Performance of polyoxymethylene plastic (POM) as a component of a tissue engineering bioreactor

Polyoxymethylene (POM, acetal homopolymer, polyacetal), commercialized as Delrin by DuPont, is an engineering resin with mechanical properties that make it useful for the prototyping and manufacture of laboratory apparatus. These properties include excellent, "metal-like," machining characteristics and dimensional stability, as well as thermal stability, which allows steam sterilization. Historically, POM has been used widely, including as a surgical implant material. For these reasons, we have used this plastic as a media-wetted component in a tissue-engineering bioreactor, with good results. However, a study by LaIuppa et al.5 suggested that POM is unsuitable for use in a cell culture environment (LaIuppa et al. J Biomed Mater Res 1997;36:347-359). POM is based on the polymerization of formaldehyde, and, in addition, contains stabilizers and/or fillers. All of these could potentially be released into the medium, e.g., as formaldehyde or other thermal breakdown products, especially upon repeated autoclaving. The cited report thus appeared plausible, although contrary to our observations. In this study, we specifically assessed whether media conditioned by long-term exposure to machined white POM had a negative effect on the proliferation and chondrogenic differentiation of human mesenchymal stem cells (MSCs). We selected this cell system, as cartilage tissue engineering is the primary application of our bioreactor system. The POM samples were steam-autoclaved 1 to 20 times, to assess the possibility of any toxic thermal breakdown product release into the media. We found that MSCs did not attach directly to machined POM. Because cells that escape from the tissue construct cannot colonize the reactor and compete for nutrients, this is a desirable characteristic of a material used in a tissue-engineering bioreactor. Furthermore, the use of POM-conditioned media had no detectable impact on the proliferation rate of MSCs measured over a one-week period; nor was any effect on chondrogenic differentiation observed at up to 3 weeks in culture. In summary, the use of POM as a culture medium-wetted component appears to be innocuous, at least for human MSCs. The contrast of these findings to those of LaIuppa et al.5 may reflect a cell-type specific sensitivity, or may be due to different handling of the material.