Microrobots or motors can be used for different applications such as drug delivery, sensing, environmental remediation, and manipulation of small objects. Among them magnetic micropillars, driven by external magnetic fields, can be used for drug delivery in the human body. Although the fabrication, the magnetic properties and the motion of these microdevices have been extensively studied, their degradation in corrosive environments of the human body has been scarcely investigated. In this work, we report on a combined microscopic, analytical and electrochemical characterization of NiCo and NiCo/Au coated micropillars (with diameter and length dimensions of 1µm and 10 µm respectively) produced by template-assisted electrodeposition. The NiCo micropillars were coated by Au in order to reduce the Ni and Co ions release (toxic elements for the human body) and enhance the biocompatibility. The long-term degradation mechanisms of the micro- structures were investigated in phosphate-buffered solution (PBS) containing bovine serum albumin (BSA) protein to simulate inflammatory conditions close to the human body environment. To this end, scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDXS), atomic force microscopy (AFM), scanning Kelvin probe force microscopy (SKPFM), and electrochemical measurements (potentiodynamic polarization, electrochemical impedance spectroscopy, and Mott–Schottky analysis) were used to reveal the chemical composition, the electronic properties, the electrochemical behaviour and the metal ion releasing process on both NiCo and NiCo/Au coated micro-pillars. SEM/EDXS chemical maps showed a homogeneous chemical composition of both NiCo and NiCo/Au coated micro-pillars. According to electrochemical results, NiCo/Au pillar showed the lower corrosion current density and higher corrosion potential (10 nA.cm-2 and -160 mV vs. Ag/AgCl3M KCl) respect to NiCo pillars with values of 40 nA.cm-2 and -300 mV vs. Ag/AgCl3M KCl. MS analysis showed that the NiCo pillars have a lower space charge region and a higher number of defects density (Nd= 5.8×1022 cm-3, n-type semiconductor character) in comparison with the NiCo/Au coated pillars (Nd= 5.1×1022 cm-3). Likewise, NiCo/Au coated micro-pillars have the higher value of flat band potential (EFB, an important parameter for electronic conductivity) than the NiCo micro-pillars. AFM and SKPFM analyses revealed the presence of some micro and nanoporosity on the Au coating covering the NiCo pillars. The difference in the Volta potential between the Au coating and the NiCo substrate can drive a preferential dissolution of the substrate though the pores (galvanic coupling).

A comprehensive insight on the degradation of NiCo and NiCo/Au coated micro-robots during immersion in simulated body fluids

Ehsan Rahimi
;
Ruben Offoiach;Lorenzo Fedrizzi;Maria Lekka
2020

Abstract

Microrobots or motors can be used for different applications such as drug delivery, sensing, environmental remediation, and manipulation of small objects. Among them magnetic micropillars, driven by external magnetic fields, can be used for drug delivery in the human body. Although the fabrication, the magnetic properties and the motion of these microdevices have been extensively studied, their degradation in corrosive environments of the human body has been scarcely investigated. In this work, we report on a combined microscopic, analytical and electrochemical characterization of NiCo and NiCo/Au coated micropillars (with diameter and length dimensions of 1µm and 10 µm respectively) produced by template-assisted electrodeposition. The NiCo micropillars were coated by Au in order to reduce the Ni and Co ions release (toxic elements for the human body) and enhance the biocompatibility. The long-term degradation mechanisms of the micro- structures were investigated in phosphate-buffered solution (PBS) containing bovine serum albumin (BSA) protein to simulate inflammatory conditions close to the human body environment. To this end, scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDXS), atomic force microscopy (AFM), scanning Kelvin probe force microscopy (SKPFM), and electrochemical measurements (potentiodynamic polarization, electrochemical impedance spectroscopy, and Mott–Schottky analysis) were used to reveal the chemical composition, the electronic properties, the electrochemical behaviour and the metal ion releasing process on both NiCo and NiCo/Au coated micro-pillars. SEM/EDXS chemical maps showed a homogeneous chemical composition of both NiCo and NiCo/Au coated micro-pillars. According to electrochemical results, NiCo/Au pillar showed the lower corrosion current density and higher corrosion potential (10 nA.cm-2 and -160 mV vs. Ag/AgCl3M KCl) respect to NiCo pillars with values of 40 nA.cm-2 and -300 mV vs. Ag/AgCl3M KCl. MS analysis showed that the NiCo pillars have a lower space charge region and a higher number of defects density (Nd= 5.8×1022 cm-3, n-type semiconductor character) in comparison with the NiCo/Au coated pillars (Nd= 5.1×1022 cm-3). Likewise, NiCo/Au coated micro-pillars have the higher value of flat band potential (EFB, an important parameter for electronic conductivity) than the NiCo micro-pillars. AFM and SKPFM analyses revealed the presence of some micro and nanoporosity on the Au coating covering the NiCo pillars. The difference in the Volta potential between the Au coating and the NiCo substrate can drive a preferential dissolution of the substrate though the pores (galvanic coupling).
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11390/1196252
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