Biomimetic hybrid nanofibrous network for building strong, robust, and high-conductivity materials

Abstract

Collagen fiber plays a critical role in load-bearing soft tissues, such as cartilage, ligament, and skin. Besides, the arrangement of collagen fibrils and other constituents in tissues contributes to the tunable mechanical properties and a variety of functions of soft tissues. Compared with collagen fibrils from macroscopic collagen fiber in nature, synthetic aramid nanofibers (ANFs) achieved from macroscale poly(paraphenylene terephthalamide) fiber also exhibit exceptional mechanical properties. Composite hydrogel consists of rigid ANFs and soft poly(vinyl alcohol) (PVA) shows promising mechanical properties compared to those of collagen-fibril-reinforced natural materials, which opens up new opportunities for fabricating abiotic soft tissues for diverse applications. However, the strengthening and toughening mechanisms of such nanocomposite network are still vague; Additionally, it is still under development to further regulate this network to achieve even greater mechanical properties matching or outperforming those of natural tissues using biomimetic approaches. Functionalization of ANF-PVA hydrogel with electrical conductivity is vital for its applications in soft electronics, energy harvest and storage devices, and soft robotics. We first focus on the mechanical behaviors of the nanofibrous network. ANF-PVA hydrogel was dried by critical point drying to generate ANF-PVA aerogel without water. The mechanical properties of the aerogel were extensively researched, and theoretical simulations were used to reveal the relationship between nanoarchitecture and macroscopic mechanical behaviors of the network. The aerogels have a typical assembled network with high nodal connectivity and strong crosslinking between nanofibers due to the interactions between the nanoscale constituents. 3D modeling of the deformation of nanofibrous network shows these features at nanofiber joints may lead to an increment of orders of magnitude on the mechanical properties. Indeed, the polymeric aerogels show both high specific tensile modulus at ~625.3 MPa cm3 g−1 and fracture energy of ~4700 J m−2. Electrical conductivity of ANF-PVA network was initially achieved by mobile ions added into the network. Further chemical crosslinking allows the elastic, conductive and mechanically strong ANF-PVA hydrogels for large-strain sensors with stable operation. Alternatively, conducting polymer of polypyrrole (PPy) was in-situ synthesized within aramid nanofibrous network, and a topological clue of nanostructured conducting pathway contributes to 2-3 orders of magnitude enhancement on the electrical conductivity of hydrogels compared with those of other PPy-based hydrogels (limited below 0.2 S cm−1). The hydrogel composed of ANF, PVA and PPy exhibits a combination of a high conductivity of ~80 S cm−1, mechanical strength of ~9.4 MPa, and stretchability of ~36%. We also demonstrate its great potential as electrodes and interconnects of soft electronics and implantable devices for tissue engineering and supercapacitors. Overall, by using the merits of biomimetic ANF-PVA nanofibrous network, we have made breakthrough on the mechanical properties of nanofibrous materials, expanding the materials toolbox for a wide range of applications. We also propose a notable approach to fabricating electroconductive hydrogels based on the hybrid assembly of polymeric nanofiber networks, combing high electrical conductivity with favorable mechanical properties for device applications.