Coatings & surface treatment 75%
Centers for Disease Control and Prevention
UTIs acquired in the hospital that are associated with a urinary catheter.
surfaces by infiltrating porous or roughened substrates with various liquid perfluorocarbons. This prevents adhesion to the underlying substrate through the formation of a stably immobilised, molecularly smooth, liquid overlayer. Non-stick, ultra-repellent and self-healing, SLIPS provided a promising starting point for a catheter coating. But it also relies on a thin liquid layer on a hard surface, and in a complex and hostile biological system that approach doesn’t work well. So, Howell proposed an infused biocompatible polymer with both liquid on the surface and a bulk of liquid beneath it. When the liquid on the surface is removed, more moves up from within the polymer to replace it. “Liquids don’t like to stay in place under force and pressure,” says Howell. “So, we wanted to get a liquid to stay in place using surface chemistry to increase the affinity of the catheter’s surface. It is more energetically favourable for the liquid to stay in contact with the catheter than for contaminants to stick to it.”
In vivo tests show that the technology works well in the body and on the laboratory bench. In mice, the liquid-infused coatings prevented 95% of proteins from binding to the catheter. “The catheter is so slippery that it also prevents damage to the bladder, which in turn prevents protein recruitment,” adds Flores-Mireles. “Now, we can target multiple pathogens with one technology, which is better than developing new vaccines or anti-microbials.”
Like SLIPS, the new technology devised by Howell and Flores-Mireles is inspired by the Nepenthes pitcher plant, which uses a liquid membrane to trap insects. Previously, state-of-the-art liquid repellent surfaces have been modelled on lotus leaves, which have rough, waxy surfaces that are extremely hydrophobic. However, materials inspired by lotus leaves are less effective against oils, cannot self-heal and are fragile under stress.
“We can target multiple pathogens with one technology, which is better than developing new vaccines or anti-microbials.”
Ana Lidia Flores-Mireles
“Nature has had billions of years to create innovative solutions,” remarks Howell, whose group, Inspired by Nature, works to understand and ultimately control biological systems through surface interactions and other environmental factors. “Imitating nature is the best way forward
90
for addressing many challenges. Our materials science can now replicate the properties of natural materials with a high degree of fidelity.” For application in catheters, the project is still in its early days. The NIH is providing funding to test the safety and efficacy of the technology, though development of a commercialised product may take five or ten more years.
Inspiration everywhere
Howell and Flores-Mireles are certainly not alone in carrying forward the potential of novel coatings to improve the performance of medical devices used inside the human body. Another recent innovation stems from a research team at the Peter the Great St. Petersburg Polytechnic University, where Maxim Maximov, of the school’s Institute of Mechanical Engineering, Materials and Transport, has led the development of a coating for titanium implants to improve osseointegration. Working with colleagues from St. Petersburg State University, Maximov’s team sought to accelerate the integration of titanium implants into bone tissue. Its solution was to create a new method for applying a uniform layer of titanium oxide, at the nanometre scale, on to titanium carcass structures that can be used as implants. A range of coatings, which were applied using atomic layer deposition (ALD) in a vacuum, were extensively tested by the research team in varying compositions, thicknesses and structures, and it was found that if a layer of silver nanoparticles was applied first, the implants benefited from the element’s natural antibacterial properties. The result is not only better osseointegration, but also a greatly reduced risk of implant rejection. According to the research group, the key factor in the successful use of silver nanoparticles is size and morphology, which need to be chosen to prevent toxicity while maintaining the material’s beneficial properties. “We started applying silver nanoparticles on the surface of titanium, since this chemical element obtains useful antibacterial properties and should reduce the risk of implant rejection,” said Denis Nazarov, Research Park, St. Petersburg State University in a statement. “However, to use the silver coatings freely, it is necessary to adjust the conditions and the general method of obtaining the material.” Whether inspired by nature or driven by the known properties of chemical elements, innovation in medical device coatings is moving forward apace. The challenges they face are numerous – ageing populations, antibiotic resistance and unprecedented reliance on implanted devices – but the research looks promising. ●
Medical Device Developments /
www.nsmedicaldevices.com
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