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Magnetic resonance(MR)imaging is an important medical diagnostic technique widely used in modern medicine for cancer diagnosis,due to its noninvasive,high spatial resolution,and nonionizing radiation source.In order to improve the MR detection sensitivity and acquire high quality MR images,contrast agents are indispensable in the clinic practice.Hybrid nanogels(NGs)are three dimensional physically or chemically crosslinked polymer particles with a unique spherical shape and of colloidal dimensions.The NGs allows a further flexible functionalization with different macromolecules,like dyes,drugs,metal-complexes and also nanoparticles,so as to be used in many fields of nanomedicine,such as imaging,controlled drug release,and hyperthermia therapy.In our previous work,N-vinylcaprolactam(VCL)has been used as main monomer to synthesize poly(N-vinylcaprolactam)(PVCL)NGs,which is temperature sensitive and exhibits a lower critical solution temperature in water.PVCL shows a great biocompatibility and is therefore suitable for medical in vivo applications.Moreover,the abundant functional groups on the surface of the PVCL NGs shells allow their incorporation with variating amounts of the Gd chelates ligands to improve the drawbacks of Gd chelates based contrast agents.In this work,we have firstly synthesized the biocompatible PVCL NGs based on the monomer VCL,the comonomer acrylic acid(AAc),and the acid degradable crosslinker 3,9-divinyl-2,4,8,10-tetraoxaspiro-[5,5]-undecane(VOU)in aqueous solution via precipitation polymerization,followed by covalently binding with Gd-DOTA-NH2 complexes to form the final PVCL NGs loaded with gadolinium(PVCL-Gd NGs)for MR imaging application.The formed PVCL-Gd NGs with a size less than 200 nm are water dispersible,well uniform,colloidally stable,and biocompatible in the given concentration range,and displayed relatively higher r1 relaxivities(6.38-7.10 mM-1 s-1)than the clinical Gd chelates contrast agents.The PVCL-Gd NGs hold the ability to enhance the MR imaging of cancer cells in vitro and tumor models in vivo.Therefore,the developed PVCL-Gd NGs could be used as a promising contrast agent candidate for T1 MR imaging in the future.