SPECIAL SECTION/ADVANCED CERAMICS: Material Innovations: Super Putty

April 1, 2009
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Nanoporous silicon could play an integral role in revolutionizing orthopedic medicine.



Biomedical engineers at The University of Texas Health Science Center at Houston are leading a multi-institution initiative to produce a bio-compatible compound designed to mend serious leg fractures. The researchers have been awarded $5.2 million in initial funding from the U.S. Department of Defense (DOD) to develop a “fracture putty” that could be used to regenerate bones shattered by roadside bombs or other explosive devices. The total value of the effort, if all phases of the development program are completed, could be up to $7.9 million.

Non-Union Fractures

If a soldier’s leg is shattered by a bomb, treatment options are currently limited. One lengthy, painful and often unsuccessful treatment option is bone grafts, where pins, plates or screws hold the grafts to healthy bone while external fixators provide support. This treatment may require multiple surgeries and recuperation periods of about a year, and the soldier still might never regain full use of the injured leg. Amputation is an option of last resort, though it is more common for those in less-advantaged or remote areas.

The new fracture putty could put an end to all of that. Researchers hope to develop a viscous material that can be quickly injected into the leg (or other affected area) on-site or molded to the affected bone in a surgical setting. Solidification would be triggered by ultraviolet light or heat, and the material would become load-bearing-even at the femoral level-over a period of approximately one week. The material would also trigger regrowth of the bone and associated soft tissue (i.e., blood vessels, nerves, muscles), and it would degrade at a controlled rate while the bone is regenerating.

“This is work that could not be done at any individual institution,” says Mauro Ferrari, Ph.D., principal investigator and deputy chairman of the Department of Biomedical Engineering, a joint venture among the UT Health Science Center at Houston, The University of Texas at Austin and The University of Texas M. D. Anderson Cancer Center. “It is the result of cross-institutional, multidisciplinary partnerships.”

Fracture Putty for Traumatic Bone Regeneration: 1. The fracture putty composite material is implanted in the site of the shattered bone. Growth factors are released from the implant and recruit the patient’s stem cells. The putty is load-bearing, so the patient is able to walk while the bone heals. 2. The fracture putty is infiltrated by cells that begin to create new bone. At the same time, the material constituting the fracture putty starts to degrade. 3. The degradation of the fracture putty gradually transfers the weight of the patient to the regenerating bone, aiding in its functional recovery. 4. Several months after injury, the architecture and function of the bone are fundamentally restored.

Puzzle Pieces

Each component in the composite material has been developed and tested independently. For example, Antonios Mikos, Ph.D., director of Rice University’s Center for Excellence in Tissue Engineering and a collaborator on the project, has shown that the polymer polypropylene fumarate can serve as a biocompatible, degradable bone replacement material in animal maxillofacial surgeries. Alone, however, it cannot provide enough strength to enable the bone to become load-bearing.

That’s where nanoporous silicon and silica come in. “Silicon has stiffness greater than steel,” says Ferrari. “We’re talking about a material that can provide tremendous mechanical reinforcement.” Silicon is also biocompatible and degradable, and it offers an additional benefit because its porous nature can be used to house some of the putty’s other necessary components.

The next two elements are biological stimuli and receptors, which go hand in hand. As Ferrari says, “Bone growth doesn’t just happen by itself.” Stimuli in the form of peptide amphiphile analogs, developed by Samuel Stupp, Ph.D., director of the Institute of BioNanotechnology in Medicine at Northwestern University and another collaborator, trigger the injured person’s own stem cells to begin the bone regeneration process. Paul Simmons, Ph.D., director of the Centre for Stem Cell Research at the Brown Foundation Institute of Molecular Medicine for the Prevention of Human Diseases (and a co-investigator), has successfully demonstrated the growth of bone to fill large missing segments in human clinical trials.

 Finally, “all of these pieces need to come together somehow,” says Ferrari. George Whitesides, Ph.D., the Woodford L. and Ann A. Flowers University Professor at Harvard University, serves as the group’s adhesives expert.

“Success on even a small part of the project has the potential to revolutionize orthopedic medicine,” says Ferrari. “It could give people with serious leg injuries an opportunity to regain full use of limbs that now require amputations or the use of permanent implants. We’re creating a living material that can be applied to crushed bones. The putty will solidify inside the body and provide support while the new bone grows.”

If fracture putty proves successful, it could also be used in emergency rooms to treat civilians injured in traffic accidents and other traumatic events involving the spine, skull and facial bones. According to Ennio Tasciotti, Ph.D., a research assistant professor in Ferrari’s lab, “The findings of this research could eventually benefit all victims of any bone-related traumatic injury and reduce the number of wartime amputations in the military, as well as in the civilian population.”

Development and Testing

The Defense Advanced Research Projects Agency (DARPA), the DOD agency that is funding the project, sponsors high-risk, high-reward research that bridges the gap between fundamental discoveries and their military and civilian use. According to DARPA Program Manager Mitchell Zakin, Ph.D., “This undertaking represents the ultimate convergence of materials science, mechanics and orthopedics. I look forward to the first results, which should present themselves in about a year or so.”

Ferrari’s team will begin the pre-clinical study by testing the mechanical and biological properties of candidate compounds in mathematical models and in vitro systems. Afterward, the compounds will be tested in several animal models. The research, called “BioNanoScaffolds for Post-Traumatic OsteoRegeneration,” runs through December 2010.

If the fracture putty works in an animal model, the next step will involve patients. “We have been in preliminary conversations with the U.S. Food and Drug Administration,” says Ferrari, who also serves as president of the Alliance for NanoHealth, a consortium that has formally partnered with the FDA to investigate the regulatory challenges of nanomedicine through the development of a public/private partnership.

Ferrari has high hopes for the future of biomedical technology development. “Serious progress will be made by institutions working together and integrating with regulatory bodies,” he says.

For more information, visit www.uth.tmc.edu.

Editor’s note: Members of the Alliance for NanoHealth include the Baylor College of Medicine; The University of Texas M.D. Anderson Cancer Center; Rice University; The University of Houston; The University of Texas Health Science Center at Houston; Texas A&M Health Science Center; The University of Texas Medical Branch; and The Methodist Hospital Research Institute. Visit www.nanohealthalliance.org for additional information.


SIDEBAR: Safety at the Nano-Scale

"My personal philosophy is that medical ethics also apply to medical research. As you know, the concept of medical ethics starts with, `First, do no harm.' Regardless of whether we are by the bedside taking care of people or in the lab cooking up materials, that's the primary concern. These materials are all extensively tested and will continue to be tested for any negative effects. And not only by us-the FDA will require very thorough safety investigations before they allow the material to go into people."

- Mauro Ferrari

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