Thermoresponsive PEDOT:PSS/PNIPAM conductive hydrogels as wearable resistive sensors for breathing pattern detection

Advances in conducting nanocomposite hydrogels for wearable biomonitoring

Recent advancements in wearable biosensors and bioelectronics have led to innovative designs for personalized health management devices, with biocompatible conducting nanocomposite hydrogels emerging as a promising building block for soft electronics engineering. In this review, we provide a comprehensive framework for advancing biosensors using these engineered nanocomposite hydrogels, highlighting their unique properties such as high electrical conductivity, flexibility, self-healing, biocompatibility, biodegradability, and tunable architecture, broadening their biomedical applications. We summarize key properties of nanocomposite hydrogels for thermal, biomechanical, electrophysiological, and biochemical sensing applications on the human body, recent progress in nanocomposite hydrogel design and synthesis, and the latest technologies in developing flexible and wearable devices. This review covers various sensor types, including strain, physiological, and electrochemical sensors, and explores their potential applications in personalized healthcare, from daily activity monitoring to versatile electronic skin applications. Furthermore, we highlight the blueprints of design, working procedures, performance, detection limits, and sensitivity of these soft devices. Finally, we address challenges, prospects, and future outlook for advanced nanocomposite hydrogels in wearable sensors, aiming to provide a comprehensive overview of their current state and future potential in healthcare applications.

Facile Formation of Multifunctional Biomimetic Hydrogel Fibers for Sensing Applications

To face the challenges in preparing hydrogel fibers with complex structures and functions, this study utilized a microfluidic coaxial co-extrusion technique to successfully form functional hydrogel fibers through rapid ionic crosslinking. Functional hydrogel fibers with complex structures, including linear fibers, core-shell structure fibers, embedded helical channels, hollow tubes, and necklaces, were generated by adjusting the composition of internal and external phases. The characteristic parameters of the hydrogel fibers (inner and outer diameter, helix generation position, pitch, etc.) were achieved by adjusting the flow rate of the internal and external phases. As biocompatible materials, hydrogel fibers were endowed with electrical conductivity, temperature sensitivity, mechanical enhancement, and freeze resistance, allowing for their use as temperature sensors for human respiratory monitoring and other biomimetic application developments. The hydrogel fibers had a conductivity of up to 22.71 S/m, a response time to respiration of 37 ms, a recovery time of 1.956 s, and could improve the strength of respiration; the tensile strength at break up to 8.081 MPa, elongation at break up to 159%, and temperature coefficient of resistance (TCR) up to -13.080% °C-1 were better than the existing related research.

Molecularly Engineered Supramolecular Thermoresponsive Hydrogels with Tunable Mechanical and Dynamic Properties

Synthetic supramolecular polymers and hydrogels in water are emerging as promising biomaterials due to their modularity and intrinsic dynamics. Here, we introduce temperature sensitivity into the nonfunctionalized benzene-1,3,5-tricarboxamide (BTA-EG4) supramolecular system by incorporating a poly(N-isopropylacrylamide)-functionalized (BTA-PNIPAM) moiety, enabling 3D cell encapsulation applications. The viscous and structural properties in the solution state as well as the mechanical and dynamic features in the gel state of BTA-PNIPAM/BTA-EG4 mixtures were investigated and modulated. In the dilute state (c ∼μM), BTA-PNIPAM acted as a chain capper below the cloud point temperature (Tcp = 24 °C) but served as a cross-linker above Tcp. At higher concentrations (c ∼mM), weak or stiff hydrogels were obtained, depending on the BTA-PNIPAM/BTA-EG4 ratio. The mixture with the highest BTA-PNIPAM ratio was ∼100 times stiffer and ∼10 times less dynamic than BTA-EG4 hydrogel. Facile cell encapsulation in 3D was realized by leveraging the temperature-sensitive sol-gel transition, opening opportunities for utilizing this hydrogel as an extracellular matrix mimic.

A highly conductive, robust, self-healable, and thermally responsive liquid metal-based hydrogel for reversible electrical switches

This study introduces a thermally responsive smart hydrogel with enhanced electrical properties achieved through volume switching. This advancement was realized by incorporating multiscale liquid metal particles (LMPs) into the PNIPAM hydrogel during polymerization, using their inherent elasticity and conductivity when deswelled. Unlike traditional conductive additives, LMPs endow the PNIPAM hydrogel with a remarkably consistent volume switching ratio, significantly enhancing electrical switching. This is attributed to the minimal nucleation effect of LMPs during polymerization and their liquid-like behavior, like vacancies in the polymeric hydrogel under compression. The PNIPAM/LMP hydrogel exhibits the highest electrical switching, with an unprecedented switch of 6.1 orders of magnitude. Even after repeated swelling/deswelling cycles that merge some LMPs and increase the conductivity when swelled, the hydrogel consistently maintains an electrical switch exceeding 4.5 orders of magnitude, which is still the highest record to date. Comprehensive measurements reveal that the hydrogel possesses robust mechanical properties, a tissue-like compression modulus, biocompatibility, and self-healing capabilities. These features make the PNIPAM/LMP hydrogel an ideal candidate for long-term implantable bioelectronics, offering a solution to the mechanical mismatch with dynamic human tissues.

Extremely-fast electrostatically-assisted direct ink writing of 2D, 2.5D and 3D functional traces of conducting polymer Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate- polyethylene oxide (PEDOT:PSS-PEO)

HYPOTHESIS

Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is an attractive conducting polymer, albeit its rheological properties are inappropriate for direct ink writing (DIW). Here it is hypothesized that a suspension of PEDOT:PSS with a non-conducting highly spinnable viscoelastic polymer, e.g., polyethylene oxide (PEO), will significantly facilitate printability and enhance the electrical conductivity (EC) of PEDOT:PSS-PEO. It is also hypothesized that high-humidity post-treatment will enhance the EC even further, and the application of the electric field can facilitate the DIW speed beyond the capabilities of current commercial 3D printers.

EXPERIMENTS

The rheological behavior of PEDOT:PSS suspensions with several non-conducting polymers was explored in the experiments. The EC of the suspensions was measured, including the effect of high-humidity post-treatment. High-speed DIW of the optimal suspension was experimentally demonstrated with the applied electric field.

FINDINGS

The findings revealed that PEO serves as a secondary dopant, and the suspension of 4.33 wt% PEDOT:PSS-52 wt% PEO possesses the EC > 15 times higher than that of PEDOT:PSS. Many 2D, 2.5D and 3D functional traces were printed at high resolution at the DIW speed up to 8.64 m/s (>10 times faster than current commercial printers), facilitated by the applied electric field. Post-treatment at 80-90% relative humidity enhanced the EC more than twice.

High-Performing Conductive Hydrogels for Wearable Applications

Conductive hydrogels have gained significant attention for their extensive applications in healthcare monitoring, wearable sensors, electronic devices, soft robotics, energy storage, and human-machine interfaces. To address the limitations of conductive hydrogels, researchers are focused on enhancing properties such as sensitivity, mechanical strength, electrical performance at low temperatures, stability, antibacterial properties, and conductivity. Composite materials, including nanoparticles, nanowires, polymers, and ionic liquids, are incorporated to improve the conductivity and mechanical strength. Biocompatibility and biosafety are emphasized for safe integration with biological tissues. Conductive hydrogels exhibit unique properties such as stretchability, self-healing, wet adhesion, anti-freezing, transparency, UV-shielding, and adjustable mechanical properties, making them suitable for specific applications. Researchers aim to develop multifunctional hydrogels with antibacterial characteristics, self-healing capabilities, transparency, UV-shielding, gas-sensing, and strain-sensitivity.

A Biomimetic Bilayer Hydrogel Actuator Based on Thermoresponsive Gelatin Methacryloyl-Poly(N-isopropylacrylamide) Hydrogel with Three-Dimensional Printability

Development of hydrogel-based actuators with programmable deformation is an important topic that arouses much attention in fundamental and applied research. Most of these actuators are nonbiodegradable or work under nonphysiological conditions. Herein, a temperature-responsive and biodegradable gelatin methacryloyl (GelMA)-poly(N-isopropylacrylamide) hydrogel (i.e., GN hydrogel) network was explored as the active layer of a bilayer actuator. Small-angle X-ray scattering (SAXS) revealed that the GN hydrogel formed a mesoglobular structure (∼230 Å) upon a thermally induced phase transition. Rheological data supported that the GN hydrogel possessed 3D printability and tunable mechanical properties. A bilayer hydrogel actuator composed of active GN and passive GelMA layers was optimized by varying the layer thickness and compositions to achieve large, reproducible, and anisotropic bending with a curvature of ∼5.5 cm^-1. Different patterns of the active layer were designed for actuation in programmable control. The 3D printed GN hydrogel constructs showed significant volume reduction (∼25-60% depending on construct design) at 37 °C with the resolution enhanced by the thermo-triggered actuation, while they were able to fully reswell at room temperature. A more intricate 3D printed butterfly actuator demonstrated the ability to mimic the wing movement through thermoresponsiveness. Furthermore, myoblasts laden in the GN hydrogel exhibited significant proliferation of ∼376% in 14 days. This study provides a new fabrication approach for developing biomimetic devices, artificial muscles, and soft robotics for biomedical applications.

Colorful Luminescence of Conjugated Polyelectrolytes Induced by Molecular Weight

Due to their distinctive intrinsic advantages, the nanoaggregates of conjugated polyelectrolytes (CPEs) are fascinating and attractive for various luminescence applications. Generally, the emission luminescence of CPEs is determined by the conjugated backbone structure, i.e., different conjugated backbones of CPEs produce emission luminescence with different emission wavelength bands. Here, we polymerized the bis(boronic ester) of benzothiadiazole and an alkyl sulfonate sodium-substituted dibromobenzothiatriazole to provide PBTBTz-SO3Na with different molecular weights via controlling the ratio of the monomer and the catalyst. Theoretically, the CPEs with the same molecular structure usually display similar photoelectronic performances. However, the resulting PBTBTz-SO3Na reveal a similar light absorption property, but different luminescence. The higher molecular weight is, the stronger the fluorescence intensity of PBTBTz-SO3Na that occurs. PBTBTz-SO3Na with different molecular weights have different colors of luminescence. It is well known that the molecular aggregates often led to weaker luminescent properties for most of the conjugated polymers. However, PBTBTz-SO3Na exhibits a higher molecular weight with an increasing molecular chain aggregation, i.e., the nanoaggregates of PBTBTz-SO3Na are beneficial to emission luminescence. This work provides a new possible chemical design of CPEs with a controllable, variable luminescence for further optoelectronics and biomedicine applications.

Highly conductive thermoresponsive silver nanowire PNIPAM nanocomposite for reversible electrical switch

Highly conductive nanocomposite hydrogels have been challenging to produce due to their high water volumes inhibiting the incorporation of an essential amount of conductive nanofillers. Furthermore, the most common fillers used, typically for easy integration, display small aspect ratios. Thus, the formation of interparticle pathways for electronic travel is limited, resulting in low conductivities. Here, we introduce ultralong silver nanowires (ULAgNWs) into a thermoresponsive, volume changing PNIPAM gel to form a nanocomposite that shows switchable electronic performance. The produced nanocomposite surpasses other PNIPAM nanocomposites by expressing the largest electrical switch ratio and the highest peak conductivity. The PNIPAM matrix possesses an interconnected microporous structure that offers a spacious network for the dispersion of nanowires while still maintaining a high volume switch ratio and excellent elastic behavior under extreme compression cycles (98% compression). The ULAgNWs significantly enhance the probability of more numerous connections forming during shrinking cycles. The high swellability displayed by the PNIPAM gel provides the ability to separate the embedded nanowires by many lengths. Together, they form a nanocomposite that can thermo-modulate its electrical properties. Moreover, the conductive PNIPAM maintains the electrical switch of 4.3-4.4 orders of magnitude with thermo-responsive cycles. Because of their high electrical conductivity and outstanding elastic behavior, these stimuli-responsive nanocomposite hydrogels may expand the prospects for conductive hydrogel applications and provide greater performance in their applications.

Toward Plant Cyborgs: Hydrogels Incorporated onto Plant Tissues Enable Programmable Shape Control

Engineered living materials (ELMs) that incorporate living organisms and synthetic materials enable advanced functional properties. Here, we seek to create plant cyborgs by combining plants or plant tissues with stimuli-responsive polymeric materials. Plant tissues with integrated shape control may find applications in regenerative medicine, and the shape control of living plants enables another dimension of adaptability and response to environmental threats, which can be applied to next-generation precision farming. In this work, we develop chemistry to integrate stimuli-responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogels with decellularized plant tissues assisted by 3D printing. We demonstrate programmable shape morphing in response to thermal cues and ultraviolet (UV) light. Specifically, by taking advantage of the extrusion-based 3D printing method, we deposit nanocomposite PNIPAM precursors onto silane-treated decellularized leaf surface with prescribed shapes and spatial control. When subjected to external stimuli, the strain mismatch generated between the swellable nanocomposite PNIPAM and nonswellable decellularized leaf enables folding and bending to occur. This strategy to integrate the plant tissues with stimuli-responsive hydrogels allows the control of leaf morphology, opening avenues for plant-based biosensors and soft actuators to enhance food security; such materials also may find applications in biomedicine as tissue-engineering scaffolds.