DIW 3D Printing of Flexible Electrochemical Implant Sensor Array Based on Silk Fibroin

The performance of biomedical implantable sensors is critically impacted by immune responses within the body, which can compromise the device's functionality and long-term measurement. Material selection and mechanical design are one of the main factors that influence these immune responses, offering a pathway for mitigation. However, the surface of living tissues is flexible, curved, and lubricous due to blood and dynamic movements. Therefore, a biocompatible and flexible substrate that can maintain its structure on the sensory part needs to be developed for biomedical sensors [1]. Silk fibroin (SF), the nature given extract, is one of the main protein polymer that performs highly stable and robust mechanical characteristic and reliable biocompatibility due to its degree of crystallization [2]. Recent study also presents the feasibility of this natural polymer as a material for direct ink writing (DIW), which is a novel manufacturing method for versatile 2D and 3D designing and thus modulating its effective modulus [3].
In this study, we explored the use of DIW 3D printer to fabricate flexible electrochemical sensors based on SF, a biocompatible material with excellent mechanical properties. The sensor design utilized interdigitated electrodes (IDEs) patterned with PEDOT:PSS, a conductive polymer known for its stability and biocompatibility. Our approach focused on the creation of a dual three-electrode system, an innovative design that enhances the sensor's sensitivity. The sensor platform were fabricated by SF solution which was extracted and dialyzed using Bombyx mori silk. The IDE structure was fabricated using a PEDOT:PSS ink with a concentration of 4 wt.%, combined with reference electrode fabricated by Ag/AgCl paste dispensed over a spin-coated SF film. This dual-electrode configuration provides a more reliable signal by amplifying the signals compared to traditional single three-electrode systems.
We employed cyclic voltammetry and chronoamperometric tests using 1 mM Fe(CN)₆ solution to evaluate the electrochemical performance of the SF IDE sensor. The oxidation potential of the sensor was confirmed at 0.57 V whereas the reduction potential was confirmed at -0.17 V. Through this contribution, the sensor demonstrated a remarkable ability to detect the Fe(CN)₆ solution, achieving a 100-fold amplification in current output compared to the single three electrode system. This significant improvement in performance highlights the advantages of the dual-electrode configuration in detecting low-concentration analytes with greater accuracy and sensitivity. The performance of sensor was demonstrated by detecting levodopa solution and dopamine solution. Furthermore, electrochemical ability of the sensor was confirmed after derformation test which includes bending and twisting test over 100 times. The mechanical characteristics of the sensor was also verified by universal tensile machine to measure the sensor’s effective modulus. After confirming the sensor’s mechanical performance, the electrochemical implant sensor array was integrated with a DIW-printed SF platform. The effective flexural and sectional modulus of the electrochemical sensor array was conducted to demonstrate a flexibility and functionality of fully integrated electrochemical sensor system. The flexibility of the SF IDE sensor array makes it well-suited for implantable applications, where the ability to conform to dynamic biological environments is essential. The use of silk fibroin not only ensures biocompatibility but also contributes to the sensor's mechanical robustness, addressing common challenges faced by traditional implantable sensors such as rigidity and potential for mechanical failure.