Researchers from the Indian Institute of Science, Bangalore, ask: Can surface mechanical attrition treatment be considered a promising technology for the development of high performance orthopaedic implants?
Metallic materials such as commercially pure Ti (cp-Ti), Ti-6Al-4V, 316L stainless steel (SS), Co-Cr-Mo alloys have found widespread use in load-bearing orthopaedic devices . These materials are preferred over ceramic and polymeric biomaterials due to their superior mechanical properties, vis-à-vis strength and toughness. Furthermore, high corrosion resistance of these materials aids in maintaining the mechanical integrity of implants and prevent toxicity resulting from the excessive dissolution of metallic ions.
The performance of an implant is critically dependent on the mechanical and biological responses of the constituent materials. Mechanical responses include strength (tensile or compressive), fatigue strength, toughness, fracture toughness, and wear. Biological responses include osseointegration, toxicity and inflammation (caused by wound healing, bacterial infection or wear debris). While an implant can fracture because of fatigue, it can fail by loosening due to poor osseointegration. Incidentally, both failure by fatigue and interaction with the surrounding bone depend on the surface properties of an implant. Therefore, a well-designed surface engineering technique can concurrently improve the mechanical and biological responses of an implant.
The current surface engineering strategies for orthopaedic implants rely on chemical and topographical surface modifications . These strategies can positively impact the biological response, but may either have no impact or a negative impact on the mechanical response. For example, the high roughness that is preferred for implant stability can degrade its fatigue resistance. Therefore, there is a need for a surface engineering strategy that can simultaneously enhance the mechanical and biological responses of an implant.
Nanocrystalline metals are known to elicit superior biological response . They can be produced by severe plastic deformation techniques. Depending on the severe plastic deformation technique, both bulk and surface nanocrystalline metals can be produced. However, several instrumentation challenges render generation of bulk nanocrystalline metals a challenging task. On the other hand, surface severe plastic deformation techniques are simpler and can be more efficient than bulk severe plastic deformation techniques in producing a nanocrystalline material. Since the nanocrystallisation is induced by plastic deformation, its higher hardness, combined with the presence of compressive residual stress, can improve the fatigue life of a material. Therefore, surface severe plastic deformation is a strategy that has the potential to concurrently augment the mechanical and biological responses of metallic biomaterials.
Surface mechanical attrition treatment (SMAT) is a surface severe plastic deformation technique that can produce a nanocrystalline surface . In this process, a metal sheet placed in an enclosed chamber, is bombarded with hard steel balls travelling in random directions with high speed. The impact of the balls severely deforms the surface of a metal and induces nanocrystallisation.
A study was designed to produce nanocrystals on the surface of cp-Ti using SMAT. The effect of nanocrystallisation was evaluated on the in vitro response of human mesenchymal stem cells (hMSCs) and fatigue life of the metal in simulated body fluid (SBF). A commercially available 2mm thick sheet of cp-Ti was used. The sheet was processed by SMAT in a custom designed setup developed by Cosmic Industrial Laboratories, Bengaluru, India. The sheet was processed using 4.75mm diameter hardened steel balls for 30 min. Microstructural characterization to confirm nanocrystallisation was performed using transmission electron microscopy (TEM). The increase in surface hardness after SMAT was measured using micro-Vickers indentation. The surface topography was characterized using optical profilometer. The passively formed surface oxide was characterized using X-ray photoelectron spectroscopy (XPS). The electronic conductivity of the surface oxide layer was measured using Mott-Schottky plots. Surface wettability was evaluated by measuring the contact angle of de-ionized water using a contact angle goniometer. The fatigue tests were performed in corrosive SBF that mimics the salt concentration of blood plasma. The presence of corrosive fluid can accelerate the process of failure by fatigue and is termed as corrosion-fatigue. The number of cycles to failure were measured at various stress levels in the fatigue tests. The details of the experiments performed are provided elsewhere .
The biological response of the hMSCs was evaluated in vitro. hMSCs derived from the bone marrow of a 25-year-old male donor were obtained commercially . The attachment and proliferation of stem cells was measured after one day, three days and seven days by quantifying the amount of double stranded DNA using the picogreen assay. The osteogenic differentiation of hMSCs cultured on the sample surfaces in the medium containing osteogenic supplements was evaluated by measuring the mineral deposited at the end of 14 days using Alizarin Red S dye. The biological response observed, was related to the modulation in protein adsorption after surface nanocrystallization. Therefore, the adsorption of model protein bovine serum albumin was quantified directly using Bradford assay and indirectly using electrochemical impedance spectroscopy (EIS).
The TEM micrograph of cp-Ti as shown in Figure 1 confirms the formation of nanocrystalline grains at the surface after SMAT. The average surface grain size was 40nm. The hardness of the surface also increased form the initial value of 1.4 GPa to 2.2 GPa. Thus, severe plastic deformation after SMAT was able to produce a hard nanocrystalline surface of cp-Ti.
The surface topographies measured using optical profilometer are shown in Figure 2. The initial samples’ surface consisted of scratches due to the surface grinding process (Figure 2a). The scratches are broken by impact of balls during SMAT as seen in the corresponding image (Figure 2b). Despite visual changes, the values of surface roughness did not change after SMAT. The roughness (Ra) was consistent in the order of 0.3µm for both initial and SMAT samples. The surface wettability increased marginally as evidenced by a 10° reduction in the water contact angle after SMAT from an initial value of 90°. More remarkable changes occurred in the physical, chemical and electronic properties of the passively formed titanium oxide layer at the surface. XPS revealed that the surface composed solely of titanium oxide after SMAT whereas it was a mixture of metallic titanium and titanium oxide in the initial condition. XPS also showed that the thickness of the oxide increased after SMAT. The change carrier density of the semi-conducting titanium oxide also increased after SMAT, as determined by the Mott-Schottky plots, implying a higher electronic conductivity.
The number of cycles to failure during fatigue in SBF before and after SMAT are listed in Table 1. The cycles to failure increased after SMAT at all the stress levels implying an improvement in the fatigue resistance after SMAT. The increase in surface hardness and generation of compressive residual stress (~ 300 MPa) contributed towards an improvement in the fatigue life after SMAT.
The DNA quantification as an indicator of cell number is compiled Figure 3. The number of cells attached at day one was higher after SMAT. The cells proliferated on the surfaces of initial and SMAT samples. The ratio of DNA content at three days versus one day and seven days versus three days was calculated as an indicator of the proliferation rate. The values of proliferation rate at three days and seven days were 2.3 and 4.0 after SMAT, higher than the corresponding values of 1.9 and 3.7 for the initial condition. Therefore, nanocrystallisation by SMAT led to an increase in the attachment and proliferation of stem cells. The higher bioactivity of stem cells on the SMAT surfaces can be beneficial in achieving a faster recovery of patients after orthopaedic surgery. Mineral content deposited by stem cells at the end of 14 days cultured in osteogenic medium was similar in both SMAT and initial condition. The two common surface attributes that affect the cell responses are roughness and wettability. While roughness can vary the way a cell attaches, the wettability affects the cell response by altering the adsorption behaviour of the proteins. Both the roughness and wettability did not change significantly after SMAT and therefore, cannot be the major factors in modulating the cell response. The protein adsorption studies revealed that the adsorption of bovine serum albumin was higher on the initial samples compared to the SMAT samples. Since wettability and roughness did not vary, the change in protein adoption is attributed to the changes in the properties of the surface oxide formed after SMAT. The changes in composition, thickness and conductivity of the surface oxide can alter the adsorption behaviour of protein thereby affecting the cell response [5,6]. It is to be emphasised here that when a cell attaches on a metallic surface, it is not in contact with the metal itself but with the overlying surface oxide. Therefore, changes in oxide layer are of prime importance in fully understanding the response of cells to a metallic biomaterial.
This work demonstrates that surface nanocrystallisation by SMAT is a unique strategy for simultaneous augmentation of the fatigue life and stem cell response to cp-Ti. The higher surface hardness combined with compressive residual stress enhanced the fatigue life in simulated body environment. The changes in properties of the surface oxide layer modified the adsorption behaviour of proteins that consequently enhanced the attachment and proliferation of stem cells. Altogether, SMAT can be considered as a promising technology, which can be used in the development of high performance orthopaedic implants.
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Sumit Bahl is currently working as a Research Associate in the Department of Materials Engineering, Indian Institute of Science, Bangalore, India, after completing his PhD from the same department in March 2018, working on surface engineering of metallic biomaterials.
Bhavya Tulasi Aleti obtained her Master of Engineering in July 2017 from the Department of Materials Engineering, Indian Institute of Science, Bangalore, India. She is currently an employee of Tata Steel, India.
Satyam Suwas is a Professor in the Department of Materials Engineering, Indian Institute of Science, Bangalore, India and with a research focus on the mechanical behavior of metals.
Kaushik Chatterjee is an Associate Professor in the Department of Materials Engineering, Indian Institute of Science, Bangalore, India, leading a research group focused on biomaterials research.