Unveiling the Potential of Natural Silk Proteins
by Anabela Veiga, CBQF/LEPABE
Natural-driven proteins are receiving increased attention due to their excellent intrinsic properties, such as biocompatibility, biodegradability, tunability, and functionalization. In particular, the use of natural silk proteins is gaining a lot of relevance in both the research and industrial community. In this article, I will unveil the potential of natural silk proteins.
The Origin and Importance of Silk
Silk is produced by several organisms, such as silkworms, spiders, mollusks, scorpions, bees, and ants. However, silk from domesticated silkworms is the most well-established and suitable source for cosmetic and biomedical applications due to its abundance, stability, and clinical track record.
Among the silk-producing lepidopteran insects belonging to Bombycidae or Saturniidae families, nearly 95% of commercial silk production is by mulberry silkworm Bombyx mori.
Sericulture includes cultivating mulberry trees, rearing silkworms on mulberry leaves to produce cocoons, and silk reeling and weaving. These processes are essential in Asia (90%), specifically China and India, containing the largest collections of well-defined genetic stocks of mutants and practical breeds. The remaining 10% of global silk production comes from Brazil, Uzbekistan, Vietnam, North Korea, and Thailand.
The Composition of Silk is Essential to Understanding Its Specific Applications
Silk is composed of two main proteins with different properties that can be applied to develop biomaterials that generate a good clinical response while being cost-effective: silk fibroin and silk sericin. Silk fibroin comprises a fibrous semi-crystalline silk core, mainly responsible for the load-bearing capacity. The outer layer of silk is made of a globular protein, silk sericin, which serves as a gluing agent and has a protective function.
While fibroin has been used for centuries in the textile industry and decades in biomaterials, providing stiffness and resistance, sericin is a by-product generated during this process in the so-called degumming procedure.
Silk sericin represents about 50,000 tons of raw silk thread processing per year. Recent studies changed the sericin paradigm, valorizing this protein and reducing the environmental impact generated. It is important to note that sericin can improve biocompatibility in vitro, increase cell adhesion and proliferation of several mammalian cells, and act as a nutritive media for cell growth.
Surprisingly, sericin’s interesting characteristics don’t stop there. Antioxidant and anti-inflammatory behaviour and the ability to stimulate collagen production are also among its many properties.
Natural Silk Proteins for Biomedical Engineering
We have witnessed silk shifting from being used mainly for apparel and fashion to being used in other vital industries such as biomedical engineering: an excellent industrial setup for sericulture in modern times!
Silk fibroin maintains mechanical tensile integrity in vitro in tissue culture conditions and exhibits slow degradation in vivo. It is used for sutures, cardiac patches, artificial ligaments, and scaffolds for bone tissue engineering.
Silk sericin is typically used for skin applications. Nevertheless, sericin-based materials have been applied in bone-related applications to study calcium phosphates' biomineralization process and mimic the organic/inorganic bone matrix.
Both silk proteins can be easily modified by chemical and physical methods to enhance their properties further. A more widespread approach is the combination of silk sericin or fibroin with other materials, forming composites to accommodate a broader spectrum of functional requirements.
As a result of silk’s intrinsic properties and tunability, innovative possibilities are rising from these proteins in recent years. New biomaterials include silk-based microcarriers for the growth of adherent cells in bioreactors. This technology allows the creation of a 3D environment similar to natural surroundings in vivo and may provide an alternative to animal studies.
Moreover, the microtissues formed by microcarriers and cells can be delivered directly to the defect site, eliminating the digestion of cells before transfer from a monolayer culture (characteristic of conventional in vitro assays).
Developing silk-based bio-inks for 3D bio-printing is also a cutting-edge application that is increasingly the subject of research and testing.
Factors to Consider When Using Natural Silk Proteins
As with all biomaterials, there are several key factors to consider when using silk proteins, such as their origin, extraction methods, and concentration techniques. Both sericulture practices and experimental/industrial procedures must be standardized and controlled to obtain silk-based materials with homogeneous and reproducible properties.
In particular, strict protocols with the desired yield and the least downstream processing and variability are required when contemplating pharmaceutical and biomedical applications.
Extraction (which can be conducted using different reaction types: acidic, alkaline, enzymes or even different conditions: time, temperature, pressure, pH) and concentration methods (dialysis, lyophilization, evaporation, among others) significantly affect the protein's biochemical properties, generating materials with different molecular weights, that can range from 10 to over 400kDa, and even distinct amino acid composition.
Hence, there is still an unmet need to develop a scalable process to supply silk proteins with controlled and tailored properties at an industrial level.
About the Author
Anabela Veiga completed the Integrated Master in Bioengineering in 2018 by the Faculty of Engineering of the University of Porto (FEUP) in collaboration with Abel Salazar Institute for Biomedical Science (ICBAS). She is doing her PhD in Biotechnology at the Faculty of Biotechnology of the Catholic University of Portugal in collaboration with the Laboratory for Process Engineering, Environment, Biotechnology and Energy (LEPABE). During this time, she was involved in a mobility internship at the Institute of Polymer Science and Technology in Madrid, Spain. She recently completed research at the Department of Biomedical Engineering, Tufts University, Boston, USA, with the support of a Fulbright Grant.
Her research focused on inorganic and composite precipitation systems, development and characterization (hydrodynamics and mass transfer) of scaled-down multiphase reactors for both continuous and batch processes and in protein-based biomaterials. In addition to Medical Biotechnology, Biomaterials and Chemical Engineering she also has interest in Health Care Economics and Management.
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