We report the formulation and mechanical characterization of double network (DN) composite hydrogels. The first network consists of covalently crosslinked poly(ethylene glycol diacrylate) (PEGDA), which forms a strong, brittle network that provides elasticity to the gel. The second network, sodium alginate, is ionically crosslinked with Ca 2+ to allow increased dissipation of mechanical energy. The novelty of this system over existing DN hydrogels is the additional incorporation of a third mesoscale network, composed of thermoresponsive poly(dimethyl siloxane) (PDMS) nanoemulsions, which undergo colloidal gelation through the bridging of the PEGDA hydrophobic end groups into the PDMS droplets. The colloidally gelled microstructures are photopolymerized into a solid hydrogel by crosslinking the precursors with ultraviolet (UV) light. Tensile mechanical experiments performed on the crosslinked DN nanoemulsion hydrogels show that their rupture stress (0.17-0.34 MPa), fracture energy (144-421 J/m 2 ), and Young's modulus (1-2.1 MPa) are comparable to similar systems in the literature. These mechanical measurements suggest that the gels may be suitable for manufacturing processes in which large shear rates and deformations are encountered. K E Y W O R D S composite hydrogels, double network hydrogels, nanoemulsions, thermoresponsive 1 | INTRODUCTION Hydrogels are a class of biomaterials that are widely used in biomedical applications, such as diagnostic devices, 1,2 tissue engineering, 3,4 and drug delivery, 5,6 due to their hydrophilicity and biocompatibility. 7-9 The physical and chemical properties of hydrogels can be tailored to specific biopharmaceutical applications. However, the adoption of conventional hydrogels composed of a single hydrophilic polymer, so-called single network (SN) hydrogels, are often limited in such applications due to a lack of mechanical strength as characterized by low Young's modulus values 10 of E~10 kPa and low fracture energies 11 of Γ = 10 0 -10 1 J/m 2 . Enhancing the mechanical strength of hydrogels is therefore important in expanding the applications of hydrogels to manufacturing processes in which large deformations are often encountered. 12 Double network (DN) hydrogels, 13 which are composed of two interpenetrating polymeric networks, are promising in overcoming the traditional mechanical limitations of SN hydrogels. The first network is typically made of a low-molecular weight polymer that provides rigidity to the system by tight covalent crosslinking. The second network is typically a polymer with high-molecular weight which is loosely crosslinked and provides flexibility to the system. 12 In 2003, Gong et al 13 developed a two-step sequential free-radical polymerization method to synthesize the first DN hydrogels, consisting of poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) as the first network and polyacrylamide (PAAm) as the second network. These DN gels achieved Γ = 10 2-3 J/m 2 and fracture tensile stresses σ rup = 10 0-1 MPa. Later, Sun et al 14 developed toug...

We report the formation of solid composite hydrogel fibers with tunable mass transport kinetics using a continuous extrusion process, in which the length scales of hydrophobic microdomains are tuned by temperature and flow conditions. The hydrophobic domains consist of surfactant-stabilized poly­(dimethylsiloxane) (PDMS) oil droplets that undergo colloidal gelation at elevated temperatures due to interdroplet bridging. The hydrophilic domains consist of photo-cross-linked poly­(ethylene glycol diacrylate) (PEGDA) molecules, which also serve as the thermogelator for the nanoemulsions. Microstructural variations are generated when the thermally gelled nanoemulsions flow through a cylindrical channel at various applied shear rates. We use confocal laser scanning microscopy and cryogenic scanning electron microscopy to demonstrate microstructural changes between the centers and edges of the fibers. The microstructures enable control over the release profile of a model active ingredient, retinol, from within the oil domains of the fibers to a fluid medium over a period of days. Our results suggest that hydrogel fibers can be produced with interconnected morphologies without the need for expensive postprocessing steps, thus opening up future avenues of manufacturing composite materials with excellent moisture-holding and mass transport properties for biomedical delivery applications.