For example, it appears that small pieces of gel break off the bulk material and are being taken up by surrounding cells, which may eventually lead to disassembly and degradation of the gel over extended periods of time. Metal-phenolic hydrogels can also be prepared by replacing titanium with zirconium as the coordinating metal. Exploring these zirconium-based hydrogels would be of interest for future studies as zirconium-containing materials have shown improved performance (even compared to wellperforming titanium-containing materials) in some biomedical applications. Titanium accumulation is low in distal tissues. Coordination-driven assembly strategies are based around non-covalent interactions, which are dynamic with a stability that depends on the local environment and the time scale. For example, TiTannic hydrogels can disassemble rapidly (within minutes to hours) if exposed to extreme pH or to competing ligands (Figure ), but remain stable for months when immersed in sterile cell culture media and PBS (Figure ). While no large changes in gel size or mass were observed during our in vivo studies (Figure ), small amounts of material leakage from the TiTannic gels could still occur. Tannic acid is a naturally abundant polyphenol (found in many plants, fruits and vegetables) and is being investigated for its intrinsic therapeutic effects and its ability to enhance the delivery of drugs. The other component, titanium, is a metal commonly used in the design of implants (e.g., for dental implants and joint replacement). Detection of increased tissue levels of titanium is of interest clinically to aid in the assessment of implant status and prognosis, where increased levels of titanium may indicate increased implant wear. In the current study, we used mass spectroscopy to evaluate titanium levels at distal tissues to the injection site (Figure ).