Conducting polymer hydrogels based on supramolecular strategies for wearable sensors

Abstract

Conductive polymer hydrogels (CPHs) are gaining considerable attention in developing wearable electronics due to their unique combination of high conductivity and softness. However, in the absence of interactions, the incompatibility between hydrophobic conductive polymers (CPs) and hydrophilic polymer networks gives rise to inadequate bonding between CPs and hydrogel matrices, thereby significantly impairing the mechanical and electrical properties of CPHs and constraining their utility in wearable electronic sensors. Therefore, to endow CPHs with good performance, it is necessary to ensure a stable and robust combination between the hydrogel network and CPs. Encouragingly, recent research has demonstrated that incorporating supramolecular interactions into CPHs enhances the polymer network interaction, improving overall CPH performance. However, a comprehensive review focusing on supramolecular CPH (SCPH) for wearable sensing applications is currently lacking. This review provides a summary of the typical supramolecular strategies employed in the development of high-performance CPHs and elucidates the properties of SCPHs that are closely associated with wearable sensors. Moreover, the review discusses the fabrication methods and classification of SCPH sensors, while also exploring the latest application scenarios for SCPH wearable sensors. Finally, it discusses the challenges of SCPH sensors and offers suggestions for future advancements.

Keywords: conductive polymer; hydrogel; supramolecular interaction; wearable sensor.

FIGURE 1

Introduce the common supramolecular strategies for preparing SCPHs, the characteristics of SCPHs, and their related applications in wearable sensing.

FIGURE 2

The mechanism diagram of supramolecular interactions. (A) Hydrogen bond. (B) Electrostatic interactions. (C) Host‐guest interactions. (D) Coordination bond.

FIGURE 3

Schematic representation of commonly used preparation techniques for SCPHs. (A) SCPHs are obtained through the direct crosslinking of CP with crosslinking agents. (B) SCPHs are synthesized via a one‐step polymerization process, wherein CP monomer and a hydrogel precursor liquid are blended and polymerized to yield SCPHs. (C) SCPHs are synthesized by utilizing a pre‐prepared hydrogel matrix as a template, wherein CP monomer is initially introduced and subsequently polymerized in situ within the hydrogel matrix to generate SCPHs.

FIGURE 4

Construction of SCPHs based on hydrogen bonds. (A) Preparation of P(AAm‐co‐HEMA)/PANI interpenetrating network hydrogels. Reproduced with permission.^{[ 59 ]} Copyright 2018, American Chemical Society. (B) The preparation process of PAAm/CMC/PPy hydrogel. Reproduced with permission. Reproduced with permission.^{[ 60 ]} Copyright 2020, Royal Society of Chemistry. (C) Preparation and schematic illustration of the structure of PANI NPs@(CS‐PAA) hydrogels.^{[ 61 ]} Copyright 2023, Elsevier. (D) Schematic representation of the synthesis mechanism of PHEMA/PVA/CNFs@PPy hydrogels and the interactions between the polymer networks. Reproduced with permission.^{[ 62 ]} Copyright 2023, Elsevier. (E) Schematic of the formation mechanism of supramolecular PANI/PSS‐UPy hydrogels, and (F) their self‐healing, freely moldable, and injectable properties. Reproduced with permission.^{[ 63 ]} Copyright 2019, American Chemical Society.

FIGURE 5

The SCPHs are prepared by electrostatic interactions. (A) The gelation process of PA crosslinked PANI. Reproduced with permission.^{[ 52 ]} Copyright 2012, National Academy of Sciences. (B) Schematic diagram of the gelation process of PEDOT: PSS as a crosslinker for crosslinking six conductive polymers. Reproduced with permission.^{[ 64 ]} Copyright 2022, Elsevier. (C) Mechanism diagram of the formation of PIC/PANI composite hydrogel, where PANI is crosslinked to the PIC matrix by PA. Reproduced with permission.^{[ 65 ]} Copyright 2018, American Chemical Society. (D) Schematic structure of HAPAA/PANI hydrogel and multiple interactions between polymer networks. Reproduced with permission.^{[ 66 ]} Copyright 2021, Royal Society of Chemistry. (E) Schematic mechanism of synthesis of composite SCPHs loaded with PPy microgel. Reproduced with permission.^{[ 67 ]} Copyright 2023, Wiley‐VCH.

FIGURE 6

The SCPHs are prepared by the host‐guest interaction. (A) Schematic synthesis of PANI‐P(AAm‐co‐AA) @Fe³⁺ hydrogel. Reproduced with permission.^{[ 74 ]} Copyright 2022, Royal Society of Chemistry. (B) Schematic illustration of the preparation of CxPy hydrogels and interactions between the polymers and water‐soluble PPy. Reproduced with permission.^{[ 76 ]} Copyright 2022, Elsevier. (C) Schematic synthesis of poly (NIPAM‐co‐β‐CD). Reproduced with permission.^{[ 77 ]} Copyright 2018, American Chemical Society. (D) Preparation of supramolecular hydrogels based on host‐guest interaction and (E) their application to supercapacitor assembly. Reproduced with permission.^{[ 78 ]} Copyright 2021, Wiley‐VCH.

FIGURE 7

The SCPHs are fabricated by coordination bonds. (A) Diagram of the formation mechanism of CS/PAA/PPy/Fe (III) hydrogels. Reproduced with permission.^{[ 87 ]} Copyright 2021, American Chemical Society. (B) Preparation procedure and schematic of the internal structure of the PAM/CS/PPy hydrogels. Reproduced with permission.^{[ 88 ]} Copyright 2018, American Chemical Society. (C) Schematic diagram of the synthesis process of PAM/CMCS‐g‐PANI/Ag hydrogels and the multiple interactions within the SCPHs network. Reproduced with permission.^{[ 89 ]} Copyright 2022, Elsevier. (D) Schematic illustration of the preparation of GCP@PPy hydrogel and the healing mechanism of dynamic Au‐SR bonds in GCP@PPy hydrogel stimulated by NIR laser and electricity. Reproduced with permission.^{[ 93 ]} Copyright 2019, Wiley‐VCH.

FIGURE 8

The conductivity of SCPHs. (A) Highly interconnected network structure of PA crosslinked PANI hydrogels. Reproduced with permission.^{[ 52 ]} Copyright 2012, National Academy of Sciences. (B) Microstructure and conductivity of six PEDOT: PSS crosslinked SCPHs. Reproduced with permission.^{[ 64 ]} Copyright 2022, Elsevier. (C) Schematic illustration of the effect of microstructure onion and electron transport in SCPHs. Reproduced with permission.^{[ 98 ]} Copyright 2020, Springer Nature.

FIGURE 9

The toughness of SCPHs. (A) Schematic strain mechanism and tensile properties of PAA/PANI hydrogels. Reproduced with permission.^{[ 99 ]} Copyright 2022, Wiley‐VCH. (B) Schematic diagram of the multiple supramolecular interactions in the hydrogel network and the excellent mechanical properties of PAM‐ALG‐PPy hydrogels. Reproduced with permission.^{[ 100 ]} Copyright 2023, Wiley‐VCH. (C) Mechanical performance of PVA/PA/PANI/GA hydrogels and their tensile photographs. Reproduced with permission.^{[ 102 ]} Copyright 2021, American Chemical Society.

FIGURE 10

The biocompatibility of SCPHs. (A) Biocompatibility of PEDOT: PSS hydrogels crosslinked with PPy. Reproduced with permission.^{[ 105 ]} Copyright 2021, American Chemical Society. (B) Viability of PC12 cells cultured on six PEDOT: PSS crosslinked hydrogels after 48 h. Reproduced with permission.^{[ 106 ]} Copyright 2023, Royal Society of Chemistry. (C) The SCPHs based on carboxymethyl CMC‐DA/PEDOT: PSS exhibit excellent biocompatibility, facilitating the proliferation of skin fibroblasts and keratinogenic cells. Reproduced with permission.^{[ 107 ]} Copyright 2021, Elsevier. (D) The γ‐GM‐P hydrogel is highly biocompatible and friendly to human skin fibroblasts. Reproduced with permission.^{[ 108 ]} Copyright 2022, Elsevier.

FIGURE 11

The self‐adhesive properties of SCPHs. (A) Adhesive performance and adhesive mechanism of PAM/CMCS‐g‐PANI/Ag DN hydrogels. Reproduced with permission.^{[ 89 ]} Copyright 2022, Elsevier. (B) Adhesion performance and adhesion instances of poly (HEAA‐co‐SBAA)/PEDOT: PSS hydrogels with various substrates. Reproduced with permission.^{[ 111 ]} Copyright 2020, Royal Society of Chemistry. (C) Self‐adhesion of PDA‐PPy‐PAM and sensing applications of the hydrogels adhering to the human body. Reproduced with permission.^{[ 115 ]} Copyright 2018, American Chemical Society. (D) The adhesion strength and adhesion mechanism of PHEMA/PVA/CNFs@PPy to different substrates. Reproduced with permission.^{[ 62 ]} Copyright 2023, Elsevier.

FIGURE 12

The self‐healing properties of SCPHs. (A) Self‐healing properties of γ‐GM‐P hydrogels formed by multiple hydrogen bonding and demonstration of their self‐healing behavior. Reproduced with permission.^{[ 108 ]} Copyright 2022, Elsevier. (B) Schematic mechanism of self‐healing and self‐healing performance of SCPHs based on coordination bonds. Reproduced with permission.^{[ 120 ]} Copyright 2017, Wiley‐VCH. (C) Photographs of self‐healing behavior of PAA/PANI hydrogels based on electrostatic interactions and tensile properties and conductivity of the hydrogels after self‐healing. Reproduced with permission.^{[ 99 ]} Copyright 2022, Wiley‐VCH. (D) Schematic illustration of the formation of dynamically crosslinked PEDOT: S‐Alg‐Ad/Pβ‐CD hydrogels through host‐guest interactions and the scheme of the hydrogel self‐healing process. Reproduced with permission.^{[ 121 ]} Copyright 2019, American Chemical Society. (E) The mechanism diagram of self‐healing by different supramolecular interactions.

FIGURE 13

The anti‐swelling properties of SCPHs. (A) Schematic diagram of multiple interactions within SF/TA@PPy hydrogel networks and their swelling curves and underwater sensing applications. Reproduced with permission.^{[ 125 ]} Copyright 2022, Elsevier. (B) Demonstration of swelling resistance of HF(PVA‐C/P) hydrogels and their swelling curves and mechanical properties after soaking in water. Reproduced with permission.^{[ 126 ]} Copyright 2021, American Chemical Society. (C) Schematic illustration of multiple hydrogen bonds within the PAA‐PANI/PVA/PDA/AOP hydrogel network and their anti‐swelling properties. Reproduced with permission.^{[ 127 ]} Copyright 2023, Elsevier. (D) Demonstration of anti‐swelling behaviors of PAAm‐SA‐PPy hydrogels. Reproduced with permission.^{[ 128 ]} Copyright 2023, Elsevier. (E) Photographs of PANI‐6/PMON hydrogel working underwater and characterization of its resistance to swelling. Reproduced with permission.^{[ 129 ]} Copyright 2021, Royal Society of Chemistry.

FIGURE 14

The fatigue resistance of SCPHs. (A) Schematic mechanism of anti‐fatigue and self‐repair of PANI‐modified PVA/PHEA hydrogels and results of anti‐fatigue experiments. Reproduced with permission.^{[ 132 ]} Copyright 2023, Wiley‐VCH. Copyright 2020, American Chemical Society. (B) Single notch tensile test of PVA/PEDOT: PSS and tensile photos after 30,000 cycles. Reproduced with permission.^{[ 24 ]} Copyright 2023, Wiley‐VCH.

FIGURE 15

Common fabrication methods for SCPH sensors: (A) Cast molding, Reproduced with permission.^{[ 133 ]} Copyright 2014, Springer Nature. (B) Direct‐write 3D printing. Reproduced with permission.^{[ 134 ]} Copyright 2017, Wiley‐VCH. (C) Liquid‐in‐liquid 3D printing. Reproduced with permission.^{[ 135 ]} Copyright 2023, Wiley‐VCH. (D) light‐cured 3D printing. Reproduced with permission.^{[ 136 ]} Copyright 2023, Wiley‐VCH. (E) In situ molding. Reproduced with permission.^{[ 137 ]} Copyright 2023, Wiley‐VCH. Classification of SCPHs: (F) Strain Sensors. (G) Pressure sensors. Reproduced with permission.^{[ 99 ]} Copyright 2022, Wiley‐VCH. (H) Sweat sensor. Reproduced with permission.^{[ 138 ]} Copyright 2023, Elsevier. (I) Electrophysiologic signal sensors. Reproduced with permission.^{[ 76 ]} Copyright 2022, Elsevier. (J) Temperature sensors. Reproduced with permission.^{[ 139 ]} Copyright 2022, Elsevier.

FIGURE 16

SCPH sensors for motion perception. (A) Examples of PVA@MXene@PPy hydrogel sensors for monitoring human motion. Reproduced with permission.^{[ 160 ]} Copyright 2023, Elsevier. (B) PANI NPs@(CS‐PAA) hydrogel sensor for monitoring Parkinsonian behavior. Reproduced with permission.^{[ 61 ]} Copyright 2023, Wiley‐VCH. (C) Schematic synthesis mechanism of SCPHs with anti‐freezing and flame‐retardant properties and their strain sensing properties at low and high temperatures. Reproduced with permission.^{[ 161 ]} Copyright 2022, American Chemical Society. (D) Application of PAM‐ALG‐PPy hydrogel sensor for underwater breaststroke detection. Reproduced with permission.^{[ 100 ]} Copyright 2023, Wiley‐VCH.

FIGURE 17

SCPH sensors for sweat analysis. (A) Real‐time analysis of K⁺ and Na⁺ in sweat by SCPH sensors during running. Reproduced with permission.^{[ 172 ]} Copyright 2021, Royal Society of Chemistry. (B) Photograph of the integrated SCPHs sweat sensor and real‐time on‐body sweat sensing. Reproduced with permission.^{[ 173 ]} Copyright 2022, Elsevier. (C) Schematic structure of a SCPH sensor integrated with a microfluidic device and its performance in detecting uric acid in sweat. Reproduced with permission.^{[ 154 ]} Copyright 2021, Elsevier. (D) Schematic diagram of the working principle of SCPHs sweat sensor and detection of tyrosine in human sweat by the sensor. Reproduced with permission.^{[ 138 ]} Copyright 2023, Elsevier. (E) Schematic diagram of the operation of a self‐powered sweat sensor and real‐time monitoring of ions in sweat by a wireless sensor worn on the human body. Reproduced with permission.^{[ 175 ]} Copyright 2022, Wiley‐VCH.

FIGURE 18

SCPH sensors for electrophysiological signal monitoring. (A) The Gel/PPy/rGO sensors are used to control robotic arms by capturing and recognizing human EMG signals. Reproduced with permission.^{[ 184 ]} Copyright 2022, Elsevier. (B) In situ formed PSP hydrogel sensors detect EMG signals on complex body surfaces (hairy areas). Reproduced with permission.^{[ 137 ]} Copyright 2023, Wiley‐VCH. (C) Photographs of PCPPM hydrogel sensors and real‐time monitoring of ECG signals by the sensors. Reproduced with permission.^{[ 185 ]} Copyright 2021, Elsevier. (D) N170 and P300 testing with dry, wet and SCPH electrodes via EEG caps. Reproduced with permission.^{[ 139 ]} Copyright 2023, Springer Nature. (E‐F) Hydrogel‐integrated wearable BMI for EEG (E) and EOG monitoring (F). Reproduced with permission.^{[ 186 ]} Copyright 2023, Wiley‐VCH.

FIGURE 19

SCPH sensors for temperature monitoring. (A) Schematic diagram of the temperature response principle of SCPH sensors and sensing of the temperature of cold and hot water and breathing gases. Reproduced with permission.^{[ 156 ]} Copyright 2021, Wiley‐VCH. (B) Near‐infrared absorption and photothermal response of PHEMA/PVA/CNFs@PPy hydrogel. Reproduced with permission.^{[ 62 ]} Copyright 2023, Elsevier. (C) The relationship between conductivity and temperature of PPFe4 hydrogels with photothermal conversion properties and the hydrogel response to water at different temperatures. Reproduced with permission.^{[ 193 ]} Copyright 2020, Royal Society of Chemistry. (D) PANI NFs temperature sensor serves as a “fever indicator” to monitor a human forehead's temperature. Reproduced with permission.^{[ 194 ]} Copyright 2020, American Chemical Society. (E) The PBA/CPA/Gly hydrogel sensors possess the ability to simultaneously monitor strain and temperature, and there are examples of utilizing the sensors to recognize temperature. Reproduced with permission.^{[ 139 ]} Copyright 2022, Elsevier.

FIGURE 20

SCPH sensors for human‐machine interaction. (A) SCPHs integrate the touch screen pen for dialing and drawing.^{[ 160 ]} Copyright 2023, Wiley‐VCH. (B) PEDOT: PSS‐PVA hydrogel remotely controls a robotic arm to execute different movement paths. Reproduced with permission.^{[ 41 ]} Copyright 2022, Wiley‐VCH. (C) SCPH sensors are employed for virtual character control by capturing EMG signals. Reproduced with permission.^{[ 184 ]} Copyright 2023, Elsevier.

FIGURE 21

SCPH sensors for soft robot sensing. (A) PEDOT: PSS/PVA hydrogel monitors the process of object grasping by a soft robotic gripper. Reproduced with permission.^{[ 41 ]} Copyright 2022, Wiley‐VCH. (B) The light‐driven feature of SCPHs and the soft machine octopus developed by SCPHs realize the self‐actuation and sensing of grasping objects. Reproduced with permission.^{[ 201 ]} Copyright 2021, American Association for the Advancement of Science. (C) The schematic diagram of the programmable shape of the SCPHs actuator, as well as the shape deformation of the SCPHs artificial octopus, fish, and gripper driven by near‐infrared light. Reproduced with permission.^{[ 202 ]} Copyright 2023, Wiley‐VCH.

FIGURE 22

SCPH sensors for information transmission. (A) The sensor transmits finger bending signals into alphabetic and numeric information via Morse code. Reproduced with permission.^{[ 208 ]} Copyright 2023, American Chemical Society. (B) The SCPH tactile sensors transmit touch signals via Morse code. Reproduced with permission.^{[ 108 ]} Copyright 2022, Elsevier. (C) The SCPH sensor encrypts and transmits finger bend signals underwater based on Morse code. Reproduced with permission.^{[ 125 ]} Copyright 2022, Elsevier. (D) The SCPH strain sensors attached to five fingers recognize sign language by monitoring gesture changes. Reproduced with permission.^{[ 209 ]} Copyright 2023, American Chemical Society. (E) Messaging is based on different individual handwriting habits. Reproduced with permission.^{[ 89 ]} Copyright 2022, Elsevier.