Elastic and stretchable functional fibers have drawn attentions from wide research field because of their unique advantages including high dynamic bending elasticity, stretchability and high mechanic strength. Lots of efforts have been made to find promising soft materials and improve the processing methods to fabricate the elastomer fibers with controllable fiber geometries and designable functionalities. Significant progress has been made and various interdisciplinary applications have been demonstrated based on their unique mechanical performance. A series of remarkable applications, involving biomedicine, optics, electronics, human machine interfaces etc., have been successfully achieved. Here, we summarize main processing methods to fabricate soft and stretchable functional fibers using different types of elastic materials, which are either widely used or specifically developed. We also introduce some representative applications of multifunctional elastic fibers to reveal this promising research area. All these reported applications indicate that the fast innovated interdisciplinary area is of great potential and inspire more remarkable ideas in fiber sensing, soft electronics, functional fiber integration and other related research fields.
Fiber breakage is found to be a ubiquitous phenomenon during the thermal treatments for electrospun nanofibers, because of the presence of solvent molecules and unrelaxed assembly of polymer chains. Here a strengthening strategy is designed by introducing a pre-heating stage for the as-spun nanofibers. At a temperature above the polymer’s glass transition temperature, the chains can get sufficiently relaxed by the aid of hot solvent, and at the same time, the solvent confined in the entangled chains can fully and steadily evaporate, but not erupt, off the nanofiber, when the temperature is just as high as the boiling point. Therefore, the nanofibers become more uniform in structure and can withstand the subsequent thermal reactions. For polyacrylonitrile-based electrospinning, such strategy can improve the final tensile strength from 42 to 112 MPa for the nanofiber films.
Parallel fibrous scaffolds play a critical role in controlling the morphology of cells to be more natural and biologically inspired. Among popular tissue engineering materials, poly(2-hydroxyethyl methacrylate) (pHEMA) has been widely investigated in conventional forms due to its biocompatibility, low toxicity, and hydrophilicity. However, the swelling of pHEMA in water remains a major concern. To address this issue, randomly oriented and aligned as-spun pHEMA nanofibrous scaffolds were first fabricated at speeds of 300 and 2000 rpm in this study, which were then post-treated using either a thermal or a freeze-drying method. In cell assays, human dermal fibroblasts (HDFs) adhered to the freeze-drying treated substrates at a significantly faster rate, whereas they had a higher cell growth rate on thermally-treated substrates. Results indicated that the structural properties of pHEMA nanofibrous scaffolds and subsequent cellular behaviors were largely dependent on post-treatment methods. Moreover, this study suggests that aligned pHEMA nanofibrous substrates tended to induce regular fibroblast orientation and unidirectionally oriented actin cytoskeletons over random pHEMA nanofibrous substrates. Such information has predictive power and provides insights into promising post-treatment methods for improving the properties of aligned pHEMA scaffolds for numerous tissue engineering applications.
The electro-thermal actuators (ETA) are smart devices that can convert electric energy into mechanical energy under electro-heating stimulation, showing great potential in the fields of soft robotics, artificial muscle and aerospace component. In this study, to build a low-voltage activating, fast responding ETA, a robust and flexible carbon nanotube film (CNTF) with excellent electrical and thermal conductivity was adopted as the conductive material. Then, an asymmetric bilayer structured ETA was manufactured by coating a thin layer of polydimethylsiloxane (PDMS) with high coefficient of thermal expansion (9.3 × 10–4 °C−1), low young’s modulus (2.07 MPa) on a thin CNTF (~ 11 μm). The as-produced CNTF/PDMS composite ETA exhibited a large deformation (bending angle ~ 324°) and high electro heating performance (351 °C) at a low driving voltage of 8 V within ~ 12 s. The actuated movement and the generated heat could be controlled by adjusting the driving voltages and showed almost the same values in 20 cycles. Furthermore, the influences of the PDMS thickness and driving voltage on CNTF/PDMS composite ETA performance were systematically investigated. The CNTF/PDMS soft robotic hand which can lift 5.1 times and crab 1.3 times of its weight demonstrated its potential capability.
Long-term in vivo monitoring of chemicals with implanted sensors has received considerable interests over the past decades owing to their significant contributions in reflecting health conditions and assistance in diagnosing diseases. However, the widely explored chemical sensors outside the body fail to meet the requirements of in vivo applications. This perspective reviews main challenges, recent advances and future directions of long-term in vivo monitoring of chemicals, related to immune response and sensing performance. Challenges in terms of the immune response caused by unstable interfaces between sensors and tissues and improper implanting methods, and the insufficient performance of chemical sensors in complex physiological environment are discussed. Therewith, recent advances in fabricating biocompatible, flexible and thin sensors, developing effective implanting methods with reduced injury and improving the sensitivity, selectivity and stability of chemical sensors for accurate monitoring in vivo are summarized. Finally, we propose the future directions to address these challenges by fiber chemical sensors through the combination of soft fiber configuration, facile implanting methods and new recognition elements, which will provide new platforms for health monitoring and physiological mechanism revealing.