Tracking　the　distribution　and　degradation　of　hydrogels　in　vivo　is　important　for　various　applications　including　tissue　engineering　and　drug　delivery.　Among　various　imaging　modalities,　fluorescence　imaging　has　attracted　intensive　attention　due　to　their　high　sensitivity,　low　cost　and　easy　operation.　Particularly,　upconversion　nanoparticles　(UCNPs)　that　emit　visible　lights　upon　near-infrared　(NIR)　light　excitation　as　tracking　probes　are　promising　in　deciphering　the　fate　of　hydrogels　after　transplantation.　Herein,　we　reported　a　facile　and　non-invasive　in　vivo　hydrogel　tracking　method　using　UCNPs,　where　the　degradation　of　hydrogels　was　determined　using　the　decrease　in　fluorescence　intensity　from　the　UCNPs　encapsulated　in　the　hydrogels.　We　found　that　the　change　in　the　fluorescence　intensity　from　the　UCNPs　was　well　consistent　with　that　of　the　fluorescein　isothiocyanate　(FITC)　covalently　conjugated　to　hydrogels　and　also　with　the　weight　change　of　the　hydrogels,　suggesting　the　accuracy　of　the　UCNPs　in　tracking　the　degradation　of　hydrogels.　Furthermore,　the　in　vivo　fluorescence　signals　were　only　observed　from　the　UCNPs　instead　of　FITC　after　implantation　for　7　days　due　to　the　deep　tissue　penetration　of　UCNPs,　demonstrating　the　capability　of　UCNPs　in　longitudinal,　consecutive　and　non-invasive　monitoring　the　in　vivo　degradation　of　hydrogels　without　causing　any　damage　to　the　major　organs　(heart,　lung,　liver　and　kidney)　of　model　rats.　This　study　thus　paves　the　way　for　monitoring　the　in　vivo　behaviors　of　biomimetic　materials　via　deep　tissue　imaging　with　great　clinical　translation　potentials.　Statement　of　Significance　Long-term　noninvasive　in　vivo　tracking　of　the　distribution　and　degradation　of　biodegradable　hydrogels　using　fluorescent　probes　is　important　in　tissue　regeneration　and　drug　delivery.　Unlike　the　widely　used　fluorescent　dyes　and　quantum　dots　(QDs)　that　suffer　from　photobleaching　and　undesired　toxicity,　upconversion　nanoparticles　(UCNPs)　with　high　stability,　deep　tissue　penetration　as　tracking　probes　are　promising　in　deciphering　the　fate　of　hydrogels　after　transplantation.　Herein,　we　reported　a　noninvasive　in　vivo　hydrogel　tracking　method　using　UCNPs　and　found　that　the　fluorescence　intensity　change　from　the　UCNPs　was　well　consistent　with　the　weight　change　of　the　hydrogels,　suggesting　the　accuracy　of　UCNPs　in　tracking　hydrogel　degradation.　This　study　provides　inspirations　on　developing　advanced　NIR　light　regulated　probes　with　great　clinical　translation　potentials.　(C)　2017　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.