Not so long ago, plasma treatments were the stuff of science fiction. In Star Trek: The Next Generation, patients visiting the sick bay were treated with a ‘dermal regenerator’ device emitting a bright, glowing light that could heal wounds and remove scars. Now, thanks to innovations in plasma processing, this futuristic technology is actually becoming a reality.

Neoplas and Adtec, creators of the kINPen MED and MicroPlaSter respectively, use ‘cold’ atmospheric plasmas to clinically treat wounds and skin conditions.

This rapidly developing area of technology is known as plasma medicine, and is just one of a broad range of applications for plasmas. These include the etching of silicon to fabricate semiconductor devices; the deposition of wear-resistant tool coatings; and biocompatible coatings for medical devices.

Matter of fact

Plasma is one of the five states of matter in physics, along with liquids, solids and gases. Plasmas are super-ionised gases that consist of various active components such as ions, electrons and photons in an electronically neutral form. Plasmas are generally divided into two types – ‘hot’ and ‘cold’. ‘Hot’ plasmas can be used for plasma spraying; for example, to deposit hydroxyapatite coatings on to dental and orthopaedic implants to facilitate greater cell adhesion to the implant.

‘Cold’ plasmas – also known as nonequilibrium plasmas – are produced at, or just above, room temperature and can be applied to increase the surface energy of polymers prior to printing or adhesive bonding. These types of plasma can be formed either under low pressure or vacuum conditions, or at atmospheric pressure. The latter area has grown significantly over the past decade, as the processes can be carried out without the need for a vacuum chamber.

“Plasmas generated using an electron beam are widely used to sterilise medical devices,” explains Professor Denis Dowling, an expert on the deposition of coatings using atmospheric plasma systems. “The plasma breaks the chains of DNA in living organisms such as bacteria, resulting in microbial death and rendering the space they inhabit sterile. If the energy within the plasma is reduced substantially, the plasma can have the effect of generating active species such as nitrous oxide and oxygen radicals, which research shows is crucial in enhancing wound healing. Hence the increased use of these low-energy plasmas in wound and skin treatments.

“Recent research has also established the potential of ‘cold’ atmospheric plasmas in the treatment of cancer. It has been demonstrated that cancerous cells are much more sensitive to plasma exposure compared with those that are nonmalignant. The plasma can thus selectively kill cancerous cells, which explains the enormous growth in interest in the field of plasma medicine and medical devices.”

Plasma partnerships

Dowling is a member of Science Foundation Ireland’s Precision Cluster consortium, which consists of academic and industry partners. The group’s objectives are to develop the scientific and technological knowledge required for present and future manufacturing applications using plasmas, with a specific emphasis on nano-scale products, process reliability, manufacturing costs and advanced materials processing. The project enables the fundamental understanding of plasma processing, combined with real-world product application in the medical device industry. Dowling is collaborating with industry partners to develop processing solutions that will facilitate the wider adoption of plasmas in manufacturing.

As part of the initiative, the consortium is investigating the effect of the plasma deposition parameters on the surface chemistry, energy, morphology and mechanical properties of the coatings on to various polymeric and metallic substrates. Key research breakthroughs so far include the significantly enhanced control of plasma processes during semiconductor device manufacturing. A further development has been the wider adoption of atmospheric plasma treatments in precision cleaning of parts, as well as for polymer surface activation during product manufacture.

“A major focus of Precision is to assist medical device manufacturers in evaluating plasma-processing technologies, as well as enhancing the efficiency of plasma processes already in use,” says Dowling. “Medtech is particularly important in Ireland, employing more than 29,000 people. The country is the second-largest exporter of medtech products in Europe.”

Advantages for manufacturers

For medical devices, a key advantage comes from enhancing the surface energy of polymers prior to adhesive bonding or coating, as Dowling explains.

“Plasmas modify the surface of the polymer only, leaving the bulk material unaffected,” he says. “Polymer exposure to the plasma can result in physical and chemical changes − particularly the introduction of oxygen functionality − including etching, surface cleaning, cross-linking and activation.”

Dowling notes that over the past few years in Ireland, medical device manufacturers have increased their use of atmospheric plasma treatments during continuous processing operations. There are also important economic benefits for the industry.

“One of the advantages of using plasma technology in manufacturing is that it facilitates the manufacture of products that are cheaper and of superior quality than those produced by competing technologies,” he says. “It also develops unique and disruptive processes. For example, we have recently developed an atmospheric plasma source for the treatment of polymer particles. This process is used to enhance the surface energy of the particles prior to their use in subsequent processing. We have taken the activated particles and used them in the extrusion of polymer filaments. These filaments were then used, in turn, to print dog-bone test parts by additive manufacturing. These exhibited an increase in tensile strength of up to 22% compared with test parts produced using the non-activated polymer.”

This increased mechanical performance is due to a combination of a higher polymer surface energy and the removal of contaminant molecules from the polymer particle surface. Thus, this novel process facilitates enhanced polymer bonding during the extrusion process.

Benefits and challenges

Two key areas of the Precision team’s research – the monitoring of atmospheric plasmas and the use of these plasmas in the deposition of functional coatings – highlight the benefits of using plasma within medical devices. However, research scientists and manufacturers also face a number of challenges in bringing plasma innovations to market.

A particular focus has been the development of methods to monitor and control low pressure and atmospheric processes. Dowling explains that for ‘cold’ atmospheric plasmas, the impetus has been on the use of optical and acoustic techniques to control, and monitor, these plasmas.

“A soundcard from a laptop computer can be used to monitor a jet plasma, as the jet emitted from the applicator nozzle produces a low whistling sound,” he says. “This changes according to substrate distances and types, so acoustic plasma metrology was found to be a relatively low-cost, non-invasive technique for the monitoring of plasma jets during surface treatments − a significant benefit.”

The team has also been looking at the use of atmospheric plasma to deposit plasma polymerised coatings using liquid monomer precursors. “The resulting nanometre-thick coatings can be used to control protein and cell adhesion,” says Dowling. “This type of technology is crucial for the fabrication of microfluidic devices and anti-fouling surfaces, as well as for sensor devices.”

Economic value

One crucial issue is the cost of parts produced by plasma processing. However, plasma treatment of surfaces to remove contaminants or to activate polymers is environmentally friendly compared with conventional solvent treatments. Plasmas are rapid, more homogeneous and produce almost no emissions. The running costs are fairly low, as most systems use inexpensive argon gas in the plasma, and a low level of input power is required to sustain the discharge. Plasma treatments are therefore increasingly being adopted as standard in the medical device sector.

Dowling says atmospheric plasma treatments are also being used for the deposition of plasma polymerised coatings. “One of the strengths of the technique is that the chemical functionality of the surface can be tailored to meet the desired end-user application,” he says. “However, the processing costs are relatively high due to a combination of a low deposition rate – typically less than 20Nm a minute – along with, for example, the low area of coverage by a single plasma jet, as well as the cost of precursor chemicals. Hence, the technology has found application in high-value areas such as microfluidics, as well as the fabrication of sensor devices.”

The use of plasmas for coatings can be relatively expensive compared with conventional wet chemical routes, but Dowling is keen to point out that “in many cases, it would be extremely difficult to obtain the required deposited functional chemistry without the use of plasmas”.

He adds, “The area of treatment can also be precisely controlled. Hence, for highvalue- added devices such as sensors or for drug-eluting surfaces – for example on stent devices – the use of controlled plasma processing is critical.” The general cost of plasma treatments has decreased significantly. Dowling cites as an example the considerable potential of ‘cold’ treatments of polymer and other particles in sectors such as catalyst fabrication.

With R&D in an exciting phase, it can only be a matter of time before plasma devices become regular medical tools.