Biomedical engineers at Purdue University have proven that bone cells attach better to metals with nanometer-scale surface features, offering hope for improved prosthetic hips, knees and other implants.
Conventional titanium alloys used in hip and knee replacements are relatively smooth – their surfaces possess bumps measured in microns – or millionths of a meter. Natural bone and other tissues, however, have rougher surfaces with bumps about 100 nanometers – or billionths of a meter – wide.
The body often reacts to the smooth artificial parts as it would to any foreign invader: It covers the parts with a fibrous tissue intended to remove the unwanted material. This fibrous tissue gets between prosthetic devices and damaged body parts, preventing prostheses from making good contact with the body parts in which they are implanted and interfering with their proper functioning.
Thomas Webster, an assistant professor of biomedical engineering, and postdoctoral researcher Jeremiah Ejiofor, have shown that materials containing the nanometer-scale bumps could be critical to keeping the body from rejecting artificial parts. The work also shows that materials containing the tiny bumps stimulate the body to regrow bone and other types of tissue.
Webster has demonstrated that human bone cells called osteoblasts generate about 60 percent more new cells when they are exposed to a titanium alloy that contains nanometer-scale features, compared to the same alloy containing micron-size surface bumps. Because bone and other tissues adhere to artificial body parts by growing new cells that attach to the implants, the experiments offer hope in developing longer lasting and more natural implants, he said.
The peer-reviewed findings were presented on Oct. 28 during the sixth annual Nanoparticles 2003 Conference in Boston.
"We believe the bone cells are basically recognizing the rougher nanometer surface and saying, 'Gee, this is a lot like what I'm used to adhering to in the body, so I am going to adhere to it and make bone,'" Webster said.
The experiments – in a field of research known as tissue engineering – were done in petri dishes, not with animals or people. Webster had demonstrated similar increases in cell growth using ceramics and various polymers, or plastics, and composites made of both ceramics and polymers, which are used in artificial body parts. He and co-workers at Purdue, including Karen Haberstroh and Riyi Shi, both assistant professors of biomedical engineering, have seen increased cell growth in cartilage and tissues from the bladder, arteries and brain when exposed to ceramics and polymers with nanometer-scale surface features.
Demonstrating the same results with metals, however, is especially important, Webster said.
"The reason we are excited about these findings is that metals are used much more than ceramics and polymers in artificial parts that are attached to bone," he said.
Webster and Ejiofor combined nanometer particles of a titanium alloy with a liquid suspension of human bone cells in petri plates. After three hours, they washed the alloys and used a microscope to count how many of the dyed cells adhered, which enabled the researchers to calculate how many cells stuck to the metal. Out of 2,500 bone cells in the suspension, about 2,300 – or more than 90 percent – were found to adhere to the metal. That compares with about 1,300 cells – or about 50 percent – adhering to metal with conventional, smoother surfaces.
"Almost all of the cells are attaching, which is pretty unheard of," Webster said. "With the conventional material you normally get about half of the cells attaching. We can do a lot better than that."
The need for better technology is growing as more artificial body parts are used, Webster said.
For example, about 152,000 hip replacement surgeries were performed in the United States in 2000, representing a 33 percent increase from 1990. The number of hip replacements by 2030 is expected to grow to 272,000 in this country alone because of elderly baby boomers.
The researchers used an alloy of titanium, aluminum and vanadium, which is commonly used in artificial joints and hip and knee replacements. They also are seeing similar increases in cell growth for commercially pure titanium and for an alloy of cobalt, chromium and molybdenum, both of which are currently used as orthopedic implants.
"The average lifetime of an implant is about 15 years, unfortunately," Webster said. "By the end of that 15 years, on average, the implant fails as bonding between the bone and the implant separates. It's not bound to anything anymore, so it becomes loose and it is very painful."
The work may help researchers use nanotechnology to design implants that last longer and work better.
"Due to these promising experiments with petri dishes, we are currently conducting more experiments and are working closely with area companies to commercialize these metals," Webster said.
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