Geometrical analysis of extrusion based (Additively Manufactured) 3D designed scaffold for bone tissue Engineering: A finite element approach

Abstract

The gap between demand and supply for the required bone graft and donor is widening day by day. Tissue engineering approach is used to overcome the limitations of bone graft substitutes. In Tissue Engineering, a biocompatible scaffold is placed inside the body, which over a period of time gets converted into the bone Extra-Cellular Matrix (ECM). Hydroxyapatite (HA) and β-Tricalcium Phosphate (β-TCP) are extensively studied to fabricate bone scaffolds for tissue engineering applications by conventional methods. However, the scaffold needs to be produced with controlled pore size, porosity, and pore interconnectivity for homing of the cells which can be achieved by additive manufacturing. Also, the scaffolds produced with conventional methods lack mechanical properties which limit the use of hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) at the recipient site. In this paper, scaffolds with different materials' compositions, different layers orientations, and pore sizes are designed as an input parameter for different scaffold architecture which has not been previously studied considering extrusion-based additive manufacturing. These parameters are considered to predict the mechanical properties of the scaffold architecture. It has been found that in all combinations the scaffold with 0°- 90°- 0°- 90° orientation layer gives Young’s modulus that is comparable to natural human bone. However, the scaffolds with 0°- 90° -0°- 90° orientation layer and 350 µm pore size gives comparatively higher effective Young’s modulus of about 30.948 GPa for 5% HA composition. In the future 0°- 90°- 0°- 90° orientation can be considered to fabricate 3D scaffold architecture using extrusion-based additive manufacturing method.

Introduction

Bones are the important part of the body that supports and maintain the body structure. Many times, bones are not able to function to their complete functional ability owing to diseases like trauma, osteoporosis, or injuries caused by an accident, etc. [1]. There are traditional techniques in which bones can be repaired by taking the bone graft from the same or different species. These are autograft, allograft, and xenograft. In an allograft, the bone graft is taken from the same body as the gold standard but it has limitations of size and donor site morbidity which includes possibilities of infection. An allograft is a technique where the bone graft is taken from the donor of the same species and in xenograft, the bone graft is taken from the other species but there is the possibility of disease transmission and immune rejection is reported in both these cases [2]. Tissue engineering is the new approach where the principles of life sciences and engineering are applied altogether to improve or repair tissue function. This approach is based on the scaffold which acts as the structural support and guiding template for the cells to generate bone extracellular matrix [3]. This scaffold is made of biomaterial having characteristics such as biocompatible, biodegradable, and bioactive. And the characteristics related to the scaffold structure are architectural design and mechanical properties which should match with the recipient [4].

The three basic categories of the material are metals, polymers, and ceramic. The metallic scaffolds have good biocompatibility, properties have good strength, and are useful in slow bone growth but they exhibit low bioactivity and poor biodegradation which may lead to secondary surgery as well as the release of metal ions that may cause toxicity. In addition to this, the superior modulus may cause stress shielding. The polymeric scaffolds are highly biocompatible, biodegradable, have low toxicity but usually, they are not suitable in load-bearing applications, show a lack of cell adherence, and lack bioactivity [5]. In addition to this, in natural polymers, “endotoxin-like impurities” may be present and synthetic polymers lack cell recognition sites. However, the ceramics have biocompatible, biodegradable, and bioactive properties, also the composition is similar to the mineral content of the recipient. But when ceramics are used alone they are hard and brittle. The faster degradation or resorption rate decreases the mechanical properties [6]. Because of these excellent properties ceramics are widely suited for bone tissue engineering either alone or in composites [7]. Also, ceramics have comparable chemical and mechanical properties to natural bone. In natural bone, hydroxyapatite is present up to 65% and the remaining structure comprises bone minerals containing collagen type-I, water, etc [8]. Recent studies show that the particle reinforcement of HA in polymer matrix improves the mechanical properties in polylactic acid (PLA) [9], [10] and coating of HA also improves the cell adherence of titanium implant for orthopaedic applications [11].

Different materials based on ceramics are available to generate the bone scaffolds using additive manufacturing techniques. In ceramics, the materials which are extensively studied for bone tissue engineering are hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) individually as well as in composites [12]. Hydroxyapatite is having the same mineral components of the bone and teeth with good physic-mechanical properties [13]. While β-TCP is well suited for bone replacement material and easy to bind with the recipient, though it has weak mechanical properties and a higher resorption rate which limits its use [14]. R. De Oliveira Lomelino et al. [15] evaluated biological behavior of granules made of 30% HA and 70% β-TCP. They found that the granules are cytocompatible and compatible with human bone cells and show a xenogenic model. Hassna R.R. Ramay et al. [16] studied gel casting method of scaffold fabrication with different compositions of HA (1%-5%) and remaining β-TCP (99%-95%). The composite with 5% HA and 95% β-TCP show good compression properties compared to other scaffolds.

S. Yamada et al. [17] compared two different scaffolds with 25% HA + 75% β-TCP and 75% HA + 25% β-TCP using rabbit bone cells to study osteoclast resorption. They concluded that in the study of 25% HA + 75% β-TCP composition, The said material is resorbed by the osteoclasts cells whereas, 75% HA + 25% β-TCP was not resorbed by osteoclasts. Also, extrusion-based manufacturing is successfully used to fabricate scaffolds of HA, β-TCP, and HA/β-TCP composites by different researchers. P. Miranda et al. [18] fabricated scaffolds of HA and β-TCP by robotics extrusion method (robocasting) and found that after immersion of the scaffolds into the simulated body fluid (SBF) the strength of HA increases whereas β-TCP decreases. Y Wang et al. [19] studied scaffolds printability with 40% HA + 60% β-TCP using phosphoric acid as the binder. The biphasic calcium phosphate, which is the composite of hydroxyapatite and β-tri-calcium phosphate supports good cell adhesion, differentiation, proliferation, and finally converts scaffold to the bone extracellular matrix [20]. From the above combination, it is evident that HA and β-TCP make a good choice of material for bone tissue engineering. Because of this many researchers have focused on the use of this composite. But the main difficulty experience with this material composition is difficult to control architecture by conventional fabrication. In addition to this, the mechanical properties of the scaffold are dependent on the architecture and through the architecture, the pore diameter, pore interconnectivity, and porosity of the structure are maintained. This architectural property is important in maintaining the health of the cells by supplying oxygen, blood, nutrients and in eliminating wastes. Also, the scaffold architecture allows the cells to attach, adhere, migrate and proliferate on the surface [21], [22], [23].

In conventional scaffold fabrication technique controlling pore size, porosity and achieving 100% interconnectivity among pores is a challenging task. And, in the contrary additive manufacturing allows the scaffold fabrication with controlled pore size, porosity, interconnected pores as well as a custom-specific design [24], [25]. In this AM technique researchers have studied structural analysis of Fused deposition modeling (FDM) [26], Stereolithography (SLA) [27], Selective laser sintering (SLS) [28], [29], [30], [31], [32], [33], [34] based structures, however, extrusion-based additive manufacturing is not attempted much. The fabrication of the ceramic scaffolds with controlled pore size, pore interconnectivity, and porosity is challenging in itself. The ceramic material is converted into the paste using the suitable binder and then it can be loaded into the syringe. Extrusion-based additive manufacturing offers a wide variety of materials that can be fabricated with only one condition that is material should be extruded through the nozzle [35]. To predict the optimal materials’ composition, pore size, layer orientations for optimal scaffold architecture in extrusion based additive manufacturing process is challenging and this paper presents FEA approach to solve this predicament.

In this paper, the biphasic calcium phosphate consisting of hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) in different compositions ratios has been used. Also, different architecture has been designed using Catia V5 software considering extrusion-based additive manufacturing method for bone tissue engineering. The scaffold is designed with varying layer orientation and pore size in the CAD model, and a different material composition ratio has been studied to evaluate mechanical properties and presented using a finite elemental approach.