Built-in microscale electrostatic fields induced by anatase–rutile-phase transition in selective areas promote osteogenesis

Bone has a built-in electric field because of the presence of piezoelectric collagen. To date, only externally applied electric fields have been used to direct cell behavior; however, these fields are not safe or practical for in vivo use. In this work, for the first time, we use a periodic microscale electric field (MEF) built into a titanium implant to induce osteogenesis. Such a MEF is generated by the periodic organization of a junction made of two parallel semiconducting TiO2 zones: anatase and rutile with lower and higher electron densities, respectively. The junctions were formed through anatase–rutile-phase transition in selective areas using laser irradiation on the implants. The in vitro and in vivo studies confirmed that the built-in MEF was an efficient electrical cue for inducing osteogenic differentiation in the absence of osteogenic supplements and promoted bone regeneration around the implants. Our work opens up a new avenue toward bone repair and regeneration using built-in MEF. Titanium implants with built-in electric fields that assist bone regeneration have been made by a team of researchers in China and the USA. The polymer collagen makes up about 90% of the organic matrix of bones. Mechanical stresses generated during movement cause collagen to generate electric fields, which promote bone formation. In the past, externally applied fields have generally been used to mimic this effect, but now Chuanbin Mao of the University of Oklahoma and Zhejiang University and co-workers have made implants with built-in electric fields. They used selective laser irradiation to produce alternating strips of two phases of titanium oxide, which have low and high electron densities; this variation in the electron density gives rise to internal electric fields. The researchers found that bone regeneration was promoted near the implants. Two parallel semiconducting TiO2 zones, anatase and rutile, are made to have lower and higher electron densities, respectively, and form periodically organized junctions through anatase–rutile-phase transition in selective areas by laser irradiation on the titanium bone implants. The periodic junctions result in the formation of a periodic microscale electric field (MEF), built-in on the implants, which in turn efficiently induces and promotes osteogenesis.