Rational Fabrication of Functionally-Graded Surfaces for Biological and Biomedical Applications

As a ubiquitous feature of the biological world, gradation, in either composition or structure, is essential to many functions and processes. Taking protein gradation as an example, it plays a pivotal role in the development and evolution of human bodies, including stimulation and direction of the outgrowth of peripheral nerves in a developing fetus. It is also critically involved in wound healing by attracting and guiding immune cells to the site of injury or infection. Another good example can be found in the tendon-to-bone enthesis that relies on gradations in composition, structure, and cell phenotype to create a gradual change in mechanical stiffness. It is these unique gradations that eliminate the high level of stress at the interface, enabling the effective transfer of mechanical load from tendon to bone. How to fabricate and utilize graded surfaces and materials has been a constant theme of research in the context of materials science, chemistry, cell biology, and biomedical engineering. In cell biology, for example, graded surfaces are employed to investigate the fundamental mechanisms related to embryo development and to elucidate cell behaviors under chemo-, hapto-, or mechano-taxis. Scaffolds based upon graded materials have also been widely explored to enhance tissue repair or regeneration by accelerating cell migration and/or controlling stem cell differentiation.