The role of surface free energy in osteoblast–biomaterial interactions

The clinical success of many orthopaedic implants relies on good integration between the implant and adjacent bone. As stabilising bone grows not only to the implant, but from it, the quick adhesion of bone forming cells called osteoblasts, their appropriate differentiation and ability to form mineralised bone are vital to achieve a good clinical outcome. Surface free energy can be thought of as a measure of the ‘unsatisfied bond energy’ resulting from ‘dangling bonds’ exposed at a material's surface. This unsatisfied bond energy affects protein adsorption and cell attachment, and thus controls the early stages of cell–biomaterial interactions and ultimately implant fixation. When water, proteins, or cells approach a surface, their surface domains align to minimise the overall surface free energy of the interface. Determining these interactions, however, is not simple. While contact angle measurements on flat surfaces can predict some surface free energy-related interactions, this is not the case when surface topography is modified. Here, the authors review how surface free energy can be altered on self-assembled monolayers, polymers, metals and ceramics and clarify the differences between measurements of surface free energy and wettability. The authors also review how surface free energy affects protein interactions and osteoblast behaviour. The result is a clearer understanding of the effect of surface free energy on cell behaviour and an unambiguous need for further studies that isolate such effects.