Inkjet-Printed Transparent Temperature Sensors based on Organic Thermoelectrics for High Temporal Resolution Temperature Sensing in Optical Neural Stimulation

Light based localized heat generation (e.g. photothermal effect) has been proposed in various biomedical engineering applications. In particular, photothermal neural stimulation, either excitatory or inhibitory control of neuronal activities such as action potential, have been suggested in neural interfaces technologies for studying neural circuits or for neurological disorder treatments.[1-3] Because the signals of interest such as action potential of nerve cells are as fast as several kHz in bandwidth, there is a strong need for optically transparent and high-speed operating temperature sensor for in-depth analysis of the effect of temperature on the high-speed biological signals. However, currently many of micro-temperature sensors can neither operate at high-speed (&gt; a few kHz) nor provide optical transparency to be compatible with the optical heat generation mechanism.<br/>In this presentation, we will report a temperature sensor based on the thermoelectric effect resulting in millisecond temporal resolution by measuring the Seebeck voltage from temperature differences. 100 nm-thick inkjet-printed poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) film was used as p-type organic thermoelectric legs having a Seebeck coefficient of 13.5 µV/K and good optical transparency of 95 % in the visible spectrum. In addition, low-temperature inkjet printing allows the realization of organic thermoelectrics with metal contacts on a 100 µm-thick flexible substrate. After depositing parylene-C for biocompatible passivation, thermo-plasmonic gold nanoparticles were introduced to optically implement high-speed local temperature change by the photothermal effect.<br/>1-mm diameter spot of the gold photothermal layer was irradiated by short pulses (&lt; 50 msec) of 785 nm near infrared laser through the transparent PEDOT:PSS thermoelectric temperature sensor. As the local temperature on one end of PEDOT:PSS layers was changed in msec time scale, the Seebeck voltage was also generated across the PEDOT:PSS layers via the thermoelectric effect. After laser irradiation, the temperature and Seebeck voltage changes were measured simultaneously using infrared thermographic camera and a data acquisition system with a low-noise voltage preamplifier (sampling frequency: 25 kHz), respectively. From the experimental results, a good linearity of generated voltage as a response of the photothermal temperature change (minimum 10 msec) was clearly observed. By applying the measured RMS voltage noise across the PEDOT:PSS to the three-sigma rule, we calculated the limit of detection (LoD) as 2.35 °C.<br/>Therefore, we believe that our device can be used to directly measure photothermal temperature changes in biomedical/bioelectronics applicatons that reqiure a fast response rate (msec or below) and a few degrees of temperature changes (e.g. most of optical biomodulation experiments). In addition, the 95 % of the transmittance of PEDOT:PSS thermoelectric temperature sensor is sufficiently applicable to optical bioimaging techniques or optical neural recordings such as calcium or voltage sensitive dye recording providing simultaneous temperature sensing.<br/><br/><b>References</b><br/>1. Rastogi, S. K. et al. Remote nongenetic optical modulation of neuronal activity using fuzzy graphene. <i>Proc. Natl. Acad. Sci</i>. 201919921 (2020).<br/>2. Nelidova, D. et al. Restoring light sensitivity using tunable near-infrared sensors. <i>Science</i> <b>368</b>, 1108–1113 (2020).<br/>3. Paris, L. et al. Millisecond infrared laser pulses depolarize and elicit action potentials on in-vitro dorsal root ganglion neurons. <i>Biomed. Opt. Express</i> <b>8</b>, 4568 (2017).