Unlocking opportunities at the nano-scale
Every time a patient receives a bone implant, a gamble is involved: will the body accept the new arrival or reject it as an invading entity? Will it become infected requiring drastic action to save it? While most implants are successful, if the dice rolls rejection or infection, it is the start of an ordeal for the patient.
As Australia’s ageing population generates more joint issues, osteoporosis-related bone fractures and treatments for bone cancers, the implant dice is being rolled more often. Fortunately, a new way of rigging the dice in the patient’s favour has emerged.
It has come from the pioneering world of biomedical engineering where researchers take advances in technology and design and apply them to medical problems. Robotic and laser surgery, super-detailed medical imaging and advanced systems to monitor vital signs have all been shaped by biomedical engineers.
Then there’s the work being done on the atomic scale by researchers like Dr Behnam Akhavan who is, among many other things, a surface engineer. It’s a field that can fly under the radar but as it creates new coatings and surfaces to reduce friction, stop corrosion and absorb or reflect sound, light and energy, surface engineering is the quiet achiever of new technologies.
Now consider the benefits of a surface engineer like Akhavan turning his eye to medical implants and the surfaces they have where rejection and infection begin.
Like all our projects, we started this by thinking backwards from the problem...We asked if our method could solve the problem. Turns out it could.
The question to answer in this case was: how can the surface of an implant be changed to minimise the chance of rejection which would also reduce the possibility of infection?
The starting point for an answer was in substances called hydrogels, which are soft and jelly-like with the ability to contain large amounts of liquid without losing their essential structure. They’re not unlike human soft tissue, so they’re often used in medical treatments. For example, a gauze dressing saturated with a hydrogel can keep the dressing from sticking to the wound surface and provide moisture and pain relief through its high-water content. They can also gently deliver infused medications.
Their jelly-like characteristics make hydrogels seem less alien to the human body which logically implied the idea that coating an implant with a hydrogel could reduce the likelihood of implant rejection. But there was a major obstacle.
“Hydrogels are inherently weak and structurally unstable,” Akhavan says. "So, they don’t easily attach to solids, making them difficult to use in mechanically demanding implant applications like cartilage and bone tissue engineering."
The challenge became engineering the surfaces of medical implants to have a more robust ability to attach to hydrogels. The solution came through plasma technology; specifically, a process called plasma immersion ion implantation, which Akhavan has used previously in various ways because it allows bespoke surface modifications to a depth of mere atoms, with plasma being the medium that allows the process to happen.
Though plasma isn’t commonly encountered on Earth (except in things like neon signs and plasma TVs), it is by far the most abundant substance in the wider universe. A gas-like substance, it is classified as the fourth state of matter after solid, liquid and gas. You could describe it as a soup of electrons and ionised particles that were once together as atoms but were torn apart by electric fields or super-high temperatures, like those found in stars.
Our own sun is essentially a giant ball of plasma, but the plasma used by Akhavan, and his team has less hellish origins, because it’s starting point is nitrogen gas (which is an inexpensive and plentiful raw material for his purpose), which is hit with electrical charges.
One of Akhavan’s early, plasma-based projects, in fact for his PhD, was around water purification. Using plasma, he modified the surface of sand so that pollutants in water – copper, zinc, oil, and other metals and contaminants - that were filtered through it would attach themselves to the plasma-modified sand and be removable.
Another project that is now reaching into the pockets of almost every person in the developed world, was about rescuing mobile phones from a looming shortage of an essential element called Indium. This hard-to-mine metal has two essential qualities that make touchscreen technologies possible; it conducts electricity (so your finger can cause a charge that tells your phone which button you’re touching), and spread thinly enough, it’s transparent, so you can see the phone screen graphics under the Indium.
But there is a problem with Indium. It is mainly produced as a by-product of zinc mining and demand for zinc has been falling to the extent that it’s predicted Indium supplies will dwindle within ten years. How will you manage without your mobile phone? Dry your tears. Akhavan and his team have come up with a process, so it won’t come to that.
Again, using plasma as the medium, they’ve sandwiched a layer of silver between two layers of tungsten oxide, bringing the necessary characteristics of transparency and electrical conductivity that Indium previously brought. The thickness of all three layers together is less than 100 nanometres, keeping in mind that a human hair is a comparatively meaty 100,000 nanometres thick.
That Akhavan is a world-leader in the field of surface engineering, has a lot to do with his skill and creativity, but also some credit is due to his businessman father who saw early on his aptitude for science.
“I remember him not letting me go with him to his shop day-to-day,” says Akhavan who was born and grew up in Iran. “Dad was afraid I might become attracted to the business life."
My parents weren’t scientists, but I think I’m only where I am now because of them.
Specifically, where Akhavan is right now, is in the University’s Faculty of Engineering in the School of Biomedical Engineering. He is also part of a team that includes people who are genuine world-leaders in using biomedical engineering to create life-transforming technologies that would have been unimaginable just a few years ago. One of those technologies is hydrogel-coated, low-rejection medical implants.
The process of surface engineering these enhanced implants (and many of the other surface enhancements that Akhavan has devised) starts with a vacuum chamber being filled with nitrogen gas at low pressure. An electrical charge causes the nitrogen electrons to break away from their atoms putting the nitrogen gas into a plasma state, so it is ready to be used for what researchers like Akhavan call ‘surface activation’.
The thing about plasma that makes it effective for surface activation, is that the mix of freewheeling charged particles makes plasma hyper-active. Place, say, a piece of polymer (like epoxy, polyester, Teflon, silk or wool) in the chamber and the plasma particles will collide with the polymer so powerfully that individual carbon atoms in the polymer will be dislodged, creating lone electrons in the structure of the polymer so the polymer itself becomes highly reactive.
A polymeric surface activated in this way becomes like sticky tape for various substances, including hydrogels.
The result is a hydrogel-attracting surface suitable for implants used in cartilage and bone repairs, artificial nerves and blood vessels. This break-through process also has uses in aeronautics, microelectronics, and other areas of medicine.
An added bonus for Akhavan, who is an avid hiker and lover of natural places (“It’s what I live for, basically.”), is the environmentally friendly nature of much of the work he does with plasma technologies.
“It’s a truly a green technique”, he says enthusiastically. “Whether your goal is activating the surface of an implantable device for attaching hydrogel or making absorbents for water purification, the one-step process happens in typically less than a few minutes, at room temperature, with no toxic acids or other chemicals and it produces literally no waste.”
Written by George Dodd for Sydney Alumni Magazine. Photography by Louise Cooper.