Functionalization of structural materials through electro-blown spinning of ultrathin and transparent silk fibroin ionotronic nanofiber skin

Abstract

Direct functionalization of structural materials can provide an elegant balance between their structure, performance, and function. However, effective direct functionalization techniques are very limited. This work reports a technique to functionalize structural materials in a non-destructive manner, wherein the surface of the materials is selectively etched without compromising their internal structure. In this technique, an ultrathin and transparent silk fibroin ionotronic nanofiber skin (SFINS) is spun on the surface of structural materials through electro-blown spinning, endowing them with electric conductivity and environmental responsiveness. This process is characterized by outstanding scalability, low cost, and high efficiency. In addition, the SFINS can be firmly bonded to the surface of different structural materials, such as glass, metals, polymers, and wood. More importantly, the electro-blown spinning process and the formed SFINS do not alter the structure, properties, and sustainability of the substrate materials. For instance, the transparency of glass and acrylic plates, the texture of wood, the color of plastics, the text of papers, the elasticity of elastomers, as well as their strength are all retained. Consequently, structural materials functionalized under the proposed process can be directly integrated into functional devices that require both structural stability and functional diversity in practice use. In this work, two prototypes are developed to demonstrate the applications of SFINS-functionalized structural materials as tactile receptors for an intelligent sorting manipulator and as a self-powered IntelliSensor for geological hazard early warning. Both examples reveal the advantages of the proposed technique in terms of the balance between the structure, performance, and function of the developed device systems.

Graphical Abstract

Introduction

In traditional materials design, structural materials and functional materials are relatively independent. For instance, steel, concrete, glass, wood, and polymers are generally regarded as structural materials [1], [2], [3]. They are primarily used to provide structural support to buildings and engineering structures or flexibility to tires and snubbers [4], [5]. Nevertheless, the rapid development of new materials increasingly requires blurring their structure and function attributes. For instance, materials used in the manufacture of intelligent devices need to bear or comply with external forces, ensuring the structural stability of the device [6], [7]. However, it is also important for them to possess environmental awareness and response functions. For this purpose, structure-function integration in a material is preferred. This can reduce the design complexity and manufacturing costs, as well as optimize the structure, performance, and function of the resultant system. Indeed, the result of natural evolution, i.e., the survival of the fittest, makes the structural and functional characteristics of most natural materials indistinguishable. For example, the spider major ampullate gland silk is the structural framework of spider webs [8]. During prey capture, the silk-based structural framework acts also as signal lines that enable the spider to locate the spatial position of the prey through the perception of the vibration frequency [9]. Moreover, it is the spider's dragline for life support [10], [11]. Similarly, the cytoskeleton, which is the structural frame of the cell, channels the cell stability and endows it with deformability [12]. Furthermore, it participates in several important life activities, such as pulling chromosomes apart during cell division and transferring vesicles and organelles during material transport [13].

Thus far, it remains challenging to find a natural-selection-like path for engineering materials and design and fabricate materials through trial-and-error tests for hundreds of millions of years. However, attempts to realize a collaborative optimization of the structure and function of materials have been launched. For instance, a series of biomimetic materials and metamaterials have been reported that can trade off between structural stability, mechanical properties, and functional diversity [14], [15], [16], [17], [18]. In addition, a variety of structural engineering strategies, e.g., origami and auxetic design, have been developed to improve the mechanical performance of functional materials [19], [20]. Nevertheless, limited by the intrinsic properties of certain materials, e.g., the brittleness of silicon semiconductors [21] and the temperature-dependent properties of polymers [22], such strategies cannot solve all the problems in practice. For example, the sensors utilized in structural engineering, industrial equipment, and environmental monitoring still depend on the independent assembly of the power supply, sensing devices, and structural supports [23], [24], [25].

To realize an elegant balance between structure, performance, and function, a series of pioneering works have attempted the direct functionalization of structural materials. For example, cotton fabrics have been coated with a layer of graphene or elastomer ink to provide compliance for batteries and energy storage devices [26], [27], [28]. Silk fibers and textiles have been carbonized into conductive sensors to monitor the motion and deformation of the human body [29], [30]. Wood has been functionalized into transparent glass, photovoltaic panels, and energy storage devices through delignification, perfusion, and/or hot pressing [31], [32], [33]. Such strategies have significantly enhanced the added value of the structural materials, and a synergetic improvement of the material structure, performance, and function has also been achieved to a certain extent. However, these methods are often destructive. For instance, the coating of cotton fabrics needs to allow the solvent to fully penetrate the fibers. This process usually results in structural defects within the fibers that compromise their mechanical properties [34], [35]. Similarly, carbonization can dramatically weaken the mechanical performance of silk, transforming them from strong-and-tough silk fibers to brittle-and-weak carbides [36]. Although the functionalization of wood can preserve or even enhance its mechanical properties, such mechanical enhancement is typically attributed to the added components, such as epoxy resins and zeolitic imidazolate framework materials [37], [38]. In fact, during the delignification process, the mechanical advantages of wood are practically eliminated [39].

In this study, an electro-blown spinning (EBS) technique is developed to silk fibroin ionotronic nanofiber skin (SFINS) on structural materials. Fig. 1 summarizes the advantages and uniqueness of using EBS to produce SFINS for the functionalization of structural materials. Henceforce, this technique will be called ESFSM. These benefits are reflected in the preparation technique, composition, structure, mechanical properties, and functionality of the SFINS, as well as in the retention of the structure and the performance of structural materials during the functionalization process [40], [41], [42]. Regarding the preparation method, we used a highly efficient and easy-to-scale-up EBS technique (Fig. 1a). Compared to electrospinning, the introduction of air-blowing has improved the sample preparation speed by 3–10 times [43], [44]. In addition, EBS is more suitable for the continuous functionalization of large-sized structural components [45]. Since SFINS has good conductivity, environmental sensitivity, conformality, and sustainability, as well as strong adhesion, frost resistance, and self-healing capabilities. The multifunctional SFINS can be spun directly onto the surface of structural materials, with the resultant structure retaining all these features. Thus, these functionalized structural materials can be directly integrated with Internet of Things for functional use. In this work, two prototypes are developed to demonstrate the application of the proposed SFINS in the tactile receptors of an intelligent sorting manipulator and the self-powered IntelliSensor of a geological hazard early warning system.

Section snippets

SFINS design and fabrication

Here, SFINS was selected as the functional component mainly based on the following considerations. First, SFINS can provide a wide range of adjustable mechanical properties, from hard and strong to soft and tough. Second, the SFINS, silk fibroin (SF), and ion components are environmentally friendly and degradable. Hence, involving them in the functionalization process will not increase the environmental burden imposed by the structural materials [46]. Third, the rheological properties of silk

Conclusions

In summary, this work presents a rational strategy for the non-destructive functionalization of structural materials. This strategy uses a high-efficiency EBS technique to spin SFINS onto the surface of structural materials, endowing them with electrical conductivity and environmental responsiveness without affecting their structural and mechanical properties. The proposed process is characterized by outstanding scalability, low cost, and high efficiency. Moreover, the developed spinning dope

Materials

Formic acid (FA; CAS 64–18–6, 98%) was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Lithium chloride (LiCl; CAS 7447–41–8, AR) was purchased from Aladdin Reagent Co., Ltd., Shanghai, China. Sodium bicarbonate (NaHCO3; CAS 144–55–8, AR) was purchased from Meilunbio Co., Ltd., Dalian, China. Bombyx mori (B. mori) silkworm cocoons were collected from the Huzhou agricultural markets, Zhejiang province, China. The commercially VHB acrylic elastomer films were purchased from

Article Source：https://www.sciencedirect.com/science/article/abs/pii/S1748013223001226?via%3Dihub