Owing to its good biocompatibility and low cost, stainless steel is one of the most widely utilized biomaterial. However, longtime assessment of stainless steel has shown problems related to material degradation, especially localized corrosion and bio-film formation. In addition, the leaching of toxic nickel and chromium ions from stainless steel leads to additional health complications. Here, we utilized submerged friction stir processing, a severe surface deformation technique for significantly enhancing its durability, bio-activity as well as antibacterial resistance. The processing was done with a wide variation in strain rates to produce tunable surface microstructure. High strain-rate processing resulted in nearly single-phase fine-grained microstructure, while slow strain-rate processing developed a dual-phase fine-grained microstructure. The bio-corrosion rate of processed steel was reduced by more than 60 \% along with significant enhancement in the pitting resistance. The processed steel showed nearly no bacterial adhesion/biofilm formation, evaluated using S. aureus and E. coli bacterial strains. Further, the processed stainless steel surface demonstrated minimum leaching of the toxic elements, significantly enhancing its appeal for bio-implant applications. The observed behavior was explained based on the formation of a stable passive layer, rich in Cr2O3, as determined using x-ray photoelectron microscopy (XPS) and increased hydrophilicity. © 2020 Elsevier B.V.

Harpreet Singh Grewal

Mechanical Engineering

(SoE) School of Engineering

Harpreet Singh

Perumal G.

<p>Shiv Nadar University is a comprehensive, multidisciplinary, research-focused, and student-centric University offering a full range of academic programs at the undergraduate, postgraduate and doctoral level. With state-of-the-art infrastructure, the University comprises of academic wings, sophisticated labs, international standard sports facilities, amphitheaters, auditorium, conference rooms and smart classrooms. The University&rsquo;s goal is to become internationally recognized for the quality of its research and creative endeavors and their applicability to improving quality of life, generating new insights and expanding the boundaries of human knowledge creativity. Committed to excellence in teaching, research and service, the University aims to serve the higher education needs of India and the world beyond.</p>

Shiv Nadar University

Colloids and Surfaces B: Biointerfaces

The leaching out of toxic elements from metallic bioimplants has serious repercussions, including allergies, peripheral neuritis, cancer, and Alzheimer's disease, leading to revision or replacement surgeries. The development of advanced structural materials with excellent biocompatibility and superior corrosion resistance in the physiological environment holds great significance. High entropy alloys (HEAs) with a huge compositional design space and outstanding mechanical and functional properties can be promising for bioimplant applications. However, microstructural heterogeneity arising from elemental segregation in these multiprinciple alloy systems is the Achilles heel in the development of next-generation HEAs. Here, we demonstrate a pathway to homogenize the microstructure of a biocompatible dual-phase HEA, comprising refractory elements, namely, MoNbTaTiZr, through severe surface deformation using stationary friction processing (SFP). The strain and temperature field during processing homogenized the elemental distribution, which was otherwise unresponsive to conventional annealing treatments. Nearly 15 min of the SFP treatment resulted in a significant elemental homogenization across dendritic and interdendritic regions, similar to a week-long annealing treatment at 1275 K. The SFP processed alloy showed a nearly six times higher biocorrosion resistance compared to its as-cast counterpart. X-ray photoelectron spectroscopy was used to investigate the nature of the oxide layer formed on the specimens. Superior corrosion behavior of the processed alloy was attributed to the formation of a stable passive layer with zirconium oxide as the primary constituent and higher hydrophobicity. Biocompatibility studies performed using the human mesenchymal stem cell line, showed higher viability for the processed HEA compared to its as-cast counterpart as well as conventional metallic biomaterials including stainless steel (SS316L) and titanium alloy (Ti6Al4V). Copyright © 2020 American Chemical Society.

ACS Applied Bio Materials

Enhanced Biocorrosion Resistance and Cellular Response of a Dual-Phase High Entropy Alloy through Reduced Elemental Heterogeneity

<p>Biofilm related bacterial infection is one of the primary causes of implant failure. Limiting bacterial adhesion and colonization of pathogenic bacteria is a challenging task in health care. Here, a highly simplistic processing technique for imparting antibacterial properties on a biomedical grade stainless steel is demonstrated. Low-temperature high strain-rate deformation achieved using submerged friction stir processing resulted in a nearly single phase ultra-fine grain structure. The processed stainless steel demonstrated improved antibacterial properties for both Gram-positive and Gram-negative bacteria, significantly impeding biofilm formation during the in vitro study. Also, the processed stainless steel showed better compatibility with human fibroblasts manifested through apparent cell spreading and proliferation. The substantial antibacterial properties of the processed steel are explained in terms of the favorable electronic characteristics of the metal-oxide and by using classical Derjaguin–Landau–Verwey–Overbeek (DLVO) and the extended DLVO (XDLVO) approach at the cell–substrate interface. © 2019, © 2019 Informa UK Limited, trading as Taylor &amp; Francis Group.</p>

Biofouling

Enhanced antibacterial properties and the cellular response of stainless steel through friction stir processing

In the current study, surface modification of austenitic stainless steel, SS316L, was done using friction stir processing. The strain rates during processing were varied over a wide range by using two different tool rotational speeds of 388 rpm and 1800 rpm. The processing was done at two different conditions of: (i) ambient cooling and (ii) submerging in liquid bath maintained at 0 °C. Friction stir processing resulted in significant grain refinement from 22 μm for the as-received alloy to 0.67 μm for the sample processed at 388 rpm under submerged cooling condition. A relationship between the final recrystallized grain size and Zener-Holloman parameter was developed as Z=72.84/d. Deformation texture for processed specimens showed strong strain rate and temperature dependence. Sample processed at high temperature and low strain-rate showed copper-type texture while low temperature and high strain-rate promoted brass-type texture. Hardness of processed specimens showed an increase while elastic modulus decreased after processing which is explained by texture evolution during processing. Tribological properties of processed specimens were evaluated using reciprocating wear tests and slurry erosion tests. The processed samples showed significant improvement in the wear and erosion resistance and were found to have strong correlation with the flow work given as hardness to elastic modulus ratio. © 2017 Elsevier B.V.