Synthetic biodegradable scaffolds are today¿s most promising candidates for bone substitution and regeneration[1]. The surgery involved in implanting synthetic bone substitutes is less invasive than for auto-grafts (which require two surgical sites), and do not have the problem of available quantities. They are also free of the rejection and disease transmission risks associated with allo- and xeno-grafts. Biodegradable scaffolds provide structural support for cell growth during regeneration of the tissue, and they are eventually resorbed, leaving only the newly-formed living tissue and the lesion fully healed. Ideal scaffolds are porous and made from an osteophilic material, so that new bone tissue can be induced to grow into the pores. The porosity must be interconnected[2] to allow cells¿ ingrowth, as well as vascularization and diffusion of nutrients. Most of the limitations associated with conventional scaffold fabrication techniques (solvent casting, fiber meshing, gas foaming, etc.) are related to a limited control over the pore structure. Fortunately, solid freeform fabrication (SFF) techniques¿stereolithography, 3D-printing, fused deposition modeling, robocasting, etc.¿can overcome these hurdles[3,4] since they build structures layer-by-layer with customized and complex 3D shapes following a computer- aided design (CAD) model. They can therefore produce the optimal pore architecture to attain the desired mechanical and diffusion properties for a given application. Moreover the CAD model can be obtained from medical scan data (computerized tomography, nuclear magnetic resonance imaging, etc.), allowing the scaffold¿s external shape to match the damaged tissue site.Currently, biodegradable scaffolds are processed either from ceramics (calcium phosphates or bioglasses) or from polymers (polylactic and polyglycolic acids, ¿-polycaprolactone, polydioxanone, etc.). Ceramic scaffolds show a greater potential for bone tissue engineering applications because of their ability to bond directly to bone tissue and their higher elastic moduli[5,6], which potentially make them more suitable to replace bone structural function in load-bearing regions of the skeleton. Among the SFF methods capable of building ceramic scaffolds, robocasting (also known as direct-write assembly, direct-ink write or micro-robotic deposition) is unique in that it uses water-based inks with minimal organic content (< 1 wt%) and requires no sacrificial support material or mold[7-9]. This technique consists of the robotic deposition of highly concentrated colloidal suspensions (inks) capable of fully supporting their own weight during assembly. Thus, a 3D structure is printed directly as a network of ink rods extruded through the deposition nozzle. With the recent development of robocasting inks made from ß-tricalcium phosphate[10] (ß-TCP) and hydroxyapatite[11-13] (HA) powders, this technique has allowed customized calcium phosphate scaffolds to be built for bone regeneration.However, despite the improvement in pore architecture achieved by robocasting, the main limitation of these ceramic scaffolds still lies in their intrinsic brittleness and the poor mechanical resistance associated to their porosity[14]. A possible approach to improve mechanical performance would be to develop a composite material by infiltration of a biodegradable polymer into the ceramic structures. The addition of a polymeric phase to a ceramic scaffold has been shown to enhance toughness[15,16] and strength[17,18] in non-SFF scaffolds (i.e., with limited control on pore architecture). Although the toughness enhancement has been attributed to crack bridging by polymeric fibrils[15], which significantly increase the fracture energy, the actual role of the polymer material in the strengthening of the scaffolds remains unclear. Some workers have tentatively suggested that reduction in scaffold porosity[17] would seem to be the most likely explanation for this effect, but probably because of the intractability of the geometries of their infiltrated scaffolds, they did not elaborate any further. The present work seek to shed light on this question by analyzing the microstructure and mechanical properties (compressive and flexural strength, toughness and reliability) of ß TCP robocast scaffolds completely impregnated with two different biodegradable polymers¿polylactic acid (PLA) and ¿ polycaprolactone (PCL)¿ by means of polymer-melt infiltration. The controlled geometry of the robocast scaffolds allows one to explore the effect of polymer infiltration on the stress field within the ceramic structure, by performing finite element modeling (FEM) simulations of the mechanical tests. Fully impregnating the structures fixes the variables porosity and amount of polymer deposited, thereby simplifying the analysis of the results. Furthermore, as will be discussed, fully-impregnated composite scaffolds might be of interest by themselves, since they could have superior mechanical properties than porous structures. An additional advantage of incorporating a polymeric material would be the possibility to use it as a biomolecule carrier (growth factors, antibiotics, anti-inflammatory drugs, etc.) for controlled delivery. However, the incorporation of such biomolecules does not allow for the extreme temperatures required for melting the polymers, thus, a less thermally aggressive process for infiltration of these biodegradable polymers into the porous ceramic scaffolds is developed and examined in this work. This method consists of the in situ polymerization of the corresponding monomers or dimers ¿ ¿ caprolactone (¿-CL) and L-lactide (LLA) are used here ¿ within the ceramic structure. Fully impregnated TCP structures fabricated by this method are analyzed and compared to those fabricated by the first process. Nonetheless, at this point, it is worth indicating that the processing conditions and the polymers used in this study are selected with a focus on optimizing the mechanical performance of the composites, and not so much at optimizing the biological performance. In particular, the selection of two biodegradable polymers with very different mechanical properties provides information about how those individual properties affect the performance of the composite scaffold.Since the preservation, at least partially, of the initial porosity is desirable for facilitating bone tissue ingrowth, a process based on in situ polymerization for coating of robocast scaffolds with PCL is developed. The mechanical performance of the resulting coated structures is evaluated under compressive loads and compared with the corresponding fully-impregnated structures. The mechanical performance of all the structures developed in this study is compared to results from the literature and with natural bone properties. The results of this work provide valuable insight into the mechanical behavior of hybrid robocast scaffolds for bone tissue engineering applications, and pave the way for future work aimed at optimizing the mechanical performance of such structures.
