Polymer-Based Microparticles in Tissue Engineering and Regenerative Medicine Mariana B. Oliveira and Joa ˜o F. Mano 3Bs Research Group-Biomaterials, Biodegradables and Biomimetics, University of Minho, AvePark, Zona Industrial da Gandra, S. Cla ´udio do Barco, Caldas das Taipas, Guimara ˜es 4806-909, Portugal Institute for Biotechnology and Bioengineering (IBB), PT Government Associated Laboratory, Guimara ˜es, Portugal DOI 10.1002/btpr.618 Published online in Wiley Online Library (wileyonlinelibrary.com). Different types of biomaterials, processed into different shapes, have been proposed as temporary support for cells in tissue engineering (TE) strategies. The manufacturing methods used in the production of particles in drug delivery strategies have been adapted for the de- velopment of microparticles in the fields of TE and regenerative medicine (RM). Micropar- ticles have been applied as building blocks and matrices for the delivery of soluble factors, aiming for the construction of TE scaffolds, either by fusion giving rise to porous scaffolds or as injectable systems for in situ scaffold formation, avoiding complicated surgery proce- dures. More recently, organ printing strategies have been developed by the fusion of hydro- gel particles with encapsulated cells, aiming the production of organs in in vitro conditions. Mesoscale self-assembly of hydrogel microblocks and the use of leachable particles in three- dimensional (3D) layer-by-layer (LbL) techniques have been suggested as well in recent works. Along with innovative applications, new perspectives are open for the use of these versatile structures, and different directions can still be followed to use all the potential that such systems can bring. This review focuses on polymeric microparticle processing techni- ques and overviews several examples and general concepts related to the use of these sys- tems in TE and RE applications. The use of materials in the development of microparticles from research to clinical applications is also discussed. V V C 2011 American Institute of Chemical Engineers Biotechnol. Prog., 000: 000–000, 2011 Keywords: microparticles, tissue engineering, biomaterials Introduction Tissue engineering (TE) is a field that applies the princi- ples of biology and engineering to the development of func- tional substitutes for damaged tissue. 1 Many of the currently proposed TE strategies are based on the use of hydrogels and porous scaffolds. 2 Advances in the field of TE and re- generative medicine (TE&RM) were possible through the de- velopment of alternative systems, which can conjugate the advantages and simultaneously the elimination of drawbacks of both kinds of systems. In this context, particles have been suggested as injectable or moldable systems in which cells can adhere and proliferate in a solid substrate. These systems offer the possibility of injecting the isolated particles into the defect. Moreover, besides the polymeric particles alone as injectable systems, encapsulated bioactive agents or in vitro preseeded cells can be delivered in the defect using particles as vehicles. The use of particles can be discussed in terms of the dimension of the objects (Figure 1). In a nanoscale perspec- tive, particles for modeling cell behavior by gene delivery have been used in cell therapy with special emphasis in the treatment of cancer and immune system diseases. 3–5 In TE&RM strategies, nanoparticles could be used to deliver bioactive substances either to the cell surrounding medium or directly into the interior of the cells by internalization. The release of proteins [including growth factors (GFs)] or low molecular weight differentiation agents can target and control the behavior and the fate of the cells, as schemati- cally represented in Figure 1A. 6–8 Despite the relevance of nanoparticles in this field, this review will focus mainly on the use of polymeric microparticles. The use of microparticles in TE&RM may have different purposes, which include (i) the incorporation of micropar- ticles in hydrogels or porous scaffolds (Figure 1B) aiming for the formation of pores, 9 (ii) the achievement of the com- plex delivery systems for macromolecules (e.g. dual release profile systems), 10 or (iii) the incorporation of osteoconduc- tive materials in the system. 11 The injection of microparticles loaded with bioactive molecules aiming for controlled deliv- ery (Figure 1C) has also been performed, relying on diffu- sion, 12 polymer degradation or using responsive polymers properties to trigger the release of the molecules. 13 These particle diameters usually range from 1 lm to 10 lm. Regarding particles with sizes varying from 10 lm to 1,000 lm, scaffolds with interconnected porosity have been obtained by the sintering or chemical agglomeration of microparticles [Figure 1E(b)]. 14–16 The use of separate par- ticles offers high surface area for cell expansion [Figure 1E(c)]. 17,18 The in situ formation of scaffolds by cell-induced aggregation of injected microparticles has also been Correspondence concerning this article should be addressed to J. F. Mano at jmano@dep.uminho.pt. V V C 2011 American Institute of Chemical Engineers 1