Original authors: Suyoung Lee, Mark Van Dyke, Minkyu Kim

Institution: Department of Materials Science and Engineering, Department of Biomedical Engineering, and the BIOS Institute at the University of Arizona in the United States

Research Topic: Focused on recombinant keratin, encompassing its synthesis methods, hierarchical assembly mechanisms (i.e., the assembly process from the molecular to the macroscopic scale), material properties, and application scenarios across various fields (such as biomedicine and materials engineering), with particular emphasis on its potential value in these domains.

Article Abstract

Keratin has garnered extensive attention due to its outstanding mechanical properties, thermal stability, and bioactive functions such as promoting hemostasis and wound healing. Traditionally, keratin is extracted from natural sources like human hair, wool, and feathers and processed into biomaterials such as thin films, hydrogels, and nanoparticles, primarily for biomedical applications. However, conventional extraction methods often result in highly heterogeneous keratin mixtures that contain residual impurities and structural damage caused by stringent purification conditions, thereby complicating the investigation of how specific keratins and their hierarchical assemblies contribute to the desired material properties.

Recombinant keratin technology effectively addresses the aforementioned challenges by enabling the synthesis of a single keratin type with high purity and batch-to-batch consistency. These advances have facilitated in-depth investigations into how keratin, across different assembly stages—from molecular components and heterodimers to intermediate filaments and their networks—influences material properties. Moreover, this technology permits precise genetic engineering, holding promise for the development of keratin variants with tailored characteristics for specific applications.

Despite its clear advantages, translating recombinant keratin into practical applications still requires overcoming key manufacturing challenges, such as optimizing large-scale production and enhancing purification efficiency. This review summarizes the current state of research on recombinant keratin, highlights recent technological advances, and explores its applications in contemporary biomaterials. Although its current applications remain more limited than those of naturally extracted keratin, recombinant keratin holds great promise for advanced materials design and other non-medical fields in the future.

1. Introduction

In biological systems, natural materials that provide protection and structural support typically exist in the form of fibrous architectures, which exhibit exceptional properties such as high tensile strength, elasticity, hardness, and toughness. These fibrous structures are widely found across diverse organisms. For example, cellulose in plants forms microfibrils, conferring high mechanical strength; chitin in the exoskeletons of invertebrates occurs as nanofiber layers, providing high tensile strength and hardness. In vertebrates, fibrin also performs a similar function, assembling hierarchically to form a robust tissue framework that ensures structural stability and mechanical integrity for protective and supportive roles.

Collagen is a major fibrous protein in vertebrates, essential for maintaining the structural integrity and mechanical properties of tissues such as skin, tendons, cartilage, and bone. Its distinctive triple-helical structure consists of three polypeptide chains that are further hierarchically assembled into protofibrils and fibrils, thereby enhancing tissue strength and resilience. Traditionally, collagen has been extracted from animal tissues—such as fish skin and bovine hide—for biomedical applications, owing to its excellent biocompatibility and its ability to promote cell adhesion and proliferation. A wide range of collagen-based products are now available on the market, including medical devices, dietary supplements, and skincare formulations.

Despite the wide-ranging applications of collagen, its extraction often results in highly heterogeneous samples containing residual impurities, which poses challenges to consistency and reproducibility in both research and clinical settings. To address these issues, recombinant collagen produced via recombinant DNA technology in yeast, bacterial, insect, or plant cells has emerged. Recombinant collagen allows precise control over the amino acid sequence, removal of non-collagenous components, and enhancement of consistency and reproducibility, making it well suited for specific applications. Moreover, high-purity recombinant collagen facilitates in-depth investigations into its assembly mechanisms, hierarchical structure, and interactions with cells, thereby advancing the development of biomimetic and bio-inspired biomaterials.

Keratin is another important fibrous protein and serves as the primary structural component of tissues such as skin, hair, nails, wool, horns, and hooves, protecting them from damage and external stimuli. Although these structures are predominantly composed of keratin, they exhibit exceptional mechanical properties—high tensile strength, elasticity, hardness, and toughness—largely attributable to keratin’s fibrous architecture and its reinforcement mechanisms. Moreover, keratin demonstrates superior resistance to enzymatic degradation (as humans lack keratinases) and higher thermal stability than collagen. Nevertheless, research on keratin remains relatively limited, possibly because it is primarily found in non-essential structural components. In addition, the human genome encodes 54 distinct keratin genes, whose structural diversity and tissue-specific functional roles are highly heterogeneous, further complicating studies. Nonetheless, these unique characteristics make keratin an exceptionally attractive biomaterial.

Keratin extracted from natural sources, such as waste hair and wool, has been widely employed in biomedical applications including hemostasis, wound healing, drug delivery, and tissue regeneration. However, the extraction process typically involves harsh conditions—such as strong acids, strong bases, and high temperature and pressure—to disrupt keratin’s dense fibrous architecture and extensive disulfide-bond network. These conditions can lead to fiber breakage and damage to cysteine residues, thereby compromising mechanical properties and structural stability. Consequently, materials reconstituted from extracted keratin often lack the hierarchical organization and specific mechanical performance—such as fracture toughness and tensile strength—characteristic of native keratin structures. To develop functionally optimized keratin-based materials, particularly fibers, it is essential to elucidate the intrinsic characteristics of hierarchical assembly in native keratin and the underlying molecular interaction mechanisms. Although extensive research has been conducted on extracted keratin, the fundamental principles governing its hierarchical assembly and reinforcement mechanisms remain incompletely understood.

Recombinant keratin offers a promising solution to overcome the limitations of conventional keratin extraction. Recombinant technology has made it possible to investigate the composition, structure, properties, and the relationships among processing and performance of keratin across multiple scales, from the molecular to the macroscopic level—ranging from the formation of assembled structures and the molecular interactions and bonding types that stabilize fibrous architectures to the environmental conditions required for their assembly. Building on this fundamental research, traditional biomaterials previously produced by extracting keratin are now increasingly being replaced by specific types of recombinant keratin, thereby expanding their applications in the biomedical field. Beyond medical applications, recombinant keratin is also attracting attention in emerging areas such as biochemistry, biophysics, soft robotics, and wearable devices. Although several reviews have been published on extracted keratin, a systematic review focusing specifically on recombinant keratin remains lacking. Therefore, this paper aims to fill this gap by providing a comprehensive overview of the biosynthesis of recombinant keratin, its self-assembly mechanisms, the structure and properties of assembled fibers and networks, material fabrication, recent applications, and future directions of development.

2. Synthesis of Recombinant Keratin

Recombinant keratin technology enables the synthesis of a single type of keratin with high purity and batch-to-batch consistency. This capability opens up opportunities for in-depth investigation of the structure–property relationships of keratin-based materials and expands their potential in biomaterials and advanced materials design. To fully capitalize on these opportunities, it is essential to adopt efficient production strategies.

2.1 Classification of Keratins and Advances in Recombinant Keratin Research

Keratin is typically classified in three ways:

According to the isoelectric point : Classified into acidic Type I and alkaline or neutral Type II;

According to organizational distribution : It is classified into soft keratin found in soft tissues such as skin and mucous membranes, and hard keratin found in hair, horns, hooves, and the like;

According to the secondary structure : It is classified into α-keratin, which has an α-helical structure, and β-keratin, which is predominantly composed of β-sheets.

The human genome encodes a total of 54 keratin genes, 28 of which are type I and 26 are type II. These keratins typically assemble into intermediate filaments (IFs) as heterodimers of type I and type II subunits, forming the structural foundation of keratin fibers and their hierarchical material organization.

The development of genetic technologies in the 1990s made it possible to express recombinant keratin, enabling the production of high-purity, impurity-free keratin that outperforms naturally extracted keratin.

Currently, research on recombinant keratin is primarily focused on keratins found in human skin and hair. For example:

Soft keratins (Cytokeratins) : For example, K9–K20 of type I and K1–K8 of type II are predominantly found in epithelial tissues;

Hard keratins (Trichocyte keratins) : For example, K31–K40 and K33a/b of Type I, and K81–K86 of Type II, are primarily found in hair and nails.

Hard keratin, owing to its higher cysteine residue content, can form more disulfide bonds, resulting in superior mechanical properties and thus attracting greater attention for biomaterial applications. In addition, mouse keratin has been used in comparative studies with human keratin. Although the gene sequences of wool and feather keratins have been elucidated and commercial products are available, research on non-human recombinant keratins remains limited, primarily because human-derived keratins are better suited for biomedical applications.

2.2 Biosynthesis of Recombinant Keratin

The biosynthesis of recombinant proteins typically involves two main stages: Expression with Purification 。

Expression system

Escherichia coli is the most widely used expression system due to its rapid growth, well-characterized genetic background, and ease of manipulation. In current research, the yield of recombinant keratin is not the primary focus; the main objective is to obtain sufficient protein for experimental use.

Expression and Purification Process

Recombinant keratin often forms in Escherichia coli. Inclusion body —that is, aggregates of misfolded proteins. Because keratin exhibits a strong propensity for self-assembly and contains a large number of cysteine residues that readily form disulfide bonds, this further promotes the formation of inclusion bodies.

To obtain functional keratin, inclusion bodies must be solubilized using strong denaturants (such as 8 M urea) and reducing agents (such as DTT or β-mercaptoethanol) to disrupt noncovalent interactions and disulfide bonds, thereby preventing premature aggregation. Maintaining a high urea concentration throughout the purification process is critical, as some keratins may begin to self-assemble even in 6 M urea.

Keratin exhibits thermal stability; once dissolved in an 8 M urea buffer, it can be handled and stored at room temperature, eliminating the need for low-temperature storage that is typically required for most recombinant proteins.

Self-Assembly Control

To achieve controlled molecular self-assembly between type I and type II keratins, it is common to employ Step-by-step dialysis : Gradually reduce the urea concentration to prevent misfolding or aggregation caused by rapid denaturation. This step facilitates the formation of correctly assembled heterodimers and intermediate filament-like structures. Following completion of molecular self-assembly, the reducing agent can be removed by dialysis or allowed to degrade naturally, thereby enabling disulfide bond formation and final structural maturation.

2.3 Challenges in Recombinant Keratin Production

Expressing differences

A significant challenge is Differences in the Expression Levels of Type I and Type II Keratins For example, the yield of type I hair keratins is typically higher than that of type II. Although the two types share approximately 30% sequence similarity, their isoelectric points (pI) differ substantially (type I: 4.5–6.0; type II: 6.5–8.5), which may affect the solubility and stability of the proteins during expression and purification. Most studies employ buffers with a pH around 8, which may be unfavorable for type II keratins and result in lower yields.

Comparison with keratin extraction

Extract keratin : Sources are diverse (e.g., wool, feathers), making acquisition straightforward; however, heterogeneity and impurities remain issues.

Recombinant keratin : It enables high-purity, highly controllable protein expression, making it suitable for both research and applications; however, production is more complex and requires optimization of the gene sequence, expression host, and culture conditions.

Common Challenges Both require multi-step downstream purification to remove impurities and stabilize the protein, which may increase costs.

2. Assembly, Structure, and Molecular Interactions of Keratin Intermediate Filaments (IFs) and Their Networks

Understanding the hierarchical assembly and molecular interactions of keratin intermediate filaments (IFs) and their networks is essential for elucidating their structural roles in advanced structures such as keratinized tissues.

3.1 General Assembly Process of Intermediate Filaments and Keratin Intermediate Filaments

Intermediate filaments are a crucial component of the cytoskeleton, assembled from various IF proteins such as keratins, vimentin, desmin, and lamins, and they confer structural stability to cells and tissues under mechanical stress.

The IF protein structure comprises an N-terminal globular head, a central α-helical rod domain, and a C-terminal unstructured tail.

Specifically, the rod domain comprises four segments (1A, 1B, 2A, and 2B) that can form coiled-coil dimers, separated by non-helical linker regions L1, L12, and L2. These structural elements assemble into stable α-helical dimers via heptad repeats and are further stabilized by salt bridges and hydrophobic interactions.

Dimeric intermediate filaments further assemble into tetramers and octamers, ultimately forming intermediate filaments with a diameter of approximately 10 nm. Subsequently, these IFs are embedded in an amorphous matrix composed of other proteins, giving rise to macrofibrils with a diameter of about 400–500 nm, which then assemble into fibers measuring roughly 6 μm in length, thereby constituting the fundamental structural core of both soft and hard tissues in vertebrates.

Characteristics of Keratin IF : Composed of heterodimers of type I and type II keratins; rich in cysteine residues that can form disulfide bonds, thereby enhancing structural stability; exhibits a complex expression pattern, particularly in hair, where it varies with the stage of differentiation, further complicating studies on their assembly.

3.2 Investigating the Assembly Mechanism of IFs Using Recombinant Keratins

3.2.1 Early Assembly Stage: Heterodimers and Tetramers

In the 1990s, Coulombe and Fuchs first used Recombinant Humanized Keratin K5 (Type II) and K14 (Type I) to study IF assembly. Gel chromatography and transmission electron microscopy (TEM) revealed that K5 and K14 form antiparallel coiled-coil heterodimers; these dimers further assemble into heterotetramers; heterodimers are the basic units of IF assembly, rather than homodimers.

Mutagenesis experiments further confirmed that introducing cysteine mutations into K8/K18 can promote the formation of homodimers, but these dimers are unable to assemble into functional IFs; only under reducing conditions, when heterodimers are formed, can successful assembly into IF structures occur. In addition, the study revealed that the 1B and 2B regions play critical roles in IF assembly: the 1B region is involved in IF formation, while the 2B region influences the stability of IFs and their associated networks.

3.2.2 Assembly Path of the IF Maturation Process

Through structural modeling and mutational analysis, the researchers elucidated the assembly pathway of IF from a tetramer to a mature fibril:

Section 1B : Mediates tetramer stability via hydrophobic stripes;

Section 2B : Mediates the assembly of octamers via antiparallel interactions, thereby promoting IF elongation;

N-terminal head and C-terminal tail : N-terminal deletion leads to a shorter IF and reduced assembly efficiency; C-terminal deletion impairs the lateral assembly and extension capabilities of the IF.

In addition, the study proposes two IF extension models:

End-to-end extension model : The octamer is connected in a reverse parallel manner via the 2B segment and extends longitudinally;

Unit-Length Fiber (ULF) Model : The octamer first assembles laterally into a 32-mer unit, which is then linked longitudinally to form the mature IF.

3.3 The Role of Molecular Interactions and Chemical Bonds in IF Assembly and Stabilization

The stability of keratin intermediate filaments and their networks depends on multiple molecular interactions:

Noncovalent interactions : hydrogen bonding, hydrophobic interactions, salt bridges; Covalent interaction : Disulfide bonds (particularly important).

Experiments demonstrate that, in the presence of high concentrations of urea and reducing agents, K5 and K14 can still form heterodimers via hydrophobic interactions; disulfide bonds play a crucial role in IF formation, and their reduction leads to a loosening of the IF structure and a reduction in branching; mutagenesis analysis reveals that hydrophobic stripes, the N-terminal pocket, and the C-terminal knob-and-pocket interface all contribute to tetramer assembly; furthermore, certain cysteine residues (such as Cys367 in K14) can form inter-IF disulfide bonds, thereby enhancing network flexibility and stability.

3.4 Impact of Pathogenic Mutations on the Structure and Function of Keratin IF

Keratin mutations disrupt its molecular interactions, leading to a spectrum of skin disorders collectively known as Keratinopathies . For example: Epidermolysis Bullosa (EBS) : The Cys367 mutation in K14 is replaced by Pro, disrupting the α-helical structure and reducing dimer stability; Palmar-plantar keratosis : The Ser233Leu mutation in K1 leads to the formation of abnormal tubular structures rather than normal IFs; Epidermolytic ichthyosis : The Leu452Pro mutation in K10 and the Ile479Phe mutation in K1 destabilize the heterodimeric structure, leading to the formation of an abnormal IF network.

These studies, employing recombinant keratin technology, have elucidated the critical roles of specific amino acids in maintaining the structure and function of IFs, thereby providing a molecular basis for understanding the pathogenesis of related dermatological disorders.

4. Keratin Intermediate Filaments (IFs): Performance and Structural Characteristics of the Network

Keratin intermediate filaments and their networks form the structural foundation of keratinized tissues; therefore, characterizing their properties and architecture is essential for understanding their macroscopic mechanical behavior. Recombinant keratin technology provides a controllable platform for investigating how specific keratin pairings, mutations, and environmental conditions influence the mechanical and structural properties of intermediate filaments and their networks.

4.1 Mechanical Behavior of Keratin IF Networks

In natural keratinized structures such as skin, hair, and horns, IFs are embedded within an amorphous protein matrix, forming a complex composite material. This structural arrangement makes it challenging to investigate the intrinsic mechanical properties of individual components, such as IFs.

To address this issue, researchers constructed IFs and networks using recombinant keratin and employed multi-scale characterization techniques to investigate their mechanical properties. The study focused on two pairs of recombinant keratins:

K5–K14 : Derived from stratified epithelium (e.g., skin), with strong resistance to mechanical stress;

K8–K18 : Derived from simple epithelium (e.g., in visceral organs), it exhibits greater flexibility.

Influence of Environmental Conditions on Mechanical Properties:

pH change : As the pH decreases from 7.4 to 7.0, the storage modulus (G′) of the K5–K14 network increases significantly, indicating a more compact structure;

NaCl concentration : Adding 10 mM NaCl can enhance network rigidity, with an effect similar to lowering the pH;

Protein concentration : G′ increases with increasing protein concentration, indicating more extensive intermolecular interactions;

Keratin type : Although K5–K14 and K8–K18 originate from different sources, their network responses under environmental changes exhibit similar trends, indicating that, at the IF level, their mechanical behavior shares common characteristics.

Effect of Mutations on Mechanical Properties:

Comparison between the K14 wild type (wt) and its mutant K14R125C: the mutant network begins to soften at low strains, with the yield point occurring earlier; in contrast, the wild-type network exhibits strain-hardening behavior under large deformations, demonstrating greater elastic resilience.

The phase angle δ indicates that the mutant network exhibits higher viscosity and lower elasticity.

Mechanical behavior of a single IF:

K5–K14 IF was stretched using atomic force microscopy (AFM): the IF could be extended to 3.2 times its original length before rupture; after stretching, the structure remained permanently deformed, with a reduction in diameter, indicating an irreversible transition from α-helical to β-sheet conformation.

This indicates that IF can act as an “energy absorber,” dissipating energy under external forces to protect cells and tissues.

Mechanical properties of the K8–K18 network:

In the presence of Mg²⁺, the G′ of the K8–K18 network is not significantly affected, unlike in the case of Na⁺; after rupture, the network can regain its original viscoelastic properties within approximately 30 minutes, demonstrating excellent self-healing capability; under high stress, the network exhibits nonlinear strengthening behavior, whereby G′ increases with increasing stress, thereby helping to maintain structural integrity.

Optical tweezers experiments demonstrate that K8–K18 IF maintains structural stiffness under cyclic deformation, exhibiting excellent mechanical toughness.

4.2 Structural Characteristics of Natural and Non-Natural Keratin Combinations

Investigating the assembly behavior of different keratin combinations can elucidate the relationships among their structural features, intermediate filament morphology, network formation, and hierarchical organization. This is of great significance for understanding their macroscopic mechanical behavior and for designing functional keratin-based materials.

4.2.1 Structural Characteristics of Natural and Non-Natural Keratin Combinations

Under physiological conditions, type I and type II keratins exhibit specific pairing patterns: for example, K5–K14 are found in the basal layer of stratified epithelia; K1–K10 are present in the suprabasal layers of the epidermis; K8–K18 are expressed in simple epithelia; and hair keratins display a complex expression profile, with multiple type I and type II keratins being expressed at different stages of hair follicle development.

Recombinant technologies can be used to study both natural and non-natural assembly behaviors: Non-natural pairing For example, the K5–K18 pairing can form wider (20–25 nm) IFs, exhibiting stronger self-assembly capability and greater network elasticity; in contrast, the K8–K14 pairing yields networks with poorer elasticity, although it can still form IF structures. These findings indicate that different keratin combinations influence the morphology and mechanical properties of IFs, which can be leveraged for material design optimization.

4.2.2 Time-Dependent Assembly Dynamics of Keratin Composites

The assembly morphologies of keratin combinations at different time points were investigated using continuous dialysis: K35–K81 formed bead-like fibrils within 1 hour and, after 40 hours, a IF network with a diameter of less than 15 nm; K35–K85 and K36–K81 initially formed only bead-like structures, which subsequently developed into networks after 12 and 24 hours, respectively. The assembly efficiency of multi-protein combinations was generally lower than that of dimeric combinations, likely due to the complexity of their interactions and the presence of a “short-board effect,” indicating that assembly efficiency is closely related to the type and ratio of the keratin combination.

4.2.3 Impact of Genetically Engineered Keratin Variants on Assembly and Properties

By introducing mutations—such as the QTY motif, which replaces hydrophobic residues Leu, Ile, and Val with hydrophilic residues Gln, Thr, and Tyr—the water solubility and self-assembly capability of keratin can be enhanced: QTY variants undergo self-assembly at neutral pH without the need for urea-gradient dialysis, forming thicker IF bundles (62–68 nm versus 46–59 nm for the wild type).

It can form both homodimers and heterodimers, with more relaxed assembly conditions.

5. Applications of Recombinant Keratin

Recombinant keratin technology enables researchers to precisely control the types and combinations of keratins, thereby facilitating the development of biomaterials with tailored properties. These materials have been explored in various well-established forms, including hydrogels, nanofibers, and nanoparticles, and have demonstrated promising potential in hemostasis, wound healing, antibacterial applications, and tissue regeneration.

5.1 Recombinant Keratin in Engineered Biomaterials

Although keratin extraction has been widely employed in biomaterials, the application of recombinant keratin remains in its early stages. Compared with native keratin, recombinant keratin offers higher purity, greater controllability, and enhanced designability, making it well suited for the fabrication of functionalized materials.

Nanofiber

Recombinant hair keratin is blended with polymers such as polycaprolactone (PCL) and gelatin to prepare electrospun nanofibers, which typically have diameters ranging from 100 to 700 nm and exhibit a tensile modulus of 6–10 MPa—values comparable to those of human skin. Fibers containing type I keratins (e.g., K31–K40) display higher mechanical strength than those containing type II keratins (e.g., K81–K86).

The introduction of keratin enhances the fiber’s extensibility and toughness.

Hydrogel

Leveraging the self-assembly properties of keratin, hydrogels were prepared in PEG buffer via dialysis. Heterodimeric combinations such as K35–K81 and K36–K81 exhibited rapid gelation, small pore sizes, and high compressive modulus; extending the dialysis time further enhanced the mechanical properties of the hydrogels. Blending with carboxymethyl cellulose (CMC) yielded composite hydrogels with a porous structure, making them suitable for drug delivery or cell culture. Moreover, photopolymerization techniques—such as the K31-PGLa-PEGDA system—can be used to fabricate antibacterial hydrogels, which are well suited for infected wounds.

Nanoparticles

Using ultrasonic dispersion, K37 or K81 is dissolved in urea and then introduced into an acidic solution to form nanoparticles with particle sizes ranging from 270 to 500 nm and surface potentials of +14 to +22 mV. By adjusting the protein concentration, both particle size and surface charge can be controlled, making these nanoparticles suitable for drug delivery systems.

5.2 Applications of Recombinant Keratin in Biomedicine

The applications of recombinant keratin in biomedicine are primarily focused on the following areas:

5.2.1 Hemostasis and Wound Healing

Hemostasis Mechanism Research : Hair keratins such as K31, K37, K81, and K86 were selected and used to construct their full-length forms, rod domains, and α-helical fragments; the α-helical structure can accelerate fibrinogen release and polymerization, thereby promoting coagulation; substituting cysteine with serine enhances water solubility, thus improving hemostatic efficacy.

Animal studies have demonstrated that soluble recombinant keratin can significantly shorten hemostasis time in a hepatic hemorrhage model.

Wound healing performance : Following the introduction of QTY mutations (which enhance water solubility), keratin can rapidly form a gel-like protective layer at wound sites; compared with native keratin, the QTY variant exhibits faster wound healing in diabetic animal models; K81 nanoparticles promote cell proliferation and migration in vitro, and K37, upon reductive treatment to enhance its water solubility, also demonstrates excellent wound-healing efficacy.

All 17 hair keratins were systematically evaluated, and K33b, K34, K39, and K84 were found to be most effective in promoting wound closure, angiogenesis, and re-epithelialization.

Certain keratins, such as K34 and K81, can also modulate macrophage polarization, exerting anti-inflammatory effects; in a spinal cord injury model, K33a nanofibers reduce the size of the lesion cavity and improve motor function by regulating M2-type microglial/macrophage polarization.

5.2.2 Implant Coatings and Antimicrobial Applications

Surface Modification of Implants ：

Inspired by the nail–skin interface, researchers engineered a K31–K81 recombinant keratin coating on titanium surfaces. Compared with native keratin extraction, the recombinant keratin forms a canonical IF structure, thereby promoting the spreading and differentiation of HaCaT cells; concomitant upregulation of early differentiation markers, such as involucrin, indicates that it enhances tissue integration and biocompatibility.

Antimicrobial Material Development ：

The antimicrobial peptide PGLa was fused to K31 to construct a photocrosslinkable hydrogel; when used alone, it exhibits no antibacterial activity, but in combination with ampicillin it restores susceptibility to multidrug-resistant bacteria such as Escherichia coli and Staphylococcus aureus. The underlying mechanism involves downregulating genes associated with efflux pumps—such as acrA, acrB, and tolC—thereby enhancing intracellular accumulation of the antibiotic. In a wound infection model caused by drug-resistant pathogens, this combination therapy markedly promotes wound healing and attenuates inflammatory responses.

6. Summary and Outlook

Recombinant technologies have significantly advanced keratin research by enabling precise control over keratin sequences, targeted mutagenesis, and the high-purity, reproducible expression of single proteins. These capabilities allow researchers to gain deep insights into how amino acid sequences, domain functions, and intermolecular interactions influence keratin assembly, morphology, and stability across multiple scales. Coupled with investigations into both natural and engineered keratin pairings, these advances have enhanced our understanding of the performance of native keratinized structures and laid the foundation for the design of biomimetic materials.

Although the application of recombinant keratin in biomaterials is still in its early stages, it has already demonstrated significant potential in areas such as wound healing, antimicrobial therapy, implant coatings, and tissue regeneration.

These studies have opened new avenues for the development of engineering materials with superior or novel properties by fully leveraging the unique structural and functional characteristics of keratin.

6.1 Summary of Research Progress

Through recombinant keratin technology, researchers have been able to: systematically investigate the hierarchical assembly mechanisms of keratin; elucidate how mutations lead to disease-associated structural disruptions; and explore the effects of different keratin combinations on material properties.

Develop functional biomaterials, such as hemostatics, wound dressings, and implant coatings.

However, current research has largely focused on cytokeratins (such as K5–K14 and K8–K18), while studies on more complex systems like hair keratins remain limited. The intricate expression patterns and ambiguous pairing relationships of hair keratins pose significant challenges for research. Moreover, although β-keratins—found in avian feathers and reptilian scales—are characterized by greater hardness and structural stability, their recombinant production and functional characterization are still severely underexplored, underscoring the need for intensified investigation in the future.

Challenges and Strategies for Practical Applications, To advance recombinant keratin from laboratory research to practical applications, the following key challenges must be addressed:

Production Scale and Cost Control

Currently, the process relies primarily on the Escherichia coli expression system, which limits product yield; therefore, it is necessary to optimize expression hosts—such as yeast, insect cells, and plants—and fermentation processes; introduce industrial-scale bioreactors to enhance yield and stability; and reduce purification costs while streamlining the purification workflow.

Regulations and Standardization

Leverage the FDA-approved experience with collagen, hyaluronic acid, and other biomaterials; establish quality standards and a safety assessment system for recombinant keratin; and proactively plan preclinical studies to facilitate regulatory approval pathways.

Materials Design and Functional Optimization

Employ AI tools such as AlphaFold and RosettaFold to predict protein structures; integrate synthetic biology tools to design functionalized keratin variants; incorporate targeting peptides, drug-binding sites, and degradation-regulating modules; thereby realizing on-demand customization of high-performance biomaterials.

6.2 Prospects for Application

Although current research on recombinant keratin is largely focused on the biomedical field, its potential in other areas also warrants attention:

Biomedical

Hemostatic materials, wound dressings, and tissue-engineering scaffolds; antibacterial hydrogels, implant coatings, and nerve-repair materials; treatment of chronic, hard-to-heal wounds such as diabetic ulcers, burns, and spinal cord injuries.

Functional Materials

Flexible biomaterials for wearable devices; structure–function integrated materials for soft robotics; reinforcing components for biodegradable plastics and eco-friendly packaging materials.

Structural Bionics

Mimic the hierarchical structures of natural materials such as hair, feathers, and horns; design artificial materials with high toughness, thermal stability, and resistance to enzymatic degradation; and develop bio-based composites suitable for extreme environments.

Conclusion

Recombinant keratin technology has ushered in unprecedented opportunities for materials science and biomedicine. By integrating biotechnology, computational design, and engineered manufacturing, we are poised to develop a new generation of high-performance, sustainable, and multifunctional keratin-based materials. These materials will not only advance the fields of regenerative medicine and bioengineering but also hold great promise in cutting-edge areas such as smart materials and green manufacturing. The key to the future lies in: Interdisciplinary collaborative innovation + engineering-scale production capabilities + seamless translation of clinical findings into industrial applications

Keratin, one of the most abundant and resilient proteins in nature, is steadily transitioning from being a “neglected structural protein” to becoming a “star multifunctional biomaterial.”