat plastic
with plasma, it can transform a two-dollar
item into a fifty-dollar product.
Some of the essential areas of plasma
treatment in the medical device industry
include printing on plastics, microfluidic
devices, bonding plastic with dissimilar
materials, treating plastic labware,
coating plastics to prevent leaching, and
facilitating R&D.
Printing on Plastics
When printing on plastics is required,
binding the ink to the surface can sometimes
be challenging; this occurs when the
print beads up on the surface or does not
sufficiently adhere to the surface. Greater
print durability may be needed, including
fade resistance even under high heat or
repeated washings. For example, to resolve
the beading issue, plasma treatment
can make the surface hydrophilic (attracted
to water). The treatment facilitates
spreading out the ink on the surface, so it
does not bead up.
For many applications, plasma treatments
are utilized to increase the surface
energy of the material. Surface energy is
defined as the sum of all intermolecular
forces on a material, the degree of attraction
or repulsion force a material surface
exerts on another material.
When a substrate has high surface energy,
it tends to attract. For this reason, adhesives
and other liquids often spread
more easily across the surface. This wettability
promotes superior adhesion using
chemical adhesives.
On the other hand, substrates with low
surface energy - such as silicone or PTFE
- are difficult to adhere to other materials
without first altering the surface to increase
the free energy. Depending on what is required,
organic silicones can also be used
to create intermediate bonding surfaces
12
with either polar or dispersive surface energy
to help printing inks adhere to the surface
of the plastic. This approach can facilitate
the durable printing of a logo on the
surface of bottles when the logo cannot
fade after the first wash, for example. Another
application includes the printing on
plastics used for syringes, which do not
bond easily with biodegradable inks that
are friendly to the human body.
Microfluidic Devices
Typically, microfluidic systems used for
medical or industrial applications transport,
mix, separate, or otherwise process
small amounts of fluids using channels
made of plastics, measuring from tens to
hundreds of microns.
Microfluidic devices usually have various
wells containing different chemistries,
either mixed or kept separate. So, it is imperative
to either maintain flow through
the channel or prevent any residual liquid
flow in the channel after the chemistry has
passed through it. With microfluidics, plasma
treatment is used to disperse liquid on
the surface to allow it to flow through easily.
Or it can make the surface more hydrophobic
(water repellent) to prevent the
fluids from clumping together in unintended
areas. When the fluids are pushed
away, this minimizes the chance of any
sticking or getting left behind.
In such cases, plasma treatment of plastic
surfaces can facilitate the smooth, precise
flow of liquids in the narrow channels.
This can be critical not only for
safety in medical procedures but also for
quality for industrial processes.
Bonding Plastic with Dissimilar
Materials
When traditional chemical adhesives
fail to sufficiently bond dissimilar types of
materials, or if medical device companies
are looking to reduce the amount of
chemical waste produced, engineers often
turn to plasma treatments to solve
complex adhesion problems.
Plasma treatment can assist the bonding
of dissimilar materials. While treating the
plastic alone can improve its binding,
treating both materials enhances the binding
of both by improving adhesive wicking
across the surface. Whether bonding metal
to plastic, silicon to glass, polymers to
other polymers [of different durometers],
biological content to [polymeric] microtiter
plates, or even bonding to PTFE, plasma
can be used to promote adhesion.
Like with printing, adhesion promotion
is achieved by increasing the surfacefree
energy through several mechanisms.
This includes precision cleaning, chemically
or physically modifying the surface,
increasing surface area by roughening,
and using primer coatings. The net effect
is a dramatic improvement in bonding.
In some cases, up to a 50x increase in
bond strength can be achieved.
Plasma Treatment of Plastic
Labware
Each year, billions of multi-well
plates, pipettes, bottles, flasks, vials, Eppendorf
tubes, culture plates, and other
polymer labware items are manufactured
for research, drug discovery, and
diagnostics testing.
Plasma can transform the surface properties of plastic to achieve aims that normally would not be feasible.
(Credit: PVA TePla)
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Medical Design Briefs, December 2022

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Medical Design Briefs - December 2022

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