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Modified Nanoporous Alumina Membrane Holds Promise for Future Implants
A scanning electron micrograph of a PEGylated platinum-coated nanoporous alumina membrane after exposure to human platelet-rich plasma. The pores are largely free of fouling, and the membrane surface does not show fibrin networks or platelet aggregations. (Photo credit: SP Adiga et al.)
From pacemakers to heart pumps to insulin dispensers, medical implants have changed the practice of medicine. But because of protein adsorption and biocompatibility issues that may arise in vivo, the successful implantation of these life-saving devices is a challenging endeavor. To overcome these obstacles, a research team from North Carolina State University (NC State; Raleigh, NC) has developed a nanoporous alumina membrane material that could be used to create an interface between human tissues and medical devices that is free of protein buildup, according Roger Narayan, the project’s team leader.
Implantable blood glucose sensors, for example, currently have membrane-tissue interfaces that are inadequate for long-term use. In addition, biofouling and inflammation undermine biosensor membrane stability. Future implants must be made of a material that prevents the body’s proteins from accumulating on the sensors. They must also be designed so that they do not induce an inflammatory response from the body that can cause them to be walled off or rejected completely. To achieve long-term implantability, a biosensor membrane material must prevent protein adsorption while exhibiting cell compatibility. In addition, it must be highly porous and thin enough to enable biosensors to respond to analyte fluctuations.
Nanoporous alumina provides several advantages over conventional materials as a sensor membrane, explains Narayan, associate professor in the joint biomedical engineering department of NC State and the University of North Carolina (Chapel Hill, NC). “These membranes can be processed with smaller pore sizes—the 10–100-nm range—and more-uniform pore sizes than polymer membranes,” notes Narayan. These characteristics enable them to be used for medical implants.
To achieve protein resistance, the research team coated the alumina membranes with diamondlike carbon thin films. Unlike many metal or polymer surfaces, according to Narayan, carbon resists protein adsorption, as shown by in vitro studies involving human platelet–rich plasma. In addition, diamondlike carbon demonstrates an exceptionally low coefficient of friction, which may facilitate fluid transport.
“Diamondlike carbon serves as a hermetic seal around the nanoporous alumina,” Narayan says. “Diamondlike carbon thin films are atomically smooth and offer low friction; they are wear resistant, corrosion resistant, and immune to scratching by third-body wear particles. In vitro studies have shown that human cells exposed to diamondlike carbon-coated membranes do not demonstrate reduced cell viability or cell growth.”
The team’s goal is to create a material that can be used in immunoisolation devices, kidney dialysis membranes, implantable biosensors, and other active medical devices that experience biocompatibility problems limiting in vivo function. Narayan remarks that the new technology has received significant commercial interest. He and his colleagues hope that their research will reach the market within the next five years.