When replacing a torn anterior cruciate ligament (ACL) or using bone tissue in spinal fusion to treat vertebral problems, the last thing that a doctor or patient wants to worry about is an infection or the structural integrity of the replacement part. For this reason tissues are screened, cleaned and processed to meet U.S. Food and Drug Administration (FDA) guidelines. During processing, however, some tissues are sterilized typically by irradiating the replacement part or tissue, and this process can undermine its strength. So, a new approach is emerging as an alternative—"supercritical" carbon dioxide, compressed to somewhere between a gaseous and liquid state.

When carbon dioxide is compressed to nearly 10 times atmospheric pressure, or around 10 kilograms per centimeter, and heated to 35 degrees Celsius, it forms what is known as a supercritical fluid—a wet gas that can permeate surfaces and has some of the dissolving properties of liquids, as well. After transplant tissue has been cleaned and sealed in its final medical packaging, these bone fragments, tendons or even skin grafts are placed in baskets within a high-pressure chamber. Once sealed, an automated system fills the chamber with CO2 and then pressurizes and heats it above the critical point where the gas forms a solvent that sterilizes the samples within a few hours. Supercritical CO2 can prompt the formation of the related carbonic acid, which, along with chemical oxidation, may kill the bacteria.

Evidence for using supercritical CO2 for sterilization dates back a decade when a team of Massachusetts Institute of Technology researchers reported in Proceedings of the National Academy of Sciences the use of this form of carbon dioxide to deactivate live bacteria. Within a year, NovaSterilis in Lansing, N.Y., formed to commercialize the process with larger automated systems that could use supercritical CO2 to sterilize tissue. Today, the company's Nova 2200 features a 20-liter sterilization chamber in which up to 60 individual allografts (transplant tissues) can be purified. To deactivate spores as well as live bacteria, the company provides sheets of paper treated with a mix of chemical additives including an acid that can be dissolved into the supercritical CO2 sterilizing fluid, much like detergent in a washing machine, NovaSterilis President and Chief Executive David Burns says. The fluid penetrates bacterial cells and kills them.

The demand for replacement bone and tendons is great—about 1.7 million musculoskeletal allograft tissues are processed in the U.S. annually, according to the American Association of Tissue Banks' (AATB) Annual Survey Report 2007. The supercritical approach is getting a toehold in this industry. Burns says that five tissue-processing companies have licensed the NovaSterilis technology. Bone, tendon and skin samples treated with the high-pressure CO2 have not shown any reduced structural integrity, according to at least two studies conducted by biomedical company researchers and published in peer-reviewed journals, offering an alternative to irradiation and providing replacement parts that reach FDA sterilization standards on par with those for medical devices.

Bathing allografts in supercritical CO2 has other practical advantages, as well. Supercritical treatment allows tissue banks to process their own products on site and within a few hours rather than shipping samples out for decontamination. Because of the expense and governmental regulations associated with using radiation, tissue banks don't irradiate their own tissues. Instead, they ship samples to facilities with the necessary licenses and expertise. That process is, however, both time consuming and expensive, says Sharon Bryce, director of tissue services for Australian Biotechnologies, a New South Wales biotech firm that expects to get approval from the Therapeutic Goods Administration (TGA), Australia's regulatory equivalent of the FDA, to use supercritical carbon dioxide to sterilize one of their bone products by the end of this year. And like irradiation, the CO2 process can be performed while the samples are sealed within their final packaging, reducing the opportunity for contaminants to be reintroduced.

The problem with high doses of gamma radiation, or harsh treatments including steam and chemicals such as ethylene oxide (a fumigant known to cause respiratory irritation and lung injury), is that they can degrade collagen and other proteins within bone and soft tissue, compromising their strength and rendering them unusable for grafts, notes AATB Chief Policy Officer Scott Brubaker. Bryce adds: "Gamma irradiation is actually quite harsh to the bone. We walk a fine tightrope in our company," using low-dose radiation to sterilize while trying to avoid structural degradation.

In fact, although low-dose irradiation is an FDA-validated sterilization method, some surgeons still prefer not to use irradiated tissue because they question the mechanical integrity of the resulting grafts, says Shawn Hunter, director of research and development at Community Tissue Services in Dayton, Ohio. This has Hunter and his colleagues testing sterilization of soft tissue allografts using supercritical carbon dioxide as an alternative to irradiation. The supercritical approach provides added control over the sterilization process and gives a tissue processor firsthand knowledge that samples have been handled appropriately, Hunter adds.

Some surgeons have been impressed with the supercritical process so far, says Bruce Humphrey, president of Integrity Bio-Surgical Systems in Andover, N.J., which distributes soft tissues, including some sterilized with supercritical CO2. Although tissue banks still have to be convinced of the value of investing in the new technology, Humphrey is optimistic about its future. "I see the process becoming a standard in the industry," he says.