Tissue engineering is a growing discipline that uses a combination of engineering disciplines to develop novel materials and therapeutic strategies for the repair or replacement of damaged tissues and organs. Current strategies typically focus on the development of tissue engineering scaffolds using hydrogels. Scaffolds are application specific and may require interfacing multiple hydrogel materials to mimic the properties of the native tissue. In addition, the resulting design must be mechanically robust so as to withstand physiological loading without failure. Thus, the success of a given application is dependent on 1) the ability to accurately characterize the properties of the native tissue and scaffold materials and 2) the ability to integrate multiple hydrogel materials in a robust manner.

One key challenge is that the influence of surface roughness on the indentation of soft, hydrated materials is poorly understood. Surface roughness is known to influence the properties of traditional engineered materials, but this influence has not been evaluated in the context of soft tissues and hydrogels with rough surfaces. This challenge is addressed through microscale indentation testing of agarose hydrogels, and articular cartilage. The mechanical properties of both rough and smooth surfaces were evaluated using a range of probe sizes, displacement rates, and indentation depths. Experiments demonstrated that rough surfaces reduce the measured indentation properties by as much as 90%.

Another key challenge is the robust integration of dissimilar hydrogels. Composite hydrogels can be used to tune the properties of a scaffold, but stresses concentrate at the interfaces between materials with dissimilar mechanical properties and may fail under loading. Functionally graded interfaces (FGIs) reduce the property mismatch at the interfaces, but the influence of the width of the FGI on the fracture toughness of hydrogel interfaces. This challenge is first addressed via single edge notch fracture tensile tests of dissimilar polymer interfaces with varying interfacial widths and will subsequently be extended to study dissimilar hydrogel interfaces. Preliminary results demonstrate a strong dependence on the dimensions of the soft phase that may be mitigated through elimination of the stress concentration at the interface between the soft phase and the steel grips used during testing.