Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Short communication Novel strategy for toughening robocast bioceramic scaffolds using polymeric cores Claudia Paredes, Francisco J. Martínez-Vázquez * , Antonia Pajares, Pedro Miranda Department of Mechanical, Energy and Materials Engineering, University of Extremadura, Avda. de Elvas s/n., 06006, Badajoz, Spain ARTICLE INFO Keywords: Composite scaffolds Ceramic Polymer Mechanical properties Robocasting ABSTRACT A novel method for the fabrication of hybrid polymer/ceramic porous scaffolds with core/shell struts is de- veloped. Robocasting with coaxial needles is used to deposit beta-tricalcium phosphate scaffolds with hollow rods, which are subsequently filled with a polycaprolactone melt by suction. The polymeric core provides outstanding improvement of the toughness of the structure in bending, with strain energy density increasing nearly two orders of magnitude and continuous polymeric fibres holding the structure together even after large deflections. Moreover, flexural strength is not significantly reduced compared to dense-strut structures; and the macroporosity of the scaffold and osteoconductivity of bioceramic surfaces are preserved. Biodegradable ceramic scaffolds are an attractive solution to re- generate damaged bone tissue due to their ability to guide, or even to induce the activation of, the cellular mechanisms involved in natural bone remodelling [1]. Such bioactive materials overcome the typical drawbacks associated to natural tissue grafts or bioinert implants [2]. They provide structural support and promote bone growth into their interconnected porous matrix while avoiding secondary surgical sites, immunogenic response or disease transmission risks. Among the bio- ceramic materials eligible for scaffold applications are calcium phos- phates [3,4], calcium sulphate [5,6], bioglasses [7,8] and glass-cera- mics [9,10]. Despite their adequate biological performance, all these materials are intrinsically very brittle, and their strength is typically low, especially when they are fabricated as highly porous scaffolds to ensure a proper interconnectivity for vascularization. The development of additive manufacturing (AM) techniques such as robocasting has contributed to a greater control over scaffolds geo- metrical features, including pore architecture (size, shape, distribution …). This has enabled a reduction in the total porosity needed to achieve the required pore interconnectivity and, thus, improved scaffolds’ load bearing capacity [11,12]. Nonetheless, strength and, especially tough- ness of bare bioceramic robocast scaffolds are still far from matching the mechanical performance of natural bone (Fig. 1). However, sig- nificant improvements are achieved on both properties by combining the inorganic scaffold with a biodegradable polymeric phase [13–17], either by full impregnation of the structure or by coating its struts. Strength and toughness are, thus, enhanced by mechanisms of defect healing and crack bridging [18,19]. Furthermore, when the im- pregnated polymer occupying the macropores is stiff enough, an addi- tional increase of strength and, hence, toughness is achieved through stress-shielding: stresses in the ceramic matrix are diminished as the polymer supports some of the load. The crack bridging mechanism is also greatly enhanced in fully impregnated structures since the poly- meric fibres occupying the macropores greatly hinder crack propaga- tion. They can, indeed, hold the structure together long after the ceramic skeleton fails, even at large deformations. This drastically en- hances the fracture energy of the hybrid material, which can even surpass that of cortical bone in compression for some cases (Fig. 1a) [8]. Performances are a little bit poorer in bending (Fig. 1b), but fully im- pregnated structures come substantially closer to natural bone values even under this most deleterious loading mode [20–26]. Nevertheless, full impregnation of ceramic scaffolds with polymers (Fig. 2a) sacrifices scaffold macroporosity and prevents, at least in- itially, colonization of the structure by new tissue. Polymeric coatings (Fig. 2b) preserve the predesigned macroporosity but still limit the biological performance of the scaffolds by insulating the bioactive ceramic surface from the physiological medium. The polymeric barrier delays the release of ions (i.e. Ca 2+ , PO 4 3- ) that are crucial for os- teoconduction and osteoinduction [27]. Moreover, degradation me- chanisms can also be hampered, thus reducing resorption rates [28]. Accordingly, hereby we propose a novel strategy to keep the bioceramic surface exposed to physiological medium by moving the tough poly- meric phase to the interior of the robocast scaffold struts, as depicted in https://doi.org/10.1016/j.ceramint.2019.06.175 Received 13 May 2019; Received in revised form 7 June 2019; Accepted 11 June 2019 * Corresponding author. E-mail addresses: clparedes89@unex.es (C. Paredes), fjmartinezv@unex.es (F.J. Martínez-Vázquez), apajares@unex.es (A. Pajares), pmiranda@unex.es (P. Miranda). Ceramics International xxx (xxxx) xxx–xxx 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Please cite this article as: Claudia Paredes, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.06.175