M. Jobin, C. Pellodi, C. Sandu, G. Benvenutti, A. Sandoz, L. Stoppini
hepia / HES-SO University of Applied Sciences of Western Switzerland,
Keywords: biocompatible TCO, electrostimulation, full wafer metrology, chemical beam epitaxy
Summary:Our first motivation is to develop a biocompatible and versatile (in terms of electrical sheet resistivity) transparent conductive oxide (TCO) which can be used as thin electrodes in biomedical devices. Indeed, the most common TCO, namely ITO (indium tin oxide) is not convenient because of its poor biocompatibility and its predictable lack of availability at a reasonable price in the near feature (indium). Titania (TiO2) based materials, on the other hand, appear as a prime choice as TiO2 is the flagship material in term of biocompatibility. Its large bang gap (3eV) makes it optically transparent in the visible range: consequently, the device can be used under an epi-fluorescence microscope during the test, which is very valuable in most experiments. Titania is a highly resistive material that can be made electrically conductive with a small concentration of Nb ranging from 2% to 10%, i.e. up to the limit of solubility. Conductivity of typically 0.4 Ohmcm has been reported which makes Nb:TiO2 competitive to ITO. We have designed a versatile device based on TiO2:Nb thin films, which can be used to study the electro-stimulation of various cell cultures, typically neurons or cardiocytes. The optimal TiO2:Nb film parameters have been obtained with the help of combinatorial Chemical Beam Epitaxy (CBE). CBE is a UHV deposition technique which enables various types of gradients such as chemical and thickness gradients. Our bio-electrodes pattern consists of three spatially separated patterns, each containing three set of electrodes. The two outer sets of electrodes measure the effects of a dc or ac electric field on the cell, while the electrode can be used the measure the effect of a dc or ac electric current. The encapsulation of the bio-electrodes on the final device is shown in Fig. 1 Apart from the integrated circuit (shown in green in Fig 1)), the other layers are made from 1mm thick PMMA layers. The fluidic channels and the openings in the PMMA layers have been made by laser machining with a CO2 laser. The assembled device is placed in an incubator and electrically connected sine wave generator. Typical signals were 50mVpp at 1 Hz. As explained above, we have used a combinatorial approach for the Nb:TiO2 thin film deposition. The 4’’ samples has been investigated with our home-made a full-wafer metrology instrument, which allows for the optical thickness and the sheet resistivity characterizations. The optical thickness nd (n is the refractive index and d is the thickness) can be obtained with the white light reflectivity spectrum. Representative results on Nb:TiO2 electrodes is shown in Fig 2a for homogeneous thicknesses . An example of the sheet resistivity mapping is shown in Fig 2b for a TiO2:Nb film with an homogeneous thickness, but with a Nb chemical gradient. The resistivity cross section [Ohm/mm] in shown in Fig 2c). In Fig 3 is shown an example of application, where the cell growths clearly depends on the electrical stimuli.