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Physics, Applied
Extensive studies have been conducted on utilising natural fibres as reinforcement in composite production. All-polymer composites have attracted much attention because of their high strength, enhanced interfacial bonding and recyclability. Silks, as a group of natural animal fibres, possess superior properties, including biocompatibility, tunability and biodegradability. However, few review articles are found on all-silk composites, and they often lack comments on the tailoring of properties through controlling the volume fraction of the matrix. To better understand the fundamental basis of the formation of silk-based composites, this review will discuss the structure and properties of silk-based composites with a focus on employing the time–temperature superposition principle to reveal the corresponding kinetic requirements of the formation process. Additionally, a variety of applications derived from silk-based composites will be explored. The benefits and constraints of each application will be presented and discussed. This review paper will provide a useful overview of research on silk-based biomaterials.
- silk-based composites
- time–temperature superposition
- biomaterials
1. Introduction
In the production of composites materials, the increase in awareness of building a sustainable future has stimulated the idea of replacing non-renewable petroleum-based polymers with bio-based constituents that have low or zero carbon emissions [1]. For example, substantial research can be found on incorporating cellulosic fibres such as flax, hemp, kenaf and bamboo as reinforcement to produce natural-fibre-reinforced polymer composites (NFRPCs) for applications in the automotive industry [2,3,4]. Researchers believe that these NFRPCs have the potential to significantly reduce the weight of vehicles by 30% and cut down the overall manufacturing cost by 20% [5,6,7]. Comparatively, silks, a group of protein-based fibres that have evolved for millions of years and have properties that often exceed man-made materials, have had limited studies as a reinforcement for composites in engineering applications [8].
For the use of these natural-fibre composites in engineering applications, studies have pointed out that NFRPCs present a few drawbacks: (i) surface incompatibility between natural fibres and polymer matrices results in poor interfacial bonding between the fibre and matrix phases and negatively impacts the physical and mechanical performance of the composites; (ii) difficulty separating the constituents during the recycling process leads to degraded performance after recycling [5,9,10]. To overcome these concerns, considerable research has focused on exploring the potential of self-reinforced composites (SRCs), also referred to as single/all-polymer composites, where both the reinforcement and matrix are composed of the same polymer [11]. In 1975, Capiati and Porter first developed all-polyethylene (all-PE) composites through partially melting the crystals within the PE fibres, providing a gradual change in the morphology between fibre and matrix and resulting in a competitive interfacial shear strength comparable to conventional glass fibre reinforced with epoxy resin composites [12]. Following the success of all-PE composites, research on SRCs began around the world. The most-reported synthetic-fibre-based SRCs are fabricated using polyolefin-based (polyethylene and/or polypropylene), polyester-based (polylactic acid and/or polyethylene terephthalate) or polyamide-based (nylon) fibres or tapes. The techniques involved in fabricating SRCs include thermal processing [13], hot compaction [14,15,16,17], cool drawing [18], etc. [19].
Following the concept of SRCs and sustainable design of composite materials, all-natural fibre composites were first introduced by Nishino et al., leading to the design of all-cellulose composites (ACCs) [20]. There are generally two methods to fabricate ACCs: (i) an impregnation method (two-step approach), where fibres are impregnated by cellulose solution; and (ii) a surface-selective dissolution method, where surfaces of fibres are partially dissolved. Undissolved inner fibre cores serve as the reinforcement phase, while the dissolved fibres serve as the matrix phase upon coagulation [21,22,23]. Subsequently, research has been carried out utilising various sources of cellulose to fabricate ACCs, including ramie [24], cotton [25,26,27], hemp [28,29], flax [30], microcrystalline cellulose [21,31] and regenerated cellulose fibres such as Lyocell and Bocell [23,32,33].
2. Silk Structure and Silk-Based Composites Properties
Silk is a common biological protein formed of a complex hierarchical structure with variable chemical compositions. The most common of these are the silk sericin and silk fibroin (SF) proteins. SF typically has a hexapeptide primary sequence dominated by glycine amino acid units, as seen in Figure 1 [67]. Raw fibre sheets of these biomaterials have inherent flaws compared to composites. Existing voids act as water channels to allow degradation by wetting, and hydrogen bonds can be broken by water molecules, which allows solvation and plasticisation [68]. This and other issues can be overcome by their inclusion in composites, which can improve material properties.
Figure 1. Illustration of the common chemical structure and amino acid sequence of a silk fibroin protein with a hexapeptide sequence.
Silk forms a complex semi-crystalline hierarchical structure in its native state. The most common commercial silk is from the silk worm, Bombyx mori, but silk is also produced by spiders and other species [69]. It exists in heavy or light chains of 390 or 25 kDa, respectively, with chaperonin-like P25 proteins in a 6:6:1 ratio displaced along fibril axes [70,71]. Natural fibres of Bombyx mori are typically formed of two filaments of SF with a surrounding matrix of gummy silk sericin to maintain integrity. The continuous phase of silk sericin is removed by degumming using hot water, alkaline or acidic solutions, or other methods. The structure can be seen in Figure 2 [69]. In Bombyx mori, SF content is 66.5–73.5 wt%, while sericin content makes up 26.5–33.5 wt% [72].
Figure 2. Illustration showing hierarchical structure of raw silk fibre with SF core and sericin coating.
The source of this silk can alter its properties with variations in strength, toughness and finish. Spiders alone can have seven different types of silk, controlled by the amino acid content of the protein. Dragline silk is rich in analine [73]. The precise effects of glycine and analine concentrations on SF crystallinity are disputed [69,73]. Shear and elongation stress also control conformation and crystallinity and, hence, material properties. Silk can reach a strength-to-weight ratio five times that of steel and three times that of Kevlar [73].
The high glycine content of SF allows for tight and stable packing of antiparallel β-sheet crystallites in SF. These are associated with the mechanical strength of silk [69]. In its native state, Bombyx mori SF has a very complex structure with differing amino acid sequences promoting high- and low-order structures [74]. This typically consists of 56±5% β-sheet crystallites and 13±5% α-helix conformations, with the remaining molecules disordered [75]. SF can form silk I, II, or III. Silk I has an α-helix or zigzag spatial conformation and is metastable. It often exists in phases within amorphous regions of semicrystalline SF and is shown to exhibit good swelling properties [69,76]. As with other crystal polymorphs, the structure has good chemical, thermal and enzymatic stability [76]. Silk II has the less soluble, stable β-sheet crystal structure, which is a monoclinic system [69]. Silk III is an unstable polymorph only seen at air–water interfaces in regenerated silk [67]. High silk II content is often associated with preferable material properties such as strength and toughness.
3. Published Methods of Tailoring Mechanical Properties of Silk Composites
Mechanical properties of NFRPCs are one of the most important characteristics for their use in engineering and biomedical fields, and it is essential for composite materials to provide adequate mechanical behaviour (tensile, flexural, impact and hardness) based on their desired applications [77,78]. The mechanical properties of NFRPCs depend on various parameters, such as fibre alignment, fibre length, fibre orientation, volume fraction of fibres, aspect ratio of fibres and fibre–matrix adhesion [79,80,81,82,83]. Studies investigating mechanical performance of NFRPCs have focused on two major aspects: (i) influences of various treatments of fibres (physical, chemical and biological), fibre content, fabrication process and external coupling agents on mechanical properties; and (ii) incorporating experimental data and well-established models to predict mechanical behaviour [84,85,86,87,88,89,90,91,92,93].
Natural silk fibres from Bombyx mori have relatively high mechanical properties: 300–740 MPa (ultimate strength), 4–26% (breaking strain), 10–17 GPa (Young’s modulus) and 70–78 MJm−3(toughness). These properties often exceed those of synthetic fibres such as nylon, Kevlar and polypropylene [54,61,94,95,96,97,98]. The variation in mechanical properties of Bombyx mori silk fibres comes from several factors, such as the food, rearing conditions and health of silkworms [99], differences in the spinning process (natural spinning, forced spinning at a controlled drawing rate and modulated spinning in an electric field) [54,100,101] and genetic modification of the silk sequence [102,103]. For the purpose of broadening silk-based applications, silk fibres generally undergo the process of degumming, dissolution and regeneration and subsequently form the formats of sponges, hydrogels, films, mats and composites for versatile applications. Research suggests that these SF materials show good biocompatibility with a series of cell types and also promote the characteristics of adhesion, proliferation, growth and functionality [104].
In particular, research on tailoring the mechanical properties of silk-based composites also follows the two major aspects addressed earlier: (i) applying experimental treatments to tune the mechanical properties in order to achieve the desired structure–property–application relationship; and (ii) using analytical models to predict mechanical performance for further facilitation of composite material design and optimisation.
4. Applications of Silk Composites
Although the fundamental research of silk composites is interesting due to the inherent complexity of the natural system and preparation conditions, they are often researched with intended applications in mind. Hence, research can either be approached as bottom-up, fundamental, blue-sky research or challenge-driven top-down research [112].
Silks have been primarily used for biomedical applications as they are perceived to be biocompatible, biodegradable and non-toxic [113]. It is of note that biocompatibility is not universal and must be specific to tissue and wound to encourage the correct immune response during healing [112]. Current uses of silk include sutures, surgical meshes and medical fabrics. Coating silk fibres with regenerated SF for use as sutures is one of the first examples of ASC use in medicine. Some future applications still being developed include tissue engineering and wound healing [112]. Microneedles also offer an exciting new development in transdermal vaccine delivery. SF microneedles offer a solution for controlled-release drug delivery with minimally invasive techniques [114,115]. As shown in works by Tsioris et al. and Stinson et al., SF microneedles provide favourable mechanical properties, biocompatibility, biodegradability, benign processing conditions and the ability to maintain the activity of biological compounds in its matrix [114,115]. This biomaterial could offer a new application for engineered biocomposites of SF in which the techniques mentioned above may confer improved toughness over simpler SF microneedle arrays.
Scaffolds of biomimetic materials are common forms of biosynergistic composites and function as host environments for cell and tissue growth and proliferation [116]. In order to be biomimetic, these engineered tissues must regulate healing phases by imitation of immunoresponse signals [117]. When preparing a composite for tissue engineering, it must provide [118,119,120,121]:
This complexity requires careful preparation of aero- or hydrogels to meet these requirements. This, then, requires engineered tissues to mimic highly variable native or artificial tissue mechanical (shear, tensile or compressive) moduli [122,123,124]:
SF is confirmed to promote cellular adhesion and proliferation of fibroblasts and keratinocytes [121,125,126]. Another benefit for biomedical uses and tissue engineering is that these materials can degrade in vivo and trigger minimal inflammatory response. They also aid in cell growth due to the intrinsic biocompatibility of SF materials [127,128]. This eliminates the need for implant-removal surgery but requires understanding of the degradation process to ensure it does not compromise efficiency [69,127,129]. Silk-based composites have also been prepared with drug retention and release capabilities that show some promise for targeted-drug-release applications [130]. Utilising ASCs could provide samples with high homogeneity, good material properties and tunable biological interactions for continued use in these applications. It is essential in biomedical applications of these composites to mimic the natural environment: for example, in the proliferation of fibroblasts on the surface of breast implants [126,131].
Wahab et al. utilised silk’s heavy-metal-adsorption properties in conjunction with low-cost bentonite clay to produce a composite capable of adsorbing lethal heavy-metal ions from aqueous solution [132]. They successfully impregnated SF with bentonite clay and achieved high monolayer adsorption values for Cd(II), Pb(II), Hg(II) and Cr(VI) [132]. Silk composites could offer future solutions in water sanitation aids as an environmentally sustainable tool.
Spider silk is seen to be of such high tensile quality, even compared to Bombyx mori silk [112], that composites reinforced with spider silk components have been thought of for use in aerospace engineering [133]. Mayank et al. prepared a transparent epoxy/spider-silk composite with material strength within the safety margins of typical standard acrylic aviation windowpanes [133]. They also showed improved impact deflection and stress loading. As a weight-reduced non-magnetic material, silk composites could offer an innovative material option for the aerospace industry [133]. It is of note that improved interfacial interactions of ASCs are seen due to chemically identical components of the matrix and the reinforcing fibres [50]. This is due to a favourable energetic interaction developed in identical biopolymers compared to phase-separated interfaces of polymer blends [134]. This interface is least influential in miscible polymer blends with favourable interactions, such as those between cellulose and silk fibroin [134]. ASCs could, therefore, reduce the likelihood of the primary failure mechanism of composites: failure at the interface of the reinforcement and the matrix. This implies the viability of ASCs in the uses mentioned throughout this article
Lastly, silk composites have also been used for decorative and restorative challenges, in line with their historic use. Cianci et al. produced a hybrid SF/cellulose solution to support and recover aged silk fabrics [71]. This forms a silk composite in which the cellulose improves silk crystallinity and material properties, allowing the sustainment of aged and degraded silk fabrics [71]. Untreated pristine silk and the same fabric treated with 1.35/1.35 wt% SF/cellulose nanocrystals dispersed in water then dried gave axial forces at breaking of ≈30 N and ≈35 N, respectively. This was a marked improvement over silk treated with single-component dispersions [71]. Other hybrid material composites have been produced for structural applications. Kimura and Aoki reinforced plywood and medium-density fibreboard with silk fabric using a polybutylene succinate matrix to create decorative laminates with improved flexibility and impact resistance [135]. ASCs could be used in a similar decorative capacity with dyed patterns from the fabric retained in the final product. As shown by Cianci et al., this could offer a use for waste silk with tangible benefits for the end product [71]. Though these improvements imply an impactful contribution from silk within these composites, it is essential to evaluate sustainability in the creation of these products. If overused, silk could contribute to environmental burdens rather than reducing them by increasing water usage and transport costs.
This entry is adapted from the peer-reviewed paper 10.3390/ma16103804
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