Biocompatible Carbon Nanotube-Based Hybrid Microfiber for Implantable Electrochemical Actuator and Flexible Electronic Applications

Background

Microfiber actuators that operate inside the body require reliable mechanics, electrical performance and biocompatibility. Carbon nanotube (CNT) fibers can actuate in biological environments without cytotoxic electrolytes, but they suffer from creep, short cycle life and low mechanical stability because the nanotubes slide past each other. HA can disperse SWCNTs and act as an ion-conducting binder, but it is degraded by hyaluronidases in the body, which leaves creep, fatigue and possible nanotube leakage unresolved.

Design and approach

An HA/SWCNT suspension at a 5:8 weight ratio was injected through a 0.2 mm needle at 50 mL/h into a rotating coagulation bath of 5% CaCl2 in 70% ethanol at 20 rpm, where calcium ions physically crosslink the HA. The fibers were then activated with 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC) and 1-hydroxybenzotriazole (HOBt) and covalently crosslinked with hexamethylenediamine (HMDA) through the HA carboxyl groups, which are also the recognition sites for hyaluronidase. The covalent HA network is intended to improve load transfer between nanotubes and to resist enzymatic degradation and nanotube leakage.

How it was evaluated

The authors compared uncrosslinked and crosslinked fibers spun from suspensions containing 4 to 8 mg/mL SWCNT. Tests covered structure (scanning electron microscopy, polarized Raman spectroscopy), enzymatic degradation in DPBS with hyaluronidase (25 U/mL) for 5 days, tensile mechanics of swollen fibers, resistivity under tension, bending and folding, cyclic voltammetry, and actuation between +1 and −1 V versus Ag/AgCl at 25 mV/s under a 4 mN load. NIH-3T3 fibroblasts were cultured on braided fibers, and fibers were implanted in the dorsal subcutaneous space of 6-week-old female C57BL/6 mice (n = 4 per group) for 1 and 3 weeks.

Key results

The dry fibers were about 50 μm in diameter with SWCNTs aligned along the fiber axis, and crosslinking did not significantly change the Raman D/G ratio (0.18 ± 0.01 uncrosslinked, 0.17 ± 0.02 crosslinked). In hyaluronidase, the supernatant absorbance after 120 h was 0.38 for uncrosslinked fibers and 0.03 for crosslinked fibers, and after the degradation test the crosslinked fibers had on average 75% higher Young's modulus and 600% higher tensile strength.

As SWCNT concentration increased from 4 to 8 mg/mL, failure strain rose from 2.0 ± 0.6 to 6.1 ± 1.6% for uncrosslinked fibers and from 4.9 ± 0.4 to 11.7 ± 2.6% for crosslinked fibers. Young's modulus increased from 0.5 ± 0.1 GPa (4 mg/mL, uncrosslinked) to 1.8 ± 0.4 GPa (7 mg/mL, crosslinked) and fell to 1.0 ± 0.5 GPa at 8 mg/mL, so 7 mg/mL was identified as the optimum concentration.

Crosslinking increased resistivity but made it more stable under deformation. In bending, resistivity increased on average by 11% for uncrosslinked and 6% for crosslinked fibers; under folding, crosslinked fibers stayed within a 10% increase over 2500 cycles and plateaued at about 14% at 3000 cycles, whereas uncrosslinked fibers reached about 65%. The capacitance reached 76.2 F/g, and crosslinking had no significant effect on it.

The fibers expanded at −1 V and contracted at +1 V in DPBS, which the authors attributed to combined double-layer and quantum mechanical effects. Crosslinked fibers recovered their strain over several cycles, whereas uncrosslinked fibers accumulated residual strain with creep-like behavior. Actuation strain of crosslinked fibers ranged from 0.04 to 0.09% and was highest at 7 mg/mL; a scan rate of 50 mV/s or lower was needed for strains of 0.06% or more, and a braided bundle of six fibers gave a stable 0.05% strain.

NIH-3T3 fibroblast viability on braided fibers remained above 80% from day 1 to day 5. In mice, hematoxylin and eosin staining showed thicker tissue around crosslinked than uncrosslinked fibers, and crosslinked fibers drew more monocytes and M1 macrophages (CD68 and CD86) at 1 week, but after 3 weeks only a few of these cells were detected around either fiber type.

How it connects to other research

The related Science report described electromechanical actuators made from single-walled carbon nanotube sheets, in which electrochemical double-layer charging causes a quantum chemical-based expansion of the nanotubes, whereas this study binds SWCNTs with crosslinked HA in wet-spun microfibers tested in DPBS, where the authors propose that double-layer and quantum mechanical effects both contribute to actuation.