[2] These materials can be inspired by nature where mechanical gradients are used commonly to improve interfaces between two dissimilar surfaces.
In contrast, a butt joint that gives a junction between materials with little to no gradient has been shown to be weaker than the homogenous components.
[8] There are many examples in nature where soft tissue and hard surfaces are connected by a mechanical gradient to improve the fracture and impact resistance.
Without the mechanically gradient in the beak, the squid would be unable to withstand the high impacts despite its hardness since it would break off from the body at the junction between the materials.
[11] Similarly, in knee and hip implants, there is a need for high integration between the strong bone and the cartilage and tissue.
Otherwise problems such as stress shielding can occur where the bone degenerates due to the implant having too strong of a modulus.