Can the venerated silk be the next-generation nanobiomaterial for biomedical-device designing, regenerative medicine and drug delivery? Prospects and hitches
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Of late, the relevance of silk in a myriad of material science and biotechnological realms has been realized, as attested by the incessantly clambering number of reports and patents in the scientific repositories. The write-up is geared off with a scrutiny into the pertinence of the basic nano-structural features of silk, christened as the ‘queen of textile’ for exemplary bioengineering applications including designing and fabrication of devices for microfluidics, optofluidics, chemo/bio sensing, etc. Then, the major thrust of this short review is directed towards comprehending the prospects of using silk-based biomaterials (e.g. scaffolds, electrospun membranes, films, hydrogels, bioinks) for tissue engineering and regenerative medicine as well as targeted delivery of various biomolecular cargoes/therapeutic agents, etc., as vouched by few avant-garde endeavours of the recent years. The write-up is entwined with a discussion on the various factors that could plausibly hinder the realization of silk as the next-generation nanobiomaterial, suggestions for some approaches to dodge and deal with the practical snags and what lies ahead!
KeywordsDrug delivery Microfluidics Nanobiomaterial Regenerative medicine Silk Tissue engineering
Witnessing the continually growing pertinence of silk as a material of choice in the biomedical domain is quite invigorating. In recent years, there has been commendable addition to the records in the scientific repositories, vouching for the immense prospects of silk in the niche of tissue engineering and regenerative medicine (TERM). Revered with the sobriquet of ‘queen of textile’, silk is making a larger-than-life journey from the traditional looms and the ramp walks to the operation theatres as well as its foray into the nano/bioengineering labs for fabrication of innovative optical, electronic, energy and microfluidic devices demand special mention [1, 2, 3]. A myriad of insects (e.g. silkworms, spiders etc.) are known to synthesize silk, the synthesis being dictated by microfluidic principles . Silk represents nature’s quintessential nanotechnological manoeuvre, attesting the evolutionary intricacies of nano-viscoelastic, nano-roughness, nano-stiffness, nano-electric and nano-tribological facets [5, 6]. Dictates of intermediate hydrophobic/hydrophilic block ratios and longer chain lengths have been corroborated with the formation of spider silk fibre and its high strength, as attested by mesoscopic modelling in concert with genetic block copolymer synthesis . Furthermore, functioning of spider silk peptide as linear nano-spring for intracellular tension-sensing has also been proposed . On the other hand, covalently integrated repetitive heavy and non-repetitive light chains as well as the crystalline and amorphous domains (with varied compositional abundance of amino acids) are the signature attributes of silkworm silk . Features of biocompatibility, resilience, mechanical robustness and tunable biodegradation profile as well as ease of processibility into multiple formats (e.g. nanoparticles, films, electrospun mats, hydrogels, etc.) have conferred special niche to silkworm silk (constituted by fibroin (SF) and the sticky hydrophilic sericin (SSC)) in particular in the domains of soft and hard tissue engineering, cosmetics, food preservation, etc. [1, 2, 9, 10]. Perusal of recent publications and patents unveils that exploration of the nanoscale attributes as well as the unique biomaterial properties of silk has paved the way to numerous advanced biomedical applications, amongst others. In the succeeding sections of this article, I have tried to cite a few exemplary endeavours that stand in testimony to the afore-stated statement, interlaced with highlights on the prospects and challenges in the niche of silk-based nanobiomaterials.
From micro/nanofluidics to optofluidics
Prospects for TERM and biocargo-delivery
As mentioned in the introductory section, silk has received an unprecedented impetus in the domain of TERM and allied biomedical applications. Some of the exemplary reports are cited underneath. To start off, silk-based approaches to address skin damage has been at the forefront of research in the last few years . As an exemplary evidence, Jadi et al.  had demonstrated that non-mulberry silk sericin could function as an effective antioxidant in ameliorating UV A and UV B radiation-induced skin damage, thereby raising the prospects of its inclusion in skin care products (Fig. 1III). Needless to say that a number of pathophysiological conditions (e.g. burns, diseases like diabetes, etc.) perturb the otherwise highly orchestrated process of wound healing. In situ 3D printing, in situ forming hydrogels, electrospraying, use of microRNA (miRNA) and small interfering RNA (siRNA)-based skin therapeutics and so on have been investigated for accelerated wound healing . Amongst others, electrospun silk-based mats have garnered considerable research thrust in the recent years. Lately, a simple coating technique was employed for functionalization of silkworm silk fibroin-based electrospun matrices with recombinant spider silk fusion proteins . Functionalization of the spider silk fusion proteins with cell-binding RGD (Arg-Gly-Asp) peptide, growth factor peptide and antimicrobial peptide conferred multifaceted features to the fabricated SF-based wound dressings (Fig. 1IV). In an extended work to this report, the researchers had recently demonstrated substantial skin tissue regeneration (in comparison to commercial Duoderm dressing and untreated wounds), mediated by spider silk fusion proteins functionalized silkworm silk-based skin dressings in alloxan-treated diabetic rabbit model . Thus, these studies attest the prospective candidature of such hybrid silk matrices to address diabetic wounds and burns.
In another recent endeavour, a bioengineered silk-blend-based full thickness angle-ply construct (Fig. 2II) was reported to mimic the native intervertebral disc (IVD) complexity . Proliferation, alignment and maturation of primary annulus fibrosus (AF) cells (porcine-origin) and differential mechanical features mediated guidance of extracellular matrix turnover were documented. This study is pertinent in the context of the fact that the conventional surgical interventions (which are highly case-dependent and not applicable to all patients) are good enough only in symptomatic pain relief, with no restitution of the biomechanical functions  and possibly eventual degeneration of adjacent parts.
On a similar note, the current treatment approaches of autologous chondrocyte implantation, matrix-assisted chondrocyte implantation or mosaicplasty to address osteochondral defects (OCD) are challenged in the context of obtainability of suitable donor tissue, donor site morbidity and the consequential poor resistance of the fibrocartilage towards shear and clinical robustness . In this regard, exploitation of biphasic scaffolds for OCD has been envisaged as a prospective solution. At this juncture, I would like to cite one of our works wherein, we had demonstrated that a silk–bioglass-based inexpensive, implantable biphasic electrospun nanofibrous composite, mimicking the osteochondral interfacial micro-niche (Fig. 2III) could offer a plausible solution for conditions like osteochondral degeneration and arthritis . The biphasic scaffold provided a bioactive, mechanically robust and porous bioglass-based osteogenic matrix to emulate the micro-milieu of the subchondral bone (that assisted directional alignment of osteoblasts) while chondrocyte condensation into functional clusters was evident on the silk mats.
Representative nanoscale silk-based therapeutic agent delivery systems
Silk protein used for fabrication of the nano-delivery system
Outcome, observed at the bio-interface
Bound insulin exhibited augmented resistance to trypsin digestion and prolonged half-life than native insulin; in vitro stability in human serum
Speedier killing of gastric cancer cell lines BGC-823 and SGC-7901; better antitumor activity than the free PTX
Apoptosis in A549 lung cancer cells
Increased hyperlipidemic activity
Quercetin’s sustained release at pH 7.4 (phosphate buffer saline) and pH 6.8 (simulated intestinal fluid)
pH responsive, surface charge reversal nanoparticles with augmented cellular entry and Dox delivery
Increased cytotoxicity in neuroblastoma cells in comparison to hepatocarcinoma cells
On the other hand, formation of strong complexes of silk with DNA is hindered due to non-cationic nature of the former. In this context, nanoscale complexes of silk chains with poly(l-lysine) copolymers, displaying electrostatic interaction with plasmid DNA (pDNA) were employed for gene-delivery into human embryonic kidney cells . Similarly, Antheraea pernyi silk fibroin (rich in cell-binding RGD sequences)/poly(ethylene imine)/DNA ternary complex exhibited higher transfection efficacy in human colorectal carcinoma (HCT 116) and human embryonic kidney (HEK 293) cells with augmented transfection being displayed in the former, attributed to copiousness of RGD binding integrins . These are just a few representative studies to illustrate the potential of silk for regenerative medicine and allied biomedical applications.
The challenges and the future direction
Albeit, there exist tremendous prospects for bench-to-bed side technology transfer of silk-based nanobiomaterials, a number of pertinent issues need to be addressed tactfully. The prime requisite lies for a scalable, economically feasible-production protocol (with quality-control) for the silk-based nanobiomaterials, propelling clinical trials and applications. In particular, some of the practical snags while resorting to wild non-mulberry silk (endowed with RGD sequence that promotes cell adhesion and proliferation) are the slow-paced seed production and assaults to the worms by a myriad of pests, predators and diseases. Indoor rearing of such worms could be a safer alternative. From the laboratory-experiments’ perspective, the conventional (toxic) lithium salts (used for mulberry silk fibroin extraction) do not hold good enough for cocoon-solubilization for non-mulberry silk protein extraction while harvesting of gland proteins is challenged by concerns of rapid aggregation and precipitation. Anionic detergents like sodium dodecyl sulphate have been forwarded as effective substitutes . On the other hand, territorial nature and hostile attribute of spiders obstruct the collection of ample volumes of their silk proteins. In this context, seri-informatics, complemented by genomic/molecular tools would be instrumental in conservation and improvement in the production of silk of superior quality. In a similar vein, deeper insight into the biochemical machinery of the silk worms (particularly the endemic varieties) and the spiders along with metabolic and cellular engineering of appropriate hosts in the context of genetic engineering could pave the way to the production of novel silk-based nanobiomaterials. Needless to say that comprehending the physicochemical characteristics of the silk proteins with respect to the genomic and proteomic information of the silkworms is of utmost importance [1, 43]. Another issue is the current incipient status of the clinical trials of silk-based nanobiomaterials [3, 44]. There lies a plethora of reports on the successful in vitro or preclinical assessments of silk-based nanobiomaterials for regenerative medicine as well as prospective delivery of biomolecular cargoes; however, in vivo studies and clinical trials are just a handful. Detailed unveiling of the pharmacokinetics, pharmacodynamics and off-target outcomes in bench studies is pre-requisite for commercial translation of silk-based nanobiomaterials.
Amalgamation of bioreactors and stem cell technology is envisaged to pave the way to more realistic applications of silk-based nanobiomaterials for TERM. What next? Possibly, the simulation and optimization strategies to fabricate transplantable bioengineered organs with befitting performance/price equilibrium. Furthermore, silk-based scaffold systems could be possibly employed for the enhanced production of medically important biocatalysts like urokinase under optimized media in bioreactors. Current research also seems to be buoyant towards organ-on-a-chip/multichannel 3D microfluidic silk-based cell culture chip platforms in near future. The realization would imply a route to evade the use of animals in drug assessments and toxin evaluation. As a conclusive statement, I would like to stress upon the fact that silk-based nanobiomaterial-research demands inter-institutional concerted activities and incentives for an innovation ecosystem to ensure a ‘silken-health for all’.
I would like to offer a note of gratitude to Dr. Biman B. Mandal, IIT Guwahati, India, for introducing me to the awe-inspiring niche of silk-bioengineering.
The author did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors while compiling this article.
Compliance with ethical standards
Conflict of interest
The author declares that he has no conflict of interest.
No studies with human or animal subjects were conducted afresh by the author for inclusion in this review article.
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