Old spark, new ideas23 May 2019
Although the use of plasma treatments for medical device coatings has been around for some time, in recent years it is enjoying something of a renaissance. Andrew Tunnicliffe talks with Professor Denis Dowling about its potential and its limitations.
From the often magnificent lightning displays in the sky or the aurora borealis, to its use in car headlights and the televisions we watch, plasma is everywhere. Said to be the fourth state of matter – the first three being solids, liquids and gasses – it is an excited gas produced by adding energy to it, causing electrons to leave the atom. While natural plasma has been in existence for as long as time itself, it wasn’t until the 1920s that American physicist Irving Langmuir first recorded and then researched its properties. He coined the name plasma to describe the ionised gas as it reminded him of a blood plasma. Since then it has been used for myriad applications, not least in industry.
For several years now the medical device sector has been increasingly turning to plasma to coat its products. In 2016, industry experts spoke of the growing use of plasma-enhanced chemical vapour deposition (PECVD) as a means of depositing coatings to protect products against corrosion and wear.
Today, plasma has become commonplace during the manufacturing of medical devices. “Almost every medical device company looking at polymer processing will have a use for plasma,” explains Professor Denis Dowling, director of the I-Form Advanced Manufacturing Research Centre at University College Dublin.
Plasma can be used to prepare surfaces prior to printing, for adhesive bonding, as well as for depositing metals and functional coatings – all of which have uses in applications ranging from the fabrication of medical device sensors, to enhancing cell attachment to metallic implants.
“There is a diverse range of either surface activation treatments, etching treatments, or functional coating treatments where the technology is applied,” Dowling adds.
Although medical devices aren’t the only products to require coatings – others include semiconductors, engine parts, and packaging, to name but a few – the challenge they pose is unique because of the nature of their use for clinical application. The ultimate aim is to increase hardness, gripping surface and resistance to wear; but with a coating that is biocompatible.
Plasma spray coatings
In recent years medical device original equipment manufacturers (OEMs) have turned their attention to hydroxyapatite coatings deposited by plasma-spraying, particularly in the field of orthopaedic implants. This coating is vital as it has the same composition as bone; this means the metal implant knits with the bone, otherwise there is potential for loosening of the device. For plasma spray coatings to perform at their optimum, the surface and its preparation is essential, as Dowling explains.
“It comes down to the surface – tailoring the surface for what you want to achieve,” he says. “For example, you can use plasma to put down a layer of diamond a few microns thick. However, if that layer goes on to a surface that is not particularly hard, effectively you won’t get the form of layer that is a harder resistance because the structure underneath doesn’t support it. It is like an egg; it has a hard shell, but it is soft underneath. So it provides some protection but it won’t take any impact.”
Therefore, Dowling says that when it comes to selecting the coating it is essential to understand how the material will perform and how the device is ultimately going to be used.
“You are tailoring the type of energy that you put in to the material properties, mechanical properties, thermo properties, of what you are applying it to and what you want to do,” he explains.
Going back to his example of diamond, he says, the plasma used to deposit it could be a microwave discharge operating at 900°C. However, if you are using a polymer or a low melting-point metal such as aluminium, it will melt and decompose before you can apply the coating. For these substrates, you would have to select a different wear-resistant coating such as metal nitrides, which could be deposited at much lower temperatures.
“You select the deposition conditions that are appropriate for the substrate you are working with,” Dowling says. “If you are dealing with polymers, generally you don’t go over 50°C or 80°C to avoid any damage to the substrate, whereas if you select ceramics you can go to much higher temperatures.”
This is why he uses the term ‘surface engineering’ a lot, but rightly so. Clearly the success of any coating is hugely dependant on understanding its compatibility with the material being used, and the ultimate application of the device.
The amount in which fabricate polymer materials are thinner with the application of plasma technology.
University College Dublin
An unbreakable bond
One of the areas plasma coating has really changed is bonding, says Dowling. On his visits to device manufacturers’ plants in Ireland, he has seen a shift towards the use of atmospheric plasmas for bonding.
“10 years ago there were very few, but now you see them routinely,” he says. “Before, they [manufacturers] would have stopped the production line and put the devices into a chamber where they were bonded. Now plasma is in the process, making it continuous and meaning you can avoid that. Increasingly, as newer devices come through, the use of these plasma processes are built in, making it is so much quicker.”
As Dowling eludes to, traditionally plasmas for use in manufacturing were applied using a vacuum process. The component was placed into a chamber, the pressure of the chamber was then reduced and gas was introduced to treat the surface by forming a discharge. Once complete, the chamber was repressurised. Today, for some plasma treatments there are atmospheric pressure alternatives thanks to technological developments in recent years.
Not only is this more cost effective, there are benefits for the environment, and health and safety.
“Polymers can be activated by dipping them into a solvent,” says Dowling. “If you took a polymer and dipped it in an alcohol you have the effect of activating the surface and, as a result, you enhance its wettability. However, you then have the problem of these organic solvents evaporating off, with health and safety issues there, whereas plasmas are much cleaner and generally very well contained.
“They are also very fast; the process can range from seconds to minutes. So they are environmentally a lot cleaner and much more controllable than conventional chemical processing.”
For medical devices, bonding is critically important. Therefore, plasmas are routinely used in order to enhance the surface of a medical device prior to the bonding of two polymers. Dowling and his colleagues are currently carrying out some significant research in this field.
“In our own research we have shown that a type of coating known as plasma polymerised can be used to deposit nanometre-thick layers that exhibit superhydrophobic properties, with a water contact angle >150°C,” he explains. “One application of these non-wetting surfaces, for example, is to protect the electronics of mobile phones when they fall into water.
The temperature at which it is possible to deposit from ceramic materials.
University College Dublin
“Another application of plasmas is to produce surfaces that wet very easily; the term is ‘activated’. We are using this approach to activate polymer particles.”
These particles were subsequently used in either injection moulding or 3D printing.
In the case of the injection moulding, the polymer particles were passed directly into the moulding equipment, while for 3D printing the particles were melted and extruded into a filament, which was then used for printing. Dowling says the parts produced exhibited an approximately 10% enhancement in mechanical performance.
This is associated with the cleaning of the particles with the removal of weakly bound water and organic molecules. Their removal increased the bonding between the particles, therefore enhancing the mechanical strength of the fabricated polymer part.
“We started out with a barrel plasma source that could treat approximately 1g of polymer particles,” says Dowling. “The capacity of the source was initially increased to 20g at laboratory scale and then to 500g for pilot scale treatment.”
One application of the developed technology is to fabricate polymer medical devices that are at least 10% thinner but exhibit the same mechanical strength.
“A high level of technical know-how is required when using plasmas for advanced coating and etching applications in sectors such as medical devices and in semiconductor processing,” says Dowling. “Manmade plasmas are usually in the form of discharges formed between two electrodes.”
That discharge is formed, under low pressure or the opposite, via myriad different systems of varying design, shape and size, but fundamentally they all are used for the same purpose; either for removing material, to etch, or for deposition. For coating, says Dowling, typically a line of sight method has been the one of choice.
“In other words, it is anything that the source can see, that the electrode can see, that gets coated,” he says.
Discharges: benefits and complexity
However, although in principle it seems simple, it isn’t.
“There are a range of different types of discharges,” says Dowling. “You engineer the type of discharge according to what type of material you want to deposit. For example, because of the energy contained in a thermo discharge, they are best suited to things that require a lot of energy to break bonds.
“If you’re thinking of a ceramic material, such as hydroxyapatite, you have to go up to about 2,000°C in order to be able to deposit from that. If you are talking about putting down a polymer layer, there is a technique known as plasma polymerisation, where you take a monomer of the polymer and spray it into the discharge, where it gets crosslinked to form the polymer layer. That is done under much lower energy conditions. So much like surface engineering, you tailor the type of discharge according to what you want to apply.”
Although atmospheric plasma has gained traction in recent years, it isn’t suitable for everything, particularly for the deposition of metal coatings.
“With coating, it comes down to the process and what is required,” Dowling explains. “Is it metal, a polymer layer, a low-friction coating and so on? So, for those applications involving metal coatings, you do have to stop and take them [devices] out; it wouldn’t be something you could put in line because it’s simply not technically feasible.”
However, when atmospheric plasma can be used, the cost benefits and more positive impact it has on the environment are a real bonus.
Although plasmas can be applied simply for applications such as polymer activation, once you move away from those simple applications and start putting down functional layers, the skill set increases considerably. The level of time and expertise required to carry out those processes would be substantial.
“At a basic level, plasmas can be used routinely as a low-tech solution,” says Dowling. “But once you are trying to achieve superior coating performance or etching, the degree of complexity of the process, and the cost associated with those processes, go up considerably. It then becomes a cost-benefit consideration. There will be a point where it may or may not be worth it in terms of the value added of the component.”
It may have some limitations but the use of plasma for medical device coating has been around for a while and is here to stay – and it may very well evolve still further.
“There is much wider adoption of plasma than previously, with a range of newer coatings that are being deposited,” Dowling concludes. “Almost all surfaces can be coated. However, the choice of coating should be appropriate.”
That is the conundrum for device manufacturers, one they seem to be getting to grips with.