Review Hydrogels for protein delivery in tissue engineering Roberta Censi a, ⁎, Piera Di Martino a , Tina Vermonden b , Wim E. Hennink b a School of Pharmacy, University of Camerino, via S. Agostino 1, 62032, Camerino (MC), Italy b Department of Pharmaceutics, Utrecht Institute for Pharmaceutical Sciences,Utrecht University, PO Box 80082, 3508 TB Utrecht, The Netherlands abstract article info Article history: Received 13 January 2012 Accepted 2 March 2012 Available online 8 March 2012 Keywords: Hydrogels Growth factors Protein delivery Tissue engineering Controlled release Tissue defects caused by diseases or trauma present enormous challenges in regenerative medicine. Recently, a better understanding of the biological processes underlying tissue repair led to the establishment of new ap- proaches in tissue engineering which comprise the combination of biodegradable scaffolds and appropriate cells together with specific environmental cues, such as growth or adhesive factors. These factors (in fact pro- teins) have to be loaded and sustainably released from the scaffolds in time. This review provides an overview of the various hydrogel technologies that have been proposed to control the release of bioactive molecules of in- terest for tissue engineering applications. In particular, after a brief introduction on bioactive protein drugs that have remarkable relevance for tissue engineering, this review will discuss their release mechanisms from hydrogels, their encapsulation and immobiliza- tion methods and will overview the main classes of hydrogel forming biomaterials used in vitro and in vivo to release them. Finally, an outlook on future directions and a glimpse into the current clinical developments are provided. © 2012 Elsevier B.V. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 680 2. Proteins in the healing process: an introduction to their delivery approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 681 3. Protein release strategies from hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 681 3.1. Physical entrapment of growth factors into hydrogels: general principles and release mechanisms . . . . . . . . . . . . . . . . . . 681 3.1.1. Synthetic polymers for the physical encapsulation of growth factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683 3.1.2. Natural polymers for the physical encapsulation of growth factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683 3.2. Covalent binding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 688 3.3. Dual/multiple delivery systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 689 4. Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 689 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 690 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 690 1. Introduction Controlled drug delivery has seen rapid advances in the last few de- cades with the introduction of novel biomaterials and technologies that found application in all fields of pharmaceutical and biomedical sciences. Particularly, with the advent of protein therapeutics, the need for con- trolled delivery systems able to enhance protein's pharmacokinetic and pharmacodynamic properties became more urgent. Nowadays, proteins are used in the treatment of many diseases but also in the area of tissue engineering, where supply of biomolecular cues that mimic the environ- ment of natural tissues and promote the communication between cells proved crucial for achieving effective tissue repair or replacement. There- fore, modern tissue engineering aims at assisting the re-growth of func- tional tissues by combining cells and engineering materials with signaling biomolecules [1,2]. Biomolecular signals that are mainly growth factors or other cytokines, chemoattractants, adhesion proteins and many others, must be locally delivered in their active form and with a sustained release profile. Among the existing technologies, hydro- gels – water-swollen, cross-linked polymer networks – have emerged as particularly promising materials for tissue engineering, as they can act both as scaffolding materials and/or releasing matrices for biologically active and cell modulating substances. Their water content, soft nature and porous structure mimic biological tissues and make them suitable to accommodate cells and to encapsulate and release water-soluble com- pounds like proteins in a controlled fashion. Journal of Controlled Release 161 (2012) 680–692 ⁎ Corresponding author. Tel.: + 39 0737 402215; fax: + 39 0737 402457. E-mail address: roberta.censi@unicam.it (R. Censi). 0168-3659/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jconrel.2012.03.002 Contents lists available at SciVerse ScienceDirect Journal of Controlled Release journal homepage: www.elsevier.com/locate/jconrel