Magnetron sputtering hydroxyapatite/PLA gyroid scaffolds for biomedical applications

Polylactic acid (PLA) as a popular polymeric biomaterial has been widely used in various biomedical applications, ranging from tissue engineering to drug-loaded systems due to its biodegradability, biocompatibility, mechanical strength, and processability. As a promising 3 dimensional (3D) printing method for developing scaffolds with complex geometries, fused deposition modelling (FDM) has become increasingly popular, in which the process parameters can be completely controlled. Porous PLA scaffolds manufactured by 3D printing possess higher accuracy in structural parameters such as porosity, pore size, pore shape, and pore arrangement compared to the scaffolds produced by conventional manufacturing methods. 3D printed PLA scaffolds have high potential to provide an appropriate environment for cell migration, differentiation, and growth.

In this study, dense PLA and porous PLA gyroid scaffolds with three different unit cell sizes of 2 mm, 2.5 mm, and 3 mm (denoted G2, G25, and G3, respectively) were manufactured via FDM. The microstructure, mechanical properties, failure mode of these scaffolds with different sample orientations are investigated. The porosity of G2, G25, and G3 measured by X-ray microtomography was 86.1±1.4%, 89.4±1.0%, and 90.3±0.4%, respectively, and these values are close to those measured by Archimedes¿ principle, which was 85.3±4.2%, 87.6±1.0%, and 89.1±1.3%, respectively. The PLA scaffolds exhibited structural anisotropy of 3.80, 2.00, and 1.04 for G2, G25, and G3, respectively. The compressive yield strength for G2, G25, and G3 was 6.1±1.2 MPa, 3.5±0.9 MPa, and 3.2±0.5 MPa in the building direction and 4.6±1.0 MPa, 3.0±0.8 MPa, and 2.7±0.6 MPa in the transverse direction, respectively. The compressive elastic modulus for G2, G25, and G3 was 176.7±3.16 MPa, 130.2±2.8 MPa, and 120.7±2.5 MPa in the building direction and 134.8±2.2 MPa, 108.9±2.1 MPa, and 108.0±1.8 MPa in the transverse direction, respectively. The tensile yield strength for G2, G25, and G3 was 3.4±1.2 MPa, 2.6±0.7 MPa, and 2.1±0.9 MPa in the building direction and 3.2±0.9 MPa, 2.1±1.1 MPa, and 2.1±0.5 MPa in the transverse direction, respectively. The tensile elastic modulus for G2, G25, and G3 was 61.5±1.4 MPa, 55.3±1.2 MPa, and 52.1±1.0 MPa in the building direction and 1500.5±1.0 MPa, 1300.2±0.6 MPa, and 12.8±0.9 MPa in the transverse direction, respectively. Compressive test results indicated that both the dense PLA and porous PLA scaffolds had elastic-plastic deformation behavior in both building and transverse directions. The scaffolds showed higher compressive elastic modulus, compressive yield strength, tensile elastic modulus, and tensile yield strength in the building direction than those in the transverse direction. These FDM-manufactured gyroid PLA scaffolds showed significantly higher values for compressive strength (up to three times) compared to other gyroid structures reported in the literature, to date.

Additionally, the surfaces of the G2 PLA scaffolds were coated with hydroxyapatite (HA) through magnetron sputtering at different deposition powers from 75 W to 200 W. The surface morphology, phases, chemical bonds of PLA/HA, mechanical properties, and surface roughness of scaffolds were investigated. Bare PLA gyroid scaffolds showed an average roughness of 9.6 µm. The surface roughness of the PLA scaffolds decreased to around 3.1 µm, 3.4 µm, 3.8 µm, and 4.3 µm after HA-coating at a deposition power of 75 W, 100 W, 150 W, and 200 W, respectively. The surface roughness of the scaffolds gradually increased with increasing deposition power after HA coating. The reduced elastic modulus and hardness of uncoated scaffolds increased after HA coating and increased with increasing deposition power. The uncoated PLA scaffolds showed a reduced elastic modulus of ~5.98 GPa and a nanohardness of ~0.27 GPa. The reduced elastic modulus of scaffolds increased from 6.23 GPa to 7.13 GPa with increasing the deposition power from 75 W to 200 W. The nanohardness of scaffolds increased from 0.39 GPa to 0.89 GPa with increasing the deposition power from 75 W to 200 W.

Due to the COVID-19 lockdown, the originally planned experiments on cell response to PLA scaffolds with and without surface modification were not performed. However, a detailed literature review on the importance of surface treatment of PLA scaffolds and its effect on cell response was carried out instead. The surface properties of biomaterials as a bone substitute play a vital role in different cell activities. Because the surface of biomaterials is the first point of contact with the host tissue, it is essential to improve the surface properties of PLA scaffolds to meet the critical clinical requirements such as biocompatibility and mechanical properties. Introducing other biopolymers and ceramics onto the surface of PLA scaffolds can be considered effective methods to achieve improved surface properties of PLA scaffold biomaterials. HA as a bioactive inorganic biomaterial is the most frequently investigated bioceramics due to its chemical similarity with the mineral component of natural bones. The major advantages of surface treatment of PLA scaffolds through the functionalisation and surface modification are the enhanced bioactivity, hydrophilicity, cell attachment, proliferation, and differentiation. Choosing an appropriate architecture for scaffolds is another factor that can be considered an effective method to achieve a suitable environment for cell activities. Highly porous scaffolds with well-interconnected pores can provide a proper environment for different cell activities such as oxygen diffusion, new cell ingrowth, migration, and differentiation.

In conclusion, this study demonstrated that porous PLA gyroid scaffolds manufactured via FDM can be anticipated as promising scaffold biomaterials for bone tissue engineering applications, by virtue of their bone-mimicking porous structure and good mechanical properties. These scaffolds with porosity ranged from 86% to 90% demonstrated excellent compressive strength. Furthermore, HA coating of these scaffolds through magnetron sputtering resulted in an increase in nanohardness and reduced elastic modulus.