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. Author manuscript; available in PMC: 2022 Aug 1.
Published in final edited form as: J Mech Behav Biomed Mater. 2021 Apr 29;120:104564. doi: 10.1016/j.jmbbm.2021.104564

Understanding wear behavior of 3D-Printed calcium phosphate-reinforced CoCrMo in biologically relevant media

Himanshu Sahasrabudhe 1,**, Kellen D Traxel 1, Amit Bandyopadhyay 1,*
PMCID: PMC8205995  NIHMSID: NIHMS1701764  PMID: 33965811

Abstract

Recent advances in the processing of wear-resistant calcium-phosphate reinforced CoCrMo composites for articulating surface applications has necessitated further investigation of performance in biological conditions relevant to patient applications. To this end, CoCrMo composites containing calcium phosphate in the form of hydroxyapatite (HA) were manufactured to study the influence of the reinforcing phase on the tribofilm formation in biologically-relevant conditions. The CoCrMo-HA composites were processed using a laser engineered net shaping (LENS™) additive manufacturing (AM) system with three distinctive compositions: CoCrMo-0%HA, CoCrMo-1%HA, and CoCrMo-3%HA. Extensive wear testing of the CoCrMo-HA composites was carried out in DMEM (cell media) and DMEM + Hyaluronic acid (found naturally in synovial fluid). Wear tests were performed at loads ranging from 5N to 20N, and wear media was measured post-test using ICP-MS techniques to release Co and Cr ions. During testing, all coefficients of friction remained in the 0.15-0.25 range, which was lower than the previously reported 0.50-0.75 range in DI water, indicating that the DMEM + hyaluronic acid media plays a significant role in reducing frictional contact. At loads higher than 15N, the HA-tribofilm exhibited a breakdown resulting in higher wear rates but still lower overall ion release than the CoCrMo control composition. Our results indicate that CoCrMo alloys with HA addition can significantly reduce wear rates and ion release even in the presence of naturally-occurring synovial-fluid friction-reducing constituents.

Keywords: Cobalt-chromium-molybdenum, laser additive manufacturing, hydroxyapatite, wear, tribofilm

1. Introduction

Increases in wear and corrosion degradation-based health issues in articulating-surfaces of implants, namely CoCrMo, coupled with patient-specific geometric needs, requires innovative materials and processing technologies to improve long-term patient health (Bandyopadhyay and Traxel, 2018; Bose et al., 2019, 2018; Sahasrabudhe et al., 2018; Tofail et al., 2018). The main issue of wear and corrosion occurs directly at the surface in contact with the joint replacement areas with strong point loadings such as hip and knee joints. When there is relative motion between the two contacting surfaces, such as a hip joint, especially in the presence of a media such as body fluid, localized adhesion is followed by fracture of the asperities. The production of wear debris when there is relative motion in between two surfaces is unavoidable, and in the case of load-bearing implants, over a long period, there is a possibility of a large amount of such debris. This can cause inflammation and swelling and pain in the joint (Coleman et al., 1974; Ferguson et al., 1960; Uo et al., 2001). Repeated loading on the worn-down/structurally imperfect implant could also cause the implant to move out of place, causing aseptic loosening (Lombardi et al., 1989; Sundfeldt et al., 2006; Wooley and Schwarz, 2004). Problems are also caused by the transport of the metallic ions released during wear, called Metallosis, which arises out of the accumulation of the metal ions such as Co and Cr in the human cells, sometimes even in distant organs such as in the kidneys or liver (Fisher et al., 2004; Kehler et al., 1999; Sundfeldt et al., 2006; Türkan et al., 2006; Wooley and Schwarz, 2004). A knee replacement with Cobased alloys can last between 10 and 15 years, depending upon the patient's health and lifestyle. Once an implant is loose or corroded, the patient must undergo revision surgery to extract the worn implant and allow for another new implant. Since more and more younger patients are getting load-bearing implants, there is a need for improvement in the current implants' life by improving their bio-tribological properties. Cobalt-based alloys are the gold standard for biomedical load-bearing implants because they maintain high hardness and low wear rates along with a low coefficient of friction and good resistance to corrosive media. However, CoCr alloys still degrade in the long-term and can create several complications, as discussed, motivating the investigation of improvement strategies for long-term performance (Sahasrabudhe et al., 2018). A proposed alternative, ceramic-on-ceramic implants, has shown popularity among young patients; however, they have required extensive revision surgeries due to fracture, infection, cup loosening, and improper positioning, indicating that a balance must be struck between wear resistance and toughness that neither the base CoCrMo alloy nor pure ceramic counterpart can achieve alone (Migaud et al., 2016). Most wear-improvement strategies involve significant modification to the base CoCrMo alloy or coating procedures that increase processing complexity. Among several investigations, a combination of nitriding, boronizing, and ion implantation has seen success in improving wear performance (Alvarez-Vera et al., 2013; Fisher et al., 2004; Kehler et al., 1999; Mu et al., 2010; Ortega-Saenz et al., 2011; Türkan et al., 2006), utilizing different gradient structures has improved wear performance (Dittrick et al., 2011), as well as various biocompatible coatings and coatings based on diamond-like carbon (Bandyopadhyay et al., 2019; Escobedo et al., 2006; Liao et al., 2011; Ortega-Saenz et al., 2011; Sheeja et al., 2005). These methods, while successful, can generally increase processing complexity and cost for end-use manufacturers and patients. Ideally, CoCrMo alloy would be modified with an existing biomaterial to provide strengthening, wear reduction simultaneously, and non-toxic to the body. Besides, for scalability in production, CoCrMo wear enhancement methods must be practical for complex geometry implants, motivating the investigation of additive-based composite manufacturing methods to meet this need (Bandyopadhyay and Traxel, 2018). Specifically, our previous work has shown that calcium phosphate-based materials such as hydroxyapatite (HA) and tricalcium phosphate (TCP) with a composition close to natural bone fulfill both the above criteria when incorporated in the standard CoCrMo alloy processed via additive manufacturing technology (Bandyopadhyay et al., 2019; Sahasrabudhe et al., 2018). Specifically, Sahasrabudhe et al. (2018) demonstrated that calcium phosphate reinforced CoCrMo alloy exhibited tribofilm formation leading to significantly reduced wear rates and Co/Cr ion release in DI water medium to a complex film formation mechanism (Sahasrabudhe et al., 2018). Bandyopadhyay et al. (2019) demonstrated that this tribofilm formation contributes to increased contact resistance between the coating and the top surface (see Figure 1) and that the presence of the tribofilm increases in vivo osteoid formation in a Sprague-Dawley rat and rabbit in vivo models, indicating the efficacy of this composite system (Bandyopadhyay et al., 2019). A remaining challenge, however, is understanding the effect of biological environments on tribofilm formation. More specifically, the presence of proteins within hyaluronic-acid containing synovial fluid in joints naturally provides a soft carbonaceous layer to reduce friction (Liao et al., 2011), provide shock absorption capability, and facilitate nutrients and waste product transport, i.e., a vastly different environment to test wear performance in comparison to DI water (Hui et al., 2012). Moreover, despite the success of HA-reinforced CoCrMo to significantly reduce the wear rates and ion release of the underlying CoCrMo alloy in DI water medium, understanding how this tribofilm that forms due to HA will interact in the presence of a synovial-fluid like a medium that already contains the right ingredients for a friction-reducing film formation is crucial to substantiating this composite as a solution for current wear-induced implant issues.

Figure 1:

Figure 1:

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Outline of LENS™ based processing and previous work showing a demonstration of tribofilm formation. LENS™ schematic and contact resistance plot modified from (Bandyopadhyay et al., 2019; Bandyopadhyay and Traxel, 2018), respectively.

To this end, the present work's main objective is to characterize the effects of the biological environment on the tribofilm formation and wear behavior of HA-reinforced CoCrMo composites produced using laser-based AM. We hypothesize that the presence of naturally-occurring proteins, represented herein by Dulbecco's Modified Eagle's Medium (DMEM), will form a natural tribofilm in addition to the one formed by the CoCrMo-HA composites and reduce the overall wear rates and ion release in comparison to previous data from DI water medium. Secondly, we hypothesize that introducing a lubricative agent to DMEM, represented by hyaluronic acid (HyaA), will further decrease the wear rates and Co/Cr ion release under loading, contributing to the effect of HA creating a synthetic tribofilm along the top surface of the composite. To evaluate these hypotheses, CoCrMo was processed via LENS™ AM technology with HA in the form of hydroxyapatite in three separate compositions: CoCrMo-0%HA, CoCrMo-1%HA, and CoCrMo-3%HA. These composite structures, also utilized in the previous work of Sahasrabudhe et al. (2018) (Sahasrabudhe et al., 2018), were subjected to linearly reciprocating wear in an environment of Dulbecco's Modified Eagle's Medium (DMEM) and hyaluronic acid (HyaA, 10% and 50% by volume) at loads ranging from 5N to 20N, to evaluate the combined effects of the environment, loading condition, and composite composition on the resulting wear performance. Coefficient of friction (COF) data was collected during testing. The resulting wear tracks were measured and analyzed via scanning electron microscopy (SEM) to identify the normalized wear rate and the presence of wear-induced tribofilm post-testing. ICP-MS was utilized to identify Co and Cr ions in the media to indicate any ion release from the metal-composite material during reciprocating testing. The results of this work can be used to help inform the design of next-generation implant materials for load-bearing articulating surface applications.

2. Materials and Methods

2.1. Composite processing via LENS™:

CoCrMo alloy powder (Biodur CCM+ Micromelt Alloy, particle size 44- 149μm, Carpenter Powder Products, Bridgeville, PA) was mixed with 0%, 1% and 3% by weight of hydroxyapatite (Ca10(PO4)6 (OH)2) powder (particle size <500nm, Berkeley Advanced Biomaterials Inc., Berkeley, CA). The premixed powder was processed using a LENS™ 750 (Optomec Inc., Albuquerque, NM) system, equipped with a CNC-controlled build plate and 500W Nd: YAG laser system. Square shaped samples of side 14mm were deposited on a Stainless Steel 410 substrate of 3mm thickness. Each square-shaped sample was ~12mm thick. The laser power of 400W and a raster scan speed of 450-600 mm/min were employed during the deposition, with the building height and build-track consistency used to evaluate the tracks' quality. Note that the same batch of samples in the present study was used in the previous work from ref. 1 (Sahasrabudhe et al., 2018).

2.2. Wear testing preparation and characterization:

Samples processed via LENS™ were cut with a low-speed diamond saw (MTI Corporation) for microstructural analysis and wet ground on SiC paper from 120 grit up to 1000 grit. Samples were then polished on 1 μm, 0.3 μm, and 0.05 μm alumina suspensions in DI water. Polished samples were cleaned in an ultrasonic bath for ~30min in a 75% ethanol solution at room temperature. Linear reciprocal pin on disk wear tests was conducted on the CoCrMo-HA composite samples at room temperature. Wear tests were performed in a Nanovea series tribometer (Nanovea Series, Nanovea, Irvine CA, USA), and COF data was also collected (Figure 2). Samples' top surface was mounted in epoxy and then ground and polished to a mirror finish. Wear tests were performed for 1000m using a silicon nitride ball (hardness ≈ 1400HV0.1, φ=3mm) as a counter material. Because the hardness of the worn ball is ~3X the coating's hardness, wear rates were assumed to occur strictly due to damage from the composites themselves, not the ceramic wear balls. Further, previous work with similar wear couples has shown Si3N4 balls with limited surface degradation in the range of 3-6% of the overall coating wear, indicating that the majority of degradation is taking place in the coating itself and not the ball (Sahasrabudhe and Bandyopadhyay, 2014). The amplitude of each reciprocating stroke was 10mm, and the reciprocating speed was 1200 mm/min, at loads of 10N, 15N, and 20N. The linear reciprocating wear tests performed here conformed to ASTM standard G133-05(2016) for the ball on flat sliding wear, with three separate tests performed at each of the load and wear media settings (ASTM Standard G-133-05, 2016), and wear rates determined via wear volume calculation from measurements of the wear scar width after testing and then divided by the wear distance of 1000m. Damage analysis was performed via scanning electron microscopy (FEI Quanta, Hillsboro, OR.) on as-tested surface wear tracks. Wear testing was performed in Dulbecco's Modified Eagle's Medium (DMEM) cell media and a mixture of DMEM and Hyaluronic Acid (HyaA). HyaA solution was made from blending sodium hyaluronate powder (Bulk Supplements, Henderson, NV) to form a 1% by weight strength solution in DI water (concentration of ~1mg/mL, comparable to actual synovial fluid HyaA concentration of 3-4mg/mL (Hui et al., 2012)). The wear testing media of DMEM and HA consisted of 1-3% of the blended solution in DMEM by volume. The three media used were DMEM+0%HyaA, DMEM+10%HyaA, and DMEM+50%HyaA. After each wear test, the wear debris medium was collected and stored for determining the concentration of metal ions leached during wear via ICP-MS. Ion concentration of Co and Cr ions was measured using Agilent 7700 Inductive Coupled Plasma-Mass Spectroscopy (ICP-MS). Standard solutions of 10ppb, 500ppb, and 1000ppb were used for calibrating the ICP-MS instrument. Table 1 below shows the viscosity (via viscometer) of the three solutions at room temperature with identical mixing procedures. A chrome steel ball with a measured density of 7.801 g/cc was used for the viscosity measurements.

Figure 2:

Figure 2:

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Characteristic COF curves for composites. (A) DMEM-0%HyAcid. (B) DMEM-10%HyAcid. (C) DMEM-50%HyAcid.

Table 1:

Viscosity of DMEM+Hyaluronic Acid mixture solutions via viscometer

Solution Viscosity (Pa-s)
DMEM + 0 Hyaluronic Acid 1.2778
DMEM + 10 Hyaluronic Acid 1.5813
DMEM + 50 Hyaluronic Acid 2.1512
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3. Results

3.1. Processing, microstructure, and phase evolution:

From previous works employing the same compositions as the current study (Bandyopadhyay et al., 2019; Sahasrabudhe et al., 2018), it was observed that AM-produced CoCrMo produces a fine dendritic structure with carbides observed along the grain boundaries (via high magnification SEM imaging and XRD analysis). The presence of HA within the composites resulted in a discontinuous cellular carbide structure and is claimed to stabilize the ε-HCP phase of CoCrMo, resulting in the increased dissolution of carbides in the microstructure. The compositions' microhardness remained constant across HA addition at roughly 550-575HV, indicating no effect of HA on the composites' hardness.

3.2. Wear tests in DMEM-0%HyaA solution:

At low loads of 5N and 10N in DMEM with no HA, the wear rate measurements revealed that by gradually increasing the concentration of HA in CoCrMo from 0% to 3%, the wear rate reduced (see Figure 3). At 5N load, the CoCrMo-0%HA composite had a wear rate of 4.09±0.20 x 10−6 mm3/Nm. For the CoCrMo-3%HA composite, the wear rate was reduced to 2.38±0.16 x 10−6 mm3/Nm. Similarly, at a load of 10N, the wear rate of CoCrMo-0%HA composite was 2.39±0.13 x 10−6 mm3/Nm, and for the CoCrMo-3%HA composite, it was 2.19±0.08 x 10−6 mm3/Nm. However, at higher loads of 15N and 20N, the wear rates of the CoCrMo-3%HA composite was found to be higher than the composite with no HA addition. Specifically, at 15N load in pure DMEM solution, the wear rate of CoCrMo-0%HA composite was 1.68±0.06 x 10−6 mm3/Nm, and the wear rate for CoCrMo- 3%HA composite was found to be 2.17±0.07 x 10−6 mm3/Nm. Similarly, at the load of 20N, the wear rate of CoCrMo-0%HA composite was 1.48±0.04 x 10−6 mm3/Nm, whereas the wear rate of CoCrMo-3%HA composite was measured to be 1.97±0.05 x 10−6 mm3/Nm. In all of these cases, the wear rates for CoCrMo-1%HA composite in pure DMEM solution are roughly between the wear rates of CoCrMo-0%HA and CoCrMo-3%HA composites. For wear tests of all the three composites at 10N, 15N, and 20N load in pure DMEM solution, the concentration of both cobalt and chromium ions was found to decrease with increasing HA concentration in the LENS™ processed composite. The significant drops in cobalt ion leaching are visible from the drop in the measured cobalt concentration from 520ppb to 392ppb at 10N load and from 695ppb to 380ppb at 15N load, and from 650ppb to 565.5 ppb at 20N load. Similar trends can be observed in the concentration of the leached chromium ions. In the wear tests where the wear rate of CoCrMo-3%HA composite increased in comparison to the CoCrMo-0%HA composite, the concentration of the leached cobalt and chromium ions was still found to reduce. The plot in Figure 2A shows the behavior of the coefficient of friction of all the CoCrMo-HA composites that were wear-tested in pure DMEM solution for a distance of 1000m. All composites showed a similar trend in which the coefficient of friction of the samples fluctuated but in a narrow range of 0.15 to 0.20. Except for a few, most samples showed a rise in an initial spike in the coefficient of friction, followed by a drop, and then consistent values throughout the rest of the test. This was observed mostly in the first ~150m of the wear test, and after that, the values were fluctuating in a narrow range. Some samples showed a gradual rise in the COF with distance; however, the rise was small from a value of 0.17 to 0.20. This rise was noted after a distance of ~600m.

Figure 3:

Figure 3:

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Wear rates and Co + Cr ion release into a solution across all testing conditions.

3.3. Wear Tests in DMEM-10%HyaA Solution:

At low loads of 5N and 10N in DMEM-10% HyaA, the wear rate measurements revealed that by gradually increasing the concentration of HA in CoCrMo from 0% to 3%, the wear rate reduced, similar to the trend observed in the wear tests done at 5N load in pure DMEM solution (see Figure 3). At 5N load, the CoCrMo-0%HA composite had a wear rate of 2.56±0.22 x 10−6 mm3/Nm. For the CoCrMo-3%HA composite, the wear rate was reduced to 2.17±0.12 x 10−6 mm3/Nm. Similarly, at a load of 10N, the wear rate of CoCrMo-0%HA composite was 2.57±0.09 x 10−6 mm3/Nm, and for the CoCrMo-3%HA composite, it was 1.55±0.01 x 10−6 mm3/Nm. However, at higher loads of 15N and 20N, the wear rates of the CoCrMo-3%HA composite were found to be slightly higher or about the same as the CoCrMo-0%HA composite. Specifically, at 15N load in DMEM-10%HyaA solution, the wear rate of CoCrMo-0%HA composite was 1.97±0.02 x 10−6 mm3/Nm, and the wear rate for CoCrMo-3%HA composite was found to be 2.08±0.04 x 10−6 mm3/Nm. Similarly, at the load of 20N, the wear rate of CoCrMo-0%HA composite was 1.95±0.05 x 10−6 mm3/Nm, whereas the wear rate of CoCrMo-3%HA composite was measured to be 1.83±0.05 x 10−6 mm3/Nm. In all of these cases, wear rates for CoCrMo-1%HA composite in pure DMEM solution are roughly between CoCrMo-0%HA and CoCrMo-3%HA composites. It can also be observed that the overall wear rates of the three LENS™ processed CoCrMo-HA composites were lower in the DMEM-10%HyaA solution than they were in the pure DMEM solution with no HyaA addition. In all of these wear tests, the wear test media after completing the wear test was analyzed for the concentration of cobalt and chromium ion release during the wear test by ICP-MS technique. For wear tests of all the three composites at 10N, 15N, and 20N load in the DMEM-10%HyaA solution, the concentration of both cobalt and chromium ions decreased with increasing HA concentration in the LENS™ processed composites.

As seen from Figure 3, when wear tests were conducted in DMEM-10%HA solution, the leached cobalt ion concentration at 10N decreased from 637.5 ppb to 327.5 ppb. A similar trend was observed for the other wear tests done at 15N and 20N, where the concentration of the leached cobalt ion was 659ppb and 658ppb respectively for the CoCrMo-0%HA composite, whereas it was 367.5ppb and 529ppb respectively for the CoCrMo-3%HA composite. The leached chromium ions concentration was also found to decrease for the LENSTM processed CoCrMo-3%HA composite compared to the LENS™ processed CoCrMo-0%HA composite. The plot in Figure 2B shows the trend of the coefficient of friction of all the LENS™ processed CoCrMo-HA composites tested in the DMEM-10%HyaA solution. The first immediate distinction that the COF trend in DMEM-10%HyaA solution as compared to the COF trend in the pure DMEM solution was that the COF values were overall fluctuating in a slightly higher range of 0.17 to 0.21 as compared to the fluctuation in the range of 0.15 to 0.20 for the tests done in pure DMEM solution. The next observation from the plot was that the COF rose sharply in the first 50-100m of the wear test, and following that, there was a small drop, further followed by a relatively stable state and small variation. In some samples, the COF values appeared to drop compared to the tests done in pure DMEM, where the COF values of some of the HA-composites showed a small increase with distance.

3.4. Wear Tests in DMEM-50%HyaA Solution:

Wear tests that were done in DMEM-50%HA solution showed more fluctuating wear rates for all the three LENS™ processed composites (see Figure 3). At the lower load of 5N, the wear rate of CoCrMo-0%HA was higher at 3.66±0.09 x 10−6 mm3/Nm compared to the CoCrMo-3%HA composite with a rate of 2.68±0.11 x 10−6 mm3/Nm. However, for the CoCrMo-1%HA sample tested under similar conditions, the wear rate was found to be higher than the other two composites at 4.10±0.11×10−6 mm3/Nm. At the load of 10N and 15N, there was a backward trend in which the wear rates were found to increase from the CoCrMo-0%HA composite to the CoCrMo-3%HA composite. At the highest testing load of 20N, the wear rate for the CoCrMo-1%HA composite was found to be the highest in between all the three composites. As seen from Figure 3, the cobalt and chromium ions' measured concentrations do not follow a regular trend, as was seen in the wear tests done in the DMEM-10%HyaA solution. However, it can still be seen that there was a reduction in the ion release concentration for the CoCrMo-1%HA composite and the CoCrMo-3%HA composite as compared to the CoCrMo-0%HA composite. For all the three loads of 10N, 15N, and 20N, the leached cobalt ions' concentration dropped for both the composites with HA addition. In general, all the leached ion concentrations measured for the wear tests done in DMEM-50%HyaA solution were lesser than the wear tests done in pure DMEM solution and lesser than the wear tests DMEM-10%HyaA solution. This indicates a lesser ion leaching effect with higher hyaluronic acid concentration. Figure 2C shows the friction coefficient of the LENS™ processed CoCrMo-HA composite when wear tested in DMEM-50%HyaA solution for a distance of 1000m. As seen from the plot, the overall COF values in the DMEM-50%HyaA solution fluctuated at values higher than the wear tests done in pure DMEM and DMEM-10%HyaA solution. The COF values in this set of wear tests fluctuated in the range of 0.18 to 0.22. There was a rise initially in the COF; however, the rise was not as sharp as seen in the wear tests conducted in the DMEM-10%HyaA solution. The rise appeared to be more gradual in the first ~100m of the wear tests, beyond which there was not a very significant drop in the coefficient of friction, and generally remained constant for the remainder of the test. For all samples, the COF values appeared to be very stable beyond a distance of 500-600m in the wear tests.

3.5. Tribofilm behavior at higher load in DMEM+HyaA solution:

Figure 4 shows the formation of tribofilm on the surface of the LENS™ processed CoCrMo-3%HA composite, and this tribofilm appeared to be similar in morphology to the tribofilm formed during wear in DI water medium (Sahasrabudhe et al., 2018), i.e., dark areas being regions of tribofilm formation. However, even if it may seem similar, the chemical nature of the tribofilm may be different when the wear tests are done in pure DMEM or DMEM+HyaA solutions. More specifically, cell media and high viscosity hyaluronic acid solution may contribute to the film formation under reciprocating load. This tribofilm was shown to be non-existent at the high load of 20N (see Figure 5), likely due to breakdown during testing under such high loading.

Figure 4:

Figure 4:

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Surface damage images of CCM + 3%HA composites at 15N load under the DMEM+50HyAcid environment.

Figure 5:

Figure 5:

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SEM Micrograph showing an absence of tribofilm in CCM-3HA composite at 20N load under the DMEM+50HyAcid environment.

4. Discussion

CoCrMo-HA composites were successfully manufactured via LENS™ based additive manufacturing technology. Extensive wear testing in biological media was performed to evaluate the efficacy of the in situ tribofilm to prevent wear-induced ion leaching and wear degradation of the base CoCrMo material. It was hypothesized that the formation of a tribofilm would significantly reduce wear-induced damage and ion release from the underlying CoCrMo material, similar to that observed in previous work in DI water media.

4.1. Development of tribofilm in HA-containing composites:

As expanded upon in (Sahasrabudhe et al., 2018), the HA-containing composites maintain calcium phosphate particles that are the softer phase in the overall structure at the articulating surface. Under loading and successive wear due to the applied force, these particles tend to wear from the coating itself and fuse to form the thin film observed in Figure 4, as evidenced by the difference between the HA-containing composite and the non-HA containing structure. As the testing continues, these fused particles begin to form the film that extends along the coating surface due to the worn ball's motion. This film deters further ion release by providing a diffusion barrier, as has been similarly evidenced in previous studies with similar alloys and cast-iron (Balagna et al., 2012; Büscher et al., 2005; Neville and Kollia-Rafailidi, 2002; Walker et al., 2013). This is mechanistically depicted in Figure 6. The tribofilm formed in the present work in the DMEM+HyaA medium is comparable to that shown in the previous work using DI water as the medium (Sahasrabudhe et al., 2018). Namely, discontinuous high-contrast (via SEM) areas where HA particles have fused under loading. This indicates that the medium has limited to no effect on the tribofilm formation, and the added benefit of DMEM proteins and a representative hyaluronic acid additive does not deter the formation of the HA tribofilm.

Figure 6:

Figure 6:

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Mechanism of action of tribofilm formation in DMEM + HyaA environment.

4.2. Effects of DMEM +HyaA mixtures on wear rates:

In general, the wear rates in pure DMEM and DMEM+HyaA mixtures were a magnitude lower than the wear rates observed in the DI water medium (Sahasrabudhe et al., 2018). This was the expected outcome since the DMEM and DMEM+HA media have a higher lubricating effect naturally occurring in the body due to the DMEM media's proteins. For the DMEM+HyaA mixtures, the lubrication effect is partly from the protein in the DMEM and partly from the gel-like nature of the HyaA solution itself. As seen from the viscosity measurements, the DMEM+HyaA mixtures are also more viscous than the pure DMEM and DI water media, potentially playing a role during testing. If the wear rates of the tests conducted in different DMEM and DMEM+HyaA mixtures are compared (Figure 3), not a very significant trend is observed across loading with the addition of hyaluronic acid, indicating that the worn ball is reaching a similar depth concerning loading in comparison to the DMEM mixture without hyaluronic acid, as well as that reported in DI water medium (Sahasrabudhe et al., 2018). In general, the DMEM+50%HyaA mixture showed the lowest wear rates, as well as the lowest concentration of the leached cobalt and chromium ions, indicating that the environmental conditions influence the wear performance due to the presence of both HA-tribofilm formation as well as any added effects from DMEM proteins and HyAcid additive. In most cases, however, the wear rates and the ion concentrations were the lowest for the CoCrMo-3%HA composites, indicating that HA is also playing a significant role in decreasing ion release and wear compared to the CoCrMo alloy on its own in biological media. In some cases, at the load of 20N, the wear rate was higher, which may have been caused due to the breakdown of the tribofilm at such a high load, without regard to different media. This rise is also observed in all the DMEM, DMEM+HA mixtures, and in some cases, the numbers were higher even for the wear test done at 15N. When the CoCrMo-1%HA and CoCrMo-3%HA composites are subjected to wear, there is a formation of a tribofilm due to the added calcium phosphate, as described earlier. The tribofilm of this nature is thought to be stable only up to a specific load. Beyond this load, there may be too much stress-induced damage, especially from a hard counter material like silicon nitride, leading to complete or partial destruction of the tribofilm under loading (see Figure 5). As soon as the tribofilm is damaged, a spike in the wear rate as a fresh surface is more susceptible to wear damage when exposed. Despite this fact, the ion release from the composite at 20N is still lower than the counterpart CoCrMo alloy in the solutions containing hyaluronic acid. The presence of this wear-resisting tribofilm is evidenced in Figure 4. In comparison to Balagna et al. (2012), which investigated cast and wrought CoCrMo tested in Bovine serum solution, the wear rates presented here are significantly lower, namely 3x10−6 to 2x10−5 (mm3/Nm) in comparison to the lowest of ~1.5x10−6 (mm3/Nm) for the CoCrMo-3%HA at 20N with a DMEM-50%HyaA (Balagna et al., 2012).

4.3. Effects of DMEM +HyaA mixtures on COF and tribofilm formation:

The coefficient of friction values for all the tests performed in pure DMEM solution and DMEM+HyaA solution were much lesser as compared to the values measured during wear test in DI water, namely 0.15-0.25 range which was lower than previously reported 0.50-0.75 range (Sahasrabudhe et al., 2018). This was expected due to the increased lubrication and increased viscosity as compared to the DI water medium. However, the COF values were slightly increased for the wear tests performed in the DMEM+HyaA solution compared to the pure DMEM solution. In addition to the increase in the values, the values also appeared to have reached a steady-state faster with lesser fluctuations in the values with distance, indicating the formation of a thin tribofilm, as evidenced in Figure 4. When the same COF values were compared for the different LENS™ processed CoCrMo-HA composites, a trend similar to that observed in the DI water wear tests was observed (Sahasrabudhe et al., 2018). The COF rose slightly with increasing the HA content, and along with the rise, the COF reach a steady-state sooner than the sample with no HA addition. This could have happened due to the direct effect of the tribofilm formation. In the alloy with no HA addition, the COF either fluctuated more or showed a steady rise after the initial drop. This could be due to the absence of the tribofilm in materials that could have caused high and low wear periods characterized by COF fluctuations. In the material with HA addition at lower loads of 5N, 10N, and 15N, there could be the formation of a tribofilm similar to that found during the wear tests in the DI water medium. However, since there are many organic constituents in the pure DMEM solution and the DMEM+HyaA solution, the nature of the tribofilm may be different. Nevertheless, this tribofilm remained effective in reducing the wear damage and successfully reduced the release of metallic ions. However, at the load of 20N, the wear rates show a slight increase and a rise in the COF for the samples tested at 20N. These factors point to a probable breakdown of the tribofilm at 20N load for the LENS™ processed CoCrMo-3%HA composites. In the related work of Doni et al. (2013), CoCrMo tested in NaCl, and dry-sliding conditions maintained COF as high as 0.3-0.4, indicating that the COF of the HA-reinforced composites here was much lower in comparison to the hot-pressed variant (Doni et al., 2013). Comparing the two different hyaluronic acid-containing solutions, it is clear that the higher amount of hyaluronic acid additive decreases the significant ion release from the surface. Despite the overall decrease, the HA-containing composites still had the lowest ion release amount compared to the CoCrMo control, indicating that HA incorporation can significantly reduce overall wear-induced damage at comparable conditions to articulating joints.

5. Conclusion

Increases in wear degradation-based health issues related to articulating-surface implant materials (namely CoCrMo), coupled with patient-specific geometric needs, require innovative materials and processing technologies to improve long-term patient health in future applications. To this end, CoCrMo composites containing calcium phosphate in the form of hydroxyapatite were manufactured to study the influence of the reinforcing phase on the tribofilm formation in biologically-relevant conditions. During testing, all coefficients of friction remained in the 0.15-0.25 range, which was much lower than the previously reported 0.50-0.75 range in DI water as well as others in the literature, indicating that the DMEM + hyaluronic acid media plays a significant role in reducing frictional contact loads in the body. At loads below 20N, each HA-reinforced composition demonstrated reduced ion release compared to the control CoCrMo material; however, at 20N load, the tribofilm exhibited a breakdown resulting in higher wear rates still lower overall Co and Cr ion release than the control CoCrMo. Our results point towards utilizing next-generation, controlled reinforcing phases in CoCrMo for improved wear resistance in articulating surface applications in the biomedical industry.

Highlights.

  • CoCrMo-CaP composites exhibit lower wear rates and coefficient of friction (COF) in DMEM media than DI water.

  • Hyaluronic acid addition to media plays a significant role in decreasing the wear rates and COF under reciprocating loading.

  • At loads higher than 15N, the CaP-tribofilm exhibited a breakdown resulting in higher wear rates but still lower overall ion release than the CoCrMo control.

6. Acknowledgement

The authors acknowledge financial support from the National Science Foundation under grant # CMMI 1934230 (PI- Bandyopadhyay) and CMMI 1538851 (PI- Bandyopadhyay). The authors also acknowledge financial support from the National Institute of Arthritis and Musculoskeletal and Skin Diseases of the National Institutes of Health under Award Number R01 AR067306. The content is solely the authors' responsibility and does not necessarily represent the National Institutes of Health's official views.

Footnotes

Conflict of interest

None.

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