Contents lists available at ScienceDirect Materials Science & Engineering C journal homepage: www.elsevier.com/locate/msec Osteogenic, anti-osteoclastogenic and immunomodulatory properties of a strontium-releasing hybrid scaffold for bone repair Ana Henriques Lourenço a,b,c , Ana Luísa Torres a,b,d , Daniela P. Vasconcelos a,b,d , Cláudia Ribeiro-Machado a,b , Judite N. Barbosa a,b,d , Mário A. Barbosa a,b,d , Cristina C. Barrias a,b,d , Cristina C. Ribeiro a,b,e, ⁎ a i3S - Instituto de Investigação e Inovação em Saúde, Universidade do Porto, Rua Alfredo Allen, 208, 4200 - 135 Porto, Portugal b INEB - Instituto de Engenharia Biomédica, Universidade do Porto, Rua Alfredo Allen, 208, 4200 - 135 Porto, Portugal c Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, s/n, 4200-465 Porto, Portugal d ICBAS - Instituto de Ciências Biomédicas de Abel Salazar, Universidade do Porto, Rua de Jorge Viterbo Ferreira n. 228, 4050-313 Porto, Portugal e ISEP – Instituto Superior de Engenharia do Porto, Instituto Politécnico do Porto, Rua Dr. António Bernardino de Almeida 431, 4249-015, Porto, Portugal ARTICLE INFO Keywords: Strontium Hydroxyapatite Alginate Osteoclasts Mesenchymal stem/stromal cells Immune response modulation ABSTRACT Strontium (Sr) is known to stimulate osteogenesis, while inhibiting osteoclastogenesis, thus encouraging re- search on its application as a therapeutic agent for bone repair/regeneration. It has been suggested that it may possess immunomodulatory properties, which might act synergistically in bone repair/regeneration processes. To further explore this hypothesis we have designed a Sr-hybrid system composed of an in situ forming Sr- crosslinked RGD-alginate hydrogel reinforced with Sr-doped hydroxyapatite (HAp) microspheres and studied its in vitro osteoinductive behaviour and in vivo inflammatory response. The Sr-hybrid scaffold acts as a dual Sr 2+ delivery system, showing a cumulative Sr 2+ release of ca. 0.3 mM after 15 days. In vitro studies using Sr 2+ concentrations within this range (0 to 3 mM Sr 2+ ) confirmed its ability to induce osteogenic differentiation of mesenchymal stem/stromal cells (MSC), as well as to reduce osteoclastogenesis and osteoclasts (OC) func- tionality. In comparison with a similar Sr-free system, the Sr-hybrid system stimulated osteogenic differentiation of MSC, while inhibiting the formation of OC. Implantation in an in vivo model of inflammation, revealed an increase in F4/80 + /CD206 + cells, highlighting its ability to modulate the inflammatory response as a pro- resolution mediator, through M2 macrophage polarization. Therefore, the Sr-hybrid system is potentially an appealing biomaterial for future clinical applications. 1. Introduction Bone is a complex and highly dynamic tissue, and the maintenance of its mass is ensured by a proper balance between bone resorption and bone formation. The deregulation of this equilibrium may lead to severe pathological conditions, such as osteoporosis, cancer and Paget's dis- ease, as well as inflammatory disorders like rheumatoid arthritis and periodontal disease [1,2]. These pathological situations are frequently associated with multiple morbidities, especially in an increasingly older population, and often lead to non-healing fractures. Autologous bone grafts remain the gold standard material for the treatment of this kind of fractures, though they present critical drawbacks, including low tissue availability and high morbidity of the secondary harvest place. Thus, the development of synthetic bone grafts that support and sti- mulate bone repair and regeneration is a major need in clinics [3]. Among those, injectable bone substitutes are attractive options, as they can be implanted through minimally invasive surgery and can easily fit into irregular bone defects, providing local support for bone repair [4]. Synthetic hydroxyapatite (HAp) has been widely used as bone-like ceramic owing to its biocompatibility and resemblance to the mineral phase of natural bone [5]. Injectable systems are often composed of ceramic particles embedded within hydrogels, somehow mimicking the composite nature of bone tissue [6]. Ceramic materials also provide strength and improve the mechanical properties of the system, as compared to hydrogels alone [7]. In turn, the hydrogel phase provides a hydrated three-dimensional (3D) environment, that may allow the en- trapment of bioactive factors or even cells, and may also support host cell colonization and new tissue ingrowth [8]. Gel-precursor solutions can act as vehicles for the ceramic particles, facilitating their injection, and preferably reticulating in situ. https://doi.org/10.1016/j.msec.2019.02.053 Received 14 June 2018; Received in revised form 4 February 2019; Accepted 15 February 2019 ⁎ Corresponding author at: i3S – Instituto de Investigação e Inovação em Saúde, Universidade do Porto, Rua Alfredo Allen, 208, 4200 – 135 Porto, Portugal. E-mail address: cribeiro@ineb.up.pt (C.C. Ribeiro). Materials Science & Engineering C 99 (2019) 1289–1303 Available online 16 February 2019 0928-4931/ © 2019 Published by Elsevier B.V. T