(Invited) More Than Moore Applications with Thin-Film-Transistors Array Substrates from Liquid Cristal Display: New Devices for Biological Cells Analyses
Authors: Tixier-Mita Agnes , Ihida Satoshi , Cathcart Grant Alexander , Shaik Faruk Azam , Toshiyoshi Hiroshi 
Published: February 2020

Thin-Film-Transistors technology has been a famous technology since 20 years for the development of Liquid-Cristal-Displays, for televisions, smart-phones or video games screens applications. Thanks to that technology, a dense array of pixels, individually controllable by an array of independent transistors, can be achieved, allowing screens with very high resolution (pixels of some ten micrometers square). The key point is the fabrication of the array of transistors by deposition technique on a substrate which are usually glass or transparent plastic. The cross-section of a TFT-LCD screen shows that it consists into three different layers attached together. The upper layer (or upper glass) is a glass covered on one side with a polarizer and on the other side, with a color filter. This one is in contact with the second layer which is the liquid crystal layer. The third layer (or lower glass) is a glass covered with a large and dense array of independent and transparent micro-electrodes (made in Indium Tin Oxide, ITO), individually controllable by the array of independant transistors. That lower glass is called here “TFT-substrate” and is the key device in the research described hereafter.

In parallel to the development of TFT technology, since about 30 years, micro-technology has seen tremendous headways thanks to the development of micro-fluidics and bio-microsystems. Bio-microsystems require transparent substrates, for the ease of optical observation through the device, and micro-fluidics to confine the liquid environment usually necessary. More recently, the integration of some electronic components, from simple micro-electrodes to more complex ISFET (Ion-Sensitive Field Effect Transistors) have widened the frame of applications in that area. However, in such devices, alignment between the electronic component and the biological entity remains a key issue and is tricky. A large and dense array of independent electronic components would avoid to care about alignment, but standard microtechnology hardly allow that solution.

The solution can be brought by the TFT technology, as it provides such an array of independent electronic components. Such substrate becomes a very interesting device for biological applications, as it will be demonstrated in this talk.

The TFT technology, at first mainly applied to LCDs, diversifies its applications in a “More than Moore” objective, proposing new and unique solutions for biological purposes: an electrically controllable transparent substrate. In addition, the transistor array, initially used only as simple switches to control the electrodes ON and OFF state, becomes an active part in the device, allowing not only DC or AC signal application, but also sensing, either through the micro-electrodes, or directly being themselves a possible array of ISFETs.

Our group aims at demonstrating the possible applications, as well as the limitations, of TFT-substrates in the field of biological cell research. The structure of the TFT-substrate is reported in Figure, as well as some experimental results. The substrate has been obtained by detaching by hand the upper glass from the lower glass of a smart phone screen, and then by cleaning it with organic cleaning, to remove the liquid crystal. Each micro-electrode is connected to the drain of each transistor. The sources of the TFTs are connected line by line to the pad of the corresponding line. The gates of the TFTs are connected column by column to the pad of the corresponding column. When one source line and one gate line are ON, the TFT at its intersection, as well as the attached electrode becomes ON. Usual TFT devices are used under DC potential. However, for biological purposes, AC potential is needed. Characterization of the output signal at the drain (the micro-electrode), when a DC voltage is applied at the gate and an AC voltage at the source, has been performed. It showed an attenuation of the output signal with the decreasing gate and an attenuation of -20dB for each decade of the frequency above 1 kHz. Despite these attenuations, several results have already been successfully obtained. Dielectrophoresis on micro-beads and on yeast cells have demonstrated a negative dielectrophoretic force on micro-beads while a positive dielectrophoretic force was obtained on yeast cells at 100 kHz. Sensing was also performed using the same substrate. Impedance measurements on yeast cells was done, showing that the impedance between two adjacent micro-electrodes was increasing, as expected, with the number of cells, at a frequency of 10 kHz. Impedance measurements were also performed on live and dead yeast cells, showing a clear decreasing of impedance, as expected, in the case of dead cells. pH sensing was also performed. Other experiments are under investigation for more dedicated purposes.

Figure 1