A research team from materials science has developed a method of connecting plastics which enables completely new applications. For example in heart valves, to which hardly any blood adheres.
Heart valves regulate the blood flow, to ensure the body is supplied with enough blood. If they don’t close properly any more, for example due to a heart attack, then artificial heart valves can fulfil the required function. But blood platelets can easily stick to the metal surfaces of conventional heart valves. In order to prevent the formation of blood clots, patients must therefore take medication for life. Certain blood-repellent plastics could offer alternative materials. However, until now they have been too soft to be used as a heart valve.
A research team from the Institute for Materials Science at Kiel University (CAU), in cooperation with the University Medical Center Schleswig-Holstein (UKSH), Campus Lübeck, has now managed to combine a soft, blood-repellent plastic with a sturdy plastic. The team is convinced it could be used for biomedical implants such as artificial heart valves in future. The research team has presented how they used the simple, purely mechanical procedure to permanently connect non-adhesive plastics for the first time in the journal Nanoscale Horizons.
Complex medical applications often require materials which fulfil very different or even contradictory requirements at the same time. It can therefore often be difficult to combine these materials with each other, such as with so-called low surface energy plastics. Due to their low surface energy, hardly anything sticks to them. Previous chemical bonding methods either chemically alter the material surfaces, or even destroy them - for this reason, they are often not suitable for biomedical applications.
Making the blood-repellent properties of low surface energy plastics usable
The Kiel team has now succeeded in using a purely mechanical process to bond the soft polymer PDMS (polydimethylsiloxane) with the highly stable polymer PEEK (polyether ether ketone). "Through a relatively simple coating method, we were able to create a polymer composite that combines the properties of both substances in an ideal way," explained Leonard Siebert, doctoral researcher in the "Functional Nanomaterials" working group at the CAU. In doing so, the surfaces of the two materials are mechanically interlocked with each other.
Through this bonding, the blood-repellent polymer PDMS became robust enough to even withstand strong pressure loads, such as those in a constantly opening and closing heart valve. Initial laboratory tests at the Department of Cardiac and Vascular Surgery at the USKH, Campus Lübeck, confirmed that there is significantly less blood platelet adhesion on the new composite material than on conventional materials such as titanium or diamond-like carbon layers, which are already used for artificial heart valves. "Plastics which are simultaneously flexible and robust could be especially interesting for so-called transcatheter valves. They are introduced into the body using a gentle, minimally invasive method, without traditional surgery, and must therefore meet special material requirements," said Professor Hans-Hinrich Sievers, UKSH, emphasising the importance the new procedure could have for medical applications.
In order to bond the two polymers PDMS and PEEK, the scientists used the capillary effect. This effect causes liquids to ascend in narrow tubes or cavities. The research team sprinkled the smooth surface of the PEEK polymer with a powder made from conventional ceramic particles of various sizes. Through heating, the plastic virtually sucked up the particles, and merged to form an extremely rough structure full of cavities. Then the researchers applied liquid PDMS to the "rugged" surface, which penetrated deep into the cavities created.
"The key to our mechanical method is the differences in the size of the particles on the nanoscale and microscale. This enables the normal round particles to form an interlocking structure, into which the soft plastic can fit perfectly. Once it’s in a dry state, it becomes firmly anchored," said materials scientist Siebert, summarising the method known as "mechanical interlocking". In this way, the research team achieved significantly higher adhesion than other methods, in which the plastics separated from each other again, even after low loads.
New bonding methods for metals and plastics
For a long time, the "Functional Nanomaterials" working group at the CAU has been investigating ways of permanently bonding plastics and metals at the nanoscale, without using conventional welding, adhesive or chemical processes of joining technology. "At first, we discovered that using a similar principle enabled metals and plastics to be joined together with microscopic barbs," explained leader Professor Rainer Adelung. "Through the further development of the process using powder particles, we can now also effectively combine plastics into completely new composite materials with innovative properties." However, this is still fundamental research. In a follow-up step, an interdisciplinary team from materials science and medicine plans to investigate the implantation of coated transcatheter valves more closely. (Editor: Guido Deussing)
Perfect polymer interlocking by spherical particles: capillary force shapes hierarchical composite undercuts, Leonard Siebert, Tim Schaller, Fabian Schütt, Sören Kaps, Jürgen Carstensen, Sindu Shree, Jörg Bahr, Yogendra Kumar Mishra, Hans-Hinrich Sievers and Rainer Adelung DOI: 10.1039/C9NH00083F Nanoscale Horizons, 2019, 4, 947-952