Over the past few decades, synthetic hydrogels have garnered significant attention in various fields such as tissue engineering [1,2], drug delivery [3], [4], [5], anti-biofouling [6], [7], [8], actuation [9], [10], [11], and sensing [12,13], owing to their eminent biocompatibility, mechanical properties, lubricity, and biodegradability [14], [15], [16]. To date, researchers have made tremendous progress in the design of hydrogels with specific functionalities [17]. However, in most technical applications, hydrogels are used separately, and there are few repots on their combination with substrates due to their relatively weak adhesion strength, especially toward substrates with complex geometries [18]. Therefore, the tough bonding of hydrogels to substrates has emerged as a promising but challenging area of research.

Strong adhesion and robust mechanical properties of hydrogel to the substrate are the key parameter for the successful preparation of hydrogel coatings. Yuk et al. achieved adhesion of more than 1000 J/m2 by a surface bridging strategy to bond tough hydrogels to nonporous surfaces [19]. However, this strategy requires the interaction of bridging molecules with the substrate to form strong bonds and the thickness of the coating is difficult to control [20]. Although countless efforts and attention being devoted to improving the adhesion of hydrogel coatings, it remains challenging to coat substrates with complex geometries uniformly (e.g., complex curvatures, hollow, or cage-like structures) [21,22]. To overcome this shortcoming, Wang et al. used adhesives to adhere dehydrated dry gel particles on large-area substrates to form a renatured hydrogel coating [23]. However, the use of toxic adhesives hinders their further application in the biomedical field. Recently, Zhao et al. developed a hydrogel coating preparation technology via the surface initiation method, which interpenetrates the substrate polymer and the hydrogel coating network at the interface of polymer substrates with complex geometries [24]. Additionally, Zhou et al. prepared uniform hydrogel lubricating coatings on diverse biomedical device surfaces via ultraviolet-triggered surface catalytically initiated radical polymerization [25]. Nevertheless, the preparation of these hydrogel coatings involves simultaneous polymerization, crosslinking and interconnection, requiring a strictly oxygen-free environment [18,26]. Therefore, the development of versatile strategies to prepare hydrogel coatings on substrates with complex geometries remians a critical demand and central challenge in the biomedical device field [27,28].

For industrial paints, polymerization is separated from crosslinking and interconnection due to the division of labor between manufacturer and user [29]. In principle, paints can be applied to a various substrates through various processes and does not require a strictly oxygen-free environment. Moreover, the toxic reagents can be separated on demand, adding to their versatility and convenience [30]. Microgels, which are polymer particles with intramolecularly crosslinked structures, have emerged as a valuable tool in the industrial paint industry, particularly in processing and construction. [31,32]. Their introduction into paints has shown to improve not only the rheological and mechanical properties of the coatings, but also the stability of pigment dispersion and chemical resistance [33,34]. Recently, microgels have also been used as an important component in double network hydrogels to enhance their mechanical properties [35,36]. Therefore, the development of microgel-based hydrogel paint strategies holds immense potential in the realm of biomedical devices. These coatings can improve the durability and stability of the devices while also allowing for greater flexibility in coating various substrates.

Herein, inspired by the principle of microgel reinforcement in industrial paints, we proposed a composite hydrogel paints (CHPs) strategy for biomedical devices surface functionalization. The CHPs were comprised of zwitterionic copolymers and microgels, both of which had reactive groups. The chemical composition, surface morphology, hydrophilicity, rheology, and mechanical properties of the CHP could be easily adjusted by physical blending, making it a highly versatile and customizable coating option. We investigated the universality and adhesion of CHPs on various substrates with different shapes, including metals, polymers, and inorganic materials. As potential application cases, we further demonstrated the application of CHPs on utility devices including surgical sutures and blood-contacting PVC tubing.