taken to dry the polymers before processing to exclude humidity. As most biodegradable polymers have been synthesized by ring opening polymerization, a thermodynamic equilibrium exists between the forward polymerization reaction and the reverse reaction that results in monomer formation. Care needs to be taken to avoid an excessively high processing temperature that may result in monomer formation during the molding and extrusion process. It must be followed carefully. Resorbable polymers can also be 3D printed. Degradation Once implanted, a biodegradable device should maintain its mechanical properties until it is no longer needed and then be absorbed by the body leaving no trace. The backbone of the polymer is hydrolytically unstable. That is, the polymer is unstable in a water based environment. This is the prevailing mechanism for the polymers degradation. This occurs in two stages. 1. Water penetrates the bulk of the device, attacking the chemical bonds in the amorphous phase and converting long polymer chains into shorter water-soluble fragments. This causes a reduction in molecular weight without the loss of physical properties as the polymer is still held together by the crystalline regions. Water penetrates the device leading to metabolization of the fragments and bulk erosion. 2. Surface erosion of the polymer occurs when the rate at which the water penetrating the device is slower than the rate of conversion of the polymer into water-soluble materials. Biomedical engineers can tailor a polymer to slowly degrade and transfer stress at the appropriate rate to surrounding tissues as they heal by balancing the chemical stability of the polymer backbone, the geometry of the device, and the presence of catalysts, additives or plasticisers. Applications Biodegradable polymers are used commercially in both the tissue engineering and drug delivery field of biomedicine. Specific applications include. Sutures Dental devices (PLGA) Orthopedic fixation devices Tissue engineering scaffolds Biodegradable vascular stents Biodegradable soft tissue anchors References Further reading Some biodegradable polymers, their properties and degradation times can be found in Table 2 in this document. An example of the structure of some of the types of polymer degradation can be viewed in Fig. 1 in this article Bellin, I., Kelch, S., Langer, R. & Lendlein, A. Polymeric triple-shape materials. Proc. Natl. Acad. Sci. U.S.A. 103, 18043-18047 (2006. Copyright (2006) National Academy of Sciences, U.S.A. Lendlein, A., Jiang, H., Jünger, O. & Langer, R. Light-induced shape-memory polymers. Nature 434, 879–882 (2005). Lendlein, A., Langer, R.: Biodegradable, Elastic Shape