Flexible and Stretchable PEDOT‐Embedded Hybrid Substrates for Bioengineering and Sensory Applications

Background

Conductive polymers combine electrical activity with polymer processability and are used in biosensors, implantable monitors, tissue scaffolds and wearables, but pristine PEDOT is stiff, brittle and insoluble, which limits its use in flexible electronics. Electrospinning produces porous mats of interconnected fibers that resemble extracellular matrix, and the degree of fiber alignment offers an indirect way to tune their mechanical properties.

Design and approach

NBR/PEGDM fibers were electrospun under UV for partial photocrosslinking and then thermally crosslinked. Random mats were collected on a stationary plate, and aligned mats were spun on a custom apparatus after adding high molecular weight poly(ethylene oxide) (PEO), which was later leached out. The mats were swollen with 3,4-ethylenedioxythiophene (EDOT) and polymerized with FeCl3 so that PEDOT forms a semi-interpenetrating network within the rubbery fibers, intended to keep the elasticity of the rubber and the conductivity of PEDOT.

How it was evaluated

The authors characterized fiber morphology and alignment by scanning electron microscopy, surface and nanoscale modulus by atomic force microscopy, and bulk mechanics by uniaxial and cyclic tensile tests with aligned mats loaded along the fiber direction. Conductivity was measured with a four-point probe, resistance was recorded during cyclic stretching and finger-joint bending, and cyclic voltammetry was run in PBS at pH 7.4. Cardiac fibroblasts and cardiomyocytes isolated from two-day-old Sprague-Dawley rats were cultured on the mats for viability, metabolic activity and immunostaining.

Key results

Aligned and random pristine fibers had similar diameters (7.0 ± 1.2 μm and 6.6 ± 1.7 μm), and the PEDOT-embedded mats were about 60 μm thick. PEDOT raised the Young's modulus from 0.7 ± 0.4 to 3.8 ± 1.8 MPa in aligned mats and from 2.3 ± 0.8 to 10.3 ± 3.0 MPa in random mats, and the nanoscale DMT modulus was also significantly higher. Elongation at break fell from 122.4 ± 12.6% to 75.1 ± 18.1% (aligned) and from 84.9 ± 11.1% to 48.3 ± 6.4% (random). Under cyclic stretching to 20% elongation, hysteresis changed between cycles 1 and 10 and did not decrease significantly thereafter, and the authors concluded that PEDOT reduced stretchability but improved shape recovery.

Conductivity was 5.8 ± 3.6 S/cm for aligned and 4.6 ± 3.1 S/cm for random PEDOT-embedded mats, which the authors read as independent of fiber orientation. During cyclic stretching at 20% strain, resistance followed the extension cycles; aligned mats gave more stable and reversible changes during the first 10 cycles, and after 10 cycles both mat types showed stable changes of about 6%.

Mounted on the finger joints of a gloved hand, the mats increased in resistance as the bending angle increased and held that value while the finger stayed bent, with larger changes for aligned than random mats at 0°, 45°, 90° and 135°. A PEDOT-embedded sheet on porcine skin also changed resistance when bent, and cyclic voltammetry in PBS led the authors to propose the mats as electrodes that do not require cytotoxic electrolytes.

Cardiac fibroblast viability after 24 hours was 79.7 ± 7.0% on aligned and 79.1 ± 9.5% on random PEDOT-embedded mats, whereas no fibroblasts were observed on pristine NBR/PEGDM mats, and the proliferation rate of cells increased significantly on the PEDOT-embedded scaffolds. Cardiomyocytes on both mat types showed sarcomeric α-actinin striations and connexin-43 (Cx-43) expression.

Open questions

The authors noted that the mats are stiffer than cardiac or skeletal muscle (10 kPa to 50 kPa), that cardiomyocytes did not clearly align on the aligned mats, and that the sheets are too thin (about 100 μm) to form thick tissue constructs without stacking with cell layers.

How it connects to other research

The related silver nanoparticle work builds stretchable printed circuits from a conductive silver network inside elastomeric SBS fibers, whereas this study polymerizes PEDOT within crosslinked NBR/PEGDM fibers as a semi-interpenetrating network and tests the mats with cardiac cells.