The use of micro- and nanospheres as functional components for bone tissue regeneration

During the last decade, the use of micro- and nanospheres as functional components for bone tissue regeneration has drawn increasing interest. Scaffolds comprising micro- and nanospheres display several advantages compared with traditional monolithic scaffolds that are related to (i) an improved control over sustained delivery of therapeutic agents, signaling biomolecules and even pluripotent stem cells, (ii) the introduction of spheres as stimulus-sensitive delivery vehicles for triggered release, (iii) the use of spheres to introduce porosity and/or improve the mechanical properties of bulk scaffolds by acting as porogen or reinforcement phase, (iv) the use of spheres as compartmentalized microreactors for dedicated biochemical processes, (v) the use of spheres as cell delivery vehicle, and, finally, (vi) the possibility of preparing injectable and/or moldable formulations to be applied by using minimally invasive surgery. This article focuses on recent developments with regard to the use of micro- and nanospheres for bone regeneration by categorizing micro-/nanospheres by material class (polymers, ceramics, and composites) as well as summarizing the main strategies that employ these spheres to improve the functionality of scaffolds for bone tissue engineering.