# How Does the Biocompatibility of Molybdenum Compare to the Gold Standard Titanium?--An In Vivo Rat
## 4 Discussion
Therefore, the degradation proceeds by the formation of an oxide layer, the molybdenum content of which is reduced over time by the release of molybdate ions. The absence of an appreciable degradation product layer thus indicates minimal dissolution. The most likely explanation for the low degradation rate is the anatomical environment to which the samples are exposed. As described previously, the molybdenum samples in the fold of the neck were enclosed in tissue capsules, which was not observed in the studies of molybdenum wires implanted into the aortic wall. The capsule restricts the exchange of fluid with the rest of the body. Within the capsule, both the degradation of molybdenum (overall reaction equation: Mo + 4 H\({}_{2}\)O \(\rightarrow\) MoO\({}_{4}\)\({}^{2-}\) + 8H\({}^{+}\) + 6e\({}^{-}\)) and the observed low-level inflammation may lead to more acidic conditions. Acidic conditions are known to slow the degradation of molybdenum [40]. The formation of the capsule may also inhibit the integration of the samples into the surrounding tissue. Based on results from earlier studies, tissue integration seems to be an important factor in determining the degradation rate, with less integrated samples degrading more slowly. For example, the degradation product layer thickness for a less integrated sample was reported to be only one-fifth of the layer thickness for a well-integrated sample in [34]. Bone healing, e.g., after a fracture, involves complex remodeling processes and depends, above all, on the body region. Complete ossification is usually observed after 6 weeks. Complete stability is normally assumed after about six months, especially in the field of maxillofacial surgery. A good guideline is that the implant should bear load for 3-6 months and then have fulfilled its function. In our previous in vitro experiments, we were able to show that molybdenum sheets under physiological conditions exhibited a mass loss of approx. 20% after 6 months [28]. Thus, a relevant loss of stability occurred, which, in theory, would fit well with the time needed for bone stability to be regained. Furthermore, this rate of degradation indicates that there is a reasonable expectation that a molybdenum-based implant will degrade within a clinically relevant time frame, e.g., 1-2 years. However, the degradation rates for encapsulated molybdenum are so low (\(<\)<1 \(\upmu\)m/year) that degradation will not occur in humanly achievable time. Animal studies in other tissues (aortic wall) have shown degradation rates corresponding to a few tens of \(\upmu\)m/year at most. Unless future experiments with other tissues show significantly higher rates, molybdenum implants should, therefore, not exceed structural sizes of 100 \(\upmu\)m, or a maximum of 150 \(\upmu\)m, to allow for degradation within a few years. The significant strength and stiffness of molybdenum are advantageous here, allowing for much thinner structure sizes than those of magnesium implants, for example. The suitability of molybdenum as a resorbable biomaterial for osteosynthesis implants may, therefore, depend on the tissue reaction at the location of implantation. Direct contact with bone tissue in osteosynthesis would most likely lead to different degradation behavior. The degradation behavior in various tissues must be studied in more detail to determine the factors influencing the degradation. This may also result in strategies (such as surface functionalization) leading to the desired level of tissue integration and favorable conditions for moderate, uniform degradation in the range of several tens of \(\upmu\)m per year.