Design of the Ti-PCL interpenetrating phase composites with the minimal surface for property enhancement of orthopedic implants

IPCs (interpenetrating phase composites) offer a promising approach to enhancing the mechanical properties of orthopedic implants, including strength, fracture toughness, and the potential for incorporating complex functionalities. In this study, Ti6Al4V-PCL IPCs were fabricated by additive manufacturing and infiltrating melted PCL into a Ti6Al4V scaffold with a minimal surface structure. The IPCs introduction modifies the specimen’s failure mode and improves its energy absorption performance through enhanced two-phase interaction and effective stress transfer. Finite element simulations demonstrate increased stress diffusion, improved energy absorption mechanisms, and a prolonged stress plateau period. Among various scaffold types, the Gyroid IPCs exhibit superior mechanical properties, making them an excellent choice for enhancing the performance and functional development of degradable implantable stents.


Introduction
Titanium alloys are widely employed in the field of medicine, particularly as the preferred choice for bone tissue replacement and fixation materials, owing to their remarkable corrosion resistance, biocompatibility, and high strength-to-weight ratio.However, the use of high-hardness metals as implant materials can lead to bone resorption, implant loosening, and failure due to the relatively low elastic modulus of the human skeletal structure.To address this issue, an increasing number of studies have begun to utilize porous titanium alloy scaffolds to fill implants, thereby reducing their elastic modulus and achieving compatibility with human bone tissue.
Human bone tissue is composed of trabeculae.TPMS (triply periodic minimal surface) is a geometric structure that exhibits advantages such as complete connectivity, high specific strength, high stiffness, and zero mean curvature.TPMS is widely found in insect wings and animal exoskeletons and shares similar structural characteristics with trabeculae [1].Therefore, researchers have incorporated TPMS into the porous design of bone scaffolds [2].Studies have shown that TPMS porous structures enhance the mechanical properties and toughness of porous scaffolds.Additionally, TPMS porous structures improve the biological performance of bone scaffolds by promoting cell attachment and growth, thereby facilitating bone regeneration and repair processes.Despite the significant improvement in the performance of titanium alloy porous scaffolds achieved through the incorporation of TPMS porous structures, the optimization of the structure alone cannot overcome the inherent insufficient toughness of titanium alloys.Therefore, further optimization is required to enhance the toughness and energy absorption properties of titanium alloys.
IPCs (Interpenetrating Phase Composites) are a class of composite materials designed to optimize performance by incorporating a mutually interpenetrating structure of different materials, thereby improving material properties and overcoming their respective limitations while harnessing the advantages of each material.IPCs have been extensively studied in various fields, demonstrating the synergistic enhancement of properties achieved by the combination of two different materials.The reinforcing effects of IPCs have been widely investigated in areas such as wear resistance, compressive strength, electrical conductivity, and energy absorption capacity, showcasing the performance benefits resulting from the synergistic interaction of the two materials.Liu Q. et al. introduced a novel concept of layered interpenetrating phase composites by infiltrating polymers into 3D-printed multilevel porous ceramic lattices, which exhibited impressive mechanical and thermal enhancements.Liu C. et al. developed a three-dimensional Al2O3-polymer composite with a densely interconnected framework that demonstrated excellent mechanical strength, integrity, and multiple thermal transfer pathways.Xiao et al. [3] incorporated epoxy resin into a metallic mesh and found that IPCs improved the peak strength, prolonged the stress plateau, and provided enhanced energy absorption capabilities.Collectively, these studies demonstrate that IPCs offer an excellent approach to constructing multifunctional materials, enhancing material properties, and creating composites with multifunctional performance.PCL (Polycaprolactone) is a chemically synthesized high-biodegradable polymer material that can undergo in vivo erosion or hydrolytic degradation, characterized by its low melting point temperature.It possesses several exceptional properties, such as biocompatibility, stiffness, mechanical elasticity, biodegradability, non-toxicity, thermal stability, rheological behavior, and viscoelasticity, making it widely utilized in clinical applications.Combining PCL with titanium alloy porous scaffolds to form a composite material holds the promise of harnessing the advantages of both materials.The good toughness of PCL allows it to absorb and disperse stresses originating from skeletal movement, thereby reducing the impact between the implant and the bone.By synergistically leveraging the properties of PCL and titanium alloy porous scaffolds, the composite material can enhance the toughness of orthopedic implants, reduce stress concentration, and provide sufficient rigidity and strength to support the normal function of the bone.Through controlling the degradation rate of PCL, newly formed bone can replace the degrading PCL, continuously integrating with the metal, thereby providing dynamically stable mechanical performance and improving the long-term stability of the implant.Furthermore, drug loading and slow release can be incorporated into PCL, enabling the development of functional titanium alloy porous implants.Therefore, the combination of PCL and titanium alloy porous scaffolds holds the potential to leverage their respective advantages, improve the performance of orthopedic implants, and offer better solutions for clinical treatments.
This study presents the design and fabrication of TI-PCL interpenetrating phase samples with a TPMS structure.The performance of the interpenetrating phase material, as well as its deformation mechanisms and failure mechanisms during compression deformation, were investigated through compression experiments and finite element simulations.

Material and method
The design of porous samples was conducted using TPMS implicit functions, and the models were generated using MATLAB.The selected scaffold structures were Gyroid, Diamond, and I-WP, with their corresponding functions as follows: The volume fractions of the designed Diamond, Gyroid, and I-WP structures were controlled based on the authors' previous study [4], with relative density of 15%, 25%, and 35%, respectively.The Ti6Al4V porous scaffold was manufactured using laser powder bed fusion with Ti-6Al-4V powder in the particle size range of 45-105 µm.After printing, the samples underwent sandblasting and ultrasonic cleaning to remove adhered powder.The completed printed samples are shown in Figure 1(a).The samples were then prepared using a hot-press machine, where the heating mold was set to 100°C.PCL particles were melted and pressed into the metal porous structure, followed by cooling under pressure and demolding to complete the sample fabrication.The manufactured samples are depicted in Figure 1(b).The IPCs are labeled as G1, D1, and I1 for the Gyroid, Diamond, and I-WP structures, respectively, with a relative density of 15%.Similarly, G1L, D1L, and I1L refer to the pure titanium scaffolds.The numbers denote the relative density of the metallic component, while the letter "L" indicates the sole lattice structure for the notation, such as G1L.Similarly, G1S represents the finite element model of IPCs.Compression experiments were conducted on both the metal porous scaffolds and IPC samples to investigate their mechanical properties.The compression tests were performed following the guidelines of the compression test standard (ISO 13314:2011).For each type of sample, three tests were conducted to ensure accuracy and account for any significant variations.Outliers were removed, and the average values were taken to obtain reliable experimental results.This approach helped ensure the accuracy and consistency of the experimental data.
To investigate the mechanical strengthening mechanisms and deformation failure mechanisms of the interpenetrating phase samples (IPCs), finite element simulations were performed for analysis.The PCL material properties were set as follows: density of 1300 kg/m³, elastic modulus of 97 MPa, and Poisson's ratio of 0.3.The plasticity and fracture parameters were determined based on compression tests of PCL.The Ti6Al4V material properties were set based on experimental data reported in [5].The elastic modulus was set to 107 GPa, and the Poisson's ratio was set to 0.3.To simulate the deformation and failure behavior, a J-C (Johnson-Cook) model was used as the damage deformation model.The equation for the J-C model is as the author's previous research [4].

Compression test
Figure 2(a-c) depicts the stress-strain curves of IPC samples with different unit cell configurations and relative densities of the metal phase.From the figures, it can be observed that when the metal porous scaffold is prepared as IPCs, the samples with lower relative densities of the metal phase exhibit more stable mechanical support during the compression process without significant stress drops.As the relative density of the metal phase increases, the load-bearing capacity of the IPC samples significantly improves, but the toughness is weakened.
Among the different structures, the Gyroid structure exhibits the most outstanding performance, showing better load-bearing capacity and toughness compared to the Diamond and I-WP structures.Conversely, the Diamond structure is weaker in terms of strength and toughness compared to the other two structures.Figure 2(d) provides a comparison between the stress-strain curves of the pure metal porous scaffold and the IPCs.It is observed that incorporating PCL into the titanium alloy porous scaffold to form IPCs significantly enhances the peak strength of the samples.G1 exhibits the highest peak strength, reaching 32 MPa, which is a 107% increase compared to G1L.Similarly, I1 shows a 139% increase compared to I1L, and D1 demonstrates a 56.6% increase compared to D1L, along with improved toughness.This results in a smoother deformation process, avoiding significant stress failure and yielding a more gradual trend in the stress-strain curve.
The unit energy absorption performance of the specimens is defined by Equation ( 4), and its curve is shown in Figure 3.We observed that the wide range of fracture failures significantly affects the energy absorption performance of the specimens.After experiencing large-scale destruction, the energy absorption capacity of the specimens decreases noticeably.This decrease demonstrates an enhanced trend with an increase in the relative metal density of the specimens.This indicates that the toughness and post-fracture failure mode of the specimens greatly determine their overall energy absorption performance.
Among all the specimens, the Gyroid structure exhibits excellent energy absorption performance, largely due to its outstanding peak strength and comparatively good toughness.Meanwhile, the Diamond and I-WP structures show similar energy absorption performance.Figure 3(d) compares the energy absorption performance of pure metal porous scaffolds with that of interpenetrating phase materials.Since G1L demonstrates the best performance among specimens with the same relative metal density, it is shown here.We can observe that the introduction of interpenetrating phases significantly enhances the energy absorption performance of the specimens.The energy absorption curve shows no significant change in slope, indicating that IPCs achieve overall stable energy absorption performance by improving the deformation and failure mode of the specimens.

Finite element analyses of IPCs
Figure 4 depicts a comparison between the stress-strain curves obtained from finite element simulation and experimental results, demonstrating that the simulation model in this study reliably reproduces the performance changes of the specimens during compression.This confirms the reliability of the simulation model employed in this research.In order to further investigate the deformation failure mechanism and the reasons behind the performance enhancement of interpenetrating phase materials during the compression process, stress contour maps and internal stress distribution of the metal scaffold in the IPCs model, are plotted at different strains, as shown in Figure 5.  From Figure 5, we can observe that at 10% strain, internal metal fractures occur in the I2 structure.In contrast, the G2 structure exhibits a uniform stress distribution within the metal scaffold, still providing good load-bearing capacity.As compression continues, the specimen collapses layer by layer along the fracture region.The stress distribution in the specimen reveals that the compressive stress in the I-WP structure mainly distributes along the axial direction.In contrast, the tensile stress mainly distributes along the radial direction.Stress concentration occurs at the single-cell node region, which may be a significant reason for its lower toughness compared to the Gyroid structure specimens.On the contrary, unlike the stress distribution observed in the previous study on pure metal porous scaffolds [6], the Gyroid scaffold in IPCs exhibits a more uniform stress distribution.The PCL effectively disperses stress, providing sufficient multi-directional support to the scaffold, which resists scaffold deformation and changes the deformation failure mode of the specimen.

Conclusions
In this study, Ti-PCL interpenetrating phase composites (IPCs) with a minimal surface structure were fabricated using hot-pressing techniques to enhance the implant stability and energy absorption performance of orthopedic implants.The mechanical properties and deformation failure mechanisms were investigated through compression tests and finite element simulations, leading to the following conclusions: 1.The synergistic effect between the Ti6Al4V scaffold and PCL in IPCs significantly improves the peak strength and energy absorption performance while optimizing the deformation failure mode of the specimens.The mutual infiltration of PCL and Ti6Al4V promotes interaction, effective stress transfer, and uniform stress distribution.
2. Increasing the relative density of the metal greatly enhances the load-bearing capacity of the specimens but at the expense of reduced toughness and pre-fracture energy absorption.
3. Experimental and simulation results demonstrate that the Gyroid IPCs exhibit a uniform internal stress distribution, which facilitates stress dispersion, prevents crack propagation, and delays scaffold fracture, resulting in a better performance than that of Diamond and I-WP structures.
In summary, IPCs hold promise for improving the mechanical performance of orthopedic implants and further advancing their functional development.However, further research is needed to explore the fatigue behavior and impact resistance of Ti-PCL IPCs, as well as the performance after altering the molecular weight of PCL and incorporating drug loading.

Figure 2 .
Figure 2. (a-c): Mechanical properties of IPC samples with different relative densities; (d): Performance comparison between IPCs and pure porous scaffolds.

Figure 4 .
Figure 4. Comparison of stress-strain curves between experimental and simulation models.

Figure 5 .
Figure 5. Tension and compression stress distribution and stress nephogram of IPCs internal scaffold.