Keratin-Based Biomaterials For Reproduced Films, Blends And Composites:Potentials And Challenges
Keratin, a naturally-derived fibrous protein, is non-toxic and biocompatible with biological tissues. It is also capable of forming self-assembled structures which can regulate cellular recognition and behavior. These qualities have made keratin a potential solution to the problems encountered in fields like wound healing, tissue engineering, drug delivery, and medical devices.
Introduction
The development of biocompatible materials has always been one of the most focused area in the biomaterials field. Because of their availability and biocompatibility, numerous researchers have chosen to explore the potentials of natural occurring polymers, such as keratin. Specifically, protein-based biomaterials have emerged as potential candidates for many biomedical and biotechnological applications due their ability to function as a synthetic extracellular matrix that facilitates cell-cell and cell-matrix interactions. Such keratin contains a defined, three-dimensional microstructure that supports cellular proliferation and cell-guided tissue formation, both of which are important characteristics for biomaterial scaffolds. In addition, the strong bioactivities and diverse physiochemical properties of proteinaceous macromolecules are attractive for other biomedical applications for which biocompatibility is essential, such as medical devices, bioactive surfaces, hygiene products, etc.
In 1982. Moll et al. (1982) used 2D isoelectric focusing and SDS-PAGE to map the keratin profiles of a large number of normal human epithelia, tumors, and cultured cells. They grouped the basic-to-neutral type II keratins as K1–K8 and the acidic type I keratins as K9–K19. Type I and II IF form the bulk of cytoplasmic epithelia and epidermal appendageal structures (i. e. , hair, wool, horns, hooves and nails). The structural subunits of both epithelial and hair keratins are two chains of differing molecular weight and composition (designated types I and II) that each contain non-helical endterminal domains and a highly-conserved, central alpha-helical domain. The type I (acidic) and type II (neutral-basic) keratin chains interact to form heterodimers, which in turn further polymerize to form 10-nm intermediate filaments. Subsequent research of these structural proteins led to the classification of mammalian keratins into two distinct groups based on their structure, function and regulation.
A research in 2006 by Schweizer et al. on keratin’s structural proteins created the current version of consensus nomenclature from the basis of the 1982 work. They added three additional categories in addition to the original two categories (type I and II): human epithelial, human hair, and nonhuman epithelial and hair keratins. Several proteins have been investigated in the development of naturally-derived biomaterials, including collagen, albumin, gelatin, fibroin and keratin. Of these, keratin-based materials have shown promise for revolutionizing the biomaterial world due to their intrinsic biocompatibility, biodegradability, mechanical durability, and natural abundance. This review discusses the literature to date on keratin research in an attempt to gain a holistic view of the biomaterials applications of keratin for reproduced films, blends and composites. Keratin properties The main interest of biomaterial research on keratin is the type I and II hair keratin because it is a easily accessible, plentiful resource compared to epithelial keratin.
The most common method to obtain hair keratin is through reducing hair fibers. Hair fibers are elongated keratinized structures that are composed of heavily cross-linked hard keratins. Each fiber is divided into three principle compartments: the cuticle, cortex, and medulla. The thin outer surface of the fiber, the cuticle, is a scaly tubular layer that consists of over-lapping flattened cells. The cuticle primarily contains beta-keratins that function to protect the hair fiber from physical and chemical damage. The major body of the hair fiber is referred to as the cortex, which is composed of many spindle-shaped cells that contain keratin filaments. Occasionally, in the very center of the hair fiber is a region called the medulla that consists of a column of loosely connected keratinized cells. Within the cortex of the hair fiber are two main groups of proteins: (1) low-sulfur, “alpha” keratins (MW 40−60 kDa) and (2) high-sulfur, matrix proteins (MW 10−25 kDa). Collectively, the hair fiber consists of 50−60% alpha keratins and 20−30% matrix proteins. The alpha keratins assemble together to form microfibrous structures known as keratin intermediate filaments (KIFs) that impart toughness to the hair fiber. The matrix proteins function primarily as a disulfide crosslinker or glue that holds the cortical superstructure together and are also termed keratin associated proteins or KAPs.
In total, there are 17 human hair keratin genes (11 type I; 6 type II) and more than 85 KAP genes that potentially contribute to the hair structure in humans. Hair morphogenesis begins in a proliferative compartment at the base of the hair follicle called the bulb. Within this region, cells divide and differentiate to form the various compartments of the hair follicle. The hair follicle is a cyclic regeneration system comprised of actively migrating and differentiating stem cells responsible for the formation and growth of hair fibers. The follicle undergoes a continuous cycle of proliferation, regression, and quiescence that is regulated by over thirty growth factors, cytokines and signaling molecules. The mature proliferation hair follicle contains a concentric series of cell sheaths, the outermost of which is called the outer root sheath (ORS), followed by a single cell layer called the companion sheath. The inner root sheath (IRS) lies adjacent to the companion layer and consists of three compartments: the Henle layer, the Huxley layer, and the IRS cuticle. The hair fiber fills the center of this multilayered cylinder, which is itself divided into cuticle, cortex and medulla. As cells within the hair shaft terminally differentiate, they extrude their organelles and become tightly packed with keratin filaments. The cysteine-rich keratins become physically crosslinked upon exposure to oxygen and give strength and flexibility to the hair shaft.
Latest Applications of Keratin-Based Materials
Keratin Films
The preparation of protein films from keratin extracted from various sources has been tested by many researchers, and the properties of the reproduced keratin film are carefully studied. Due to keratin’s non-toxicity, its potentiality as food packaging is considered. In 2014, Song, Nak-Bum, et al. did a series of experiments testing the applicability of films made from extraction of chicken feather protein, a poultry industry byproduct. The extracted protein from the chicken feather was mainly composed of beta-keratin, a major structural fibrous protein. Composite films were prepared by adding various amounts of gelatin (0. 5, 1. 0, 1. 5, and 2. 0%) into the CFP film forming solution. The TBARS wasdetermined by mixing the sample with 10 mL of 7. 5% trichloroacetic acid and homogenized for 3 min using a Stomacher. The suspension was filtered, and 5 mL of the filtrate was added to 5 mL of TBARS reagent (0. 02 M 2-thiobarbituric acid in distilled water). The mixture was immersed in a boiling water bath at 95 °C for 45 min, cooled with water, and the absorbance was read at 539 nm using a spectrophotometer. The TBARS value wasrecorded as mg malonaldehyde (MDA) per kg sample.
CFP films containing gelatin had significantly higher tensile strength (TS) and elongation at break (E) and lower water vapor permeabilitythan the control. The TS of the films showed an increasing trend with the increasing amount of gelatin. Noticeably, TS increased from 1. 35 to 15. 31 MPa as the inclusion of gelatin into the films increased from 0 to 2%. There was a substantial increase in the E value upon the incorporation of gelatin. In conclusion, the oxidative substances, such as PV and TBARS, were reduced to a great extent in the smoked salmon by the CFP/gelatin composite film containing clove oil. Therefore, the CFP/gelatin composite film containing 1. 5% clove oil should be considered a potential active packaging material for smoked salmon preservation.
Keratin Blends
Since regenerated keratin materials degrade in both vitro (by trypsin) and in vivo, researchers are interested in manufacturing keratin blends, especially keratin/poly(ethylene oxide) (PEO) blend nanofibers for biomedical usages. Pure keratin’s processing and application are always restricted by it’s fragility and poor mechanical properties. Thus, PEO, a non-degradable polymer with good compatibility and low toxicity, is added to the blend to improve the overall strength of keratin. In 2007, Tonin, C. , et al. extracted keratin from wool fibers into aqueous solutions of m-bisulphite and urea and casted the keratin solution with PEO solution in several blending ratios. Through morphological, thermal and spectroscopic analysis of the different blending ratios, the researchers discovered that PEO will form the thermally more stable -sheet conformation in samples with comparable amount of PEO and keratin. They states the PEO/keratin blends appear to be a promising candidate for the production of new bio-compatible materials suitable for a number of applications in different.
Keratin Composite
Like many other naturally derived protein biomaterials from the extracellular matrix, keratin is known to interact with integrin which enable it to support cellular attachment, proliferation and migration. Therefore, many researchers are looking to utilize keratin composite hydrogels as wound dressings.
In 2018, Zhai, Mingcui, et al. experimented with Keratin-chitosan/n-ZnO nanocomposite (KCBZNs) hydrogels. They fabricated porous KCBZNs bandages and placed the bandages over controlled skin wounds of 1. 5 cm2 on one month old Sprague-Dawley (SD) rats. The in vivo assessments in SD rats exposed that nanocomposite bandages increased the wound curing with assisted for quicker skin cell construction along with collagen development. Hence, the researchers concluded that their acquired experimental data strongly supports to utilize of this nanocomposite hydrogels for burn wounds. [
Conclusion
It would appear that keratin biomaterials have been in the collective conscience of materialsresearchers for many decades, yet there are no keratin biomaterials currently in clinical use. Thiscomprehensive review has shown an impressive level of activity, diversity, and ingenuity, albeit at arelatively low level compared to other mainstream biomaterials. Keratin biomaterials possess manydistinct advantages over conventional biomolecules, including a unique chemistry afforded by theirhigh sulfur content, remarkable biocompatibility, propensity for self-assembly, and intrinsic cellularrecognition. As these properties become better understood, controlled and exploited, many biomedical applications of keratin biomaterials will make their way into clinical trials.
