Conducting polymer-based flexible electronics for high-quality monitoring of bioelectricity

• Main Author: Kong, Lingxuan
• Other Authors: Chen Peng
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2025
• Subjects: Chemistry; Engineering; Medicine, Health and Life Sciences; Flexible bioelectronic; Conducting polymer; Bio-electrode interface; Human-machine interaction; Wearable; Electrophysiology
• Online Access: https://hdl.handle.net/10356/182355
• Institution: Nanyang Technological University

Biopotentials, which encompass action potentials and plant-specific biopotentials, play a crucial role in cell-to-cell communication and propagate within ionically conductive bio-tissues. Monitoring electrophysiology signals using electronic devices offers a real-time diagnostic approach for various diseases. The emergence of conducting polymer (CP)-based flexible bioelectronics has revolutionized high-quality signal acquisition by seamlessly interfacing with bio-tissues. In this thesis, we explore CP-based bioelectronics in the form of microneedle (MN) patch and hygroscopic adhesive, and investigate their performance in electrophysiology monitoring. Firstly, we introduce the concept of a CP-based dual-conductive stiff-morphing MN patch (DuoMorph) designed for in planta electrophysiology monitoring. By integrating CP PEDOT:PSS with thermoplastic poly lactic-co-glycolic acid (PLGA), we create a thermos-pressed DuoMorph capable of penetrating poorly conductive epidermal barriers in a minimally invasive manner. The stiffness-morphability of the DuoMorph ensures a low mechanical mismatch with soft plant tissues, effectively suppressing potential foreign body responses. Additionally, the addition of dimethyl sulfoxide (DMSO) further enhances the conductivity of CP. In vitro electrochemical impedance spectroscopy conducted on agarose-based epidermal phantoms and Epipremnum aureum leaves reveals significantly lower bio-electronic interface impedance compared to invasive metallic electrodes. This improvement is attributed to the low charge transfer resistance (Rct) and high double-layer capacitance (Cdl) of PEDOT:PSS. Demonstrations on living Epipremnum aureum plants successfully capture variation potentials induced by mechanical wounding and flame burning. Furthermore, the “lock” effect induced by sap adsorption ensures a better signal-tonoise ratio (SNR) and improved anti-interference capability compared to metallic electrodes. Finally, the DUOMORPH coated with a cation-selective membrane enables real-time in situ ion fluctuation capture, highlighting its potential as a versatile sensing platform for multiple bio-signals. Secondly, inspired by spider silk, we developed an electronically conductive hygroscopic adhesive (HydroAd) that enables a chronic bio-electronic interface. Unlike conventional wet electrodes, which fail due to electrolyte volatilization, and dry electrodes, which provide poor signal quality due to high interface impedance, HydroAd offers significant advantages. HydroAd contains an electrolyte that moistens the stratum corneum, reducing interface impedance to levels lower than those of commercial wet electrodes (CWE). It also maintains a water balance between evaporation and adsorption over long-term period. The incorporation of a crosslinkable charged ionic liquid into CP enhances its electrical properties. Specifically, this modification improves conductivity and increases charge storage and injection capacity by facilitating CP conductive network. The crosslinkable ionic liquid also acts as plasticizer that improves CP’s stretchability. Additionally, the ionic liquid’s hygroscopic nature allows it to function as a superabsorbent, leading to strong electrostatic interactions with water molecules. Gelatin methacryloyl (GelMA) enhances HydroAd's adhesiveness, and by adjusting the composition of the initiator and crosslinker, its adhesiveness can be optimized. Notably, HydroAd maintains a resistance variation of approximately 2.5% and an impedance variation below 10% during cyclic stretching tests. It achieves stable electrical properties and adhesiveness over a record-breaking six-month period. Applications in biopotential monitoring, such as electrocardiogram (ECG), electroencephalogram (EEG), and electrooculogram (EOG), demonstrate that HydroAd offers better signal acquisition quality, with no SNR change observed after a six-month drying process. This highlights its potential as a chronic bio-electronic platform. Finally, we explored the mechanism behind the remarkable water permeability of the HydroAd. Compared to conventional hygroscopic compounds used in electronically conductive adhesives (such as LiCl and uncrosslinked choline-based ionic liquids), HydroAd exhibits even higher water permeability. It surpasses not only conventional encapsulation materials like PDMS (Polydimethylsiloxane), PI (Polyimide), and highly permeable thermoplastic polyurethane (TPU) but also the CWE. Raman spectroscopy and differential scanning calorimetry confirmed that after crosslinking, the charged network within HydroAd modulates the water state, increasing the ratio of intermediate water (loosely bound) to bound water (strongly bound), denoted as the I/B ratio. This adjustment decreases the overall water enthalpy, resulting in improved electrode breathability. On sweaty skin, HydroAd’s high permeability enables rapid water evaporation and adhesion recovery within just 30 minutes. Further investigation revealed that the high I/B ratio plays a crucial role in regulating both electrochemical and mechanical properties. Specifically, it leads to decreased electrical impedance and enhanced adhesion. By triggering the formation of electrochemical interface, the higher content of intermediate water enhances the Cdl while suppressing electrolyte resistance (Rs) and Rct. To demonstrate its practical application, we designed a 10-channel HydroAd electrode array for surface electromyography (sEMG)-based gesture recognition. Remarkably, HydroAd-based gesture recognition achieves an impressive prediction accuracy of over 95%, surpassing both metallic dry electrodes and CWE. Even after a six-month drying process, its performance remains unchanged. This positions HydroAd as an ideal platform for advanced human-machine interaction.