(Invited) Inkjet Printing of Impedance Sensors for Living Tissues Monitoring

One of the most promising techniques recently applied to minimally invasive living tissues analysis is electrical impedance spectroscopy (EIS). This methodology is able to determine the state of the cells present in an organ by measuring variations in the impedance recorded between electrodes placed in contact with them [1]. The advantages of this approach with respect to invasive techniques like biopsy or others can be immediately individuated: no need to perform surgery on the patient, fast response, easiness and low cost. Moreover, by using EIS, different states (involving for example metabolic toxicity, perfusion loss or cancer development) can be reliably determined. Typically, EIS is performed placing electrodes directly inside the tissue of interest. The most convenient way to do this is by inserting microneedles containing the electrodes through the skin and directly into the target organ. Moreover, microneedles can be used also to deliver drugs in case of need [2]. For this reason, current research is focusing deeply on the development of microneedles characterized by the highest possible biocompatibility and the lowest invasivity. Interesting examples of micromachined needles exist in literature [3], but they all make use of standard microfabrication techniques with limited flexibility and high cost during the fabrication process. In the present paper, we discuss the production of EIS microneedles by inkjet printing coupled with electrodeposition. Both these techniques are more flexible than current state-of-the-art methodologies like lithography or sputtering. The shape of the devices is inspired to existing microneedle EIS sensors [3] and adapted to inkjet printing. To produce the sensors, SU-8 is printed to form a mask for the electrodes on a copper substrate. Pt is subsequently electrodeposited selectively inside the mask to form conductive tracks and planar electrodes for EIS. SU-8 is then inkjet printed on the electrodes to form the structural part of the sensor. Finally, the device is flipped and SU-8 is inkjet printed to form a mask on the surface of the sensor able to isolate the tracks from the sensing electrodes. At the end of the manufacturing process, sensors are characterized from the morphological and functional point of view. [1] P. Heroux, M. Bourdages, Ann. Biomed. Eng. 22(3), 328-337 (1994) [2] Y.C. Kim, J.H. Park, M.R. Prausnitz, Adv. Drug Deliv. Rev. 64(14), 1547-1568 (2012) [3] M. Tijero et al., Biosens. Bioelectron. 24, 2410-2416 (2009)