2000014 (1 of 9) © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmattechnol.de FULL PAPER Highly Linear and Stable Flexible Temperature Sensors Based on Laser-Induced Carbonization of Polyimide Substrates for Personal Mobile Monitoring Srinivas Gandla, Muhammad Naqi, Mingoo Lee, Jung Joon Lee, Yoochan Won, Pavan Pujar, Junchul Kim, Sunghoo Lee,* and Sunkook Kim* DOI: 10.1002/admt.202000014 diferent structural forms have been syn- thesized to fabricate various electronic devices and sensors due to their unique functional properties. Despite its unique features, straightforward synthesis, and most importantly patterning, of high quality (high conductivity) carbon struc- tures in a scalable approach is quite chal- lenging. Toward this, initial groundworks have already shown promising routes to produce highly conductive carbon struc- tures. Over the past few decades, decrease in the electrical resistivity of polymers have been reported through ion beam bombardment, pyrolysis by thermal radia- tion, and visible laser ablation by photo- thermal or photophysical. [1–5] Among these, initial observations took place to convert polyimide substrates into electrically conductive flms (10 16 to 3 × 10 -3  Ω cm) is by ion beam bombardment in the keV range, ascribed as beam-induced carbonization, in which the chemical structure of the polymer material is forced to drastically convert into highly cross-linked carbon structures. On the other hand, pyrolysis process that undergo thermo- chemical decomposition of polymer material in the absence of oxygen yields carbon. Although considerable electrical resistivi- ties down to 10 -2 Ω cm have been achieved by these methods, they are time-consuming in the case of ion beams or have poor surface quality and inefcient to produce spatial resolution. [6] Later, lasers have evolved to directly write desired carbon pat- terns on polyimide substrates without any additional process or treatment to the substrate. [7–9] Thereafter, wide variety of lasers with diferent wavelengths have been studied to tune the physical and chemical properties of the PI flms for various applications. [10–18] Similarly, blending polymers or nanoparticles or doping with polyimide material or with other polymers fol- lowed by laser-induced graphene have also been reported. [19–24] Among these, temperature sensors produced by laser-induced carbonization, more importantly exhibiting stable and linear response to temperature have not been reported yet. The demand of real-time monitoring systems based on fexible sensors is rapidly increasing in the biomedical feld specifcally in the healthcare industry. As it is known that the thermoregulation system of human body maintains homeo- stasis boundaries and body temperature plays a vital role in human healthcare settings. [25] A human body temperature Wearable on-skin electronic devices that can monitor temperature in real time are of signifcant interest for personalized mobile health monitoring. Here, a fexible temperature sensor directly patterned by laser-induced carboniza- tion on Kapton polyimide flms integrated with fexible printed circuit boards is reported. The proposed sensor design possessing high resistance values exhibits high-linear and stable response to temperatures when integrated with fexible printed circuit boards (FPCBs) to enable continuous monitoring. The anisotropic conductive flm bonding technique is used to obtain the stable real-time monitoring data under various complex environments. The sensor integration with a wearable patch based FPCB establishes conformal contacts with human skin and allows wireless sensing capabilities smoothly in real time. This kind of approach can enable multifunctional sensors to be directly laser patterned on FPCBs without any additional interfacing. Dr. S. Gandla, M. Naqi, Dr. J. J. Lee, Y. Won, Dr. P. Pujar, Prof. S. Kim Multifunctional Nano Bio-Electronics Lab Department of Advanced Materials and Science Engineering Sungkyunkwan University Suwon, Gyeonggi-do 16419, South Korea E-mail: intel0616@gmail.com M. Lee, J. Kim, Dr. S. Lee Korea Electronics Technology Institute (KETI) Seongnam, Gyeonggi-do 13509, South Korea E-mail: 2sungho@gmail.com The ORCID identifcation number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admt.202000014. 1. Introduction Flexible electronic devices with the newly developed nano- materials through straightforward laser writing have been emerged with unique functionality. Unlike, conventional rigid electronics, fexibility ofers thin, light-weight, low-cost, mechanical stability, and conformability. As of now, the most widely used unconventional substrates for fexible electronic devices include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyimide (PI). Among these, PI has been a strong candidate for interfacing with fexible printed circuit board electronics due to its high thermal stability, excel- lent mechanical stability, and dielectric properties. Thus, fex- ible electronics devices with PI as a substrate material have been widely reported. On the other hand, elemental carbon in Adv. Mater. Technol. 2020, 2000014