What is the best way to prepare medical parts for gluing?

Louise Smyth

In the medical device industry, plasma treatments are often utilised to solve problems that cannot be resolved other ways.  

Whether attempting to improve adhesion of dissimilar materials, depositing coatings, cleaning surfaces, applying a protective coating that repels water for example or biological fluids or enhancing surface wettability or functional coating, plasma treatments often add significant value to medical devices.

For many medical device manufacturers, the application of plasma-applied coatings and surface treatments can play a significant role in new product development, but also when upgrading legacy medical devices under 510(k) guidelines. 
As a result, the industry is aggressively investigating and applying plasma to products such as well plates, microfluids, IOL’s, stainless-steel guide wires, cardiac pacemaker or implantable cardioverter defibrillators (ICD), catheters, stents, and vascular surgical tools.  

“Medical device manufacturers are continuously looking for unique ways to technologically advance their products to be the market leader,” said Michael Barden of PVA TePla, a company that designs and manufactures plasma systems, provides surface modification research, and also serves as a contract processing centre. “Often, the top tier products incorporate some form of advanced coating to functionalise the surface.”

However, when medical device manufacturers want plasma-treated parts or components without having to invest in in-house equipment, the solution is to utilise a contract processor. In this approach, parts are sent in, treated and returned within 1-2 days, or up to a week for larger volumes. For small or infrequent batches, this can lower the price per part significantly.

The most common application for contract processing is to improve the bonding power of chemical adhesives, whether metal to plastic, silicon to glass, polymers to other polymers, biological content to microtiter plates and even bonding to PTFE. However, it also is used often to clean, activate, chemically graft and deposit a wide range of chemistries on the surface of parts or components.

According to Barden, the companies that tend to opt for contract processing generally fall into two categories. 

The first has little or no experience with plasma treatments, but may have heard that it can resolve issues or add value to products.

“For many customers, plasma is not the first thing they have tried, but they have read or heard ‘you may be able to use plasma’ to solve a specific problem,” explained Barden. “They have a target for their material or the chemistry, but very often they don’t know exactly how to get there. So, it can be somewhat experimental.”

When this is the case, the initial capital costs may be a deterrent. Even large medical device manufacturers may opt to avoid purchasing equipment initially if the product is new, or a new version, and production quantities are unknown.

The other type of inquires received are from companies where outsourcing is a core concept of their corporate philosophy. This could also be true for medical device companies that want to develop the product, but only assemble and do final packaging in house, along with contract sub-assemblers.

Why are plasma treatments useful?

According to Barden, many contract processing projects involve using plasma to create a high-energy surface to resolve bonding issues with chemical adhesives.  

Plasma treatments are utilised to increase the surface free energy of the material to be bonded. When a substrate has a 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.

Adhesion promotion can be achieved by increasing the surface free energy through several mechanisms, including precision cleaning, chemically or physically modifying the surface, increasing surface area by roughening and primer coatings.

The net effect is a tremendous improvement in bonding. In some cases, up to a 50 times bond strength improvement can be achieved.

On the other hand, substrates that have a low surface energy, such as PTFE, are difficult to adhere to other materials without first selectively altering the surface to increase the free energy.

“When a surface is really hydrophobic, like Teflon, it’s very difficult to bond to it,” said Barden. “If you apply a liquid or adhesive, by nature it pools and will not spread effectively across the surface.”

There are several plasma methods employed to increase surface energy, including physical and chemical plasmas along with PECVD coating surfaces.  

Silicon, another material widely used in the industry for its low surface energy, often is utilised as an over-moulding compound to protect electronic boards from extreme conditions.  

Unfortunately, the topography of a PCB means the silicone must bond to many types of materials, including polymers, metals, alloys, ceramics and the FR-4 board itself, all of which have unique surface energies and chemistries.  

Without proper adhesion, silicone can begin to delaminate, not only at the edges of the PCB board but also in the form of small air pockets on, or around, components. This can lead to moisture ingress and subsequent corrosion or electrical shorts.

“In terms of surface energy, the best strategy is to deposit a thin film coating over everything so the silicone only has to bond to one surface energy,” continued Barden.  “A process using plasma can basically harmonise all of the many surfaces and turn it into one.”

To accomplish this, PVA TePla has developed a specific process starting with a precision cleaning/surface activation treatment followed by the deposition of an inert chemical primer that serves as a tie layer for the over-molding and provides a uniform surface energy for the silicone to bond. 

Gas plasma can also provide surface conditioning of in vitro diagnostic platforms prior to the adsorption of biological molecules (protein/antibody, cells, carbohydrate, etc.) or biomimetic polymers. This includes precision cleaning of the substrate at the molecular level, along with raising the surface energy to improve surface assimilation of the intended content.

“Microtiter and multiwell plates are often made of polystyrene, which is extremely hydrophobic. Water will bead on it,” said Barden. “However, if you treat polystyrene with oxygen plasma it will become very hydrophilic, so liquid spreads everywhere. This allows aqueous solutions containing biological content to spread and deliver biomolecules to the surface while providing a hydrogen bonding platform to adhere them.”  

However, some in vitro diagnostic substrates require more selective surface chemistry to immobilise a customer’s proprietary molecules. For this, PVA TePla has recently developed methods for chemically functionalising various polymer platforms for the selective adhesion promotion and conjugation of bio-active molecules.  

This is achieved by providing particular chemical functionality or linker chemistries at the surface, allowing conjugation of a wide variety of molecules ranging from small molecule drugs to peptides to larger biopolymers such as carbohydrates and antibodies. Amino, carboxylic, hydroxyl and epoxy functionalities are important examples of the chemistries that are readily obtainable using a gas plasma surface treatment.

Contract processors should ideally bring a combination of resources for plasma R&D and experience with many customer types in markets beyond medical devices. Working with a contract processor also has tremendous advantages when it comes to tapping into the years of technical expertise applying a variety of plasma treatments.  This can often speed research and development efforts significantly.

In many ways, the application of plasma is the domain of chemists and other scientists.  Companies like PVA TePla, for example, have three Ph.D. scientists and surface, polymer, physical, bio and organic chemists, as well as engineers, plasma physicists and metallurgists on staff.

Additional specifications for a process for electronic part, anti-static so when it comes out of the plasma because it’s an electron emitter, it can accumulate static; anti-static fans can be used to remove static on the parts when the chamber is opened.

“Depending on the application and complexity, customers face strict industry requirements and often have unique product of processing requirements,” explains Barden. “Generally, we have existing solutions for most of the things, so usually that is not a concern. 

Even the way the parts are delivered to the contract processor, or how they are packaged and returned can be modified as needed.  

Parts can be sent in specific counts in bags or in specifically designed pre-loaded trays, for example. Once treated, the parts are typically packaged in polyethylene bags, but for customers with electronic devices like PCBs, antistatic bags can be specified to protect components from electrostatic discharge (ESD).

As for turnaround time, it depends on the quantity and complexity of the processing.  However, it can be quite fast. A single batch can be completed in an hour, and large batches of, say, 1,500 parts can be treated and shipped within a day or two. 

For medical device manufacturers interested in treating parts and components with plasma, but do not want to make the investment just yet, or require only small batches, outsourcing it to a qualified contract processor could be the solution they are seeking.


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