## 1 Introduction  
Immunodiagnosis is a method to detect various diseases, based on the specific immune response between antigen and antibody [1; 2]. Based on different labeling signals, various immunodiagnosis methods are derived. The immunodiagnostic methods include: enzyme-linked immunosorbent assay (ELISA) [3], immunofluorescence technology, radioimmunoassay (RIA), immunomodolol gold labeling (ICS), chemiluminescence immunoassay (CLIA) [4; 5], etc. At present, CLIA and ELISA are widely used, especially in clinical diagnosis and experimental research [6]. They have become the mainstream of immunodiagnostic technology and have a particularly important position in modern medicine. The traditional immunoassay methods in these laboratories are not suitable for rapid on-site testing, due to the long sample preparation time and the need for laboratory personnel with professional background. The electrochemical immunosensor provide the possibilities for label-free detection, detecting electrical signals instead of optical signals. They have been widely applied in biological detections at different scales, including but not limited to, organs, tissues, cells and biomolecules. Therefore, the electrochemical immunoassay has attracted extensive attention because of its detection specificity, simple operation, and small size [7; 8; 9].  
Electrochemical impedance spectroscopy (EIS) is a non-destructive steady-state technique that is capable of probing the relaxation phenomena over a range of frequencies [9]. It is widely adopted to analyze electrode process dynamics and clarify corrosion mechanisms [10; 11]. The electrochemical impedance biosensor is an analysis device that converts biological signals into electrical impedance signals [12; 13; 14]. It analyzes the changes of resistance and capacitance signals on the electrode surface to observe the process of the adsorption and interaction of biomolecules on the electrode surface ranging over enzymes, proteins, DNA, bacteria [10; 15], etc. However, most current electrochemical impedance sensors are based on rigid, non-deformable substrates, resulting in that [16; 17; 18], sensors can hardly fit with the measured object faced with complicated test environments such as three-dimensional and bendable objects, thus, leading to measurement errors. The situation above limits the wider applications of the device in the field of biosensing detection [19; 20]. Therefore, in order to better adapt to the test environment and reduce the measurement errors, flexible electrodes, with excellent performance, is desperately needed [19; 21].  
In this paper, we designed a novel 3 \(\times\) 3 interdigital electrode array (with minimum electrode spacing of 20 \(\upmu\)m) on the flexible and sensitive micro-immunosensor, which was fabricated by photolithography on flexible PEN film. The process flow chart of the device can be found in the supplementary file Figure S1. Using this system, we detected Phosphate Buffer Saline PBS of different concentrations and IgG of low concentration, moreover, a relatively wide linear detection interval of 0.01-10 ng/mL is obtained, which verified the feasibility of flexible PEN -based electrode for immunoassay. In further research, the system can be applied in complex environments such as three-dimensional biological tissues and can detect multiple proteins simultaneously for disease diagnosis.