Austenitic stainless steels and titanium alloys are the two widely used metallic materials for biomedical applications, including devices for bone fixation, partial/total joint replacement and spring clips for the repair of aneurysmal defects. These materials are corrosion resistant and have the necessary mechanical strength and biocompatibility. [1][2][3] A potentially transformative approach to favorably modulate and improve the cellular response of biomaterials is to utilize nanograined (NG)/ultrafine-grained (UFG) materials in lieu of conventional coarse-grained materials. Ultrafine structures may provide benefits of enhanced cellular attachment, stimulate metabolic activity, and up-regulate protein formation. [4][5][6][7][8][9] These properties provide the motivation to study bulk nanostructured metals with the aim to enhance metabolic compatibility and cellular response, in addition to the use of thinner bioimplants and high strength/weight ratio, especially for bone growth. The focus here is to combine fundamental aspects of materials science, engineering, and cellular and molecular biology to favorably modulate cell-substrate response between preosteoblasts and NG/UFG austenitic stainless steels. We have used NG/UFG austenitic stainless steel processed by a novel procedure involving controlled phase reversion of strain-induced martensite in a cold rolled austenitic stainless steel. [10][11][12][13] The cellular response of NG/UFG austenitic stainless steel is compared with conventional coarse-grained austenitic stainless steel. Experiments on the effect of NG/UFG structures have shown that the proliferation, morphology and spread of pre-osteoblasts are favorably modulated and significantly different from conventional coarse-grained austenitic stainless steel. Additionally, immunofluorescence studies demonstrated stronger vinculin signals associated with actin stress fibers in the outer regions of the cells and cellular extensions on NG/UFG stainless steel substrate. These observations suggest enhanced cell-substrate interaction and activity. The differences in cellular response between NG/UFG and coarsegrained stainless steel substrates are attributed to grain size effects and hydrophilicity of steel substrates. These intriguing observations open up a new avenue for nanostructured materials with combined benefits of biological and mechanical properties (high strength/weight ratio).
Metallic materials with sub-micron to nanometer-sized grains provide surfaces that are different from conventional polycrystalline materials because of the large proportion of grain boundaries with high free energy. [14] Thermo-mechanical processing (TMP) is one of the primary methods to achieve grain refinement in metals. [15][16][17][18] Grain refinement limits imposed by conventional TMP can be overcome by the application of extensive plastic deformation, which leads to the formation of submicron or ultrafine grain structures in metallic materials. [19,20] The laboratory scale methods that have generally been adopted to obtain NG/UFG materials include equal channel angular pressing (ECAP), [20,21] accumulative roll bonding (ARB), [22][23][24] high pressure torsion (HPT), [5,6,25,26] multiple compression, [27] and upsetting extrusion. [28] However, the ductility of the materials produced by these methods is low compared to the coarse-grained materials. High strength-ductility combination is an important mechanical property requirement for the long-term stability of metallic bioimplants. In this context, the work presented here is of particular significance because high ductility was obtained in NG/UFG material. NG/UFG austenitic stainless steel was produced through controlled phase-reversion of strain-induced martensite in a cold rolled austenitic stainless steel. Multi-pass cold deformation (40-65%) of austenite at room temperature led to strain-induced transformation of austenite (face-centered cubic g) to dislocation cell-type martensite (bcc a 0 ), which upon annealing in the temperature range of 650-850 8C transforms back to austenite via diffusional reversion mechanism. The temperature-time annealing sequence depends on the percentage of cold deformation (see experimental for details). Forming experiments carried out using conventional low-rate hydraulic bulging and by high velocity electro-hydraulic impulse methods for the NG/UFG austenitic stainless steel proved that the formability of the fine-grained steel was adequate for various applications that require high strength and good formability. In both the tests, the height of the highest solid dome was equal (36 mm) and that was achieved using 80 bar pressure in the conventional test and by 13.8 kJ energy in the high velocity test. Electro-hydraulic impulse forming into a mold was also tried. The COMMUNICATION