Porous scaffolds with tailored reactivity modulate in-vitro osteoblast responses Guocheng Wang a , Zufu Lu a , Dennis Dwarte b , Hala Zreiqat a, ⁎ a Biomaterials and Tissue Engineering Research Unit, School of AMME, The University of Sydney, Australia b Australian Centre for Microscopy & Microanalysis, The University of Sydney, Australia abstract article info Article history: Received 22 July 2011 Received in revised form 24 February 2012 Accepted 28 April 2012 Available online 5 May 2012 Keywords: Calcium silicate Hardystonite Primary human osteoblasts Zinc Reactivity CaSiO 3 (CS) ceramic has been extensively studied for biomedical applications. The main advantages are its ability to induce bone-like apatite formation and the beneficial effects of the dissolution products on the bone cells, resulting from high reactivity of CS in liquid solutions. However, the high reactivity also results in a rapid degra- dation rate and accordingly leads to a high pH value in the body fluid, adversely affecting bone cell responses, especially when CS is used as a highly porous scaffold. In this study, we provide an approach to minimize this pH-dependent cell damage and maximize the beneficial effects of the dissolution products of the CS scaffold by adding chemically stable and biocompatible Zn-containing hardystonite (Ca 2 ZnSi 2 O 7 , HT) into the CS scaffold, the resultant composite scaffold is referred to as HT–CS. We investigated the responses of primary human oste- oblasts (HOBs) to the CS, HT and the HT–CS scaffolds. HOBs on HT and HT–CS scaffolds attached better than on the CS scaffold. HOBs cultured on the HT–CS scaffolds expressed higher gene expression levels for Runx-2, osteopontin (OPN), osteocalcin (OCN), bone sialoprotein (BSP), and collagen type I (Col-I) and enhanced alkaline phosphatase (ALP) activity compared to those on the CS and HT scaffolds. The higher activity of the HOBs cultured on the HT–CS scaffold was ascribed to the moderate pH variation and the dissolution products containing Ca, Si and Zn. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Choosing an appropriate chemical composition is of great impor- tance in designing scaffolds for bone tissue engineering. The significant effects of calcium (Ca) on bone formation are well-known [1]. Trace el- ements, such as silicon (Si) and zinc (Zn), also play important roles in bone remodeling and bone regeneration. Si is an essential trace element for metabolic processes associated with the development of bone and connective tissues [2]. It plays important roles during the early stage of bone formation and the calcified process [2,3], by increasing the mRNA expression of osteoblastic genes, such as type I collagen [4,5]. Zn is another important essential trace element in the human body with significant effects on bone formation [6]. At the cellular level, Zn plays a significant role in enhancing osteoblast proliferation [7], increas- ing the alkaline phosphatase activity and DNA content in bone tissues [8,9], as well as selectively inhibiting osteoclast functions [10]. Inspired by the beneficial effects of these elements, attempts have been made to incorporate these elements into scaffolds for bone tis- sue regeneration. Both Si substituted-hydroxyapatite (Si-HAp) and α-tricalcium phosphate (Si-α-TCP) exhibited enhanced bone appo- sition, bone in-growth and cell-mediated degradation compared to stoichiometric HAp controls [11–17]. Encouraging results have also been obtained with Zn modified bioglass [18], glass-ceramics [19,20], calcium phosphate [21–23] and calcium sulfate [24]. Collectively, these results suggest that chemical modification with these trace el- ements has the potential to improve the quality of the current existing biomaterials. In recent years, calcium silicate ceramics (CaSiO 3 , referred to as CS in this study) have been widely studied as a potential biomaterial for bone tissue engineering due to their ability to induce bone-like apatite formation in simulated body fluid (SBF) and the beneficial effects of their dissolution effects on osteogenesis [25–27]. The apatite formation ability of CaSiO 3 ceramics is largely due to their high reactivity which can cause preferential release of Ca ions from the ceramics and an in- crease in the pH value of the SBF solution. Studies into the effect of dis- solution products of Ca–Si based ceramics or coatings showed that Ca and Si ions support osteoblast adhesion and enhance cell proliferation and differentiation in cell culture medium with physiological pH level [28,29]. However, when used as tissue engineering scaffolds, the CS degrades at a much higher rate as the specific surface area of a porous scaffold is significantly higher than ceramic coatings or disks. This high degradation rate is likely to cause the collapse of the scaffold's structure prior to the formation of sufficient bone extracellular matrix. Additionally, the big deviation in pH values from the physiological level caused by excessive dissolution products of CS scaffolds may cause damage to surrounding cells [30,31]. Hardystonite (Ca 2 ZnSi 2 O 7 , HT) is a more chemically stable material, which can be synthesized by the addition of Zn into CS. Its ability to re- lease a certain amount of Zn ions is supposed to contribute to the good Materials Science and Engineering C 32 (2012) 1818–1826 ⁎ Corresponding author at: Biomaterials and Tissue Engineering Research Unit, School of AMME, The University of Sydney, Sydney 2006, Australia. Tel.: +61 2 93512392; fax: + 61 2 93517060. E-mail address: hala.zreiqat@sydney.edu.au (H. Zreiqat). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2012.04.068 Contents lists available at SciVerse ScienceDirect Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec