In spite of the numerous benefits offered by nanophase materials in general, there are major concerns in the possible use of these materials in the human body due to cytotoxicity. [20] Recently, it has been reported that hepatocyte viability in the presence of CdSe quantum dots [21] and keratinocyte viability in the presence of single-walled carbon nanotubes [22] are adversely affected by the presence of these nanomaterials. In this context, the results from this work, indicating the non-cytotoxic behavior of Si NWs with regard to fibroblast-cell viability, are encouraging and useful for future biomedical applications. Additional in-vitro assays involving orthopedically relevant osteoblast cell lines are in progress.
This work demonstrates the ability of Si NWs to facilitate the growth of uniform, synthetic bone coatings along their surface and to support the facile proliferation of fibroblast cells in their presence. Unlike most biodegradable polymers and ceramics used in orthopedics, nanophase Si in the form of Si NWs is responsive to electrical bias. Therefore, these results set the stage for the fabrication of a broad range of semiconducting Sibased materials of potential value in orthopedics and beyond.
Experimental
Synthesis of Silicon Nanowires (Si NWs): Previously cleaned 1.5 cm 0.5 cm pieces of low resistivity (0.008±0.02 X cm), Sb-doped Si <100> wafers were used as substrates for Si NW synthesis. As an adhesion layer, drops of a 0.1 % (w/v) poly(L-lysine) solution (Ted Pella, Inc.) were first introduced onto the surface and allowed to dry, followed by addition of a layer of 5 nm gold nanocrystals at an approximate concentration of 7.5 10 12 particles per 1.5 cm 0.5 cm wafer. This wafer was then placed inside an alumina boat in a custombuilt quartz reactor tube equipped with a 6 cm oven. The catalyst was annealed at 600 C inside the reactor under a He flow (3000 sccm) for ~2 h and then exposed to SiH 4 (0.5 % in He) at a flow rate of 40 sccm for 5 min. A dense network of Si NWs with diameters in the range of 120±180 nm and lengths up to 150 lm was obtained.
Electrochemical Instrumentation: Cathodic-bias experiments were performed in an electrochemical cell where the Si wafer with attached Si NWs was the working electrode, platinum foil was the counter electrode, and simulated body fluid (SBF) solution was the electrolyte. SBF was prepared by mixing 1.37 M NaCl, 0.03 M KCl, 0.042 M NaHCO 3 , 0.01 M K 2 HPO 4 , 0.015 M MgCl 2 , 0.025 M CaCl 2 , and 0.005 M Na 2 SO 4 solutions into a buffer of tris(hydroxymethyl)aminomethane and adjusted with HCl to pH 7.30. A cathodic bias of 1.1 mA cm ±2 (Keithley Model 236 Source Measure unit) was applied to the Si wafer either for a continuous 3 h period, or alternatively, for a bias for 1 h followed by a 1 week soak in SBF under zero bias. At the end of the experiment, the wafer was rinsed with deionized water and air-dried before characterization.
Cell-Proliferation Experiments: The biocompatibility of Si NWs was studied by monitoring the in-vitro cell-proliferation assays of human kidney fibroblasts in the presence of Si NWs. Human kidney fibroblasts from cell line 293 (American Type Culture Collection) were harvested in Dulbecco's Modified Eagle's Medium (DMEM) containing 10 % fetal bovine serum (FBS). The proliferation of cells was measured by counting the number of cells using a Bright-Line Hemacytometer. The initial cell density was 1.00 10 4 cells mL ±1 , to which 0.067 mg Si NWs was added. The cells were counted on days 3, 5, and 7 to monitor the proliferation. By day 7, the cell density of fibroblasts had grown to a value of 1.36 10 6 cells mL ±1 , compared to an average value of 1.15 10 6 cells mL ±1 for those wells lacking Si NWs (control).