© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com REVIEW Natural-Based Nanocomposites for Bone Tissue Engineering and Regenerative Medicine: A Review Sandra Pina,* Joaquim M. Oliveira, and Rui L. Reis DOI: 10.1002/adma.201403354 1. Introduction Tissue engineering and regenerative medicine (TERM) is a multidisciplinary field of research that employs principles of chemistry, biology, and engineering sciences towards growth, development, and regeneration of damaged tissues or organs. [1] It involves scaffolds combined with cells and suitable biochem- ical signals, which promote the design of new organs and tissues. Among all tissues in the body, bone is the most widely investigated for tissue engineering due to its high potential for regeneration. Bone graft materials, as autografts and allografts, Tissue engineering and regenerative medicine has been providing exciting technologies for the development of functional substitutes aimed to repair and regenerate damaged tissues and organs. Inspired by the hierarchical nature of bone, nanostructured biomaterials are gaining a singular attention for tissue engineering, owing their ability to promote cell adhesion and proliferation, and hence new bone growth, compared with conventional microsized materials. Of particular interest are nanocomposites involving biopolymeric matrices and bioactive nanosized fillers. Biodegradability, high mechanical strength, and osteointegration and formation of ligamentous tissue are properties required for such materials. Biopolymers are advantageous due to their similarities with extracellular matrices, specific degradation rates, and good biological perfor- mance. By its turn, calcium phosphates possess favorable osteoconductivity, resorbability, and biocompatibility. Herein, an overview on the available natural polymer/calcium phosphate nanocomposite materials, their design, and prop- erties is presented. Scaffolds, hydrogels, and fibers as biomimetic strategies for tissue engineering, and processing methodologies are described. The specific biological properties of the nanocomposites, as well as their interaction with cells, including the use of bioactive molecules, are highlighted. Nanocompos- ites in vivo studies using animal models are also reviewed and discussed. Dr. S. Pina, Dr. J. M. Oliveira, Prof. R. L. Reis 3B´s Research Group – Biomaterials Biodegradables and Biomimetics University of Minho Headquarters of the European Institute of Excellence on Tissue Engineering and Regenerative Medicine AvePark 4806–909, Caldas das Taipas, Guimarães, Portugal E-mail: sandra.pina@dep.uminho.pt Dr. S. Pina, Dr. J. M. Oliveira, Prof. R. L. Reis ICVS/3B’s – PT Government Associate Laboratory Braga/Guimarães, Portugal have been used to repair bone fractures and other defects, because of their oste- oinductive and osteoconductive character- istics. [2] Nevertheless, concern issues are associated with the risk of disease transfer, infection, chronic pain, possible immuno- genicity, deficient supply, and increase of operative time and cost. [3] Despite their scarcity, biomaterials that are able to mimic the structural, mechan- ical, and biological properties of natural tissues have been attracting a significant attention. Meaningful progress has been made in designing and processing new materials in order to properly address cell activity. This is an important issue to be considered as regeneration pro- cesses involve achieving the desired cell function, i.e., stimulate specific cellular responses and activate genes that stimu- late cells differentiation and extracellular matrix (ECM) production for enhancing the regeneration of the damaged tissues. Inspired by the nature of bone, three- dimensional, structurally hierarchical con- structs and nanocomposites that can com- prise several levels of organization, i.e., from the macroscopic tissue arrangement down to the molecular arrangement of pro- teins have been required. [4] These nanostructured materials can provide enhanced mechanical performance and allow suitable transduction of the mechanical stimuli to the cellular level. [5] Nanocomposites involving biodegradable and biopolymeric matrices and bioactive/resorbable nanofillers have been con- sidered as a strategy for tissue engineering and regeneration ( Figure 1). The fillers with nanosized features can intensely change the physical properties of the polymer matrix, allowing for the engineering of improved biomaterials that the individual materials cannot attain. The nanoparticles have a large surface area when compared to the conventional microsized fillers, which can form a tight interface with the polymeric matrices, offering improved mechanical properties, while maintaining the favourable osteoconductivity and biocompatibility of the fillers, thus influencing protein adsorption, cells adhesion, pro- liferation and differentiation for new tissue formation. [6] Biodegradable polymers from natural origin, like polysaccha- rides (e.g., cellulose, chitin, glycosaminoglycans) and proteins (e.g., collagen, silk, fibrinogen, elastin) hold significant simi- larities with the ECM, chemical versatility, and good biological performance without toxicity or immunological reactions. [7–9] On the other hand, bioresorbable fillers, such as calcium Adv. Mater. 2015, DOI: 10.1002/adma.201403354 www.advmat.de www.MaterialsViews.com