Enhancement of the Mechanical Properties of Hydrogels with Continuous Fibrous Reinforcement

Reinforcing mechanically weak hydrogels with fibers is a promising route to obtain strong and tough materials for biomedical applications while retaining a favorable cell environment. The resulting hierarchical structure recreates structural elements of natural tissues such as articular cartilage, with fiber diameters ranging from the nano- to microscale. Through control of properties such as the fiber diameter, orientation, and porosity, it is possible to design materials which display the nonlinear, synergistic mechanical behavior observed in natural tissues. In order to fully exploit these advantages, it is necessary to understand the structure-property relationships in fiber-reinforced hydrogels. However, there are currently limited models which capture their complex mechanical properties. The majority of reported fiber-reinforced hydrogels contain fibers obtained by electrospinning, which allows for limited spatial control over the fiber scaffold and limits the scope for systematic mechanical testing studies. Nevertheless, new manufacturing techniques such as melt electrowriting and bioprinting have emerged, which allow for increased control over fiber deposition and the potential for future investigations on the effect of specific structural features on mechanical properties. In this review, we therefore explore the mechanics of fiber-reinforced hydrogels, and the evolution of their design and manufacture from replicating specific features of biological tissues to more complex structures, by taking advantage of design principles from both tough hydrogels and fiber-reinforced composites. By highlighting the overlap between these fields, it is possible to identify the remaining challenges and opportunities for the development of effective biomedical devices.