Material properties and cell compatibility of poly(γ-glutamic acid)-keratin hydrogels

Synthetic and biopolymers-based antimicrobial hybrid hydrogels: a focused review

The constantly accelerating occurrence of microbial infections and their antibiotic resistance has spurred advancement in the field of material sciences and has guided the development of novel materials with anti-bacterial properties. To address the clinical exigencies, the material of choice should be biodegradable, biocompatible, and able to offer prolonged antibacterial effects. As an attractive option, hydrogels have been explored globally as a potent biomaterial platform that can furnish essential antibacterial attributes owing to its three-dimensional (3D) hydrophilic polymeric network, adequate biocompatibility, and cellular adhesion. The current review focuses on the utilization of different antimicrobial hydrogels based on their sources (natural and synthetic). Further, the review also highlights the strategies for the generation of hydrogels with their advantages and disadvantages and their applications in different biomedical fields. Finally, the prospects in the development of hydrogels-based antimicrobial biomaterials are discussed along with some key challenges encountered during their development and clinical translation.

A review of the current state of natural biomaterials in wound healing applications

Skin, the largest biological organ, consists of three main parts: the epidermis, dermis, and subcutaneous tissue. Wounds are abnormal wounds in various forms, such as lacerations, burns, chronic wounds, diabetic wounds, acute wounds, and fractures. The wound healing process is dynamic, complex, and lengthy in four stages involving cells, macrophages, and growth factors. Wound dressing refers to a substance that covers the surface of a wound to prevent infection and secondary damage. Biomaterials applied in wound management have advanced significantly. Natural biomaterials are increasingly used due to their advantages including biomimicry of ECM, convenient accessibility, and involvement in native wound healing. However, there are still limitations such as low mechanical properties and expensive extraction methods. Therefore, their combination with synthetic biomaterials and/or adding bioactive agents has become an option for researchers in this field. In the present study, the stages of natural wound healing and the effect of biomaterials on its direction, type, and level will be investigated. Then, different types of polysaccharides and proteins were selected as desirable natural biomaterials, polymers as synthetic biomaterials with variable and suitable properties, and bioactive agents as effective additives. In the following, the structure of selected biomaterials, their extraction and production methods, their participation in wound healing, and quality control techniques of biomaterials-based wound dressings will be discussed.

Wool keratin/zeolitic imidazolate framework-8 composite shape memory sponge with synergistic hemostatic performance for rapid hemorrhage control

Uncontrollable hemorrhage and subsequent wound infection pose severe threats to life, especially in the case of deep, non-compressible, massive bleeding. Here, a wool keratin/zeolitic imidazolate framework-8 (WK/ZIF-8) composite shape memory sponge is prepared by incorporating ZIF-8 nanoparticles into wool keratin. The combination of keratin and ZIF-8 particles not only reduces the effect of ZIF-8 particles on cell viability but also bolsters the mechanical properties of the keratin sponge and endows it with antibacterial efficacy. Due to the synergistic effect of the excellent hemostatic performance of keratin and Zn2+ release from ZIF-8 nanoparticles, the porous structure suitable for blood cell adhesion and the shape recovery ability of sponges, the WK/ZIF-8 composite sponge exhibits superior hemostatic performance to commercial medical sponges in SD rat and rabbit hemorrhage models. In addition, in vitro and in vivo antibacterial experiments demonstrate the anti-infection activity of the composite sponge. Overall, the WK/ZIF-8 composite sponge provides a promising approach to rapidly control bleeding and promote wound healing.

Antimicrobial activity of gamma-poly (glutamic acid), a preservative coating for cherries

We investigated the minimum inhibitory concentration (MIC), antibacterial activity, and preservation ability of four molar masses of γ-polyglutamic acid (PGA) against Escherichia coli, Bacillus subtilis, and yeast. The antibacterial mechanism was determined based on the cell structure, membrane permeability, and microscopic morphology of the microorganisms. We then measured the weight loss, decay rate, total acid, catalase activity, peroxidase activity, and malondialdehyde content toward the possible use of PGA as a preservative coating for cherries. When the molar mass was greater than 700 kDa, the MIC for Escherichia coli and Bacillus subtilis was less than 2.5 mg/mL. The mechanism of action of the four molar masses of PGA was different with respect to the three microbial species, but a higher molar mass of PGA corresponded to stronger inhibition against the microbes. PGA of 2000 kDa molar mass damaged the microbial cellular structure, resulting in excretion of alkaline phosphatase, but PGA of 1.5 kDa molar mass affected the membrane permeability and the amount of soluble sugar. Scanning electron microscopy indicated the inhibitory effect of PGA. The antibacterial mechanism of PGA was related to the molar mass of PGA and the microbial membrane structure. Compared with the control, a PGA coating effectively inhibit the spoilage rate, delay the ripening, and prolong the shelf life of cherries.

Bioinspired Robust Keratin Hydrogels for Biomedical Applications

Although keratins are robust in nature, hydrogels producing their extracts exhibit poor mechanical properties due to the complicated composition and ineffective self-assembly. Here we report a bioinspired strategy to fabricate robust keratin hydrogels based on mechanism study through recombinant proteins. Homotypic and heterotypic self-assembly of selected type I and type II keratins in different combinations was conducted to identify crucial domain structures for the process, their kinetics, and relationship with the mechanical strength of hydrogels. Segments with best performance were isolated and used to construct novel assembling units. The new design outperformed combinations of native proteins in mechanical properties and in biomedical applications such as controlled drug release and skin regeneration. Our approach not only elucidated the critical structural domains and underlying mechanisms for keratin self-assembly but also opens an avenue toward the rational design of robust keratin hydrogels for biomedical applications.

Keratin-based wound dressings: From waste to wealth

Keratin is a natural protein with a high content of cysteine residues (7-13%) and is widely found in hair, wool, horns, hooves, and nails. Keratin possesses abundant cell-binding motifs such as leucine-aspartate-valine (LDV), glutamate-aspartate-serine (EDS), and arginine-glycine-aspartate (RGD), which benefit cell attachment and proliferation. It has been confirmed that keratin plays important roles in every stage of wound healing, including hemostasis, inflammation, proliferation, and remodeling, making keratin-based materials good candidates for wound dressings. In combination with synthetic and natural polymers, keratin-based wound dressings in the forms of films, hydrogels, and nanofibers can be achieved with improved mechanical properties. This review focuses on the recent development of keratin-based wound dressings. Firstly, the physicochemical and biological properties of keratin, are systematically discussed. Secondly, the role of keratin in wound healing is proposed. Thirdly, the applications of keratin-based wound dressings are summarized, in terms of the forms and functionalization. Finally, the current challenges and future development of keratin-based wound dressings are presented.

Sustainable Applications of Animal Waste Proteins

Currently, the growth of the global population leads to an increase in demand for agricultural products. Expanding the obtaining and consumption of food products results in a scale up in the amount of by-products formed, the development of processing methods for which is becoming an urgent task of modern science. Collagen and keratin make up a significant part of the animal origin protein waste, and the potential for their biotechnological application is almost inexhaustible. The specific fibrillar structure allows collagen and keratin to be in demand in bioengineering in various forms and formats, as a basis for obtaining hydrogels, nanoparticles and scaffolds for regenerative medicine and targeted drug delivery, films for the development of biodegradable packaging materials, etc. This review describes the variety of sustainable sources of collagen and keratin and the beneficial application multiformity of these proteins.

Recent advances of 3D hydrogel culture systems for mesenchymal stem cell-based therapy and cell behavior regulation

Mesenchymal stem cells (MSCs) have been increasingly recognized as resources for disease treatments and regenerative medicine. Meanwhile, the unique chemical and physical properties of hydrogels provide innate advantages to achieve...

Repair of Peripheral Nerve Injury Using Hydrogels Based on Self-Assembled Peptides

Peripheral nerve injury often occurs in young adults and is characterized by complex regeneration mechanisms, poor prognosis, and slow recovery, which not only creates psychological obstacles for the patients but also causes a significant burden on society, making it a fundamental problem in clinical medicine. Various steps are needed to promote regeneration of the peripheral nerve. As a bioremediation material, self-assembled peptide (SAP) hydrogels have attracted international attention. They can not only be designed with different characteristics but also be applied in the repair of peripheral nerve injury by promoting cell proliferation or drug-loaded sustained release. SAP hydrogels are widely used in tissue engineering and have become the focus of research. They have extensive application prospects and are of great potential biological value. In this paper, the application of SAP hydrogel in peripheral nerve injury repair is reviewed, and the latest progress in peptide composites and fabrication techniques are discussed.

Amino acid derived biopolymers: Recent advances and biomedical applications

Over the past few years, amino acids (AA) have emerged as promising biomaterials for the synthesis of functional polymers. Owing to the diversity of functional groups in amino acids, various polymerization methods may be used to make a wide range of well-defined functional amino-acid/peptide-based optically active polymers with varying polymer lengths, compositions, and designs. When incorporated with chirality and self-assembly, they offer a wide range of applications and are particularly appealing in the field of drug delivery, tissue engineering, and biosensing. There are several classes of these polymers that include polyamides (PA), polyesters (PE), poly(ester-amide)s (PEA)s, polyurethanes (PU)s, poly(depsipeptide)s (PDP)s, etc. They offer the ability to control functionality, conjugation, crosslinking, stimuli responsiveness, and tuneable mechanical/thermal properties. In this review, we present the recent advancements in the synthesis strategies for obtaining these amino acid-derived bio-macromolecules, their self-assembly properties, and the wealth of prevalent applications.

Injectable poly(γ-glutamic acid)-based biodegradable hydrogels with tunable gelation rate and mechanical strength

Polypeptide-based hydrogels have potential applications in polymer therapeutics and regenerative medicine. However, designing reliable polypeptide-based hydrogels with a rapid injection time and controllable stiffness for clinical applications remains a challenge. Herein, a class of injectable poly(γ-glutamic acid) (PGA)-based hydrogels were constructed using furfurylamine and tyramine-modified PGA (PGA-Fa-Tyr) and the crosslinker dimaleimide poly(ethylene glycol) (MAL-PEG-MAL), through a facile strategy combining enzymatic crosslinking and Diels-Alder (DA) reaction. The injectable hydrogels could be quickly gelatinized and the gelation time, ranging from 10 to 95 s, could be controlled by varying the hydrogen peroxide (H2O2) concentration. Compared with hydrogels formed by single enzymatic crosslinking, the compressive stress and strain of the injectable hydrogels were remarkably enhanced because of the occurrence of the subsequent DA reaction in the hydrogels, suggesting the DA network imparted an outstanding toughening effect on the hydrogels. Furthermore, the mechanical strength, swelling ratio, pore size, and degradation behavior of the injectable hydrogels could be easily controlled by changing the molar ratios of H2O2/Tyr or furan/maleimide. More importantly, injectable hydrogels encapsulating bovine serum albumin exhibited sustained release behavior. Thus, the developed hydrogels hold great potential for applications in biomedical fields, such as tissue engineering and cell/drug delivery.

Hydrogels for Large-Scale Expansion of Stem Cells

Stem cells demonstrate considerable promise for various preclinical and clinical applications, including drug screening, disease treatments, and regenerative medicine. Producing high-quality and large amounts of stem cells is in demand for these applications. Despite challenges, as hydrogel-based cell culture technology has developed, tremendous progress has been made in stem cell expansion and directed differentiation. Hydrogels are soft materials with abundant water. Many hydrogel properties, including biodegradability, mechanical strength, and porosity, have been shown to play essential roles in regulating stem cell proliferation and differentiation. The biochemical and physical properties of hydrogels can be specifically tailored to mimic the native microenvironment that various stem cells reside in vivo. A few hydrogel-based systems have been developed for successful stem cell cultures and expansion in vitro. In this review, we summarize various types of hydrogels that have been designed to effectively enhance the proliferation of hematopoietic stem cells (HSCs), mesenchymal stem/stromal cells (MSCs), and pluripotent stem cells (PSCs), respectively. According to each stem cell type's preference, we also discuss strategies for fabricating hydrogels with biochemical and mechanical cues and other characteristics representing microenvironments of stem cells in vivo. STATEMENT OF SIGNIFICANCE: In this review article we summarize current progress on the construction of hydrogel systems for the culture and expansion of various stem cells, including hematopoietic stem cells (HSCs), mesenchymal stem/stromal cells (MSCs), and pluripotent stem cells (PSCs). The Significance includes: (1) Provide detailed discussion on the stem cell niches that should be considered for stem cell in vitro expansion. (2) Summarize various strategies to construct hydrogels that can largely recapture the microenvironment of native stem cells. (3) Suggest a few future directions that can be implemented to improve current in vitro stem cell expansion systems.

Role of Hydrogels in Bone Tissue Engineering: How Properties Shape Regeneration

Bone defect that resulted from trauma, tumors, and other reasons is believed as a common clinical problem, which exists mainly in post-traumatic healing. Additionally, autologous/allogeneic transplantation, bone tissue engineering attracts increasing attention due to the existing problem of the limited donor. The applications of biomaterials can be considered as a rising and promising strategy for bone regeneration. Especially, hydrogel is featured with hydrophilic characteristic, good biocompatibility, and porous structure, which shows unique properties for bone regeneration. The main properties of hydrogel such as surface property, adhesive property, mechanical property, porosity, and degradation property, generally present influences on the migration, proliferation, and differentiation of mesenchymal stem cells exclusively or in combination, which consequently affect the regeneration of bones. This review mainly focuses on the theme: "how properties of hydrogel shape bone regeneration." Moreover, the latest progress achieved in the above mentioned direction is further discussed. Despite the fascinating advances researchers have made, certain potential challenges continue to exist in the research field, which need to be addressed for accelerating the clinical translation of hydrogel in bone regeneration.

From fabric to tissue: Recovered wool keratin/polyvinylpyrrolidone biocomposite fibers as artificial scaffold platform

Keratin extracted from wool fibers has recently gained attention as an abundant source of renewable, biocompatible material for tissue engineering and drug delivery applications. However, keratin extraction and processing generally require a copious use of chemicals, not only bearing consequences for the environment but also possibly compromising the envisioned biological outcome. In this study, we present, for the first time, keratin-PVP biocomposite fibers obtained via an all-water co-electrospinning process and explored their properties modulation as a result of different thermal crosslinking treatments. The protein-based fibers featured homogenous morphologies and average diameters in the range of 170-290 nm. The thermomechanical stability and response to a wet environment can be tuned by acting on the curing time; this can be achieved without affecting the 3D fibrous network nor the intrinsic hydrophilic behavior of the material. More interestingly, our protein-based membranes treated at 170 °C for 18 h successfully sustained the attachment and growth of primary human dermal fibroblasts, a cellular model which can recapitulate more faithfully the physiological human tissue conditions. Our proposed approach can be viewed as pivotal in designing tunable protein-based scaffolds for the next generation of skin tissue growth devices.