Marine mussels have superb abilities to adhere to a wide range of substrates including rock, metal, and wood, while subjected to harsh environmental conditions, such as ocean waves and predator encounters. This adhesion is achieved through specialized structures that consist of an extensible thread that terminates in an adhesive plaque.  Although the chemical nature of the proteins that mediate the bonding at the interface is understood, little is known of the structural origins of the adhesive performance. We hypothesize that the presence of multiple material phases within the thread-plaque structures is one of the key reasons for their superb load-bearing capabilities. Both the thread and plaque have layered structures, with a stiff outer cuticle and soft inner core which is void-filled and porous. In this experimental study, we seek to understand how this composite structure contributes to the quality of adhesion by examining the modes of separation at the interface of the structure and a substrate, and by quantifying the toughness of the structures by examining their deformation under applied load. To achieve this, we developed new manufacturing methods based on multistage casting of polydimethylsiloxane(PDMS) into custom-designed 3D-printed molds which mimic the natural mussel structures. PDMS was selected due to its biological compatibility and tunable stiffness, which allows for an outer stiff phase surrounding a compliant core. Two-phase mussel-inspired structures were successfully generated, tested using tensile loading, and imaged to determine the failure mechanisms. Preliminary results suggest that the presence of the soft core may improve toughness by resisting the propagation of cracks across the interface between structure and substrate. We hope that our results will lead to robust manufacturing methods for developing high-strength, high-toughness, biocompatible, adhesive structures with potential applications in medicine and soft robotics.