Femur fractures, often resulting from trauma or osteoporosis, pose significant challenges due to their effect on mobility and life quality. Metallic implants like titanium and stainless steel, despite their strength and biocompatibility, present problems related to stress shielding, altered biomechanics, and limitations in diagnostic imaging. This research suggests the use of biocompatible epoxy composites fortified with kevlar fibers (KF), carbon fibers (CF), hybrid fibers, and flax as potential replacements for metallic implants to address these issues. Our examination of the biomechanical reactions of these composites under tensile and flexural stresses revealed that kevlar fiber composites demonstrated superior performance, exhibiting exceptional mechanical properties with a maximum tensile strength of 283.5 MPa and flexural strengths of 53 MPa and 90.4 MPa for the first and second modes, respectively, at a 24% volume fraction. While flax fibers offer the advantage of being natural, their performance was found to be subpar. Carbon and hybrid fiber composites showed performance similar to flax but inferior to kevlar. Interestingly, the inclusion of kevlar in hybrid composites enhanced performance compared to carbon composites. All composites experienced a 50% reduction in ductility when transitioning from the first to the second flexural mode, but this was offset by a significant increase in flexural strength. These findings suggest that kevlar fiber-reinforced composites, despite addressing the problems associated with metallic implants, show promise as an alternative material for femur implants due to their superior mechanical properties. Further research is required for clinical application to optimize fiber mixtures, enhance composite structures, and assess in vivo biocompatibility.
Article
Fiber reinforced epoxy composites for femur fractures: a mechanical investigation
https://doi.org/10.21203/rs.3.rs-4555096/v1
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Orthopaedic Implants
Kevlar Fibers
Carbon Fibers
Hybrid Fibers
Flax
Polymer Composites
Epoxy
Mechanical strength
The quest for substances with improved mechanical characteristics and biocompatibility has resulted in significant progress in the orthopaedic implant sector in recent years [1, 2]. Traditional materials such as cobalt chrome, stainless steel, and titanium alloys have been vital in fulfilling the requirements of load-bearing applications, particularly for Femur Plate Implants, Fig. 1. Nonetheless, the increasing need for implants with enhanced mechanical performance, reduced weight, and improved patient results has spurred the exploration of novel materials and composite systems [3–6].
Reinforced polymer matrix composites, also known as RPMCs, represent a promising avenue in the quest for advanced orthopedic implant [7–9]. By combining high-performance fibers such as carbon, flux, and kevlar with the advantageous properties of polymers, RPMCs have the potential to revolutionize the orthopaedic implant material industry [10, 11]. The exceptional mechanical properties that these fibers confer to the polymer matrix enable the creation of implants that surpass the limitations of conventional materials [12–17].
Ensuring the reliability and longevity of orthopaedic implants during usage is heavily dependent on their mechanical behavior. The ability of the implant to withstand physiological loads and dynamic stresses encountered in daily activities is influenced by several key factors, such as tensile strength, fatigue resistance, and flexural performance [10, 11, 18]. Tensile strength is a basic mechanical property that defines an implant’s resistance to tensile forces. Flexural performance indicates the implant’s resilience to the bending forces that are inherent in the human body’s movements in everyday scenarios [19–21].
To evaluate the potential of RPMCs as replacements for conventional materials, a comprehensive understanding of their mechanical performance in orthopaedic applications is crucial. This study aims to investigate the mechanical behavior of RPMCs fortified with kevlar, flux, and carbon fibers for their application in the repair of femoral bone fractures using stabilizing plate implants.
By conducting an in-depth analysis of the flexural performance of these composite implants, valuable insights can be gained about their suitability as alternatives to traditional materials. In addition to aiding in the selection and optimization of implant materials, knowledge of the mechanical performance of these RPMCs will pave the way for improved patient outcomes, more effective implants, and a reduction in surgical complications.
Biocompatible Epoxy Resins:
Epoxy resins, known for their exceptional mechanical properties, chemical resistance, and ease of processing, have found widespread use across various industries. However, initial applications were limited to non-biological environments due to concerns about their biocompatibility. Through careful modifications and advancements in resin composition, researchers have successfully tailored epoxy resins to meet stringent biocompatibility standards, thus paving the way for their use within the human body [22–24].
The key attribute of biocompatible epoxy resin is its ability to coexist with biological tissues without eliciting cytotoxic or adverse immune responses. Several methods are employed to achieve this compatibility, such as the utilization of biocompatible raw materials, the elimination of harmful additives, and the enhancement of resin curing processes. As a result of these modifications, the epoxy resin becomes an ideal candidate for medical applications as it exhibits minimal toxicity and maintains its structural integrity, see Table 1, in a biological environment [25, 26].
| Property | Value |
|---|---|
| Tensile Strength (MPa) | 179 |
| Tensile modulus (GPa) | 10.4 |
| Elongation at Break (%) | - |
| Density (g/cm³) | 1.1 |
Flax:
Flax-woven fiber is a versatile and sustainable material. It is appreciated for its environmentally friendly farming practices, ability to decompose, and unique aesthetic appeal, despite its mechanical strength being lower than some other options as seen in Table 2. Research efforts are focused on overcoming performance limitations while maintaining its inherent advantages, positioning flax-woven fiber as a promising candidate for a sustainable future across various industries [27].
| Property | Value |
|---|---|
| Tensile Strength (MPa) | 61 |
| Tensile modulus (GPa) | 7 |
| Elongation at Break (%) | 1.5 |
| Density (g/cm³) | 1.45 |
Carbon Fiber:
The tensile strength and stiffness of carbon fibers are outstanding, see Table 3, surpassing a variety of traditional materials like metals and ceramics. Carbon fibers consist of slender filaments of high-strength carbon. When integrated into polymer matrices, carbon fibers enhance the strength of composites, boosting their ability to bear loads and resulting in impressive strength-to-weight ratios [17, 28].
| Property | Value |
|---|---|
| Tensile Strength (MPa) | 3100 |
| Tensile modulus (GPa) | 230 |
| Elongation at Break (%) | 1.8 |
| Density (g/cm³) | 1.79 |
Kevlar Fibers:
Traditional metallic implants often exert excessive weight and pressure on the adjacent bone, leading to an increased likelihood of issues such as stress shielding, implant failure, and bone resorption. Kevlar fiber composites present a solution by reducing the weight of the implant while maintaining or potentially improving its mechanical properties, as seen in Table 4.This alleviates bone stress, enhances patient comfort, and facilitates more efficient rehabilitation [15].
| Property | Value |
|---|---|
| Tensile Strength (MPa) | 3800 |
| Tensile modulus (GPa) | 131 |
| Elongation at Break (%) | 2.4 |
| Density (g/cm³) | 1.44 |
Hybrid CF-Kevlar:
Carbon-Kevlar hybrid woven fabric is a type of composite material that merges the attributes of both carbon fiber and Kevlar fiber. This fabric is created by interlacing strands of carbon and Kevlar fiber in a specific arrangement, resulting in a fabric with enhanced mechanical properties.
Mechanical testing
For investigating new materials for a femur implant, several mechanical tests are crucial to ensuring the material's suitability for the application. Tensile Testing measures the material's strength and ductility under tension, helping to understand its response to pulling forces and assess its ability to withstand femur stresses. Flexural Testing evaluates the material's bending capability, crucial given the femur's bending forces during activities like walking or running. By conducting these mechanical tests, researchers and manufacturers can thoroughly evaluate the performance, durability, and biocompatibility of new materials for femur implants, ensuring their safety and effectiveness in clinical use.
Tension test:
The static tensile tests were implemented using a universal material testing machine with a maximum load of 30 tons. The tests were carried out at the strain rates of (10 mm/min). Figure 2 shows specimen dimension used in tension test.
Flexural test:
Given the extraordinary flexibility and wide range of motion inherent in the human body, the creation of an implant capable of withstanding such varied stresses requires comprehensive analysis from various perspectives. For example, while the implant may be subjected to flexural stress, its vulnerability to this stress can change significantly when the direction is altered, as depicted in Figs. 3 and 4. Simply changing the stress direction by 90⁰ can have a profound impact on the flexural forces the implant can resist. A universal material testing machine was utilized in a 3-point bending setup, and the tests were conducted at strain rates of 10 mm/min.
The governing formula for determining the stress applied to a rectangular test sample, Test mode 1, when subjecting specimens to a 3-point bending setup, is as follows:
Regarding the second mode, seen in Fig. 4, it is apparent that the moment of inertia will differ from the first due to a 90° rotation, leading to significantly increased values of flexural stress that the test sample can endure.
Tension
Figure 5 illustrates the stress strain curve of four different materials that are proposed as alternative implant materials: kevlar, hybrid, carbon fibre, and flax.
It’s clear that all fiber materials have an increasing behavior in their tensile strength when increasing the volume fraction. However, in flax fibers, both 8 and 16 vol. % have displayed convergence in performance.
It can be deduced that KF exhibits the highest tensile strength, indicating its superior ability to resist tearing under tensile stress. Moreover, during the test, the failure of the KF composite was catastrophic and sudden.
The hybrid material follows CF in terms of tensile strength and exhibits almost a similar performance under strain; unlike KF, there was a tearing sound in the hybrid composite that could be detected, which made the failure point more anticipated. This is due to the presence of CF within the composite. CF, while less in tensile strength than KF and the hybrid material, outperforms flax, which trails behind the other three materials with the lowest tensile strength. For further understanding, Fig. 6 illustrates the effect of volume fraction on ultimate tensile stress for KF, hyprid, CF, and flax at a strain rate of 10 mm/min.
Figure 6 illustrates the relationship between volume fractions of reinforcement fibers and ultimate tensile stress. As the volume fraction increases from 0–24%, all materials exhibit an increase in stress, indicating that they become durable. Among them, KF stands out with the highest stress values across all volume fractions, reaching 283.5 Mpa at 24 vol. %, leaving a big gap between other proposed composites, making it the most robust of the four materials.
Flexural Test Mode 1
Figure 7 presents a comprehensive analysis of the relationship between deflection and flexural stress for different composite materials at varying volume fractions, 8, 16, and 24%. The materials under study include pure, flax, CF, KF, and hybrid.
It’s crucial to understand that in flexural stress, the upper section undergoes compression (above the neutral axis), while the lower section is subjected to tension (below the neutral axis). In the context of the first mode of the bending test, the fibers oriented in the transverse direction play a more significant role in responding to the bending load. As a result, when analyzing the curves in Fig. 7, they display a zig-zag pattern, indicative of the ongoing rupture of fibers along the transverse direction.
Figure 8 depicts the influence of the volume fraction of fiber reinforcement on flexural strength. As the volume fraction rises from 8 to 24%, all composites demonstrate a significant increase in flexural force, indicating a positive relationship between the volume fraction and the flexural stress of the proposed composites. KF appears to consistently display the greatest flexural force across all volume fractions (8, 16, and 24%), making it the most resilient to flexural bending.
Flax, CF, and hybrid fibers exhibit increasing performance, with hybrid typically surpassing flax and CF. In terms of flexural elastic modulus, a similar trend as above is observed, as it escalates with the introduction of reinforcement fibers, starting with flax, then CF, then hybrid, culminating at the apex, which is KF fibers achieving 53 MPa at a 24% volume fraction.
Flexural Testing Mode 2
At first glance of Fig. 9, two significant insights can be drawn. First, the performance of the fiber within the epoxy matrix reflects the pattern seen in the initial bending mode. Second, there is a substantial increase in flexural forces across different volume fractions. This rise is in line with expectations, originating from the 90⁰ rotation of the implant, leading to an increased area moment of inertia. As a result, the composite suggested for the implant demonstrates improved resistance to flexural forces. However, this comes at the cost of reduced ductility, which has significantly decreased by about 50% compared to the first mode.
In Fig. 10, at volume fractions of 8 and 16%, the performance of KF and hybrid fibers was relatively similar. However, a notable peak emerged for KF at a volume fraction of 24%, reaching a value of 90.4 MPa.
To understand and examine the fracture mechanism of the proposed compositions, a microscope was used to image the fracture surface at the microscale, Fig. 11, for each of the fiber types used.
Figure. 11 illustrates the fracture zone of each proposed fibers. In Fig. 11(a), flax appears as bundles made of smaller fibers intertwined together, forming a large cohesive bundle, in contrast to carbon fiber CF, hybrid, and KF, that consists of small fibers woven together. Flax fibers stand out due to their larger size compared to other fibers, resulting in a distinct behaviour upon detachment from the epoxy matrix. As these fibers separate, they carry substantial segments of epoxy, appeared in the red coloration. This is in stark contrast to the behaviour observed with other fibers, where small epoxy particles on the surface, highlighted in blue, as depicted in Fig. 11(b), (c) and (d).
Upon examining the fracture shape of the fibers themselves, indicated by the green colour, a crucial observation emerges regarding the fundamental difference in fracture mechanisms between CF and KF. Previous conclusions drawn from tensile and bending tests, whether in the first or second mode, have consistently demonstrated KF superior strength compared to CF. This assertion finds further support in the images captured. In Fig. 11, (b), (c), and (d), it becomes apparent that during tension tests, KF undergo slight elongation, see Table 4, until reaching the fracture point. Upon complete separation, these fibers undergo a spring-back effect, resulting in a characteristic arching shape at the fracture area. On the other hand, the CF has straight fiber breakage, showing a brittle fracture characteristic, see Table 3.
The assessment of the performance of composites reinforced with KF fibers via mechanical testing uncovers a potentially beneficial substitute for traditional metal implants. This substitute tackles a key issue related to stress shielding, a phenomenon triggered by the excessive rigidity intrinsic to metal implants. The composites, distinguished by their diminished rigidity, bypass this problem without sacrificing the necessary mechanical strength vital for the implant’s effective operation during the bone recovery process. As a result, the integration of KF into the composites presents itself as a promising approach, providing a balanced trade-off between rigidity and mechanical strength, thereby improving the overall effectiveness of orthopaedic implants.
Reinforcing epoxy with woven fibers has presented promising results that can address the issues regarding the traditional implants used. These are the conclusions reached:
The use of woven fibers to reinforce epoxy has shown encouraging results that can address the problems associated with traditional implants. The following conclusions were drawn:
1- KF excelled in all conducted tests, demonstrating superior mechanical properties. It recorded an ultimate tensile strength of 284 MPa in tension. In terms of flexural modes, it achieved 53 MPa and 90 MPa for the first and second modes, respectively, at a volume fraction of 24%.
2 - While flax fibers have the benefit of being natural, making them more appropriate for the intended application, their performance in resisting mechanical stresses was inferior compared to other fiber types.
3 - CF and hybrid fibers showed average performance, surpassing flax fibers but falling behind KF. It’s noteworthy that in the case of hybrid fibers, the inclusion of KF with carbon enhanced the performance of the composites compared to the composites containing only CF.
4 - The flexibility of nearly all composites decreased by 50% when the flexural test mode was changed from the first mode to the second mode, but this was compensated by a significant increase in flexural strength.
Compliance with Ethical Standards:
The authors declare that they have no conflict of interest.
Author Contribution
All authors reviewed the manuscript
Data Availability
Data is provided within the manuscript
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No competing interests reported.