3D-printed biodegradable composite scaffolds with significantly enhanced mechanical properties via the combination of binder jetting and capillary rise infiltration process

https://doi.org/10.1016/j.addma.2021.101988Get rights and content

Abstract

For hard tissue engineering applications, biodegradable composite scaffolds have been extensively investigated because of their satisfactory mechanical properties and biocompatibility. Recently, 3D printing processes have received substantial attention in the tissue engineering field because of their ability to be customized for tissues that have suffered different types of loss or damage for each patient. However, previous studies on material extrusion-based techniques lack flexibility in the filler loading amount and cannot fulfill requirements that aim to enhance mechanical properties and biocompatibility. Herein, we propose a biodegradable polymer-based composite scaffolds with high ceramic loadings fabricated using the binder jetting (BJ) technique conjugated with capillary rise infiltration. A calcium sulfate hemihydrate (CSH) scaffold was fabricated using BJ-based 3D printing. Thereafter, CSH was transformed into biphasic calcium phosphate (BCP) using hydrothermal treatment, followed by heat treatment. Melted polycaprolactone (PCL) was infiltrated in the resulting BCP scaffold. BCP was then completely dispersed in the PCL matrix, and the calculated PCL loading in the BCP matrix exceeded 40 vol%. The PCL/BCP composite scaffold demonstrated the highest compressive strength, moduli, and toughness with the fracture mode shifted from brittle to less brittle. Moreover, a stable PCL/BCP surface promotes initial cell responses and shows sufficient proliferation and differentiation of pre-osteoblast cells.

Introduction

In tissue engineering aimed at replacing or restoring the lost or damaged tissues in the human body, 3D tissue engineering scaffolds are intended to act as cell growth platforms for cell repair and simultaneously act as a physical support for lost tissues [1], [2]. An ideal scaffold would transfer its role to the restored organization and degrade after completing its role of organizing recovery because there would be no requirement for secondary operation for scaffold removal [3]. Moreover, it should have an appropriate porous structure that can allow cell adherence, vascularization, and nutrient and metabolite diffusion [4], [5]. For hard tissue scaffolds that replace hard tissues, including bones and teeth, the scaffolds must possess mechanical and biological properties that are similar to those of native physiological tissues at the defect site [6].

Biodegradable, polymer-based biomaterial scaffolds are specifically used to regenerate bone tissues after traumas or tooth extractions [7], [8], [9]. This not only overcomes the stress concentration phenomenon, which is a disadvantage of the conventional bio-inert metals (such as titanium or cobalt chromium alloys, or ceramics such as brittle alumina and zirconia) but also benefits from polymer material characteristics, including biocompatibility and biodegradability [10], [11], [12]. Synthetic polymers with excellent physical properties are extensively used to fabricate polymer scaffolds. The typically used biodegradable polymers include polylactic acid (PLA), polyethylene glycol, and polycaprolactone (PCL) [13]. Among these, PCL, which is a biodegradable polymer, is extensively used because of its unique physical strength; thus, it is employed for hard tissue engineering in various domains [14], [15], [16]. One of the advantages of biodegradable polymer scaffolds is that they are easy to process. They are produced through various approaches, including solvent casting with particulate leaching, gas forming, phase separation, and emulsion freeze-drying; accordingly they can be applied in various fields. Recently, the 3D printing process, called additive manufacturing, has received considerable attention in the tissue engineering field because of its ability to design and fabricate complex structures with high reliability [17], [18], [19]. The 3D printing technique, combined with computer-aided design (CAD) systems, can be customized for tissues that have suffered different types of loss or damage for each patient [20], [21], [22], [23].

Generally, from an application viewpoint in hard tissue engineering, the mechanical properties of biodegradable polymer scaffolds are inferior to those of other materials such as ceramics and metals [24]. Moreover, the biocompatibility of biodegradable polymer scaffolds must be enhanced to broaden their applications for hard tissue engineering. Therefore, to overcome these limitations while retaining the advantages of biodegradable polymer scaffolds, polymer-based composite scaffolds have been considered as promising candidate materials with enhanced mechanical and biological properties [25], [26], [27], [28]. Among polymer-ceramic composite scaffolds, calcium-phosphate-based (CaP) ceramic materials, including hydroxyapatite (HA), tricalcium phosphate (TCP), and biphasic calcium phosphates (BCP) (which is a combination of HA and TCP), are most commonly used as fillers or reinforcements because of their excellent biocompatibility, osteoconductive characteristics, and satisfactory mechanical properties [29], [30], [31], [32], [33], [34]. Based on these advantages, PCL/CaP-based polymer-ceramic composite have been extensively studied [35]. According to several previous studies, when PCL/CaP composite scaffolds are developed, the desired cell properties and physical properties can be enhanced, and other polymers, such as PLA, collagen, starch, could be added to further improve the desired properties [36], [37], [38], [39]. However, to facilitate extrusion for performing 3D printing through such an extrusion process (such as fused filament fabrication (FFF)), the filler concentration in the polymer matrix was up to ~30–40 wt% owing to the printability; thus, the enhancement in the mechanical properties was limited [27], [40], [41]. In addition, there are several papers that have shown reinforcement of binder jetted BCP or HA scaffolds by coating them with PCL solutions [42], [43], [44]. With the viscous nature of PCL solutions it is difficult to fill the micropores inside the scaffolds which can limit the enhancement in mechanical properties.

Therefore, in this study, we propose a different approach for fabricating the PCL/CaP composite with significantly enhanced mechanical properties. PCL, a polymer material, was used both as a matrix and a filler, and CaP was used as the primary material for the structure. The mechanical integrity of the scaffold was enhanced using the polymer to fill the pores of the ceramic structure. By fabricating a composite material using the idea, we hypothesized the final product could meet the requirements that can enhance both mechanical and biological properties. To confirm our hypothesis, we designed the experiment with binder jetting incorporated with the capillary rise infiltration (CaRI) process as follows. First, a calcium sulfate hemihydrate (CSH) scaffold designed by CAD was fabricated using a BJ-based 3D printing. Through a hydrothermal treatment process, followed by heat treatment, CSH was transformed into BCP, which offered two advantages: high bioactivity of HA and biodegradability of TCP. Finally, using the CaRI process, the transformed BCP scaffold was filled with melted PCL. The structural morphologies and chemical structures of the PCL/BCP composite scaffolds were examined using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and micro-computed tomography (μ-CT). The surface topography and roughness of produced PCL/BCP composite scaffolds were evaluated using confocal laser microscopy (CLM). Moreover, the mechanical behaviors of scaffolds were evaluated using mechanical compression tests. Furthermore, the biocompatibility of as-prepared scaffolds was observed by in-vitro cell attachment, proliferation, and differentiation tests using pre-osteoblast cells.

Section snippets

Sample preparation

Commercially available CSH powder (zp150) and 2-pyrrolidone (zb63) supplied by 3D Systems Inc. (USA) were used as starting materials for 3D printing. The structures of the desired CSH scaffolds, i.e., with ~50% porosity and 1000 µm pore size, were modeled using SolidWorks 2013 (Solid Works Corporation, Santa Monica, CA, USA) and exported as an STL file to the printer. Modeling was then performed by layering a 12-mm3 of cube with alternating 1000-μm strut and 1000-μm spacing in each plane. For

Characterization of the 3D printed BCP/PCL scaffolds

Fig. 1 shows the FESEM images of surface morphology and shows the optical image of the overall shape of 3D-printed scaffolds. From the optical image of the inset, all specimens appear to have the same overall shape and there is no significant difference. The original scaffold printed by CSH had a well-controlled porous structure that matched the initial design, which had a pore size of ~1000 µm, generated in a layer-by-layer manner without any cracks or defects (Fig. 1(A)). After hydrothermal

Conclusion

In this study, novel PCL/BCP composite scaffolds with significantly enhanced mechanical properties in which a ceramic reinforcing strut was first formed, followed by polymer infiltration using the BJ-based 3D printing technique conjugated with the CaRI process. By sequential hydrothermal and heat treatments, the 3D-printed CSH scaffold was successfully transformed into the BCP scaffold. Melted PCL was infiltrated into the micropores (57% porosity) of the resulting BCP scaffold by capillary

CRediT authorship contribution statement

Ji-Ho Ahn: Conceptualization, Methodology, Validation, Investigation. Jinyoung Kim: Methodology, Validation, Investigation, Writing - original draft. Ginam Han: Validation, Investigation. DongEung Kim: Investigation. Kwang-Hee Cheon: Investigation. Hyun Lee: Validation. Hyoun-Ee Kim: Supervision, Project administration. Young-Jig Kim: Project administration. Tae-Sik Jang: Conceptualization, Methodology, Writing - review & editing. Hyun-Do Jung: Conceptualization, Methodology, Writing - original

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by The Catholic University of Korea, Research Fund, 2020 and the Basic Science Research Program [No. 2020R1F1A1072103] through the National Research Foundation of Korea funded by the Korea government (MSIT).

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