The Physiological Inorganic Polymers Biosilica and Polyphosphate as Key Drivers for Biomedical Materials in Regenerative Nanomedicine

There is a need for novel nanomaterials with properties not yet exploited in regenerative nanomedicine. Based on lessons learned from the oldest metazoan phylum, sponges, it has been recognized that two previously ignored or insufficiently recognized principles play an essential role in tissue regeneration, including biomineral formation/repair and wound healing. Firstly, the dependence on enzymes as a driving force and secondly, the availability of metabolic energy. The discovery of enzymatic synthesis and regenerative activity of amorphous biosilica that builds the mineral skeleton of siliceous sponges formed the basis for the development of successful strategies for the treatment of osteochondral impairments in humans. In addition, the elucidation of the functional significance of a second regeneratively active inorganic material, namely inorganic polyphosphate (polyP) and its amorphous nanoparticles, present from sponges to humans, has pushed forward the development of innovative materials for both soft (skin, cartilage) and hard tissue (bone) repair. This energy-rich molecule exhibits a property not shown by any other biopolymer: the delivery of metabolic energy, even extracellularly, necessary for the ATP-dependent tissue regeneration. This review summarizes the latest developments in nanobiomaterials based on these two evolutionarily old, regeneratively active materials, amorphous silica and amorphous polyP, highlighting their specific, partly unique properties and mode of action, and discussing their possible applications in human therapy. The results of initial proof-of-concept studies on patients demonstrating complete healing of chronic wounds are outlined.

Egyptian propolis extract for functionalization of cellulose nanofiber/poly(vinyl alcohol) porous hydrogel along with characterization and biological applications

Bee propolis is one of the most common natural extracts and has gained significant interest in biomedicine due to its high content of phenolic acids and flavonoids, which are responsible for the antioxidant activity of natural products. The present study report that the propolis extract (PE) was produced by ethanol in the surrounding environment. The obtained PE was added at different concentrations to cellulose nanofiber (CNF)/poly(vinyl alcohol) (PVA), and subjected to freezing thawing and freeze drying methods to develop porous bioactive matrices. Scanning electron microscope (SEM) observations displayed that the prepared samples had an interconnected porous structure with pore sizes in the range of 10-100 μm. The high performance liquid chromatography (HPLC) results of PE showed around 18 polyphenol compounds, with the highest amounts of hesperetin (183.7 µg/mL), chlorogenic acid (96.9 µg/mL) and caffeic acid (90.2 µg/mL). The antibacterial activity results indicated that both PE and PE-functionalized hydrogels exhibited a potential antimicrobial effects against Escherichia coli, Salmonella typhimurium, Streptococcus mutans, and Candida albicans. The in vitro test cell culture experiments indicated that the cells on the PE-functionalized hydrogels had the greatest viability, adhesion, and spreading of cells. Altogether, these data highlight the interesting effect of propolis bio-functionalization to enhance the biological features of CNF/PVA hydrogel as a functional matrix for biomedical applications.

Efficient and ecological leather processing: replacement of lime and sulphide with dispase assisted by 1-allyl-3-methylimidazolium chloride

Leather is a collagen-based biomass prepared from raw skins or hides by a series of unit operations, in which the unhairing and fiber opening are extremely important operations. However, the conventional Na2S/Ca(OH)2 system used in unhairing and fiber opening has given rise to the pollution to the environment. It is necessary to develop substitute technology for the Na2S/Ca(OH)2. In the present study, 1-allyl-3-methylimidazolium chloride ([AMIm]Cl) was used to cooperate with dispase for cycle unhairing and one-pot beamhouse to recycle waste bovine hides and compared with conventional processing. During those processes, the mechanism of [AMIm]Cl-dispase synergistic unhairing and collagen fibers opening were studied. Besides, plant hazard, organic matter and [AMIm]Cl of wastewater from [AMIm]Cl-dispase process were respectively investigated and separated to evaluate the environmental and economic benefits of the [AMIm]Cl-dispase process. As a result, enzyme activity after unhairing by [AMIm]Cl-diapase system for using 5 times is higher than that by KCl-dispase system, and needs lower unhairing time, which is because of rapid penetration of [AMIm]Cl-dispase solution in bovine hides. For this reason, the tensile strength and elastic modulus of tanned leather from [AMIm]Cl-dispase process are higher than those from the KCl-diapase and conventional processes, and its hydrothermal shrinkage temperature is comparable to that of the conventional one. Because of the 58.13% lower wastewater discharge (WD), 66.60% lower total solids (TS), 97.23% lower ammonia nitrogen (NH3-N), non-toxic wastewater and organic matter recovery in wastewater are reached from [AMIm]Cl-dispase process, which is expected to be an alternative to the conventional process to reduce environmental pollution and realize the sustainable development of technology for leather manufacturing.

Graphical abstract

Catalyst-Free Click Chemistry for Engineering Chondroitin Sulfate-Multiarmed PEG Hydrogels for Skin Tissue Engineering

The quest for an ideal biomaterial perfectly matching the microenvironment of the surrounding tissues and cells is an endless challenge within biomedical research, in addition to integrating this with a facile and sustainable technology for its preparation. Engineering hydrogels through click chemistry would promote the sustainable invention of tailor-made hydrogels. Herein, we disclose a versatile and facile catalyst-free click chemistry for the generation of an innovative hydrogel by combining chondroitin sulfate (CS) and polyethylene glycol (PEG). Various multi-armed PEG-Norbornene (A-PEG-N) with different molecular sizes were investigated to generate crosslinked copolymers with tunable rheological and mechanical properties. The crosslinked and mechanically stable porous hydrogels could be generated by simply mixing the two clickable Tetrazine-CS (TCS) and A-PEG-N components, generating a self-standing hydrogel within minutes. The leading candidate (TCS-8A-PEG-N (40 kD)), based on the mechanical and biocompatibility results, was further employed as a scaffold to improve wound closure and blood flow in vivo. The hydrogel demonstrated not only enhanced blood perfusion and an increased number of blood vessels, but also desirable fibrous matrix orientation and normal collagen deposition. Taken together, these results demonstrate the potential of the hydrogel to improve wound repair and hold promise for in situ skin tissue engineering applications.

Inorganic Polymeric Materials for Injured Tissue Repair: Biocatalytic Formation and Exploitation

Two biocatalytically produced inorganic biomaterials show great potential for use in regenerative medicine but also other medical applications: bio-silica and bio-polyphosphate (bio-polyP or polyP). Biosilica is synthesized by a group of enzymes called silicateins, which mediate the formation of amorphous hydrated silica from monomeric precursors. The polymeric silicic acid formed by these enzymes, which have been cloned from various siliceous sponge species, then undergoes a maturation process to form a solid biosilica material. The second biomaterial, polyP, has the extraordinary property that it not only has morphogenetic activity similar to biosilica, i.e., can induce cell differentiation through specific gene expression, but also provides metabolic energy through enzymatic cleavage of its high-energy phosphoanhydride bonds. This reaction is catalyzed by alkaline phosphatase, a ubiquitous enzyme that, in combination with adenylate kinase, forms adenosine triphosphate (ATP) from polyP. This article attempts to highlight the biomedical importance of the inorganic polymeric materials biosilica and polyP as well as the enzymes silicatein and alkaline phosphatase, which are involved in their metabolism or mediate their biological activity.

Biomimetic Polyphosphate Materials: Toward Application in Regenerative Medicine

In recent years, inorganic polyphosphate (polyP) has attracted increasing attention as a biomedical polymer or biomaterial with a great potential for application in regenerative medicine, in particular in the fields of tissue engineering and repair. The interest in polyP is based on two properties of this physiological polymer that make polyP stand out from other polymers: polyP has morphogenetic activity by inducing cell differentiation through specific gene expression, and it functions as an energy store and donor of metabolic energy, especially in the extracellular matrix or in the extracellular space. No other biopolymer applicable in tissue regeneration/repair is known that is endowed with this combination of properties. In addition, polyP can be fabricated both in the form of a biologically active coacervate and as biomimetic amorphous polyP nano/microparticles, which are stable and are activated by transformation into the coacervate phase after contact with protein/body fluids. PolyP can be used in the form of various metal salts and in combination with various hydrogel-forming polymers, whereby (even printable) hybrid materials with defined porosities and mechanical and biological properties can be produced, which can even be loaded with cells for 3D cell printing or with drugs and support the growth and differentiation of (stem) cells as well as cell migration/microvascularization. Potential applications in therapy of bone, cartilage and eye disorders/injuries and wound healing are summarized and possible mechanisms are discussed.

Collagen-based biocomposites inspired by bone hierarchical structures for advanced bone regeneration: ongoing research and perspectives

Bone is a hard-connective tissue composed of matrix, cells and bioactive factors with a hierarchical structure, where the matrix is mainly composed of type I collagen and hydroxyapatite. Collagen fibers assembled by collagen are the template for mineralization and make an important contribution to bone formation and the bone remodeling process. Therefore, collagen has been widely clinically used for bone/cartilage defect regeneration. However, pure collagen implants, such as collagen scaffolds or sponges, have limitations in the bone/cartilage regeneration process due to their poor mechanical properties and osteoinductivity. Different forms of collagen-based composites prepared by incorporating natural/artificial polymers or bioactive inorganic substances are characterized by their interconnected porous structure and promoting cell adhesion, while they improve the mechanical strength, structural stability and osteogenic activities of the collagen matrix. In this review, various forms of collagen-based biocomposites, such as scaffolds, sponges, microspheres/nanoparticles, films and microfibers/nanofibers prepared by natural/synthetic polymers, bioactive ceramics and carbon-based materials compounded with collagen are reviewed. In addition, the application of collagen-based biocomposites as cytokine, cell or drug (genes, proteins, peptides and chemosynthetic) delivery platforms for proangiogenesis and bone/cartilage tissue regeneration is also discussed. Finally, the potential application, research and development direction of collagen-based biocomposites in future bone/cartilage tissue regeneration are discussed.

Gadolinium-doped mesoporous calcium silicate/chitosan scaffolds enhanced bone regeneration ability

Chitosan (CTS) and mesoporous calcium silicate (MCS) have been developed for bone defect healing; however, their bone regeneration capacity still does not satisfy the patients with bone diseases. Gadolinium (Gd) is accumulated in human bones, and plays a beneficial role in regulating cell performance and bone regeneration. We firstly constructed Gd-doped MCS/CTS (Gd-MCS/CTS) scaffolds by a lyophilization technology. The interconnected arrangement of CTS films lead to forming macropores by using ice crystals as templates during the lyophilization procedure, and the Gd-MCS nanoparticles dispersed uniformly on the macropore walls. The biocompatible chemical components and hierarchical pores facilitated the attachment and spreading of rat bone marrow-derived mesenchymal stem cells (rBMSCs). Interestingly, the Gd dopants in the scaffolds effectively activated the Wnt/β-catenin signaling pathway, resulting in excellent cell proliferation and osteogenic differentiation capacities. The osteogenic-related genes such as alkaline phosphatase (ALP), runt-related transcription factor 2 (Runx2) and collagen type1 (COL-1) were remarkably up-regulated by Gd-MCS scaffolds as compared with MCS scaffolds, and their expression levels increased in a positive correlation with Gd doping amounts. Moreover, in vivo rat cranial defect tests further confirmed that Gd-MCS/CTS scaffolds significantly stimulated collagen deposition and new bone formation. The exciting finding suggested the beneficial effects of Gd³⁺ ions on osteogenic differentiation and new bone regeneration, and Gd-MCS/CTS scaffolds can be employed as a novel platform for bone defect healing.

Mass-Production and Characterizations of Polyvinyl Alcohol/Sodium Alginate/Graphene Porous Nanofiber Membranes Using Needleless Dynamic Linear Electrospinning

The aim of this study was to investigate the feasibility of large-scale preparation of porous polyvinyl alcohol/sodium alginate/graphene (Gr) (Gr-AP) nanofiber membranes using a copper wire needleless dynamic linear electrode electrospinning machine. Furthermore, the effects of Gr concentrations (0, 0.0375, 0.075, 0.25, 0.5, and 0.75 wt.%) on the morphology, electrical, hydrophilicity and thermal properties were evaluated. Results indicate that the dynamic linear electrospun Gr-AP membranes have a high yield of 1.25 g/h and are composed of porous finer nanofibers with a diameter of 141 ± 31 nm. Gr improved the morphology, homogeneity, hydrophobicity and thermal stability of Gr-AP nanofiber membranes. The critical conductive threshold is 0.075 wt.% for Gr, which provides the nanofiber membranes with an even distribution of diameter, an optimal conductivity, good hydrophilicity, appropriate specific surface area and optimal thermal stability. Therefore, needleless dynamic linear electrospinning is beneficial to produce high quality Gr-AP porous nanofiber membranes, and the optimal parameters can be used in artificial nerve conduits and serve as a valuable reference for mass production of nanofiber membranes.

Amorphous polyphosphate, a smart bioinspired nano-/bio-material for bone and cartilage regeneration: towards a new paradigm in tissue engineering

Physiological amorphous polyphosphate nano/micro-particles, injectable and implantable, attract and stimulate MSCs into implants for tissue regeneration.

Uptake of polyphosphate microparticles in vitro (SaOS-2 and HUVEC cells) followed by an increase of the intracellular ATP pool size

Recently two approaches were reported that addressed a vitally important problem in regenerative medicine, i. e. the successful treatment of wounds even under diabetic conditions. Accordingly, these studies with diabetic rabbits [Sarojini et al. PLoS One 2017, 12(4):e0174899] and diabetic mice [Müller et al. Polymers 2017, 9, 300] identified a novel (potential) target for the acceleration of wound healing in diabetes. Both studies propose a raise of the intracellular metabolic energy status via exogenous administration either of ATP, encapsulated into lipid vesicles, or of polyphosphate (polyP) micro-/nanoparticles. Recently this physiological polymer, polyP, was found to release metabolic energy in form of ATP into both the extra- and also intra-cellular space. In the present work the uptake mechanism of the amorphous polyP microparticles "Ca-polyP-MP" has been described and found to be a clathrin-dependent endocytosis import, based on inhibition studies with the inhibitor trifluoperazine, which blocks the clathrin-dependent endocytosis import. The experiments had been performed with SaOS-2 cells, by studying the uptake and distribution of the electron-dense particles into the cells, and with HUVEC cells, for analysis of the intracellular accumulation of polyP, visualized by fluorescent staining of polyP. Concurrently with the uptake of particular polyP the intracellular ATP level increased as well. In contrast to "Ca-polyP-MP" the soluble polyP, administered as "Na-polyP[Ca2+]", did not cause an increase in the intracellular Ca2+ level, suggesting a different mode of action of these two forms of polyP. Based on existing data on the effect of polyP and ATP on the induction of vascularization during wound repair, both groups (Sarojini et al. and Müller et al.) propose that the acceleration of wound repair is based on an increased metabolic energy supply directly to the regenerating wound area.

Enhancement of Wound Healing in Normal and Diabetic Mice by Topical Application of Amorphous Polyphosphate. Superior Effect of a Host⁻Guest Composite Material Composed of Collagen (Host) and Polyphosphate (Guest)

The effect of polyphosphate (polyP) microparticles on wound healing was tested both in vitro and in a mice model in vivo. Two approaches were used: pure salts of polyphosphate, fabricated as amorphous microparticles (MPs, consisting of calcium and magnesium salts of polyP, "Ca⁻polyp-MPs" and "Mg⁻polyp-MPs"), and host⁻guest composite particles, prepared from amorphous collagen (host) and polyphosphate (guest), termed "col/polyp-MPs". Animal experiments with polyP on healing of excisional wounds were performed using both normal mice and diabetic mice. After a healing period of 7 days "Ca⁻polyp-MP" significantly improved re-epithelialization in normal mice from 31% (control) to 72% (polyP microparticle-treated). Importantly, in diabetic mice, particularly the host⁻guest particles "col/polyp-MP", increased the rate of re-epithelialization to ≈40% (control, 23%). In addition, those particles increased the expression of COL-I and COL-III as well as the expression the α-smooth muscle actin and the plasminogen activator inhibitor-1. We propose that "Ca⁻polyp-MPs", and particularly the host⁻guest "col/polyp-MPs" are useful for topical treatment of wounds.